Reductive dechlorination of trichloroacetic acid (TCAA) by electrochemical process over Pd-In/Al2O3 catalyst

Article abstract of DOI:10.1016/j.electacta.2017.02.071

Electrochemical reduction treatment was found to be a promising method for dechlorination of Trichloroacetic acid (TCAA), and acceleration of electron transfer or enhancement of the concentration of atomic H* significantly improve the electrochemical dechlorination process. Bimetallic Pd-based catalysts have the unique property of simultaneously catalyzing the production of atomic H* and reducing target pollutants. Herein, a bimetallic Pd–In electrocatalyst with atomic ratio of 1:1 was evenly deposited on an Al₂O₃ substrate, and the bimetallic Pd-In structure was confirmed via X-ray photoelectron spectroscopy (XPS). Electrochemical removal of trichloroacetic acid (TCAA) by the Pd-In/Al₂O₃ catalyst was performed in a three-dimensional reactor. 94% of TCAA with the initial concentration of 500?μg?L^?1 could be degraded within 30?min under a relatively low current density (0.9?mA?cm^?2). In contrast to the presence of refractory intermediates (dichloroacetic acid (DCAA)) found in the Pd/Al₂O₃ system, TCAA could be thoroughly reduced to monochloroacetic acid (MCAA) using Pd-In/Al₂O₃ catalysts. According to scavenger experiments, an electron transfer process and atomic H* formation function both existed in the TCAA reduction process, and the enhanced indirect atomic H* reduction process (confirmed by ESR signals) played a chief role in the TCAA removal. Moreover, the synergistic effects of Pd and In were proven to be able to enhance both direct electron transfer and indirect atomic H* formation, indicating a promising prospect for bimetallic electrochemical reduction treatment.

Full text of DOI:10.1016/j.electacta.2017.02.071

Electrochimica Acta 232 (2017) 13–21
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Reductive dechlorination of trichloroacetic acid (TCAA) by
electrochemical process over Pd-In/Al O catalyst
2
3
Yanzhen Liu , Ran Mao , Yating Tong , Huachun Lan *, Gong Zhang , Huijuan Liub,c,
Jiuhui Qu
a
a,b
a
a
a,
a
a,b
Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085,
PR China
b
University of Chinese Academy of Sciences, Beijing, 100049, PR China
State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, PR
c
China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 14 October 2016
Electrochemical reduction treatment was found to be a promising method for dechlorination of
Trichloroacetic acid (TCAA), and acceleration of electron transfer or enhancement of the concentration of
atomic H* significantly improve the electrochemical dechlorination process. Bimetallic Pd-based
catalysts have the unique property of simultaneously catalyzing the production of atomic H* and
reducing target pollutants. Herein, a bimetallic Pd–In electrocatalyst with atomic ratio of 1:1 was evenly
Received in revised form 6 February 2017
Accepted 14 February 2017
Available online 16 February 2017
Keywords:
Electrocatalytic dechlorination
TCAA removal
deposited on an Al O3 substrate, and the bimetallic Pd-In structure was confirmed via X-ray
2
photoelectron spectroscopy (XPS). Electrochemical removal of trichloroacetic acid (TCAA) by the Pd-
In/Al
2
O
3
catalyst was performed in a three-dimensional reactor. 94% of TCAA with the initial
Pd-In/
g-Al
2
O
3
ꢀ1
concentration of 500
mg L could be degraded within 30 min under a relatively low current density
ꢀ
2
(
0.9 mA cm ). In contrast to the presence of refractory intermediates (dichloroacetic acid (DCAA)) found
in the Pd/Al system, TCAA could be thoroughly reduced to monochloroacetic acid (MCAA) using Pd-In/
Al catalysts. According to scavenger experiments, an electron transfer process and atomic H*
2 3
O
2 3
O
formation function both existed in the TCAA reduction process, and the enhanced indirect atomic H*
reduction process (confirmed by ESR signals) played a chief role in the TCAA removal. Moreover, the
synergistic effects of Pd and In were proven to be able to enhance both direct electron transfer and
indirect atomic H* formation, indicating a promising prospect for bimetallic electrochemical reduction
treatment.
