Electrocatalytic reduction of low concentration CO2

Article abstract of DOI:10.1039/c8sc04124e

Utilization of low concentration CO₂ contained in the exhaust gases from various industries and thermal power stations without the need for energy-consuming concentration processes should be an important technology for solving global warming and the shortage of fossil resources. Here we report the direct electrocatalytic reduction of low concentration CO₂ by a Re(i)-complex catalyst that possesses CO₂-capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO₂ reduction were almost the same when using Ar gas containing 10% CO₂ or when using pure CO₂ gas, and the selectivity of CO formation was very high (98% at 10% CO₂). At a concentration of 1% CO₂, the Re(i) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO₂ gas. The electrocatalysis was due to the efficient insertion of CO₂ into the Re(i)-O bond in fac-[Re(dmb)(CO)₃{OC₂H₄N(C₂H₄OH)₂}] (dmb = 4,4′-dimethyl-2,2′-bipyridine).

Full text of DOI:10.1039/c8sc04124e

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Electrocatalytic reduction of low concentration
CO2†
Cite this: DOI: 10.1039/c8sc04124e
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have been paid for by the Royal Society
of Chemistry
Hiromu Kumagai, ‡ Tetsuya Nishikawa,‡ Hiroki Koizumi, Taiki Yatsu, Go Sahara,
*
Yasuomi Yamazaki, § Yusuke Tamaki and Osamu Ishitani
Utilization of low concentration CO2 contained in the exhaust gases from various industries and thermal
power stations without the need for energy-consuming concentration processes should be an important
technology for solving global warming and the shortage of fossil resources. Here we report the direct
electrocatalytic reduction of low concentration CO2 by a Re(I)-complex catalyst that possesses CO2-
capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO2
reduction were almost the same when using Ar gas containing 10% CO2 or when using pure CO2 gas,
and the selectivity of CO formation was very high (98% at 10% CO2). At a concentration of 1% CO2, the
Re(I) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic
efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO2
gas. The electrocatalysis was due to the efficient insertion of CO2 into the Re(I)–O bond in
fac-[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (dmb ¼ 4,40-dimethyl-2,20-bipyridine).
Received 17th September 2018
Accepted 11th November 2018
DOI: 10.1039/c8sc04124e
rsc.li/chemical-science
using amines,7,8 MOFs,9–13 and lters,14 these processes
consume extra energy. To the best of our knowledge, the elec-
Introduction
trocatalytic reduction of CO2 at low concentration has not been
reported yet.
Efforts to solve global warming problems caused by the
increasing CO2 concentration in the atmosphere have focused
on carbon capture and storage (CCS). However, the depletion of
fossil resources as energy sources and as chemical raw materials
is another extended problem. Both of these serious problems
have the same origin; there are no practical methods to utilize
the large volumes of CO2 produced in daily life and industrially,
as a main carbon source. However, in nature, photosynthesis
uses the low concentration of CO2 in the atmosphere as the
carbon source to produce high-energy organic materials using
sunlight as an energy source. The electrocatalytic reduction of
CO2 has been widely investigated,1–5 but most of these reports
targeted pure CO2 despite the fact that exhaust gases from heavy
industries only contain low concentrations of CO2; for example,
3–13% in the case of re power plants.6 Although condensation
technologies of low concentration CO2 can be achieved by
There are several reports of metal complexes being used to
x low concentration CO2. Orchin et al. determined that CO2
insertion into the Mn–O bond of fac-[Mn(CO)3(P–P)(OEt)] (P–P:
diphenylphosphino ethane) to generate fac-[Mn(CO)3(P–P)
{OC(O)OEt}] was possible using pure CO2 gas as well as air.15
Umakoshi et al. reported the formation of a methyl carbonate
complex from [Ru(CNC)(bpy)(OMe)](PF6) (CNC: 2,6-bis(tert-
butylimidazol-2-ylidene)pyridine) in ambient air.16
Recently, we reported the CO2-capturing ability of a Re(I)
complex with deprotonated triethanolamine as a monodentate
ligand, i.e., fac-[Re(bpy)(CO)3{OC2H4N(C2H4OH)2}] (bpy ¼ 2,20-
bipyridine), to react with CO2, giving the aminoethylcarbonato
complex fac-[Re(bpy) (CO)3{OC(O)OC2H4N(C2H4OH)2}] (Scheme
1).17 Since this equilibrium reaction has a large equilibrium
constant (K ¼ 1.7 ꢀ 103 Mꢁ1), fac-[Re(bpy)(CO)3{OC2H4N(C2-
H4OH)2}] can efficiently capture low concentration CO2. 93% of
the Re complex can be converted to the aminoethylcarbonato
complex by bubbling Ar gas containing only 10% CO2 through
the solution containing the Re complex.
Department of Chemistry, School of Science, Tokyo Institute of Technology, O-okayama
2-12-1-NE-1, Meguro-ku, Tokyo 152-8550, Japan. E-mail: ishitani@chem.titech.ac.jp
† Electronic supplementary information (ESI) available: Curve tting of the FT-IR
spectrum under 1% CO2 in DMF–TEOA, detailed cyclic voltammograms of the
Re(I) complex, picture of the setup for the bulk electrolysis, time courses of
current in the bulk electrolysis, results of the analysis (FT-IR, CVs, and UHPLCs)
aer electrolysis for experiments under 1% CO2 in DMF and under 10% CO2 in
DMF–TEOA, and FT-IR spectra of the Re complex before and aer adding
TPAHBF4. See DOI: 10.1039/c8sc04124e
‡ These authors contributed equally to this work.
§ Present address: Faculty of Science and Technology, Department of Materials
and Life Science, Seikei University, 3-3-1 Kichijojikitamachi, Musashino, Tokyo,
180-8633, Japan.
Scheme 1 CO2 capture by fac-[Re(bpy) (CO)3{OC2H4N(C2H4OH)2}].
