Electrochemical dechlorination of chlorinated hydrocarbons - Electrochemical reduction of chloroform in acetonitrile/water mixtures at high current density

Article abstract of DOI:10.1246/cl.2003.230

Electrochemical reduction of chloroform was studied in 0.1 M (1 M = 1 mol dm^-3) tetraethyl ammonium perchlorate solutions of acetonitrile-water mixture using various metal electrodes. High water concentration (1 M) promoted the electrolysis with the products mainly CH₄ and CH₂Cl₂. The partial current density of CH₄ formation amounted to 0.3 A cm^-2 at an Ag electrode.

Full text of DOI:10.1246/cl.2003.230

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30
Chemistry Letters Vol.32, No.3 (2003)
Electrochemical Dechlorination of Chlorinated Hydrocarbons – Electrochemical Reduction of
Chloroform in Acetonitrile/Water Mixtures at High Current Density
ꢀ
Yoshio Hori, Kazumi Murata, and Takayoshi Oku
Department of Materials Technology, Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522
(Received November 26, 2002;CL-021012)
Electrochemical reduction of chloroform was studied in
ꢁ3
0
.1 M (1 M = 1 mol dm ) tetraethyl ammonium perchlorate
solutions of acetonitrile–water mixture using various metal
electrodes. High water concentration (1 M) promoted the elec-
trolysis with the products mainly CH4 and CH2Cl2. The partial
ꢁ
2
current density of CH4 formation amounted to 0.3 A cm at an
Ag electrode.
Electrochemical reduction of chlorinated hydrocarbons may
contribute to a new detoxification process operated safely with a
compact device at ambient temperature and pressure. However,
the low solubility of chlorinated hydrocarbons limits the electro-
Figure 1. A time course of the current density during the
electrochemical reduction of CHCl3 at an Ag electrode at
þ
ꢁ
2:58 V vs Fc/Fc . Electrolyte: 0.1 M TEAP in acetoni-
trile. H O: 8 mM. CHCl : 20 mM.
2
3
ꢁ2
1{6
lysis current less than 1 mA cm
in aqueous solutions.
controlled potential electrolysis of chloroform at an Ag electrode
at ꢁ2:58 V with H2O concentration of 8 mM. The current density
steeply dropped in 5 min. Yellowish brown film was found to
cover the electrode surface after the electrolysis. The film was
insulative, although the substance was not identified. The rapid
drop of the current density in Figure 1 may be attributed to the film
formation. The current decay and the film formation were
observed for all the metal electrodes in the present study. Such
Application of nonaqueous solutions enhances the current density
as reported for reduction of 2,4-dichlorophenoxyacetic acid (2,4-
7
8
D) and CFC-113 in methanol based electrolytes with current
2
ꢁ
density of 100 mA cm or more. However, the electrodes are
9
deteriorated by formation of thick film or metal organic
1
0
products on the electrode surface during the electrolysis in
nonaqueous solutions, suppressing the current density. This paper
demonstrates electrochemical reduction of chloroform at high
current density in 0.1 M tetraethylammonium perchlorate
9
film formations were previously reported by others.
CH4 and CH2Cl2 were the main products in the present
measurements with small fraction of C2H4 less than 2% in the
current efficiency. An electrochemical reaction applied to
calculation of the current efficiency is exemplified for CH4
formation as below. Similar equations are used for other products.
(
TEAP) solutions of acetonitrile(AN) – water mixture. We also
attempt to reveal the catalytic activity of metal electrodes for
electrochemical dechlorination of chlorinated hydrocarbons.
