Molecular Electrochemical Catalysis of the CO2-to-CO Conversion with a Co Complex: A Cyclic Voltammetry Mechanistic Investigation

Article abstract of DOI:10.1021/acs.organomet.8b00555

The electrochemical catalytic reduction of CO₂ into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2′-quaterpyridine complex [Co^II(qpy)(H₂O)₂]²⁺ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H₂ as by product, along with a turnover frequency of 1.2 × 10⁴ s^-1 for CO production at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO₂ and the C-O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO₂, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at low overpotential (turnover frequency of 3 × 10⁴ s^-1 at η = 0.3 V) and 95% selectivity for CO production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify, activate, and characterize several pathways for the CO₂-to-CO conversion and to decipher Co electrochemical reactivity toward CO₂.

Full text of DOI:10.1021/acs.organomet.8b00555

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Molecular Electrochemical Catalysis of the CO2‑to-CO Conversion
with a Co Complex: A Cyclic Voltammetry Mechanistic Investigation
†
‡
†
Claudio Cometto, Lingjing Chen, Elodie Anxolabehere-Mallart, Claire Fave,† Tai-Chu Lau,§
́
̀
and Marc Robert*,†
†
́
́
́
Sorbonne Paris Cite, Laboratoire d’Electrochimie Moleculaire, UMR 7591 CNRS, Universite Paris Diderot, 15 rue Jean-Antoine de
Baïf, CEDEX 13 F-75205 Paris, France
‡School of Environment and Civil Engineering, Dongguan University of Technology, Guangdong 523808, China
§Department of Chemistry, Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon
Tong, Hong Kong, China
S
* Supporting Information
ABSTRACT: The electrochemical catalytic reduction of CO2 into CO
could be achieved with excellent selectivity and rate in acetonitrile in the
presence of phenol with cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine complex
[CoII(qpy)(H2O)2]2+ (Co) acting as a molecular catalyst. Upon using
cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic
pathways have been identified. At a low concentration of phenol (<1 M),
catalysis mainly occurs after the reduction of Co with three electrons. In
that case, the selectivity for CO production is ca. 80% with 20% of H2 as by
product, along with a turnover frequency of 1.2 × 104 s−1 for CO
production at an overpotential η of ca. 0.6 V. The triply reduced active
species binds to CO2 and the C−O bond is cleaved thanks to the acid. At
very large concentration of phenol (3 M), another pathway becomes
predominant: the doubly reduced species binds to CO2, while its reductive
protonation leads to CO formation. As already shown, this later process is
endowed with fast rate at low overpotential (turnover frequency of 3 × 104 s−1 at η = 0.3 V) and 95% selectivity for CO
production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify,
activate, and characterize several pathways for the CO2-to-CO conversion and to decipher Co electrochemical reactivity toward
CO2.
a
Scheme 1. Structure of the Catalyst (Co)
1. INTRODUCTION
The electrochemical reduction of CO2 has attracted in the
recent years a lot of attention, due to the possibility of
producing value added chemical as well as fuels, such as
methanol or light hydrocarbons.1,2 It may have an enormous
impact on our industry (electrification of the industry) as well
as on our societies.3,4 In this perspective, CO2 would be used
as a renewable feedstock for storing unlimited, free solar
energy (initially transformed into electricity) before reutilizing
it for various applications (heating, transportation, etc.).
Molecular electrocatalysts are mainly capable of transforming
CO2 with two electrons and two protons into either CO or
formic acid, and they have been recently reviewed with great
care.5−12 Catalysts employing noble metals (e.g., Ru, Re, Pd)
as well as earth-abundant metals (e.g., Co, Mn, Fe) have been
used, although the later ones are less numerous. Among those
recently proposed for CO2-to-CO catalytic conversion, the
cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine [CoII(qpy)(H2O)2]2+
(Scheme 1, noted Co) is one of the most promising examples
in terms of selectivity, durability, and catalytic rate, being active
in acetonitrile (ACN) as solvent both in electrochemical13 and
a
−
Counter ions (2 ClO4 ) omitted for clarity.
