Electrocatalytic dechlorination of chloropicolinic acid mixtures by using palladium-modified metal cathodes in aqueous solutions

Article abstract of DOI:10.1016/j.electacta.2016.06.001

In China, chloropicolinic acid (ClPA) mixtures comprising 3,5,6-trichloropicolinic acid, 3,6-dichloropicolinic acid (3,6-D), 3-ClPA, and 6-ClPA are discharged as organic wastes at a rate of approximately 300 tons per year. In this work, we developed an aqueous phase electrocatalytic hydrogenation (ECH) system based on Pd catalyst to dechlorinate the ClPA mixtures into picolinic acid (PA) at room temperature. Firstly, we evaluated the influence of cathode support and Pd loading on the catalytic performance of cathodes, as well as the effects of operating parameters on the intermediate product selectivity and dechlorination efficiency of the ECH process with 3,6-D as the target compound. Secondly, we analyzed the ECH dechlorination mechanism of 3,6-D with regard to the surface condition of cathode and catholyte pH, and the rate-limiting step of the dechlorination process was also discussed. Finally, we assessed the practicability of the ECH system to dechlorinate the ClPA mixtures into PA by using a plate-and-frame cell. Results demonstrated that Pd/Ni foam cathodes with Pd loading of 2.25-3.6 mg cm^-2 exhibited the optimum ECH dechlorination performance, and the basic aqueous solution and high 3,6-D concentration favored the ECH process. The ClPA mixtures with 47 g L^-1 concentration (the total concentration of ClPAs was approximately 250 mM) can be selectively dechlorinated into PA with 99% yield, 76.3% current efficiency, and 2.47 kW h kg^-1 PA specific electric energy consumption at a current density of 208 A m^-2 in a 1.25 M NaOH aqueous solution.

Full text of DOI:10.1016/j.electacta.2016.06.001

Accepted Manuscript
Title: Electrocatalytic dechlorination of chloropicolinic acid
mixtures by using palladium-modified metal cathodes in
aqueous solutions
Author: Hongxing Ma Yinghua Xu Xufen Ding Qi Liu
Chun-An Ma
PII:
S0013-4686(16)31318-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.06.001
EA 27440
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24-2-2016
30-5-2016
1-6-2016
Please cite this article as: Hongxing Ma, Yinghua Xu, Xufen Ding, Qi Liu,
Chun-An Ma, Electrocatalytic dechlorination of chloropicolinic acid mixtures by
using palladium-modified metal cathodes in aqueous solutions, Electrochimica Acta
http://dx.doi.org/10.1016/j.electacta.2016.06.001
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Electrocatalytic dechlorination of chloropicolinic acid
mixtures by using palladium-modified metal cathodes in
aqueous solutions
Hongxing Maa,Yinghua Xua*, Xufen Dinga, Qi Liub, Chun-An Maa*
aState Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou
City, Zhejiang 310032, China
bCollege of Life and Environmental Science, Hangzhou Normal University,
Hangzhou City, Zhejiang 310036, China
*Corresponding author. Tel & Fax: +86-0571-88320830. E-mail: xuyh@zjut.edu.cn
(Y. Xu), science@zjut.edu.cn (C. Ma)
1

Graphical abstract
ABSTRACT
In China,
chloropicolinic
acid
(ClPA)
mixtures
comprising
3,5,6-trichloropicolinic acid, 3,6-dichloropicolinic acid (3,6-D), 3-ClPA, and 6-ClPA
are discharged as organic wastes at a rate of approximately 300 tons per year. In this
work, we developed an aqueous phase electrocatalytic hydrogenation (ECH) system
based on Pd catalyst to dechlorinate the ClPA mixtures into picolinic acid (PA) at
room temperature. Firstly, we evaluated the influence of cathode support and Pd
loading on the catalytic performance of cathodes, as well as the effects of operating
parameters on the intermediate product selectivity and dechlorination efficiency of the
ECH process with 3,6-D as the target compound. Secondly, we analyzed the ECH
dechlorination mechanism of 3,6-D with regard to the surface condition of cathode
and catholyte pH, and the rate-limiting step of the dechlorination process was also
discussed. Finally, we assessed the practicability of the ECH system to dechlorinate
the ClPA mixtures into PA by using a plate-and-frame cell. Results demonstrated that
Pd/Ni foam cathodes with Pd loading of 2.25–3.6 mg cm-2 exhibited the optimum
ECH dechlorination performance, and the basic aqueous solution and high 3,6-D
concentration favored the ECH process. The ClPA mixtures with 47 g L-1
concentration (the total concentration of ClPAs was approximately 250 mM) can be
selectively dechlorinated into PA with 99% yield, 76.3% current efficiency, and 2.47
2

kW·h·kg−1 PA specific electric energy consumption at a current density of 208 A m−2
in a 1.25 M NaOH aqueous solution.
Keywords: 3,6-Dichloropicolinic acid; High concentration; Electrocatalytic
dechlorination; Pd/Ni foam; Dechlorination mechanism
1. Introduction
China is one of the largest producers of 3,6-dichloropicolinic acid (3,6-D), a
selective herbicide used to control broadleaf weeds, particularly thistles and clovers.
Recently, the total production of 3,6-D in China has reached up to 3000 tons per year
[1-3], and past trends indicate further increase in global production and application of
this
herbicide.
Since
2010,
the
electrochemical
reduction
of
3,4,5,6-tetrachloropicolinic acid has become the main source of 3,6-D in China
because of its high yield and relatively low environmental contamination [4].
However, the process have to discharge chloropicolinic acid (ClPA) mixtures
containing 3,5,6-trichloropicolinic acid (3,5,6-T), 3,6-D, 3-ClPA, and 6-ClPA as
organic wastes. In a typical electrochemical synthesis process, these compounds are
present in ratios of 1 wt%, 82 wt%, 7 wt%, and 6 wt%, resulting in approximately 0.1
ton of ClPA mixtures per ton of pure 3,6-D product.
Several oxidation techniques such as photocatalysis [5], electro-Fenton [6],
electron beam treatment [7], and UV/H2O2 or ozone treatment [8] have been
suggested for the disposal of 3,6-D. Therefore, these techniques may also be
applicable to the degradation of ClPA mixtures. In principle, however, reductive
dechlorination techniques are more suitable than oxidation techniques in treating
ClPA mixtures because of the possibility of transforming these mixtures into highly
valuable products, such as picolinic acid (PA), the price of which amounts to
approximately 30,000 USD per ton. Numerous reduction techniques have been
3

