Optimisation of electrocatalytic dechlorination of 2,4- dichlorophenoxyacetic acid on a roughened silver-palladium cathode

Article abstract of DOI:10.1016/j.electacta.2013.02.068

Electrocatalytic hydrogenolysis (ECH) dechlorination of 2,4-dichlorophenoxyacetic acid (2,4-D) in an aqueous solution was investigated at room temperature using a roughened silver-palladium cathode (Pd/Ag(r) cathode) in batch-mode electrolysis experiments. The Pd/Ag(r) cathode was prepared by galvanic replacement reaction (GRR) of a roughened silver (Ag(r)) electrode with PdCl₂ solution. The effect of preparation conditions on the catalytic activity and stability of the Pd/Ag(r) cathode and of operating parameters on the rate and current efficiency (CE) of the ECH dechlorination reaction were evaluated. In particular, the ECH dechlorination mechanism of 2,4-D was analysed with regard to the dependence of dechlorination efficiency on the different operating parameters. Moreover, preliminary assessments of product selectivity and carbon mass balance of the dechlorination reaction were carried out. The results demonstrate that a moderate GRR time and GRR temperature favoured the catalytic activity and cathode stability and that a basic aqueous solution without ethanol, high 2,4-D concentration, and moderate current density had the most beneficial effects on the dechlorination process. Under the optimised conditions, 25 mM of 2,4-D could be selectively dechlorinated to phenoxyacetic acid with 85% yield and 66% CE at 298 K after 6 h electrolysis. The only products generated during the electrolysis process were phenoxyacetic acid, 2-chlorophenoxyacetic acid, and 4-chlorophenoxyacetic acid.

Full text of DOI:10.1016/j.electacta.2013.02.068

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Electrochimica Acta 96 (2013) 90–96
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Optimisation of electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid
on a roughened silver–palladium cathode
Ying Hua Xu, Qian Qian Cai, Hong Xing Ma, Yan He, Hong Zhang, Chun An Ma∗
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou City,
Zhejiang 310032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Electrocatalytic hydrogenolysis (ECH) dechlorination of 2,4-dichlorophenoxyacetic acid (2,4-D) in an
aqueous solution was investigated at room temperature using a roughened silver–palladium cathode
(Pd/Ag(r) cathode) in batch-mode electrolysis experiments. The Pd/Ag(r) cathode was prepared by gal-
vanic replacement reaction (GRR) of a roughened silver (Ag(r)) electrode with PdCl2 solution. The effect
of preparation conditions on the catalytic activity and stability of the Pd/Ag(r) cathode and of operating
parameters on the rate and current efficiency (CE) of the ECH dechlorination reaction were evaluated. In
particular, the ECH dechlorination mechanism of 2,4-D was analysed with regard to the dependence of
dechlorination efficiency on the different operating parameters. Moreover, preliminary assessments of
product selectivity and carbon mass balance of the dechlorination reaction were carried out. The results
demonstrate that a moderate GRR time and GRR temperature favoured the catalytic activity and cath-
ode stability and that a basic aqueous solution without ethanol, high 2,4-D concentration, and moderate
current density had the most beneficial effects on the dechlorination process. Under the optimised con-
ditions, 25 mM of 2,4-D could be selectively dechlorinated to phenoxyacetic acid with 85% yield and
66% CE at 298 K after 6 h electrolysis. The only products generated during the electrolysis process were
phenoxyacetic acid, 2-chlorophenoxyacetic acid, and 4-chlorophenoxyacetic acid.
Received 2 December 2012
Received in revised form 8 February 2013
Accepted 14 February 2013
Available online 21 February 2013
Keywords:
Electrochemical hydrodechlorination
Palladium/silver cathode
Dechlorination mechanism
2,4-Dichlorophenoxyacetic acid
Chlorinated aromatic compounds
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
[7,8]. This technology ensures complete removal of the Cl atoms
under mild experimental conditions (aqueous medium, ambient
Chlorinated aromatic compounds (CACs) are widespread envi-
ronmental contaminants found in both groundwater and soil. Many
of these are toxic, persistent, and strongly resistant to chemical,
physical, and biological degradation. They gradually accumulate
in the environment, achieving concentrations that are hazardous
to living organisms, including humans [1,2]. Therefore, the devel-
opment of an efficient and cost–effective technology for the
detoxification and destruction of CACs is highly sought after.
