Acetaldehyde as an Intermediate in the Electroreduction of Carbon Monoxide to Ethanol on Oxide-Derived Copper

Article abstract of DOI:10.1002/anie.201508851

Oxide-derived copper (OD-Cu) electrodes exhibit unprecedented CO reduction performance towards liquid fuels, producing ethanol and acetate with >50% Faradaic efficiency at -0.3 V (vs. RHE). By using static headspace-gas chromatography for liquid phase analysis, we identify acetaldehyde as a minor product and key intermediate in the electroreduction of CO to ethanol on OD-Cu electrodes. Acetaldehyde is produced with a Faradaic efficiency of ≈5% at -0.33 V (vs. RHE). We show that acetaldehyde forms at low steady-state concentrations, and that free acetaldehyde is difficult to detect in alkaline solutions using NMR spectroscopy, requiring alternative methods for detection and quantification. Our results represent an important step towards understanding the CO reduction mechanism on OD-Cu electrodes.

Full text of DOI:10.1002/anie.201508851

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International Edition: DOI: 10.1002/anie.201508851
German Edition: DOI: 10.1002/ange.201508851
Electrochemistry
Acetaldehyde as an Intermediate in the Electroreduction of Carbon
Monoxide to Ethanol on Oxide-Derived Copper
Erlend Bertheussen, Arnau Verdaguer-Casadevall, Davide Ravasio, Joseph H. Montoya,
Daniel B. Trimarco, Claudie Roy, Sebastian Meier, Jürgen Wendland, Jens K. Nørskov,
Ifan E. L. Stephens,* and Ib Chorkendorff*
Abstract: Oxide-derived copper (OD-Cu) electrodes exhibit
unprecedented CO reduction performance towards liquid
fuels, producing ethanol and acetate with > 50% Faradaic
efficiency at À0.3 V (vs. RHE). By using static headspace-gas
chromatography for liquid phase analysis, we identify acetal-
dehyde as a minor product and key intermediate in the
electroreduction of CO to ethanol on OD-Cu electrodes.
Acetaldehyde is produced with a Faradaic efficiency of ꢀ 5%
at À0.33 V (vs. RHE). We show that acetaldehyde forms at low
steady-state concentrations, and that free acetaldehyde is
difficult to detect in alkaline solutions using NMR spectrosco-
py, requiring alternative methods for detection and quantifica-
tion. Our results represent an important step towards under-
standing the CO reduction mechanism on OD-Cu electrodes.
that potentials of À1 V (vs. RHE), or overpotentials, h >
ꢀ 1.0 V, are needed to produce significant amounts of C2
products, that is, above 5% Faradaic efficiency with a current
density of 1 mAcmÀ2 or higher.[2–6]
A viable route forward is to split the reaction into two
sequences; reducing CO2 to CO at first, and then reducing CO
to the desired product in a second step. Since CO is a key
intermediate in the reduction of CO2 to alcohols and hydro-
carbons, CO reduction can be used as a proxy for under-
standing trends in CO2 reduction.[2,4] Several catalysts have
been reported to reduce CO2 to CO efficiently and selec-
tively,[7–11] but the second step remains a challenge owing to
multi-electron transfer involving several reaction intermedi-
ates.[12] This calls for development of new catalysts with
improved energy efficiency and selectivity for CO reduction
towards valuable compounds. Kanan and co-workers recently
achieved a breakthrough in this area; they showed that
oxidation and subsequent reduction of polycrystalline copper
yields a high surface area metallic copper electrode with
unprecedented CO electroreduction performance.[13,14]
Oxide-derived copper (OD-Cu) has a Faradaic efficiency
U
tilization of CO2 as a feedstock for producing fuels and
commodity chemicals is a highly promising technology for
reducing the anthropogenic carbon footprint. Capture of CO2
from point sources or ambient air, followed by reduction,
gives an opportunity to close the carbon cycle.[1] Electro-
chemical technology provides a means of achieving this, as
electrochemical devices can operate at ambient conditions,
with minimal capital investment, and with fast start–stop
cycles enabling coupling to intermittent energy sources. To
date, implementation of this technology is hindered by a lack
of electrocatalysts capable of converting CO2 into energy-rich
products in an efficient and selective manner. Copper is the
only pure metal that is active for CO2 reduction towards
hydrocarbons and alcohols.[2] However, high overpotentials
are needed and a variety of products are formed. Measure-
ments on planar extended surfaces of Cu electrodes showed
towards ethanol as high as 43% at À0.3 V, h ꢀ 500 mV (U0
CO/
CH3CH2OH = 0.18 V), and a total Faradaic efficiency towards CO
reduction products of 57%, with a total geometric current
density of ꢀ 0.3 mAcmÀ2.
