Structure- and Potential-Dependent Cation Effects on CO Reduction at Copper Single-Crystal Electrodes

Article abstract of DOI:10.1021/jacs.7b10142

The complexity of the electrocatalytic reduction of CO to CH₄ and C₂H₄ on copper electrodes prevents a straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst's selectivity over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111), and Cu(polycrystalline) electrodes in 0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and -1.5 V vs RHE. We used online electrochemical mass spectrometry and high-performance liquid chromatography to determine the product distribution as a function of electrode structure, cation size, and applied potential. First, cation effects are potential dependent, as larger cations increase the selectivity of all electrodes toward ethylene at E > -0.45 V vs RHE, but methane is favored at more negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C₂H₄ formation depends on the electrode structure and cation size, whereas that for CH₄ does not. Fourier Transform infrared spectroscopy (FTIR) and density functional theory help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate-determining step to methane and ethylene formation is CO hydrogenation, which is considerably easier in the presence of alkaline cations for a CO dimer compared to a CO monomer. For Li⁺ and Na⁺, the stabilization is such that hydrogenated dimers are observable with FTIR at low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity toward specific products.

Full text of DOI:10.1021/jacs.7b10142

Subscriber access provided by GRIFFITH UNIVERSITY
Article
Structure and potential dependent cation effects on
CO reduction at copper single-crystal electrodes
Elena Perez-Gallent, Giulia Marcandalli, Marta Costa Figueiredo, Federico Calle-Vallejo, and Marc T.M. Koper
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10142 • Publication Date (Web): 24 Oct 2017
Downloaded from http://pubs.acs.org on October 24, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Structure and potential dependent cation effects on CO reduction at
copper single-crystal electrodes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Elena Pérez-Gallent, Giulia Marcandalli, Marta Costa Figueiredo, Federico Calle-Vallejo* and Marc
T. M. Koper*
Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.
CO reduction • Electrocatalysis • Structural sensitivity • Alkaline cations • OLEMS • HPLC • FTIR • DFT
ABSTRACT: The complexity of the electrocatalytic reduction of CO to CH4 and C2H4 on copper electrodes prevents a
straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and
electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst’s selectivity
over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111) and Cu(polycrystalline) electrodes in
0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and -1.5 V vs RHE. We used Online
Electrochemical Mass Spectrometry (OLEMS) and High-Performance Liquid Chromatography (HPLC) to determine the product
distribution as a function of electrode structure, cation size and applied potential. First, cation effects are potential dependent, as
larger cations increase the selectivity of all electrodes towards ethylene at E > -0.45 V vs RHE, but methane is favored at more
negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C2H4 formation depends on the electrode
structure and cation size, whereas that for CH4 does not. Fourier Transform Infrared Spectroscopy (FTIR) and Density Functional
Theory (DFT) help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate determining
step to methane and ethylene formation is CO hydrogenation, which is considerably easier in presence of alkaline cations for a CO
dimer compared to a CO monomer. For Li+ and Na+, the stabilization is such that hydrogenated dimers are observable with FTIR at
low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity towards specific products.
1. Introduction
Electrocatalytic reduction of CO2 is an attractive
strategy for the conversion of renewable energy into fuels,
which helps in closing the biogeochemical carbon cycle.
most efficient for the conversion of CO2 to C2H4, Cu(111)
favors the formation of CH4 and HCOOH, and Cu(110) gives
the highest current efficiencies for secondary C2 products
(e.g. acetic acid, acetaldehyde and ethanol). Similar results
were obtained by Schouten et al. during the reduction of
CO8. While Hori et al concluded that the introduction of
steps on (100) terraces enhances C2H4 evolution and
suppresses CH4 formation7, Schouten et al. attributed the
selective formation of C2H4 to pristine (100) terraces8.
Moreover, they showed that CO reduction to C2H4 takes
place preferentially at Cu(100) electrodes without
simultaneous CH4 evolution, which indicates that the
reaction paths towards CH4 and C2H4 bifurcate in the early
stages of CO reduction9. On the other hand, it has been
shown that C2 species such as ethylene and ethanol are
formed in a common pathway that bifurcates at the late
stages of the reaction10-12. The favorable formation of C2H4
on Cu(100) is supported by density functional theory
(DFT) calculations, which demonstrate that C-C bond
formation proceeds via CO dimerization and has a lower
activation barrier on Cu(100) than on Cu(111).13-14 In
addition to structural effects, there is an important role of
the electrolyte, especially through the pH. The onset
potential on the NHE scale of CH4 evolution depends on pH,
while C2H4 evolution does not. Hori et al concluded that CO
Several metals and other types of electrodes have been
1
studied for the electrochemical reduction of CO2
.
However, copper remains the only metal to produce
hydrocarbons (primarily methane and ethylene) with
reasonable faradaic efficiencies2-3. For instance, Jaramillo
et al reported the formation of 16 different species from
CO2 reduction in aqueous bicarbonate solution, where
complex molecules including C2 (e.g. acetaldehyde, acetate,
ethylene glycol, glycolaldehyde) and C3 species (e.g. n-
propanol, propionaldehyde, allyl alcohol, acetone) were
detected with current efficiencies lower than 5 % 4. Several
studies of Cu-based catalysts5-6 have shown higher
selectivity for C2 products. However, the mechanistic
reasons for their selectivity remain elusive. Here we
examine the combined role of electrolyte cations, potential
window and catalyst structure on the selectivity towards
C1 vs C2 products during CO reduction on Cu.