in photochemical conditions under visible light irradiation.14
This complex (having 2 Cl atoms as ligand in axial position
instead of the water molecules) was recently incorporated in a
catalytic ink comprising a mixture of multiwalled carbon
nanotubes plus a minimal amount of Nafion and was deposited
onto porous carbon paper.15 It led to exceptional catalytic
activity for CO production in pure water at close to neutral pH
(7.3) with 99% selectivity and current densities up to ca. 20
Special Issue: Organometallic Electrochemistry: Redox Catalysis
Going the Smart Way
Received: August 2, 2018
© XXXX American Chemical Society
A
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mA/cm2 (geometric area) at a low overpotential of 425 mV. It
is currently the most active and selective molecular supported
catalyst for the electrochemical reduction of CO2 in water. We
have reported a preliminary study on the catalytic mechanism
in organic solvent in the presence of a highly concentrated
proton source (3 M phenol).13 The role of the acid is to boost
the C−O bond cleavage. At such high phenol concentration, a
first reduction wave produces the CoI one-electron-reduced-
species (OERS) that reversibly binds two molecules of phenol
in axial position. Upon a second reduction process at potential
close to ca. −1.25 V vs SCE, the catalyst was shown to activate
CO2 and lead to a maximum turnover frequency of 3.3 × 104
s−1 at an overpotential as low as 300 mV. However, a more
detailed mechanistic study probing the effect and the
importance of the applied electric potential and acid
concentration on the catalytic pathway is missing. In this
connection, in a preliminary study on [CoII(qpy)(H2O)2]2+,
Che et al. have reported a catalytic process starting at ca. −1.7
V vs SCE in the absence of any added acid at a potential
corresponding to a reduction wave that was assigned to ligand
reduction, suggesting that the CO2 may be activated at various
potentials by different species.16 Different catalytic pathways
may thus operate depending on the acid content of the
solution and the applied potential.
Figure 2. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) under Ar: (a) without added H2O, (black); + 1% H2O, (red); +
10% H2O (blue) (scan rate 2.5 V/s), (b) with 10% added H2O: 0.1
V/s (black); 0.5 V/s (red); 2.5 V/s (blue), 5 V/s (pink). A glassy
carbon electrode (3 mm diameter) was used as working electrode.
molecules from the one-electron-reduced CoI species (C + E
mechanism),17 as already hypothesized by Che et al.16 The
following reactions summarize the two redox processes:
In this study, we have dissected in detail the reaction
pathways for the electroreduction of CO2 into CO as a
function of phenol addition to the solution and of the potential
applied to the electrode upon using cyclic voltammetry from
0.1 to 500 V s−1 along with complementary controlled-
potential electrolysis. It appeared that CO2 could be activated
at various potentials depending on the phenol concentration
from a doubly or a triply reduced catalyst, respectively.
Mechanisms and catalytic rate constants for both processes
have been obtained.
[CoII(qpy)(H2O)2]2+ + e− V [CoI(qpy)(H2O)2]+
[CoI(qpy)(H2O)2]+ V [CoI(qpy)]+ + 2H2O
[CoI(qpy)]+ + e− V [CoI(qpy)•−]
(1)
(2)
(3)
The second reduction appears localized on the ligand,13 as
suggested by DFT calculation (spin-density analysis) so that
the neutral[Co(qpy)] (noted [CoI(qpy)•−] in reaction (3))
can be viewed as a COI species coordinated by a ligand radical
anion. This doubly reduced species undergoes adsorption on
the electrode surface, as inferred from the sharp desorption
peak observed upon reoxidation scan (oxidation peak at ca.
−0.78 V vs SCE, Figure 1) and from a second desorption peak
(Ep = −1.51 V vs SCE, Figure 1) that corresponds to the third
reduction wave of the complex. The adsorption/desorption
phenomena made it difficult to obtain qualitative and
quantitative data for Co reduction from the analysis of cyclic
voltammetry (CV) experiments at low scan rate (i.e., typically
0.1 V/s). However, when raising the scan rate, the charge
passing through the electrode surface diminishes, and the
amount of adsorbed species could thus be minimized. This is
what can be seen from Figure 3, where a scan rate of up to 70
V/s was applied, leading to a purely diffusion-controlled third
reduction peak (see Figure S1). A standard redox potential E0
= −1.51 V vs SCE was obtained from this reduction wave that
corresponds to the generation of the anionic [Co(qpy)]−
species.
