Electrochemical CO Reduction Builds Solvent Water into Oxygenate Products

Article abstract of DOI:10.1021/jacs.8b03986

Numerous studies have examined the electrochemical reduction of CO (COR) to oxygenates (e.g., ethanol). None have considered the possibility that oxygen in the product might arise from water rather than from CO. To test this assumption, C¹⁶O reduction was performed in H₂¹⁸O electrolyte. Surprisingly, we found that 60-70% of the ethanol contained ¹⁸O, which must have originated from the solvent. We extended our previous all-solvent density functional theory metadynamics calculations to consider the possibility of incorporating water, and indeed, we found a new mechanism involving a Grotthuss chain of six water molecules in a concerted reaction with the?C-CH intermediate to form?CH-CH(¹⁸OH), subsequently leading to (¹⁸O)ethanol. This competes with the formation of ethylene that also arises from?C-CH. These unforeseen results suggest that all previous studies of COR under aqueous conditions must be reexamined.

Full text of DOI:10.1021/jacs.8b03986

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Communication
Electrochemical CO reduction builds solvent water into oxygenate products
Yanwei Lum, Tao Cheng, William A. Goddard, and Joel W. Ager
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03986 • Publication Date (Web): 16 Jul 2018
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Electrochemical CO reduction builds solvent water into oxygenate
products
Yanwei Lum†,‡,⊥, Tao Cheng§,∥,⊥, William A. Goddard III*,§,∥ and Joel W. Ager*,†,‡
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†Joint Center for Artificial Photosynthesis and Materials Sciences Division, Lawrence Berkeley National Laboratory, Cali-
fornia 94720, United States.
‡Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States.
§Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, California 91125, United States.
∥Materials and Process Simulation Center (MC139-74), California Institute of Technology, Pasadena, California 91125,
United States.
Supporting Information Placeholder
vent water on the Cu surface plays an essential role in the mecha-
nisms by providing hydrogen to the intermediates and products.
This role of surface water, which involves transferring hydrogen
to these intermediates, often through a Grotthuss mechanism in-
volving other solvent waters, was a new result. In our previous
QM calculations18 for CO reduction on Cu, we found that the
pathway for ethanol formation proceeds through *(OH)C-CH; an
intermediate after 4 e- transfers, which then subsequently either
reduces to *C-CH (leading to ethylene formation with a free ener-
gy barrier of 0.61 eV) or to *H(OH)C-CH (leading to ethanol with
a free energy barrier of 0.67 eV). However, we did not consider
the possibility that solvent water could provide the O in the prod-
ucts, and we assumed that all the O atoms in the oxygenate
(CnHmOx) products come from the original CO molecule being
reduced. In fact, this is a common feature of all current proposed
mechanisms, with recent work from Head-Gordon and co-workers
predicting that none of the oxygenate products should possess
oxygen from the solvent water.23
ABSTRACT: Numerous studies have examined the electro-
chemical reduction of CO (COR) to oxygenates (e.g. ethanol).
None considered the possibility that oxygen in the product might
arise from water rather than from CO. To test this assumption,
C16O reduction was performed in H218O electrolyte. Surprisingly,
we find that 60-70% of the ethanol has 18O, which must origi-
nate from the solvent. We extended our previous all solvent den-
sity functional theory metadynamics calculations to consider the
possibility of incorporating water, and indeed we find a new
mechanism involving a Grotthuss chain of six H2Os in a concert-
ed reaction with the *C-CH intermediate to form *CH-CH(18OH),
subsequently leading to 18O ethanol. This competes with the for-
mation of ethylene that also arises from *C-CH. These unforeseen
results suggest that all previous studies of COR under aqueous
conditions must be reexamined.