Hori et al showed that CO2 reduction on Cu
electrodes is structure sensitive7: Cu(100) electrodes are
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 11
reduction to CH4 proceeds as a series of concerted proton- terms of a variation in the potential in the outer Helmholtz
1
2
3
4
5
6
7
8
electron transfers, in contrast with C2H4 evolution for
which the rate limiting step only involves an early electron
plane (OHP), which originates from a difference in the
hydration number of the different cations. Larger cations
are less hydrated and expected to adsorb more easily on
the cathode surface, shifting the potential to more positive
values thereby steering the selectivity towards C2H4
instead of CH4. Such experimental observations were
confirmed by Kyriacou et al.20. Bell and coworkers
explained cation effects on CO2 reduction in terms of the
different pKa values for cation hydrolysis, which lower the
local pH at the surface from Li+ to Cs+ and lead to an
increase in CO2 concentration near the electrode surface.21
However, this model cannot explain the fact that similar
cation effects are observed during CO reduction10, the
concentration of which is not affected by pH.
transfer, justifying its pH-independent onset15-16
. We
showed previously with DFT calculations that the electron
transfer to form a negatively charged (CO)2 dimer is the
potential-limiting step of CO reduction to C2H4 on
Cu(100),11 in contrast with studies that assume only
concerted proton-electron transfers17, which cannot
explain the pH independence of C2H4 formation.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In addition to pH, the activity and selectivity of Cu
for CO2 reduction also depends on the nature of the anions
and/or cations in the electrolyte. Strasser et al showed that
the selectivity of the major products of CO2 reduction
depends on the size and concentration of halides18: while
Cl- and Br- enhance the production of CO, I- lowers CO
evolution and increases the selectivity towards methane.
The effects were attributed to halide adsorption on copper,
which alters the negative charge on the surface and favors
the protonation of CO towards CH4. Furthermore, Lee et al
showed that the presence of Cl- enhances the catalytic
activity toward multiple C2-C4 species on Cu-oxide derived
catalysts, due to the presumed advantageous affinity
between reaction intermediates and catalytic surface in
presence of Cl-19. Hori et al reported that alkaline cations
affect the selectivity of CO2 and CO reduction on
polycrystalline copper10, so that larger cations favor the
formation of C2 and C3 species such as C2H4, C2H5OH and
C3H7OH. Cation effects were explained by Hori et al in
All previous studies concerning cation effects in
the reduction of CO2 and CO on copper have used only
polycrystalline electrodes and did not cast light on their
potential dependence. In the following, we will argue that
such effects depend on the electrode structure, the applied
potential and the size of the cation. To this end, we used
two single-crystalline copper surfaces (Cu(100), Cu(111))
together with polycrystalline Cu in LiOH, NaOH, KOH,
RbOH and CsOH solutions. Online Electrochemical Mass
Spectrometry (OLEMS) and High-Performance Liquid
Chromatography (HPLC) were used to investigate the
product distribution over a wide potential range. In
addition, in situ Fourier Transform Infrared (FTIR)
spectroscopy and Density Functional Theory (DFT)
Cu (111)
Cu (100)
Cu (poly)
8x10-10
6
8x10-10
6
8x10-10
6
d)
a)
g)
4
2
4
2
0
4
2
0
C2H4
C2H4
m/z=26
C2H4
m/z=26
m/z=26
e)
b)
h)
2.0x10-9
1.5
2.0x10-9
1.5
2.0x10-9
1.5
1.0
0.5
0.0
1.0
0.5
0.0
CH4
1.0
0.5
0.0
CH4
CH4
m/z=15
m/z=15
m/z=15
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
50
40
30
20
10
0
c)
50
40
30
20
10
0
50
40
30
20
10
0
i)
f)
LiOH
NaOH
KOH
RbOH
CsOH
LiOH
NaOH
KOH
RbOH
CsOH
LiOH
NaOH
KOH
RbOH
CsOH
C2H4 / CH4
(m/z=26) / (m/z=15)
C2H4 / CH4
C2H4 / CH4
(m/z=26) / (m/z=15)
(m/z=26) / (m/z=15)
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
E vs. RHE / V
E vs. RHE / V
E vs. RHE / V
Figure 1. OLEMS mass fragments measured during CO reduction associated with the formation C2H4 (top panel, a, d and g) and
CH4 (middle panel, b, e, and h) on a) Cu(100) , b) Cu(111) and c) polycrystalline Cu for different 0.1 M alkaline hydroxide
solutions. Bottom panel (c, f and i) shows the potential-dependent ratio (m/z = 26) / (m/z = 15) of OLEMS mass fragments
associated with the formation of C2H4 and CH4 during CO reduction.
ACS Paragon Plus Environment

Page 3 of 11
Journal of the American Chemical Society
calculations are used to identify early reaction both an onset potential of approximately -0.8 V. Similar
1
2
3
4
5
6
7
8
intermediates of CO reduction on Cu(100) and explain the
cation-mediated enhancement of ethylene formation over
methane. Understanding how the structural and potential-
dependent cation effects impact the catalytic performance
provides insight for devising efficient and selective
catalysts for CO reduction.
results were obtained by Nilsson et al,5 who found onset
potentials for methane and ethylene on Cu(111) close to -
0.9 V. However, in this work ethylene formation on
Cu(111) starts at lower overpotentials (-0.4 to -0.5 V),
which we attribute to the higher sensitivity achieved by
using a larger OLEMS tip in combination with the non-
meniscus configuration. Although it is not possible to rule
out the contribution of other facets present on the
electrode, the significantly higher activity and lower onset
potential of Cu(100) surface for ethylene formation is also
clearly observed in this work.