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In addition, Re(I) diimine carbonyl complexes having a fac-
[Re(N^N) (CO)3X] structure are well-known catalysts for CO2
reduction in electrochemical systems.18–24 By combining these
catalytic and CO2-capturing properties, the construction of
a catalytic system, which can directly utilize low concentration
CO2 as a substrate for CO2 reduction could be expected.
Herein, we report the electrochemical reduction of low
concentration CO2 with high selectivity, high faradaic effi-
ciency, and good durability using a Re(I) mononuclear complex
with CO2-capturing ability.
Fig. 1 FT-IR spectra of the Re(I) complexes (5.0 mM) in a DMF–TEOA
(5 : 1 v/v) solution containing 0.1 M Et4NBF4 after purging with Ar (a),
and Ar containing various concentrations of CO2 (b). Black asterisk (*),
blue triangle (:) and red circle (C) display the peaks which corre-
spond to Re-DMF+, Re-TEOA, and Re-CO2TEOA, respectively.
Results and discussion
First, we conrmed the ability of the Re(I) complex to capture
CO2 to form the aminoethylcarbonato complex in co-existence
with a supporting electrolyte (Et4NBF4) for electrochemical
reactions (Scheme 2). The fac-[Re(dmb)(CO)3(MeCN)]+ (Re-
MeCN+: dmb ¼ 4,40-dimethyl-bpy) complex was selected as
a starting complex, which can be converted to the ligand
substituted complex fac-[Re(dmb)(CO)3(DMF)]+ (Re-DMF+: DMF
¼ N,N0-dimethylformamide) by dissolving in a DMF solution
containing Et4NBF4 (0.1 M), due to the lower stability of the
isolated Re-DMF+. Part of this DMF complex was converted into
a complex with a deprotonated triethanolamine ligand, i.e., fac-
[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (Re-TEOA) by adding trie-
thanolamine (TEOA) to the DMF solution (DMF : TEOA ¼ 5 : 1
v/v). Fig. 1(a) shows the stretching vibrational bands of the CO
ligands of Re-DMF+ (nCO ¼ 2028 cmꢁ1, 1921 cmꢁ1, and
1910 cmꢁ1) and Re-TEOA (nCO ¼ 2005 cmꢁ1, 1895 cmꢁ1, and
1878 cmꢁ1) in the mixed solution under an Ar atmosphere. The
ratio of Re-DMF+ and Re-TEOA was calculated to be 1 : 2.4 by
curve tting and changed according to the concentration of
TEOA; therefore, this ligand substitution reaction is in equi-
librium (eqn (1)).
Analysis of these data using eqn (1) and (2) supplied the
equilibrium constants of both reactions as K1 ¼ (6.4 ꢂ 0.2) ꢀ
10ꢁ2 in eqn 1 and K2 ¼ (1.6 ꢂ 0.1) ꢀ 103 Mꢁ1 in eqn (2). This
clearly shows that Re-TEOA has high CO2 capturing ability even
in the presence of 0.1 M Et4NBF4. We note that a similar value
(K2¼ (1.8 ꢂ 0.1) ꢀ 103 Mꢁ1) was obtained in the absence of the
electrolyte. Table 1 summarizes the distribution of the propor-
tions of these complexes under 1% and 10% CO2 atmospheres.
Notably, 67% of the Re(I) complex existed as Re-CO2TEOA even
under a 1% CO2 atmosphere (curve-ttings are shown in
Fig. S1†), where the solution contains only 2.2 mM CO2 while
the saturating concentration of CO2 is 0.14 M.25 Thus, we can
assume that the CO2 capturing property of Re-TEOA is appli-
cable to the electrochemical reduction of low concentration
CO2.
Fig. 2 shows the cyclic voltammograms (CVs) of the Re(I)
complex(es) in DMF and in a DMF–TEOA (5 : 1 v/v) mixed
solution containing the electrolyte, which were measured both
under an Ar and a CO2 atmosphere. Under an Ar atmosphere,
the CV of Re-DMF+ in DMF shows a chemically quasi-reversible
reduction wave (E1/2 ¼ ꢁ1.66 V) attributed to an electron
injection into the p* orbital of the diimine ligand of Re-DMF+,
and a subsequent irreversible reduction wave at Ep ¼ ꢁ1.8 V,
which is attributed to the reduction of the metal center Re(+/0)19
(Fig. 2(a), further details in the ESI (Fig. S2(a)†). Under a CO2
atmosphere, Re-DMF+ shows a catalytic current at a more
negative potential than the second reduction wave.
Re-DMF+ + 2TEOA % Re-TEOA + TEOA-H+ + DMF (1)
On introducing pure CO2 into the solution (bubbling for 1
min), all the nCO bands of Re-DMF+ and Re-TEOA disappeared
and a set of three new peaks appeared at nCO ¼ 2019 cmꢁ1
,
1913 cmꢁ1, and 1890 cmꢁ1. Fig. 1(b) shows the nCO bands in Ar
containing various concentrations of CO2. This clearly indicates
that CO2 insertion into the Re–O bond is also in equilibrium
(eqn (2)).
In the DMF–TEOA mixed solution, both Re-DMF+ and Re-
TEOA are present under an Ar atmosphere, as described above.
Certainly, the CV of this solution showed a broader “rst”
reduction wave at E ¼ ꢁ1.6 to ꢁ1.9 V (Fig. 2(b), and a detailed
Re-TEOA + CO2 % Re-CO2TEOA
(2)
Table 1 Distribution of the proportions of the Re(I) complexes in
a DMF–TEOA (5 : 1 v/v) solution containing 0.1 M Et4NBF4 after
purging with Ar containing various concentrations of CO2
Re-DMF+
Re-TEOA
Re-CO2TEOA
100% CO2
10% CO2
1% CO2
Trace
3%
15%
Trace
3%
18%
>99%
94%
67%
Scheme 2 Ligand substitution and CO2 capture by the Re(I) complex
in a DMF–TEOA mixed solution.
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and the square root of the dissolved CO2 concentration. This
good linearity suggests that the diffusion of CO2 as a substrate is
a rate-determining step in the electrochemical reaction at
ꢁ1.95 V.26 In contrast, the current at ꢁ1.82 V was almost inde-
pendent of the concentration of CO2. This suggests that the
rate-determining step of the electrochemical reaction occurring
at this potential is not dependent on the concentration of CO2.
