Syngas Production with a Highly-Robust Nickel(II) Homogeneous Electrocatalyst in a Water-Containing System

Article abstract of DOI:10.1021/acscatal.8b02044

Syngas (CO and H₂) is an essential raw material for producing various chemicals in industry. The reduction of CO₂ in a water-containing system can serve as a more sustainable pathway to obtain syngas than the transformation of fossil

Full text of DOI:10.1021/acscatal.8b02044

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Article
Syngas Production with a Highly-Robust Nickel(II)
Homogeneous Electrocatalyst in a Water-Containing System
Jia-Wei Wang, Hai-Hua Huang, Jia-Kai Sun, Di-Chang Zhong, and Tong-Bu Lu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02044 • Publication Date (Web): 09 Jul 2018
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Syngas Production with a Highly-Robust Nickel(II) Homogeneous
Electrocatalyst in a Water-Containing System
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JiaꢀWei Wang,a HaiꢀHua Huang,a JiaꢀKai Sun,a DiꢀChang Zhong,*b and TongꢀBu Lu*ab
a MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun
YatꢀSen University, Guangzhou 510275, China
b Institute for New Energy Materials and Low Carbon Technologies, School of Material Science
& Engineering, Tianjin University of Technology, Tianjin 300384, China
* To whom correspondence should be addressed.
Eꢀmail: zhong_dichang@hotmail.com; lutongbu@mail.sysu.edu.cn.
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ABSTRACT: Syngas (CO and H2) is an essential raw material for producing various chemicals
in industry. The reduction of CO2 in a waterꢀcontaining system can serve as a more sustainable
pathway to obtain syngas than the transformation of fossil fuels, while the modulation of the
H2/CO ratios is a challenge. Herein a nickel(II) tripodal complex is employed as a homogeneous
electrocatalyst for CO2 and H2O reduction. With this catalyst, selective CO formation with
negligible H2 evolution can be accomplished in the presence of 5.0 M H2O in
N,N’ꢀdimethylformamide (DMF). By further varying the applied potentials, the H2/CO ratio can
be delicately tuned. The catalyst is appreciably robust with a high turnꢀover number of 1.9 × 106
in 1 day operation with negligible deactivation, which can be attributed to the redox innocence of
the used ligand. Based on the results of electrochemistry and DFT calculation, the catalytic
mechanism is proposed.
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KEYWORDS: carbon dioxide reduction, water reduction, syngas, mononuclear nickel complex,
electrocatalysis, high stability, redox innocence
INTRODUCTION
Synthetic gas (syngas), the mixture of carbon monoxide (CO) and hydrogen (H2), is a key
chemical feedstock in the production of chemical products, such as methanol, dimethyl ether,
acetic acid, etc.1ꢀ3 Nonꢀrenewable fossil fuels, especially natural gas (mainly CH4) and coal, are
the dominant sources of syngas production in current industry.2 Carbon dioxide (CO2) is viewed
as an abundant carbon source in preparation of valueꢀadded chemicals.4ꢀ5 In particular, the
utilization of solar energy or electricity to drive the reduction of both CO2 and H2O to gain
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syngas can be a promising renewable pathway for chemical production. This process requires
biꢀfunctional catalysts to reduce both CO2 and H2O. It should be mentioned that the design of
efficient catalysts for CO2 reduction presents a challenge,6 as the high thermodynamic stability of
CO2 makes its activation and reduction energyꢀdemanding.7ꢀ8 Recently, metal complexes as
molecular, homogeneous catalysts for CO2 reduction, have been considered as attractive
candidates due to the following reasons:9ꢀ13 (1) homogeneous catalysts have shown high activity,
stability and selectivity for the certain reactions; (2) catalytic intermediates of homogeneous
catalysts are relatively feasible to spectroscopic, crystallographic and theoretical
characterization, enabling the verification of catalytic mechanisms; (3) homogeneous catalysts
possess wellꢀdefined structures that can be deliberately optimized by synthetic methods; (4)
immobilizing homogeneous catalysts on various substrates can be a promising method to prepare
atomically dispersed heterogeneous catalysts with remarkably improved catalytic efficiency.
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In terms of syngas production, the H2/CO ratio is a key factor for the synthesis of downstream
products: syngas with 1:1 H2/CO ratio is needed in the hydroformylation to yield aldehyde14 and
the one with 2:1 is the required composition in FischerꢀTropsch hydrocarbon synthesis.1 In
modern technologies, the modulation of syngas composition relies on the additional waterꢀgas
shift reaction, which makes the entire process more costly.3 The coꢀreduction of CO2 and H2O to
obtain adjustable H2/CO ratios may eliminate the requirement for intermediate adjustments in
syngas composition, which, however, still remains a significant challenge. Besides the lethargic
activation of CO2,8 the reduction of H+/H2O to H2 is thermodynamically favored in contrast to
the CO2ꢀtoꢀCO conversion (ꢀ0.41 vs. ꢀ0.53 V vs. NHE at pH 7, Equations 1 and 2).6 A significant
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CO2 + 2e− + 2H+ → CO + H2O
2H+ + 2e− → H2
E0 = ꢀ0.53 V (1)
E0 = ꢀ0.41 V (2)
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preference of H2 evolution over CO production in waterꢀcontaining systems makes the H2/CO
ratio less controllable. To overcome this problem, the catalyst kinetically favoring the reduction
of CO2 over H+/H2O should be designed. Moreover, although selective CO2 reduction to CO
with homogeneous catalysts have been reported,15ꢀ19 effective strategies and systems to easily
control the H2/CO ratio are still limited.20 Kubiak and coꢀworkers21 constructed a system to
produce syngas by combining a homogeneous CO2ꢀreduction catalyst, Re(bpy)(CO)3Cl (bpy =
2,2’ꢀbipyridine), with an illuminated pꢀSi that acted as a photoelectrochemical heterogeneous
H2Oꢀreduction catalyst. Based on the remarkable catalytic activity of the rhenium complex
toward CO2ꢀtoꢀCO conversion, the addition of water increased H2 evolution and thus modulated
the H2/CO ratio. Meyer et al22 used a ruthenium complex, [Ru(tpy)(Mebimꢀpy)(H2O)](PF6)2 (tpy
= 2,2’:6’,2’’ꢀterpyridine; Mebimꢀpy = 3ꢀmethylꢀ1ꢀpyridylꢀbenzimidazolꢀ2ꢀylidene), as a
homogeneous CO2/H2Oꢀreduction electrocatalyst for syngas production. The H2/CO ratios were
regulated by varying the applied potentials and pH values. Obviously, simple and lowꢀcost
catalytic systems for syngas production with controllable H2/CO ratio are much desired.
