Electrochemical reduction of CO at a copper electrode

Article abstract of DOI:10.1021/jp970284i

CO₂ is electrochemically reduced to CH₄, C₂H₄, and alcohols in aqueous electrolytes at Cu electrode with high current density. CO₂ is initially reduced to adsorbed CO and further to hydrocarbons and alcohols. This paper describes macroscopic electrolytic reduction of CO at a Cu electrode in various electrolyte solutions in order to reveal the unique properties of Cu electrode in comparison with Fe and Ni electrodes. The reaction products from the Cu electrode are CH₄, C₂H₄, C₂H₅OH, n-C₃H₇OH, CH₃CHO, and C₂H₅CHO. Neither C₂H₆ nor CH₃OH is produced. CH₄ is favorably produced in aqueous KHCO₃ of high concentrations (e.g. 0.3 mol dm^-3), whereas C₂H₄ and C₂H₅OH are produced in dilute KHCO₃ solutions (0.03 mol dm^-3). Such product selectivity is derived from the electrogenerated OH^- in the cathodic reaction, as is the case in the CO₂ reduction. The partial current densities of CH₄, C₂H₄, and C₂H₅OH are correlated with the electrode potential. Tafel relationships hold for C₂H₄ and C₂H₅OH irrespective of pH. The partial current of CH₄ formation is proportional to proton activity and also follows the Tafel relationship. The transfer coefficients are 0.35 for C₂H₄ formation and 1.33 for CH₄ formation. These facts indicate that the reaction paths of CH₄ and C₂H₄ formations are separated at an early stage of CO reduction. Ethanol and n-propanol may be reduced from acetaldehyde and propionaldehyde intermediately formed in the reaction. The molecular reaction path is discussed with regard to these experimental results.

Full text of DOI:10.1021/jp970284i

J. Phys. Chem. B 1997, 101, 7075-7081
7075
Electrochemical Reduction of CO at a Copper Electrode
Yoshio Hori,* Ryutaro Takahashi, Yuzuru Yoshinami, and Akira Murata
Department of Applied Chemistry, Faculty of Engineering, Chiba UniVersity,
Yayoi-cho, Inage-ku, Chiba 263, Japan
X
ReceiVed: January 22, 1997; In Final Form: May 15, 1997
CO
2
is electrochemically reduced to CH
4 2 4
, C H , and alcohols in aqueous electrolytes at Cu electrode with
high current density. CO
2
is initially reduced to adsorbed CO and further to hydrocarbons and alcohols.
This paper describes macroscopic electrolytic reduction of CO at a Cu electrode in various electrolyte solutions
in order to reveal the unique properties of Cu electrode in comparison with Fe and Ni electrodes. The reaction
products from the Cu electrode are CH
nor CH OH is produced. CH is favorably produced in aqueous KHCO
OH are produced in dilute KHCO solutions (0.03 mol dm ). Such
product selectivity is derived from the electrogenerated OH in the cathodic reaction, as is the case in the
CO reduction. The partial current densities of CH , C , and C OH are correlated with the electrode
potential. Tafel relationships hold for C and C OH irrespective of pH. The partial current of CH
formation is proportional to proton activity and also follows the Tafel relationship. The transfer coefficients
are 0.35 for C formation and 1.33 for CH formation. These facts indicate that the reaction paths of CH
and C formations are separated at an early stage of CO reduction. Ethanol and n-propanol may be reduced
4
, C
2
H
4
, C
2
H
5
OH, n-C
3
7 3 2 5
H OH, CH CHO, and C H CHO. Neither
C
2
H
6
3
3
4
3
of high concentrations (e.g.
-
-3
0.3 mol dm ), whereas C
H
2 4
and C
2
H
5
3
-
2
4
2
H
4
2 5
H
H
2 4
H
2 5
4
H
2 4
4
4
2 4
H
from acetaldehyde and propionaldehyde intermediately formed in the reaction. The molecular reaction path
is discussed with regard to these experimental results.
1
3
1
. Introduction
voltammetric, chronopotentiometric, and infrared spectroscopic
1
4
measurements. Adsorbed CO markedly prevents hydrogen
evolution in aqueous solution as well as other redox reactions
and underpotential deposition of metals. In-situ infrared absorp-
tion spectroscopy revealed that adsorbed CO is present at the
Cu electrode surface at potentials more negative than -0.7 V
Vs SHE.
CO2 is electrochemically reduced to CO, formic acid, and
hydrocarbons on metal electrodes.1,2 Copper electrode uniquely
reduces CO2 to methane, ethylene, and alcohols at high current
densities in aqueous electrolytes,
1
-3
as confirmed by many
workers.4 This reaction is of great interest from the viewpoint
of environment, energy, and natural resources, since this reaction
may be applied to load-leveling technology as a novel energy
storage process in which electrical energy is converted to
chemical energy and Vice Versa. Three major problems are
pointed out for this process; high overpotential, low product
selectivity, and transport problems.7 Among them, the transport
problems may be basically solved by electrolysis of CO2 by a
-6
Various metal electrodes reduce CO2 to CO, i.e. Au, Ag, Zn,
1
5
Pd, and Pt. Among these metals, Cu can effectively reduce
CO further to hydrocarbons and alcohols. Thus the electro-
chemical process of CO reduction at a Cu electrode is of great
interest from the scientific viewpoint as well.