reported for the dechlorination of chlorinated organic compounds (COCs); these
strategies include microbial dechlorination [9], zero-valent metal [10] or modified
zero-valent metal reduction [11], and catalytic hydrodechlorination [12]. Among these,
catalytic hydrodechlorination with Pd/Al2O3 catalyst has been proven to be an
effective method to transform 3,6-D into PA (77% yield) [12].
Compared with the above methods, electrocatalytic hydrogenation (ECH)
presents a significant potential to transform ClPA mixtures into PA in an industrial
scale because ECH provides mild reaction conditions without the secondary pollution
effects associated with excess reagents. In the past 10 years, ECH dechlorination with
Pd as electrocatalyst has been proven to be one of the most attractive methods for the
disposal of hazardous COCs [13-19]. In accordance with the literature [15-17], the
ECH dechlorination process is described in Eqs. (1)–(4) (where M refers to Pd or
supports):
2 H3O+ or 2 H2O + 2 e- + Pd ↔ 2 HadsPd + 2 H2O or 2 (1)
OH-
R-Cl + M ↔ (R-Cl)adsM
(2)
(3)
(4)
2 HadsPd + (R-Cl)adsM → (R-H)adsM + HCl + Pd
(R-H)adsM ↔ R-H + M
In virtue of the specific interactions between Pd and hydrogen, a strongly reducing
adsorbed hydrogen (Hads) can be electro-generated in situ at very positive potentials
and is quite active toward the hydrodechlorination of C-Cl. Almost all types of C-Cl,
including Csp3-Cl (aliphatic chlorides [18]) and Csp2-Cl (aryl [17] or alkenyl chlorides
[19]), can be hydrogenated with Hads. These two facts make it possible to dechlorinate
various COCs with very low electric energy consumption. In addition, the Pd
electrocatalyst support exerts a major effect on the ECH dechlorination performance
of Pd-modified cathodes because it influences the electronic structure of the catalyst
(Pd) [20] and may change the adsorption behavior of the reactant (R-Cl) [21].
Therefore, numerous materials have been investigated as potential electrocatalyst
supports. These items mainly include different metal materials such as Ti mesh [22],
Ti/TiO2 nanotubes [19], Fe gauze [23], Ag mesh [16,17], Cu foam [24], and Ni foam
4

[13,14,24-27]; various carbon-based materials such as glassy carbon (GC) [15],
reticulated vitreous carbon (RVC) [28], MoOx-modified GC [29], activated carbon
fibre/cloth [30,23,31], granular graphite [32], carbon nanotubes [33] and grapheme
[34-35]; and other composite materials such as polypyrrole [36], polypyrrole/Ni foam
[37-39], and surfactant/polypyrrole/meshed Ti [40].
In this paper, we primarily report the ECH dechlorination of 3,6-D (as the
representative of ClPA mixtures) in aqueous solutions at room temperature by using
Pd/Ni foam cathodes. High concentrations of 3,6-D (25–300 mM) were used in our
work for the practicability of ECH dechlorination. Our study mainly aims to develop a
highly efficient ECH dechlorination system (by optimizing the ECH dechlorination
conditions, such as cathode materials, pH of catholyte, initial concentration of 3,6-D,
and applied current density) and evaluate the practicability of this system to convert
ClPA mixtures into PA. Furthermore, this research aims to determine the reaction
mechanism of 3,6-D and the rate-limiting step in the ECH dechlorination system. To
our knowledge, this study is the first to employ an ECH technique with Pd-modified
cathode in the dechlorination of ClPA. In addition, studies on Pd-modified cathodes
have focused predominantly on the in-situ treatment of ubiquitous COCs existing in
trace amounts; however, the use of the ECH dechlorination technique to dispose high
concentrations of COCs (>70 mM) has not been reported to date [31].
2. Materials and methods
2.1. Chemicals
Ni foams (1.0 mm thickness), Cu foams (1.0 mm thickness), and Ag mesh (open
area 37%) were obtained from Cells Electrochemistry Experiment Equipment Co.,
Ltd., China (www.hzcell.com). 3,5,6-T, 3,6-D, 3-ClPA, 6-ClPA, and PA with purity of
98%–99% and ClPA mixtures comprising 3,5,6-T (1.3 wt%), 3,6-D (81.6 wt%),
3-ClPA (6.5 wt%), 6-ClPA (6.4 wt%), and PA (0.4 wt%) were obtained from
Zhejiang Avilive Chemical Co., Ltd., China (www.avilive.com) and used as received.
PdCl2 (99.5 wt%), NaOH, HCl, H2SO4, KH2PO4, H3PO4, acetone, and ethanol, all
5

with 97 wt%–99 wt% purity, were obtained from Aladdin Reagent Co., China and
used as received. Acetonitrile and methanol (HPLC grade) used for HPLC analysis
were purchased from National Medicines Co., Ltd., China. H2O with resistivity of
18.2 MΩ·cm was obtained from a Millipore Milli-Q system for the preparation of all
solutions.
2.2. Electrode modification
Pd-modified nickel (Pd/Ni) foam, silver (Pd/Ag) mesh, and copper (Pd/Cu) foam
electrodes were prepared by electroless deposition, which was conducted
spontaneously via a galvanic replacement reaction until the yellow PdCl2 solution
turned colorless with the vial capped and the solution stirred. Prior to Pd modification,
three electrode supports were degreased and cleansed in acetone for 20 min under
ultrasonic vibration and then etched in a diluted H2SO4 (80 g L−1) solution for 5 min
to remove surface native oxides. After thorough rinsing with deionized water, the
supports were immersed into a vial with 50 mL aqueous solution of 0.1 M HCl
containing different concentrations of PdCl2 at 25 °C. A number of electrodes were
deposited under exactly the same conditions to ensure reproducibility. After Pd
modification, the electrodes were kept in ethanol for later use without drying.
2.3. Apparatus
The surface morphology and composition of the abovementioned electrodes
were determined by SEM (Scanning Electron Microscope, Hitachi S-4700 II)
equipped with EDS (Energy Dispersive Spectrometer, Thermo NOANVANTAGE
ESI) and XRD (X-Ray Diffraction, Thermo ARLSCINTAG X’TRA, 45 kV and 40
mA, Cu Kα). XPS (KRATOS AXIS ULTRA DLD) with a monochromatized
aluminium X-ray source was used to confirm the Pd element surface concentration of
Pd/Ni foam. The binding energies were calculated with C1s peak (284.8 eV) as
internal standard.
6

Cycle voltammetry (CV) and electrolysis experiments were conducted with a
PAR 283 potentiostat and a DC-regulated power supply (HZC-061), respectively. A
three-electrode cell composed of a Ag/AgCl reference electrode, a Pt (2 cm  2 cm)
counter electrode, and a Pd/Ni disk working electrode (3 mm diameter) was used for
the CV experiments. The potentials of the CV experiments were reported relative to
the Ag/AgCl/saturated KCl aqueous solution reference.
A conventional two-compartment glass H-cell separated by a Nafion-117
membrane was used in the electrolysis experiments to obtain the optimized ECH
dechlorination conditions. The cathode and anode were manufactured from a material
consisting of Ni foam, Ag mesh, Cu foam, Pd/Ni foam, Pd/Ag mesh or Pd/Cu foam
(Projected area: 2 cm  6 cm), and graphite sheet (2 cm  6 cm). The volume of the
catholyte and anolyte was 50 mL each, and the catholyte was stirred during
electrolysis.
Density functional theory (DFT) was used to calculate C-Cl bond length of the
3,6-D molecular and to simulate its optimized molecular structure with the hybrid
exchange functional of B3LYP [41]. For C, N, O, Cl and H, and the basis sets [42]
were 6-311+ G (d, p). Gaussian 03 software package [43] was used to perform all the
calculations and simulations.
A plate-and-frame cell with a Nafion-117 membrane was selected to assess the
practicability of the ECH dechlorination system and convert the ClPA mixtures into
PA. A piece of Pd/Ni foam (Projected area: 7 cm  13 cm) in a 1.25 M NaOH
aqueous solution containing 47 g L−1 ClPA mixtures (total concentration of ClPAs:
approximately 250 mM) was used as the cathode, and a stainless steel mesh (7 cm 
13 cm) in a 1.25 M NaOH aqueous solution was used as the anode. The schematic
construction of the electrolysis device using the plate-and-frame cell is shown in Fig.
1. The volume of the catholyte and anolyte was 700 mL each.
All CV and electrolysis experiments were performed at 25 °C and a 1.25 M
NaOH or 0.75 M H2SO4 aqueous solution was used as anolyte in all electrolysis
experiments.
7

