Electrochemical hydrogenation of CFC-13 using metal-supported gas diffusion electrodes

Article abstract of DOI:10.1021/es980122f

Chlorofluorocarbons(CFCs) are known to cause the depletion of the ozone layer at the stratosphere. A large amount of CFCs still in use should be collected and retreated to harmless compounds to the environment. Electrochemical hydrogenation of CFC-13 (CClF₃), which is the most stable of the C1 chlorofluorocarbons, was attempted with 13 kinds of metal-supported porous carbon gas diffusion electrodes (GDEs). In the case of the electrolysis at a Cu-supported GDE, both dechlorination and defluorination of CFC-13 proceeded, and methane was produced. With a Ag-supported GDE, dechlorination proceeded selectively, and HFC-23 (CHF₃) was produced. The faradaic efficiency for hydrogenation of CFC-13 depends on the current density, the pressure of CFC-13, the composition of the electrolyte solution, and the potential imposed on the GDE. If these factors are optimized, the faradaic efficiency for hydrogenation of CFC-13 was improved up to 77.7%. Chlorofluorocarbons(CFCs) are known to cause the depletion of the ozone layer at the stratosphere. A large amount of CFCs still in use should be collected and retreated to harmless compounds to the environment. Electrochemical hydrogenation of CFC-13 (CCIF₃), which is the most stable of the C1 chlorofluorocarbons, was attempted with 13 kinds of metal-supported porous carbon gas diffusion electrodes (GDEs). In the case of the electrolysis at a Cu-supported GDE, both dechlorination and defluorination of CFC-13 proceeded, and methane was produced. With a Ag-supported GDE, dechlorination proceeded selectively, and HFC-23 (CHF₃) was produced. The faradaic efficiency for hydrogenation of CFC-13 depends on the current density, the pressure of CFC-13, the composition of the electrolyte solution, and the potential imposed on the GDE. If these factors are optimized, the faradaic efficiency for hydrogenation of CFC-13 was improved up to 77.7%.

Full text of DOI:10.1021/es980122f

Environ. Sci. Technol. 1998, 32, 4005-4009
2 3
oxidative decomposition on an Fe O catalyst supported on
Electrochemical Hydrogenation of
CFC-13 Using Metal-Supported Gas
Diffusion Electrodes
an activated carbon carrier (10). They reported that the
temperature at which the conversion of CFC-13 reached 100%
was more than 100 °C higher (570 °C) than that of CFC-12.
3
Hydrogenation of CFC-13 to HFC-23 (CHF ) by hydrogen
was reported to proceed in a Cu tube and a Pd-lined tube
at 650-720 °C with 70% efficiency (11). Hydrogenation of
CFC-13 is also reported to be caused by methanol, used as
a hydrogen source, in a Ni tube at 550 °C with a 66%
conversion (13). Decomposition of CFC-13 was not so noted
as that of CFC-12 because the amount of CFC-13 used was
much less than that of CFC-12. However, CFC-13, which is
more stable than CFC-12, is produced via disproportionation
of CFC-12 (14) and the process of decomposition of CFC-12
N O R I YU K I S O N O YA M A * A N D
T A D A YO S H I S A K A T A
Department of Electronic Chemistry, Interdisciplinary
Graduate School of Science and Engineering,
Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8502, Japan
(
15). Therefore, decomposition of CFC-13 is also necessary
to decompose CFC-12 completely.
Chlorofluorocarbons(CFCs) are known to cause the
depletion of the ozone layer at the stratosphere. A large
amount of CFCs still in use should be collected and
retreated to harmless compounds to the environment.
Electrochemical hydrogenation of CFC-13 (CClF3), which is
the most stable of the C1 chlorofluorocarbons, was
attempted with 13 kinds of metal-supported porous carbon
gas diffusion electrodes (GDEs). In the case of the
electrolysis at a Cu-supported GDE, both dechlorination
and defluorination of CFC-13 proceeded, and methane was
produced. With a Ag-supported GDE, dechlorination
proceeded selectively, and HFC-23 (CHF3) was produced.