Additionally, the longevity of those reported homogeneous catalysts for CO2/H2Oꢀreduction are
not satisfactory, which usually suffers from COꢀpoisoning,23 catalyst polymerization24ꢀ25 or
ligand hydrogenation/decomposition,7,26ꢀ27 causing low turnꢀover numbers (TONs) and thus
restricting their further applications.
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To address the above issues, a nickel(II) tripodal complex, [NiII(Me3NTB)(CH3CN)2](BF4)2 (1;
Me3NTB = tris(Nꢀmethylbenzimidazolꢀ2ꢀylmethyl)amine, Figure 1), was prepared as a
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homogeneous electrocatalyst for CO2ꢀtoꢀCO conversion in DMF/H2O electrolyte. During
electrocatalysis, the ratio of H2 and CO in the products can be effectively controlled by adding
appropriate amounts of water or varying applied potentials in the system. Benefitting from the
redox innocence of Me3NTB ligand, 1 can function as a highly robust catalyst for CO2ꢀtoꢀCO
conversion with a high TON.
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Figure 1. (a) The schematic structure of 1. (b) The crystallographic structure of 1. Hydrogen
atoms, counter anions and guest solvents were omitted, probability = 30%.
RESULTS AND DISCUSSION
Synthesis and Structure. 1 was obtained as violet crystals from a CH3CN solution of
Ni(BF4)2·6H2O and Me3NTB. The result of Xꢀray structural analysis reveals that the structure of
ꢀ
1 contains a [NiII(Me3NTB)(CH3CN)2]2+ cation, two BF4 counter anions and three guest
acetonitrile molecules (Table S1). As shown in Figure 1, the nickel(II) center is sixꢀcoordinated
to four N atoms from the tripodal ligand and two N atoms from two CH3CN molecules, forming
a slightly distorted octahedral geometry. Dissolved in DMF solution, 1 remains a mononuclear
structure based on the result of electroꢀspray ionization mass spectrometry (ESIꢀMS, Figure S1).
The UVꢀVis spectrum of 1 in DMF solution exhibits a characteristic band at ca. 600 nm (Figure
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S2), which can be assigned to the dꢀd transition of NiII. This absorbance remained unchanged in
the presence of water after 1 day of aging, showing the high stability of 1 in the solution.
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Cyclic Voltammetry under Argon. Initially, the redox property of 1 was investigated by
cyclic voltammetry in anhydrous DMF in the presence of 0.1 M tetra(nꢀbutyl)ammonium
hexafluorophosphate (TBAPF6) at a glassyꢀcarbon (GC) disc electrode (1 mm diameter) in
absence of proton source. Unless otherwise stated, all potentials were footnoted as versus normal
hydrogen electrode (vs NHE). The cyclic voltammogram (CV) of 1 under argon featured two
reduction peaks at ꢀ1.33 and ꢀ1.63 V, respectively (Figure 2). The two peaks are assigned as
metalꢀcentered NiII/I and NiI/0 reduction, respectively, as the Me3NTB ligand cannot be reduced
at the same conditions (Figure S3, Equations 3 and 4). According to the RandlesꢀSevcik analysis
at the quasiꢀreversible NiII/I couple at E1/2 = ꢀ1.29 V (ca. 80 mV peakꢀtoꢀpeak separation), 1 is
freely diffusing in solution with a diffusion coefficient of D = 1.2 × 10−6 cm2/s (see SI and Figure
S4 for details).28
Figure 2. CVs scanned at potentials from 0 to (a) ꢀ1.5 V and (b) ꢀ1.8 V with 1.0 mM 1 in a 0.1
M TBAPF6 DMF solution under 1 atm argon (black) and CO2 (violet) at a scan rate of 100 mV/s.
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II
−
I
2+
+
[Ni (Me3NTB)] + e → [Ni (Me3NTB)]
Ep = ꢀ1.33 V (3)
Ep = ꢀ1.63 V (4)
I
−
0
+
[Ni (Me3NTB)] + e → [Ni (Me3NTB)]
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Cyclic Voltammetry under CO2. Subsequently, the cyclic voltammetry of 1 under 1 atm CO2
in an anhydrous DMF solution were recorded. As shown in Figure 2a, the CV of 1 displayed that
the NiII/I redox couple became irreversible and the reduction peak shifted positively by ca. 80
mV (at ꢀ1.25 V), followed by a sharp increase of the current that peaked at ꢀ1.60 V. The positive
shift of the NiII/I reduction suggests the binding of CO2 to the NiI species of 1 (Equation 5).23 The
irreversibility and current increase may result from a rapid, irreversible chemical step following
the binding of CO2, most probably catalytic CO2 reduction,23 which was clarified by
controlledꢀpotential electrolysis (CPE, see below) experiments.
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I
+
+
[Ni (Me3 NTB)] + CO2 → [Ni(CO2 )(Me3NTB)]
(5)
The catalytic CO2 reduction reaction is first order in the concentration of catalyst ([1]), as a
linear relationship by plotting catalytic currents (icat) at ꢀ1.60 V versus [1] (Figure S5).
Additionally, plotting icat versus the square root of [CO2] revealed a linear relationship (Figure
S6a), showing that the catalytic reaction is also first order in [CO2]. The reaction constant of CO2
binding (KCO2) can be calculated following the Equation (6).
RT
ꢀECO
=
ln{1 + [CO2 ]KCO }
(6)
2
2
nF
Equation (6) illustrates an ERCcat mechanism, where electron transfer and onward/backward
reactions are substantially fast and KCO2 is relatively large.29 Although in our case the reverse
reaction of CO2 binding is slow due to the absence of NiII/I oxidation wave under CO2
atmosphere (Figure 2), this equation can still be applied here to estimate the KCO2 according to
the work of Sampson et al.30 Since the observed shift in potential of the cathodic peak (ꢁECO2) at
low [CO2] is a linear function of ln[CO2] with a slope (22.4) near 26 mV/decade (Figure S7), a
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KCO2 of ca. 350 M−1 can be tentatively estimated according to the Equation 6.23,29ꢀ30 This value is
considerably higher than those wellꢀdefined electrocatalysts for CO2 reduction, such as
[Ni(cyclam)]2+ (6 Mꢀ1; cyclam = 1,4,8,11ꢀtetraazacyclotetradecane)23 and [Mn(mesbpy)(CO)3]+
(46±10 Mꢀ1; mesbpy = 6,6’ꢀdimesitylꢀ2,2’ꢀbipyridine),30 demonstrating a high affinity of the
catalyst to CO2. This result also infers its high selectivity toward CO2 reduction rather than
hydrogen evolution. The facile CO2ꢀbinding is attributable to the open site of Me3NTB ligand
platform. In addition, the strong basicity of methylated benzimidazole Nꢀdonors31 offers
substantial electron density to the NiI center and also facilitates the binding of CO2 at the carbon
atom.