©
2017 Elsevier Ltd. All rights reserved.
1
. Introduction
completely dechlorinated to acetic acid over supported Pd
catalysts on ZrO , which could convert H to atomic H*. However,
liquid-phase catalytic hydrogenation requires constant H supple-
2
2
Trichloroacetic acid (TCAA) is one of the largest fractions of
chloroacetic acids (CAAs) that always exists in chlorinated water
1–3]. TCAA has been proven to be carcinogenic and poses potential
2
mentation, causing hidden danger for storage and transportation.
Alternatively, the electrochemical treatment of TCAA was found to
be more promising as it ensures the selective removal of chlorine
atoms from TCAA, while no toxic byproducts are produced [11–14].
Two main steps, namely, direct dechlorination by electron transfer
and an indirect process induced by atomic H*, take place during the
electrocatalytic reduction process [15]. Generally, in contrast to the
low reduction efficiency of electron transfer, the catalytically
produced atomic H* plays a predominant role in dechlorination. Li
et al. [13] studied the electrochemical dechlorination activity of
CAAs with electrodeposited Fe- and Pd/Fe-modified carbon paper
as cathodes. The results showed that while only 58% of TCAA could
be removed within 30 min at pH 7.0 with the bias potential of
[
risks to human health [4,5], and a 0.1 mg/L standard for TCAA in
drinking water has already been promulgated. To meet this
limitation, numerous treatment methods, including biodegrada-
tion, photocatalysis, advanced oxidation, and Fenton reaction, have
recently been explored [6–8]. Among these removal approaches,
catalytic hydrogenation has been proven to be an efficient method
to remove TCAA [9]. Zhou et al. [10] found that CAAs could be
*
Corresponding author.
E-mail address: lhclan@126.com (H. Lan).
http://dx.doi.org/10.1016/j.electacta.2017.02.071
013-4686/© 2017 Elsevier Ltd. All rights reserved.
0

14
Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
ꢀ
1.5 V using a Fe-C electrode through the direct electron transfer
Microscope (TEM) images and Energy Dispersive Spectrometer
(EDS) images were obtained with Transmission Electron
process, 96% of TCAA could be removed using a Pd/Fe-C electrode
under the same experimental conditions where atomic H* were
formed. Thus, the fabrication of suitable catalysts that can produce
higher amounts of atomic H* has great potential for practical
applications.
a
Microscope (JEM-2100F Field Emission Electron Microscope, JEOL
Ltd.). The specific surface area (BET), pore diameter, and pore
volume were measured by the nitrogen adsorption method with a
surface area analyzer, model ASAP 2020 HD88 (Micromeritics Co.).
Surface potential was characterized with a Nano Particle Sizing &
Zeta Potential Analyzer (DelsaNano C, Beckman Coulter Ltd.). The
Palladium catalysts have been proven to have the unique
property of activating H
2
and catalyzing the electrochemical
O to produce adsorbed atomic H*, which is a
much stronger reducing agent than H in catalytic reduction
+
reduction of H or H
2
X-ray photoelectron spectroscopy (XPS) data for particle electro-
TM
2
des were collected with a PHI Quantera SXM
Scanning X-ray
TM
reactions [16]. Some studies found that coupling two metals with
different oxidizing abilities could enhance the generation of
electrons [17], and some chemical reactions were reported to show
better activity and selectivity when using bimetallic and trimetallic
catalysts compared with monometallic Pd catalysts [18–20]. The
chemical environment of Pd could be modified by a second metal
addition [21,22], and the second metal, such as Au, Cu, Bi, Sn etc.
Microprobe
2.3. Three-dimensional electrochemical reactor and batch experiment
The electrochemical dechlorination of TCAA by Pd-In/Al was
(F ULVAC-PHI. INC.).