2. Results and discussion
2.1. OLEMS and HPLC
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cu(111), Cu(100) and Cu(poly) were characterized by
voltammetry before and after experiments to control the
morphology of the surface22, see Figure S8. The activity
and selectivity of the three electrodes towards CO
reduction were investigated with OLEMS by varying the
On polycrystalline Cu (Fig. 1g-i) the onset
potential for CH4 production is around -0.65 V regardless
of the cation in solution. The plateau is less pronounced
than on the single crystals, and the trend with the size of
the cation is less evident. C2H4 formation is strongly
dependent on the cation, with the smallest cation (Li+)
showing essentially no ethylene formation and the largest
cation Cs+ showing the strongest selectivity towards
ethylene. Though we remark again that it is currently
impossible to perform fully quantitative selectivity
measurements using OLEMS, the results in Figure 1 allow
us to conclude that (i) larger cations enhance CO reduction
to ethylene at low overpotentials, and that the effect is
significantly stronger on Cu(100), (ii) larger cations
enhance methane production at high overpotentials.
alkaline cation in the 0.1
M hydroxide supporting
electrolyte. A linear sweep voltammetry between 0 and -
1.5 V vs RHE at a scan rate of 1 mVs-1 was carried out while
simultaneously the volatile products were detected with
OLEMS. Figure 1 displays the volatile products formed
during the reduction of CO on Cu(100), Cu(111) and
polycrystalline Cu for different alkaline hydroxide
solutions of identical concentration (0.1 M). Figure 1a-1c
shows the results of CO reduction for Cu(100). The middle
panel (b) shows the mass fragment m/z = 15 associated
with CH4, and the top panel (a) shows the mass fragment
m/z = 26 associated with C2H4. It is important to note that
the reported amounts of products formed are in fact lower
than the amounts expected if a purification process of the
electrolyte would have been performed, according to the
results obtained by Surendranath23. The onset potential for
CH4 at ca. -0.65 V is independent of the cation in solution.
For all cations, except Cs+, the formation of methane
reaches a plateau around -0.9 V vs RHE. The general trend
is that larger cations increase methane production. Figure
1b also shows that the formation of C2H4 on Cu(100) starts
at ca. -0.3 V regardless of the cation. The amount of
ethylene formed, as well as its formation rate, increase
with the size of the cation. Especially Cs+ shows a
significant increase in ethylene production.
To support these conclusions, we consider in
Figure
1 bottom panels (c, f and i) the potential
dependence of the ratio of the mass signals corresponding
to ethylene (m/z = 26) and methane (m/z = 15) for the
three different Cu electrodes. The ratio was plotted as
I(m/z = 26)/I(m/z = 15), so a high value expresses a larger
production of ethylene over methane. Since at potentials
more positive than -0.65 V there is no methane production
and the value of the ratio C2/C1 is infinite, the ratio was
only calculated in the potential region where both
products coexist or the amount of C2H4 detected is null. At
low overpotentials, all copper surfaces show a higher
selectivity towards ethylene with increasing cation radius,
with the highest selectivity achieved for Cu(100),
especially with Rb+ and Cs+ in solution. In particular, for a
fixed potential of -0.75 V, the ethylene/methane ratio for
Cu(100) are 1.59 for Li+, 3.93 for Na+, 4.32 for K+, 7.54 for
Rb+ and 14.8 for Cs+ (see Figure S2a). These values show a
clear cation effect towards ethylene formation that
monotonically follows the cation sizes. In addition, the
selectivity for ethylene is enhanced in a larger potential
range when larger cations are in solution. Figure S2b
shows that larger cations maintain the same selectivity for
ethylene at more negative potentials compared to smaller
cations: the potentials for which a fixed value of 5 for the
On Cu(111) (Fig 1d-f), the onset potential for CH4
is ca. -0.65 V regardless of the cation, with trends in
activity similar to Cu(100). (Rb+ deviates from this trend
but, as mentioned in Section S7 in the SI, RbOH frequently
showed problems with purity and reproducibility). The
formation of C2H4 starts around -0.4 V regardless of the
cation, which is 0.1 V more negative than on Cu(100). The
amount of ethylene formed, as well as its formation rate, is
highest with Cs+ in the electrolyte. It is important to point
out the differences observed with the previous work by
Schouten et al,9 where the formation of ethylene and
methane on Cu(111) displayed a similar profile having
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 11
ethylene/methane ratio are -0.70, -0.72, -0.72, -0.74 and - The minor liquid products obtained during CO
1
2
3
4
5
6
7
8
0.79 V for Li+, Na+, K+, Rb+, and Cs+. Similar tendencies are
observed for Cu(111) and polycrystalline Cu but with
significantly lower ratios. The ratio C2/C1 on Cu(111) and
polycrystalline Cu in LiOH solution is almost zero over the
whole potential range in which both species coexist,
indicating low selectivity for ethylene formation over
methane in this electrolyte.
reduction on polycrystalline copper and their dependence
on the nature of the cation were collected and analyzed
with HPLC. Due to the low amount of products formed,
chronoamperometry experiments for 2 h were carried out
using a large copper disk (16.85 mm diameter). Given the
long duration of this set of experiments, we did not
perform these experiments with single-crystal electrodes,
since the stability of the surface structure may be
compromised. Chronoamperometry experiments were
carried out at three different potentials: -0.5, -0.7 and -0.9
9
Figure S1 show the mass fragment m/z = 2
associated with the formation of H2 from the competitive
hydrogen evolution reaction (HER). For all copper
surfaces, hydrogen evolution starts at ca. -0.4 V for all
different cations except Cs+, for which it starts at slightly
less negative potentials. The amount of hydrogen produced
as well as its formation rate increases with the size of the
cation in the electrolyte for all copper electrodes.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
V
vs RHE, with different alkaline hydroxides. The
concentrations of the obtained products and their cation
dependence are summarized in Figure S3. The only liquid
product detected for CO reduction at -0.5 V was formic
acid. At more negative potentials (-0.7 v and -0.9 V) the
products obtained were: formic acid as C1 product; acetic
acid, glycolic acid, ethylene glycol and ethanol as C2
Cu (100) , 0.1 M NaOH , CO atmosphere
Cu (100) , 0.1 M KOH , CO atmosphere
c)
Cu (100) , 0.1 M LiOH , CO atmosphere b)
a)
1708 cm-1
1191 cm-1
1627 cm-1
1191 cm-1
0.0005 a.u.