Electrochemical CO2 reduction (CO2R) has emerged as a prom-
ising technology to utilize increasingly cheaper renewable elec-
tricity to convert CO2 into useful chemicals and fuels.1–6 In this
context, Cu-based catalysts are currently the most promising for
driving CO2R to produce significant amounts of multicarbon oxy-
genates and hydrocarbons such as ethanol and ethylene.7–10 Ena-
bling the deterministic design of more selective and efficient cata-
lysts requires understanding of the reaction mechanisms to predict
how changes in the catalysts and electrolyte can modify the kinet-
ics and products. Indeed, a number of theoretical papers have
been published explaining how the experimentally observed
changes in products depend on pH, applied potential, and pres-
ence of counter ions.11–14 It is generally accepted that on various
Cu surfaces, CO2 reduces first to CO.15,16 At low pH, CO can
further reduce to *HCO or *OCH and then to *CH2OH, leading to
methane or methanol formation.14,17 At pH> 7, CO can undergo
C-C coupling to generate a *CO-CO dimer,14,17–21 which then
forms *OC-COH.22 Subsequent steps leading to ethylene and
ethanol have been further studied in quantum mechanics (QM)
based theory papers.17–19,23–25
We tested this critical assumption experimentally by carrying
out reduction of C16O in H218O electrolyte on various Cu surfaces
and quantifying the isotopic composition of the products using gas
chromatography-mass spectrometry (see SI for more details). An
important reason that CO reduction and not CO2 reduction was
performed is because CO2 is known to rapidly equilibrate with
water to form bicarbonate.26 Therefore, dissolved CO2 would
likely incorporate 18O from the solvent, resulting in 18O in the
products. In contrast, CO does not exchange O with water (see SI
for more details). The reduction of C16O in 0.05 M K2CO3 (pH
11.3) electrolyte was carried out with different Cu orientations:
Cu (111), Cu (100) and Cu (751) at a potential of -0.64 V vs RHE.
Analysis of the isotopic composition of the products (Figure 1a)
reveals that the majority of the ethanol, acetate and 1-propanol
are 18O enriched. In order to ensure that incorporation of 18O into
the products were not solely due to homogenous reactions occur-
ring in the bulk of the electrolyte (e.g. Cannizzaro reactions27), a
series of control experiments were conducted (see SI for more
details). Control experiments were also performed to ensure that
the mass spectrometer has similar detection sensitivities for 16O vs
18O fragments (see SI for more details).
Recently, we published the first complete determination of the
atomistic reaction mechanism for reduction of CO on Cu (100)
using QM based metadynamics in full solvent to determine the
free-energy barriers and kinetics at 298 K.18 We showed that sol-
For all three Cu surfaces, the fraction of ethanol with 18O is
around 66% and that for 1-propanol is around 72%. Acetate pos-
sesses 2 oxygen atoms and therefore may have 3 different config-
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urations: 16O16O, 16O18O and 18O18O acetate. 18O18O acetate was
ingly, allyl alcohol and methanol were not enriched with 18O,
which suggests that the mechanisms for their formation may be
very different. These findings are summarized in a chart in Figure
1b, which sorts the products into those with 18O and those without
18O.
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not observed on any of the Cu surfaces and the distribution of
16O16O versus 16O18O depends on the Cu orientation, with Cu
(100) producing the highest fraction of 16O18O (>90%). Addition-
ally, allyl alcohol (prop-2-en-1-ol) was detected as a product on
all surfaces and methanol only on the Cu (111) surface. Interest-
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Figure 1. Fraction of product with 18O for ethanol (red), acetate (yellow) and 1-propanol (green) for C16O reduction in H218O electrolyte
with: (a) different Cu orientations tested at c.a. -0.65V vs RHE, (c) Cu (100) at different pH and (d) Cu (100) at different applied poten-
tials. (b): A chart showing products with an 18O pathway (ethanol, acetate and 1-propanol) and products without an 18O pathway (methanol
and allyl alcohol). Note: 18O18O acetate was never observed as a product. Faradaic efficiency data are available in the SI.