2 3
O
carried out in a three-dimensional electrochemical batch reactor,
where a proton-exchange membrane (Nafion 117, Dupont) divided
the reactor into a cathode cell and anode cell. A 50 mL aqueous
[18–20,23], acts as a promoter for the Pd-based catalysts. For
instance, the polymer-protected Au/Pd bimetal alloy, with an Au
core structure, exhibited a higher catalytic activity for selective
hydrogenation of 1,3-cyclooctadiene than that of monometallic Au
or Pd catalyst [20]. In addition, the reduction efficiency of nitrates
solution composed of a certain content of TCAA (500
otherwise specified) was added to the cathode cell, and 50 mL
Na SO solution (2 mM) was added into the anode cell corre-
spondingly. Solution pH was adjusted with diluted H SO and
mg/L, unless
2
4
2
4
by Pd/SnO
nitrates and N-Nitrosodimethylamine could be effectively re-
moved after indium was added to Pd/Al , owing to the formation
2
was greatly enhanced after Sn addition [24]. Notably,
NaOH solutions (guaranteed grade) before each batch experiment.
0.9 mA/cm current density (with a 6.8 V potential) was applied to
the reactor which was powered by a DC power supply source
2
2
O
3
of Pd-In intermetallic compounds [25,26]. However, studies on the
electrochemical reduction of TCAA by Pd-In suspended electrodes
are still rare.
2
(AMERLLPS302A, Dahua instrument corporation of Beijing). RuO /
Ti electrodes with the same effective geometric surface area
2
(10 cm ) and 2 cm separation were employed as the cathode and
In this study, Pd-In mixtures with different atomic ratios were
x y 2 3
anode, and a certain quantity of Pd -In /Al O (1.0 g/L unless
evenly deposited on Al
2
O
3
substrates via coprecipitation followed
emphasized) was added into the cathode cell as the particle
electrode (stirring rate: 800 rpm, room temperature). Pure
Nitrogen/oxygen was purged to modify the DO concentration
(0.2 and 30 mg/L, 7.42 mg/L “air saturated solution”). Heating bath
method was applied to control the reaction temperature (303.15 K
and 273.15 K, room temperature ranged from 288.15 to 291.15 K).
by H reduction, and their reduction efficiency toward TCAA was
2
evaluated in a three-dimensional electrochemical reactor having
better electroreduction efficiency compared to the particle-free
system. TEM, XRD, XPS, BET and Zeta potential analysis were
carried out to study the synergistic effect between Pd and In. Then,
the effects of applied current density, catalyst dosage, initial TCAA
concentration and solution pH on TCAA reduction over Pd-In/Al
2
O
3
2.4. Analysis method
were also studied in detail. Moreover, scavenger and DO impact
experiments on the TCAA direct electron transfer and indirect
atomic H* process were studied. On the basis of the experimental
results, we proposed a reaction mechanism for TCAA reduction
The concentrations of TCAA, dichloroacetic acid (DCAA) and
monochloroacetic acid (MCAA) were measured by an ion
chromatograph (IC) (Dionex 2000) equipped with an IonPac AS-
19 anion-exchange analytical column and an IonPac AG19 guard
column. The mobile phase eluent of the IC was KOH solution. The
chromatograms of TCAA, DCAA and MCAA were collected under
gradient elution conditions (0.0-30.0 min: 10.0 mM KOH; 30.1-
37.0 min: 35.0 mM KOH; 37.1-43.0 min: 10.0 mM KOH, flow rate:
2 3
using Pd-In/Al O .
2. Materials and methods
2
.1. Preparation of Pd-In/Al O particle electrodes
2 3
1
.0 mL/min).
0
The particle electrodes were prepared by a coprecipitation
method. -Al was immersed in a 0.1 M hydrochloric acid
solution containing a certain content of PdCl and InCl 4H O, then
Atomic H* was trapped by 5,5 -dimethyl-1-pirroline-N-oxide
(DMPO), and the signals were further detected by a Bruker model
300E electron spin resonance (ESR) spectrometer, using a quanta-
Ray Nd:YAG laser system for the irradiation source.
g
2 3
O
2
3
ꢁ
2
the mixture was ultrasonically treated (360 min) and left to stand
ꢂ
overnight. After drying at 120 C, the obtained samples were
further calcined at 300 C (120 min), followed by reduction in H
atmosphere (200 C; 300 min; 100 mL/min flow rate). For all the
catalysts, the initial palladium loading amount was 3 wt. %, and the
final obtained Pd-In catalyst with the initial palladium-to-indium
ꢂ
2
3. Results and discussion
ꢂ
3.1. Characterization of Pd-In/
g
-Al
2
O
3
particle electrodes
ꢂ
ꢂ
ꢂ
atomic ratio of 1:1 was denoted as Pd-In/Al
recorded as Pd -In /Al (x: y refers to palladium-to-indium
ratio). Pd/Al and In/Al with 3 wt. % metal loading amounts
were also prepared under the same conditions.