Among the different types of potential methods for treating
CAC-containing effluents, electrochemical techniques are consid-
ered to be intrinsically milder and more efficient. Electro-oxidation
[3,4] and electro-reduction processes [5,6] can both be used for
breaking the C Cl bond within the CACs. During the oxidation
processes, undesired reactions may arise because of the forma-
tion of reactive radicals, either on the electrode surface or in
the solutions. However, cathodic processes, especially electrocat-
alytic hydrogenolysis (ECH) using Pd-modified cathodes, have been
shown to achieve the goal without producing toxic by-products
temperature, and no chemical additives). The resulting Cl-free
organic compounds are usually less toxic and thus can be dis-
posed by using more convenient and economic methods such as
biodegradation. However, it is difficult to achieve a high current
efficiency (CE) using conventional Pd-modified cathodes to dechlo-
rinate CACs. For example, previous studies have demonstrated
that less than 20% current could be effectively utilised in the ECH
dechlorination of chlorinated biphenyls [9] and chlorinated phe-
noxyacetic acid (PAA) [8,10–12]. Concomitant reduction of H2O is
the main cause of such low CE, which not only drastically increases
electrical energy consumption but also results in an abundance of
volatile H2. These two drawbacks to ECH dechlorination technology
largely limit its application in the practical disposal of CACs.
Cathode substrates have a major effect on the efficiency of
ECH dechlorination of chlorinated compounds, and therefore, large
numbers of materials with a highly porous structure have been
investigated as potential catalyst supports (i.e. cathode substrates).
These include various carbon-based materials such as activated car-
bon fibre/cloth/felt [8,10,7], reticulated vitreous glassy carbon [13],
MoOx-modified glassy carbon [14], granular graphite [15], and car-
bon nanotubes [16]; different metal materials such as Ti mesh [17],
Fe gauze [18], and Ni foam [9,19]; and other substances such as
∗
Corresponding author. Tel.: +86 0571 88320830; fax: +86 0571 88320830.
E-mail address: science@zjut.edu.cn (C.A. Ma).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.02.068

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Y.H. Xu et al. / Electrochimica Acta 96 (2013) 90–96
91
polypyrrole [20], polypyrrole/Ni foam composite [21], and surfac-
tant/polypyrrole/meshed Ti [22].
2 mm) was used for the voltammetric investigations with IR com-
pensation (10 ꢀ). The Pd/Ag(r) electrode was prepared using a GRR
time of 60 min and a GRR temperature of 298 1 K. Electrolysis
experiments were carried out in a two-compartment cell divided
by a cation-exchange membrane (Nafion® 117), with catholyte
and anolyte volumes of 40 cm3. The cathode consisted of a rect-
angular palladised Ag metal mesh (4 cm × 3 cm × 0.1 cm), and a Pt
sheet (4 cm × 3 cm × 0.05 cm) in 0.5 M NaOH aqueous solution was
used as the anode in order to provide a harmless counter reac-
tion (2OH−–2e− → (1/2O2) + H2O). The catholyte was stirred during
the electrolysis process. All electrochemical experiments were car-
ried out at 298 1 K, and all potentials are reported as relative
to SCE.
Ag, as a cathode catalyst, has recently been receiving much
attention for its extraordinary electrocatalytic activity towards
the reduction of halogenated organic compounds [23–27]. The
electrocatalytic process is probably related to the formation of a
bridge-like R· · ·X· · ·Ag adsorbed intermediate, although the cat-
alytic role of Ag in the reduction process still needs to be further
explored. Simonet’s group chose Ag as the cathode substrate for
preparing a new cathode material, palladised Ag [28–30], that
exhibited greater electrocatalytic activity towards the reduction of
alkyl bromides and alkyl iodides than an electrode consisting of
Ag alone. In addition, the Song group developed a new Pd/Ag/Ni
foam cathode that showed very high CE for the dechlorination of
dichloroacetic acid [31].