The faradaic efficiency for hydrogenation of CFC-13 depends
on the current density, the pressure of CFC-13, the
composition of the electrolyte solution, and the potential
imposed on the GDE. If these factors are optimized, the
faradaic efficiency for hydrogenation of CFC-13 was
improved up to 77.7%.
In this work, we carried out electrochemical hydrogena-
tion of CFC-13 using metal-supported GDEs at room tem-
perature. The catalytic activity of hydrogenation of CFC-13
and products depended on the kind of metals supported on
GDEs. Methane was produced from an electrolysis of CFC-
1
3 at a Cu-supported GDE. By using a Ag-supported GDE,
dechlorination of CFC-13 proceeded selectively, and HFC-
23 (CHF ) was produced.
3
Experimental Section
All the electrolyses were carried out in a stainless steel
autoclave as described in a previous paper (4). A cell
equipped with GDEs was made of poly(vinyl chloride), which
is resistant to corrosion by HF. GDEs and Ru- and Pt-
supported GDEs were purchased from Tanaka Noble Metal
Ltd. and used as working electrodes. Other metals (Ni, Zn,
Ag, Cu, Pd, Pb, Co, Fe, Sn, Cr, and In) were supported on the
GDEs by the impregnation method (16). The potential of
the working electrode was measured with respect to an Ag/
AgCl/ saturated KCl reference electrode. A Pt wire was used
as a counter electrode. The electrolyte solution was a 1:1
volume mixture of water and methanol containing 1 M NaOH
Introduction
Chlorofluorocarbons (CFCs) are known to cause the depletion
of the ozone layer at the stratosphere. Recently, the
production of specified CFCs was discontinued. However,
a large amount of CFCs are still in use as refrigerants, etc.
It is a serious subject to recover these CFCs and to convert
them into harmless compounds to the environment. Gen-
erally, decomposition of CFCs requires high energy, e.g., high
temperature (1), plasma (2), supercritical state (3), etc. In
a previous paper, we reported electrochemical hydrogenation
(
(
GR Wako Pure Chemical) unless otherwise noted. Methanol
GR Wako Pure Chemical) was used without further puri-
2
fication. Purified N gas was bubbled into the solution for
at least 20 min to remove dissolved oxygen. CFC-13 (Syouwa
Denko) was introduced directly into the autoclave. Elec-
trolyses were carried out galvanostatically (passage of 250
°
C) with the aid of a potentiostat-galvanostat (Hokuto model
HA-305) connected in series with a coulomb-ampere-hour
meter (Hokuto model HF-201). The potential of a cathode
was corrected with an IR compensation instrument (Hokuto
model HI-203). The sampled gas from the autoclave was
analyzed by gas chromatography. An Ohkura GC-802
instrument equipped with an activated carbon column (4
mm × 2 m) and a thermal conductivity detector (TCD) for
2 2
of CFC-12 (CCl F ) using metal-supported gas diffusion
electrodes (GDEs) (4). A GDE has very porous structure and
consists of two regions; a reaction region made of a
hydrophilic carbon and a gas diffusion region made of a
hydrophobic carbon (5, 6). Electrolytic solution penetrates
only into the reaction region, and a gaseous reactant is
supplied through the gas diffusion region rapidly. Therefore,
a GDE is appropriate for the electrochemical reaction with
gaseous reactants. With this system, hydrogenation of CFC-
2
H , an Ohkura GC-202 instrument equipped with a VZ-10
column (4 mm × 2 m), and a flame ionization detector (FID)
for hydrocarbons, an Ohkura GC-103 instrument equipped
with a Porapak QS column (4 mm × 2 m), and a FID for
HFC-23 were used for this purpose. Identification of
fluorocarbons produced in electrolyses was curried out with
a gas chromatograph-mass spectrometer (Hitachi model
M-80) equipped with a Porapak Q column (4 mm × 2 m).
1
2 proceeds in a very high faradaic efficiency at room
temperature. In this paper, we attempted hydrogenation of
CFC-13, which is the most stable and difficult to decompose
among C1 chlorofluorocarbons (3, 7-9), to extend this
electrochemical method.