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Cyclic Voltammetry with Proton Addition. It can be noticed that, under CO2 and dry
condition, the catalytic current at the NiII/I reduction wave (ꢀ1.25 V) was not significant and a
more drastic current appeared at potentials more negative than ꢀ1.50 V (Figure 2), indicating that
the catalytic reduction reaction following the CO2 binding is slow. Based on our previous
findings32 and other reports,33ꢀ34 the slow catalytic reduction reaction may be ascribed to the fact
that the cleavage of the CꢀO bond in the [NiꢀCO2]+ adduct is unfavorable without proton source.
Thus we were intended to add water as a proton source in the system of 1 to improve its
performance.23,33,35 Along with the addition of water, the CV of 1 under argon showed that both
of the peak potentials of NiII/I and NiI/0 reduction remained unchanged, which demonstrates that
no protonꢀdependent transfer was involved in the two redox processes.23 However, their currents
would increase along with increasing [H2O] (Figure 3a). In particular, the peak current at NiI/0
increased dramatically at [H2O] ≥ 7.0 M. The increase in current suggests that the Ni0 species of
1 can reduce protons to evolve hydrogen from water (Equation 7).
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0
+
II
2 +
Ni (Me3NTB) + 2H → [Ni (Me3NTB)] + H2
(7)
When CO2 was introduced into this waterꢀcontaining catalytic system, the peak position of the
catalytic wave (at 1.60 V in anhydrous DMF) was positively shifted (Figure 3b). The shift of the
catalytic peak (ꢁE) reached to a maximum of ca. 0.23 V with [H2O] ≥ 8.0 M, and the current at
NiII/I peak (ꢀ1.25 V) was also enhanced substantially. Similar positive shifts were found in the
CVs of 1 upon the addition of two other Brønsted acids, 2,2,2ꢀtrifluoroethanol (TFE) and phenol
(Figure S8). These observations demonstrate that the addition of protons thermodynamically
facilitates the CO2 reduction process and thus lowers the catalytic potential. This facilitation is
likely to undergo a concerted protonꢀcoupled electron transfer (PCET) involving a protonation
on the NiꢀCO2 adduct to activate the formation of product (Equation 8).23,33 The above findings
indicate that, assisted by the water addition, the catalytic reduction of CO2 would be
thermodynamically favored via a protonation pathway.
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I
−
ꢀ
+
+
[Ni (Me3NTB)] + H2O + CO2 + e → [Ni(COOH)(Me3NTB)] + OH
(8)
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Figure 3. CVs of 1.0 mM 1 in a 0.1 M TBAPF6 DMF solution under 1 atm (a) argon and (b)
CO2 in the presence of water ([H2O] = 0 ~ 11.0 M), scan rate = 100 mV/s.
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Scheme 1. Proposed mechanism of CO2ꢀtoꢀCO conversion catalyzed by 1.
Proposed Mechanism. Based on some reported studies32,36 and the experimental results,
including the fact that the main product of catalytic CO2 reduction with 1 is CO (see below), a
catalytic mechanism of 1 can be tentatively proposed as Scheme 1, which is further clarified by
DFT calculations. At the beginning, the NiI species is generated by oneꢀelectron reduction of 1,
followed by the dissociation of acetonitrile molecules and then binding with CO2 to form a
[NiꢀCO2]+ species. The NiII/NiI reduction potential was estimated as ꢀ1.27 V, close to the
experimental value (ꢀ1.33 V). Next, the weak Brønsted acid (i.e. water) protonates the NiꢀCO2
intermediate to yield a NiꢀCOOH species, and thus facilitates the cleavage of its CꢀOH bond to
produce CO.33,37 The calculation reveals that the energy barrier of NiꢀCO2 formation (TS 1; 7.7
kcal/mol) is lower than that of COꢀcleavage step (TS 2; 8.9 kcal/mol). This result indicates that
the COꢀcleavage process is the rateꢀdetermining step rather than the CO2ꢀbinding process, which,
as mentioned before, can be attributed to the high affinity of the NiI intermediate with CO2.
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Modulation of Product Selectivity. To determine the composition of CO2 reduction product,
CPE experiments were carried out with 1.0 mM 1 in a DMF solution with different
concentrations of water at ꢀ1.24 V in a oneꢀroom cell. As shown in Table 1 (Entry 1), after CPE
for 6 h in an ‘anhydrous’ DMF, the Faradaic efficiency of 1 for CO production [denoted as
F(CO)] was determined as ~82%. With increasing water (3.0 ~ 11.0 M), the F(CO) values can be
improved to above 90%, concomitant with decreasing amounts of H2. With 5 M H2O in DMF,
the F(CO) reached the highest value (~ 96%) (Entry 6 in Table 1). These results show that the
CO selectivity and the F(CO) of 1 can be readily improved by the addition of water into this
system.
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Table 1. Electrocatalytic reduction of CO2 by 1.a
Entry
1
[H2O] (M)
Q (C)
n(CO) (ꢁmol)
13.2 ± 0.4
13.9 ± 0.4
13.8 ± 0.5
16.8 ± 0.6
20.5 ± 0.4
21.8 ± 0.8
22.8 ± 0.7
24.8 ± 1.2
25.3 ± 1.7
9.2 ± 0.5
n(H2) (ꢁmol)
2.51 ± 0.21
1.63 ± 0.11
1.12 ± 0.21
0.70 ± 0.05
0.81 ± 0.10
0.70 ± 0.08
1.27 ± 0.15
2.04 ± 0.32
2.07 ± 0.24
1.70 ± 0.20
0.83 ± 0.10
0.20 ± 0.05
CO selectivity
F(CO) (%)
0
1.0
2.0
3.0
4.0
5.0
6.0
8.0
11.0
0
3.1 ± 0.2
3.2 ± 0.2
3.0 ± 0.3
3.6 ± 0.3
4.3 ± 0.2
4.4 ± 0.3
4.8 ± 0.5
5.3 ± 0.3
5.6 ± 0.6
2.2 ± 0.4
3.8 ± 0.5
3.2 ± 0.3
84
90
92
96
96
98
95
93
92
84
96
98
82
84
89
90
92
96
92
90
88
81
95
48
2
3
4
5
6
7
8
9
10b
11b
12c
5.0
0
18.7 ± 1.0
8.0 ± 0.8
a
CPE experiments were operated with 1.0 mM 1 in a 0.1 M TBAPF6 DMF solution at Ecat
=
b
ꢀ1.24 V for 6 h in a oneꢀroom cell. A carbon rod was used as the counter electrode instead of
the Pt wire. c CH3CN was employed as the solvent instead of DMF.