2.4. Analytical methods
A Waters HPLC system was used to analyze the dechlorinated products and the
remaining reactants at room temperature. The system was assembled with a symmetry
column (250 mm length × 4.6 mm i.d., 5 μm particle size), an injection valve fitted
with a 20 μL sample loop, and a Waters 2996 Photodiode Array Detector (λ 275 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. Calibration curves of
standards were used to determine the concentrations of the product and the remaining
reactants. The precision of the mass balance used in the measurement was ca.
97%–103% of the nominal value.
PA yield was calculated as the molar ratio of PA produced to the initial amount
of 3,6-D. The conversion of 3,6-D was determined as the ratio of the amount of 3,6-D
eliminated to the initial amount prepared. Current efficiency (CE, %) in the
electrolysis of 3,6-D was calculated using the following equation:
(n1CPA  n2 (C3ClPA  C6ClPA ))FV
CE 
I t
where n1 and n2 are the electron transfer numbers per molecule of PA (n1 = 4) and
ClPA (n2 = 2), respectively; △CPA, △C3-ClPA, and △C6-ClPA are the concentration
differentials (M) of PA, 3-ClPA, and 6-ClPA during △t, respectively; F is the
Faraday constant (96500 C·mol−1); V is the volume of the total catholyte (0.05 L); and
I is the applied current.
A similar calculation method was used to obtain the yield of PA and the CE of
the electrolysis of the ClPA mixtures. The specific electric energy consumption
(SEEC, kW·h·kg−1 PA) of the electrolysis process was calculated using the following
equation:
I t U
SEEC 
CPA V  M
where I is 1.89 A, △t is 12 h, U is the average cell voltage (2.3 V), V is 0.7 L, and M
is the relative molecular mass of PA.
8

3. Results and discussion
3.1. Effect of cathode material
3.1.1. Effect of cathode supports
We initially evaluated the effect of cathode supports on the dechlorination of
3,6-D through a series of electrolysis experiments. As shown in Table 1, higher CE
and PA yield were obtained from the Pd/Ni foam cathode than those from the Pd/Ag
mesh and Pd/Cu foam cathodes. A very small amount of 3,6-D was mainly
dechlorinated into 6-ClPA when the Ni foam, Ag mesh, and Cu foam were used as
cathodes. These results suggest that Pd is a powerful electrocatalyst for the
dechlorination of 3,6-D, and the Pd/Ni foam is the most feasible cathode material for
the dechlorination of 3,6-D among the three Pd-modified metal cathodes.
In addition, Cl at C6 of 3,6-D exhibited higher reductive activity than Cl at C3
on three Pd-modified metal cathodes, but an opposite activity order was exhibited on
three metal cathodes. Pyridinic nitrogen is a stronger electron-withdrawing group than
COO-; thus, Cl at C6 of 3,6-D would be more active than Cl at C3 of the isolated
3,6-D molecule. Therefore, the selectivity of the dechlorination reaction on the three
metal cathodes was opposite to the directing effect of pyridinic nitrogen. This
phenomenon would be interesting from the mechanistic viewpoint; however, this
occurrence is not possible to analyze on the basis of the currently available data.
3.1.2. Effect of Pd loading
Considering that the Pd/Ni foam was the most feasible cathode material among
the three Pd-modified metal cathodes, the effect of Pd loading on the dechlorination
performance of the prepared Pd/Ni foam was further evaluated. As shown in Fig. 2,
the conversion of 3,6-D reached 83% with the use of Pd/Ni foam cathode, which has
very low Pd loading (0.45 mg cm-2); whereas the optimal CE was achieved when the
Pd/Ni foam cathodes with high Pd loading (2.25–3.6 mg cm-2) was used. These results
indicate that the dechlorination activity of the Pd/Ni foam cathodes strongly depend
on Pd loading, and a loading of 2.25 mg cm-2 favors the performance of the cathode to
9

save Pd use.
3.1.3. Dechlorination stability of the Pd/Ni foam optimized
The dechlorination stability of the optimized Pd/Ni foam was evaluated by six
recycled electrolysis experiments. As shown in Fig. 3, only slight reductions were
observed in the yield of PA and CE over the six cycles of electrolysis experiments.
This result implies that the dechlorination activity of the optimized Pd/Ni foam was
almost stable in the 36 hours of electrolysis experiments.
3.2. Effect of operating parameters
3.2.1. Effect of catholyte pH
The catholyte pH exhibited a surprising “V” effect on the dechlorination of
3,6-D (Table 2). Moderate dechlorination efficiencies were obtained in neutral and
weak acidic aqueous solutions (Entries 4 and 5), whereas very high dechlorination
efficiencies were achieved in 1.25 M NaOH and 0.75 M H2SO4 (Entries 1 and 7).
To reveal this “V” effect, the dynamic curves of reactant and product
concentrations, as well as that of CE, during the dechlorination were separately
determined in 1.25 M NaOH or 0.75 M H2SO4 (Fig. 4). Low concentration (96 mM)
of 3,6-D was used to ensure its complete dissolution in the acidic solution before the
dechlorination experiment started. Over 100% CE was achieved in 0.75 M H2SO4
(Fig. 4B’) in the early stage of the dechlorination experiment (≤ 2 h). According to
Faraday's law, the highest CE of an electrolysis experiment is 100%. Therefore, we
assumed that a Pd-modified zero-valent Ni dechlorination process occurred in 0.75 M
H2SO4, in which Ni was the reducing agent and Pd was the catalyst. To confirm this
theory, we performed separate experiments using Ni foam or Pd/Ni foam as the
potential reducing agent and/or catalyst under identical conditions. A considerable
reduction of 3,6-D to PA with a conversion of 9.7% and a yield of 9.3% was observed
when the Pd/Ni foam was used, whereas no obvious reduction of 3,6-D was observed
where the Ni foam was used after six hours of reaction. The abovementioned results
strongly suggest that the Pd-modified zero-valent Ni dechlorination process coexisted
with the electrochemical dechlorination of 3,6-D in 0.75 M H2SO4, which is one of
10