2.4. Analytical methods
Recently, we prepared a roughened Pd/Ag (Pd/Ag(r)) cathode
using a galvanic replacement reaction (GRR) and demonstrated
its excellent electrocatalytic activity towards the ECH dechlori-
nation of 2,4-dichlorophenoxyacetic acid (2,4-D) [32]. Here, this
work is taken forward with the aim of optimising various factors
that might affect the electrocatalytic activity and stability of the
Pd/Ag(r) cathode and the rate and CE of the dechlorination reaction,
including GRR time, GRR temperature, electrolyte composition, ini-
tial concentration of 2,4-D, and current density. In addition, product
selectivity and carbon mass balance of the dechlorination reaction
were investigated, and the reaction mechanism was analysed in
detail from the perspective of the dependence of operating param-
eters on dechlorination efficiency.
Quantitative analysis of the dechlorinated products and the
remaining 2,4-D was performed at room temperature by using
a
Waters HPLC system with a symmetry column (250 mm
length × 4.6 mm i.d., 5 ␮m particle size), an injection valve fitted
with 20 ␮L sample loop, and a Waters 2996 Photodiode Array
Detector (ꢁ 270 nm). Isocratic elution was used with a mobile phase
of acetonitrile/methanol/H2O (1:3:6, v/v) containing 30 mM H3PO4
at a flow rate of 1 mL min−1. Product concentrations were deter-
mined by using calibration curves of standards. The precision of
the PAA mass balance in the measurement was ca. 95–105% of the
nominal value.
The conversion of 2,4-D was calculated as the ratio of the amount
of 2,4-D eliminated to the initial amount prepared. The yield of PAA
was determined as the molar ratio of PAA produced to the initial
amount of 2,4-D. The CE of dechlorination 2,4-D was calculated
from the following equation:
2. Experimental
2.1. Chemicals
(n1ꢂCPAA + n2(ꢂC2-ClPAA + ꢂC4-ClPAA))FV
Ag (99.9%, mesh; open area 37% or plate) was obtained from
Cells Electrochemistry Experiment Equipments Co., Ltd., China
(www.hzcell.com). PdCl2 (99.5%), 2,4-D, 2-chlorophenoxyacetic
acid (2-ClPAA), 4-chlorophenoxyacetic acid (4-ClPAA), PAA, NaOH,
NaCl, HCl, H2SO4, Na2SO4, KH2PO4, and ethanol, all with 97–99%
purity, were obtained from the Aladdin Reagent Co., China, and
used as received. Acetonitrile and methanol (HPLC-grade) were
purchased from the National Medicines Co. Ltd., China. All solu-
tions were prepared using H2O with a resistivity of 18.2 Mꢀ cm
obtained from a Millipore Milli-Q system.
CE =
I × ꢂt
where n1 and n2 are the electron transfer numbers per molecule
of PAA (n1 = 4) and ClPAA (n2 = 2), respectively; ꢂCPAA, ꢂC2-ClPAA
and ꢂC4-ClPAA are the concentration differentials (M) of PAA, 2-
ClPAA, and 4-ClPAA, respectively, during ꢂt; F is Faraday’s constant
(96,500 A s mol−1); V is the volume of the total catholyte (0.04 L);
and I is the applied current density.
,
3. Results and discussion
2.2. Electrode modification
3.1. Effect of preparation conditions
Ag was degreased with 10% NaOH, boiled in concentrated HCl
for 10 min, and then washed with H2O. Roughened Ag (Ag(r)) elec-
trodes were obtained using an oxidation–reduction cycle (ORC)
process in an aqueous solution of 0.5 M NaOH and 0.5 M NaCl, at a
scanning rate of 10 mV s−1, in the range −0.3 to 0.7 V, within 400 s,
at 298 1 K, as described in Ref. [23]. The resulting Ag(r) cathodes
were immersed in 50 mL aqueous solution of 0.1 M HCl containing
2.0 mM PdCl2 for different lengths of time (GRR time) at different
temperatures (GRR temperature) to prepare the Pd/Ag(r) cathodes.