Electrochemical reduction of CO2 at a Cu electrode yields
C2H4 more favorably with relatively low overpotential than
8
9
gas diffusion electrode, under high pressure or by electrolysis
of liquid CO2 in methanol solution under high pressure.
3
CH4. The selectivity between CH4 and C2H4 in CO reduction
1
0
1
2
depends on the pH of the electrolytes; C2H4 formation is
favored in relatively high pH solutions as compared with CH4.
Electrolyte cations also affect the selectivity between CH4 and
The problem involved with the electrode process still remains
unsolved at present; the energy conversion efficiency of the
copper electrode process is ca. 40% mainly due to high cathodic
overpotentials.7 Development of excellent electrocatalysts with
low overpotential is thus necessary for this purpose. Not many
metallic electrodes effectively reduce CO2 to hydrocarbons or
alcohols in spite of many attempts to search useful electrocata-
lysts. Other than the Cu electrode, it has recently been reported
that silver can reduce CO2 to CH4, C2H4, and C2H5OH at high
current efficiency in pulsed electrolysis accompanied with
anodic potential bias.11 It is thus important to understand the
fundamental aspects of the Cu electrode process in the elec-
trochemical reduction of CO2.
1
6
C2H4.
This article attempts to reveal comprehensively the electro-
chemical process of the reduction of CO to CH4, C2H4, and
alcohols at a Cu electrode as functions of the pH and electrode
potential. The reaction mechanism will be discussed on the
basis of these experimental results.
2. Experimental Section
The metal electrodes used in the present study are sheets of
Cu (purity, 99.999%) donated by Sumitomo Metals & Mining,
Co. Ltd., and Fe (99.9%) and Ni (99.9%) purchased from
Furuuchi Kagaku, Co. Ltd. Each metal sheet was cut into a 20
CO is also reduced at Cu electrodes, giving similar product
distribution.3
,12
One can thus expect that the electrochemical
×
20 mm electrode with a lead strip of the same metal attached.
reduction of CO2 to hydrocarbons proceeds with intermediate
formation of CO. During the electrochemical reduction of CO2,
adsorbed CO is formed at the Cu electrode as demonstrated by
The electrodes were polished with fine emery paper (#1500)
and subsequently electrolytically polished in 85% phosphoric
acid. Surface oxide film which may be formed after the
electropolishing, was cathodically reduced in the electrolytic
*
Corresponding author. Fax +81 432 90 3401; e-mail hori@
-
6
-2
cell for ca. 2 min by 30 × 10 A cm prior to the
planet.tc.chiba-u.ac.jp.
X
Abstract published in AdVance ACS Abstracts, August 15, 1997.
measurements. The electrochemical measurements were con-
S1089-5647(97)00284-8 CCC: $14.00 © 1997 American Chemical Society

7
076 J. Phys. Chem. B, Vol. 101, No. 36, 1997
TABLE 1: Faradaic Efficiencies of Various Products Affected by KHCO
Faradaic efficiencies/%
Hori et al.
Concentrationa
3
KHCO
3
/mol/L
potential/V Vs SHE
CH
4
C
2
H
4
EtOH
n-PrOH
MeCHO
EtCHO
H
2
total
2 4 4
C H /CH
0
0
0
0
0
.03
.05
.1
.2
.3
-1.38
-1.36
-1.36
-1.35
-1.35
16.2
18.5
22.3
23.2
32.4
28.1
19.1
21.7
15.8
15.3
13.1
7.6
7.1
7.1
6.3
3.1
1.9
2.2
2.2
3.5
2.1
1.6
1.4
0.7
0.4
2.7
1.3
0.5
0.6
0.8
40.1
49.4
41.7
51.5
44.8
105.4
99.4
96.9
101.1
103.5
1.73
1.03
0.973
0.681
0.472
a
Total ionic concentration was kept at 0.3 mol/L by adding KCl. Temperature: 19 °C. Current density: 2.5 mA cm-2
.
TABLE 2: Faradaic Efficiencies of Various Products from Cu, Fe, and Ni Electrodesa
Faradaic efficiencies/%
electrode
potential/V
CH
4
C
H
2 6
C
2
H
4
MeOH
EtOH
n-PrOH
MeCHO
EtCHO
H
2
total
Electrolyte: 0.1 M KHCO b
3
Cu
Fe
Ni
-1.36
-1.38
-1.49
22.3
1.1
1.1
0.00
0.4
0.3
21.7
0.1
0.2
0.00
0.1
7.1
tr.
tr.
2.2
0.00
tr.
1.4
tr.
0.1
0.5
tr.
tr.
41.7
96.0
100.3
96.9
97.7
102.5
c
0.5
Electrolyte: 0.1 M KH
PO
2 4
+ 0.1 M K
2
HPO
0.00
tr.
4
(pH 6.8)
Cu
Fe
Ni
-1.24
-1.32
-1.14
15.6
2.6
1.5
0.00
0.8
4.1
0.2
0.1
0.00
0.1
0.2
0.3
0.2
0.1
0.00
tr.
0.1
0.00
tr.
tr.