2
O
3
, while others were
As shown in Fig. 1(a), diffraction peaks at 40.1 , 46.7 and 68.1
x
y
2
O
3
assigned to the face centered cubic (fcc) crystallographic structure
catalyst [27,28], and
the peak at 40.1 shifts slightly to lower angle at 39.2 for Pd-In/
2
O
3
2
O
3
of metallic Pd were observed for the Pd/Al
2
O
3
ꢂ
ꢂ
2 3
Al O catalysts, related to the formation of the Pd-In bimetallic
2
.2. Particle electrode characterization
alloy [29]. The dark spots in the TEM picture (Fig. 1(b)) were
denoted as Pd or In particles with atomic numbers. Though it was
difficult to distinguish whether the Pd, In particles was just located
X-ray powder diffraction (XRD) patterns were measured using
ꢂ
ꢂ
Ni-filtered Cu K
a
irradiation from 5 to 90 (in 2
u
) on an X’Pert PRO
2 3
between the Al O multi-layers or scattered in the inner channels
Powder diffractometer (PANalytical Co.). Transmission Electron
of Al , the EDS images (Fig. 1(c)) indicated that Pd and In
2 3
O

Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
15
Fig. 1. (a) X-ray diffractograms for Al
In/Al
2
O
3
, Pd/Al
2
O
3
, and Pd
x
-In /Al
y 2
O
3
; (b) TEM image of Pd-In/Al
2
O
3
; (c) EDS images for Pd-In/Al
2
O
3
element mapping; (d) BET analysis of Pd-
2 3
O .
particles were dispersed uniformly on the surface of Al
shown in inset Fig. 1(b), a distinct lattice structure with a lattice
spacing around 2.2 Å was observed, which may derive from the
2
O
3
. As
surface area of the catalysts decreases with increasing Pd-In
loading amount, while the pore diameter increases correspond-
2 3 2 3
ingly. The pore volume of Al O and Pd-In/Al O were both around
3
(
111) plane of the Pd-In bimetallic alloy (the lattice spacings for Pd
0.3 cm /g, it was reasonable to speculate that the decreased
2
and In are 2.2458 Å [27] and 2.25680 Å [30] respectively); besides,
some of In particles cover the same sites as Pd particles (shown in
Fig. S1), further confirmed the potential of Pd-In alloy formation.
BET analyses of the catalysts are listed in Table S1. The specific
specific surface area (71.61 vs 60.77 m /g) was probably resulted
from the increased pore diameter (from 16.72 to 23.16 nm). The
increased pore diameter in the Al
2 3
O substrate had much more
opportunities to be entered by those nano-seized metallic Pd and
Fig. 2. XPS survey spectrum for (a
1
) Pd/Al
2
O
3
, (a
2
) In/Al
2
O
3
, (b) Pd-In/Al
2
O
3
, (c) Pd
1
-In0.5/Al
2
O
3
, (d) Pd
1
-In
2
/Al
3, (d) Pd
2
O
3
; and XPS spectra of Pd in (a) Pd/Al
-In /Al
2 3 2 3
O , (b) Pd-In/Al O , (c)
Pd -In0.5/Al , (d) Pd -In /Al ; and XPS spectra of In in (a) In/Al
1
2
O
3
1
2
2
O
3
2
O3, (b) Pd-In/Al
2
O
3
, (c) Pd
1
-In0.5/Al
2
O
1
2
2 3.
O

16
Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
In, so that the inner channels were readily to be blocked,
while excess addition of In would restrain the catalyst activity. As
In itself did not have the ability for TCAA reduction, an excessive In
concentration would decrease the isolated Pd sites [36] and block
decreasing in the specific surface area of the catalysts. As shown
2
in Fig.1(d), Pd-In/Al
2
O
3
has a mesoporous structure with 60.77 m /
g BET area and 17.56 nm average pore size, giving a type-IV
isotherm with a type-H3 loop.