Pd loading on the surface of the Pd/Ag(r) cathodes was indirectly
determined using inductively coupled plasma mass spectrometry
(ICP-MS) (PerkinElmer, Elan DRC-e).
3.1.1. Effect of GRR time
A series of Pd/Ag(r) cathodes were prepared using GRR with
Ag(r) electrodes and a solution of PdCl2 for different GRR times.
As shown in Fig. 1A, Pd loading of the Pd/Ag(r) cathodes rapidly
increased during the initial 30 min, before reaching a quasi-steady-
state value. Figs. 1B–D illustrates the effect of GRR time on the
dechlorination rate, as evaluated by 2,4-D conversion, PAA yield,
and the CE of the Pd/Ag(r) cathodes. As GRR time increased
to 60 min, a notable increase in the dechlorination rate and CE
was observed. The values then reached a plateau, before slightly
decreasing after 90 min. These results imply that the catalytic activ-
ity of the Pd/Ag(r) cathodes strongly depend on Pd loading, with
moderate loading favouring the catalytic activity.
2.3. Apparatus
Linear sweep voltammograms (LSVs) and electrolysis experi-
ments were carried out with a PAR 273A potentiostat. A three-
electrode cell composed of an aqueous saturated calomel electrode
(SCE) as reference electrode, a platinum sheet (0.5 cm × 1 cm) as the
counter electrode, and a Pd/Ag(r) electrode as working electrode (Ø
3.1.2. Reactivation of Pd/Ag(r) electrode
In general, the activity of a catalytic material decreases over
the course of the reaction. Therefore, a preliminarily evaluation
of the stability of the prepared Pd/Ag(r) electrode towards ECH
dechlorination of 2,4-D was carried out by a prolonged electrolysis

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92
Y.H. Xu et al. / Electrochimica Acta 96 (2013) 90–96
Fig. 1. Effect of GRR time on Pd loading and ECH chlorination of 2,4-D. Cathode: Pd/Ag(r) mesh (GRR temperature = 298 1 K); anode: Pt sheet; catholyte: 0.5 M NaOH + 25 mM
2,4-D; and anolyte: 0.5 M NaOH solution. Applied current density: 4.2 mA cm−2 (50 mA); time of electrolysis: 3 h; temperature: 298 1 K.
experiment. As shown in Fig. 2A, during the initial 2 h, the concen-
tration (c) of 2,4-D decreased linearly with electrolysis time, while
that of ClPAA and PAA showed the opposite behaviour. At longer
times, especially after 3 h, the concentration of 2,4-D, ClPAA, and
PAA remained almost unchanged.
Highly similar results were previously observed for the elec-
trocatalytic reduction of 2,4-D on a Pd-modified glassy carbon
electrode by the Casella group [11], with the results attributed
to the strong adsorption of the reaction products such as chlo-
ride species, which gradually poison the catalyst. On the basis of
this deactivation hypothesis, a convenient surface-cleaning pro-
cess (which polarised the Pd/Ag(r) electrode with an anode current
of 5 mA for 100 s) was used to reactivate the Pd/Ag(r) electrode.
Fig. 2B shows the results of an electrolysis experiment in which
the Pd/Ag(r) cathode was bihourly reactivated by the surface-
cleaning process. It can be seen that almost 100% conversion of
2,4-D was achieved, with an 85% yield of PAA after 6 h elec-
trolysis. This result indicates that the catalytic activity of the
Pd/Ag(r) cathode can be kept constant using this surface-cleaning
process.