There are few reports of decomposition of CFC-13 as far
as we know (10-13). Okazaki and Kurosaki carried out
Results and Discussion
One of the differences between CFC-12 and CFC-13 is the
solubility in water as well as stability. The solubility of CFC-
12 in water is 0.026 g/ 100 g, whereas CFC-13 is hardly soluble
*
Corresponding author Tel: +81-45-924-5400; fax: +81-45-924-
5
489; e-mail: sonoyama@echem.titech.ac.jp.
1
0.1021/es980122f CCC: $15.00

1998 Am erican Chem ical Society
VOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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Published on Web 10/29/1998

TABLE 1. Faradaic Efficiencies for the Hydrogenation of CFC-13 and for Product Formation in Electrolyses of CFC-13 at Various
a
Metal-Supported Gas Diffusion Electrodes
faradaic efficiency (%)
supported metal
Eb (V)
hydrogenation
CH4
CHF3
H2
total
Ni
Pt
-1.27
-0.92
-1.47
-1.13
-1.00
-1.03
-1.9
-1.51
-1.35
-1.30
-1.49
-1.45
-1.53
0.8
1.7
4.9
60.3
51.2
2.0
1.3
0.5
1.0
0.3
6.6
0.5
2.1
0.3
1.3
1.3
3.3
21.0
1.5
0.5
0.4
3.6
57.0
30.2
0.5
1.3
nd
0.5
nd
6.0
nd
80.1
79.3
78.8
19.1
39.3
69.2
83.0
77.2
73.4
71.5
67.1
72.3
79.0
80.9
81.0
83.7
79.4
90.5
71.2
84.3
77.7
74.4
71.8
73.7
72.8
81.1
Zn
Ag
Cu
Ru
Pd
Pb
Co
Fe
Sn
Cr
In
c
nd
0.5
0.5
0.3
0.6
0.5
0.3
1.8
a
Under the conditions: current density 31.9 m A cm -2 at 30 atm of CFC-13 and at room tem perature. b Potential (V) corrected with an IR
com pensation instrum ent (vs Ag/AgCl). c Not detected.
in water (17). By use of a GDE, a gaseous reactant is supplied
to the GDE through the gas diffusion layer and reacts at the
catalyst layer of the electrode efficiently (5, 6). However, it
seems difficult that an insoluble gas reacts efficiently at the
GDE because the actual electrode reaction occurs at the
interface between gas and liquid phases. In our electro-
chemical system, the adsorbed hydrogen on the electrode
that hydrogenates CFCs is obtained from the reduction of
water. Therefore, water is an indispensable solvent to the
electrolyte solution in our system. As the electrolyte solution,
we adopted a 1:1 volume mixture of water and methanol in
which CFCs are relatively soluble, containing 1 M NaOH.
Faradaic efficiencies for hydrogenation of CFC-13 and
that for the formation of products in electrolyses at 13 kinds
of metal-supported GDEs are summarized in Table 1.
Faradaic efficiency is defined as the ratio of the charge used
for hydrogenation or the formation of the product to the
total charge passed during the electrolysis. None of the metal-
supported GDEs showed electrocatalytic activity for hydro-
genation of CFC-13 except for Cu- and Ag-supported GDEs;
i.e., the main product of electrolyses at metal-supported GDEs
other than Cu and Ag was hydrogen produced by the
reduction of water. Ag- and Cu-supported GDEs carried out
hydrogenation of CFC-13 with a faradaic efficiency of 51-
6
2
0%. The efficiency for hydrogenation of CFC-13 was about
5% lower than that of CFC-12 (4). Instead of hydrogenation
of CFC, the faradaic efficiency for hydrogen formation
increased. Products of electrolyses were also dependent on
the kind of metals that were supported on GDEs. At a Cu-
supported GDE, methane was produced with a faradaic
efficiency of 21.0%. This result indicates that the C-F bond
of CFC-13 can be broken at room temperature in an
electrolysis at the metal-supported GDE. Among the C1
chlorofluorocarbons, whose C-F bond is known to be very
strong, the C-F bond of CFC-13 is the strongest one, and a
lot of energy is needed to break it (2, 7-9). There has been
no report of decomposition of CFC-13 at room temperature
as far as we know. Therefore, a Cu-supported GDE has a
high electrocatalytic activity even for decomposition of CFC-
FIGURE 1. Relationship between current density and the faradaic
efficiencies for the hydrogenation of CFC-13 and for product formation
at (a) Cu-supported GDE and (b) Ag-supported GDE at 30 atm of
CFC-13 in 1:1 mixed solvent of methanol and water. b, hydrogenation
of CFC-13; O, methane formation; 3, HFC-23 formation; ), hydrogen
formation.