73.0
93.8
89.7
93.0
97.7
92.1
0.5
tr.
a
Potentials Vs SHE. Temperature: 19 °C. Current density: 2.5 mA cm-2. b pH is estimated at 9.0 at the electrode surface during the reactions.
c
tr. indicates a current efficiency between 0.01 and 0.05%.
ducted at 19 °C within ( 0.5 °C. The electrode potential was
current. Thus the partial currents will not be affected by the
transport process of CO to the electrode.
measured using an Ag/AgCl reference electrode and is given
with respect to SHE in this article. The electrode potential was
corrected for the IR drop between the Luggin capillary tip and
the electrode by the current interrupter technique.
3. Results and Discussion
3
.1. Product Distribution Affected by Concentration of
The catholytes were purified by pre-electrolysis prior to the
electrochemical measurements with a Pt black cathode under
purified argon atmosphere overnight. The pre-electrolysis is
effective and necessary to maintain the activity of Cu electrodes,
preventing “deactivation of Cu electrode”, which is described
in several recent publications.6 The effect of pre-electrolysis
will be reported in detail elsewhere. CO of high-purity grade
-
HCO3 . Table 1 tabulates Faradaic efficiencies of the products
in the cathodic reduction of CO at a constant current density of
-2
2
.5 mA cm in KHCO3 solution of various concentrations. The
-3
total ionic concentration was kept at 0.3 mol dm by addition
of KCl.
Methane and ethylene are produced together with H2 formed
from water electrolysis as a competing parallel reaction. Various
soluble products are formed as well in the electrolytes; ethanol,
n-propanol, acetaldehyde, and propionaldehyde. Formaldehyde
is not formed. The Faradaic yields of C H and C H OH are
(99.9%) was used after passing through KOH solution.
Electrolytic measurements at constant currents or constant
potentials were conducted for more than 30 min with K2HPO4-
2
4
2
5
KOH, phosphate, or borate buffer solution with the total
3 4
high in dilute KHCO solutions, whereas the CH yield increases
-
3
electrolyte concentration 0.2 mol dm . CO was continuously
with increase of KHCO concentration. The Faradaic produc-
3
-
1
sparged into the catholyte (flow rate: ca. 50-140 mL min )
during the electrolyses; the catholyte was stirred at ca. 680 rpm
with a magnetic stirrer. Gaseous products in the effluent gas
from the cell were analyzed at 5 min intervals by gas
chromatographs. The concentration of gaseous products in the
effluent gas remained virtually constant during the electrolyses.
Products soluble in the electrolyte solution were analyzed by a
liquid chromatograph and a gas chromatograph using a PEG
tion ratio C H /CH significantly decreases with increase of
2
4
4
KHCO concentration.
3
Table 2 presents the result of the Cu electrode obtained in
phosphate buffer solution (KH PO + K HPO , pH 6.8); C H
2
4
2
4
2
4
and C H OH remarkably drop, whereas CH is not much
2
5
4
different compared with the value obtained in KHCO solutions.
3
No other soluble product was produced in phosphate buffer
solution.
4
00 column after the electrolyses. The limit of the quantitative
The dependence of C2H4/CH4 on HCO3- concentration in
-6
analysis was 0.6 ppm for CH4, 0.3 ppm for C2H4, and 3 × 10
electrochemical reduction of CO2 was discussed in our previous
publication in terms of the local high-pH region close to the
-
3
mol dm for C2H5OH and n-C3H7OH. Other details were
3
described in the previous publication from our laboratory.
3
-
electrode. The process is briefly described. OH is generated
in cathodic reactions that involve water molecules such as CO
The current limited by the diffusion of CO to the electrode
-
2
-
was estimated as 4.1 mA cm under the present experimental
conditions; CO at 1 atm and the temperature at 19 °C. The
diffusion layer thickness, 0.0025 cm, was determined from a
and CO2 reduction. OH thus produced will be instantaneously
neutralized by HCO3 . The pH value will not rise at the
electrode if a sufficient amount of HCO3 is supplied by
diffusion in concentrated HCO3 solution. However, the
transport of HCO3 to the electrode will not be enough in dilute
HCO3 solutions, and the pH will be enhanced in the neighbor-
-
-
3
-
-
measurement of the limiting current of reduction of Fe(CN)4
-
under identical hydraulic conditions with an equivalent geo-
metrical size of a Pt plate electrode. Five electrons were
assumed for reduction of one CO molecule as an average. All
the present electrolytic measurements of CO reduction were
conducted under the conditions that the sum of the partial
currents of CO reduction was below 20% of the limiting current;
most of the measurements were below 10% of the limiting
-
hood of the electrode. The pH values at the electrode are
-
2
estimated for a current density of 2.5 mA cm in various
KHCO3 concentrations saturated with CO according to the
3
procedure described in our previous publication: 9.5 in 0.03
-
3
-3
-3
mol dm ; 9.0 in 0.1 mol dm ; 8.5 in 0.3 mol dm .

Electrochemical Reduction of CO at a Cu Electrode
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7077
Thus the present results are compatible with the fact that CH4
formation in the electrochemical reduction of CO is favored in
low-pH electrolytes, and C2H4 in high-pH ones as reported
3
previously. The present results further confirm that the C2H4/
-
CH4 ratio in CO reduction is affected by HCO3 concentration
in a similar way with the electrochemical reduction of CO2.