0.0005 a.u.
0.0005 a.u.
1630 cm-1
1677 cm-1
1701 cm-1
-0.2 V
-0.15 V
-0.1 V
1191 cm-1
-0.2 V
1600 cm-1
-0.15 V
-0.1 V
-0.15 V
-0.05 V
0 V
-0.1 V
-0.05 V
0 V
-0.05 V
0 V
+0.05 V
+0.1 V
+0.05 V
+0.1 V
+0.05 V
+0.1 V
2000 1800 1600 1400 1200
wavenumber / cm-1
2000 1800 1600 1400 1200
wavenumber / cm-1
2000 1800 1600 1400 1200
wavenumber / cm-1
Cu (111) , 0.1 M LiOH , CO atmosphere
d) Cu (100) , 0.1 M RbOH , CO atmosphere
1635 cm-1
0.0005a.u.
Cu (100) , 0.1 M CsOH , CO atmosphere
e)
f)
1615 cm-1
0.0005 a.u.
0.0005 a.u.
1407 cm-1
1407 cm-1
1728 cm-1
-0.2 V
-0.2 V
-0.2 V
-0.15 V
-0.1 V
-0.15 V
-0.15 V
-0.1 V
-0.1 V
-0.05 V
0 V
-0.05 V
0 V
-0.05 V
0 V
+0.05 V
+0.1 V
+0.05 V
+0.1 V
+0.05 V
+0.1 V
2000 1800 1600 1400 1200
wavenumber / cm-1
2000 1800 1600 1400 1200
wavenumber / cm-1
2000 1800 1600 1400 1200
wavenumber / cm-1
Figure 2. Potential-dependent absorbance spectra of Cu(100) in CO atmosphere in 0.1 M solution of a) LiOH, b) NaOH, c) KOH, d)
RbOH, e) CsOH. f) LiOH on Cu(111), provided for comparison. Reference spectrum recorded at +0.1 V vs. RHE.
ACS Paragon Plus Environment

Page 5 of 11
Journal of the American Chemical Society
products; and propionaldehyde, 1-propanol and allyl starts, as this destabilizes the thin layer formed between
1
2
3
4
5
6
7
8
alcohol as C3 products. Such C2 and C3 products obtained
during CO reduction have been reported previously4, 24. In
general, larger cations such as Cs+ promote CO reduction to
C2+ compounds compared to small cations (Li+ and Na+), in
agreement with the results of Hori et al 10. A detailed
description of the concentration of the products depending
on the cation in solution can be found in the SI, section S2.
the electrode and the prism of the spectrometer resulting
in unstable spectra. These limitations are further explained
in section S7 of the Supporting Information. CO reduction
in different alkaline hydroxide solutions was also studied
on Cu(111) (a representative spectrum is shown in Fig. 2f).
The FTIR spectra for CO interacting with Cu(100) exhibit
two common bands for all alkaline electrolytes. The first
band, in the range of 1635-1600 cm-1, corresponds to the
O-H bending mode of H2O. This band causes fluctuations in
the baseline of the spectra making it difficult to identify
other bands in this wavenumber range. The second band,
in the range of 1730-1670 cm-1, corresponds to the C-O
stretching27-29 of CO adsorbed on hollow sites on Cu(100)
9
Further analysis by 1H-NMR was carried out for
the samples obtained after 2 h of chronoamperometry at -
0.9 V with Na-, K- and Cs-containing electrolytes (Figure S4
in the SI). The 1H-NMR spectra confirmed the products
detected with HPLC and their higher concentration with K+
and Cs+ compared to Na+. In addition, methanol was also
detected as a reduction product of CO for those three
cations. Identification of methanol was not possible with
HPLC due to an overlap with the intense peaks of the
eluent.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
30-33
. Apart from these two bands, two other bands arise
depending on the electrolyte used. For Li, Na and K
hydroxides, a band at 1191 cm-1 attributed to the C-O
stretching of a hydrogenated dimer (OCCOH)26 is observed.
Note that the assignment of this band to the hydrogenated
dimer and the exclusion of other species were discussed
extensively in our previous paper.26 Besides, in section S8
in the SI we also provide arguments to discard the
presence of oxalates. In contrast, the band at 1191 cm-1 is
not observed when Rb+ and Cs+ are in solution, a feature
that will be explained in the next section with DFT
calculations. The intensity of the band assigned to C-O
stretching of the hydrogenated dimer diminishes from Li
to K (Figs. 2a-c). For Rb and Cs (Figs. 2d-e), a band at 1407
cm-1 is observed, which according to the transmission
spectra obtained for various species in solution might
correspond to formaldehyde. Formaldehyde has been
suggested to be an intermediate of the reduction of CO to
CH4 on Cu(211)34-35. The spectra obtained during the
reduction of CO on Cu(111) (Fig. 2f) also shows the band at
1407 cm-1. However, on Cu(111) this band is more intense
for smaller cations than for larger cations.
Note that we were unable to consistently detect
aldehydes as products, while acetaldehyde and
propionaldehyde have been reported to be products of CO2
reduction on copper4. This is probably due to the fact that
our experiments were carried out at pH 13, and it is known
that aldehydes are unstable at such high pH and
disproportionate following a Cannizzaro reaction, giving
the corresponding carboxylic acid and alcohol25.
