Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes

Article abstract of DOI:10.1016/S1381-1169(03)00016-5

Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M KHCO₃ aqueous solution at constant current density 5 mA cm^-2; the electrodes employed are Cu(S)-[n(1 0 0) x (1 1

Full text of DOI:10.1016/S1381-1169(03)00016-5

Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
Electrochemical reduction of carbon dioxide at various
series of copper single crystal electrodes
Y. Hori∗, I. Takahashi, O. Koga, N. Hoshi
Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received 9 April 2002; received in revised form 20 May 2002; accepted 20 May 2002
Abstract
Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M
KHCO3 aqueous solution at constant current density 5 mA cm−2; the electrodes employed are Cu(S)-[n(1 0 0) × (1 1 1)],
Cu(S)-[n(1 0 0)×(1 1 0)], Cu(S)-[n(1 1 1)×(1 0 0)], Cu(S)-[n(1 1 1)×(1 1 1)] and Cu(S)-[n(1 1 0)×(1 0 0)]. The electrodes
based on (1 0 0) terrace surface give ethylene as the major product. The ethylene formation is further promoted by the
introduction of (1 1 1) or (1 1 0) steps to the (1 0 0) basal plane. The highest C2H4 to CH4 formation ratio amounts to 10
in terms of the current efficiency for the (7 1 1) (=4(1 0 0) − (1 1 1)) surface as compared with the value 0.2 for the (1 1 1)
electrode. The n(1 1 1) − (1 1 1) surfaces favor the formations of acetic acid, acetaldehyde and ethyl alcohol with the increase
of the (1 1 1) step atom density. CH4 formation at the n(1 1 1) − (1 1 1) electrodes decreases with the increase of the (1 1 1)
step atom density. The n(1 1 1) − (1 0 0) surfaces give higher gaseous products; the major product is CH4 with lower fraction
of C2+ compounds.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Single crystal; Copper; CO2; Electrochemical reduction; Electrocatalysis
1. Introduction
platinum single crystal electrodes [4], and other plat-
inum metals (Ir, Rh and Pd) [5–7]. We revealed that
the electrocatalytic activity in CO2 reduction is greatly
enhanced by introduction of steps and kinks to the
atomically flat surfaces such as (1 1 1) and (1 0 0).
Carbon dioxide is electrochemically reduced to
methane, ethylene and alcohols at copper electrode
[8,9]. Mechanism of the unique electrocatalytic prop-
erty of copper has not yet been revealed. A prelimi-
nary study was communicated by Frese with regard
to CH4 formation on copper single crystals [10]. We
also reported the electrochemical reduction of CO2 at
single crystal copper electrodes of low index planes.
CH4 formation is favored at Cu(1 1 1) and C2H4 at
Cu(1 0 0) among low index planes [11]. Several reac-
tions may proceed competitively at copper electrode,
Structure sensitive reactions on heterogeneous sur-
faces have been studied using single crystal metal
interfaces. Many studies with single crystal surfaces
have been focused on gas–solid reactions [1], and lim-
ited number of electrochemical reactions have been
carried out other than adsorption of anions on elec-
trodes [2]. Electrochemical reduction of CO2 to ad-
sorbed CO at platinum single crystal electrodes were
studied by Yeager and co-workers [3]. Our laboratory
further extended the studies to high index planes of
∗
Corresponding author. Tel.: +81-43-290-3382;
fax: +81-43-290-3382.
E-mail address: hori@tc.chiba-u.ac.jp (Y. Hori).
1381-1169/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00016-5

40
Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
and the electrocatalytic activity for the individual re-
actions may depend on the atomic configuration of
the electrode surface. Thus, the product distribution in
CO2 reduction at Cu single crystal electrodes varies
greatly with the crystal orientation of the electrode.
We further extended the study in order to find a good
electrocatalyst in CO2 reduction. In the course of this
study, we showed that introduction of step sites to the
Cu(1 0 0) basal plane leads to remarkably enhanced
formation of C2H4 [12]. However, the reproducibility
of the electrolysis was not very high at some crystal
surfaces, particularly Cu(1 1 0) and the related crystal
orientation electrodes. We examined the surface treat-
ment procedures of the electrode, and successfully im-
proved the reproducibility of the electrolysis results
[13]. Using the newly established surface treatment of
the electrodes, we comprehensively studied the elec-
trochemical reduction of CO2 at single crystal copper
electrodes of various orientations. This article reports
the results, comparing the electrocatalytic properties
of various series of the crystal orientations in the elec-
trochemical reduction of CO2.
mograms are characteristic of the crystal orienta-
tions which can be utilized as the fingerprint. In the
present study, we thus verified the crystal orientations
of the copper single crystal electrodes prior to the
electrolysis on the basis of the voltammograms of
the charge displacement adsorption of CO in 0.1 M
K2HPO4 + 0.1 M KH2PO4 (pH 6.8) at 0 ◦C using as
the fingerprints.