Tables 1 and 2 further indicate that the Faradaic efficiencies
of C2H4 and the soluble products at the Cu electrode depend
similarly upon pH at the electrode surface, whereas CH4
formation depends differently upon pH.
3
.2. Comparison of Cu, Fe, and Ni Electrodes with
Regard to the Fischer-Tropsch Reaction. 3.2.1. Reduction
Products from Cu, Fe, and Ni Electrodes. Table 2 presents
the reaction products in the electrochemical reduction of CO
with the Cu, Fe, and Ni electrodes at constant current 2.5 mA
-
2
cm . The Cu electrode shows very high activity. The Fe and
Ni electrodes are much less active, but they can still reduce
CO, forming small amounts of CH4, C2H6, C2H4, CH3OH, and
C2H5OH. The product distributions from the Fe and Ni
electrodes are not affected by pH at the electrode surface. The
Faradaic efficiency of C2H6 and CH3OH mostly exceeds that
of C2H4 and C2H5OH respectively.
2 4
Figure 1. Partial current densities of C H formation in electrochemical
reduction of CO correlated with the electrode potential in various
electrolyte solutions: pH 6.0-6.3 (O), pH 7.1-7.7 (4), pH 8.0-8.6
(0), pH 8.7-8.9 (×), pH 9.0-9.3 (b), pH 10.5-11.3 (2), pH 12.2
(9).
These experimental facts strongly suggest that adsorption of
The Fischer-Tropsch reaction yields hydrocarbons and
alcohols from CO-H2 gas mixtures. Ni, Fe, Co, Ru, Rh, Pd,
Pt, and Ir are active catalysts for this reaction, whereas Cu is
hydrogen species is structure sensitive. In rationalization of
the product distribution on the Cu electrode, we presume that
hydrogen species composed of two hydrogen atoms is involved
with the CO reduction on smooth copper electrodes. Atomically
adsorbed hydrogen may be involved on roughened copper
surfaces as well as nickel and iron electrodes. Surface roughen-
ing likely introduced surface defects such as steps and vacancies
favorable for reaction of adsorbed hydrogen atoms.
1
7
inert. This reaction, as is well-known, proceeds with inter-
mediate formation of carbon and hydrogen atoms adsorbed on
17
the catalyst surface. These intermediates are converted to CH2
and further to hydrocarbons and alcohols. In analogy with the
Fischer-Tropsch reaction, the electrochemical reduction of CO
at Fe and Ni electrodes may proceed with intermediate formation
of carbon and hydrogen atoms adsorbed on the electrodes
leading to the formation of hydrocarbons and alcohols.
3
.2.2. Catalytic ActiVity in the Electrochemical Reduction
of CO and CO2. The Cu electrode is markedly active in CO
reduction compared with Ni and Fe as shown in Table 2. We
previously demonstrated that the adsorption strength of CO on
an electrode relates closely with the electrocatalytic activity of
electrode metals in CO and CO2 reduction. Adsorption of CO
on Cu is exothermic, and the heat of adsorption is much lower
than that on Ni and Fe. Such moderate strength of CO
adsorption bond to Cu facilitates the reduction of CO, leading
Since Cu is inert in the Fischer-Tropsch reaction, we expect
that the electrochemical reduction of CO proceeds with different
intermediate species. On the Cu electrode, neither ethane nor
-
3
methanol was produced in both 0.1 mol dm KHCO3 and
phosphate buffer solutions. Hydrogen is adsorbed on the copper
surface in a gas/solid system with some activation energy, and
1
8,19
the heat of adsorption is much lower than on Ni and Fe.
21
to efficient formation of hydrocarbons.
Thus the concentration of adsorbed hydrogen species on a
copper electrode in aqueous solution will be much lower than
that on nickel and iron. The ethylene formation would naturally
be more advantageous than ethane formation due to the limited
supply of adsorbed hydrogen compared with Ni and Fe
electrodes. However, the Cu electrode surface will be enriched
with adsorbed hydrogen species during CO reduction at low
pH, at which methane formation and hydrogen evolution are
predominant. Ethane was not detected under such conditions;
the limit of detection was 0.05% or less in the Faradaic
efficiency. The Cu electrode thus shows a unique electrocata-
lytic feature in addition to the high efficiency in hydrocarbon
formation in CO and CO2 reduction, distinguished from other
metal electrodes.
We have reported infrared absorption spectra of the CO
molecule adsorbed on Cu, Ni and Fe in a separate paper. The
infrared absorption of C-O stretching occurs in the following
order in wavenumber units: Cu > Ni(linear) > Fe > Ni-
bridged). In accordance with Blyholder’s relation between the
22
(
C-O stretching band frequency and the adsorption bond
23
strength of adsorbed CO, weak adsorption will lead to high
C-O stretching frequency, approaching the value of the free
CO molecule. Thus the order of the C-O stretching frequency,
or the reverse of the adsorption strength of CO, again agrees
with the activity order in CO reduction, Cu > Ni = Fe.