the reasons for the “V” effect of the catholyte pH.
In addition, the product selectivity was completely different at the two pH levels
during the electrolysis process (Fig. 4A and 4B), although very similar dechlorination
efficiencies (over 90% 3,6-D had been transformed into PA) were obtained at the two
pH levels after six hours of the reaction. Obviously, two Cl atoms in the 3,6-D
molecule were eliminated stepwise in 1.25 M NaOH with 3-ClPA as main
intermediate product, whereas the atoms were removed together in one step in 0.75 M
H2SO4 (i.e., two Cl atoms of 3,6-D were dechlorinated at the same stage before the
desorption of the monochlorinated PA into the bulk catholyte). Explanation will be
proposed for the differences in the product selectivity in Section 3.3.3.
Considering the solubility of 3,6-D and the dechlorination efficiency, we chose
1.25 M NaOH aqueous solution as the catholyte in the succeeding experiments.
3.2.2. Effect of 3,6-D initial concentration
Table 3 shows the effect of the initial concentration of 3,6-D on the
dechlorination process. Increasing the initial concentration of 3,6-D from 25 mM to
200 mM resulted in an increase in CE from 17.6% to 95.3%; the CE was kept
constantly high at approximately 96% as the concentration of 3,6-D increased from
200 mM to 300 mM. These results suggest that high initial concentrations (200–300
mM) are favorable for the dechlorination of 3,6-D, which is an important conclusion
because the dechlorination of high concentration COCs (> 70mM) on Pd-modified
cathodes has not been reported so far. High concentration of COCs may retard the
dechlorination reaction of the Pd-modified electrode [31], which is probably one of
the reasons why studies using Pd-modified electrodes focus predominantly on the
dechlorination of low concentration COCs.
Moreover, nearly 100% PA yield was achieved at the two low initial
concentrations of the reactant (25 and 50 mM), although the applied charge was
greater than the theoretical charge. This result suggests that the dechlorination system
developed (Pd/Ni foam with a loading of 2.25 mg cm-2 as the cathode and 1.25 M
NaOH aqueous solution as the catholyte) here is more selective than the catalytic
11

hydrodechlorination system using the catalyst of Pd/Al2O3 [12]; the latter not only
dechlorinated 3,6-D to PA but also hydrogenated PA to pipecolinic acid. Similar
results were reported for the catalytic hydrodechlorination of 4-chlorophenol; phenol
was the sole hydrodechlorination product from a Pd/Ni foam catalyst [44], whereas
the produced phenol was further hydrogenated to cyclohexanone and cyclohexanol
over a Pd/Al2O3 catalyst [45]. Additionally, Calvo et al. [46] found that phenol was
the only product detected in the catalytic hydrodechlorination of 4-chlorophenol on
unsupported Pd nanoparticles. They considered that Al2O3 affected the selectivity of
hydrodechlorination reactions because of its interaction with Pd, provoking changes
in the electronic density of Pd that would influence the strength of adsorption on the
Pd active sites.
3.2.3. Effect of applied current density
In an attempt to study the effect of current density on dechlorination efficiency, a
series of electrolysis experiments was carried out. As shown in Fig. 5, the highest
dechlorination efficiency was obtained at the current density of 208 A m−2 (geometric
area), especially during the later periods of electrolysis. The increase of current
density from 83 A m−2 to 208 A m−2 resulted in the slight decrease of CE and the
substantial increase of PA yield, whereas the increase of current density from 208 A
m−2 to 250 A m−2 led to the decrease of both CE and PA yield at the end of the
electrolysis experiments.
3.3. ECH dechlorination mechanism
3.3.1. CV evidence
Fig. 6 shows the CV curves of Pd/Ni disk electrode in 1.25 M NaOH aqueous
solution in the absence or presence of 3,6-D. In the absence of 3,6-D (Fig. 6A), the
reductive peak (C1) associated with the electro-adsorption of hydrogen (Eq. 1,
Section 1) and the reductive current response (C2) attributed to the absorption and
evolution of hydrogen were detected in the negative potential going scan; in addition,
a oxidative peak (A3) originated from the oxidation of particularly favourable sites on
12

the palladium surface, two oxidative peaks of adsorbed hydrogen (A2 and A2’) and an
oxidative peak of absorbed hydrogen (A1) were observed in the positive potential
going scan [47]. In the presence of 3,6-D (Fig. 6B), three oxidative peaks (A3, A2 and
A2’) disappeared and the peak current of A1 decreased continuously with the rise of
3,6-D concentration in the electrolyte. Moreover, it is noted that a small double-layer
charging current was present in the potential range between -0.15 and -0.35 V with
the addition of 3,6-D (Fig. 6B), indicating the adsorption of 3,6-D over the Pd/Ni
electrode [Eq. 2, Section 1] [48]. This shows the typical CV curves of the ECH
dechlorination processes at Pd-modified electrodes [15,17], which strongly suggests
that the dechlorination of 3,6-D follows the stated typical ECH mechanism in Section
1.
The potentials of the Pd/Ni foam cathode were more positive than −1.25 V in
most aforementioned electrolysis experiments (Section 3.1. and 3.2.. Fig. 7 shows the
typical CP curves of Pd/Ni foam cathodes in a 1.25 M NaOH aqueous solution in the
presence of 250 mM 3,6-D. The applied potential during electrolysis mainly ranged
from -1.0 V to -1.25 V.). Therefore, the CV curves (Fig. 6) with a limited negative
potential of −1.25 V are valid to reflect the dechlorination mechanism of 3,6-D on the
Pd/Ni foam cathode.
3.3.2. Surface morphology, composition and phase of Pd/Ni cathode
According to the stated ECH mechanism, the relative rates of hydrogenation (Eq.
3) and H2 desorption steps (Eqs. 5 and 6) determines the dechlorination efficiency of
Pd/Ni foam cathode; thus, the factors that influence these two steps would affect the
dechlorination efficiency of 3,6-D [16].
HadsPd + H2O + e- → H2 + OH-
HadsPd + HadsPd ↔ H2
(5)
(6)
Both the hydrogenation and H2 desorption steps occurred on the surface of Pd/Ni
cathode. Therefore, the surface morphology, composition and phase of Pd/Ni cathode
must be very important factors that strongly influence the dechlorination efficiency of
Pd/Ni foam cathode. As shown in the SEM pictures of Fig 8, the surface of Ni foam
13

was covered with an obvious film after the metallic replacement reaction. Because
EDS and XPS (Fig 8) gave a similar surface Ni/Pd ratio of approximate 3/7 for the
Pd/Ni foam, it is believed that the thickness of the film was up to a few microns, and
the film had a uniform Ni/Pd ratio in depth. In addition, given that no diffraction peak
corresponding to Pd-Ni ally was observed in the XRD patter of the Pd/Ni foam (Fig.
9), the possibility of having a Pd-Ni alloy was omitted. Based on these results, the
surface of Pd or the interface of Pd-Ni bimetallic contact was assumed to be the main
reaction site for the ECH dechlorination of 3,6-D. Unfortunately, it is not possible to
discriminate between these two reaction sites on the basis of the data so far available.
3.3.3. Effect of catholyte pH
Catholyte pH not only influences the adsorption of H (Eq. 1) but may also
change the molecular structure and adsorption behavior of 3,6-D [49], making it
another important factor that can significantly change the rate of hydrogenation step
and dechlorination route. Two Cl atoms in the 3,6-D molecule were eliminated
stepwise in 1.25 M NaOH, whereas the atoms were removed together in one step in
0.75 M H2SO4 (Section 3.2.1.). In order to rationalize the abovementioned significant
difference in the dechlorination route, the molecular structure and two C-Cl bonds
length of 3,6-D were calculated by DFT. In the alkaline environment, 3,6-D is in an
anionic form. The bond length of C6-Cl is longer than that of C3-Cl for 0.027 Å (Table
4). In strong acidic solution, however, 3,6-D is cationic as carboxylate ion and the
pyridinic nitrogen are both protonated [49]. In contrast, the bond length difference of
C6-Cl and C3-Cl are very small (only 0.011 Å). On the basis of these results, we
believe that the different molecular structure of 3,6-D in different pH solutions was
one of the essential reasons for the different dechlorination route.
The difference in dechlorination route may also resulted from a change in the
surface adsorption geometry of 3,6-D on the used Pd/Ni cathode along with the
change of pH value. In 1.25 M NaOH, 3,6-D was mainly adsorbed on the cathode
surface through Cl at C6 to minimize the electrostatic repulsion between 3,6-D and the
cathode as the COO- of 3,6-D is completely unprotonated [50]. Thus, the Cl at C6 was
14