2 3
the channels of the Al O substrate (confirmed by BET analysis),
thus reducing the TCAA removal efficiency.
In addition, XPS analyses were carried out to study the influence
of In introduction on the Pd valence state, and calibration of XPS
spectra was performed referencing to C1s peak at 284.8 eV from
the adventitious carbon. As shown in Fig. 2, Pd and In were
detected in all the Pd-In catalysts. The Pd 3d spectra exhibit a spin-
orbit doublet at 335.47 eV (3d5/2) and 340.72 eV (3d3/2), indicting
The reduction products of TCAA over different particle electro-
des were further studied. As shown in Fig. 4, 40.0% of TCAA could
be reduced to DCAA in 30 min without using any catalysts. The
TCAA reduction efficiency increased to 67.0%, 84.0% and 93.2% after
addition of Al
respectively. Notably, MCAA was detected during the electrochem-
ical process when Pd-In/Al was used as the particle electrode,
2 3 2 3 2 3
O , Pd/Al O and Pd-In/Al O particle electrodes
the formation of metallic Pd in Pd/Al
Pd/Al catalyst, the Pd 3d5/2 spectrum shifted towards a lower
value (335.31 eV) for Pd-In/Al , while an opposite shift was
observed for Pd -In0.5/Al and Pd -In /Al . The low binding
energy of the Pd 3d5/2 peaks for the Pd-In/Al catalyst indicated
2
O
3
[31]; Compared with the
2 3
O
2
O
3
and around 42% DCAA and 16% TCAA were detected in the 30 min
batch experiment. However, no MCAA was formed when using
2 3
O
1
2
O
3
1
2
2
O
3
Al
Al
2
O
O
3
or Pd/Al
2 3
O under the same experimental conditions. As In/
2 3
O
2
3
had no effect on TCAA reduction (Fig. 3), we could reasonably
that metallic Pd existed in a more electron-rich state, which was
beneficial for atomic H* formation; meanwhile, a decrease in the
indium 3d5/2 binding energy was detected, indicating that a Pd-In
infer that In acts as a “promoter” metal for Pd-based catalysts
[25,37] and the 1:1 initial Pd to In atomic ratio produces the most
pronounced synergistic effect between metallic Pd and In on TCAA
electroreduction. According to the XPS analysis, metallic Pd existed
bimetallic alloy was formed in the Pd-In/Al
Moreover, the PdCl remaining in Pd/Al changed into PdO for all
the Pd-In/Al catalysts, showing that In could change the
chemical environment of Pd [34].
2 3
O catalysts [32,33].
2
2
O
3
in a more electron-rich state for Pd-In/Al
catalysts due to the modification by In, and the electron-rich state
was beneficial for atomic H* formation. So, Pd-In/Al was shown
2 3
O than in the other
2 3
O
2 3
O
to be a promising catalyst for the electrochemical removal of TCAA.