2. RESULTS AND DISCUSSION
The cyclic voltammetry of [Co(qpy)(H2O)2]2+ under Ar
atmosphere (Figure 1) shows a first reversible one-electron
reduction wave centered on the metal with E0 (CoII/CoI) =
1
−0.565 V vs SCE, followed by a second one with a standard
redox potential E0 = −0.795 V vs SCE (see below). Upon
2
increasing the H2O concentration or the scan rate (Figure 2),
the second reduction wave evolves to a small plateau-shape
current, indicating that it is preceded by the loss of water
As already described,13 upon addition of phenol (PhOH) to
a solution of Co, [CoI(qpy)]+ binds two PhOH molecules:
[CoI(qpy)]+ + 2PhOH V [CoI(qpy)(PhOH)2]+
(4)
with an equilibrium constant K of 0.18 M−2. The intensity of
the second reduction wave decreases with increasing
concentration of phenol (Figure 4, inset), due to limited
formation of uncoordinated [CoI(qpy)]+. The new species
[CoI(qpy)(PhOH)2]+ is in turn reduced at ca. −1.25 V,
corresponding to a new reduction wave that grows with
[PhOH] (Figure 4). At the largest concentration of phenol
Figure 1. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) under Ar, scan rate 0.1 V/s. A glassy carbon electrode (3 mm
diameter) was used as working electrode. The gray trace is restricted
to the first redox process (eq 1).
B
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Figure 3. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) under Ar: scan rate 5 V/s (black), 20 V/s (blue), 40 V/s (green),
70 V/s (purple). A glassy carbon electrode (1 mm diameter) was used
as working electrode.
Figure 5. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) under CO2 atmosphere, scan rate 0.1 V/s, in the absence (black)
and presence of 10 mM (red), 30 mM (blue), 130 mM (magenta),
210 mM (green), and 800 mM (orange) PhOH. A glassy carbon
electrode (3 mm diameter) was used as working electrode.
increasing [PhOH] suggests the occurrence of several catalytic
processes.
By increasing the scan rate between 50 and 500 V/s for very
high phenol concentrations (1−3 M), the catalytic peak was
distinctively split into two different waves (Figure 6a). The first
Figure 4. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) under Ar, scan rate 0.1 V/s, in the absence (black) and presence
of 1 M (red), and 1.5 M (blue) and 2 M (magenta) PhOH. Each CV
curve is normalized to the first reduction peak current in order to take
into account viscosity change and dilution effect. Inset: zoom of the
first and second reduction waves. A glassy carbon electrode (3 mm
diameter) was used as working electrode.
Figure 6. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) solution under CO2 atmosphere: (a) +1.5 M PhOH at 50 V/s
(black), 150 V/s (blue), 250 V/s (green), 350 V/s (purple), 450 V/s
(wine), and 500 V/s (red). (b) +1 M (black), 1.5 M (red), 2 M
(blue), and 3 M (magenta) PhOH at ν = 250 V/s. A glassy carbon (1
mm diameter) was used as working electrode.
employed (3 M), a marginal shift of the UV absorption band of
Co was observed (Figure S2), showing the interaction between
PhOH molecules and [CoII(qpy)(H2O)2]2+ is negligible.
When a solution of Co is purged with CO2 in absence of
added acid, no catalysis occurs upon the first three cathodic
peaks. Conversely a catalytic wave is produced at more
negative potential (potentials negative to ca. −1.62 V vs SCE,
Figure S3), as already reported by Che et al.16 We did not
investigate this wave further. Upon gradually increasing the
phenol concentration, a current enhancement is observed upon
the third reduction peak (Figure 5).
Concomitantly, adsorption phenomena decrease as attested
by the progressive disappearance of the reoxidation peak at ca.
−0.8 V vs SCE, showing that phenol limits interactions
between the electro-generated intermediates and the electrode
surface. Indeed, formation of [CoI(qpy)(PhOH)2]+ instead of
[CoI(qpy)]+ upon the first reduction wave avoids
[CoI(qpy)•−] production upon the second reduction wave,
which is the species prone to adsorption. The complex shape
and continuous positive shift of the catalytic wave upon
one is centered at ca. −1.25 V vs SCE (indicated by an arrow
in Figure 6a), while the second one is observed at ca. −1.55 V.
Both catalytic waves show plateau current at 500 V/s (Figure
6a). For a fixed scan rate of 250 V/s, the first catalytic wave
becomes the only process to be observed for 3 M phenol
(Figure 6b). Concomitantly, the reductive wave at −0.95 V vs
SCE corresponding to the formation of [CoI(qpy)•−] almost
disappears. These data suggest that two catalytic pathways
could be activated by adding phenol as proton source, at
overpotentials close to ca. 0.3 and 0.6 V respectively (E0
CO2/CO
= −0.96 V vs SCE in this solvent conditions).18 The first, most
positive one is triggered by the reduction of [CoI(qpy)-
(PhOH)2]+, while the second one requires the reduction of
C
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[Co(qpy)] to [Co(qpy)]−. In our previous contribution, we
have demonstrated that the first process is highly selective for
CO production (catalytic selectivity > 95%).13 Upon perform-
ing a controlled potential electrolysis at −1.6 V vs SCE in the
presence of 0.1 M PhOH (Figure S4), conditions into which
the second catalytic process is dominant, CO is the main
product (72% Faradaic efficiency FE, 77% selectivity) with
however a significant amount of H2 (20% FE).