3.3. Partial Currents of the Products in the Electrochemi-
cal Reduction of CO. The partial currents of the products are
calculated from the Faradaic efficiency and the total electrolysis
current with various electrolytes of pH 6.0-12.2. The loga-
rithms of the partial currents of CH4, or log i(CH4), plotted
against the potential are widely scattered. However, the values
of log i(CH4) that were obtained in the electrolyte solutions of
the same composition were relatively well correlated. This fact
strongly suggests that i(CH4) depends on pH.
Figures 1 and 2 illustrate the partial currents for C2H4 and
C2H5OH as a function of the electrode potential. Figures 1 and
2 show relatively good correlations regardless of pH. Thus the
partial currents for C2H4 and C2H5OH do not depend on the
pH of the electrolytes.
A surface-roughened Cu electrode was prepared by oxidation
at 350 °C in the air and subsequent reduction with H2 below
2
0
2
00 °C. Repeated oxidation reduction treatments enhanced
the surface area of the Cu electrode by more than 2 orders of
magnitude according to a measurement of the double-layer
charging current. The surface-roughened Cu electrode yielded
a small amount of C2H6 with ca. 1% of Faradaic efficiency in
-
2
CO2 reduction at a constant current of 5 mA cm . No C2H6
is formed from CO or CO2 reduction with smooth Cu electrodes
electrochemically polished in 85% phosphoric acid or etched
with dilute HCl solution.

7078 J. Phys. Chem. B, Vol. 101, No. 36, 1997
Hori et al.
2 5
Figure 2. Partial current densities of C H OH formation in electro-
chemical reduction of CO correlated with the electrode potential in
various electrolyte solutions: pH 6.0-6.3 (O), pH 7.1-7.7 (4), pH
8
.0-8.6 (0), pH 8.7-8.9 (×), pH 9.0-9.3 (b), pH 10.5-11.3 (2),
Figure 3. log i(CH
4
) + pH correlated with the electrode potential:
pH 12.2 (9).
pH 6.0-6.3 (O), pH 7.1-7.7 (4), pH 8.0-8.6 (0), pH 8.7-8.9 (×),
pH 9.0-9.3 (b), pH 10.5-11.3 (2), pH 12.2 (9).
The Tafel equation will hold in electrochemical reactions
when an electrochemical process determines the overall rate.
Figure 1 gives a linear correlation between log i and E, and the
partial current of C2H4 formation is expressed in the form of
the Tafel equation.
i(C H ) ) A exp[-R F(E - E°)/RT]
(1)
2
4
e
where A is a constant, Re is the transfer coefficient for C2H4
formation, and the other symbols have the usual meanings. Re
is estimated at 0.35 from Figure 1.
If the partial current for CH4 is proportional to jth power of
+
proton activity [H ] and expressed in the form of the Tafel
equation, the following equation may be obtained
+
j
i(CH ) ) B[H ] exp[-R F(E - E°)/RT]
(2)
4
m
where B is a constant and Rm is the transfer coefficient in CH4
formation. Taking the logarithm of both sides of the equation,
one obtains
+
log i(CH ) - j log[H ] ) -R F(E - E°)/2.303RT + log B
4
Figure 4. log i(CH ) + 2pH correlated with the electrode potential:
4
m
pH 6.0-6.3 (O), pH 7.1-7.7 (4), pH 8.0-8.6 (0), pH 8.7-8.9 (×),
(3)
pH 9.0-9.3 (b), pH 10.5-11.3 (2), pH 12.2 (9).
or
log i(CH ) + pH )
decomposed on Ni, Pd, Fe, Co, Ru, and Cu at 1 atm and 25-
200 °C, forming C2H4 and N2 as the predominant products in
each case. The principal reaction is a simple dimerization of
CH2 fragments formed in the decomposition of CH2N2. When
CH2N2 mixed with H2 was passed over these materials, the
nature of the product changed markedly. CH4, C2H6, and higher
hydrocarbons up to C18 are produced on the metals other than
Cu. The authors concluded that CH2 groups were polymerized
on these surfaces, initiated by metal-hydride bonds (adsorbed
hydrogen). However, Cu metal still gave C2H4 as the predomi-
nant product in the reaction of CH2N2 and H2. Cu does not
readily dissociatively chemisorb H2, and adsorbed hydrogen is
not sufficiently provided to the initiation step in the oligomer-
ization of CH2. Thus the reaction of CH2N2 and H2 on Cu leads
predominantly to dimerization and ethylene formation.
4
-
R F(E - E°)/2.303RT + log B for j ) 1 (4)
m
or
log i(CH ) + 2pH )
4
-
R F(E - E°)/2.303RT + log B for j ) 2 (5)
m
Figure 3 presents the relationship between log i(CH4) + pH
and E, and Figure 4 that between log i(CH4) + 2pH and E.
Figure 3 apparently gives a well-correlated straight line, whereas
Figure 4 gives a curved line. Thus only j ) 1 fits the
experimental data with the Rm values 1.33 as calculated from
the linear correlation of Figure 3.
Hydrogen evolution reaction at the Cu electrode proceeds in
3.4. Hydrogen Species. In connection with the mechanism
2
5,26
the Heyrovsky reaction.
of the Fischer-Tropsch reactions, Brady and Pettit studied
gaseous reactions of diazomethane on transition metal surfaces.