From these results we decided to quantitate the products from
bulk electrolysis at ꢁ1.82 V under 100%, 10%, and 1% CO2
atmospheres.
An H-shaped electrolytic cell, in which the reduction and
oxidation chambers are partitioned by a proton exchange
membrane, was used for the electrocatalytic reaction (Fig. S3†).
A DMF–TEOA (5 : 1 v/v) solution containing a 0.5 mM Re(I)
complex and Et4NBF4 (0.1 M) as an electrolyte was added to the
cathodic chamber, while the solution for the anodic chamber
contained the electrolyte and tetrabutylammonium acetate
(TBAOAc) as an electron donor. A potential of ꢁ1.82 V vs. the Ag/
AgNO3 (10 mM) reference electrode was applied to a carbon
mesh working electrode for 24 h, while bubbling with Ar gas
containing various concentrations of CO2. Fig. 4(a) shows the
time courses of the electrolysis products under 100% CO2 in
a solution in which most of the Re(I) complex was converted to
Re-CO2TEOA. A substantially stable reductive current of about
1.3 mA was observed for 24 h (Fig. S4(a)†), and CO was produced
as the main reduction product with very small amounts of H2
Fig. 2 CVs of the Re(I) complex(es) (total 0.5 mM) (a) in DMF and (b) in
the DMF–TEOA (5 : 1 v/v) mixed solution, containing Et4NBF4 (0.1 M) as
the supporting electrolyte at a sweep rate of 200 mV sꢁ1. Blue and red
lines show the CVs under CO2 and Ar, respectively. Dotted lines
indicate CVs for the corresponding solutions without the Re(I)
complex. A glassy carbon electrode (diameter: 3 mm), an Ag/AgNO3
(10 mM) electrode and a Pt wire were used as working, reference, and
counter electrodes, respectively.
CV in Fig. S2(b)†), which should contain two reduction waves,
one from each Re complex; the reduction potential of Re-TEOA
should be more negative than that of Re-DMF+ because of the
charge difference. Conversely, under a CO2 atmosphere, the
onset potential of the rst reduction wave was shied by about
70 mV to a more negative value (Ep ¼ ꢁ1.8 V) because the Re
complexes should be converted to Re-CO2TEOA which has no
charge. The cathodic catalytic wave was obtained when the
sweeping potential was slightly negative of the rst reduction
wave. It should be noted that the onset potential of this catalytic
wave (E ꢃ ꢁ1.82 V) was almost the same as in the case for
Re-DMF+ in the CO2-purged DMF solution (Fig. 2(a)).
The catalytic current decreased at lower concentrations of
CO2 as shown in Fig. 3(a). In the case of 1% CO2, the “rst”
reduction wave was slightly different compared to the other two
cases. This reduction could also be caused by the presence of
Re-DMF+ in addition to Re-CO2TEOA and Re-TEOA. Fig. 3(b)
shows the relationship between the catalytic current at ꢁ1.95 V
Fig. 4 Time courses of the generation of CO (red square), H2 (blue
triangle), and formic acid (green diamond) with half of the electrons
Fig. 3 (a) CVs of the Re(I) complex (0.5 mM) in a DMF–TEOA mixed flowing (dotted line) during bulk electrolysis using the Re(I) catalyst (0.5
solution (5 : 1 v/v) containing Et4NBF4 (0.1 M) under 100% (blue), 10% mM) at ꢁ1.82 V in a DMF–TEOA solution containing Et4NBF4 (0.1 M)
(purple), and 1% (maroon) CO2 atmospheres and under an Ar atmo- under (a) 100%, (b) 10%, and (c) 1% CO2 atmospheres (10 mL minꢁ1).
sphere (red). The sweep rate was 200 mV sꢁ1. A glassy carbon elec- Total amounts of CO and H2 are also indicated by black squares. (d)
trode (diameter: 3 mm), an Ag/AgNO3 (10 mM) electrode and a Pt wire The relationships of TONCO for a 24 h reaction with the concentrations
were used as working, reference, and counter electrodes, respectively. of Re-CO2TEOA in the solution (red circle) and CO2 (black circle).
(b) Relationship between the catalytic current at ꢁ1.95 V and the Corresponding time courses of the current during bulk electrolysis are
square root of the dissolved CO2 concentration.
shown in Fig. S4.†
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and HCOOH as byproducts. The faradaic efficiency (h) of each ꢁ1.76 V under 10% CO2, respectively. Even at a more positive
product was hCO ¼ 87%, hH2 ¼ 2% and hHCOOH ¼ 1%. The potential in the rst reduction wave, a highly selective and
turnover number for the formation of CO (TONCO) was 12.4 catalytic reaction proceeded (hCO ¼ 84%, TONCO ¼ 4.1, and
aer the 24 h electrolysis and the speed of CO formation did not selectivity of CO formation ¼ 98% aer 24 h electrolysis). This
slow down during the electrolysis.
concludes that Re-CO2TEOA efficiently electrocatalyzes CO2
Fig. 4(b) and (c) show the time courses of the electrolysis reduction to produce CO with high selectivity even at its rst
products under 10% and 1% CO2, respectively. Under 10% CO2 reduction potential under low concentration of CO2.
