Electrochem ical D ecom positionof
CFC-12U sing G as D iffusion
N O R I Y U K I S O N O Y A M A * A N D
T A D A Y O 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, Japan
Chlorofluorocarbons (CFCs) were known to cause the
depletion of the ozone layer at the stratosphere. A large
amount of CFCs is still in use as a refrigerant or still
presentintheplastic forms. TheseCFCsshouldbecollected
Electrochemical decomposition of dichlorodifluoromethane
gas diffusion electrodes (GDEs). Ag-, Cu-, In-, and Pb-
supported GDEs showed high electrocatalytic activity of
decomposition of CFC-12. Especially Cu-, In-, and Pb-
supported GDEs showed almost 100% efficiency without
GDEs causeddefluorinationof CFC-12as well as dechlo-
rination and produced methane mainly. Pb-supported
GDE induced only dechlorination of CFC-12 and produced
at 370 mA cm-2. The partial current density of HFC-32
formation at Pb-supported GDE was not saturated even at
Chlorofluorocarbons (CFCs) were known to cause the deple-
tion of the ozone layer at the stratosphere (1), and the
demanded (2). Recently, the production of specified CFCs
was discontinued. However, a large amount of CFCs is still
in use as a refrigerant or still present in the plastic forms.
These CFCs should be collected and retreated to harmless
compounds to the environment. At present, the treatment
of chlorofluorocarbons is mainly carried out by the incinera-
tion method. BecauseCFCsarevery persistent compounds
originally, thecombustion ofCFCsneedshigh temperature.
The cost of the construction of this high-temperature
incinerator is very high. Moreover, HF and HCl generated
by the combustion of chlorofluorocarbons corrode the
incinerator and shorten its lifetime. To improve these
disadvantages, the catalytic combustion method at low
temperature (3, 4) and the catalytic hydrogenation method
(5-8) have been studied. However, these methods are still
in the experimental stage, because the catalyst is poisoned
by HF and HCl.
The electrochemical method has the following advan-
tages: (a) Thereactioncanproceedunderamildercondition
than other methods. (b) The secondary pollution hardly
occurs, because reactive chemical agents are not used in
treatment. In spite of these advantages, electrochemical
decomposition of CFCs is hardly studied. Only Tezuka and
Iwasaki (9) reported dechlorination of difluorotetrachloro-
Hg-poor, Pt, and graphite paste electrodes as a cathode.
Recently, we have carried out electrochemical reductive
decomposition of chloroform in aqueous solution using 15
kinds of metal electrodes (10). By using Ag, Cu, Pd, Pb and
metals that have high electrocatalytic activity of dechlori-
nation of chloroform to the decomposition of difluoro-
dichloromethane (CFC-12). Reductive decomposition of
CFC-12 was carried out at room temperature using gas
diffusion electrodes (GDEs), which are suitable for the
electrochemical reaction of compounds in gas phase. GDE
consists of two regions: a hydrophilic reaction region and
penetrates only into the reaction region, and a gaseous
reactantissuppliedthroughthegasdiffusion region rapidly.
Electrochemical reactions occur on the metal fine grains
supported on the reaction region. GDE is porous and has
a very large reaction area. Therefore, it is appropriate for
electrochemical reaction at ahigh current density and often
ofGDE fordecomposition ofCFC-12depended on thekinds
of metals that were supported on GDE. By using Cu-, Ag-,
occurred, and difluoromethane(HFC-32) was produced in
Experim ental Section
Electrolyseswerecarried out in astainlesssteel autoclaveas
shown in Figure1. All electrolyseswerecarried out at 7atm
ofpoly(vinyl chloride),which isresistanttocorrosion byHF.
Tanaka Noble Metal Ltd. and used as working electrodes.
been described in a previous paper (16). A GDE (1 cm in
diameter) was fixed to the cell with a cap made of phenol
resin. The electrode potential of a cathode was measured
A Pt wire was used as an anode. The aqueous electrolyte
was 1.0 M NaOH (GR Wako Pure Chemical), which was
purified by pre-electrolysis with a Pt black cathode to
eliminate heavy metal impurities.
bubbled into the solution at least for 20 min to remove the
dissolved oxygen. CFC-12 (Mitsui-Dupon Fluorochemical
Ltd) wasintroduceddirectlyintotheautoclave. Electrolyses
were carried out galvanostatically (passage of 250 C) using
a potentiostat-galvanostat (Hokuto Model HA-501) con-
nected 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.
instrument equipped with an activated carbon column (4
mm ×2m) and athermal conduction detector(TCD) forH2,
(4 mm × 2 m) and a flame ionization detector (FID) for
VOL. 32, NO. 3, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9375
Purified N2 gas was
An Ohkura GC-802
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Environ. Sci. Technol. 1998, 32, 375-378
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1998 American Chemical Society
hydrocarbons, and an OhkuraGC-103instrumentequipped
with a Porapak QS column (4 mm × 2 m) and a FID for
HCFC-22 and HFC-32 were used for this purpose. The
carried out with a gas chromatography-mass spectrometer
(Hitachi Model M-80) equipped with a Porapak Q column
(4 mm × 2 m).