2.2. FTIR
FTIR spectra were recorded during the early
stages of CO reduction on Cu(100) and Cu(111) in different
alkaline hydroxide solutions, to gain insight into the
dependence of the reaction mechanism on to the surface
structure of the electrode as well as the cation in the
electrolyte. Recent FTIR experiments carried out in Li-
containing electrolytes in our group indicated a structure-
sensitive process in the early stages of CO reduction26,
which together with DFT calculations led us to hypothesize
the formation of a hydrogenated CO dimer intermediate in
the pathway leading to ethylene, in agreement with
previous experimental8 and theoretical13 studies. Figure
2a-f shows the potential-dependent absorbance spectra of
Cu(100) and Cu(111) under CO atmosphere for different
0.1 M alkaline hydroxide solutions. The spectra recorded
on Cu(100) in LiOH solution were previously reported by
our group26.
2.3. DFT calculations
To rationalize some of the observed cation effects,
we resort now to DFT calculations. We will focus on
Cu(100), as ethylene is formed at low overpotentials and
the FTIR results in Figure 2 suggest the presence of a
hydrogenated dimer intermediate. Since CO hydrogenation
is critical for both methane17 and ethylene production,11, 36
,
in the following we will focus only on this step. Figure 3
shows the energetics of the first electrochemical steps in
the reduction of one CO molecule to C1 species and two CO
molecules to C2 species in vacuum and in presence of Li, Na
and Cs (the energies shown are the averages of the
separate values found for the three cations, for details see
Figures S11 and S12). The first proton-electron transfer for
a single CO molecule proceeds as:
The reference spectrum was taken at +0.1 V vs
RHE and additional spectra are provided for +0.05, 0.00, -
0.05, -0.10, -0.15 and -0.2 V vs. RHE. Due to experimental
limitations, it was not possible to record spectra at more
negative potentials where the hydrogen evolution reaction
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 11
*+ CO(g) + H+ + e− →*CHO
(0)
1
2
This step consists of CO adsorption and
3
4
hydrogenation. On the other hand, the first proton-electron
transfer for two CO molecules proceeds as:
5
6
*+ 2CO(g) + H+ + e− →*OCCOH
(0)
7
8
9
This step comprises successive CO adsorption
(denoted *CO and 2*CO in Figure 3), dimerization (*OCCO),
and
hydrogenation
(*OCCOH).
The
adsorption
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
configurations of C1 and C2 species are provided in Figures
S9 and S10.
From Figure 3 it is clear that all intermediates,
namely *CO, 2*CO, *OCCO, *CHO and *OCCOH, are
significantly stabilized by the presence of the alkaline
cations, but the strength of the effect depends both on the
particular adsorbate. Essentially, the overall cation effect is
due to the larger stabilization of adsorbates containing C-C
bonds (*OCCO and *OCCOH) with respect to the C1
adsorbates (*CO, 2*CO, *CHO). Importantly, the presence of
the cations changes the binding mode of the CO dimer, as
shown in Figure S9, and the adsorption sites of the
adsorbates (Figures S9 and S10).
Figure 3. Energetics of the first electrochemical steps of CO
reduction for the C1 and C2 pathways on Cu(100) at 0 V vs
RHE. a) In vacuum, and b) with cations, averaging the
energies for Li, Na and Cs in Figures S11 and S12. While in
vacuum both pathways are highly endothermic, the C2
pathway is remarkably promoted by alkaline cations.
Note that both steps described by Equations (1)
and (2) are highly endothermic in the absence of cations,
so that the reaction energies for *CO hydrogenation are
0.73 and 0.87 eV for the C1 and C2 pathways (see Tables S2
and S3). In presence of the cations, the energetics of *CO
hydrogenation are lowered from 0.73 to 0.54 eV. This
change is dwarfed by the dramatic lowering in the C2
pathway from 0.87 to 0.18 eV. This attests to a substantial
enhancing effect of the alkaline cations by decreasing the
energy barriers for *CO reductive coupling. The
enhancement of *OCCO with respect to 2*CO clearly
illustrates cation effects: the adsorption energy of *OCCO is
made more negative by the cations by ~1.2 eV, whereas
the adsorption energy of 2*CO is stabilized by ~0.4 eV. In
sum, Figure 3b explains well the experimentally observed
preference of Cu(100) for ethylene formation over
methane.
This shows that cation effects can be averaged to observe
overall trends, but important subtleties pertaining to each
cation can only be captured by separate analyses. In
addition to the quantitative considerations on the cation
effect provided in Figure 3, in section S6 in the SI we also
discuss some qualitative features of Li+, Na+ and Cs+ co-
adsorption with C1 and C2 species.
2.4. Mechanistic implications
The OLEMS results in Figure 1 suggest a relation
between m/z = 15 and m/z = 26 (associated with methane
and ethylene formation, respectively). Figure 4 illustrates
the relation between these two masses for the specific case
of CO reduction on Cu(100), although the trend is also
observable for Cu(111) and polycrystalline Cu (see Figure
S5 in the SI). Importantly, the mass fragment associated
with the formation of ethylene drops when the signal
associated with the formation of methane starts to rise.
Moreover, the m/z = 15 signal increases faster in the
electrolytes for which the m/z = 26 signal decreases faster,
leading to a delay in the potential where the maximum
current for methane is observed, depending on the size of
the cation. For example, while the decay in ethylene
formation in LiOH is acute and the rise of the mass
fragment associated with methane is steep, in CsOH
solutions the decay of ethylene and the formation of
There is also another manifestation of the cation
effect, related to the stability of *OCCOH. Similar to the
dimer, the hydrogenated counterpart is significantly
stabilized (1.16 eV in average) by the presence of alkaline
cations. Following the model of Nørskov et al17 in which
the onset potential is linked to the largest uphill reaction
energy in a given pathway, the potential to go from 2*CO to
*OCCOH is -0.10 V for Li+, -0.16 for Na+, and -0.28 eV for Cs+
(see Figure S12). Therefore, the hydrogenated dimer
should only be observable with FTIR at low potentials (> -
0.2 V) in presence of Li+ and Na+, but not in the case of Cs+,
in agreement with the experimental results in Figure 2.
