Angewandte
Communications
Chemie
atmosphere when the pH was raised from 13 to 14 (see
entries 6 and 10 in Table 1). Indeed, methylene glycol (the
volume affects the formaldehyde concentration and thus its
reaction rate), the typical uncertainty is in the range of 10–
20%.
[21]
aqueous form of formaldehyde) has a pK of about 13 and is
a
mostly deprotonated at pH 14. This is supported by a similar
drop in methanol production (< 3% FE) when starting with
formaldehyde as a substrate at pH 14. Interestingly, when
methanol itself was used as a starting substrate (20 mm,
controlled potential electrolysis at ꢁ0.64 V vs. RHE and
pH 13 for 2 h), no traces of other products such as methane
were detected from GC analysis of the headspace, further
All previous results converge to a simple sequential
strategy for optimizing methanol production. CoPc can first
efficiently catalyze the electrochemical CO -to-CO conver-
2
sion with a high FE (95%) in a flow cell with current densities
ꢁ
2
[28]
up to 150 mAcm as we recently demonstrated. Pure CO
can then be used as a reactant under basic conditions (pH 13)
and be reduced to CH OH with 14.3% efficiency (740 mV
3
illustrating the selectivity of the catalysis towards CH OH
formation.
overpotential). The catalyst is the same for each step, while
the pH and the electrode potential are adjusted to maximize
each partial reduction process. From the total number of
transferred electrons, a global Faradaic efficiency of 19.5% is
calculated and the chemical selectivity is about 7.5% (Fig-
ure S8).
3
To assess the molecular nature of the catalysis and
examine the possibility that the observed reactivity is due to
[22]
decomposition of CoPc into metallic Co nanoparticles,
a series of control experiments were performed with various
films in CO-saturated solutions at pH 13. The first film was
prepared by replacing CoPc with an electrodeposited CoCl2
film with an equivalent amount of Co atoms. The second film
was obtained by replacing CoPc with an identical concen-
tration of cobalt quaterpyridine (CoQpy), a more fragile
In summary, cobalt phthalocyanine was used as a catalyst
for the electrochemical conversion of CO2 to methanol,
thanks to the unlocking of the CO-to-methanol step. The
simplicity of the catalyst, the facile procedure for preparing
the catalytic electrode, and the low loading amounts of the
catalyst make the process versatile and easy to implement.
Beyond this proof of principle, a tuning of the ligand will
rapidly lead to improved performances and will be guided by
mechanistic studies that are currently being done in our
laboratories. We have thus shown that the electrochemical
multi-electron-multi-proton reduction of CO2 beyond CO
and formate can be achieved and controlled with a molecular
catalyst. This study illuminates a new field of research for
employing earth-abundant metal-based molecular complexes
bearing simple ligand structures as cascade electrocatalysts
[
23]
catalyst
that demetalates at negative potentials. Finally,
a third film was made with CoPc, but was subjected to a more
negative potential (E = ꢁ0.99 V vs. RHE) to accelerate
catalyst decomposition. In all these experiments, no CH OH
3
was found in the catholyte after electrolysis, indicating that
the aforementioned catalysis of CO to CH OH is indeed
2
3
a molecular-driven process. This is also supported by the fact
that an anodic scan of the catalytic film right after electrolysis
failed to detect any oxidative stripping peak that would
correspond to the oxidation of electrodeposited metallic Co
(
Figure S6). The Co K-edge X-ray absorption near-edge
for liquid-fuel production from CO and renewable electricity
2
structure (XANES) spectra of the CoPc starting complex as
well as the CoPc-MWCNT electrodes before and after
electrolysis at ꢁ0.64 V vs. RHE under CO atmosphere are
in mild aqueous conditions.
shown in Figure 1d. The three spectra present the typical Acknowledgements
features expected for cobalt phthalocyanine complexes:
a first, low-intensity pre-edge peak at 7710 eV which is
We thank Dr. S. G. Derouich (Univ. Paris Diderot) for SEM
assigned to the 1s!3d/4p transition and a second intense
analysis, and G. Le Faucheur and M.-Y. Pinart (Univ. Paris
Diderot, Chemistry Department) for performing HPLC
analysis. This work was supported by the French National
Agency for Research (ANR-16-CE05-0010-01). M.W. thanks
the China Scholarship Council for her PhD fellowship (CSC
student number 201606220034). Partial financial support to
M.R. from the Institut Universitaire de France (IUF) is also
gratefully acknowledged.
peak at 7716 eV for the 1s!4p transition which is character-
z
[24,25]
istic to Co–N4 interactions.
Only slight changes are
observed after catalysis, which can be attributed to changes in
[24]
the interactions between CoPc and the MWCNTs.
A
comparison of the spectrum obtained after electrolysis with
those of reference cobalt species further demonstrates the
intactness of the CoPc-MWCNT hybrid (Figure S7). How-
ever, it should be noted that the catalytic activity progres-
sively decreased at longer times, as observed from the drop of
the Faradaic yield for methanol production after a couple of Conflict of interest
hours. It may be due to reductive hydrogenation of the C=N
double bonds of the phthalocyanine core, which has already
The authors declare no conflict of interest.
[26]
been previously reported. The exact mechanism of deac-
tivation is under active study but we already know that basic
media is not detrimental since a similar cobalt phthalocyanine
has recently been shown to be stable for more than 10 h under
Keywords: carbon-monoxide reduction · cobalt phthalocyanine ·
electrochemistry · methanol · molecular catalyst
strongly alkaline conditions (pH 14) for the CO -to-CO step
2
[
27]
at a gas diffusion electrode.
Because of the rather fast
deactivation and variation in formaldehyde concentration
over time (the ratio between electrode surface and electrolyte
4
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Angew. Chem. Int. Ed. 2019, 58, 1 – 6
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