The underlying reasons for the high performance of OD-
Cu electrodes remain unknown. Our own temperature
programmed desorption (TPD) experiments show that the
activity correlates with the presence of strong binding sites,
which in turn correlates with the presence of grain bounda-
ries.[15] Importantly, the mechanism for ethanol production
[*] E. Bertheussen, Dr. A. Verdaguer-Casadevall, D. B. Trimarco, C. Roy,
Prof. Dr. I. E. L. Stephens, Prof. Dr. I. Chorkendorff
Department of Physics, Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
Prof. Dr. J. K. Nørskov
SUNCAT Center for Catalysis and Interface Science
SLAC National Accelerator Laboratory
2675 Sand Hill Road, Menlo Park, CA 94025 (USA)
E-mail: ifan@fysik.dtu.dk
Prof. Dr. I. E. L. Stephens
ibchork@fysik.dtu.dk
Department of Mechanical Engineering
Massachusetts Institute of Technology (MIT)
77 Massachusetts Avenue, Cambridge, MA 02319 (USA)
Dr. D. Ravasio, Prof. Dr. J. Wendland
Carlsberg Laboratory
Gamle Carlsberg vej 4, 1799 København V (Denmark)
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201508851.
Dr. J. H. Montoya
SUNCAT Center for Catalysis and Interface Science
Department of Chemical Engineering, Stanford University
443 Via Ortega, Stanford, CA 94305 (USA)
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no modifica-
tions or adaptations are made.
Dr. S. Meier
Department of Chemistry, Technical University of Denmark
2800 Kongens Lyngby (Denmark)
1450
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has not been determined yet. In this work, we reveal the role
earlier works.[14,16] The parasitic evolution of hydrogen
of acetaldehyde as a likely reaction intermediate and product
of CO reduction on OD-Cu electrodes.
accounts for the remaining ꢀ 40%. Interestingly, we observed
that acetaldehyde, which is previously unreported on OD-Cu,
is produced with a current efficiency of 5% at À0.33 V, or h
ꢀ 400 mV (U0CO/CH3CHO = 0.10 V). The fact that we can only
observe C2 products corresponds well with the previous
observations that such products are favored on Cu-based
electrodes at low overpotentials under alkaline condi-
tions.[14,17] The total Faradaic efficiency at À0.28 and
À0.33 V comes, within the experimental error, to ꢀ 100%;
we attribute the error to uncertainties in the calibration as
well as minor leaks. At À0.39 V, ꢀ 20% of the charge is not
accounted for; this is likely to be caused by the high current
density towards hydrogen evolution, where some of the H2
escapes the cell. Figure 1b shows the partial current densities
to the various products, normalized to the geometrical and
specific electrode surface area. The overall current densities
are two to three times higher than those reported by Li et al.
at all potentials.[14] This discrepancy could be caused by
differences in mass transport, which is a function of the cell
design. Alternatively, the greater capacitance observed for
the electrodes produced in the current study could indicate
that they have a higher surface area (section S.2).
Oxide-derived Cu electrodes were produced following the
procedure outlined by Li et al. (Supporting Information,
section S.1).[14] The resulting electrodes had a roughness
factor of 87 Æ 10 (section S.2). Electrochemical CO reduction
was carried out in CO-saturated 0.1m KOH electrolyte. We
used three approaches to analyze the reaction products. The
gas phase composition over the electrolyte was measured by
gas chromatography (GC). The liquid phase products were
analyzed by extracting liquid samples and using a combination
of nuclear magnetic resonance (NMR) spectroscopy and
static headspace gas chromatography (HS-GC), the latter
coupled with either a flame ionization detector (FID) or
a mass spectrometer (MS). Figure 1a shows the Faradaic
Earlier studies on extended surfaces of Cu have suggested
that acetylaldehyde is an intermediate for ethanol production
from CO2 or CO reduction;[3,18] Hori et al. reported that on
polycrystalline Cu, acetaldehyde is formed from CO at
potentials negative of À0.83 V RHE, albeit with a low
selectivity of 0.5%. They also showed that acetaldehyde can
be reduced to ethanol,[16] suggesting it is a soluble intermedi-
ate in the reaction (incidentally acetaldehyde is a key
intermediate in ethanol oxidation, the reverse reaction[19]).
To investigate the possibility that acetaldehyde is an
intermediate during CO reduction on OD-Cu, we tested its
hydrogenation by introducing 10 mm CH3CHO in Ar-satu-
rated 0.1m KOH. As shown in Figure 2, ethanol is produced
Figure 1. CO reduction on oxide-derived Cu electrodes. (a) Faradaic
efficiency and (b) current density normalized to geometric and specific
surface area for the individual products. Measurements were carried
out in CO saturated (1.1 bar) 0.1m KOH. Note: The unbalanced
contribution to the current is only added at À0.39 V, since the total
Faradaic efficiency at the other two potentials are ꢀ100% within the
experimental error. The data are based on at least three independent
measurements for each potential. At À0.28 V, electrolysis was carried
out until the accumulated charge reached ꢀ5 C, and at À0.33 and
À0.39 V until ꢀ10 C.
efficiency for CO reduction on oxide-derived Cu electrodes.