They showed that CH2N2 diluted in He rapidly and quantitatively
24
+
-
H(ads) + H + e f H
(6)
2

Electrochemical Reduction of CO at a Cu Electrode
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7079
i ) i exp[-RF(E - E°)/RT] (10)
Horiuti et al. advocated in accordance with their quantum
mechanical calculations that the Heyrovsky reaction proceeds
0
+
with intermediate species H2 , which follows formation of
where i0 denotes the exchange current, R is the transfer
coefficient and other symbols have their usual meanings.
R is related to the symmetry factor of the rate-determining
adsorbed hydrogen.27
+
-
31
step â
H + e f H(ads)
(7)
(8)
+
+
R ) s/ν + âr
(11)
H(ads) + H f H (ads)
2
where s is the number of electrons passing before the rate-
determining step, and ν is the stoichiometric number and r the
number of electrons both in the rate-determining step. Conway
and Bockris showed that r should be 1 in electrochemical charge
transfers in accordance with their theoretical calculations of heats
Reactions 7 and 8 readily proceed, and the rate-determining step
is
+
2
-
H (ads) + e f H
(9)
2
3
2
of activation.
The value of ν in ethylene formation will be 1 or 2, as shown
later in connection with the reaction scheme. The transfer
coefficient of ethylene formation Re is 0.35 as determined from
Figure 1. Thus the value of s must be zero, and Re is equal to
âe, where the suffix e denotes ethylene formation. The partial
current of ethylene formation is not affected by proton activity
as described above. Therefore the rate-determining step of
ethylene formation may be the first electron transfer process or
simply the electron transfer to adsorbed CO.
+
If step 9 is the rate-determining step, H2 (ads) will be
abundant on the electrode surface. However, the presence of
+
H2 was disputed by many workers and has not yet received
experimental confirmation.2
8, 29
The process may be preferably
considered as a combined proton and electron transfer (eq 6),
similar to the discharge step 7, in which the active surface site
is an adsorbed hydrogen atom rather than a bare metal atom.
In this model, H2 (ads) is a transient transition state, rather than
a stable species with a comparatively long life time.
2
8
+
In CO-saturated solution, hydrogen evolution is greatly
prevented at Cu electrodes, and the coverage of CO on the
electrode surface amounts to 90% at -1.0 V,1 where the
reduction of CO starts. We presume under such a specific
-
•-
CO + e f CO
(12)
3
We previously depicted a reaction scheme of the electro-
chemical reduction of CO2 in analogy with Fischer-Tropsch3
+
situation that the existence of H2 (ads) is possible at the sites
reaction, in which atomic carbon is a precursor of hydrocarbons.
not covered with CO, or the hydrogen species reacts as a group
Intermediate formation of carbon at a Cu electrode surface in
electrochemical reduction of CO2 was reported on the basis of
XPS measurement, but it has not yet been confirmed by other
+
of H(ads) + H in stead of H(ads) in the subsequent reactions.
+
+
Thus H2 (ads) or H(ads) + H is involved with CO reduction,
6a
leading to the unique product distribution in CO reduction on
Cu electrodes. The hydrogen species, H2 (ads), or combined
H(ads) + H is tentatively represented as H2 (ads) for brevity
hereafter.
workers.
+
Kim et al. discussed electrochemical methanation of adsorbed
CO during CO2 reduction on the basis of their thermochemical
+
+
calculations. They concluded that CO is thermally dissociated
3.5. Rate-Determining Step of Ethylene Formation. We
to form surface carbon atoms at the Ru electrode.5 As
reported in the previous paper that adsorption of CO on a Cu
electrode is accompanied with an electron transfer according
to voltammetric measurements.14 In-situ infrared spectroscopy
at a Cu electrode showed an absorption of adsorbed CO at 2087
cm at -0.95 V, which corresponds to CO linearly adsorbed
on a metal surface without electron transfer to a CO molecule.
We suggested some cluster-like substance as an intermediate
species adsorbed on the surface in CO reduction on the basis
of the voltammetric measurements.
mentioned in the previous section, formation of surface carbon
atoms has been established as an intermediate step in the
Fischer-Tropsch reaction. Thus it is accepted that a surface
17
carbon atom is formed as a precursor to hydrocarbons in the
electrochemical reduction of CO on Ru, Ni, and Fe, catalytically
active metals in the Fischer-Tropsch reaction.
Copper is inert in the Fischer-Tropsch reaction. The infrared
spectroscopic measurements showed that CO is weakly adsorbed
on Cu with a C-O stretching band frequency closer to that of
a gaseous CO molecule than on Ni and Fe. Thus thermal
dissociation of CO to C and O atoms will not readily take place
on Cu.
-
1
However, electrolytic measurements using single-crystal
copper electrodes revealed that the Cu(111) surface can reduce
CO2 and CO to methane, whereas any charge transfer voltam-
3
0
metric peak does not appear with a Cu(111) electrode. We
then carried out an in-situ infrared spectroscopic measurements
of a Cu electrode surface, and observed that anion and CO are
competitively adsorbed on the surface. Thus we presumed that
the apparent charge transfer is derived from charge displacement
of specifically adsorbed anions by the adsorption of CO on the
electrode surface. The details will appear elsewhere. At present
we cannot propose a definite intermediate species for the CO
reduction. We discuss the reaction scheme under a hypothesis
that adsorbed CO molecules are involved in the reaction.