1
3. By using a Ag-supported GDE, we produced HFC-23
(
CHF ) with a faradaic efficiency of 57.0%, and methane was
3
hardly produced. This result shows that dechlorination of
CFC-13 proceeds selectively at a Ag-supported GDE.
From the results mentioned above, it became clear that
Cu- and Ag-supported GDEs have electrocatalytic activity
for hydrogenation of CFC-13. However, the activity is not
so high under the present condition as in the case of CFC-12.
Therefore, we sought good conditions for the electrolysis of
CFC-13 at metal-supported GDEs by investigating the effect
of current density, pressure of CFC-13, and electrolyte
solution on the faradaic efficiency for hydrogenation of CFC-
13. Figure 1 shows the relationship between the current
density and faradaic efficiencies for hydrogenation and for
product formation in electrolyses of CFC-13 at Ag- and Cu-
supported GDEs. At a Cu-supported GDE, the faradaic
efficiencies for hydrogenation of CFC-13 decreased with an
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 24, 1998

FIGURE 2. Relationship between pressure of CFC-13 and the faradaic
efficiencies for the hydrogenation and for product formation of CFC-
FIGURE 3. Relationship between the percentage of methanol in the
electrolyte solution and the faradaic efficiencies for the hydrogena-
tion of CFC-13 and for product formation at (a) Cu-supported GDE
and (b) Ag-supported GDE at 31.9 mA cm at 30 atm of CFC-13. b,
hydrogenation of CFC-13; O, methane formation; 3, HFC-23 formation;
1
3 at (a) Cu-supported GDE and (b) Ag-supported GDE at 31.9 mA
-
2
cm in 1:1 mixed solvent of methanol and water. b, hydrogenation
of CFC-13; O, methane formation; 3, HFC-23 formation; ), hydrogen
formation.
-
2
), hydrogen formation.
increase in the current density and reached almost 0% at 380
Judging from the effect of the pressure of CFC-13 on the
-
2
mA cm
. The faradaic efficiency for the formation of
efficiency for hydrogenation mentioned above, the propor-
tion of methanol to water in the electrolyte solution is
expected to affect the faradaic efficiency for hydrogenation.
CFC-13 is relatively more soluble in methanol than in water.
Therefore, the faradaic efficiency for hydrogenation of CFC-
hydrogen increased with an increase in current density.
Hydrogen was the major product in the region where the
-
2
current density was higher than 50 mA cm . In the case of
a Ag-supported GDE, the faradaic efficiency for hydroge-
nation also decreased with an increase in the current density
with the slower rate of decrease in the faradaic efficiency
than at the Cu-supported GDE; the faradaic efficiency for
1
3 is expected to increase with an increase in the ratio of
methanol in the electrolyte solution. Figure 3 shows the
dependence of faradaic efficiencies for hydrogenation and
for product formation in electrolyses of CFC-13 at Cu- and
Ag-supported GDEs on the percentage of methanol in the
electrolyte solution. At a Ag-supported GDE, the faradaic
efficiency for hydrogenation of CFC-13 was 50% in water
and increased with an increase in the percentage of methanol
in the electrolyte solution, as was expected. In an electrolyte
solution containing 70% methanol, the faradaic efficiency
for hydrogenation of CFC-13 increased to 77.7%, with a high
selectivity for HFC-23 formation (74.1% in faradaic efficiency).
