Environ. Sci. Technol. 1997, 31, 1074-1078
the electrode surface to assess the role of hydrogen or
Electrochemical Dechlorination of
4-Chlorophenol to Phenol
I . F R A N C I S C H E N G , †
Q U I N T U S F E R N A N D O , * , † A N D N I C K O R T E ‡
adsorbed hydrogen (Pd‚H2) on the reductive dechlorination
reactions. This has helped in the evaluation of the role of
surface-bound Pd in its role not only as a hydrogen evolution
catalyst but also as a facilitator for the reductive dechlorination
of organics. The experiments described below were per-
formed as a result of the observation that the Pd/ Fe system
was able to effect the rapid hydrodehalogenation of TCE as
opposed to Fe alone (4). The hydrogen evolution charac-
teristics of platinized iron and palladized iron should be
similar, but the rate of hydrodechlorination by Pt/ Fe was
much slower than that observed with Pd/ Fe. This property
may be due to the ability of palladium to absorb hydrogen;
Pd metal in conjunction with hydrogen gas has been used
over the years in hydrogenation schemes (6-9, 16). The
organic compound that we have used to follow the course of
the dechlorination reaction is 4-chlorophenol (4-CP). This
compound was selected for its relative non-volatility and
aqueous solubility of both the reactant and the product
(phenol). Moreover, 4-CP and phenol can be conveniently
determined at parts per million levels in aqueous solutions
by HPLC.
Several types of cathodic materials have been studied for
the catalytic decomposition of halogenated organics (10-
12). Zhang and Rusling utilized either carbon cloth or Pb
cathodes for the dechlorination of polychlorinated biphenyls
(13). The authors found that Pb cathodes were superior to
the former and suggested that this was due to the higher
hydrogen overpotential of Pb. Matheson and Tratnyek
proposed that it should be possible to rapidly dehalogenate
chloro-organics with hydrogen gas if a suitable dechlorination
catalyst is used (2). Such a process would take place via the
following reaction:
Department of Chemistry, University of Arizona,
Tucson, Arizona 85721, and Environmental Sciences Division,
Oak Ridge National Laboratory, Grand Junction Office,
Grand Junction, Colorado 85103
We have hypothesized that hydrogen gas intercalated in
a palladium lattice is the powerful reducing agent that
reductively dechlorinates chlorinated organic compounds
that are adsorbed on the surface of palladized electrodes.
We have shown that dechlorination of 4-chlorophenol to
phenol occurs rapidly on palladized carbon cloth or pal-
ladized graphite electrodes. The reactions on the palladized
carbon cloth and graphite depend on the adsorption of
the chlorinated organic compound on the carbon surface and
the reaction with hydrogen at the palladium/carbon
interface. Palladium was much more effective in promoting
the dechlorination reaction than platinum, probably because
of its ability to intercalate hydrogen in its lattice.
Introduction
The observation that elemental iron dechlorinates certain
toxic volatile organic compounds has led to a revival in the
investigation of metals that may be useful for the reductive
dechlorination of organic compounds that are considered to
be environmental hazards (1, 2). The reactivity of the Fe0/
Fe2+ system, with a standard potential of -0.42 V, has been
attributed to its ability to drive a large number of dechlorin-
ation reactions of halo-organic compounds. However, the
use of zero-valent iron often results in only partial dechlo-
rination in which the reaction products are sometimes more
toxic than the reactants (5). Recently, a method for the rapid
aqueous reduction of trichloroethylene and polychlorinated
biphenyls to hydrocarbons by palladized iron (Pd/ Fe) has
been demonstrated (3-5). We have proposed that the high
reactivity of the Pd/ Fe bimetallic system can be attributed to
the following sequence of reactions: (i) the evolution of
hydrogen gas by the reduction of water molecules by
elemental iron, i.e., the “corrosion” of iron in water; (ii) the
adsorption of the evolved hydrogen gas by Pd0, and the
formation of the powerful reducing species, Pd‚H2, i.e.,
hydrogen gas intercalated into the elemental palladium lattice;
and (iii) the reduction of the chlorinated organic compound
that is adsorbed on the bimetallic Pd/ Fe surface.
H2 + RX f RH + H+ + X-
(1)
Recently, Kulikov and co-workers used Zn-modified carbon
cloth for the reductive dechlorination of lindane in aqueous
solution (12). In their proposed mechanism, lindane was
adsorbed on the carbon cloth surface and then reduced by
zinc metal. The metallic Zn islands were regenerated by the
2+. No mention was made
electrochemical reduction of Zn
however of the possibility that lindane may be dechlorinated
by hydrogen gas. This route cannot be discounted since zinc
metal is a prodigious producer of H2 in aqueous solutions.
Experimental Section
Materials and Chem icals. Carbon cloth (99.9%) was supplied
by Aesar (Ward Hill, MA). Graphite electrode rods, (6.15 mm
diameter) designated as ultra-purity “F”, were obtained from
Ultra Carbon Corporation (Bay City, MI). Iron wire of 0.25
mm (99.9%, Aldrich, Milwaukee, WI), K2PdCl6 (99%, Aldrich),
K2PtCl6 (99.99%, Aldrich), sodium acetate (Fisher, Pittsburgh,
PA), methanol (HPLC grade, Fisher), and palladium gauze
(Aesar) were all used as received. Phenol (99.5%, EM Science,
Gibbstown, NJ) and 4-chlorophenol (99%+, Aldrich) were
used without purification. All water was of 18.2 MΩ‚cm
quality obtained from a Millipore-Q system.
Metal Modification of Iron Wire, Carbon Cloth, and
Graphite Electrodes. Palladized graphite rods and carbon
cloth electrodes were prepared in the following manner. A
2.2 cm2 (1.0 cm length) area of graphite rod was exposed to
a 35.0 mL solution of 1.8 mM K2PdCl6 (25.0 mg total) in 0.050
M sodium acetate-acetic acid buffer (pH 5.0) by tightly
wrapping Teflon tape to the remainder of the electrode (see
Figure 1). Palladium was electrodeposited at a constant
current of 5.6 mA until the characteristic yellow color of the
The principal variables that govern these dechlorination
reactions are the surface area of the bimetallic Pd/ Fe system
and the amount of the species Pd‚H2 that is available for the
reduction reactions. We have examined the cathodic reduc-
tion of a chlorinated organic compound at electrodes of
known surface area in the presence and the absence of evolved
hydrogen gas. We have also employed galvanostatic condi-
tions, i.e., we have maintained a constant hydrogen flux at
* Author to whom all correspondence should be addressed.
Telephone: (520)-621-2105; fax: (520)-621-8407; e-mail: fernandq@
ccit.arizona.edu.
† University of Arizona.
‡ Oak Ridge National Laboratory.
9
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1997 Am erican Chem ical Society