The formation of hydrocarbons and other products from CO
at a Cu electrode will proceed in a series of consecutive
multistep process. We assume that a rate-determining step
determines the overall rate of the reaction, and the steps prior
to the rate-determining one are in equilibrium. The partial
current for each product will follow the Tafel relationship, as
given below for the cathodic current
By electronation (eq 12) in the gas phase, however, CO is
-
33
readily decomposed to carbon atoms and O ions. Thus CO
-
is possibly decomposed to a carbon atom and an O ion, when
an electron is transferred to CO adsorbed on a Cu electrode.
Kim et al. proposed “electrochemical splitting of CO” in CO
reduction on a Cu electrode. They assumed the elementary step
of surface carbon atom formation on a Cu electrode.5
-
-
CO(ads) + H O + e f C(ads) + OH(ads) + OH (13)
2
They calculated the binding energies of adspecies, such as CO,
C, and OH, and rationalized the onset potential value in the
voltammogram of CO reduction.
Thus the electrochemical splitting of CO may possibly take
place as an intermediate step in the present ethylene formation
in the electrochemical reduction of CO on Cu. Since the
+
+
ethylene formation is independent of pH, H2 (ads) or H
+

7080 J. Phys. Chem. B, Vol. 101, No. 36, 1997
Hori et al.
H(ads) will not be involved in the process. Thus the rate-
determining step of ethylene formation may be eq 13 followed
by electronation of OH(ads).
TABLE 3: Current Efficiencies of the Products in
Electrochemical Reduction of Aldehydes at a Cu Electrode
a
in 0.1 M KHCO
3
products (%)
alcohols
-1.23 89.8 0.7 0.1 10.1 (CH OH)
-1.09 6.3 0.0 0.1 91.8 (C OH)
-1.12 45.1 0.2 0.0 57.7 (n-C OH)
concn/ potential/
-
-
OH + e f OH
(14)
aldehyde
HCHO
mM
V
H
2
CH
4
2 4
C H
61
18
27
3
C(ads) may further be reduced to CH2, the presence of which
is confirmed in the study of the dissociation of CH2N2, as
mentioned above.
CH
3
CHO
CHO
2 5
H
H
3 7
2
4
C
2
H
5
a
Potentials Vs SHE. Temperature 18 °C, 2.5 mA cm-2. pH is
estimated at 9.0 at the electrode surface during the reactions.
+
-
C(ads) + H2 + e f CH2
(15)
3
.6. Rate-Determining Step of Methane Formation.
Methane formation is proportional to proton activity with the
transfer coefficient 1.33 as shown in Figure 3. ν for methane
formation may be taken as 1, and r should be 1 as discussed
above. The transfer coefficient between 1 and 2 means on the
basis of eq 11 that an electron transfer process is in equilibrium
prior to the rate-determining step. We attempt to rationalize
the following reaction scheme by introducing a hypothetical
reaction intermediate COH.
The reaction
+
-
CO(ads) + H + e h COH(ads)
(16)
may be in electrochemical equilibrium. The coverage θ(COH)
will be given by
+
θ(COH) ) K θ(CO)[H ] exp(-FE/RT)
(17)
i
with Ki the equilibrium constant of eq 16, under the assumption
that θ(COH) is very low in comparison with θ(CO). The rate-
determining process is
-
-
COH(ads) + e f [COH] (RDS)
(18)
The reaction will further proceed consecutively. The rest of
the process may be expressed comprehensively as shown below:
-
+
-
-
[
COH] + 2H2 + 2e f CH + OH
(19)
4
The rate of methane formation is given by
Figure 5. Molecular reaction pathway diagram of the electrochemical
+
+
+
reduction of CO. H
f with a ∧ through it denotes a rate-determining step. The H
2
represents H
2
or a combined H(ads) + H . The
R(CH ) ) k θ(COH) exp(-â FE/RT) )
2
O and
4
i
m
-
OH resulting from the reactions are excluded for brevity.
+
k K θ(CO)[H ] exp[-(1 + â )FE/RT] (20)
i
i
m
of HCHO, CH3CHO, and C2H5CHO using the Cu electrode in
.1 M KHCO3. The results are given in Table 3. CH3CHO
was easily reduced to C2H5OH at the current density of 2.5 mA
cm , and C2H5CHO to n-C3H7OH. HCHO was reduced to
CH3OH with less current efficiency.
Table 1 shows that the alcohol formations are accompanied
by the corresponding aldehydes. We can reasonably conclude
that the alcohols are yielded with intermediate formation of the
corresponding aldehydes. Neither HCHO nor CH3OH is
produced at the Cu electrode.
where ki denotes the rate constant and âm the symmetry factor
in the electrode process (eq 18). θ(CO) can be taken as 1, and
the partial current i(CH4) is written as
0
-
2
+
i(CH ) ) FR(CH ) ) Fk K [H ] exp[-(1 + â )FE/RT]
4
4
i
i
m
(21)
We obtain Rm ) 1 + âm as a result of comparing eqs 21 and 4.
The symmetry factor âm is between 0 and 1 or probably close
to 0.5. Thus eq 21 is compatible with the experimental transfer
coefficient 1.33.