According to these data, two different catalytic mechanisms
(Scheme 2) could be proposed for the CO2 reduction
Scheme 2. Mechanism for the Electrochemical Reduction of
a
CO2 into CO with Co
Figure 7. CVs of Co (0.5 mM) in an ACN solution + NBu4PF6 (0.1
M) and PhOH (1 M) under CO2 atmosphere, at v = 50 V/s(black),
100 V/s (red), 150 V/s (blue), 200 V/s (magenta), 250 V/s (olive),
300 V/s (navy), 350 V/s (violet), 400 V/s (purple), 450 V/s (wine),
and 500 V/s (dark yellow). Inset: variation of the catalytic current
observed at potentials negative to −1.5 V (obtained after subtraction
of the background current value) as a function of the scan rate; see
Figure S5 for further details. A glassy carbon (1 mm diameter) was
used as working electrode.
is due to CO2 reduction (the remaining charge is essentially
directed to H2 evolution, assuming similar selectivity as found
in potential controlled electrolysis). This current is related to
the rate constant kcat of the catalysis through the following eq 5
corresponding to the mechanism sketched in Scheme 2
(pathway b):19
iplateau = 2FS × Cc0at
×
D
×
kcat
cat
(5)
where S is the electrode surface, C0 is the catalyst
cat
concentration, Dcat its diffusion coefficient, and v is the scan
rate.
a
After normalization by the one electron diffusion current of
the catalyst (obtained from the first wave reoxidation peak,
Figure 7):
Water molecules in the initial complex are omitted for clarity.
catalyzed by Co, depending on the applied potential and the
amount of PhOH added to the solution. For a very large
concentration of phenol (3 M), [CoI(qpy)(PhOH)2]+ is
produced upon the one-electron reduction of Co (pathway
a). Although this species is formally considered as the
precursor of the active species [CoI(qpy)•−(PhOH)2],
deprotonation of one or two phenolic groups should not be
excluded. Pathway b is dominant for a smaller amount of
added acid (<1.5 M). In this case, [CoI(qpy)]+ is reduced to
[CoI(qpy)•−] (E0 = −0.795 V vs SCE). Reduction of this later
species (E0 = −1.51 V vs SCE) to [Co(qpy)]− triggers the
catalysis. The partial loss of selectivity detected during
electrolysis at −1.6 V vs SCE with 0.1 M phenol can be
attributed to the formation of the highly basic species
[Co(qpy)]− that may be protonated to an hydride species
and further evolve hydrogen, this reaction thus being in
competition with CO2 reduction.
At 1 M PhOH, a catalytic plateau independent of the scan
rate is observed at large scan rate for pathway b, as shown in
Figure 7. It is indicative of pure kinetic conditions resulting
from mutual compensation between the catalyst diffusion and
fast catalytic rate.18 Catalysis is purely homogeneous as
confirmed by the freely diffusive nature of the third reduction
wave (see Figure S1). The catalytic current could be obtained
from the CVs: Of the plateau current, 72% (Figures 7 and S5)
Fv
RT
ip0eak = 0.446 × FS × Cc0at
It led to
×
D
×
cat
(6)
2
i
j
y
z
i
1
Fv
RT
plateau
ip0eak
j
j
z
z
kcat
=
×
×
j
z
j
j
z
z
(2 × 2.24)2
(7)
k
{
and thus kcat = 1.2 × 104 s−1, showing the remarkable efficiency
of the CO2 reduction process (see the Supporting
Information). The rate for catalysis is close to the rate of
pathway a (3.3 × 104 s−1 at PhOH 3 M) previously obtained.