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Several experiments were operated to verify the origins of the significant hydrogen evolution
at anhydrous conditions. The results are included in Table 1. First, no significant changes were
observed when a carbon rod was used as the counter electrode. Especially, the results in Entry 10
indicate that the replacement of counter electrode could not suppress the hydrogen evolution in
the absence of water. These results infer that the contamination of Pt ions from the Pt anode38ꢀ39
is less likely to induce the hydrogen evolution. Then, we used CH3CN instead of DMF as the
electrolyte. As shown in Entry 12, the Faradaic efficiency and yield of CO production were
relatively low in contrast to those in the case with DMF. Such differences induced by solvents
were also reported.18 We tentatively attribute it to the stronger coordination ability of CH3CN,
which hinders the substrate (CO2) from binding to the metal catalytic center and subsequent
catalysis. Importantly, negligible hydrogen was evolved with CH3CN as the solvent, suggesting
that the presence of DMF is an origin of significant hydrogen evolution under anhydrous
conditions.
10
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45
46
47
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49
50
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52
53
54
55
56
57
58
59
60
With the above results, we speculate that the DMF might be oxidatively decomposed at the Pt
electrode in the oneꢀroom cell. Such oxidative degradation at the Pt anode may generate proton
source and facilitate the hydrogen evolution. However, due to the competition of water oxidation,
the degradation of the waterꢀcontaining electrolyte is much slighter. That is one of the reasons
why the CO selectivity was enhanced along with adding water. To confirm this speculation, we
utilized a Nafion membrane to isolate the cathode and anode, followed by the CPE experiments
at different concentrations of water. This setꢀup can avoid the contamination of degraded
electrolyte at the cathode. As summarized in Table S2, different from those in the oneꢀroom cell,
the results from the twoꢀroom cell reveal that the hydrogen evolution became increasingly
significant in the CPE along with increasing water. More importantly, it can be found that little
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hydrogen was detected during the CPE in dry DMF, achieving a CO selectivity of 99%. These
results support that the oxidation degradation of electrolyte at the anode is a dominant origin of
significant hydrogen evolution in dry DMF, which can be avoided by using a twoꢀroom cell. At
present, we consider that one of the decomposition pathways is to generate formate, and the
corresponding dimethylamine40 (susceptible to further oxidation). However, the decomposition
of DMF is complicated and the detailed identification of the products is still on the progress. On
the other side, the F(CO) values still revealed an improvement along with the addition of water.
This observation can be attributed to the facilitation of the PCETꢀinvolved catalysis by water
addition (Equation 8).
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In both types of electrolytic cells, although a relatively large amount of water (approximately
20% volume ratio) was added in the DMF system, the hydrogen evolution was not significant,
with COꢀselectivity values over 89%. The F(CO) values were even enhanced by adding
appropriate amounts of water. Based on our experimental observations and the results from
Savéant et al,41 a feasible interpretation of the factors favorable to CO formation against H2
evolution in the presence of water can be described as follows. First, the high KCO2 of NiI species
of 1 indicates its higher intrinsic affinity for CO2, indicating that CO2 is the more advantageous
substrate than H+ in this catalytic reaction. Second, the thermodynamic barrier for CO production
by 1 is readily lowered by the addition of water, as observed in the CVs along with water
addition, where the NiIꢀinvolved catalytic wave was positively shifted due to the participation of
PCET (Figure 3 and Equation 8). We consider this factor readily improves the F(CO) along with
ꢀ
the water addition. Third, the reaction that converts CO2 and H2O into HCO3 and H+, is
thermodynamically difficult with an equilibrium of approximately 10ꢀ6,37 leading to a much
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lower concentration of H+ than CO2 in DMF/H2O system. In all, the above factors favor the
CO2ꢀtoꢀCO pathway and restrict the H2ꢀevolution one, resulting in the high CO selectivity in the
presence of water.
10
11
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59
60
To further investigate the factors that influence the product selectivity of 1, we also conducted
CPE measurements in DMF/H2O solution ([H2O] = 5.0 M) at varying potentials (ꢀ1.19 ~ ꢀ1.49
V). The results shown in Table 2 illustrate that when the reduction potential was varied from
ꢀ1.19 to ꢀ1.24 V, the CO yield and selectivity were improved. From ꢀ1.24 to ꢀ1.49 V, the CO
yield was also enhanced, however, more hydrogen was evolved and the selectivity of CO
formation markedly decreased. Further negatively increasing the potential to ꢀ1.44 and ꢀ1.49 V,
substantial amounts of syngas products with H2/CO ratio of 1:1 and 2:1 could be generated,
respectively (Entry 4 and 5 in Table 2). This can be attributed to the fact that these applied
potentials are increasingly closer to the NiI/0 reduction, as observed in previous CVs (Figure 3).
The formed Ni0 species possesses stronger affinity to H2O over CO2 and tends to proceed the
reduction of H2O to H2 (Equation 7), leading to the syngas production with higher H2/CO ratios.
Therefore, by controlling the applied potentials in the system with 1, the ratio of the products of
electrocatalytic CO2 reduction and H2O reduction can be tuned. To the best of our knowledge,
this is the first catalytic system that can produce syngas with tunable compositions with a
molecular catalyst based on earthꢀabundant elements.
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Table 2. Electrocatalytic reduction of CO2 by 1 at ꢀ1.19 ~ ꢀ1.49 V.a
6
7
8
9
n(CO)
n(H2)
Entry
Ecat (V)
Q (C)
H2:CO F(CO) (%) F(H2) (%)
(ꢁmol)
(ꢁmol)
1
2
3
4
ꢀ1.19
ꢀ1.24
ꢀ1.34
ꢀ1.44
ꢀ1.49
2.2 ± 0.1
4.4 ± 0.3
6.6 ± 0.4
6.9 ± 0.3
7.3 ± 0.4
10.0 ± 0.2
21.3 ± 0.8
29.3 ± 0.8
14.8 ± 0.4
10.1 ± 0.7
1.02 ± 0.04
0.72 ± 0.08
3.82 ± 0.10
14.5 ± 0.2
20.4 ± 0.5
1:9
1:50
1:9
1:1
2:1
88
96
86
41
27
8.9
3.2
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59
60
41
5
54
a
CPE experiments were operated with 1.0 mM 1 in a 0.1 M TBAPF6 DMF solution in the
presence of 5.0 M H2O for 6 h.
After CPE experiments at ꢀ1.19 ~ ꢀ1.49 V with 1 for 6 h, besides CO and H2, no formaldehyde
and oxalate but a trace amount of formate (~1 ꢁmol) was detected by capillary electrophoresis.
We noticed that a similar amount of formate was also discovered in the electrolyte after the CPE
experiment without 1. Thus, the formation of the formate is attributable to the decomposition of
DMF rather than 1. In the meantime, during all CPE experiments in the absence of 1, no CO but
the aforementioned formate and certain amounts of hydrogen can be detected. These results
further demonstrate that the CO generation originates from the CO2 reduction by 1.