3.8. Molecular Reaction Pathway Diagram. We proposed
a molecular reaction pathway diagram for the electrochemical
3.7. Formation of Aldehydes and Alcohols. Figure 2
3
presents the partial current density of ethanol formation in
various pH solutions as a function of the potential. The partial
currents are low and not well correlated. If we attempt a linear
correlation, the value of transfer coefficient is ca. 0.6 and the
partial current is independent of pH. Ethanol may be formed
in a reaction process similar to ethylene formation. A small
amount of propanol was also produced in addition to ethanol.
Aldehydes are electrochemically reduced to the corresponding
alcohols at some metal electrodes.34 We studied the reduction
reduction of CO2 at a Cu electrode. The diagram showed that
both CH4 and C2H4 were formed with common intermediate
species of CH2 adsorbed on the electrode surface. Alcohol
formations were supposed to proceed with insertion of CO to
adsorbed CH2 with reference to ethanol formation in the gas
phase from CO + H2 catalyzed by Rh/TiO2 reported by
3
5
Takeuchi and Katzer.
In the light of the present experimental results, we revise the
molecular reaction pathway diagram as illustrated in Figure 5.

Electrochemical Reduction of CO at a Cu Electrode
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7081
The reaction paths of methane and ethylene formations are
separated at the very early stage of CO reduction in accordance
with the discussion given above. We presume that insertion of
(8) (a) Cook, R. L.; MacDuff, R. C.; Sammels, A. F. J. Electrochem.
Soc. 1990, 137, 607. (b) Ikeda, S.; Ito, T.; Azuma, K.; Ito, K.; Noda, H.
Denki Kagaku 1995, 63, 303. (c) Schwarz, M.; Cook, V. R. L.; Kehoe, K.;
MacDuff, R. C.; Patel, J.; Sammels, A. F. J. Electrochem. Soc. 1993, 140,
614.
(9) (a) Ito, K; Ikeda, S; Okabe, M. Denki Kagaku 1980, 48, 247. (b)
Hara, K.; Tsuneto, A.; Kudo, A.; Sakata, T. J. Electrochem. Soc. 1994,
+
CO to CH2(ads) leads to formation of CH2-CO(ads). H2 or
+
H(ads) + H will be involved in all the hydrogenation processes.
CH2-CO(ads) will be reduced to CH2dCH2.
1
1
41, 2097. (c) Hara, K.; Kudo, A.; Sakata, T. J. Electrochem. Soc. 1995,
42, L57.
CH -CO(ads) + 2H2+ + 2 e f CH dCH + H O (22)
-
(10) Saeki, T.; Hashimoto, K.; Kimura, N.; Omata, K.; Fujishima, A.
Chem. Lett. 1995, 361.
2
2
2
2
(
(
11) Shiratsuchi, R.; Nogami, G. J. Electrochem. Soc. 1996, 143, 582.
12) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Am. Chem. Soc.
Ethylene may possibly be formed by dimerization of CH2. CH2-
CO(ads) will be further reduced to acetaldehyde and eventually
to ethanol.
1
1
987, 109, 5022.
(13) Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc., Faraday Trans.
991, 87, 125.
(14) (a) Hori, Y.; Murata, A.; Tsukamoto, T.; Wakebe, H.; Koga, O.;
A small fraction of acetaldehyde may be transformed to vinyl
alcohol CH2dCHOH by keto-enol tautomerization equilibrium.
Yamazaki, H. Electrochim. Acta 1994, 39, 2495. (b) Hori, Y.; Koga, O.;
Yamazaki, H.; Matsuo, T. Electrochim. Acta 1995, 40, 2617.
(15) Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrochim. Acta
1994, 39, 1833.
Vinyl alcohol is labile but will be possibly present at low
concentration.36 An intermediate substance vinyl alcohol may
(
16) Hori, Y.; Murata, A. Bull. Chem. Soc. Jpn. 1991, 64, 123.
be hydrogenated to CH3-CH(ads), a homologue of CH2(ads).
CH dCHOH + H2+ + e f CH -CH(ads) + H O (23)
(17) Vannice, M. A. CATALYSIS: Science and Technology; Anderson,
J. R., Ed.; Springer-Verlag: Berlin, 1982, Vol. 3, p 140.
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(
-
2
3
2
(
John Wiley & Sons, New York, 1994; p 308.
(20) Koga, O.; Nakama, K.; Murata, A.; Hori, Y. Denki Kagaku 1989,
57, 1137.
CO is inserted in to CH3-CH(ads) in a similar way as CH2(ads),
further reduced to CH3-CH2-CHO and eventually to n-
propanol.
(
21) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. Chem. Lett. 1987,
665.
22) Koga, O.; Matsuo, T.; Yamazaki, H.; Hori, Y. Submitted for
publication.
1
(
Acknowledgment. This work was partially supported by the
Grant-in-Aid for Scientific Research on Priority Areas (Catalysis
(
(
(
23) Blyholder, G.; Allen, A. C. J. Am Chem. Soc. 1969, 91, 3158.
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07242212) from the Ministry of Education, Science and Culture
of Japan.
1
967; pp 517, 577.
(
(
26) Gerischer, H.; Mehl, W. Z. Elektrochem. 1955, 59, 1049.
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