3.1.3. Effect of GRR temperature
Twelve electrolysis experiments using three Pd/Ag(r) cathodes
(GRR temperature = 283, 298, 313 K) were carried out to assess the
effect of GRR temperature on the electrocatalytic activity and sta-
bility of Pd/Ag(r) cathodes. As shown in Fig. 3, the two Pd/Ag(r)
cathodes with lower GRR temperature (283, 298 K) gave excellent
catalytic stability in four sequent electrolysis experiments, whereas
Pd/Ag(r) cathode with the highest GRR temperature (313 K) lost
its electrocatalytic activity in last three electrolysis experiments
though it exhibited the best electrocatalytic activity in its first
electrolysis experiment. Generally, increasing GRR temperature
not only accelerates the rate of a GRR but also changes sur-
face characteristics (morphology, phase state and elements) of a
bimetallic catalyst [33,34], therefore the great difference of cat-
alytic capability between three Pd/Ag(r) cathodes probably were
caused by the changes of surface characteristics of them. The
dependence between catalytic capability of a Pd/Ag(r) cathode
and its surface characteristics is an interesting topic, however it
is not possible to analyses it on the basis of the data so far avail-
able.
Fig. 2. Effect of reactivation on the electrocatalytic activity of a Pd/Ag(r) cathode
during ECH dechlorination of 2,4-D. Cathode: Pd/Ag(r) mesh (GRR tempera-
ture = 298 1 K, GRR time = 60 min). (A) The Pd/Ag(r) cathode was not reactivated.
(B) The Pd/Ag(r) cathode was bihourly reactivated in the catholyte. Time of electrol-
ysis is 6 h and all other electrolysis conditions were the same as those in Fig. 1.

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Y.H. Xu et al. / Electrochimica Acta 96 (2013) 90–96
Table 2
93
Effect of 2,4-D initial concentration on ECH dechlorination of 2,4-D. Cathode:
Pd/Ag(r) mesh (GRR temperature = 298 1 K, GRR time = 60 min). Catholyte: 40 mL
solution of 0.5 M NaOH + X mM 2,4-D. All other electrolysis conditions were the same
as those in Fig. 1.
Initial concentration
Conversion
of 2,4-D (%)
Yield of
PAA (%)
CE (%)
(mmol L−1
)
5
10
25
30
40
99.0
95.8
92.3
90.2
84.4
60.3
51.7
40.8
37.7
28.4
11.4
21.1
47.6
54.8
64.5
3.2.2. Effect of 2,4-D initial concentration
LSVs obtained using Pd/Ag(r) as the working electrode show the
influence of 2,4-D concentration on current–potential data in 0.5 M
NaOH aqueous solution (Fig. 4A). Herein, the working electrode
was prepared by immersing a Ag(r) electrode into 50 mL solution of
2.0 mM PdCl2 + 0.1 M HCl for 60 min. As can be seen in the figure, a
current plateau located between −0.6 and −1.3 V, which evidently
originated from the reduction of 2,4-D, steadily increased in current
density with increasing 2,4-D concentration. This indicates that a
higher initial concentration (<40 mM) is favourable for the reduc-
tion of 2,4-D. In addition, the peak at approximately −1.17 V was
used to assess the dependence of peak current on the concentra-
tion of 2,4-D (Fig. 4B). It is clear that these two parameters are not
proportional to each other, thereby indicating that the reduction
process is not under complete diffusion control.
In order to evaluate the effect of 2,4-D concentration on the
reduction process in more detail, five electrolysis experiments with
different initial concentrations of reactant (5, 10, 25, 30, and 40 mM)
were carried out. As shown in Table 2, the increase in the initial
concentration of 2,4-D from 5 to 40 mM led to an increase in CE
from 11.4% to 64.5% as well as an increase in dechlorination rate.
This result provided further evidence to support the findings of the
LSV experiments, where a similar improvement in dechlorination
efficiency was found at higher 2,4-D concentrations.