dechlorinated preferentially. In 0.75 M H2SO4, however, electrostatic repulsion
became weaker because of the protonation of COO- and pyridinic nitrogen [49,50].
Therefore, both the Cl at C6 and the Cl at C3 of 3,6-D can be adsorbed on the cathode,
thereby eliminating the two Cl atoms in the molecule of 3,6-D in one step. Based on
the discussion regarding the CV data, the dechlorination route and the electrode
surface characteristic, the ECH dechlorination mechanism of 3,6-D on the Pd/Ni foam
cathode is proposed in Fig. 10.
3.4. Rate-limiting step
3.4.1. CV evidence
According to the stated ECH mechanism in Section 1, the hydrogenation step
(Eq. 3) in the presence of 3,6-D would consume the adsorbed hydrogen, reduce the
concentration of adsorbed hydrogen, and accelerate the electro-adsorption of
hydrogen. This explains why the addition of 3,6-D increased the reductive peak (C1)
at the expense of the oxidative peaks (A1, A2, A2’ and A3) (Fig. 6B). The current of
the reductive peak (C1) shows a trend of first increased quickly and then increased
slowly when the concentration of 3,6-D increased from 0 mM to 100 mM and then
from 100 mM to 250 mM, suggesting that the ECH dechlorination process was
kinetically limited by a surface reaction step at the high reactant concentration
whereas partially by a diffusion step of 3,6-D from bulk catholyte to the cathode
surface at the low reactant concentration.
3.4.2. Effect of 3,6-D initial concentration
Four surface reaction steps (Eqs. 1–4) are included in the ECH dechlorination
process of 3,6-D. Among these steps, the adsorption of 3,6-D was influenced directly
by the 3,6-D concentration in the bulk catholyte. Therefore, the concentration of
3,6-D is an important factor that can change the rate-limiting step of the
dechlorination process.
As shown in Table 3, over 95% CE was achieved at the initial 3,6-D
concentration range of 200 mM to 300 mM (Entries 5–7). This result demonstrates
15

that the H2 desorption step (Eqs. [5] and [6]) was almost completely restricted during
most of the ECH experiments, which implies that the adsorbed hydrogen was
immediately consumed by a very fast hydrogenation step and the concentration of
adsorbed hydrogen dropped to near zero on the Pd/Ni foam cathode. Simply put, the
generation of adsorbed hydrogen was the rate-limiting step of the dechlorination
process under this circumstance. The high concentration of 3,6-D in the bulk catholyte
inevitably led to high concentration of 3,6-D on the surface of the Pd/Ni foam cathode
because the ECH dechlorination process was not kinetically limited by the diffusion
step of 3,6-D. The high concentration of 3,6-D on the surface of the Pd/Ni foam
cathode was favorable for the adsorption of 3,6-D, thereby accelerating the
hydrogenation step.
At the initial 3,6-D concentration range of 25 mM to 50 mM (Table 3), however,
the CE obtained was very low (17.6%–35.7%). This result demonstrates that the
concentration of adsorbed hydrogen on the Pd/Ni foam cathode was quite high during
the ECH experiments. In other words, the ECH dechlorination process was not limited
by the generation of adsorbed hydrogen under such a condition. Using the same
theory, low concentrations of 3,6-D in the bulk catholyte inevitably resulted in low
concentrations of 3,6-D on the surface of the Pd/Ni foam cathode, which is
unfavorable for both the adsorption of 3,6-D and the hydrogenation step. Therefore,
these two steps or the diffusion step of 3,6-D from bulk catholyte to the cathode
surface were most likely the rate-limiting steps of the dechlorination in the presence
of low 3,6-D concentrations.
3.4.3. Effect of applied current density
The current density would affect the ECH dechlorination by directly controlling
the generation rate of adsorbed hydrogen. Therefore, current density is another
important factor that might change the rate-limiting step of the ECH dechlorination
process.
As shown in Fig. 5(A), over 94% CE was achieved during the whole electrolysis
experiment of 250 mM 3,6-D at the current density of 83 A m−2. This result suggests
16

that the hydrogenation step was faster than the generation of adsorbed hydrogen under
such a condition, and the ECH dechlorination process was kinetically controlled by
the generation of adsorbed hydrogen.
At the high current density of 250 A m−2, however, a parabolic drop was
observed in the CE when electrolysis time was extended. Over 95% CE was achieved
during the first two hours of the electrolysis experiment (Fig. 5A). Therefore, the
generation of adsorbed hydrogen kinetically controlled the ECH dechlorination at this
period of time. A very low CE (lower than 30%) was obtained during the last hour of
the electrolysis experiment (Fig. 5C), which indicates that the generation rate of the
adsorbed hydrogen was faster than that of the hydrogenation step. Therefore, the
hydrogenation step and/or adsorption of 3,6-D was most likely the rate-limiting step
of the dechlorination process under such a condition. The shift of the rate-limiting
step during the electrolysis experiment resulted from the decrease of 3,6-D
concentration in the catholyte when the electrolysis time was extended (Fig. 5D),
thereby decreasing the rate of adsorption of 3,6-D and the hydrogenation step.
3.5. ECH dechlorination of the ClPA mixtures
Using the optimized experimental conditions (7 cm × 13 cm Pd/Ni foam with a
Pd loading of 2.25 mg cm-2 used as the cathode, 700 mL 1.25 M NaOH aqueous
solution as the catholyte, 208 A m−2 as the applied current density), the practicability
of the ECH dechlorination system to covert the ClPA mixtures to PA was assessed
using a plate-and-frame cell at a flow rate of 1.2 L/min.
After ECH dechlorination for 12 h, four ClPAs in the catholyte (total
concentration of ClPAs was approximately 250 mM) were converted to PA with 99%
PA yield, 76.3% CE, and 2.47 kW·h·kg−1 PA SEEC. These technical indexes indicate
that the ECH dechlorination system is likely to be high economic value, considering
that the price of commercial electric energy and PA is approximately 0.12 USD
per·(kW·h)−1 and 30 USD per kg in China, respectively. Besides electric energy, the
cost of electrode materials (especially the cost of Pd), chemical reagents used for pH
adjustment should be carefully assessed, if we plan to put the ECH dechlorination
17

