Facile fabrication of nanostructured Pd-Fe bimetallic thin films and their electrodechlorination activity

Article abstract of DOI:10.1016/j.molcata.2011.09.004

A series of nanostructured Pd-Fe bimetallic thin films with the performance of fast and high-efficiency electrochemical reductive dechlorination were directly formed on glassy carbon electrodes (GCEs) through galvanic replacement of partial Fe⁰ nanoparticles by Pd(II) ions. The composition and morphology of such Pd-Fe thin films are strongly dependent on the sacrificial Fe film templates fabricated by a template-free, double-potential step electrodeposition technique. The size, shape and morphology of the Fe nanoparticles that constitute the Fe thin films can be controlled well by adjusting the concentration of Fe²⁺ ions in electrolyte solutions. Due to the good synergistic effect between Pd and Fe, the as-prepared Pd-Fe thin films with the novel microstructure display a high reactivity towards the electrochemical reductive dechlorination of carbon tetrachloride (CT), which closely related to the composition of Pd-Fe thin films, applied potential and temperature. The dechlorination reaction of CT complied with pseudo-first-order kinetics.

Full text of DOI:10.1016/j.molcata.2011.09.004

Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Facile fabrication of nanostructured Pd–Fe bimetallic thin films and their
electrodechlorination activity
a
b
c
a
a,d,∗
Cuicui Qiu , Xiaoqiang Dong , Minghu Huang , Sihui Wang , Houyi Ma
a
Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
Sinopec Research Institute of Petroleum Engineering, Beijing 100101, China
School of the Ocean, Harbin Institute of Technology at Weihai, Weihai 264209, China
b
c
d
Beijing National Laboratory for Molecular Science, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2011
Received in revised form 4 September 2011
Accepted 8 September 2011
Available online 16 September 2011
A series of nanostructured Pd–Fe bimetallic thin films with the performance of fast and high-efficiency
electrochemical reductive dechlorination were directly formed on glassy carbon electrodes (GCEs)
through galvanic replacement of partial Fe0 nanoparticles by Pd(II) ions. The composition and morphol-
ogy of such Pd–Fe thin films are strongly dependent on the sacrificial Fe film templates fabricated by
a template-free, double-potential step electrodeposition technique. The size, shape and morphology of
the Fe nanoparticles that constitute the Fe thin films can be controlled well by adjusting the concen-
tration of Fe ions in electrolyte solutions. Due to the good synergistic effect between Pd and Fe, the
as-prepared Pd–Fe thin films with the novel microstructure display a high reactivity towards the electro-
chemical reductive dechlorination of carbon tetrachloride (CT), which closely related to the composition
of Pd–Fe thin films, applied potential and temperature. The dechlorination reaction of CT complied with
pseudo-first-order kinetics.
Keywords:
Palladium
Iron
Bimetallic film
Electro-dechlorination activity
Carbon tetrachloride (CT)
2
+
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
in advanced water treatment due to the high cost and espe-
cially the formation of secondary intractable contaminants. The
distinguished features of electrochemical dechlorination methods
include rapid reaction rate, mild reaction conditions, low apparatus
cost, and especially the green processing technology that does not
need additional chemical additives [8].
Many chlorinated organic compounds (COCs), such as carbon
tetrachloride (CT), trichloroethylene (TCE), and other organochlo-
rine contaminants listed at the Stockholm Convention on Persistent
Organic Pollutants (2001), are highly toxic, persistent and bioac-
cumulative. Once discharged into the environment, a great threat
will be posed to human heath and ecological environment [1,2].
Although it has been abandoned in extinguishing systems and
refrigeration equipments due to its potential hazard to ozone
depletion [3], CT is still widely used as a solvent in chemical plants
and labs at the present time, which leads to huge amounts of
residues in groundwater or in public water systems [4,5]. It is very
urgent to develop effective methods of converting COCs to non-
toxic or less toxic nonchlorinated analogues. An effective means to
eliminate COCs is to remove the elemental chlorine by converting
organic chlorine into inorganic chloride ions, which occurs with the
formation of nontoxic or low-toxic parent hydrocarbon compounds
Though during the past decade, due to abound resources, low
0
cost and considerable activity, zero-valent iron (ZVI or Fe ), espe-
cially nano-Fe [9,10], was widely used as a potent electron donor or
reductant for the hydrodechlorination of COCs, it should be noted
that the practical application of Fe in the dechlorination of COCs
has been considerably limited due to its intrinsic defects [11]. It
is of interest that these disadvantages can be efficiently elimi-
nated when using Fe-based bimetallic materials (e.g. Fe/Pd [12,13],
Fe/Ni [14,15], Fe/Al [16], Fe/Au [17], Fe/Pt [11]) as substitutes for
0
monometallic Fe . The incorporation of the lower hydrogen over-
potential metals (e.g. Pd, Pt, etc.) not only increases the density of
surface reactive sites, but also prevents the corrosion products from
accumulating at the reactive sites on the Fe0 surface. Because of
outstanding adsorbing/absorbing hydrogen capacity under ambi-
ent condition, Pd is considered to be one of the most effective
hydrodechlorination catalysts [18–20]. Moreover, the combination
of Pd and Fe makes it possible to prepare high performance dechlo-
rination catalysts. The existing studies have demonstrated that the
Pd–Fe bimetallic catalysts exhibit the higher dechlorination activity
[17,21] than other bimetallic catalysts. In view of this, the aim of our
[
6,7].
Much effort has been devoted to the degradation of COCs, but
most of the dechlorination methods (e.g. activated carbon adsorp-
tion, physical separation, and photocatalytic oxidation) are limited
∗ Corresponding author. Tel.: +86 531 88364959; fax: +86 531 88564464.
E-mail address: hyma@sdu.edu.cn (H. Ma).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.09.004