The electrolyte solution 0.1 M KHCO3 for the elec-
trochemical reduction of CO2 were prepared from su-
per pure grade chemicals (Nacarai Tesque), purified
by pre-electrolysis using a Pt black cathode overnight
prior to the measurements. The electrolyses were con-
ducted at a constant current density of 5 mA cm−2 for
1 h with CO2 gas bubbled through the electrolyte so-
lution continuously. CO2 gas, of the purity higher than
99.99%, passed through an activated copper column
and a silica gel column to remove trace amount of
gaseous impurities. The electrolysis cell was stirred
by a magnetic stirrer (ca. 260 rpm) and thermostated
at 18 ◦C. The gas sample of the effluent CO2 from
the electrolysis cell was taken every 10 min, analyzed
by gas chromatography. Soluble products were an-
alyzed by an ion chromatograph (Toyo Soda) and
a gas chromatograph-mass spectrometer (Shimadzu
GC-MS-QP5050) after the electrolyses. More experi-
mental details are published elsewhere [13].
2. Experimental
Spherical copper single crystals (10 mm diameter)
attached with a copper stick lead were prepared from
99.999% copper metal using a graphite crucible. Af-
ter orientation of crystals by the X-ray Laue back
reflection method, the surface of the electrode was
polished mechanically to mirror finish with diamond
paste down to 0.25 ␮m, then electrochemically pol-
ished in a mixture of concentrated phosphoric acid
and sulfuric acid. Rinsed with 0.1 mM HClO4 solution
prepared from Merck Suprapur grade chemicals and
ultrapure water from Milli Q low TOC (Millipore),
the electrode was transferred to a Pyrex electrolysis
cell with the electrode surface protected by a drop
of 0.1 mM HClO4. The electrode was quickly set in
the electrolysis cell in 15 s according to the hanging
meniscus configuration.
3. Results and discussion
3.1. Voltammograms of copper single crystal
electrodes
Fig. 1 shows voltammograms of the series of
Cu(S)-[n(1 0 0) × (1 1 1)] electrodes composed of n
atomic rows of (1 0 0) terrace and one atomic height
of (1 1 1) step. Figs. 2 and 3 gives voltammograms
of Cu(S)-[n(1 0 0) × (1 1 0)] and Cu(S)-[n(1 1 1) ×
(1 0 0)], respectively. Voltammograms from the
Cu(S)-[n(1 1 1)×(1 1 1)] and Cu(S)-[n(1 1 0)×(1 0 0)]
are not displayed here which were published in our
previous paper [13].
The Cu(1 0 0) gives a sharp peak at −0.71 V with
a small shoulder at −0.73 V as shown in Fig. 1.
The Cu(3 1 1) (n = 2 in n(1 0 0) − (1 1 1)) gives a
single peak at −0.82 V. The other two electrodes of
Cu(S)-[n(1 0 0) × (1 1 1)] show the intermediate fea-
We previously reported the charge displacement
adsorption of CO at the electrodes of poly- and single
crystal copper in some electrolyte solutions [14–16].
Reproducible cyclic voltammograms are obtained,
corresponding to the displacement of specifically
adsorbed anion by adsorption of CO; the voltam-

Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
41
Fig. 1. Cyclic voltammograms of the Cu(S)-[n(1 0 0) × (1 1 1)] electrodes obtained in CO saturated 0.1 M K2HPO4 + 0.1 M KH2PO4 (pH
6.8) at 0 ◦C. The potentials of the peaks and shoulders are indicated.
tures between the Cu(1 0 0) and the Cu(3 1 1). With the
increase of (1 1 1) step atom density or the decrease
of n value, the sharp peak at −0.71 V diminishes and
the broad peaks at −0.73 to −0.78 V grow.
may be included in the redox peaks. The shape of the
voltammograms of the whole series will be discussed
in connection with the surface atom configuration in
more detail elsewhere.