In conclusion, we have shown that the electrocatalytic
reduction of CO2 into CO by Co could occur through two
different mechanisms, depending on the phenol content in the
solution. Both pathways necessitate the assistance of the weak
proton source to boost the C−O bond cleavage. The most
positive pathway is activated with an overpotential η of ca. 300
mV and is predominant at very large concentration of acid (3
M). The second one occurs with η = 600 mV and is activated
at smaller acid concentrations. Both processes are endowed
with rate constant in the range of 104 s−1, illustrating the high
efficiency of the catalyst. By adjusting the acid concentration
and the scan rate, both mechanisms could be characterized
D
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independently from analysis of the CVs, showing the power of
cyclic voltammetry to access mechanistic and kinetic data for
catalytic processes.
CV, CPE, catalytic rate constant estimation for Co
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: robert@univ-paris-diderot.fr.
ORCID
■
EXPERIMENTAL SECTION
■
Chemicals. Acetonitrile (Acros, > 99.9%, extra dry over molecular
sieves) and supporting electrolyte NBu4PF6 (Fluka, purriss.) were
used as received. Co was synthesized as previously reported.14 Phenol
was purchased from Sigma-Aldrich and used without further
purification.
́
̀
Elodie Anxolabehere-Mallart: 0000-0002-8708-802X
Claire Fave: 0000-0001-8146-8702
Electrochemistry and Spectroscopic Analysis. All the experi-
ments have been performed by using dry acetonitrile as solvent and
NBu4PF6 as supporting electrolyte. Before each experiment, the
solution was purged with Ar or CO2 for 20 min.
Tai-Chu Lau: 0000-0002-0867-9746
Marc Robert: 0000-0001-7042-4106
Notes
The authors declare no competing financial interest.
Cyclic Voltammetry. The working electrode was a 3 mm
diameter glassy carbon (Tokai) disk carefully polished using diamond
paste of various size (from 15 to 1 μm) and ultrasonically rinsed in
absolute ethanol and dried before use. The counter electrode was a
platinum wire, and the reference electrode was an aqueous SCE
electrode. All experiments were carried out under argon or carbon
dioxide atmosphere at 20 °C, the double-walled jacketed cell being
temperature-controlled by circulation of water. CV data were
obtained by use of a Metrohm AUTOLAB instrument. Ohmic drop
was compensated using the positive feedback compensation
implemented in the instrument. For high speed cyclic voltammetry,
a 1 mm diameter glassy carbon was used as working electrode, and a
homemade potentiostat was used for ohmic drop compensation.
Controlled Potential Preparative Scale Electrolysis. Elec-
trolysis reactions were performed using a Princeton Applied Research
(PARSTAT 2273) potentiostat. Experiments were carried out in a
two-compartment cell with a glassy carbon plate as working electrode.
The volume of the solution was 3 mL, and active surface area was 1.8
cm2. The reference electrode was an aqueous SCE electrode, and the
counter electrode was a platinum wire positioned in a bridge
separated from the cathodic compartment by a ceramic frit,
containing a 2% H2O + 0.1 M NBu4PF6 ACN solution. The
electrolysis solution was purged with CO2 during 20 min prior to
electrolysis. The ohmic drop between working electrode and reference
electrode was minimized by dipping the former one directly in the
solution and positioning it close to the working electrode. All CPE
experiments have been performed under stirring.
Gas Chromatography. Analyses from the gas evolved in the
headspace during electrolysis were carried out with an Agilent
Technologies 7820A GC system equipped with a thermal
conductivity detector. CO and H2 production were quantitatively
assessed using a CP-CarboPlot P7 capillary column (27.46 m in
length and 25 μm internal diameter). Temperature was held at 150 °C
for the detector and 34 °C for the oven. The carrier gas was argon
flowing at 9.5 mL/min at constant pressure of 0.5 bar. Injection was
performed via a 250 μL gas-tight container (Hamilton). These
conditions allowed for separation of both H2, O2, N2, CO, and CO2.
Calibration curves for H2 and CO were determined separately by
injecting known quantities of pure gas.
ACKNOWLEDGMENTS
■
The work described in this paper was supported by Hong
Kong University Grants Committee Area of Excellence
Scheme (AoE/P-03-08), the National Science Foundation of
China (No. 21703034) and the French National Agency for
Research (ANR-16-CE05-0010-01). A Ph.D. fellowship to
́
́
C.C. from Universite Sorbonne Paris Cite (USPC) is gratefully
acknowledged. Partial financial support to M.R. from the
Institut Universitaire de France (IUF) is gratefully acknowl-
́
edged. G. Thoraval (Universite Paris Diderot) is warmly
thanked for the design and production of the glassy carbon
electrodes.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00555.
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DOI: 10.1021/acs.organomet.8b00555
Organometallics XXXX, XXX, XXX−XXX