C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
57
Table 1
The effect of Fe2 concentration on the composition , EASAb and RE for electrochemical reductive dechlorination of CT in H2SO4 solutions on the Pd–Fe thin films. (For removal
+
a
efficiency test: potential: −0.2 V; time: 30 min; temperature: 303 K.)
Fe2+ concentration (mM)
Pd/Fe weight ratio
Pd/Fe atomic ratio
EASA
RE (%)
5
99.75/0.25 = 399
85.49/14.51 ≈ 5.9
83.03/16.97 ≈ 4.9
18.31/81.69 ≈ 0.2
99.52/0.48 ≈ 207.3
75.52/24.48 ≈ 3.1
71.98/28.02 ≈ 2.6
10.52/89.48 ≈ 0.1
0.33
0.44
0.53
1.70
84.0
97.4
51.9
40.7
1
1
2
0
5
5
a
The compositions of as-prepared bimetallic films were determined according to EDS analyses.
EASA of Pd–Fe bimetallic thin films was calculated by means of oxygen adsorption method.
b
paper is to design and fabricate high performance Pd–Fe bimetallic
dechlorination catalysts using double-potential step electrodepo-
sition and galvanic replacement methods. Taking electrochemical
reductive dechlorination of CT as the research object, we investi-
gated the removal efficiencies of the as-prepared bimetallic Pd–Fe
nanocatalysts under different conditions. The present study will
provide a new path to fabricate low-temperature high-efficiency
nanocatalysts for the dechlorination of COCs. The obtained results
are also of fundamental importance to give an in-depth under-
standing of reductive dechlorination mechanism of COCs on the
Pd–Fe bimetallic catalysts.
workstation. The GCEs modified with monometallic Fe or bimetal-
lic Pd–Fe thin films were selected as working electrodes. A bright
Pt plate (1.0 cm × 2.0 cm) and a saturated calomel electrode (SCE)
were selected as the counter electrode and the reference electrode,
respectively. In this paper, the potentials were all referred to the
SCE. The reference electrode was led to the surface of the work-
ing electrode through a Luggin capillary. The electrolyte solutions
were deaerated by bubbling N2 through the solutions for 10 min
before electrochemical tests, and a blanket of N2 was maintained
during electrochemical measurements. Electrochemical reductive
dechlorination of CT on the nanostructured Pd–Fe thin films was
studied by cyclic voltammetry and potentiostatic electrolysis in
−
1
2
. Experimental
0.5 M H2SO4 containing 30 mg L
peratures.
CT solutions at different tem-
All reagents were of AR grade and were used as received
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
All of the aqueous solutions were prepared with ultrapure water
As a complicated reaction for CT dechlorination, the forma-
tion of various possible intermediates and products (e.g. CO2, CO
and HCOO ) leads to the measurement difficulty, usually causing
−
(
>18 Mꢀ cm). The glassy carbon electrodes (GCEs, ꢁ = 4 mm) were
a low total carbon mass balance in analysis [22]. In the cur-