ACS Paragon Plus Environment

Page 7 of 11
Journal of the American Chemical Society
methane are both more gradual. In this order of ideas, copper single crystals (see Figure S7), whereas on
1
2
3
4
5
6
7
8
larger cations enhance the selectivity towards C2H4 over a
wider potential range.
polycrystalline copper the onset potential varies alongside
the cation size, being -0.6 V for Li+ and Na+, -0.4 V for K+
and -0.35 V for Rb+ and Cs+. On the other hand, the onset
potential for methane formation is independent of both
cation size and surface structure. In the range from -0.3 V
to -0.65 V, larger cations enhance the formation of
ethylene, whereas at potentials more negative than -0.65 V
the formation of methane is favored. Figure 4 suggests that
this phenomenon is due to a shutting down of the C2
pathway at large overpotentials in which the C1 pathway
becomes favorable.
Cu (100)
800x10-12
2x10-9
600
400
m/z=15
m/z=26
1
200
0
Cs
0
1.0x10-9
0.5
9
300x10-12
200
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
m/z=15
m/z=26
100
Rb
0
0.0
600x10-12
150x10-12
400
200
0
100
50
m/z=15
m/z=26
K
0
150x10-12
400x10-12
100
50
0
m/z=15
m/z=26
200
Na
0
300x10-12
200
200x10-12
100
m/z=15
m/z=26
100
0
Li
0
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Figure 5. Schematics of the structure- and potential-
dependent cation effects for CO reduction towards methane
and ethylene in alkaline hydroxide electrolytes (pH=13).
Potentials vs RHE. E0 are standard equilibrium potentials.
E vs RHE / V
Figure 4. OLEMS mass fragments associated with the
reduction products formed during CO reduction on Cu
(100) electrode in different 0.1 M alkaline hydroxide
solutions. Dashed lines correspond with m/z=26 associated
with the formation of ethylene, plotted against the right axis
and full lines correspond with m/z=15 associated with the
formation of methane, plotted against the left axis. Vertical
lines highlight the potential at which the m/z=26 signal
associated with ethylene starts to decay.
Hori et al explained the differences in product
distribution on the nature of the cation based on the
potential distribution at the electrode surface in terms of
changes in the outer Helmholtz plane (OHP) potential10.
The OHP potential varies with the cations according to
their particular adsorption features. Specific adsorption of
cations supposedly shifts the OHP potential to more
positive values, the OHP potential being higher for Cs+ than
for Li+ in view of their dissimilar hydration shells. In this
model, a more negative OHP potential translates into a
higher H+ concentration, such that the pH at the electrode
will decrease as the size of the cation decreases. At higher
pH values the formation of ethylene vs methane is
enhanced.
This behavior could be understood if, for instance,
methane formation would be the result of C2H4 reduction.
However, we discarded this hypothesis because ethylene
reduction experiments did not lead to the formation of
methane (see Figure S6). It is also important to note that
ethane was not detected as a reduction product of ethylene
in these experiments.
Therefore, we believe that a cation-, potential-,
and facet-dependent picture such as the one in Figure 5 is
needed to portray the mechanistic effects of alkaline
cations on CO reduction. The figure shows a schematic
representation of the structure- and potential-dependency
of the cation effects for CO reduction towards the two main
products, methane and ethylene. The onset potential for
ethylene formation depends on the facet, being lower for
copper single crystals than for polycrystalline copper. In
addition, the onset potential for ethylene is not affected by
the cation nature when CO reduction is performed on
On the other hand, in the model proposed by Bell
et al21 for the cation enhancement of CO2 reduction, the pH
near the electrode is lower when larger cations are in
solution, leading to a higher local CO2 concentration that
results into a higher cathodic activity. They attribute the
decrease of pH when larger cations are present to a
decrease of the pKa for the cations hydrolysis. When the
pKa is sufficiently low, hydrated cations serve as buffering
agents, decreasing the pH near the electrode and thereby
ACS Paragon Plus Environment

Journal of the American Chemical Society
increasing the local concentration of CO2. Note that this
Page 8 of 11
3. Conclusions
1
2
3
4
5
6
7
8
model is in agreement with experimental data for
potentials lower than -1.1 V vs RHE. However, this model
cannot explain the fact that similar cation effects are
observed during CO reduction10, the concentration of
which is not affected by (local) pH. In their model, Bell et al
explained that the hydrolysis of hydrated cations is only
effective in mildly basic or acidic electrolytes. However,
our experimental results showed a cation enhancement for
the production of ethylene during CO reduction in strongly
alkaline media (pH=13).
In this paper, we have shown that the combined
effect of alkaline cations and catalyst morphology can steer
the selectivity of CO reduction towards ethylene or
methane, depending on the potential. Specifically, our
results suggest the following conclusions:
1) In general, larger cations enhance CO reduction to
ethylene at low overpotentials, especially on Cu(100). With
smaller cations in solution, CO reduction on Cu(111) and
Cu(polycrystalline) shows low selectivity for ethylene
formation over methane over the whole potential range in
which both species coexist. The formation of other minor
C2 and C3 products (such as acetic acid, glycolic acid,
ethanol and propanol) is also enhanced by the presence of
larger cations.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Our explanation of cation effects is based on
Figures 3, S11, and S12. We believe that cations are
essentially catalytic promoters, their presence altering
substantially the free-energy landscape of CO reduction.