The faradaic efficiencies of main products by electrolyses
using 12kindsof metal-supported GDEsaresummarized in
Table 1, where faradaic efficiency is defined as the ratio of
charge used for the formation of the product to the total
chargeofelectrolysis. Asshown in Table1,thetotalfaradaic
efficiency of each metal-supported GDE varied widely. For
easy comparison of the activity of metal-supported GDEs,
12 and the selectivity of products by
Efris the efficiency of decomposition of CFC-12, Secand Sed
are the selectivity of the production of hydrocarbons and
that oftheproduction ofdifluoromethane(HFC-32), where,
FEc, FEd, FEH and FET are the faradaic efficiencies of
hydrocarbons, HFC-32, hydrogen, and the total of them,
respectively.As shown in Table 1, the efficiencies of
electrolyses were largely dependent on the kind of metals
supported on the GDEs. Ag, Cu, Pb, and In showed very
high activity in the decomposition of CFC-12. Especially,
Cu-, Pb-, and In-supported GDEsdecomposed CFC-12with
almost 100% efficiencies. Electrolysis at each active GDE
was carried out at least three times with an error of at most
5% in faradaic efficiency. Other metals (Ni, Pt, Ru, Pd, Co,
Cr, Zn, and Sn) showed low or no activity. The products of
electrolyses were also dependent on the supported metals.
Cu-, In-, Ag-, and Zn-supported GDEs produced methane,
chlorodifluoromethane (HCFC-22), and HFC-32. Ag-, Cu-,
low efficiencies. Pb-supported GDE produced HFC-32with
In-, Ag-, and Zn-supported GDE are able to cause deflori-
nation of CFC-12 as well as dechlorination. The C-F bond
of CFCs is very strong and difficult to cut off. Generally, a
very high energy [e.g., high temperature (17), plasma (18),
supercritical state (19), etc.] is needed to break it. The
experimental condition of the present study is 7 atm and
room temperature. As far as we know, this is the mildest
condition of CFC-12decomposition in which theC-F bond
of CFC-12 can be broken. This suggests that Cu-, In-, Ag-,
and Zn-supported GDEs have very strong electrocatalytic
activity of electrochemical decomposition of CFC-12.
In the electrochemical reaction in aqueous medium,
reductive reactions on the electrode compete with the
reduction of water. Therefore, the products of electrolyses
are expected to depend on the hydrogen overvoltage of the
electrodes. Hydrogen overvoltage means the potential of a
of the hydrogen overvoltage is largely dependent on the
metals used as the electrode. In a general electrochemical
reaction, the electrode with a low hydrogen overvoltage
mainly produces hydrogen, and the electrode with a high
tendency is adapted to electrolysis of CFC-12 to a certain
extent. As shown in Table 1, Pt-, Pd-, and Ru-supported
GDEs with low hydrogen overvoltage produced H2 with
greater than 90% efficiencies, while Zn-, Sn-, and Pb-
supported GDEs with high hydrogen overvoltage indicated
the activity of electro-decomposition of CFC-12 to some
extent. However, thistendency isnot adapted to themetals
with intermediate hydrogen overvoltages. For Cu, Ni, and
Ag, the order of the hydrogen overvoltage is Cu > Ni > Ag
(20). As was shown in Table 1, Ag- and Cu-supported GDEs
have a high activity of decomposition of CFC-12, while Ni-
supported GDE has no activity. In addition, Zn- and Sn-
activity of decomposition of CFC-12 is affected by factors
other than hydrogen overvoltage. One factor would be the
degree of adsorption of CFC-12 on the surface of the fine
grainsof metal. Thedetailed mechanism of decomposition
of CFC-12 cannot be obtained from the data in this paper.