Fig. 3. Effect of GRR temperature on the electrocatalytic activity and stability
of Pd/Ag(r) cathodes. Cathode: (A) Pd/Ag(r) mesh (GRR temperature = 283 1 K,
GRR time = 300 min); (B) Pd/Ag(r) mesh (GRR temperature = 298 1 K, GRR
time = 60 min); (C) Pd/Ag(r) mesh (GRR temperature = 313 1 K, GRR time = 20 min).
All other conditions were the same as those in Fig. 2B.
3.2. Effect of operating parameters
3.2.1. Effect of electrolyte composition
Among the various operating parameters, the electrolyte com-
position showed the most obvious effect on the dechlorination
efficiency of the Pd/Ag(r) cathode towards the ECH dechlorination
of 2,4-D. Table 1 shows the results of the reduction process carried
out in seven different electrolytes. By assessing the dechlorination
rate and CE, the following conclusions were drawn:
1. Cl− is detrimental to the ECH dechlorination. Na2SO4 enhanced
the dechlorination rate by increasing the conductivity of the
electrolyte solution, while NaCl did not have a similar positive
effect on the process.
3.2.3. Effect of current density
2. OH− can drastically promote the ECH process when ethanol
is absent from the electrolyte solutions. This positive effect
of OH− probably originates from its neutralisation towards
the adsorbed HCl produced by a catalytic hydrogenation step
between adsorbed 2,4-D and chemisorbed hydrogen [32]. The
adsorbed HCl would block the reaction sites if it was not quickly
removed from the cathode surface.
3. Ethanol deactivates the Pd/Ag(r) cathode if an aqueous alkaline
solution is used for the ECH dechlorination of 2,4-D. It has been
previously reported that ethanol can dissociatively chemisorb
on Ag to form Ag ethoxide at room temperature [35]. Because an
aqueous alkaline solution favours the production of an ethoxide
ion, the poisonous effect of ethanol is attributed to its substantial
adsorption on Ag in the form of Ag ethoxide.
Fig. 5A–C shows the effect of current density on the dechlo-
rination efficiency, as evaluated from the conversion of 2,4-D,
PAA yield, and CE, during the ECH dechlorination of 2,4-D. The
best performance was achieved in the current density range
2.1–4.2 mA cm−2 (geometric area), especially during the middle
and later periods of electrolysis. An increase to a higher current den-
sity (6.3 or 8.3 mA cm−2) caused a rapid drop in dechlorination rate.
Similar results were observed for the ECH dechlorination of chlo-
rinated phenols using a Pd-modified Ti cathode by the Scott group
[18]. They attributed this phenomenon to higher H2 gas generation
in the electrode structure, restricting the access of the liquid, and
thus limiting mass transport of the reactant (H2 gas isolation effect).
In order to confirm this hypothesis, the potentials of the Pd/Ag(r)
electrode under different current densities were compared during
Table 1
Effect of electrolyte composition on the ECH dechlorination of 2,4-D. Cathode: Pd/Ag(r) mesh (GRR temperature = 298 1 K, GRR time = 60 min). All other electrolysis conditions
were the same as those in Fig. 1.
Entry
Catholyte
pH
Conversion of 2,4-D (%)
Yield of PAA (%)
CE (%)
1
2
3
4
5
6
7
25 mM 2,4-D + 0.5 M NaOH + 0.5 M Na2SO4
25 mM 2,4-D + 0.5 M NaOH
13.3–13.4
13.3–13.4
13.3–13.4
8.3–12.5
6.1–6.5
96.9
91.3
90.3
68.3
42.8
31.3
17.6
49.1
42.8
37.5
17.5
1.9
52.0
47.8
45.6
30.6
16.0
11.6
6.5
25 mM 2,4-D + 0.5 M NaOH + 0.5 M NaCl
25 mM 2,4-D + 0.5 M NaOH + 0.5 M K2HPO4
25 mM 2,4-D + 0.5 M KH2PO4 + M NaOH + 0.5 M ethanol
25 mM 2,4-D + 0.5 M NaOH + 0.5 M ethanol
25 mM 2,4-D + 0.5 M H2SO4 + 0.5 M ethanol
13.4–13.4
0.2–0.3
1.2
0.5