They especially stabilize C2 species by means of strong O-
cation interactions, justified by the strong tendency of
those species (e.g. *OCCO and *OCCOH) to be negatively
charged, unlike isolated *CO monomers. Our perspective
on cation effects is in line with that of Janik and
coworkers37, who explicitly included cation, anion and
solvation effects in their calculations and reported similar
effects for CO2 reduction in presence of K+. The averaging
in Figure 3 is close to that of Nørskov and coworkers38-39
who have shown that cations at the double layer induce
local field effects that alter the adsorption energies.
Although overall cation effects can be averaged, we stress
that subtle yet important details such as differences in
adsorption sites, adsorption configurations (Figures S9
and S10) and onset potentials as a function of cation size
(Figures S11 and S12) are only captured when modelling
the cations explicitly.
2) A correlation between the decline of ethylene formation
and the onset potential for methane formation was
observed. Furthermore, methane formation reaches its
plateau when ethylene formation drops to zero. This
suggests that the C2 pathway is blocked by the
enhancement of the C1 pathway. This correlation depends
on the cation size, so that larger cations enhance the
selectivity towards ethylene over a wider potential range.
3) For the two major products, methane and ethylene,
differences in the onset potential were found as a function
of the cation size and the surface structure. While the onset
potential of ethylene formation depends on these two
factors, the onset potential of methane does not.
4) FTIR and DFT calculations were used to gain insight into
the origin of cation effects on Cu(100). As described
elsewhere,26 FTIR suggests the presence of a hydrogenated
dimer intermediate (OCCOH) at low overpotentials. The
formation of this intermediate depends on the size of the
cation, so that the hydrogenated dimer can be detected
with FTIR in presence of Li+, Na+ and K+, but not in
presence of Rb+ or Cs+. DFT calculations explain that the
potential necessary to form *OCCOH from *CO in presence
of Cs+ is more negative compared to Li+ or Na+. Besides, the
adsorption energies of species containing C-C bonds are
dramatically stabilized by cations with respect to C1
species, justifying the selectivity towards ethylene at low
overpotentials.
Finally, it is important to note that the present
work is devoted only to “fully metallic” Cu electrodes,
while “oxide-derived” Cu electrodes also exist and, due to
their high activity for CO reduction, are the subject of
5,
extensive research
40-42. “Fully metallic” Cu(100) and
oxide-derived Cu electrodes produce both large amounts
of C2 products, although the former is inclined towards
ethylene, while the latter favors ethanol. Our previous
works provided a plausible explanation for such dissimilar
behavior: there is a selectivity-determining intermediate in
the CO reduction pathway to C2 products, namely
*CH2CHO11. If hydrogenation leads to *CH3CHO (i.e.
adsorbed acetaldehyde), ethanol is the major final product
12. Conversely, if hydrogenation leads to *CH2CH2O, then
ethylene is favored 11. Whereas acetaldehyde is reduced to
ethanol at larger rates at steps and grain boundaries12, 43
compared to terraces, pristine (100) terraces are known to
5) The role of cations in CO reduction is that of a catalytic
promoter, changing the free energy landscape of CO
reduction and specifically stabilizing certain intermediates,
especially those with a favorable (electrostatic) interaction
with the cation. Larger cations such as Cs promote
pathways with these intermediates more effectively than
smaller cations such as Li.
favor ethylene production 8-9
.
ACS Paragon Plus Environment

Page 9 of 11
Journal of the American Chemical Society
(11) Calle-Vallejo, F.; Koper, M. Angew. Chem. Int. Ed. 2013,
52, 7282-7285.
4. Methods
1
(12) Ledezma-Yanez, I.; Gallent, E. P.; Koper, M. T.; Calle-
Vallejo, F. Catal. Today 2016, 262, 90-94.
(13) Li, H.; Li, Y.; Koper, M. T.; Calle-Vallejo, F. J. Am. Chem. Soc.
2014, 136, 15694-15701.
(14) Montoya, J. H.; Shi, C.; Chan, K.; Nørskov, J. K. J. Phys.
Chem. Lett. 2015, 6, 2032-2037.
(15) Maier, G.; Rohr, C. Liebigs Ann. 1996, 1996, 307-309.
(16) Calle-Vallejo, F.; Ignacio Martinez, J.; Rossmeisl, J. Phys.
Chem. Chem. Phys. 2011, 13, 15639-15643.
(17) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.;
Nørskov, J. K. Energy Environ. Sci. 2010, 3, 1311-1315.
(18) Varela, A. S.; Ju, W.; Reier, T.; Strasser, P. ACS Catal. 2016,
6, 2136-2144.
(19) Lee, S.; Kim, D.; Lee, J. Angew. Chem. 2015, 127, 14914-
14918.
(20) Kyriacou, G.; Anagnostopoulos, A. J. Appl. Electrochem.
1993, 23, 483-486.
(21) Singh, M. R.; Kwon, Y.; Lum, Y.; Ager III, J. W.; Bell, A. T. J.
Am. Chem. Soc. 2016, 138, 13006-13012.
(22) Schouten, K. J. P.; Gallent, E. P.; Koper, M. T. J. Electroanal.
Chem. 2013, 699, 6-9.
(23) Wuttig, A.; Surendranath, Y. ACS Catal. 2015, 5, 4479-
4484.
(24) Hori, Y.; Takahashi, R.; Yoshinami, Y.; Murata, A. J. Phys.
Chem. B 1997, 101, 7075-7081.
(25) Birdja, Y. Y.; Koper, M. T. M. J. Am. Chem. Soc. 2017, 139,
2030-2034.