system into industrial applications.
High current density and high CE are very important for practical electrolysis
techniques, which not only reduce the dosage of catalyst but also increase the
space-time yield of an electrolysis reactor. As shown in the ECH dechlorination of the
ClPA mixtures, high CE (76.3%) was achieved at an acceptable current density (208 A
m−2) when high concentration (250 mM) of reactants was used. These results suggest
that the ECH dechlorination system developed in this work can be potentially used in
industrial applications for the dechlorination of ClPA mixtures to PA and even for the
disposal of other high concentration COCs that are soluble in basic aqueous solutions,
such as chlorinated phenols, chlorinated phenoxyacetic acids, and chloracetic acids.
4. Conclusion
A highly efficient ECH dechlorination system (1.25 M NaOH aqueous solution
as the catholyte, Pd/Ni foam as the cathode) was developed for the dechlorination of a
ClPA mixtures into PA. This goal was achieved by using 3,6-D as the target
compound and optimizing the Pd loading, its cathode support (Ni foam, Ag mesh, and
Cu foam), and operating parameters, such as catholyte pH, initial reactant
concentration, and applied current density. We also investigated the dechlorination
mechanism of 3,6-D with regard to the surface condition of cathode and catholyte pH,
the rate-limiting step of the dechlorination process, and the practicability of the
optimized ECH dechlorination system to convert the ClPA mixtures into PA. The
main findings are as follows:
(1) The activity of six metal cathodes during the dechlorination of 3,6-D in 0.5 M
NaOH aqueous solution decreased in the following order: Pd/Ni foam > Pd/Ag mesh
≈ Pd/Cu foam >> Ni foam ≈ Ag mesh ≈ Cu foam, and the Pd/Ni foam cathodes
with a Pd loading of 2.25–3.6 mg cm-2 exhibited the optimum dechlorination
performance. The dechlorination of 3,6-D on the Pd/Ni foam cathodes was confirmed
as a typical ECH process, and the surface of Pd or the interface of Pd-Ni bimetallic
contact was probably the main ECH site.
18

(2) The dechlorination efficiency increased with the rise of the initial
concentration of 3,6-D (from 25–200 mM). The initial 3,6-D concentration region of
200–300 mM exhibited the highest dechlorination efficiency, which indicates that
higher initial concentrations (< 300 mM) favor the ECH dechlorination of 3,6-D on
the Pd/Ni foam cathode. The current density also considerably affected the
dechlorination reactions, as a moderate current density (208 A m−2) exhibited the
highest dechlorination efficiency.
(3) The intermediate product selectivity and efficiency of the ECH
dechlorination system strongly depended on the catholyte pH values. The highest
dechlorination efficiency was achieved in 1.25 M NaOH or 0.75 M H2SO4. Two Cl
atoms in the 3,6-D molecule were eliminated stepwise in 1.25 M NaOH, whereas the
atoms were removed together in one step in 0.75 M H2SO4. This result may be
associated with the change in the molecular structure or the surface adsorption
geometries of 3,6-D on the Pd/Ni foam cathode.
(4) The rate-limiting step of the dechlorination process changed along with the
variation of reactant concentration in the catholyte and applied current density. The
dechlorination process was kinetically limited by the generation of adsorbed hydrogen
at low applied current density, whereas it may has been kinetically limited by the
hydrogenation step and/or adsorption of reactants or the diffusion step of 3,6-D from
bulk catholyte to the cathode surface at low concentration of reactants in return.
(5) The practicability research demonstrated that the 47 g L-1 ClPA mixtures
(total concentration of ClPAs: approximately 250 mM) can be converted into PA with
99% yield, 76.3% CE, and 2.47 kW·h·kg−1 PA SEEC in the optimized ECH
dechlorination system. This finding indicated that the ECH dechlorination system
developed in this work presents a considerable potential in industrial applications.
For future studies, this ECH dechlorination system can be adjusted for the
disposal of other high-concentration COCs, which are soluble in basic aqueous
solutions, such as chlorinated phenols, chlorinated phenoxyacetic acids, and
chloracetic acids.
19

Acknowledgments
This work was supported by the National Natural Science Foundation of China
(21106133, 21207028, and 21576238), the Natural Science Foundation of Zhejiang
Province, China (LY16B060012), and the National Basic Research Program of China
(973 Program) (2012CB722604).
References
[1] Public announcement of project acceptance accomplishing environmental
protection installation, Environmental Protection Agency, Zhejiang Province, 2013,
http://www.zjepb.gov.cn/root14/xxgk/xzxk/xzxkspgs/jsxmhjbhssjgys/201311/t201311
20_294868.html.
[2] Public announcement of project acceptance accomplishing environmental
protection installation, Environmental Protection Agency, Mianyang City, 2014,
http://www.my.gov.cn/MYGOV/150650805483995136/20140815/1117055.html.
[3] Public announcement of project acceptance accomplishing environmental
protection installation, Environmental Protection Agency, Zhangjiakou City, 2014,
http://www.zjkepb.gov.cn/show.aspx?id=12179&cid=35.
[4] C.A. Ma, Y.H. Xu, Y.Q. Chu, X.B. Mao, F.M. Zhao, Y.H. Zhu, Electrochemical
synthesis of 3,6-dichloropicolinic acid and its industrialization, CIES J. 61 (2010)
699–703.
[5] D.V. Sojic, V.B. Anderluh, D.Z. Orcic, B.F. Abramovic, Photodegradation of
clopyralid in TiO2 suspensions: Identification of intermediates and reaction pathways,
J. Hazard. Mater. 168 (2009) 94–101.
[6] K. Westphal, R. Saliger, D. Jager, L. Teevs, L. Teevs, U. Pruße, Degradation of
clopyralid by the Fenton reaction, Ind. Eng. Chem. Res. 52 (2013) 13924–13929.
[7] G. Xu, T. Bu, M. Wu, J. Zheng, N. Liu, L. Wang, Electron beam induced
degradation of clopyralid in aqueous solutions, J. Radioanal. Nucl. Chem.288 (2011)
20

759–764.
[8] C. Tizaoui, K. Mezughi, R. Bickley, Heterogeneous photocatalytic removal of the
herbicide clopyralid and its comparison with UV/H2O2 and ozone oxidation
techniques, Desalination 273 (2011) 197–204.
[9] S.Q. Wang, J.Z. He, Dechlorination of commercial PCBs and other multiple
halogenated compounds by a sediment-free culture containing dehalococcoides and
dehalobacter, Environ. Sci. Technol. 47 (2013) 10526–10534.
[10] J. Farrell, M. Kason, N. Melitas, T. Li, Investigation of the long-term
performance of zero-valent iron for reductive dechlorination of trichloroethylene,
Environ. Sci. Technol.34 (2000) 514–521.
[11] Y. Kim, E.R. Carraway, Dechlorination of pentachlorophenol by zero valent iron
and modified zero valent irons, Environ. Sci. Technol. 34 (2000) 2014–2017.
[12] L. Teevs, K.-D. Vorlp, U. Pruße, Model study on the aqueous-phase
hydrodechlorination of clopyralid on noble metal catalysts, Catal. Commun. 14 (2011)
96–100.
[13] C. Sun, Z.M. Lou, Y. Liu, R.Q. Fu, X.X. Zhou, Z. Zhang, S.A. Baig, X.H. Xu,
Influence of environmental factors on the electrocatalytic dechlorination of
2,4-dichlorophenoxyacetic acid on nTiN doped Pd/Ni foam electrode, Chem. Eng. J.
281 (2015) 183–191.
[14] C. Sun, S.A. Baig, Z.M. Lou, J. Zhu, Z.X. Wang, X. Li, J.H. Wu, Y.F. Zhang,
X.H. Xu, Electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid using
nanosized titanium nitride doped palladium/nickel foam electrodes in aqueous
solutions, Appl. Cata. B: Environ. 158–159 (2014) 38–47.
[15] I.G. Casella, M. Contursi, Electrocatalytic reduction of chlorophenoxy acids at
palladium-modified glassy carbon electrodes, Electrochim. Acta 52 (2007)
7028–7034.
[16] Y.H. Xu, Q.Q. Cai, H.X. Ma, Y. He, H. Zhang, C.A. Ma, Optimisation of
electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid on a roughened
silver–palladium cathode, Electrochim. Acta 96 (2013) 90–96.
[17] C.A. Ma, H. Ma, Y.H. Xu, Y.Q. Chu, F.M. Zhao, The roughened
21