The main features of the experimental procedures were
described in an article published from our laboratory. The
electrodes were wires or foils of 99.99% purity with apparent
1
1
ꢁ
ꢁ
ꢁ
CHCl3 þ 3H2O þ 6e ¼ CH4 þ 3Cl þ 3OH
Table 1 presents the current efficiencies of the products
obtained in the controlled potential reduction of chloroform. The
2
surface area ca. 0.6 cm , etched in 1:1 mixture of nitric acid and
water. Electrolysis cell was a one compartment type with the
anolyte and catholyte not separated. The potential is given with
respect to the ferrocene redox potential, whereas the reference
electrode was Ag/Ag (0.01 M AgNO3 in AN). The liqid junction
potential was corrected. The ohmic drop between the Luggin
capillary tip and the electrode was also corrected.
þ
Table 1. Electrochemical reduction of CHCl3
Electrode H2O/
Current efficiency/%
Av. C. D.a
ꢁ2
Metal
mM
CH4
CH2Cl2
H2
/ mA cm
The gaseous products were analyzed with gas chromato-
graphs. The gaseous products were taken from the gas phase, after
the gaseous products in the electrolyte were purged out to the gas
phase by supersonic irradiation. The products dissolved in the
liquid phase were analyzed by a gas chromatograph or a gas
chromatograph-mass spectrometer (Shimadzu QP5050).
Cyclic voltammograms were measured with various metal
electrodes with water concentration of 10 mM under argon
atmosphere. The cathodic current obtained with the electrolyte
solution containing 5 mM CHCl3 starts at much more positive
potential at all the electrodes than those without chloroform. Thus
the reduction of chloroform apparently proceeds more favorably
than hydrogen evolution at all the electrodes.
Ag
Zn
Pb
Cd
Cu
Fe
Au
Ni
Pt
8.2
4.5
14.9
4.9
16.2
2.7
3.7
7.4
60.3
53.6
37.9
20.1
3.7
1.6
0.7
0.2
0.2
3.7
7.4
0.2
1.9
0.7
0.5
0.7
1.6
0.8
1.5
3.8
0.4
2.13
2.11
2.43
3.03
1.05
0.64
1.42
0.64
0.58
1.13
23.9
28.1
52.1
43.2
43.2
51.0
48.7
45.5
7.5
3.9
Sn
0.2
Electrolyte: 0.1 M TEAP/ AN, CHCl3: 10 mM (20 mM for
Ag). Electrode potential: ꢁ2:88 V vs Fc/Fc for Zn and
þ
ꢁ
2:58 V for other electrodes. Total electricity in the electro-
lyses: 3 to 16 C. Av. C. D.: Average current density.
Figure 1 shows a time course of the current density in a
a
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.3 (2003)
231
electrode potential was ꢁ2:88 V for Zn, and ꢁ2:58 V for other
electrodes. The concentration of chloroform was 20 mM for the
Ag electrode, and 10 mM for the other electrodes. The current
density changed during the electrolysis as mentioned above, and
the average current density are given inTable 1. The total value of
the current efficiencies are less than 100%. The rest of the current
efficiencies may be attributed to the brown film formation on the
electrode, other products not given in Table 1, and gaseous
products dissolved in the electrolyte solution not successfully
recovered by the present experimental procedures.
We studied the effect of H2O on the reduction of chloroform
at the Ag electrode. Electrolyses in the electrolytes with H2O
concentration exceeding 970 mM did not cause film formation on
the electrode;the current density remained stably high during the
electrolyses. Table 2 shows that higher H2O concentration as
well as chloroform concentration significantly enhances the
current density. The average current density amounted to
Table 3. Electrochemical reduction of CHCl3 in the electrolytes
with high H2O concentration
a
Electrode
Metal
Current efficiency/%
Av. C. D. /
ꢁ2
CH4
CH2Cl2
CH3Cl
H2
mA cm
Ag
Cd
Cu
In
Pd
Zn
Pt
Au
Ni
Fe
Sn
Ti
58.5
53.4
43.6
38.7
41.7
49.4
29.2
26.0
8.7
6.7
2.6
2.5
1.4
7.3
7.7
1.3
7.8
2.3
2.4
0.9
3.9
0.2
0.8
0.1
10.5
0.2
2.1
0.4
18.8
6.9
121.0
73.5
90.3
74.6
56.3
68.7
190.2
68.5
35.1
36.3
3.6
20.7
20.2
19.5
33.5
45.1
31.4
44.6
82.7
80.2
98.2
39.0
9.1
17.0
0.6
2.0
0.3
0.7
37.5
1.8
ꢁ
2
3
58 mA cm with the partial current of CH4 formation exceed-
Electrolyte: 0.1 M TEAP/ AN, H2O: 1 M, CHCl3: 50 mM.