Next, the effect of pH and potential was investigated for the Cu
(100) surface. A potential of around -0.53V vs RHE was applied
at different pH; 11.3 (0.05 M K2CO3), 13 (0.1 M KOH) and 14
(1.0 M KOH). Figure 1c shows that changing the pH has no ef-
fect on the 18O composition of ethanol, which remains at around
64%. On the other hand, the 18O composition of acetate and 1-
H218O (solvent) could contribute 18O to the product. The experi-
mental results clearly demonstrate the existence of two ethanol
formation pathways (16O pathway and 18O pathway). Therefore,
the H218O must react with a C2 intermediate which has lost both
its oxygen atoms: either *C-CH, *C-CH2 or *HC-CH2. We con-
sidered that the most plausible C2 intermediate to react with 18O
water is adsorbed ethynyl (*C-CH).
propanol are significantly affected by pH. For 1-propanol, the 18
O
Thus, we explored the possibility of a two-step *CH-CH(18OH)
formation:
1. First, one surface H218O might provide a proton (H+) to form
*HC-CH plus surface 18OH (*18OH) via proton-coupled elec-
tron transfer (PCET).
2. Second, this might be followed by *18OH extracting an H+
from a solvent H218O to deliver the 18OH to form *CH-
CH(18OH) via PCET.
composition rises from 73% at pH 11.3 to 91% at pH 14. Howev-
er, for acetate, the 16O18O composition decreases from 90% at pH
11.3 to 66% at pH 14. Keeping the pH constant at 11.3 and chang-
ing the potential (Figure 1d) has no effect on the isotopic compo-
sition of the products. Additionally, changing the potential or pH
does not result in any 18O18O acetate formation.
Summarizing the experiments, by using 18O labeling of the sol-
vent we have discovered that that the majority of the ethanol,
acetate and 1-propanol produced by COR on single crystal Cu
surfaces possess 18O, showing conclusively that solvent water
plays a dominant role in their formation. As a result of this
unexpected finding, all previous mechanisms for the formation of
CnHmOx oxygenates require reexamination because H2O as the
dominant source of O for the formation of these products has been
overlooked.
However, the free energy barrier for the first step is 1.09 eV while
the that for the 2nd step is 1.22 eV. These barriers are much larger
than the values of 0.61 and 0.67 eV that we found previously to
produce ethene and ethanol. Thus we conclude that this mecha-
nism does not explain the large amount of 18O ethanol observed.
We then investigated a concerted pathway of water addition
reaction via Grotthuss water chain in which the water at C end
provide H+ to C (in *C-CH) while water at CH end simultaneous-
ly providing 18OH– to CH (in *C-CH) which are connected by the
Stimulated by the experimental results, we used quantum me-
chanics (QM) metadynamics in with full solvent (5 layers) to
determine the free energy barriers at 298 K to investigate how
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hydrogen bond network through bridging water molecules. We
non-electrochemical reaction from QM metadynamics snapshots
are as shown in Figure 2 (see also supplementary movie 1).”
considered several possible such chains with the best involves 6
water molecules, leading to a free energy reaction barrier for this
reaction of 0.81 eV. We also examined this reaction for chemi-
sorbed ethyne (*HC-CH) to form *CH2-CH(18OH) and we find a
slightly higher barrier of 0.84 eV. Finally, we also examined the
reaction where *C-CH (ethynyl) forms *C(18OH)H-CH, where we
find a barrier of 0.91 eV. Thus, we distinguish the formation of
*CH-CH(18OH) from *C-CH via water addition as the most pos-
sible mechanism attributes to C2H5(18OH) formation, which we
refer as Grotthuss Chain Ethynyl Concerted Hydrolysis (GECH),
a most unexpected and unprecedented reaction which has never
been reported before. The critical steps of such unprecedented
After the formation of *CH-CH(18OH) from *C-CH via wa-
ter addition, the remaining steps toward C2H5(18OH) formation
and the related energetics are as shown in Figure 3. GECH is ex-
pected to be independent of pH and applied potential. In the SI,
we report a simulation with explicit consideration of 1 M NaOH
(pH = 14) where we found the free energy barrier of 0.87 eV,
supporting this claim. The experimental results in Figures 1c and
d do not show a large dependence on either pH or potential, sup-
porting our claim the GECH is responsible for the formation of
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18O
ethanol.
Figure 2. The reactive trajectory for Grotthuss Chain Ethynyl Concerted Hydrolysis (GECH) the of *C-CH to *CH-CH(OH) from full
solvent quantum mechanics molecular metadynamics free energy calculations. All the 6 waters in the Grotthuss chain are shown in full.
