Electrochemical reduction of CCl2F2 on Nafion solid polymer electrolyte
composite electrodes
E. Delli, S. Kouloumtzoglou, G. Kyriacou*† and C. Lambrou
Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece
The electrochemical reduction of CCl2F2 (CFC-12) was
carried out at Pd, Au, Cu and Ag cathodes, which were
chemically deposited on Nafion 117 (H+ form) membrane;
the main electrolysis product at 21.0 V vs. Ag/AgCl at Au,
Pd and Cu was CH4, with current efficiencies (CE) of 14, 15
and 47% respectively, while at Ag cathode, in addition to
CH4, a considerable quantity of CH2F2 (CE 60%) was also
detected, which might be used as a new technology
refrigerant.
ionization detector and a Porapak QS 1/8 in, 4 m column at 120
°C.
The voltammogram at the Ag electrode, between 20.7 and
21.6 V vs. Ag/AgCl (Fig. 2), showed that the reduction of
CCl2F2 was taking place at potentials more negative than 20. 8
V, while hydrogen was produced at cathodic potentials more
negative than 21.3 V. This value is in accordance with
hydrogen evolution at Ag wire at the same pH value.6
Based on the voltammetry results, constant potential elec-
trolysis was performed in the region 20.8 to 21.6 V vs. Ag/
AgCl. The main products of the electrolysis were CH2F2 and
CH4 at all potentials examined. CH2F2 is a compound of great
practical importance, due to its application as a new technology
refrigerant which does not cause ozone depletion.7 Small
amounts of CHClF2, CH3F and HCF2CF2H were also detected
in the reaction products.
Fig. 3 illustrates the rate of CF2H2 production vs. cathodic
potential where a sharp maximum was observed at about 21.4
V. The rate of CH4 production was slightly increased at
potentials more negative than 21.4 V. The CEs of CF2H2 and
CH4 formation at 21.4 V were 60 and 30%, respectively.
Repeated experiments showed that the distribution of products
was as shown by the curves of Fig. 3, with a reproducibility of
about 15%. The form of these curves can be explained by a
The production of chlorofluorocarbons (CFCs) has been
stopped since 1996, in line with the Montreal Protocol, because
of their ability to react in ways that destroy tropospheric ozone.1
At present, almost 2 3 106 tonnes of these compounds are
stored in various freezing devices, the bigger percentage of
which is CFC-12.2 These large quantities must be destroyed or
preferentially converted to other useful products. Recently
Cabot and co-workers3,4 achieved the electrosynthesis of
trifluoroethene and difluoroethene from 1,1,2-trichloro-
1,2,2-trifluoroethane (CFC-113) in organic solvents.
This work deals with the possible conversion of CFC-12 to
non-polluting substances via electrochemical reduction at
metallic electrodes which are deposited on Nafion 117 (H+
form) membrane. These electrodes enable us to perform
electrochemical reactions with reactants that are insoluble in
water, without employing organic solvents.
consecutive
reaction
mechanism
corresponding
to
CCl2F2?CH2F2?CH4.
For the metal deposition, aqueous 0.1
M
solutions of AgNO3,
Theoretical analysis of consecutive electrochemical reactions
showed that the effect of potential on the selectivity and yield of
the intermediate compound could be considered analogous to
that of temperature in the conventional chemical reactions. In
this case the selectivity of the intermediate product vs. potential
HAuCl4, Pd(OAc)2 and CuSO4 were used, combined with 10%
aqueous NaBH4 solution, using the method previously de-
scribed for copper.5 The charged side of the metal surface of the
membrane, having an apparent effective area of 2.35 cm2 was in
contact with CFC-12, while the other side was in contact with 2
M
aqueous NaOH solution (10 cm3), as shown in Fig. 1. As a
reference, an Ag/AgCl electrode was used. The analysis of the
products was performed by gas chromatography using a flame
Fig. 1 Exploded view of the electrolysis cell: (A) gas inlet, (B) gas outlet,
(C) PTFE gasket, (D) silicone screen spacer, (E) Nafion SPE electrode, (F)
reference electrode, (G) electrolyte chamber and (H) Pt anode
Fig. 2 Polarization curves at silver cathode, in 2
mV s21 (a) with N2 and (b) with CFC-12
M KOH electrolyte at 10
Chem. Commun., 1998
1693