(26) Pérez-Gallent, E.; Figueiredo, M. C.; Calle-Vallejo, F.;
Koper, M. Angew. Chem. 2017, 129, 3675-3678.
(27) Hori, Y.; Koga, O.; Watanabe, Y.; Matsuo, T. Electrochim.
Acta 1998, 44, 1389-1395.
(28) Ryberg, R. Surf. Sci. 1982, 114, 627-641.
(29) Gajdoš, M.; Hafner, J. Surf. Sci. 2005, 590, 117-126.
(30) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095-
5102.
(31) Arán-Ais, R. M.; Figueiredo, M. C.; Vidal-Iglesias, F. J.;
Climent, V.; Herrero, E.; Feliu, J. M. Electrochim. Acta 2011, 58,
184-192.
(32) Shaw, S. K.; Berná, A.; Feliu, J. M.; Nichols, R. J.; Jacob, T.;
Schiffrin, D. J. Phys. Chem. Chem. Phys. 2011, 13, 5242-5251.
(33) Koper, M. T.; van Santen, R. A. J. Electroanal. Chem. 1999,
476, 64-70.
(34) Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem.
Soc. 1989, 136, 1982-1984.
(35) DeWulf, D. W.; Jin, T.; Bard, A. J. J. Electrochem. Soc. 1989,
136, 1686-1691.
(36) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Am. Chem.
Soc. 1987, 109, 5022-5023
(37) Akhade, S. A.; McCrum, I. T.; Janik, M. J. J. Electrochem.
Soc. 2016, 163, F477-F484.
(38) Sandberg, R. B.; Montoya, J. H.; Chan, K.; Nørskov, J. K.
Surf. Sci. 2016, 654, 56-62.
(39) Chen, L. D.; Urushihara, M.; Chan, K.; Nørskov, J. K. ACS
Catal. 2016, 6, 7133-7139.
2
3
4
5
6
7
8
9
4.1. Experimental
Experimental methods are explained in detail in section S7
of the Supporting Information.
4.2. Computational
The DFT total energies were calculated with VASP,44
making use of the projector augmented wave (PAW)
method45 and the PBE exchange-correlation functional.46
Further details are provided in section S6 in the SI.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Supporting Information. Hydrogen evolution reaction, HPLC
measurements, 1H-NMR spectra, additional mechanistic
implications, electrode surface characterization, additional
computational details, experimental methods, exclusion of the
formation of oxalates . This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* F. C.-V.: f.calle.vallejo@chem.leidenuniv.nl
* M. T. M. K.: m.koper@chem.leidenuniv.nl
Author Contributions
E. P.-G. and G. M. made the electrochemical experiments. E. P.-
G. and M. C. F. carried out the FTIR experiments. F. C.-V. made
the DFT calculations. The manuscript was written by E. P.-G.
and F. C. V., incorporating contributions from all authors. All
authors have given approval to the final version of the
manuscript.
ACKNOWLEDGMENT
FCV acknowledges funding from NWO (Veni project number
722.014.009). The use of supercomputing facilities at
SURFsara was sponsored by NWO Physical Sciences, with
financial support by NWO.
REFERENCES
(1) Hori, Y., In Modern aspects of electrochemistry, Springer:
New York, 2008; pp 89-189.
(2) Hori, Y.; Kikuchi, K.; Murata, A.; Suzuki, S. Chem. Lett.
1986, 15, 897-898.
(3) Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 14, 1695-
1698.
(4) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Energy
Environ. Sci. 2012, 5, 7050-7059.
(5) Roberts, F. S.; Kuhl, K. P.; Nilsson, A. Angew. Chem. Int. Ed.
2015, 54, 5179-5182.
(6) Li, C. W.; Ciston, J.; Kanan, M. W. Nature 2014, 508, 504-
507.
(7) Hori, Y.; Takahashi, I.; Koga, O.; Hoshi, N. J. Mol. Catal. A:
Chem. 2003, 199, 39-47.
(8) Schouten, K. J. P.; Pe
(40) Kim, Y.-G.; Javier, A.; Baricuatro, J. H.; Soriaga, M. P.
Electrocatalysis 2016, 7, 391-399.
(41) Verdaguer-Casadevall, A.; Li, C. W.; Johansson, T. P.;
Scott, S. B.; McKeown, J. T.; Kumar, M.; Stephens, I. E.; Kanan,
M. W.; Chorkendorff, I. J. Am. Chem. Soc. 2015, 137, 9808-
9811.
́rez Gallent, E.; Koper, M. T. ACS Catal.
2013, 3, 1292-1295.
(9) Schouten, K. J. P.; Qin, Z.; Gallent, E. P.; Koper, M. T. M. J.
Am. Chem. Soc. 2012, 134, 9864-9867.
(10) Murata, A.; Hori, Y. Bull. Chem. Soc. Jpn. 1991, 64, 123-
127.
(42) Gao, D.; Zegkinoglou, I.; Divins, N. J.; Scholten, F.; Sinev, I.;
Grosse, P.; Roldan Cuenya, B. ACS nano 2017, 11, 4825-4831.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 11
(43) Bertheussen, E.; Verdaguer-Casadevall, A.; Ravasio, D.;
1
2
3
4
5
6
7
8
Montoya, J. H.; Trimarco, D. B.; Roy, C.; Meier, S.; Wendland, J.;
Nørskov, J. K.; Stephens, I. E. Angew. Chem. Int. Ed. 2016, 55,
1450-1454.
(44) Kresse, G.; Furthmüller, J. Physical Review B 1996, 54,
11169-11186.
(45) Kresse, G.; Joubert, D. Physical Review B 1999, 59, 1758-
1775.
(46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Physical Review
Letters 1996, 77, 3865-3868.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

Page 11 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
S11
ACS Paragon Plus Environment