Fig. 2 shows a series of the voltammograms of
Cu(S)-[n(1 0 0) × (1 1 0)]. The features of the voltam-
mograms vary with the increase of the step atom den-
sity or the decrease of n value in a way similar to
Cu(S)-[n(1 0 0) × (1 1 1)] series.
3.2. Electrolysis with electrodes of Cu(S)-[n(1 0 0)
× (1 1 1)] and Cu(S)-[n(1 0 0) × (1 1 0)] series
The shape of the voltammograms of Cu(S)-[n(1 0 0)×
(1 1 1)] and Cu(S)-[n(1 0 0) × (1 1 0)], measured after
the electrolysis for 1 h, did not change greatly and
showed identical features with those before the elec-
trolysis. Thus, the crystal orientation of the electrodes
remains stable during the electrolysis.
Table 1 demonstrates the product distributions in
terms of the current efficiency together with n value.
The results from the Cu(1 1 1) is added to Table 1
Fig. 3 presents the voltammograms of Cu(S)-
[n(1 1 1)×(1 0 0)]. The trend is different from Figs. 1
and 2. Cu(1 1 1) does not give any peak in the mea-
sured potential range. With the increase of the step
atom density, a single cathodic peak grows at −0.79
to −0.82 V, giving a couple of large redox waves
at Cu(3 1 1). The cathodic peak is greater than the
anodic one; the electricity of the hydrogen evolution

42
Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
Fig. 2. Cyclic voltammograms of the Cu(S)-[n(1 0 0) × (1 1 0)] electrodes obtained in CO saturated 0.1 M K2HPO4 + 0.1 M KH2PO4 (pH
6.8) at 0 ◦C. The potentials of the peaks and shoulders are indicated.
Table 1
Product distribution in the electrochemical reduction of CO2 at a series of copper single crystal electrodes Cu(S)-[n(1 0 0) × (1 1 1)]
Crystal
orientation
na Potential V Current efficiency (%)
C2H4/
CH4
vs. SHE
CH4 C2H4 CO H2 MeD EtOH PrD AlOH PrOH HCOOH CH3COOH C2+ Gas Total
total
(1 0 0)
(11 1 1)
(9 1 1)
(7 1 1)
(5 1 1)
(3 1 1)
(1 1 1)
∞ −1.40
30.4 40.4 0.9 6.8 1.6
8.9 50.2 1.8 8.8 1.1 14.4 3.3 1.0
5.7 50.9 tr 12.7 1.2 16.9 2.4 2.5
5.0 50.0 1.1 15.6 1.2 7.4 5.2 2.2
11.4 39.0 1.9 18.1 1.4 12.2 3.0 1.6
9.7 2.8 0.8
1.5
2.3
4.5
4.6
3.3
1.5
0.0
3.0
3.2
3.5
4.6
8.8
1.0
2.1
1.1
0.9
0.8
0.6
1.5
57.8 71.7 98.9 1.3
74.4 60.9 97.2 5.6
79.5 56.6 101.4 8.9
71.5 56.1 97.8 10.0
61.3 52.3 101.5 3.4
33.0 62.4 98.9 0.7
15.8 61.0 96.3 0.2
6
5
4
3
2
–
−1.37
−1.36
−1.34
−1.36
−1.37
−1.55
36.0 23.8 2.6 13.3 1.1
46.3 8.3 6.4 16.3 2.1
3.3 2.3 0.4
2.6 0.6 0.7
14.0
11.5
Electrolyte solution: 0.1 M KHCO3; current density: 5 mA cm−2. MeD: acetaldehyde; EtOH: ethanol; PrD: propionaldehyde; AlOH: allyl
alcohol; PrOH: propanol. C2+ contains all the substances which have more than two carbon atoms. The gas total contains all the gaseous
products other than H2; tr: <0.05%.
a n in Cu(S)-[n(1 0 0) × (1 1 1)].

Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
43
Fig. 3. Cyclic voltammograms of the Cu(S)-[n(1 1 1) × (1 0 0)] electrodes obtained in CO saturated 0.1 M K2HPO4 + 0.1 M KH2PO4 (pH
6.8) at 0 ◦C. The potentials of the peaks and shoulders are indicated.
for comparison. CH4, C2H4 and CO were produced
as the gaseous products with small amount of H2.