ꢁ2
þ
a
ing 300 mA cm . Such a high current density, equivalent to that
of the industrial electrolytic process, encourages further intensive
studies aiming at industrial applications of electrochemical
detoxification of chlorinated hydrocarbons.
Electrode potential: ꢁ2:58 V vs Fc/Fc . Av. C. D.: Average
current density.
The metal electrodes may be classified into 3 groups in
accordance with the product selectivity. Ag electrode reduces
chloroform to CH4 at high current density, i.e. to the highest
degree of reduction. Ni, Fe, Sn and Ti reduce chloroform to
CH2Cl2 as the major product;the degree of reduction is lower, and
the current densities are lower than the other electrodes. Cd, Cu,
Pb, In, Zn, Pt and Au belong to the intermediate group;these
electrodes yield both CH4 and CH2Cl2 with comparable yields.
The current densities are also intermediate between the two
groups with the exception of Pt. The trend of the product
selectivity of metal electrodes agrees roughly with Sonoyama et
Table 2. Electrochemical reduction of CHCl3 at an Ag electrode
Av. C. D.b
a
Conc. /mM
Current efficiency/%
ꢁ2
H2O CHCl3 CH4 CH2Cl2 CH3Cl H2 /mM cm
c
2
9
44
73
10
10
50
100
100
65.5
55.4
58.5
79.8
78.5
n. a.
n. a.
6.7
n. a.
n. a.
2.6
2.6
2.7
0.1
2.8
0.8
0.4
0.5
21
31
121
279
358
1
1
1
008
036
993
9.2
10.4
1
al. s’ obtained in aqueous media. The product selectivity CH4/
Electrolyte: 0.1 M TEAP/ AN. Electrode potential: ꢁ2:58 V
þ
c
a
b
CH2Cl2 in the present study is in parallel with the current density.
The electrocatalytic activity for dechlorination of chloroform
may be evaluated by the selectivity and the current density.
vs Fc/Fc . Conc.: Concentration. Av. C. D.: Average current
density. n. a.: not analyzed.
We studied the electrochemical reduction of chloroform at
other metal electrodes in 0.1 M TEAP/AN with 1000 mM H2O.
No film formation was observed on the electrodes;the current
density was nearly constant during the electrolyses. Table 3
presents the results of controlled potential electrolyses at various
metal electrodes at ꢁ2:58 V with 1000 mM H2O and 50 mM
CHCl3. Higher water concentration evidently promotes the
reduction of chloroform at all the metal electrodes.
Mr. K. Sasaki carried out part of the experimental work.
References
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2
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3
4
5
A control electrolysis experiment was conducted at ꢁ2:58 V
with the Ag electrode in 0.1 M TEAP/AN with 1 M H2O without
chloroform. The average current density was as low as
ꢁ
2
ꢁ
1:3 mA cm , and the gaseous product was H2 with trace
amounts of CH4 and C2H4 probably produced from decomposi-
tion of AN or TEAP. Therefore the products in the present study
were obtained from the reduction of chloroform. Another control
measurement was tested;no electrolysis current was given with
the Ag electrode and using identical procedures. Neither CH4 nor
CH2Cl2 formation was observed at all.
6
7
8
9
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1
media. Rondinini et al. also reported high activity of Ag for
reduction of organic halides incomparison with glassy carbon and
9
Hg electrodes.