In a solution that contains more than 70% methanol, the
electrolyte (1 M NaOH) was not soluble. The moderate
activity for hydrogenation of CFC-13 in water could be
attributed to a certain extent of the concentration of CFC-13
in water under high pressure. At a Cu-supported GDE, the
faradaic efficiency for hydrogenation of CFC-13 increased
with an increase in the percentage of methanol in the
electrolyte solution until 50%. In the solution of 70%
methanol, the faradaic efficiency for hydrogenation decreased
-
2
hydrogenation was approximately 55% at 120 mA cm and
-
2
2
5% at 380 mA cm
.
Figure 2 shows the relationship between the pressure of
CFC-13 and the faradaic efficiencies for hydrogenation and
for product formation in electrolyses of CFC-13 at Cu- and
Ag-supported GDEs. At both Ag- and Cu-supported GDEs,
hydrogenation of CFC-13 hardly occurred at atmospheric
pressure. Faradaic efficiencies for the formation of products
in the electrochemical hydrogenation of CFC-13 increased
with an increase in the pressure of CFC-13. This result could
be attributable to the increase in the concentration of CFC-
1
3 in the electrolyte solution with an increase in the pressure
of CFC-13. Because of the low solubility of CFC-13, the
production of hydrogen should be dominant at atmospheric
pressure. As the concentration of CFC-13 in the solution
increases in proportion to the pressure of CFC-13, according
to Henry’s law, reduction of CFC-13 would proceed more
efficiently.
VOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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negative with the increase in the ratio of methanol in the
electrolyte solution due to the higher electrical resistance of
a methanol solution as compared with that of an aqueous
solution. The potential imposed on a Cu-supported GDE
changed from -1.5 V in 50% methanol electrolyte solution
to -3.2 V in 70% methanol electrolyte solution, whereas the
potential on a Ag-supported GDE changed from -1.4 V in
5
0% methanol to -2.6 V in 70% methanol. As was shown
in Figure 4, a Ag-supported GDE is able to hydrogenate CFC-
3 even at -3 V. Therefore, the faradaic efficiency for
1
hydrogenation of CFC-13 at a Ag-supported GDE should
increase with an increase in the percentage of methanol in
the electrolyte solution because of the increase in the
concentration of CFC-13 in the electrolyte solution, even in
7
0% methanol electrolyte solution (Figure 3). At a Cu-
supported GDE in the 70% methanol solution, it is disad-
vantageous to hydrogenate CFC-13 at -3.2 V, and the faradaic
efficiency for hydrogenation decreases suddenly as was
shown in Figure 3 in spite of the increase in the concentration
of CFC-13 in the electrolyte solution.
In a previous paper (4), we presumed that the mechanism
for the electrochemical hydrogenation of CFC-12 is as
follows: (i) the adsorption of a CFC on the surface of the fine
-
grain of metal on a GDE, (ii) the elimination of Cl ion induced
by electron transfer from the GDE, and (iii) the elimination
-
of F ion induced by electron transfer from the GDE. The
second and third processes would be accompanied by
hydrogenation due to the adsorbed hydrogen on the electrode
surface. The first process would determine the electrocata-
lytic activity of metals supported on GDEs, and the third
process would determine the final products of the electrolysis.
The results of hydrogenation of CFC-13 can also be explained
by this mechanism. In the electrochemical hydrogenation
of CFC-12, a Cu-supported GDE produced methane and HFC-
3
2 2
2 (CH F ) with faradaic efficiencies of 36.6% and 15.5%,
FIGURE 4. Relationship between the potential imposed on a GDE
vs Ag/AgCl and the faradaic efficiencies for the hydrogenation of
CFC-13 and for product formation at (a) Cu-supported GDE and (b)
Ag-supported GDE at 30 atm of CFC-13 in 1:1 mixed solvent of
methanol and water. b, hydrogenation of CFC-13; O, methane
formation; 3, HFC-23 formation; ), hydrogen formation.
respectively. At a Ag-supported GDE, methane and HFC-32
were produced with faradaic efficiencies of 16.3% and 50.8%,
respectively. HFC-32 was produced by the selective elec-
trochemical dechlorination of CFC-12 in a manner similar
to the production of HFC-23 in the electrolysis of CFC-13.