Acetaldehyde, ethanol, propionaldehyde, allyl alco-
hol, n-propanol, formic acid and acetic acid were de-
tected as soluble products in the electrolyte solution.
The total current efficiency amounted approximately
to 100%. This fact verifies that the major products
were accurately analyzed in the present experimental
procedures.
The product distribution is significantly affected by
the crystal orientation. The Cu(1 1 1) electrode gives
rise to CH4 formation with the current efficiency as
high as 46% at highly negative potential. Ethylene
formation on the Cu(1 1 1) remains <10%.
face. Thus the C2H4/CH4 value ranges two orders of
magnitude.
Table 2 shows the results of the series Cu(S)-
[n(1 0 0) × (1 1 0)], in which (1 0 0) basal plane is
modified by introduction of (1 1 0) steps. The features
are similar to Table 1. C2H4 formation once increases
and then decreases with the decrease of n value; CH4
steeply decreases and increases again. C2H4/CH4
value takes 7 at n = 8. CH4 formation is favorable
at the Cu(2 1 0) which has the highest dangling bond
density in the face centered cubic metal. It is inter-
esting that the Cu(2 1 0) gives a product distribution
similar to the Cu(1 1 1) which has the lowest dangling
bond density.
The Cu(1 0 0) electrode gives relatively high yield
of C2H4 among low index planes. C2H4 formation in-
creases at n = 4 as shown Table 1, and decreases for
n below 4. C2+ value, which contains C2H4, ethanol,
allyl alcohol, n-propanol, acetaldehyde, propionalde-
hyde and acetic acid, shows a similar trend with C2H4.
C2+ exceeds 70% at n = 6–4. CH4 formation steeply
diminishes simultaneously, taking a minimum at n =
4. The selectivity ratio C2H4/CH4 in terms of the cur-
rent efficiency reaches 10 at the Cu(7 1 1). This value
is 1.3 at the (1 0 0) surface, and 0.2 at the (1 1 1) sur-
Fig. 4 shows the log (C2H4)/(CH4) value and the
electrode potential as a function of the angle with ref-
erence to Cu(1 0 0). The log (C2H4)/(CH4) value takes
a maximum at (7 1 1) and (8 1 0) in n(1 0 0) − (1 1 1)
and n(1 0 0) − (1 1 0) series, respectively. The intro-
duction of step atoms into (1 0 0) terrace, regardless of
(1 1 1) step or (1 1 0) step, promotes C2H4 formation,
and severely suppresses CH4 formation. Fig. 5 depicts
rigid ball models for (7 1 1) and (6 1 0). The crystal
orientations with similar length of (1 0 0) terrace are
the most active in ethylene formation.

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Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
Table 2
Product distribution in the electrochemical reduction of CO2 at a series of copper single crystal electrodes Cu(S)-[n(1 0 0) × (1 1 0)]
Crystal
orientation
na Potential V Current efficiency (%)
C2H4/
CH4
vs. SHE
CH4 C2H4 CO H2 MeD EtOH PrD AlOH PrOH HCOOH CH3COOH C2+ Gas Total
total
(1 0 0)
(8 1 0)
(6 1 0)
(5 1 0)
(3 1 0)
(2 1 0)
∞
8
6
5
3
−1.40
−1.38
−1.37
−1.38
−1.42
−1.52
30.4 40.4 0.9 6.8 1.6
6.4 45.1 1.4 8.7 0.9
7.6 44.7 0.9 9.0 1.5
8.1 42.3 2.1 10.5 3.0
17.7 34.6 0.0 12.8 1.7
64.0 13.4 2.2 7.0 0.9
9.7 2.8 0.8
26.0 1.1 0.9
26.0 1.2 1.3
26.1 2.6 1.7
29.9 2.6 0.9
6.6 0.6 0.2
1.5
1.9
2.0
1.7
1.9
0.5
3.0
1.5
1.4
2.8
2.7
5.5
1.0
1.6
1.6
2.1
1.6
0.7
57.8 71.7 98.9 1.3
77.6 52.9 95.6 7.1
78.3 53.2 97.8 5.9
79.5 52.5 103.1 5.2
73.2 52.3 106.4 2.0
22.9 79.6 101.6 0.2
2
Electrolyte solution: 0.1 M KHCO3; current density: 5 mA cm−2. MeD: acetaldehyde; EtOH: ethanol; PrD: propionaldehyde; AlOH: allyl
alcohol; PrOH: propanol. C2+ contains all the substances which have more than two carbon atoms. The gas total contains all the gaseous
products other than H2; tr: <0.05%.
a n in Cu(S)-[n(1 0 0) × (1 1 0)].