where 91% of the Re(I) complex exists as Re-CO2TEOA, the Re(I)
To investigate the role of TEOA, electrocatalytic CO2 reduc-
complex still maintained excellent catalytic activity similar to tion was carried out in a DMF solution containing Re-DMF+ and
that under 100% CO2 (hCO ¼ 95%, TONCO ¼ 11.5, and selectivity Et4NBF4 as the electrolyte in the absence of TEOA under 100%
of CO formation ¼ 98% aer 24 h electrolysis). Even when the CO2 and 1% CO2 atmospheres. The applied potential was
concentration of CO2 was lowered to only 1%, a stable reduction ꢁ1.82 V vs. Ag/AgNO3 which corresponds to the second reduc-
current and the corresponding catalytic CO generation were tion wave of Re-DMF+ (Fig. 2(a)). Fig. 6 shows the time courses of
observed with a high faradaic efficiency (hCO ¼ 85%). The the generation of the products. Under the 100% CO2 atmo-
TONCO was 8.4 aer 24 h electrolysis, which was lower than sphere, the observed current in the initial stage was greater than
under 10% and 100% CO2 atmospheres. Notably, the dissolved that in the presence of TEOA; 4.6 mA was observed (1.5 mA in
CO2 concentrations in the DMF–TEOA mixed solution using the presence of TEOA) because the two-electron reduction of Re-
10% and 1% CO2 were 16 mM and 2.2 mM, respectively, which DMF+ proceeded (Fig. S5(a)†). However, it drastically decreased
are much lower than under a 100% CO2 atmosphere (140 during the rst 60 min, and became 0.27 mA aer 24 h elec-
mM).17,25 It is noteworthy that TEOA cannot accumulate CO2 trolysis with the corresponding selective CO production
although diethanolamine and ethanolamine can capture CO2 (Fig. 6(a)). The faradaic efficiency of CO was 51%, which was
giving carbamate (R1R2NCOOH).8 However, the electrocatalytic much lower than that in the presence of TEOA. Although the Re-
CO2 reduction by using the Re(I) complex proceeded to generate DMF+ has three CO ligands, the TON of the CO formation was
CO selectively, and the generation rate was maintained at 93% less than three. It is noteworthy that the saturated concentra-
for 10% CO2 and at 67% for 1% CO2 compared to that for 100% tion of CO2 in DMF (0.20 M) was higher than that in the DMF–
CO2. Fig. 4(d) shows the relationship between the TONCO aer TEOA (5 : 1 v/v) mixed solution.27 Under the 1% CO2 atmo-
24 h electrolysis and the concentration of Re-CO2TEOA (red) sphere, the faradaic efficiency of CO formation was very low
and the concentration of CO2 (black) in the reaction solution. even in the initial stage of the electrolysis (Fig. 6(b) and S5(b)†).
The concentration of Re-CO2TEOA indicates a good linearity The TON for CO formation was 0.6 aer electrolysis for 24 h.
with the TONCO, while the concentration of dissolved CO2 did The CV and FT-IR spectrum measured aer the electrolysis
not show such a good correlation. These results strongly suggest (Fig. S6†) clearly indicate that most of the Re complex had
that Re-CO2TEOA electrocatalyzed the CO2 reduction through decomposed to electrochemically inactive species. In a nutshell,
the reduction of CO2 captured by the complex.
Re-DMF+ did not work as an electrocatalyst under these reaction
Additionally, the electrolysis at ꢁ1.76 V under 10% CO2 was conditions. From these results, we can conclude that the elec-
conducted to conrm the onset of the reduction of low trocatalytic reduction of low concentration CO2 does not
concentration CO2. Fig. 5(a) and (b) display the magnied CV of proceed in DMF in the absence of TEOA.
the Re complex under 10% CO2 in the DMF–TEOA mixed
It has been reported that the addition of a proton source to
solution and the time courses of the electrolysis products at the reaction solution sometimes accelerates the electrocatalytic
Fig. 5 (a) CV of the Re(I) complex (0.5 mM) in a DMF–TEOA mixed
solution (5 : 1 v/v) containing Et4NBF4 (0.1 M) under a 10% CO2 Fig. 6 Time courses of the generation of CO (red square), H2 (blue
atmosphere. Measurement conditions were the same as those in Fig. 3. triangle), sum of CO and H2 (black-filled square), and formic acid
(b) Time courses of the generation of CO (red square), H2 (blue (green diamond) with half of the electrons flowing (dotted line) during
triangle), and formic acid (green diamond) with half of the electrons bulk electrolysis using Re-DMF+ (0.5 mM) at ꢁ1.82 V in a DMF solution
flowing (dotted line) during bulk electrolysis using the Re(I) catalyst (0.5 containing Et4NBF4 (0.1 M) under (a) 100% and (b) 1% CO2 atmo-
mM) at ꢁ1.76 V in a DMF–TEOA solution containing Et4NBF4 (0.1 M) spheres (10 mL minꢁ1). Corresponding time courses of the current
under a 10% CO2 atmosphere (10 mL minꢁ1).
during bulk electrolysis are shown in Fig. S5.†
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CO2 reduction because the protons work as O2ꢁ receptors from
the two-electron reduced CO2 (eqn (3)).28,29
We conducted bulk electrolysis experiments of the DMF
solutions containing TPAHBF4 (25 mM) and 0.5 mM Re-DMF+ at
EBE ¼ ꢁ1.82 V vs. Ag/AgNO3 under 100% CO2 and 1% CO2
atmospheres. Fig. 8 shows the time courses of product gener-
ation during electrolysis. In the case of the 100% CO2 atmo-
2CO2 + 2eꢁ + H+/ CO + HCO3
(3)
ꢁ
Since in the current system there is a possibility that the
reduction of low concentration CO2 could proceed due to the
effect of the hydroxy protons of TEOA, the addition of tri(n-
propyl) ammonium tetrauoroborate (TPAHBF4) as a proton
source into the reaction solution instead of TEOA was investi-
gated. Since alcohols possibly form a corresponding CO2 adduct
complex by coordinating to the Re complex, TPAHBF4 which
itself and the deprotonated form of which have low coordi-
nating ability to the Re center was used as a proton source. The
IR spectrum did not change on adding TPAHBF4 (25 mM) to
a DMF solution containing Re-DMF+ (5.0 mM) and Et4NBF4
(0.1 M) under an Ar atmosphere (Fig. S7†), suggesting that the
Re(I) complex does not undergo a ligand substitution reaction
with TPAHBF4.