3.2. Efficient electroreduction of TCAA by Pd-In/Al
2 3
O with different In
to Pd atomic ratios
3.3. Effect of different parameters on electrochemical removal of TCAA
2 3
by Pd-In/Al O
As the amount of the promoter metal would greatly influence
the activity of Pd-based bimetallic catalysts [35], the effect of the In
to Pd atomic ratio on TCAA electroreduction was optimized first. As
shown in Fig. 3(a), the removal efficiency of TCAA by Pd-based
catalysts was increased to a maximum value, and then decreased
The influences of current density, catalyst dosage, initial
concentration, solution pH and dissolved oxygen (DO) on the
electrochemical reduction of Pd-In/Al O were further investigat-
2 3
ed. As shown in Fig. 5(a), the TCAA removal rate increased greatly
2
with increasing atomic ratio of In to Pd. While In/Al
almost no activity for TCAA reduction, 94% of TCAA could be
reduced in 30 min by Pd-In/Al which had the highest
2
O
3
showed
with the current density from 0.3 to 0.9 mA/cm , and then
increased slightly when the current density further increased to
2
2
O
3
,
1.2 mA/cm . With the increase of current density, more electrons
electrochemical activity towards TCAA removal. All the reduction
processes were shown to follow pseudo-first order kinetics. The
reaction rate constants of the catalysts followed the order In/
were available for H* formation and TCAA reduction [38], thus
enhancing the TCAA removal efficiency. However, the removal
efficiency of TCAA did not change much in the first ten minutes
2
Al
2
O
3
< Pd
1
2
-In /Al
2
O
3
2
< Pd/Al O
3
< Pd
1
-In0.5/Al
2
O
3
< Pd-In/Al
2
O
3
,
when the current density rose above 0.9 mA/cm . This can be
and the catalyst activity increased from 114.29, 672.28, 839.33,
explained by the formation of profuse hydrogen bubbles at the
electrode surface. Although the presence of bubbles did not
damage the electrode, they could still interfere with TCAA diffusion
to the catalyst [39]. In addition, as active Pd-In sites are vital for
ꢀ1
ꢀ1
840.28 to 931.34
mg gCat
h
respectively, as shown in Fig. 3(b).
This result indicated that the reduction ability of Pd-based
catalysts could be enhanced by adding a proper amount of In,
Fig. 3. (a) Effect of atomic ratio of palladium and indium on the removal of TCAA (catalyst dosage = 1.0 g/L, initial TCAA concentration = 500 mg/L, current density = 0.9 mA/
2
2 4
cm , initial pH = 7.0, 2 mM Na SO , room temperature); (b) the pseudo-first-order kinetics equations of the catalysts.

Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
17
Fig. 4. Intermediates and final products of reductive dechlorination of TCAA through (a) control (without catalyst); (b) Al
2
O
3
; (c) Pd/Al
2
O
3
; and (d) Pd-In/Al
2
O
3
catalyst
2
(
catalyst dosage = 1.0 g/L, initial TCAA concentration = 500
m
g/L, current density = 0.9 mA/cm , initial pH = 7.0, 2 mM Na
2
SO
4
, room temperature).
TCAA removal [40], the TCAA electrochemical removal rate was
observed to increase gradually as the Pd-In/Al dosage increased
289.15 (room temperature) and 303.15 K respectively, exhibiting a
positive correlation to the temperatures.
2 3
O
from 0.2 g/L to 1.0 g/L (Fig. 5(b)), and the removal rate achieved a
relatively stable value when more catalyst (1.5 g/L) was used,
where the current density restricted the further enhancement of
TCAA reduction.
In addition, as DO is a ubiquitous kind of oxidant that can
compete with TCAA in consuming reducing agents such as
electrons and atomic H*, thus suppressing the removal efficiency
of TCAA [42], the electrochemical reduction of TCAA by Pd-In/
Moreover, the catalytic activity of Pd-In/Al
TCAA removal was more than doubled when the initial TCAA
concentration increased from 200 to 500 g/L (298.9, 451.5, 544.4
for TCAA with the initial concentration of
g/L, Fig. S2), attributed to the positive
correlation between the TCAA removal rate and the surface
coverage of adsorbed TCAA [41]. The corresponding pseudo-first
order kinetics reaction rate constant increased gradually from
2
O
3
catalysts for
2 3
Al O was also investigated as a function of DO content. As shown
in Fig. 5(d), the TCAA reduction efficiency was highly dependent on
the DO content. When the DO concentrationwas reduced from 7.42
to less than 0.20 mg/L, the TCAA concentration decreased sharply
in the first 5 min and was totally removed in just 15 min, and the
reaction constant increased 6-fold compared to that in the
presence of DO of 7.42 mg/L. Comparatively, when the DO content
was above 30 mg/L, almost no TCAA was removed in the 30 min
batch experiment, while the reaction constant decreased 31-fold
compared to that with the DO concentration of 7.42 mg/L (0.5802,
m
ꢀ
1
ꢀ1
and 752.9
2
m
g gCat
h
00, 300, 400 and 500
m
ꢀ
1
0
.0540, 0.0550, and 0.0561 to 0.0613 min
Furthermore, the effect of initial solution pH on TCAA removal
was not significant, and the pseudo-first order kinetics reaction
.