The electrode potentials both for Cu(S)-[n(1 0 0) ×
(1 1 1)] and Cu(S)-[n(1 0 0) × (1 1 0)] do not vary to a
great extent, but the value takes a maximum at (7 1 1)
or (6 1 0), respectively, which gives the highest C2/C1
value among the respective series. The electrode po-
tential takes the most negative value at Cu(2 1 0) and
Cu(1 1 1) which give high CH4 formation.
3.3. Electrolysis with electrodes of Cu(S)-[n(1 1 1)
× (1 0 0)] series
The results from the Cu(S)-[n(1 1 1) × (1 0 0)] se-
ries is tabulated in Table 3. Introduction of (1 0 0)
steps to (1 1 1) terrace does not make drastic change in
the product distribution from that of the (1 1 1) elec-
trode. The electrodes of this series are distinguished
by high gaseous products and low C2+. The forma-
tion of aldehydes, alcohols and acetic acid is very low
as compared with other series of the electrodes. CH4
formation rises and then drops with the increase of
Fig. 4. Variation of log (C2H4/CH4) in terms of the current effi-
ciency and the electrode potential with the angle of the crystal
orientation with the reference of Cu(1 0 0).
Fig. 5. Rigid sphere models of (7 1 1), (6 1 0), (3 3 2) and (7 5 5).

Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
45
Table 3
Product distribution in the electrochemical reduction of CO2 at a series of copper single crystal electrodes Cu(S)-[n(1 1 1) × (1 0 0)]
Crystal
orientation
na Potential V Current efficiency (%)
C2H4/
CH4
vs. SHE
CH4 C2H4 CO H2 MeD EtOH PrD AlOH PrOH HCOOH CH3COOH C2+ Gas Total
total
(1 1 1)
(11 9 9)
(7 5 5)
(5 3 3)
(2 1 1)
(3 1 1)
∞ −1.55
10 −1.48
46.3 8.3 6.4 16.3 2.1 2.6
62.4 10.2 4.9 7.2 0.5 5.9
62.9 11.5 4.4 6.9 0.6 5.3
62.9 13.0 3.0 4.7 0.6 1.9
45.6 17.8 2.1 11.2 0.3 3.4
36.0 23.8 2.6 13.3 1.1 3.3
0.6 0.7
1.6 0.0
1.2 0.7
0.7 0.5
2.0 0.7
2.3 0.4
0.0
0.2
0.6
0.4
1.3
1.5
11.5
7.9
12.3
9.7
13.6
14.0
1.5
0.8
0.5
0.5
0.5
0.6
15.8 61.0 96.3 0.2
19.2 77.5 101.6 0.2
20.4 78.8 106.9 0.2
17.6 78.9 97.9 0.2
26.0 65.5 98.5 0.4
33.0 62.4 98.9 0.7
6
4
3
2
−1.43
−1.42
−1.38
−1.37
Electrolyte solution: 0.1 M KHCO3; current density: 5 mA cm−2. MeD: acetaldehyde; EtOH: ethanol; PrD: propionaldehyde; AlOH: allyl
alcohol; PrOH: propanol. C2+ contains all the substances which have more than two carbon atoms. The gas total contains all the gaseous
products other than H2; tr: <0.05%.
a n in Cu(S)-[n(1 1 1) × (1 0 0)].
the step atom density, once taking a maximum 60%
at n = 10–4. C2H4 shows a gradual increase with the
step atom density. The variation of log (C2H4)/(CH4)
is shown in Fig. 4. A rigid ball model of this series is
exemplified by (7 5 5) in Fig. 5.
The (1 1 0) electrode is distinguished by high yields
of CH3CHO, C2H5OH and CH3COOH with very low
yield of CH4. Drastic changes are remarked for CH4,
CH3CHO, C2H5OH and CH3COOH among the both
crystal orientation series. C2H4 and HCOOH show
much less variation among the two crystal orientation
series.