Results resemble those reported by Kanan and co-workers.[14]
At À0.33 V (vs. RHE; unless otherwise stated, all potentials
in this work are given against the RHE scale), the current
efficiency to ethanol and acetate is 25% for each, and 1% of
the charge results in ethylene and ethane. Our observation
that C2H4 and C2H6 is formed on OD-Cu is consistent with
Figure 2. Faradaic efficiency for CH3CHO hydrogenation at different
potentials. Measurements were carried out in Ar saturated (1.1 bar)
0.1m KOH with 10 mm of acetaldehyde. Note: The charge not
accounted for is attributed to higher carbon oxygenates that have not
been quantified.
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with a Faradaic efficiency of ꢀ 30% at À0.33 V. Significant
production of C3H7CHO also occurred. We did not observe
this compound during our CO reduction experiments, possi-
bly because of the lower acetaldehyde concentrations. How-
ever, we cannot discard the possibility that C3H7CHO is
produced in concentrations below the detection limit from
CO reduction. For lower added concentrations of acetalde-
hyde, much less ethanol and butanal were produced, possibly
because of poorer mass transport of acetaldehyde to the
electrode. This makes measurements at the concentrations
observed for CO reduction ( ꢀ 120 mm) difficult. Interestingly,
performing the same experiment with acetate did not yield
any detectable CO reduction products; this suggests that
acetate cannot be reduced further when it is formed at the
electrode. Figure 3 shows the concentration of acetaldehyde
and ethanol from CO reduction at À0.33 V as a function of
total electrolysis charge. Whereas ethanol concentration
scales linearly with charge, the acetaldehyde concentration
quickly saturates at ꢀ 120 mm. We hypothesize that this is
caused by the formation of aqueous acetaldehyde and its
further reduction to ethanol reaching equilibrium.
In Figure 4, we provide the results of density functional
theory (DFT) calculations of intermediates proceeding from
À
adsorbed *OCCHO, a likely intermediate in the C C
coupling pathway.[17,20] The most favorable thermodynamic
pathway towards ethanol on Cu(211), highlighted in black
proceeds directly through this intermediate. Using other
copper facets could alter the reactivity of the surface, but is
unlikely to change the overall shape of the reaction path-
way.[12] Aqueous acetaldehyde can be seen as an intermediate,
as previously reported.[21] The step following the formation of
CH3CHO, a proton-coupled electron transfer producing
adsorbed *OCH2CH3, is uphill at 0.0 V vs. RHE. However,
it is downhill at À0.3 V, and will be even more exergonic with
increasing concentrations of CH3CHO. The lower free energy
found for aqueous acetaldehyde compared to its adsorbed
counterpart ( ꢀ 0.7 eV) yields a thermodynamic sink, sup-
porting the experimental observation that it is released from
the electrode and is present in the electrolyte in detectable
concentrations.
Typically, NMR spectroscopy is used for detection of
liquid products.[3,14,22] Liquid state NMR spectroscopy
requires analytes to be dissolved in a liquid phase for
adequate detection. Alternative techniques can overcome
this limitation. For instance, HS-GC heats up the liquid
samples, in this case to 708C, and relies on the volatility of
compounds for identification.[22] This is particularly useful for
products that evaporate readily, or that decompose into such
compounds upon heating. Acetaldehyde has a boiling point of
Figure 3. Acetaldehyde and ethanol concentration from CO reduction
at À0.33 V showed as a function of the charge involved in each
measurement. Measurements were carried out in 0.1m KOH.
Figure 4. Free energy diagram for reduction of C C coupled intermediate *OCCHO on Cu(211). The free energy for CH3CH2OH(g) was calculated
at 1 bar and the free energy of CH3CHO(aq) was calculated at a concentration of 100 mm. The steps marked with black represent the
thermodynamically most favorable pathway. The gray represent other potential intermediates of higher energy.
À
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Figure 5. Liquid product analysis for a representative CO reduction sample from electrolysis at À0.33 V (black) as well as two solutions containing
250 mm CH3CHO, EtOH and AcOÀ in 0.1m KOH (gray), one made from a 40%, and the other from a 100% acetaldehyde precursor. (a) HS-GC-
FID chromatograms. (b) HS-GC-MS mass spectrum obtained from the acetaldehyde peak (at ꢀ1.5 min) of a CO reduction measurement,
compared to a database reference mass spectrum for acetaldehyde. Inset: HS-GC-MS chromatograms (c) NMR spectra.