ꢀ1
0.0026 and 0.0839 min for DO concentrations of 0.20, 7.42 and
30 mg/L respectively). That is to say, DO is a strong competitor for
TCAA in this three-dimensional electrochemical reactor.
ꢀ
1
rate constant of Pd-In/Al
at all studied pH values from 3.0 to 9.0 (Fig. S3). As shown in Fig. S4,
the Pd-In/Al catalyst was positively charged when pH was
2 3
O for 20 min of reaction was 0.04 min
2
O
3
As the important indicator for the efficient catalyst, the
lower than 9.0 and the surface zeta potential changed only slightly
when pH ranged from 3.0 to 9.0. Therefore, the adsorption of TCAA
would not change much with changing pH, weaken the impact of
the pH influence. As more than 80% of TCAA was removed after
reaction for 30 min, TCAA could be effectively removed by Pd-In/
2 3
reusability of Pd-In/Al O was also evaluated. The used Pd-In/
Al
2
ꢂ
O
3
catalysts was recovered by centrifuging followed by drying at
ꢂ
80 C. After reduction in H
regenerated Pd-In/Al
2
atmosphere at 200 C for 300 min, the
2
O
3
was applied to reduce TCAA. After three
cycles, more than 80% of TCAA with the initial concentration of
0.5 mg/L could be reduced in 30 min, so that the TCAA was
decreased below the standard (0.1 mg/L) (Fig. S6). The stability of
Al
2
O
3
over a wide pH range.
The effect of temperature to TCAA removal efficiency was
further investigated. As shown in Fig. S5, 80.28%, 94.01% and
5.23% of TCAA could be reduced within 30 min under 273.15,
2 3
Pd-In/Al O is essential for reducing the cost in the practical
9
applications.

18
Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
Fig. 5. Effect of (a) current density, (b) dosage, (c) initial concentration, (d) DO concentration on the electrochemical removal of TCAA (catalyst dosage = 1.0 g/L, initial TCAA
2
concentration = 500
m
g/L, current density = 0.9 mA/cm , initial pH = 7.0, 2 mM Na
2
4 2 3
SO unless emphasized); inset shows (c) the reaction rate constant of Pd-In/Al O under
different initial TCAA concentration; (d) the reaction rate constant of Pd-In/Al
2
O
3
under different DO concentration.
3
.4. Proposed mechanisms of TCAA electroreduction on Pd-In/Al
2
O
3
addition, scavenger experiments using Tert-BuOH to quench
atomic H* were carried out to further study the TCAA reduction
process (shown in Fig. 6(b) and (c)). Blending Tert-BuOH (10 mM)
into the TCAA reduction system inhibited the TCAA removal
significantly. While the TCAA removal rate was 69.0% and 80.5% for
The adsorption efficiency of TCAA by different catalysts was
tested without electricity. No TCAA was adsorbed by Al and In/
Al , and 3.0% and 2.3% of TCAA could be adsorbed by Pd/Al
and Pd-In/Al respectively (shown in Fig. S7). This result agreed
with the zeta potential analysis (Fig. S4), where all the catalysts
were positively charged and differed slightly at neutral pH, but the
2 3
O
2
O
3
2 3
O
2
O
3
Pd/Al
BuOH, the electroreduction efficiency decreased by 49.2% and
51.2% for Pd/Al and Pd-In/Al respectively, accounting for
2 3 2 3
O and Pd-In/Al O at 25 min respectively without Tert-
2
O
3
2 3
O
potentials still followed the order Al
DCAA and MCAA adsorption by Pd-In/Al
shown in Fig. S8, no adsorption of DCAA and MCAA was observed in
the 30 min batch experiment. After adding 0.9 mA/cm2 current
density, the TCAA was dechlorinated to DCAA or further to MCAA
2
O
3
< Pd-In/Al
2
O
3
< Pd/Al
2
O
3
.