3.4. Electrolysis with electrodes of Cu(S)-[n(1 1 1)
Fig. 6 shows a correlation plot between the
current efficiencies of CH3CHO + C2H5OH and
CH3COOH for Cu(S)-[n(1 1 1) × (1 1 1)] and
Cu(S)-[n(1 1 0) × (1 0 0)] series. This correlation
suggests that CH3CHO, C2H5OH and CH3COOH
may be formed from the same intermediate species.
× (1 1 1)] and Cu(S)-[n(1 1 0) × (1 0 0)] series
The whole experimental results of the two series
were published elsewhere [13]. The results from the
electrodes of some selected crystal orientations are
reproduced in Table 4 for the sake of comparison.
Table 4
Product distribution in the electrochemical reduction of CO2 at a series of copper single crystal electrodes Cu(S)-[n(1 1 1) × (1 1 1)] and
Cu(S)-[n(1 1 0) × (1 0 0)]
Crystal
orientation
n
Potential
V vs. SHE
Current efficiency (%)
C2H4/
CH4
CH4 C2H4 CO H2 MeD EtOH PrD AlOH PrOH HCOOH CH3COOH C2+ Gas Total
total
n(1 1 1) − (1 1 1)
(1 1 1)
(3 3 2)
(3 3 1)
(1 1 0)
∞ −1.55
46.3 8.3
39.6 9.9
13.8 16.6
6.4 16.3 2.1 2.6 0.6 0.7
6.1 10.3 5.1 7.1 0.2 0.3
7.7 5.7 7.1 15.6 0.5 0.0
0.0
0.2
0.4
11.5
9.4
9.1
1.5
3.4
7.5
15.8 61.0 96.3 0.2
6
3
2
−1.51
−1.55
−1.58
26.2 55.6 91.6 0.3
47.7 38.1 84.0 1.2
66.0 34.3 99.9 2.0
6.9 13.5 13.9 3.1 19.9 10.5 1.3
tr
0.04 10.1
20.8
n(1 1 0) − (1 0 0)
(6 5 0)
(3 2 0)
(2 1 0)
6
3
2
−1.59
−1.52
−1.52
10.5 16.2 14.5 2.5 16.2 10.9 0.8
tr
0.06
0.4
0.5
6.1
5.8
5.5
20.6
4.8
0.7
64.8 41.2 98.4 1.5
29.5 71.5 98.4 0.3
22.9 79.6 101.6 0.2
52.4 13.7
64.0 13.4
5.4 5.3 3.2 6.5 0.6 0.3
2.2 7.0 0.9 6.6 0.6 0.2
Electrolyte solution: 0.1 M KHCO3; current density: 5 mA cm−2. MeD: acetaldehyde; EtOH: ethanol; PrD: propionaldehyde; AlOH: allyl
alcohol; PrOH: propanol. C2+ contains all the substances which have more than two carbon atoms. The gas total contains all the gaseous
products other than H2; tr: <0.05%.

46
Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
is much lower than in Cu(S)-[n(1 1 1) × (1 0 0)] se-
ries. CH3CHO, C2H5OH and CH3COOH formation in
Cu(S)-[n(1 1 1)×(1 1 1)] series is much higher than in
Cu(S)-[n(1 1 1)×(1 0 0)] series. This fact and the cor-
relations shown in Figs. 6 and 7 suggest that some pre-
cursor leading to form CH3COOH may be favorably
formed at the (1 1 1) step sites on Cu(S)-[n(1 1 1) ×
(1 1 1)] series. CH3COOH may be further reduced se-
quentially to CH3CHO and C2H5OH.
3.6. Unique electrocatalytic property of Cu(1 1 0)
Fig. 6. Correlation of the sum of the current efficiency of CH3CHO
and C2H5OH with that of CH3COOH for Cu(S)-[n(1 1 1) × (1 1 1)]
and Cu(S)-[n(1 1 0) × (1 0 0)] electrodes.
Cu(1 1 0) shows a unique electrocatalytic activity in
the formation of CH3CHO, C2H5OH and CH3COOH
among Cu(S)-[n(1 1 1) × (1 1 1)] series; the sum of
CH3CHO, C2H5OH and CH3COOH for Cu(1 1 0)
(n = 2) is significantly higher than that for Cu(3 3 1)
(n = 3) as shown in Table 4. (1 1 0) provides closely
adjacent (1 1 1) step lines. Such adjacent (1 1 1) step
lines may synergetically promote the formation of the
intermediate species for CH3COOH, CH3CHO and
C2H5OH, and simultaneously suppress the formation
of CH4.