However, judging from the products of electrolyses, decom-
position of CFC-12would consist of threestages; i.e., (1) the
(2) the elimination of Cl-ion induced by electron transfer
from the GDE, and (3) the elimination of F-ion induced by
electron transfer from the GDE. The second and the third
processes would be accompanied by hydrogenation caused
by the adsorbed hydrogen on the electrode surface. The
first processwould determinetheelectrocatalytic activity of
metals supported on GDEs, and the third process would
determine the final products of electrolyses. Pb seems to
have very low electrocatalytic activity of defluorination.
The dependence of faradaic efficiencies of products of
was shown in Figure 2. Electrolysis at each current density
FIGURE 1. Stainless steel autoclave and the electrolysis cell for
the electrochemical reduction of CFC-12 at 7 atm.
3769ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 3, 1998
was carried out at least twice with an error of 5% at most in
with the increase in the current density until 400 mA cm-2
and then decreased. Faradaic efficiency of hydrogen in-
creased with the increase in the current density. Faradaic
efficiencies of HCFC-22 and HFC-32 were almost indepen-
dent of the current density. Figure 3 is the relationship
between the partial current density of products formation
and thecurrent density at aCu-supported GDE. Thepartial
by current density. It means the amount of charge that is
in a second. The uncertainty of partial current density is
within 5% of the value of current density at each point. The
partial current density of methane formation increased in
proportion to the current density and was saturated at 500
mA cm-2. The maximum of the partial current density of
methane formation was 290 mA cm-2. The dependence of
a Pb supported GDE was shown in Figure 4. Faradaic
efficiency of HFC-32 formation was almost independent of
products were also independent of the current density and
almost 0%. It should be noted that a Pb-supported GDE
keptquiteahigh selectivity in theformation ofHFC-32even
at a Pb-supported GDE was shown in Figure 5. The partial
to the current density and did not reach saturation even at
650 mA cm-2. The partial current density of the formation
of other products (methane, H2, and HCFC-22) increased
graduallywiththeincreasein thecurrentdensity. However,
those values were much lower than that of HFC-32.
For the actual treatment of CFC-12, the conversion to
economically valuablecompoundsismostdesirable. In the
system of this paper, Pb-supported GDE is the most ap-
propriate electrode. Pb-supported GDE was found to
produce HFC-32 with high selectivity even at a high current
density. HFC-32 is the one of the substitutes of CFCs with
no ozone depletion potential. The catalytic conversion of
CFC-12 to HFC-32 has already been reported (7, 8). This
catalysis to convert CFC-12 into HFC-32. Electrochemical
decomposition method requires only aqueous solution as
the hydrogen source, a Pb-supported GDE, and the electric
current. In addition, HF and HCl, which deactivate the
catalysis, are able to be neutralized and fixed in solution
TABLE1. Faradaic Efficiency andEfficiency andSelectivity of Products Definedby Eqs 1-3 of Electrochem ical Decom positionof
CFC-12 U sing Various M etal-SupportedG as DiffusionElectrodes
faradaic efficiency (%)
aUnderthecondition: currentdensity 63.7mA cm-2at7atm ofCFC-12andatroom temperature.bPotential(V)correctedwithanIR compensation
instrument (vs Ag/AgCl).cDecomposition of CFC-12.dNot detected.
FIGURE 2. Current density dependence of faradaic efficiencies of
products using Cu-supported GDE. (O) methane, (4) HFC-32, (0)
HCFC-22, (]) H2.
32, (0) HCFC-22, (]) H2.
VOL. 32, NO. 3, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9377
immediately by adding alkaline to the electrolytic solution.
These advantages of the electrochemical method suggest
it can be appropriate for the conversion of CFC-12 to HFC-
32 in small scale.
From the above results, it is concluded as follows: Ag-,
Cu-, In-, and Pb-supported GDEs have high electrocatalytic
activity in electrochemical decomposition of CFC-12. Ag-,
Cu-, In-, and Zn-supported GDEs decompose CFC-12 into
selectivity. With the increase in the current density, the
partial current density of methane formation by Cu-sup-
ported GDE arrived at saturation.
produced HFC-32 in high selectivity even at 650 mA cm-2.
depletion potential. Therefore, Pb-supported GDE is the
most desirable in metal-supported GDEs for the actual
treatment of CFC-12.
We thank for Mr. Tohru Mori for carrying out the GC-MS
analyses. This study was partially financially supported by
the 1st Toyota High-Tech Research Grant Program.
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Received for review May 13, 1997. Revised manuscript re-
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FIGURE 4. Current density dependence of faradaic efficiencies of
products using Pb-supported GDE. (O) methane, (4) HFC-32, (0)
HCFC-22, (]) H2.
32, (0) HCFC-22, (]) H2.
3789ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 3, 1998