Investigation of Catalyst Robustness. The stability of 1 was initially checked by cyclic
voltammetry before and after CPE. It can be seen that a small current enhancement was obtained
after the CPE at anhydrous conditions in the oneꢀroom cell, while no significant change took
place by using the twoꢀroom cell (Figure S9a). These observations further indicate that additional
proton source was generated in the oneꢀroom cell and enhanced the catalytic current, which
could be prevented by the use of twoꢀroom cell. More importantly, the two reduction waves are
still wellꢀdefined in both cases, demonstrating the high robustness of 1 during electrocatalytic
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CO2 reduction. Moreover, with the addition of 5 M H2O, negligible changes were observed in the
CVs (Figure S9b) and the characteristic dꢀd transition band in the UVꢀVis spectra (Figure S10)
before and after CPE in the oneꢀroom cell for 6 h. These results further indicate that the addition
of water can diminish the DMF decomposition and that 1 is stable during electrocatalysis. Also,
the results of 13C labeling experiment demonstrate that the dominant CO product originates from
CO2 rather than the decomposition of 1 or the organic electrolyte (Figure S11). Furthermore, no
nickel residue was found on the surface of working electrode after CPE, as determined by the
energy dispersive Xꢀray spectroscopy (EDX, Figure S12), which clearly demonstrates that no
nickelꢀbased heterogeneous catalyst is involved in electrocatalysis of 1.42 We also conducted the
EDX measurements on the working electrodes used in CPE at ꢀ1.44 and ꢀ1.49 V (Figure S13).
No deposit was found in either working electrode after CPE, which further demonstrates the high
stability of 1 during CPE process.
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To further check the stability of 1, the CPE experiment by 1 was elongated up to 24 h. The
result shows that a relatively stable current density (j = 0.12 mA/cm2, Figure S14) was obtained
along with an accumulated charge of 22.2 C during a 24 h CPE. As shown in Figure 4, 104.6
ꢁmol of CO was detected, corresponding to a Faradaic efficiency of 91 ± 5%, concomitant with a
trace amount of H2 (~2%) and formate (~1%). Based on the number of catalyst molecules in the
catalytic system, a total turnꢀover number (TON) value of 10.4 is calculated. This value,
however, is considered as a lowered estimate, as only a small fraction of catalyst molecules that
interact with the electrode are contributing to catalysis.43 Therefore, we applied Equation (2) to
calculate the intrinsic catalytic rate (kcat = 757.3 sꢀ1) of 1 based on the number of catalyst
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molecules in the diffusion layer of the cathode.44 In Equation 9, [1] is the concentration of 1 (1.0
mM), E is the applied potential in CPE
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Figure 4. Chronocoulometric plot (purple line and black pentagon) and the yields of CO (black
star) and H2 (black circle) obtained in a 24 h CPE at ꢀ1.24 V in DMF solution at a GC plate (1.5
cm2) in the presence of 1.0 mM 1, 0.1 M TBAPF6 and 5.0 M H2O under 1 atm CO2.
1
1
[1]D2 (2kcat )2 F
j =
TOF =
TON =
(9)
(10)
(11)
F
1+ exp[
(E − Ecat )]
RT
kcat
F
1+ exp[
(E − Ecat )]
RT
kcat
× t
F
1+ exp[
(E − Ecat )]
RT
(−1.24 V), Ecat is the standard potential for catalysis (namely the potential of NiII/I reduction at
−1.33 V), T is the environmental temperature (298.15 K), and j is the averaged electrolytic
current density (0.12 mA/cm2). Since the catalytic current remained stable during 24 h
electrolysis, a TOF of 22.1 sꢀ1 and a remarkable TON of 1.9 × 106 could be calculated based on
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Equations 10 and 11.45 To date, among the reported nickel electrocatalysts for CO2ꢀtoꢀCO
conversion, [Ni(cyclam)]2+ 46 and its derivatives47ꢀ49 are a type of wellꢀknown efficient catalysts
at mercuryꢀbased electrodes, which, however, beyond the scope of homogeneous catalysis.50
Herein, featuring high values of TON and Faradaic efficiency for CO production, the catalytic
performance of 1 is competent to those pioneering mononuclear nickelꢀbased homogeneous
electrocatalysts toward CO2ꢀtoꢀCO conversion (Table S2).26,50ꢀ51 These results demonstrate that 1
is a highly selective and stable catalyst for electrocatalytic CO2ꢀtoꢀCO conversion. Furthermore,
by controlling the applied potentials, the syngas with diverse ratio of H2/CO could be produced,
which further makes 1 outstanding among reported nickelꢀbased homogeneous electrocatalysts.
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To reveal the high robustness of 1, a nickel tripodal complex of [Ni(NTB)(CH3CN)2](BF4)2 (2;
NTB = tris((1Hꢀbenzo[d]imidazolꢀ2ꢀyl)methyl)amine, Figure 5) with similar structure to 1 was
designed and synthesized. The CV (Figure S15a) and CPE results (Table 3) indicate that 2 can
also be used as an electrocatalyst for CO2ꢀtoꢀCO conversion in DMF. However, 2 was unstable
during the electrocatalytic process, as indicated by its low overall Faradaic efficiencies (< 60%,
Table 3), decreased catalytic current (Figure S16) and Niꢀbased residues on the used working
electrodes (Figure S17) after CPE at ꢀ1.24 V for 6 h. The instability could be attributed to its
redoxꢀactive NTB ligand, as verified by the CV of NTB ligand, where a large irreversible
reduction wave appeared at potentials more negative than ꢀ1.2 V (Figure S18). In contrast,
Me3NTB cannot be reduced at potentials less negative than ꢀ1.8 V in DMF (Figure S18). The
redox inertness of Me3NTB is due to the methyl groups on the imidazole fragments.52 This
finding was further supported by the parallel experiments on other Ni(II) complexes with
redoxꢀactive pyridyl53 fragments (Figure 5). Although they were found to be effective in
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electrocatalytic CO2 reduction (Figure S15), all of them would decompose during
electrocatalysis (Table 3). On the basis of the above results, it can be concluded that the high
stability of 1 during catalysis is due to its redoxꢀinnocent ligand. This observation is presently
limited in nickelꢀbased catalysts for CO2 reduction, which, however, should be instructive in the
design of more efficient and robust molecular catalysts for electrochemical CO2 reduction in
future.