From the CV of the DMF solution containing TPAHBF4
(25 mM) and 0.5 mM Re-DMF+ under an Ar atmosphere, an
irreversible reduction wave around ꢁ1.6 V and a following large
catalytic reduction wave starting from around ꢁ1.9 V were
observed (Fig. 7(a)). The reduction current near ꢁ1.9 V could be
assigned to the catalytic generation of H2 catalyzed by the
reductive derivatives from Re-DMF+. Two-electron reduction of
Re-DMF+ should eliminate the DMF ligand, and it would react
with a proton to generate fac-[Re(dmb)(CO)3H] (Re-H), which
reacts with another proton producing H2. Fig. 7(b) shows the
CVs of the same solution in the presence of 1% and 100% CO2,
where the CV measured under the Ar atmosphere is also shown.
Under the 100% CO2 atmosphere, the rst reduction wave
around ꢁ1.6 V showed no obvious change regardless of the
existence of CO2, while a new reduction wave was observed
between ꢁ1.8 V and ꢁ2 V. Under the 1% CO2 atmosphere, on
the other hand, the CV was similar to that measured under
the Ar atmosphere, especially at potentials more positive
than ꢁ1.9 V.
sphere, CO was mainly produced with hCO ¼ 64% and TONCO
¼
16.3 aer 24 h electrolysis (Fig. 8(a)), indicating the progress of
the electrocatalytic reduction of CO2. The faradaic efficiencies of
H2 and HCOOH were hH2 ¼ 1% and hHCOOH ¼ 17%, respectively
(TONH2 ¼ 0.2, TONHCOOH ¼ 4.4). Under the 1% CO2 atmo-
sphere, on the other hand, no CO production was observed,
while catalytic amounts of H2 (TONH2 ¼ 6.1, hH2 ¼ 35%) and
HCOOH (TONHCOOH ¼ 6.1, hHCOOH ¼ 27%) were produced
(Fig. 8(b)). These results clearly indicate that although the
proton source accelerates CO2 reduction at a high concentration
of CO2, it does not assist CO formation via reduction of low
concentration CO2. It is interesting that formate was catalyti-
cally produced even by using the Re complex as a catalyst
because most of the reported electrocatalytic systems using
Re(I) complexes as catalysts selectively produced CO but not the
formate species even in the presence of proton sources.30–32 It
should be considered that only pure CO2 was used in these
reports.
Table 2 summarizes the bulk electrolysis results. The elec-
trocatalysis of the Re complexes strongly depended on the
reaction conditions as described above. Under 100% CO2, the
catalytic formation of CO was the main reaction (entries 1, 5 and
7), and the co-existence of the proton source, i.e., TEOA or
TPAHBF4, accelerated the rates of CO formation (entries 1 and
7). At low concentrations of CO2 (entries 3, 6 and 8), the pres-
ence of TEOA in the reaction solution was essential for CO
formation. Even at a more positive potential (E ¼ ꢁ1.76 V) in the
rst reduction wave, a highly selective catalytic reaction pro-
ceeded (entry 4). It should be noted that the proton source did
not positively affect the CO formation in the 1% CO2 atmo-
sphere. These results also clearly indicate that the CO2
capturing ability of Re-TEOA to yield Re-CO2TEOA is essential
for the reduction of low concentration CO2.
Fig. 7 (a) CVs of Re-DMF+ (0.5 mM) in Ar-purged DMF with (red) or
without (black line) 25 mM TPAHBF4 as a proton source (dashed line: Fig. 8 Time courses of the generated CO (red square), H2 (blue
without Re-DMF+). (b) CVs of Re-DMF+ (0.5 mM) in the solution triangle), sum of CO and H2 (black-filled square), and formic acid
containing 100% CO2 (red), 1% CO2 (maroon line), and pure Ar (black (green diamond) with half of the electrons flowing (dotted line) during
dotted line). The solutions contain Et4NBF4 (0.1 M) as the supporting bulk electrolysis using Re-DMF+ (0.5 mM) at ꢁ1.82 V in the DMF
electrolyte. The sweep rate was 200 mV sꢁ1. A glassy carbon electrode solution containing Et4NBF4 (0.1 M) and TPAHBF4 (25 mM) under (a)
(diameter: 3 mm), an Ag/AgNO3 (10 mM) electrode and a Pt wire were 100% and (b) 1% CO2 atmospheres (10 mL minꢁ1). Corresponding time
used as working, reference, and counter electrodes, respectively.
courses of the current during bulk electrolysis are shown in Fig. S8.†
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Table 2 Reduction products during constant potential electrolysisa
Products/mmol
(TON based on the Re complex)
Faradaic efficiency/%
Consumed
Additive
Conc. of CO2/%
electrons/mmol
CO
H2
HCOOH
CO
H2
HCOOH
1
2
3
4b
5
6
7
8
TEOA
TEOA
TEOA
TEOA
—
100
10
1
10
100
1
1195
1025
826
203
454
300
2132
1482
522 (12.4)
485 (11.5)
351 (8.4)
170 (4.1)
116 (2.8)
25 (0.6)
11.9 (0.28)
1.2 (0.03)
0.8 (0.02)
0.5 (0.01)
0.4 (0.01)
1.7 (0.04)
9 (0.2)
8 (0.2)
10 (0.2)
21 (0.5)
3 (0.1)
n.d.
6 (0.1)
186 (4.4)
201 (4.8)
87
95
85
84
51
17
64
ꢃ0
2.0
1
2
5
2
0.2
0.2
0.3
0.2
1.2
1
ꢃ0
—
4
TPAHBF4
TPAHBF4
100
1
685 (16.3)
n.d.
17
27
256 (6.1)
35
a
Solutions containing the Re complex (0.5 mM) and Et4NBF4 (0.1 M) under EBE ¼ ꢁ1.82 V vs. Ag/AgNO3 for 24 h. b Electrolysis under EBE ¼ ꢁ1.76 V
vs. Ag/AgNO3 for 24 h. n.d.: not detected.