(3 1 1), expressed in 2(1 1 1) − (1 0 0) or 2(1 0 0)
− (1 1 1), has two adjacent step lines, i.e. (1 1 1) step
lines and (1 0 0) ones. The product distribution of the
(3 1 1) differs significantly from the (1 1 0) (=2(1 1 1)
− (1 1 1)) which is composed of adjacent (1 1 1) step
lines. The (3 1 1) gives very low yields of CH3CHO,
C2H5OH and CH3COOH with relatively high yield of
CH4 as shown in Table 3. Thus, the (1 1 1) step sites
adjacent to (1 1 1) structure will favor the formation
of a precursor leading to CH3COOH.
Fig. 7 illustrates another correlation of CH3CHO +
C2H5OH + CH3COOH versus CH4 for the two crys-
tal orientation series. A good correlation curve is
obtained. We examined a similar correlation for the
other series of the electrodes, but no such correlation
was observed for the others.
3.5. Comparison of the product distribution between
Cu(S)-[n(1 1 1) × (1 1 1)] and Cu(S)-[n(1 1 1) × (1
0 0)] series
The two crystal orientations have the surfaces of
similar atomic configuration; the structure of the step
site is different between the two. However, the prod-
uct distribution from both the series differs to a great
extent. If one compares the product distribution of
the two series at the same n value in Tables 3 and
4, CH4 formation in Cu(S)-[n(1 1 1) × (1 1 1)] series
The Cu(S)-[n(1 1 0) × (1 0 0)] series provides adja-
cent (1 1 1) step lines the length of which is limited by
the presence of (1 0 0) step lines. Table 4 shows that the
formation of CH3CHO, C2H5OH and CH3COOH de-
creases at n below 3. Thus, the unique electrocatalytic
property of (1 1 0) structure appears for the (1 1 1) step
lines with a certain length.
4. Conclusion
Electrocatalytic activity of copper single crystal
electrodes in the electrochemical reduction of carbon
dioxide were compared using the following series
of the crystal orientations; Cu(S)-[n(1 0 0) × (1 1 1)],
Fig. 7. Correlation of the sum of the current efficiency of
CH3COOH, CH3CHO and C2H5OH with that of CH4 for
Cu(S)-[n(1 1 1) × (1 1 1)] and Cu(S)-[n(1 1 0) × (1 0 0)] electrodes.

Y. Hori et al. / Journal of Molecular Catalysis A: Chemical 199 (2003) 39–47
47
Cu(S)-[n(1 0 0) × (1 1 0)], Cu(S)-[n(1 1 1) × (1 0 0)],
Cu(S)-[n(1 1 1) × (1 1 1)] and Cu(S)-[n(1 1 0) ×
(1 0 0)]. The electrolyses were conducted in 0.1 M
KHCO3 aqueous solution at constant current density
5 mA cm−2. The reaction selectivity varied greatly
with the crystal orientation. The electrocatalytic ac-
tivity of C2H4 formation on Cu(S)-[n(1 0 0) × (1 1 1)]
and Cu(S)-[n(1 0 0) × (1 1 0)] is further activated
by introduction of a certain amount of the step
atoms. Gaseous products are predominantly formed
at Cu(S)-[n(1 1 1) × (1 0 0)]. The CH4 formation is
enhanced in this series. Thus the C2H4/CH4 value in
the current efficiency varied in two orders of magni-
tude. The Cu(S)-[n(1 1 1) × (1 1 1)] electrodes give
high amount of C2+ substances, and the (1 1 0) elec-
trode derived from Cu(S)-[n(1 1 1)×(1 1 1)] uniquely
produces high yield of CH3COOH, CH3CHO and
C2H5OH. The (1 1 1) step line adjacent to (1 1 1)
structure may be effective for formation of the precur-
sor for CH3COOH. The precursor may be sequentially
reduced to CH3COOH, CH3CHO and C2H5OH.
the Ministry of Education, Science and Culture of
Japan.
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A part of the experimental works were conducted
by R. Miura. This work was supported by Research
Institute of Innovative Technology for the Earth of
Japan (RITE), New Energy and Industrial Technol-
ogy Development Organization (NEDO), and the
Grant-in-Aid for Scientific Research (12650805) from