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Figure 5. Investigated Ni(II) complexes (1ꢀ5) for electrocatalytic CO2 reduction in DMF. TPA =
tris(2ꢀpyridylmethyl)amine; MEP = N,N’ꢀdimethylꢀN,N’ꢀbis(pyridinꢀ2ꢀylmethyl)ꢀ1,2ꢀdiaminoꢀ
ethane; MCP = N,N’ꢀdimethylꢀN,N’ꢀbis(pyridinꢀ2ꢀylmethyl)ꢀ1,2ꢀdiaminocyclohexane.
Table 3. Electrocatalytic reduction of CO2 by Ni complexes.a
Entry Catalysts
Ecat
Q (C)
n(CO)
F(CO)
n(H2)
F(H2)
Deposit
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2
3
4
5
6
7
8
9
(V)
(µmol)
(%)
82
96
47
56
19
42
(µmol)
(%)
16
1
2b
3
ꢀ1.24
ꢀ1.24
ꢀ1.24
ꢀ1.24
ꢀ1.55
ꢀ1.55
3.1 ± 0.2
4.4 ± 0.3
4.3 ± 0.6
4.9 ± 0.3
5.0 ± 0.5
3.5 ± 0.3
13.2 ± 0.4
21.8 ± 0.8
10.5 ± 1.5
14.1 ± 0.7
4.9 ± 0.4
7.6 ± 0.9
2.51 ± 0.21
0.70 ± 0.08
2.40 ± 0.30
2.71 ± 0.20
1.49 ± 0.10
3.10 ± 0.40
No
No
1
1
2
3
4
5
3.1
11
10
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45
46
47
48
49
50
51
52
53
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58
59
60
Yes
Yes
Yes
Yes
4
11
5
5.7
17
6
a CPE experiments were operated with 1.0 mM catalyst in a 0.1 M TBAPF6 DMF solution for 6
b
h. The generated amount of formate after each entry of CPE was less than 0.1 ꢁmol. CPE was
conducted in the presence of 5.0 M H2O.
CONCLUSION
In conclusion, we have prepared a nickel(II) tripodal complex which can be utilized as a
homogeneous electrocatalyst for syngas production in a waterꢀcontaining medium. It has been
found that both adding water and varying the applied potential can well tune the composition of
evolved syngas. Moreover, this catalyst can be operated with high stability that is contributed by
its robust ligand. Its earthꢀabundance and synthetic ease are also notable. We believe that this
study can open an avenue for the development of excellent catalysts and shed light on the control
of product composition in CO2 reduction reaction.
EXPERIMENTAL SECTION
Materials. Me3NTB,54 NTB,55 nickel complexes (2,56 3,57 458 and 559) were synthesized
according to the literature methods. TBAPF6 (99%+, J&K) was dried under vacuum overnight
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2
3
4
5
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7
8
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with P2O5. CO2 (99.999%), argon (99.999%), 13CO2 (99%), DMF (99.8%, water < 0.005%,
J&K), CH3CN (99.8%, water < 0.005%, J&K), MilliꢀQ ultrapure water and other chemicals are
commercially available and used without further purification. Electrodes and other
electrochemical accessories were all purchased from GaussUnion Co., Ltd.
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Characterization. Xꢀray single crystal diffraction data for 1 were obtained on an Agilent
Technologies Gemini A Ultra system at ꢀ123 °C, with Cu/Kα radiation (λ = 1.54178 Å).
Electrochemical measurements were performed using a CHI 760E electrochemical workstation.
All potentials were referenced against Ag/AgNO3 reference electrode (0.10 M), and converted
to NHE by adding 0.60 V to the measured potentials. Unless otherwise stated, all potentials were
footnoted as vs NHE. Gas composition was analyzed on an Agilent 7820A gas chromatography
equipped with a thermal conductivity detector and a TDXꢀ01 packed column. The gas products
13
of CO2 isotopic experiment were checked by an Agilent 7890 GCꢀMS. The liquid products of
the reaction system were analyzed by capillary electrophoresis. UVꢀVis spectra were recorded on
a Shimadzu UVꢀ3600 spectrophotometer. EDX spectra were collected on a field emission
scanning electron microscopes (FEI, Quanta 400 or Quanta EFG 250). All the calculations were
carried out with Gaussian 09 program.60 All the structures were optimized at the BP8661/BSI
level of theory (BSI designated the basis set combination of SDD62 for Ni atom and 6ꢀ31g(d) for
nonꢀmetal atoms).
Synthesis of 1. Ni(BF4)2·6H2O (0.34 g, 1.0 mmol) and Me3NTB (0.45 g, 1.0 mmol) were
dissolved in 10 mL CH3CN to form a clear violet solution. Violet crystals (84% yield),
1⋅(CH3CN)3 (formulated based on the crystal structure), were isolated from slow diffusion of
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ether into the CH3CN solution in 2 d. After drying under vacuum, violet powder of 1 would be
obtained. Calcd for C31H36B2F8N9Ni (1): C 48.55, H 4.73, N 16.44%; found: C 48.65, H 4.60, N
16.45%.
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ASSOCIATED CONTENT
Supporting Information.
Supplementary experimental detail, additional figures and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors.
*Eꢀmail: zhong_dichang@hotmail.com (D. ꢀC. Zhong)
*Eꢀmail: lutongbu@mail.sysu.edu.cn (T. ꢀB. Lu)
ACKNOWLEDGMENTS
We are grateful for the financial support from National Key R&D Program of China
(2017YFA0700104) and NSFC (21331007 and 21790052).
REFERENCES
(1) Dry, M. E. The FischerꢀTropsch Process: 1950ꢀ2000. Catal. Today 2002, 71, 227ꢀ241.
(2) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. Recent Advances in the Conversion of Methane
to Synthesis Gas. Catal. Today 1995, 23, 3ꢀ15.
(3) Wender, I. Reactions of Synthesis Gas. Fuel Process. Technol. 1996, 48, 189ꢀ297.
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(4) Hollingsworth, N.; Taylor, S. F.; Galante, M. T.; Jacquemin, J.; Longo, C.; Holt, K. B.; de
Leeuw, N. H.; Hardacre, C. Reduction of Carbon Dioxide to Formate at Low Overpotential
Using a Superbase Ionic Liquid. Angew. Chem. Int. Ed. 2015, 54, 14164ꢀ14168.
(5) Wang, W. H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. CO2 Hydrogenation
to Formate and Methanol as an Alternative to Photoꢀ and Electrochemical CO2 Reduction. Chem.
Rev. 2015, 115, 12936ꢀ12973.
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15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(6) Schneider, J.; Jia, H.; Muckerman, J. T.; Fujita, E. Thermodynamics and Kinetics of CO2,
CO, and H+ Binding to the Metal Centre of CO2 Reduction Catalysts. Chem. Soc. Rev. 2012, 41,
2036ꢀ2051.