The other 48 water molecules not involved in the chain are faded out for clarity. This intermediate CH-CH(OH) subsequently forms etha-
nol as shown in the orange pathway in Figure 3. We examined a number of possible pathways involving various numbers of waters, with
this being the most favorable. (a) initial reactants, (b) transition state (free energy barrier: 0.81 eV) and (c) final products (exothermic by -
0.12 eV). The colors are C in gray, O in red, the H involved in the Grotthuss chain proton transfer) are in yellow, and other H are white.
Since *(16OH)C-CH is a common intermediate for forming ei-
ther 16O ethanol or to the sum of ethylene and 18O ethanol (see
Figure 3), the predicted energy barriers at 298 K (0.67 and 0.61
eV respectively) can be used to estimate the ratio of the sum of
ethylene plus 18O ethanol product to 16O ethanol. Based on the
Arrhenius equation, this ratio was calculated to be 11 which is in
excellent agreement with our experiments, which yield a ratio of
14 and a calculated energy difference in barriers of 0.066 eV (see
Figure S28 for calculation details). Similarly, *C-CH is a common
intermediate for forming both 18O ethanol and ethylene and the
predicted activation energies at 298K (0.81 and 0.61 eV) can be
compared to the observed ratio of 0.15, which implies that the
difference in barriers is 0.049 eV. This difference in experiment
and theory suggests that we may not have exhausted on all the
pathways for the GECH mechanism.
Figure 3. The mechanistic pathways for CO reduction predicted
from full solvent quantum mechanics based molecular metady-
namics to obtain free energy reaction barriers at 298K. The path-
ways of ethylene formation (black) and 16O ethanol (blue) are
from ref 18. The 18O ethanol formation pathway (orange) is a
newly discovered mechanism (GECH) reported here.
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In summary, our QM based metadynamics show that 18O etha-
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of Electrochemistry; Springer New York: New York, NY, 2008;
pp 89–189.
nol results from a solvent based concerted hydrolysis of *C-CH
(chemisorbed ethyne) to *CH-CH(OH), in which the added H and
OH are derived from waters at the opposite ends of a 6 molecule
Grotthuss chain (GECH). This is a brand-new mechanism, which
is independent of pH and applied potential and may provide new
approaches to designing nanoscale structures and compositions in
which the energy and orientation of the chemisorbed ethynyl in-
termediates are used to promote the solvent water induced ethanol
or other CnHmOx oxygenate products.
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In this work, the main focus was to understand the formation of
18O ethanol was because it is the most abundant 18O oxygenate
produced. Subsequent work will examine the C3 product pathways
for 1-propanol and allyl alcohol formation as well as the acetate
pathways, which are evidently more complicated. Since we now
know that incorporation of 18O is critical in the formation of oxy-
genates, it is paramount to use this technique to investigate other
catalyst systems used for COR such as bimetallic systems and
oxide-derived Cu.28–32 For example, oxide-derived Cu catalysts
have been shown to yield a high selectivity towards oxygenates
versus hydrocarbons.31 It is expected that such experiments will
lead to new insights on how oxygenate formation mechanisms
might be different on these catalysts. Finally, our discovery of
concerted solvent water incorporation of O into oxygenates may
have implications for many other oxygen insertion processes.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website. Synthesis, characterization, experi-
mental methods and control experiments.
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AUTHOR INFORMATION
Corresponding Author
* wag@wag.caltech.edu
* jwager@lbl.gov
Author Contributions
⊥These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This material is based upon work performed by the Joint Center
for Artificial Photosynthesis, a DOE Energy Innovation Hub,
supported through the Office of Science of the U.S. Department
of Energy under Award Number DE-SC0004993. Y.L. acknowl-
edges the support of an A*STAR National Science Scholarship.
The quantum mechanics (QM) calculations used the resources of
the Extreme Science and Engineering Discovery Environment
(XSEDE) which is supported by National Science Foundation
grant number ACI-1053575. We thank Lingfei Wei for technical
illustrations.
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Li, C. W.; Ciston, J.; Kanan, M. W. Nature 2014, 508, 504–507.
Roberts, F. S.; Kuhl, K. P.; Nilsson, A. Angew. Chemie 2015,
127, 5268–5271.
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TOC graphic, correctly sized
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