silver-palladium cathode for electrocatalytic reductive dechlorination of
2,4-Dichlorophenoxyacetic acid, Electrochem. Commun. 11 (2009) 2133–2136.
[18] W.J. Li, H.Y. Ma, L.H. Huang, Y. Ding, Well-defined nanoporous palladium for
electrochemical reductive dechlorination, Phys. Chem. Chem. Phys. 13 ( 2011)
5565–5568.
[19] W.J. Xie, S.H. Yuan, X.H. Mao, W. Hu, P. Liao, M. Tong, A.N. Alshawabkeh,
Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in
groundwater, Water Res.47 (2013) 3573–3582.
[20] I.S. Park, K.S. Lee, S.J. Yoo, Y.H. Cho, Y.E. Sung, Electrocatalytic properties of
Pd clusters on Au nanoparticles in formic acid electro-oxidation, Electrochim. Acta 55
(2010) 4339–4345.
[21] I.F. Cheng, Q. Fernando, N. Korte, Electrochemical dechlorination of
4-chlorophenol to phenol, Environ. Sci. Technol.31 (1997) 1074–1078.
[22] Z. Sun,; H. Ge, X. Hu, Y. Peng, Electrocatalytic Dechlorination of Chloroform in
Aqueous Solution on Palladium/Titanium Electrode, Chem. Eng. Technol. 32(2009)
134–139.
[23] H. Cheng, K. Scott, P.A. Christensen, Electrochemical Hydrodehalogenation of
Chlorinated Phenols in Aqueous Solutions II. Effect of Operating Parameters, J.
Electrochem. Soc. 150 (2003) D25-D29.
[24] B. Yang, G. Yu, D.M. Shuai, Electrocatalytichydrodechlorination of
4-chlorobiphenyl in aqueous solution using palladized nickel foam cathode,
Chemosphere 67 (2007) 1361-1367.
[25] B. Yang, G. Yu, J. Huang, Electrocatalytic Hydrodechlorination of
2,4,5-Trichlorobiphenyl on a Palladium-Modified Nickel Foam Cathode, Environ. Sci.
Technol. 41 (2007) 7503-7508.
[26] K.R. Zhu, C. Sun, H. Chen, S.A. Baig, T.T. Sheng, X.H. Xu, Enhanced catalytic
hydrodechlorination of 2,4-dichlorophenoxyacetic acid by nanoscale zero valent iron
with electrochemical technique using a palladium/nickel foam electrode, Chem. Eng.
J. 223 (2013) 192–199.
[27] Z.Q. He, J.J. Sun, J. Wei, Q. Wang, C.X. Huang, J.M. Chen, S. Song, Effect of
22

silver or copper middle layer on the performance of palladium modified nickel foam
electrodes in the 2-chlorobiphenyl dechlorination, J. Hazard. Mater.250–251 (2013)
181–189.
[28] P. Dabo, A. Cyr, F. Laplante, F. Jean, H. J. Menard, Lessard, Electrocatalytic
dehydrochlorination of pentachlorophenol to phenol or cyclohexanol, Environ. Sci.
Technol. 34 (2000) 1265–1268.
[29] G. Chen, Z.Y. Wang, D.G. Xia, Electrochemically codeposited
palladium/molybdenum oxide electrode for electrocatalytic reductive dechlorination
of 4-chlorophenol, Electrochem. Commun. 6 (2004) 268–272.
[30] M. Kuroboshi, H. Idei, N. Kara, Y. Kokui, H. Yanaka, Electroreduction of aryl
halides loaded on palladium-immobilized activated carbon, Electrochemistry 81
(2013) 359–361.
[31] A.I. Tsyganok, K. Otsuka, Selective dechlorination of chlorinated phenoxy
herbicides in aqueous medium by electrocatalytic reduction over palladium-loaded
carbon felt, Appl. Cata. B: Environ. 22 (1999) 15–26.
[32] Y.X. Fang, S.R. Al-Abed, Electrocatalytic dechlorination of a PCB congener at a
palladized granular-graphite-packed electrode: Reaction equilibrium and mechanism,
Appl. Cata. B: Environ.80 (2008) 327–334.
[33] C.Y. Cui, X. Quan, H.T. Yu, Y.H. Han, Electrocatalytic hydrodehalogenation of
pentachlorophenol at palladized multiwalled carbon nanotubes electrode, Appl. Cata.
B: Environ. 80 (2008) 122-128.
[34] Q. Shi, H. Wang, S. Liu, Z. Bian, Electrocatalytic degradation of
2,4-dichlorophenol using a Pd/grapheme gas-diffusion electrode, RSC Adv., 4 (2014)
56263-56272.
[35] Q. Shi, H. Wang, S. Liu, L. Pang, Z. Bian, Electrocatalytic reduction-oxidation
of chlorinated phenols using a nanostructured Pd-Fe modified graphene catalyst,
Electrochim. Acta 178 (2015) 92-100.
[36] G. Chen, Z.Y. Wang, T. Yang, D.D. Huang, D.G. Xia, Electrocatalytic
hydrogenation of 4-chlorophenol on the glassy carbon electrode modified by
composite polypyrrole/palladium film, J. Phys. Chem. B 110 (2006) 4863-4868.
23

[37] Z.R. Sun, H. Ge, X. Hu, Y.Z. Peng, Preparation of foam-nickel composite
electrode and its application to 2,4-dichlorophenol dechlorination in aqueous solution,
Sep. Purif. Technol. 72 (2010) 133-139.
[38] Z.R. Sun, X.F. Wei, H.T. Shen, X. Hu, Preparation and evaluation of
Pd/polymeric
pyrrole-sodium
lauryl
sulfonate/foam-Ni
electrode
for
2,4-dichlorophenol dechlorination in aqueous solution, Electrochim. Acta 129 (2014)
433–440.
[39] Z.R. Sun, X.F. Wei, Y.B. Han, S.Tong, X. Hu, Complete dechlorination of
2,4-dichlorophenol in aqueous solution on palladium/polymeric pyrrole-cetyltrimethyl
ammonium bromide/foam-nickelcomposite electrode, J. Hazard. Mater.244–245(2013)
187–194.
[40] Z.R. Sun, H.T. Shen, X.F. Wei, X. Hu, Electrocatalytic hydrogenolysis of
chlorophenols in aqueous solution on Pd58Ni42 cathode modified with PPy and SDBS,
Chem. Eng. J. 241 (2014) 433–442.
[41] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange-correlation functional
using the Coulomb-attenuating method (CAM-B3LYP), Chem. Phys. Lett. 393 (2004)
51-57.
[42] R. Krishnan, J. S. Binkley, R. Seeger, J.A. Pople, Self-consistent molecular
orbital methods. XX. A basis set for correlated wave functions, J. Chem. Phys.72
(1980) 650-654.
[43] M.J. Frisch, G.W. Trucks, H. B. Schlegel, et al. Gaussian 03, Revision C.01;
Gaussian Inc.: Pittsburgh, PA, 2003.
[44] S. Wang, B. Yang, T.T. Zhang, G. Yu, S.B. Deng, J. Huang, Catalytic
Hydrodechlorination of 4-Chlorophenol in an Aqueous Solution with Pd/Ni Catalyst
and Formic Acid, Ind. Eng. Chem. Res. 49 (2010), 4561–4565.
[45] E. Diaz, J.A. Casas, A.F. Mohedano, L. Calvo, M. A. Gilarranz, J.J. Rodriguez,
Kinetics of the Hydrodechlorination of 4-Chlorophenol in Water Using Pd, Pt, and
Rh/Al2O3 Catalysts, Ind. Eng. Chem. Res. 47 (2008), 3840–3846.
[46] J.A. Baeza, L. Calvo, M.A. Gilarranz, A.F. Mohedano, J.A. Casas, J.J. Rodriguez,
Catalytic behavior of size-controlled palladium nanoparticles in the
24