(7) Hammouche, M.; Lexa, D.; Momenteau, M.; Savéant, J. M. Chemical Catalysis of
Electrochemical Reactions. Homogeneous Catalysis of the Electrochemical Reduction of Carbon
Dioxide by Iron("0") Porphyrins. Role of the Addition of Magnesium Cations. J. Am. Chem. Soc.
1991, 113, 8455ꢀ8466.
(8) Savéant, J. M. Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects.
Chem. Rev. 2008, 108, 2348ꢀ2378.
(9) Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. Electrons, Photons, Protons and
EarthꢀAbundant Metal Complexes for Molecular Catalysis of CO2 Reduction. ACS Catal. 2017,
7, 70ꢀ88.
(10) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic Reduction of CO2 Using Metal
Complexes. J. Photochem. Photobiol., C 2015, 25, 106ꢀ137.
(11) Wang, J.ꢀW.; Liu, W.ꢀJ.; Zhong, D.ꢀC.; Lu, T.ꢀB. Nickel Complexes as Molecular Catalysts
for Water Splitting and CO2 Reduction. Coord. Chem. Rev. 2017, DOI:
10.1016/j.ccr.2017.12.009, in press.
24
ACS Paragon Plus Environment

Page 25 of 33
ACS Catalysis
1
2
3
4
5
6
7
8
9
(12) Sampson, M. D.; Kubiak, C. P. Manganese Electrocatalysts with Bulky Bipyridine Ligands:
Utilizing Lewis Acids to Promote Carbon Dioxide Reduction at Low Overpotentials. J. Am.
Chem. Soc. 2016, 138, 1386ꢀ1393.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(13) Mondal, B.; Rana, A.; Sen, P.; Dey, A. Intermediates Involved in the 2eꢀ/2H+ Reduction of
CO2 to CO by Iron(0) Porphyrin. J. Am. Chem. Soc. 2015, 137, 11214ꢀ11217.
(14) Franke, R.; Selent, D.; Borner, A. Applied Hydroformylation. Chem. Rev. 2012, 112,
5675ꢀ5732.
(15) Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J. M. ThroughꢀSpace Charge Interaction
Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst
of CO2ꢀtoꢀCO Electrochemical Conversion. J. Am. Chem. Soc. 2016, 138, 16639ꢀ16644.
(16) Hong, D.; Tsukakoshi, Y.; Kotani, H.; Ishizuka, T.; Kojima, T. VisibleꢀLightꢀDriven
Photocatalytic CO2 Reduction by a Ni(II) Complex Bearing a Bioinspired Tetradentate Ligand
for Selective CO Production. J. Am. Chem. Soc. 2017, 139, 6538ꢀ6541.
(17) Ouyang, T.; Huang, H. H.; Wang, J. W.; Zhong, D. C.; Lu, T. B. A Dinuclear Cobalt
Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient VisibleꢀLight
Driven CO2 Reduction to CO in CH3CN/H2O Solution. Angew. Chem. Int. Ed. 2017, 56,
738ꢀ743.
(18) Guo, Z.; Cheng, S.; Cometto, C.; AnxolabehereꢀMallart, E.; Ng, S. M.; Ko, C. C.; Liu, G.;
Chen, L.; Robert, M.; Lau, T. C. Highly Efficient and Selective Photocatalytic CO2 Reduction by
Iron and Cobalt Quaterpyridine Complexes. J. Am. Chem. Soc. 2016, 138, 9413ꢀ9416.
(19) Dey, S.; Ahmed, M. E.; Dey, A. Activation of Co(I) State in a CobaltꢀDithiolato Catalyst for
Selective and Efficient CO2 Reduction to CO. Inorg. Chem. 2018, 57, 5939ꢀ5947.
25
ACS Paragon Plus Environment

ACS Catalysis
Page 26 of 33
1
2
3
4
5
6
7
8
9
(20) Chen, L.; Guo, Z.; Wei, X. G.; Gallenkamp, C.; Bonin, J.; AnxolabehereꢀMallart, E.; Lau,
K. C.; Lau, T. C.; Robert, M. Molecular Catalysis of the Electrochemical and Photochemical
Reduction of CO2 with EarthꢀAbundant Metal Complexes. Selective Production of CO Vs
HCOOH by Switching of the Metal Center. J. Am. Chem. Soc. 2015, 137, 10918ꢀ10921.
(21) Kumar, B.; Smieja, J. M.; Sasayama, A. F.; Kubiak, C. P. Tunable, LightꢀAssisted
CoꢀGeneration of CO and H2 from CO2 and H2O by Re(bipyꢀtBu)(CO)3Cl and pꢀSi in
NonꢀAqueous Medium. Chem. Commun. 2012, 48, 272ꢀ274.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(22) Kang, P.; Chen, Z.; Nayak, A.; Zhang, S.; Meyer, T. J. Single Catalyst Electrocatalytic
Reduction of CO2 in Water to H2+CO Syngas Mixtures with Water Oxidation to O2. Energy
Environ. Sci. 2014, 7, 4007ꢀ4012.
(23) Froehlich, J. D.; Kubiak, C. P. The Homogeneous Reduction of CO2 by [Ni(Cyclam)]+:
Increased Catalytic Rates with the Addition of a CO Scavenger. J. Am. Chem. Soc. 2015, 137,
3565ꢀ3573.
(24) Smieja, J. M.; Sampson, M. D.; Grice, K. A.; Benson, E. E.; Froehlich, J. D.; Kubiak, C. P.
Manganese as a Substitute for Rhenium in CO2 Reduction Catalysts: The Importance of Acids.
Inorg. Chem. 2013, 52, 2484ꢀ2491.
(25) Ishida, H.; Terada, T.; Tanaka, K.; Tanaka, T. Photochemical Carbon Dioxide Reduction
Catalyzed by Bis(2,2'ꢀBipyridine)Dicarbonylruthenium2+ Using Triethanolamine and
1ꢀBenzylꢀ1,4ꢀDihydronicotinamide as an Electron Donor. Inorg. Chem. 1990, 29, 905ꢀ911.
(26) Elgrishi, N.; Chambers, M. B.; Artero, V.; Fontecave, M. Terpyridine Complexes of First
Row Transition Metals and Electrochemical Reduction of CO2 to CO. Phys. Chem. Chem. Phys.
2014, 16, 13635ꢀ13644.
26
ACS Paragon Plus Environment

Page 27 of 33
ACS Catalysis
1
2
3
4
5
6
7
8
9
(27) Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich, L.; Gross, Z. Reduction
of Cobalt and Iron Corroles and Catalyzed Reduction of CO2. J. Phys. Chem. A 2002, 106,
4772ꢀ4778.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(28) Han, Y.; Wu, Y.; Lai, W.; Cao, R. Electrocatalytic Water Oxidation by a WaterꢀSoluble
Nickel Porphyrin Complex at Neutral pH with Low Overpotential. Inorg. Chem. 2015, 54,
5604ꢀ5613.