20.98C under standard conditions.[23] However, no trace of
acetaldehyde was observed in the gas phase, which might be
due to deviations from standard conditions and low concen-
trations. In Figure 5, a typical sample from CO reduction at
À0.33 V is compared with two different solutions containing
the same liquid products as the sample, analyzed with HS-
GC-FID, HS-GC-MS, and NMR spectroscopy. For the two
GC techniques, acetaldehyde can be clearly identified, both
for the sample and the two prepared solutions. HS-GC-MS
enables positive identification of acetaldehyde at mass 44. In
contrast, acetaldehyde only appears in NMR through its diol
form in the solution made from 40% acetaldehyde, observed
as a doublet at ꢀ 1.25 ppm. Neither the solution made from
a 100% acetaldehyde precursor, nor the CO reduction sample
shows any major signals that could be attributed to this
compound, implying that routine NMR spectroscopy is not
a reliable technique to identify acetaldehyde under basic
conditions.
an acid-catalyzed depolymerization process. Acetaldehyde
chemistry is discussed further in section S.3, but is beyond the
scope of this study.
The appearance of an additional product should, in
principle, be observable through the Faradaic efficiency not
reaching 100%. However, the uncertainty in the analysis
techniques could lead to minor products being overlooked.
In summary, we present evidence that acetaldehyde is
a product and key intermediate in the electroreduction of
carbon monoxide on oxide-derived copper. Detection of
acetaldehyde in alkaline solutions using NMR spectroscopy is
challenging, and thus, identification is performed using HS-
GC. Our results highlight the importance of using comple-
mentary methods for product detection from electrochemical
reactions. Identification of acetaldehyde represents a first
step in elucidating the CO reduction mechanism on OD-Cu
electrodes and paves the way for the design of improved
catalysts.
The organic chemistry of acetaldehyde, particularly in
alkaline solutions, is complex.[24] Acetaldehyde occurs in
various hydrated, aggregated, and polymerized states in
solution (see section S.3 for further discussion). We hypothe-
size that the main difficulty in detecting acetaldehyde with
NMR spectroscopy is polymerization, possibly caused by
a high local concentration at the electrode. This could yield
insoluble compounds invisible to NMR and/or significant
signal broadening. Three key observations support this
hypothesis: a) For concentrated solutions in alkaline media,
a yellow precipitate can be observed (Supporting Informa-
tion, Figure S.6b), which could be attributed to an aggregated
or polymerized form. b) Acetaldehyde standards aged in 0.1m
KOH and extensive NMR signal accumulation in high-field
NMR yield spectra consistent with the presence of acetalde-
hyde and its condensation products formed upon aging
(Figure S.5). Since acetaldehyde occurs as a minor steady-
state intermediate (Figure 3), the detection of acetaldehyde
by more routine NMR methods, on the other hand, remains
difficult. c) Upon acidification of the 0.1m KOH solution from
pH 13 to pH 1, visible but broadened signals appear in the
NMR spectra (Figure S.6a). We attribute their presence to
ethyl acetate, an acetaldehyde dimer, which could result from
Experimental Section
Rectangular electrodes of 5 ” 10 mm were cut and electropolished.
They were annealed in air at 5008C for an hour and cooled to room
temperature, before being mounted in the electrochemical cell where
the Cu2O surface layer was reduced chronopotentiometrically at
7 mAcmÀ2. Electrode capacitance measured by cyclic voltammetry
was used to determine the roughness of the electrodes.
The working electrode (WE) and counter electrode (CE)
compartments were separated by a Nafion 117 membrane and the
WE and reference electrode (RE) compartments were separated by
an ion-conducting ceramic frit. A gold mesh was used as CE, while an
Hg/HgSO4 electrode was used as reference, calibrated to the
reversible hydrogen electrode (RHE) in the same electrolyte. CO
reduction was carried out potentiostatically at the desired potential.
85% of the uncompensated ohmic drop was corrected for in the
software, as measured through electrochemical impedance spectros-
copy.[25] If significant, the remaining 15% was corrected for after the
experiment. The ohmic drop typically ranged from 20 to 50 W,
depending on the WE position relative to the RE.
During measurements, CO reduction products accumulate in the
electrolyte and headspace of the WE compartment. The gaseous
product composition was analyzed by a GC mounted with a flame
ionization detector (FID) and a thermal conductivity detector (TCD).
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Liquid products were analyzed through HS-GC and NMR spectros-
copy.
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We acknowledge funding from Danish Council for Strategic
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acknowledge the assistance of Casper Hoeck and Charlotte
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