To clarify any changes in the Re complexes during electrol- substituted by MeOH and/or H2O in the mobile phase of the
ysis, IR spectra, CVs, and liquid chromatograms of the reaction liquid chromatography (Fig. 9(c)). In addition, a smaller peak
solutions using an ODS column (UHPLC) were recorded before attributable to the formate complex (fac-[Re(dmb)(CO)3{OC(O)
and aer the electrolysis. In the case of the DMF–TEOA mixed H}], Re-OCHO) was observed, in 13% conversion yield. 0.9% of
solution under the 100% CO2 atmosphere, i.e., only Re-CO2- the total electrons were consumed in the formation of Re-
TEOA was in solution before electrolysis, no obvious change of OCHO, i.e., hFE ¼ 0.9% since formation of formate from CO2
the nCO bands in the IR spectrum was observed even aer the requires two electrons. Notably, since the nCO bands and the CV
electrolysis for 24 h (Fig. 9(a)). In the CV, the shape was of Re-OCHO are very similar to those of Re-CO2TEOA, the
unchanged and the reduction current attributed to the Re UHPLC analyses were required for the detection of Re-OCHO.
complexes was slightly reduced aer electrolysis (Fig. 9(b)). In The same analyses were carried out in the case using 10% CO2
UHPLC, the main peak attributed to the starting Re complexes and showed similar results. The starting Re complexes were
was observed, where the axial ligands of Re-CO2TEOA should be detected as the main component by UHPLC and Re-OCHO was
produced in a 4% yield (hFE ¼ 0.3%, Fig. S9(c)†). In the case
using 1% CO2, on the other hand, the yield of Re-OCHO
increased to 24% (hFE ¼ 2.4%, Fig. 10). It has been reported
that formate complexes are formed through electrochemical
Fig.
9 (a) FT-IR spectra, (b) CVs, and (c) UHPLCs of the Re(I)
complexes before and after the electrolysis at ꢁ1.82 V under a 100%
CO2 atmosphere in DMF–TEOA containing Et4NBF4 (0.1 M) as the
supporting electrolyte (black lines: before electrolysis, red lines: after
electrolysis). The FT-IR spectra and CVs were measured by using the Fig. 10 (a) FT-IR spectra, (b) CVs, and (c) UHPLCs of the Re(I)
reaction solutions without any treatment. UHPLC: an ODS column: complexes before and after electrolysis at ꢁ1.82 V under a 1% CO2
ldet ¼ 400 nm, a MeOH–H2O mixed solution containing KH2PO4 atmosphere in DMF–TEOA containing Et4NBF4 (0.1 M) as the sup-
buffer of pH 5.9 (1 : 1 v/v) was used as the eluent. The blue line shows porting electrolyte (black lines: before electrolysis, red lines: after
the analysis result of the isolated Re-OCHO as a reference.
electrolysis). Measurement conditions were the same as those in Fig. 9.
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CO2 reduction using fac-[Re(N^N)(CO)3L] type complexes
as catalysts by CO2 insertion into the hydride complexes
(fac-[Re(N^N)(CO)3H]) which are two-electron reduction prod-
ucts of the starting Re complexes.33,34 In the present system, the
coordinatively unsaturated complex might be generated
through the reduction of Re-DMF+ and/or Re-TEOA and might
react with a proton to produce Re-H. Since it has also been re-
ꢁ
ported that desorption of HCO2 from the formate complexes
via their one-electron reduction proceeded,35 the production of
formate in the present electrocatalytic systems may be produced
via similar processes. It should be noted that the formation
reactions of Re-OCHO and HCOOH are very minor processes in
the whole electrolysis. These results clearly show that during
electrolysis the Re complexes acted as stable catalysts even
under the 1% CO2 atmosphere.
Fig. 12 (a) FT-IR spectra and (b) UHPLCs of the Re(I) complexes before
and after electrolysis at ꢁ1.82 V with the existence of TPAHBF4 (25
mM) under a 1% CO2 atmosphere in DMF–TEOA containing Et4NBF4
(0.1 M) as the supporting electrolyte (black lines: before electrolysis,
red lines: after electrolysis). Measurement conditions were the same as
those in Fig. 9.
In the IR spectra recorded just aer electrolysis under the
10% CO2 and 1% CO2 atmospheres, Re-TEOA was observed in
the ratio of 6% and 18% in all complexes (Fig. S9(a)† and 10(a)),
while Re-DMF+ was not. This was also conrmed from their CVs
(Fig. S9(b)† and 10(b)). These observations can be explained
because the reaction solution should become more basic during
electrolysis because the reductive conversion of CO2 to CO leads
to the formation of bicarbonate species as well (eqn (3)). This
should move the equilibrium between Re-DMF+ and Re-TEOA to
the Re-TEOA side. Thus, these results reveal that most of the
Re(I) complexes were maintained even aer electrolysis at low
concentrations of CO2 for 24 h except for the formation of Re-
OCHO.
Therefore, it was concluded that Re-DMF+ was converted to Re-
OCHO during electrolysis in the presence of TPAHBF4. These
results also suggest that Re-H was generated as an intermediate
during electrolysis and insertion of CO2 into the Re-H bond gave
Re-OCHO. It is noteworthy that, even under the 1% CO2 atmo-
sphere, the Re-DMF+ was converted in good yield and HCOOH
was catalytically produced (TONHCOOH ¼ 4.8 aer 24 h elec-
trolysis). Therefore, the electrochemical formation of formate
from a low concentration of CO2 by the Re complexes is
possible.
Experimental
Materials
Similar analyses of the electrolyzed solutions containing
TPAHBF4 instead of TEOA were conducted (Fig. 11 and 12).
Although the CO formation rates were very different between
the 100% and 1% CO2 atmospheres as described above (entries
7 and 8 in Table 2), the UHPLC and FT-IR spectra aer the
electrolysis were similar to each other. In the UHPLC, the peak
attributed to Re-OCHO was mainly observed (Fig. 11(b) and
12(b)). The conversion yields based on the Re-DMF+ used were
73% (100% CO2) and 64% (1% CO2), respectively. The FT-IR
results also supported that most of the Re-DMF+ was con-
verted to Re-OCHO; only the peaks attributed to Re-OCHO were
˚
DMF was dried with 4 A molecular sieves for several days and
then distilled under reduced pressure. TEOA was distilled under
vacuum. These compounds were stored under an Ar atmo-
sphere before use. The electrolyte, Et4NBF4 was recrystallized
from MeCN/ethyl acetate, and then dried under vacuum at 373
K overnight before use. Other reagents and solvents were of
commercial grade quality and were used without further
purication.
observed at vCO ¼ 2017 cmꢁ1, 1912 cmꢁ1, and 1888 cmꢁ1
.