In the electrochemical hydrogenation of CFC-13, as sum-
marized in Table 1, faradaic efficiencies for methane forma-
tion and HFC-23 formation were 21.0% and 30.2% at a Cu-
supported GDE and 3.3% and 57.0% at a Ag-supported GDE,
respectively. For these metals, the faradaic efficiency for
methane formation in hydrogenation of CFC-13 decreased
as compared to that of CFC-12, whereas the faradaic efficiency
for formation of HFCs (the faradaic efficiency for the selective
dechlorination) increased. This change in electrocatalytic
activity is explained according to the mechanism mentioned
above. (i) CFC-13 would be adsorbed on the surface of these
metals to a certain extent. (ii) Dechlorination would proceed
efficiently, because dechlorination requires a smaller energy
than does defluorination. (iii) Due to the stronger C-F bond
of CFC-13 than that of CFC-12 (7-9), the defluorination of
CFC-13 would proceed less efficiently than in the case of
CFC-12. As a result of these three steps, the efficiency for
the dechlorination increased, whereas that of the defluori-
nation decreased. Pb and In, which showed no electro-
catalytic activity for hydrogenation of CFC-13, exhibited high
activity for the hydrogenation of CFC-12. Concerning these
metals, the change in electrocatalytic activity might be
attributable to the degree of adsorption on the surface of
metals, i.e., CFC-13 seems hardly to be adsorbed on the
surface of In and Pb.
suddenly. The activity of a Cu-supported GDE in water is
lower than that of a Ag-supported GDE. This suggests that
the adsorption of CFC-13 on the surface of Cu is weaker than
that of Ag. The result in the solution of 70% methanol cannot
be explained by the concentration of CFC-13. This would
be elucidated by knowledge of the potential imposed on the
GDEs during electrolyses. Figure 4 is the relationship
between the steady potential during the electrolyses and the
faradaic efficiencies for hydrogenation and for product
formation. The faradaic efficiencies in Figure 1 are plotted
again in Figure 4 vs the potential imposed on the GDE during
electrolyses. At a Ag-supported GDE, the efficiency for
hydrogenation of CFC-13 decreased gradually as the potential
imposed on the cathode GDE was made more negative, until
it reached -4.4 V. When the cathode potential was made
more negative, the efficiency for hydrogenation of CFC-13
decreased steeply. The efficiency for hydrogenation of CFC-
1
3 at a Cu-supported GDE decreased more rapidly with an
increasingly negative potential than at a Ag-supported GDE.
The efficiency for hydrogenation at a Cu-supported GDE at
-
4.3 V was about 20%, whereas that at a Ag-supported GDE
was 50% at -4.4 V; i.e., a Ag-supported GDE is able to
hydrogenate CFC-13 more efficiently at very negative po-
tential than a Cu-supported GDE. The increasing negative
potential shown in Figure 4 is accompanied by an increase
in the current density at the GDE. The increasing negative
potential is also related to the change in the electrolyte
solution. The potential imposed on a GDE became more
From the results mentioned above, it is concluded as
follows. Cu- and Ag-supported GDEs show electrocatalytic
activity for hydrogenation of CFC-13. At a Cu-supported
GDE, methane and HFC-23 are produced, whereas HFC-23
is produced selectively at a Ag-supported GDE. Optimizing
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 24, 1998

some of the experimental factors causes the faradaic efficiency
for hydrogenation of CFC-13 to be improved up to 77.7%.
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Acknowledgments
(
10) Okazaki, S.; Kurosaki, A. Chem. Lett. 1989, 1901-1904.
We thank for Mr. Tohru Mori for carrying out the GC-MS
analyses. This study was financially supported partly by the
first Toyota High-Tech Research Grant Program.
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Received for review February 5, 1998. Revised manuscript
received July 2, 1998. Accepted September 24, 1998.
1
3
ES980122F
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