(29) Fujita, E.; Creutz, C.; Sutin, N.; Szalda, D. J. Carbon Dioxide Activation by Cobalt(I)
Macrocycles: Factors Affecting Carbon Dioxide and Carbon Monoxide Binding. J. Am. Chem.
Soc. 1991, 113, 343ꢀ353.
(30) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C.
P. Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of
Carbon Dioxide: Eliminating Dimerization and Altering Catalysis. J. Am. Chem. Soc. 2014, 136,
5460ꢀ5471.
(31) Catalan, J.; Claramunt, R. M.; Elguero, J.; Laynez, J.; Menendez, M.; Anvia, F.; Quian, J.
H.; Taagepera, M.; Taft, R. W. Basicity and Acidity of Azoles: The Annelation Effect in Azoles.
J. Am. Chem. Soc. 1988, 110, 4105ꢀ4111.
(32) Wang, J. W.; Huang, H. H.; Sun, J. K.; Ouyang, T.; Zhong, D. C.; Lu, T. B. Electrocatalytic
and Photocatalytic Reduction of CO2 to CO by Cobalt(II) Tripodal Complexes: Low
Overpotentials, High Efficiency and Selectivity. ChemSusChem 2018, 11, 1025ꢀ1031.
(33) Bourrez, M.; Molton, F.; ChardonꢀNoblat, S.; Deronzier, A. [Mn(Bipyridyl)(CO)3Br]: An
Abundant Metal Carbonyl Complex as Efficient Electrocatalyst for CO2 Reduction. Angew.
Chem. Int. Ed. 2011, 50, 9903ꢀ9906.
27
ACS Paragon Plus Environment

ACS Catalysis
Page 28 of 33
1
2
3
4
5
6
7
8
9
(34) Chapovetsky, A.; Do, T. H.; Haiges, R.; Takase, M. K.; Marinescu, S. C. ProtonꢀAssisted
Reduction of CO2 by Cobalt Aminopyridine Macrocycles. J. Am. Chem. Soc. 2016, 138,
5765ꢀ5768.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(35) Sato, S.; Morikawa, T.; Kajino, T.; Ishitani, O. A Highly Efficient Mononuclear Iridium
Complex Photocatalyst for CO2 Reduction under Visible Light. Angew. Chem. Int. Ed. 2013, 52,
988ꢀ992.
(36) Song, J.; Klein, E. L.; Neese, F.; Ye, S. The Mechanism of Homogeneous CO2 Reduction
by Ni(Cyclam): Product Selectivity, Concerted ProtonꢀElectron Transfer and CꢀO Bond
Cleavage. Inorg. Chem. 2014, 53, 7500ꢀ7507.
(37) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J. M. A Local Proton Source Enhances CO2
Electroreduction to CO by a Molecular Fe Catalyst. Science 2012, 338, 90ꢀ94.
(38) Chen, R.; Yang, C.; Cai, W.; Wang, H.ꢀY.; Miao, J.; Zhang, L.; Chen, S.; Liu, B. Use of
Platinum as the Counter Electrode to Study the Activity of Nonprecious Metal Catalysts for the
Hydrogen Evolution Reaction. ACS Energy Lett. 2017, 2, 1070ꢀ1075.
(39) Zhang, F.ꢀS.; Wang, J.ꢀW.; Luo, J.; Liu, R.ꢀR.; Zhang, Z.ꢀM.; He, C.ꢀT.; Lu, T.ꢀB.
Extraction of Nickel from NiFeꢀLDH into Ni2P@NiFe Hydroxide as a Bifunctional
Electrocatalyst for Efficient Overall Water Splitting. Chem. Sci. 2018, 9, 1375ꢀ1384.
(40) Huang, Y. L.; Zhong, D. C.; Jiang, L.; Gong, Y. N.; Lu, T. B. Two LiꢀZn ClusterꢀBased
MetalꢀOrganic Frameworks: Strong H2/CO2 Binding and High Selectivity to CO2. Inorg. Chem.
2017, 56, 705ꢀ708.
(41) Costentin, C.; Robert, M.; Savéant, J. M.; Tatin, A. Efficient and Selective Molecular
Catalyst for the CO2ꢀtoꢀCO Electrochemical Conversion in Water. Proc. Natl. Acad. Sci. U. S. A.
2015, 112, 6882ꢀ6886.
28
ACS Paragon Plus Environment

Page 29 of 33
ACS Catalysis
1
2
3
4
5
6
7
8
9
(42) Wang, J.ꢀW.; Sahoo, P.; Lu, T.ꢀB. Reinvestigation of Water Oxidation Catalyzed by a
Dinuclear Cobalt Polypyridine Complex: Identification of CoOx as a Real Heterogeneous
Catalyst. ACS Catal. 2016, 6, 5062ꢀ5068.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(43) Zhang, P.; Wang, M.; Yang, Y.; Yao, T.; Sun, L. A Molecular Copper Catalyst for
Electrochemical Water Reduction with a Large HydrogenꢀGeneration Rate Constant in Aqueous
Solution. Angew. Chem. Int. Ed. 2014, 53, 13803ꢀ13807.
(44) Costentin, C.; Savéant, J.ꢀM. Multielectron, Multistep Molecular Catalysis of
Electrochemical Reactions: Benchmarking of Homogeneous Catalysts. ChemElectroChem 2014,
1, 1226ꢀ1236.
(45) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J. M. Turnover Numbers, Turnover
Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic
Voltammetry and PreparativeꢀScale Electrolysis. J. Am. Chem. Soc. 2012, 134, 11235ꢀ11242.
(46) Beley, M.; Collin, J. P.; Ruppert, R.; Sauvage, J. P. Electrocatalytic Reduction of Carbon
Dioxide by Nickel Cyclam2+ in Water: Study of the Factors Affecting the Efficiency and the
Selectivity of the Process. J. Am. Chem. Soc. 1986, 108, 7461ꢀ7467.
(47) Collin, J. P.; Jouaiti, A.; Sauvage, J. P. Electrocatalytic Properties of
(Tetraazacyclotetradecane)Nickel2+ and Ni2(Biscyclam)4+ with Respect to Carbon Dioxide and
Water Reduction. Inorg. Chem. 1988, 27, 1986ꢀ1990.
(48) Schneider, J.; Jia, H. F.; Kobiro, K.; Cabelli, D. E.; Muckerman, J. T.; Fujita, E. Nickel(II)
Macrocycles: Highly Efficient Electrocatalysts for the Selective Reduction of CO2 to CO.
Energy Environ. Sci. 2012, 5, 9502ꢀ9510.
29
ACS Paragon Plus Environment