Synthesis
fac-Re(dmb)(CO)3Br was synthesized according to a previously
reported method.36
fac-[Re(dmb)(CO)3(MeCN)](PF6) (Re-MeCN+). An MeCN
solution containing Re(dmb)(CO)3Br (0.677 g, 1.27 mmol) and
AgPF6 (0.323 g, 1.28 mmol) was reuxed in the dark overnight,
and then exposed to room light for 1 h. Aer precipitation, AgBr
was removed by ltration with Celite, and the solution was
evaporated under reduced pressure. The residual solid was
dissolved in an NH4PF6-saturated mixed solution (MeCN/H2O ¼
1 : 1 v/v). MeCN was evaporated slowly to precipitate the product
as a yellow powder. Yield: 0.771 g (95%). FT-IR (in MeCN)
vCO/cmꢁ1: 2040, 1935. ESI-MS (in MeCN): m/z: 496 [M ꢁ PF6ꢁ]+.
Anal. calcd for C17H15N3F6O3PRe: C, 31.88; H, 2.36; N, 6.56.
Fig. 11 (a) FT-IR spectra and (b) UHPLCs of the Re(I) complexes before
and after electrolysis at ꢁ1.82 V with the existence of TPAHBF4
(25 mM) under a 100% CO2 atmosphere in DMF–TEOA containing
Et4NBF4 (0.1 M) as the supporting electrolyte (black lines: before
electrolysis, red lines: after electrolysis). Measurement conditions were
the same as those in Fig. 9.
1
Found: C, 31.60; H, 2.24; N, 6.73. H NMR (400 MHz, CD3CN,
ppm) d ¼ 8.83 (d, 2H, J ¼ 5.5 Hz, H6, 60), 8.33 (br, 2H, H3, 30),
7.54 (dd, 2H, J ¼ 5.5, 1.0 Hz, H5, 50), 2.05 (s, 6H, CH3).
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fac-[Re(dmb)(CO)3(DMF)](PF6) (Re-DMF+). A DMF solution
(70 mL) containing Re-MeCN (26.9 mg, 42 mmol) was kept at
room temperature for 3 h under an Ar atmosphere and dim
light. FT-IR (in DMF) vCO/cmꢁ1: 2028, 1921, 1911.
Manager II Soware).25 For the calculation of K1 in eqn (1)
(ligand substitution reaction), Re-MeCN+ (10.0 mM) was dis-
solved in DMF (0.5 mL) and this solution was kept at room
temperature in the dark for several hours (>3 h) to convert Re-
MeCN+ into Re-DMF+ completely. Aer adding a DMF solution
(0.5 mL) containing TEOA (2.48 M) and Et4NBF4 (0.2 M) to this
solution, the mixed solution was kept at room temperature in
the dark for 2 h (the concentration of the total Re complex,
TEOA and Et4NBF4 is 5.0 mM, 1.26 M and 0.1 M, respectively)
and the IR spectra of the equilibrium mixtures Re-MeCN+ and
Re-TEOA were measured. The K1 value was calculated using eqn
(4) and the obtained concentrations of each Re(I) complex. This
experiment was repeated three times, and the average value and
the standard deviation were obtained.
fac-[Re(dmb)(CO)3{OC(O)H}] (Re-OCHO).
A
H2O–EtOH
mixed solution (1 : 1 v/v, 50 mL) containing fac-[Re(CO)3(dmb)
Br] (0.320 g, 0.599 mmol) and NaOCHO (4.01 g, 60.0 mmol) was
reuxed overnight under dim light. The solution was slowly
evaporated under reduced pressure until all of the EtOH was
removed. Extraction with dichloromethane was performed
three times. The organic phase was evaporated under reduced
pressure. The residual yellow solid was recrystallized from
water–acetone, and then dried overnight under vacuum at 333
K. Yield: 0.182 g (61%). FT-IR (in CH2Cl2): vCO/cmꢁ1: 2017, 1912,
1888. 1H NMR (400 MHz, CDCl3, ppm) d ¼ 8.97 (d, 2H, J ¼
5.6 Hz, H6, 60), 8.10 (d, 1H, CHO), 7.95 (br, 2H, H3, 30), 7.33 (dd,
2H, J ¼ 5.6, 1.2 Hz, H5, 50), 2.57 (s, 6H, CH3).
½Re-TEOAꢄ½DMFꢄ½H-TEOA+ꢄ
K1 ¼
2
½Re-DMF+ꢄ½TEOAꢄ
(4)
2
½Re-TEOAꢄ ½DMFꢄ
Tri-n-propylammonium tetrauoroborate (TPAHBF4).
A
z
2
½Re-DMF+ꢄ½TEOAꢄ
HBF4 aqueous solution (42%, 85 mL) was added to the 200 mL
plastic ask. n-tripropylamine (105 mL, 0.53 mol) was added
dropwise to the solution at 273 K to obtain a white solid. Aer the
solid was ltered, the crude product was recrystallized from ethyl
acetate and dried under vacuum at room temperature overnight
before use. Yield: 44.7 g (36.5%). ESI-MS (in MeCN): m/z: 144 [M ꢁ
BF4ꢁ]+, 1H-NMR (400 MHz, CDCl3, ppm) d ¼ 3.09–3.04 (m, 6H, N–
CH2), 1.83–1.74 (m, 6H, –CH2–CH3), 1.03 (t, J ¼ 7.3 Hz, 9H, –CH3).