nearly 2/3 (71.3% and 63.6% respectively) of TCAA removal without
Tert-BuOH. Meanwhile, 16.7% inhibition was also detected for the
control experiment (no particle electrodes). It can be speculated
that the indirect atomic H* process played a more dominant role
2 3
O
was also detected. As
than direct electron transfer in TCAA reduction by Pd-In/Al
2 3
O
(
Pd-In/Al
2
O
3
only, Fig. 4). However, a decrease in the summed
catalysts, and was found to be more prominent for Pd/Al . As
2 3
O
concentration of MCAA, DCAA and TCAA along the batch experi-
ments was detected for all the catalysts. For instance, the sum of
MCAA (16%), DCAA (42%) and TCAA (6%) accounted for 64% of the
introducing In into Pd-based catalysts could enhance the atomic H*
formation, this may explain why the TCAA removal efficiency was
higher for Pd-In/Al
TCAA could be reduced by Pd/Al
2 3
lower than the value of 36.4% for Pd-In/Al O
2
O
3
than Pd/Al
2
O
3
. Comparatively, 28.7% of
after atomic H* was inhibited,
. That is to say, the
total CAAs at 30 min for Pd-In/Al
could be adsorbed by Pd-In/Al
2
O
3
, and since little of the CAAs
2 3
O
2
O
3
, the decrease of CAA concentra-
_
tion was due to the formation of other intermediates like Cl
2
CCOOH
direct electron reduction of TCAA was enhanced after In addition,
indicating that the direct electron transfer was also promoted by
the introduction of In into the Pd-based catalyst.
_
and ClCHCOOH.
As TCAA could be electroreduced both by electrons and atomic
H* [43,44], the electroreduction mechanism of TCAA reduction by
Furthermore, we investigated the intermediates and final
Pd-In/Al
characteristic peaks of DMPO-H for atomic H* [39] were detected,
and the peak intensities followed the order Pd-In/Al > Pd/Al
while no such signal was detected when Al or no catalyst was
used. This result confirmed the formation of atomic H* in the TCAA
electroreduction process in the three-dimension electrochemistry
system using Pd-based catalysts as the particle electrode. The
atomic H* reduction of TCAA could be further enhanced by
introducing In into Pd-based catalysts, further confirming that a
synergistic effect between Pd and In occurred in the catalyst. In
2
O
3
was investigated further. As Fig. 6(a) shows, the nine
2 3
products of the electroreduction of TCAA by Pd-In/Al O after
elimination of DO from the system, since DO is a strong competitor
for TCAA, which would hinder both the direct electron transfer and
indirect atomic H* process for the electroreduction of TCAA as
discussed above. As shown in Fig. 6(d), TCAA decreased rapidly
while DCAA as the dechlorination intermediate increased initially,
and then decreased slightly along the course of reaction;
meanwhile the MCAA concentration increased along with the
reduction of TCAA. As only DCAA was formed without the Pd-In
catalyst (Fig. 4), the detected MCAA was formed via the indirect
2
O
3
2 3
O ,
2 3
O

Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
19
2
Fig. 6. (a) the DMPO spin-trapping ESR spectra with different catalysts at the current density of 0.9 mA/cm ; (b) electrochemical reduction of TCAA in different systems with
4 4
and without C H10O; (c) TCAA removal under different C H10O concentrations at the reaction time of 25 min; (d) intermediates and final products of reductive dechlorination
2
of TCAA by Pd-In/Al
temperature).
2
O
3
without DO (catalyst dosage = 1.0 g/L, initial TCAA concentration = 500
m
g/L, current density = 0.9 mA/cm , initial pH = 7.0, 2 mM Na
2
SO
4
, room
atomic H* function, while DCAA could also be formed via direct
electron transfer. As shown in the schematic diagram (Fig. 7), the
direct electroreduction may follow the pathway:
_
+
ꢀ
Cl
2
CCOOH + H + e ! Cl
2
CHCOOH
(2)
ꢀ
C_ COOH + Clꢀ
Cl
3
CCOOH + e ! Cl
2
(1)
2 3
Fig. 7. Schematic diagram: reaction mechanism for TCAA reduction over Pd-In/Al O .

2
0
Y. Liu et al. / Electrochimica Acta 232 (2017) 13–21
While the indirect electroreduction of TCAA by Pd-In/Al
be described as:
2
O
3
can
(3)
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