hydrodechlorination of 4-chlorophenol in aqueous phase, J. Catal. 293 (2012) 85–93.
[47] G. Denuault, C. Milhano, D. Pletcher, Mesoporous palladium–the surface
electrochemistry of palladium in aqueous sodium hydroxide and the cathodic
reduction of nitrite, Phys. Chem. Chem. Phys. 7 (2005) 3545-3551.
[48] S.L. Yau, Y.G. Kim, K.J. Itaya, In situ scanning tunneling microscopy of
benzene adsorbed on Rh (111) and Pt (111) in HF solution. J. Am. Chem. Soc. 118,
(1996) 7795−7803.
[49] Y.H. Xu, X.F. Ding, H.X. Ma, Y.Q. Chu, C.A. Ma, Selective
hydrodechlorination of 3,5,6-trichloropicolinic acid at an activated silver cathode:
Synthesis of 3,5-dichloropicolinic acid, Electrochim. Acta 151 (2015) 284–288.
[50] M.C. Corredor, J.M.R. Mellado, M.R. Montoya, EC(EE) process in the reduction
of the herbicide clopyralid on mercury electrodes, Electrochim. Acta 51 (2006)
4302–4308.
Figure captions
Fig. 1. Schematic construction of the electrolysis device by using a plate-and-frame cell.
Fig. 2. Effect of Pd loading on the electrolysis of 3,6-D. Cathode: Pd/Ni foams (Projected area: 2
cm × 6 cm); Catholyte: 50 mL 0.5 M NaOH + 0.25 M 3,6-D; Anode: graphite sheet (2 cm × 6 cm
× 0.5 cm); Anolyte: 50 mL 1.25 M NaOH; Applied current: 0.25 A (208 A m−2); Electrolysis time:
6 h; Temperature: 25 °C. CE was calculated with △t = 6 h.
Fig. 3. Stability of Pd/Ni foam cathode over six cycles of electrolysis. The projected area of the
Pd/Ni foam cathode: 2 cm × 6 cm; Pd loading: 2.25 mg cm-2; Catholyte: 50 mL 0.5 M NaOH +
0.25 M 3,6-D; Anode: graphite sheet (2 cm × 6 cm × 0.5 cm); Anolyte: 50 mL 1.25 M NaOH;
Applied current: 0.25 A (208 A m−2); Electrolysis time of each run: 6 h; Temperature: 25 °C. CE
was calculated with △t = 6 h.
Fig. 4. Effect of pH on product selectivity and CE during the electrolysis of 3,6-D. Cathode: Pd/Ni
foams (Projected area: 2 cm × 6 cm, Pd loading: 2.25 mg cm-2); Catholyte: (A and A’) 50 mL 1.25
M NaOH + 96 mM 3,6-D, (B and B’) 50 mL 0.75 M H2SO4 + 96 mM 3,6-D+ 20% v/v Ethanol;
Anode: graphite sheet (2 cm × 6 cm × 0.5 cm); Anolyte: 50 mL 1.25 M NaOH for A, and 50 mL
0.75 M H2SO4 for B; Applied current: 0.1 A; Temperature: 25 °C; CE was calculated with △t = 1
h.
25

Fig. 5. Effect of current density on CE, yield of PA, and conversion of 3,6-D during the
electrolysis of 3,6-D. (A) Partially enlarged drawing of (C). Cathode: Pd/Ni foams (Projected area:
2 cm × 6 cm, Pd loading: 2.25 mg cm-2); Catholyte: 50 mL 1.25 M NaOH + 250 mM 3,6-D;
Anode: graphite sheet (2 cm × 6 cm × 0.5 cm); Anolyte: 50 mL 1.25 M NaOH; Temperature:
25 °C; CE was calculated with △t = 1 h.
Fig. 6. CV curves of Pd/Ni disk (3 mm diameter) in a 1.25 M NaOH aqueous solution (A) with
different negative limit; (B) with various concentrations of 3,6-D, at a scan rate of 100 mV·s−1.
Fig. 7. Typical chronopotentiometry (CP) curves during the electrolysis at 25°C. Cathode: Pd/Ni
foams (Projected area: 2 cm × 6 cm, Pd loading: 2.25 mg cm-2); Catholyte: 50 mL 1.25 M NaOH
+ 0.25 M 3,6-D; Anode: graphite sheet (2 cm × 6 cm × 0.5 cm); Anolyte: 50 mL 1.25 M NaOH;
Applied current: 0.25 A (208 A m−2).
Fig. 8. SEM images and EDS of a Ni foam and a Pd/Ni foam, and XPS images of Pd/Ni foam (Pd
loading: 2.25 mg cm-2).
Fig. 9. XRD patterns of (A) a Ni foam and (B) a Pd/Ni foam (Pd loading: 2.25 mg cm-2).
Fig. 10. Proposed mechanisms for the ECH dechlorination of 3,6-D.
Power supply
A: Pd/Ni foam cathode
0 0 0 0
V
0 0 0 0 A
B: Nafion-117 membrance
C: Stainless steel mesh anode
_
+
Anolyte tank
Catholyte tank
A
B C
P O W E R
0 0 0
Water bath
Peristaltic pump
Peristaltic pump
Fig. 1
26

100
90
80
70
60
50
100
90
80
70
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pd loading / mg cm-2
Fig. 2
110
100
90
Yield of PA
CE
80
70
60
50
1
2
3
4
5
6
Run
Fig. 3
27

90
60
30
0
PA
B
A
3-ClPA
6-ClPA
3,6-D
All
120
A'
B'
100
80
0
2
4
6
0
2
4
6
Electrolysis time / h
Fig. 4
28

105
100
95
83 A m-2
100
80
60
40
20
0
(B)
125 A m-2
166 A m-2
208 A m-2
250 A m-2
(A)
90
85
100
100
80
60
40
20
80
60
40
20
(D)
4
(C)
2
4
6
2
6
Electrolysis time / h
Fig. 5
4
2
0
10
Blank
C2
Blank
25 mM 3,6-D
100 mM 3,6-D
250 mM 3,6-D
8
6
C1
4
2
-2
-4
0
A3
A2'
A2
A1
-0.3 -0.6 -0.9 -1.2
-2
-4
(B)
-0.3 -0.6 -0.9 -1.2
(A)
E / V vs. Ag/AgCl
Fig. 6
29