For the calculation of K2 in eqn (2) (CO2-capture reaction),
Re-MeCN+ (6.0 mM) was dissolved in DMF (1.0 mL) and this
solution was kept at room temperature in the dark for several
hours (>3 h). Aer adding 1.0 mL of a DMF solution containing
TEOA (2.52 M) and Et4NBF4 (0.2 M) to this solution, the mixed
solution was kept at room temperature in the dark for 2 h (the
concentration of the total Re complex, TEOA and Et4NBF4 is
3.0 mM, 1.26 M and 0.1 M respectively at this time). Then,
a CO2-saturated DMF solution (30 mL, the CO2 concentration is
0.20 M)27 was added to this mixed solution in a shielded tube
(the volume of this tube is 2.0 mL) by using a micro-syringe in
a glove box. Aer keeping at room temperature in the dark for
2 h, the IR spectra were measured. The K2 value was calculated
using eqn (5) and the obtained concentrations of each Re(I)
complex. This experiment was also repeated three times, and
the average value and the standard deviation were obtained.
General procedure
1H NMR spectra were recorded in CDCl3 or CD3CN on a JEOL
ECA400-II system at 400 MHz. Transmission IR spectra in
CH2Cl2 or the reaction solution were recorded on a JASCO FT/
IR-610 or FT/IR-6600 spectrometer at 1 cmꢁ1. Electrospray
ionization-mass spectroscopy (ESI-MS) was performed using
a Shimadzu LC-MS-2010 A system with MeCN as the mobile
phase. Solutions containing 5 mM complexes and 0.1 M
Et4NBF4 were prepared for the FT-IR measurements, and
a solution containing only 0.1 M Et4NBF4 was used for the
background measurements. Ultra high performance chro-
matograms (UHPLC) of the Re complexes were recorded on
a SHIMADZU UHPLC Nexera X2 with an ODS column (Waters
Acquity: 150 mm ꢀ 2.1 mm i.d.), a Shimadzu DGU-20A degas-
ser, an LC-30AD pump, an SPD-M30A UV-vis photodiode-array
detector, and a Rheodyne 7125 injector. The eluent was a 1 : 1
(v/v) mixture of methanol and water containing 0.05 M KH2PO4
½Re-CO2TEOAꢄ
K2 ¼
(5)
½Re-TEOAꢄ½CO2ꢄ
Cyclic voltammetry
CV measurements were performed using a typical three elec-
trode conguration with an ALS CHI-760e potentiostat. The
experiments were carried out in a vessel containing ca. 5 mL of
a sample solution using a glassy carbon disk (diameter ¼ 3 mm)
as the working electrode. An Ag/AgNO3 electrode (Ag wire
immersed in DMF containing 0.01 M AgNO3 and 0.1 M Et4NBF4,
separated from the chamber by vycol® glass) was used as
a reference electrode. The platinum wire was employed as
a counter electrode.
(pH 5.9), and the ow rate was 0.2 mL minꢁ1
.
Calculation of equilibrium constants
We conducted preparation procedures of the solutions for the
measurements in a glove box (UNICO, UN-650F) combined with
a gas circulation dehydration device (DGE-05) under Ar (water
content was less than 376 ppm). We obtained concentrations of
the complexes in solution, using the peak areas of nCO at the
highest wavenumber (2000–2060 cmꢁ1) in the IR spectra. The
Bulk electrolysis
peak areas for each complex were obtained by curve-tting Bulk electrolysis was carried out in an H-shaped electro-
using a linear combination of the Gaussian and the Lor- chemical cell (Fig. S3†) with a Hokuto Denko HZ-7000 poten-
entzian functions using the bundled soware (JASCO Spectra tiostat using a reticulated vitreous carbon mesh (RVC®, 20 PPI,
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2.0 ꢀ 2.5 ꢀ 0.64 cm3 volume immersed in the solution) as the conditions, however, Re-DMF+ was converted to Re-OCHO via
working electrode. An Ag/AgNO3 electrode (Ag wire immersed in CO2 insertion into the electrochemically produced Re-H, and
DMF containing 0.01 M AgNO3 and 0.1 M Et4NBF4, separated formate was catalytically produced even at low concentrations
from the chamber by vycol® glass) was used as a reference of CO2.
electrode. The platinum grid was employed as a counter elec-
trode, which was divided from the cathodic chamber by
a Naon® peruorinated membrane (Naon® 324, Aldrich).
Conflicts of interest
The reference electrode was placed close to the working elec- There are no conicts to declare.
trode so as to minimize the ohmic drop. The working electrode
was washed with acetone and dried for 3 h in a vacuum, and
Acknowledgements
then electrochemically pre-reduced at ꢁ1.82 V over 3 h in DMF
solution containing only the Et4NBF4 electrolyte in the same This work was supported by JST CREST Grant Number
atmosphere as the desired reaction. The cathodic chamber was JPMJCR13L1 in “Molecular Technology”. We acknowledge Prof.
´
lled with an 84 mL portion of the complex solution. The anodic Marc Robert (Universite Paris Diderot) and Prof. Alain Der-
chamber was lled with 84 mL of the DMF–TEOA solution (5 : 1 onzier (Universite Joseph Fourier) for useful discussion.
v/v) which contained the electrolyte and tetrabutylammonium
acetate (TBAOAc) as an electron donor. Before measurement,
the cathodic chamber had been purged with the gas containing
Notes and references
a specic concentration of CO2, while the anodic chamber had
been purged with Ar over 1 h. The cathodic chamber of the
electrochemical cell was attached to the gas ow system. The
gas ow was controlled by a mass ow controller, and the
possibility of the effect of back pressure can be negligible since
the gas pressure in the upstream was only 0.2 MPa and that in
the downstream should be 1 atm, which was released to the
ambient pressure. The solution had been continuously bubbled
with Ar gas containing various concentrations of CO2 (100%,
10%, 1%, and 0%), and the headspace of the reaction space had
been continuously exchanged by the steady rate of the conveyer
gas. The gas products (CO and H2) from the reaction space in
the vented conveyer gas were detected by using a gas chro-
matograph (Agilent 490 micro-GC) equipped with a thermal
conductivity detector. HCOOH in the liquid phase was detected
by Photal CAPI-3300I capillary electrophoresis aer each reac-
tion. The amount of the produced HCOOH by the electrolysis
was calculated by subtracting the HCOOH amount produced in
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