rent study, only the change of CT concentration was analyzed to
investigate the dechlorination performance of Pd–Fe bimetallic
catalysts. The concentration of CT in the electrolytes before and
after potentiostatic electrolysis was analyzed by gas chromatogra-
phy/mass spectrometry (GC/MS)-QP2010. The gas chromatography
was equipped with a DB-5 MS capillary column (30 m × 0.25 mm,
0.25 ␮m thickness). The column temperature was maintained at
308 K, while the temperature of injection port and ion-trap detec-
tor was 473 K. Ultra-pure He was used as the carrier gas at a flow
rate of 1.1 mL min and a split ratio of 60:1. Before each GC/MS
testing, the CT samples were placed in thermostatic water bath
at 313 K for 1 h. The change of CT concentration in the solutions
before and after potentiostatic electrolysis can be used to deter-
mine the removal rates of carbon tetrachloride. Removal efficiency
(RE) was calculated as the ratio of the decrease of CT concentration
to the initial CT concentration (i.e. RE (%) = (1 − C/C0) × 100), where
C denotes the concentration of CT after electrolysis, and C0 is the
initial concentration of CT).
used as substrate electrodes. Before each experiment, a GCE was
firstly ground with emery papers of decreasing particle sizes to
#
sequently, the electrode was cleaned ultrasonically in HNO3 (1:1),
washed with ultrapure water, and finally rinsed with ethanol.
Bulk Fe electrode was made from 99.99% pure Fe wire
3500 finish, and then polished with 0.05 ␮m alumina slurry. Sub-
(
ꢁ = 2.0 mm). The Fe wire was embedded in epoxide resin mould,
leaving its cross-section only exposed to the electrolytic solutions.
The bulk Fe electrode was treated with the similar procedure used
for GCEs except it was not cleaned ultrasonically in HNO3.
The nanostructured Fe thin films were directly fabricated on
GCE substrates (denoted as Fe/GCE) by a double-potential step
electrodeposition in a solution of 250 mM NaCl + FeCl2 (with dif-
ferent concentrations) at 303 K. The concentration of FeCl2 was
selected as 5, 10, 15, and 25 mM, respectively. All Fe films were
prepared by applying two potential pulses in succession: a neg-
ative reducing nucleation pulse (−1.15 V) for 1 s and a growth
pulse (−1.05 V) for 1000 s. The potentials were controlled by
a computer-controlled electrochemical workstation (CHI 650A).
Assuming identical faradaic efficiency for the deposition process,
the amount of Fe deposited can be estimated by integrating the
charge needed to reduce Fe2 ions.
−
1
Structure and surface morphology of Fe films and bimetallic
Pd–Fe films were observed by field emission scanning electron
microscope (FESEM, JEOL JSM-6700F). The composition of these
bimetallic films was analyzed by INCAX-sight energy dispersive X-
ray spectrometer equipped on the FESEM. The weight ratios and
atomic ratios of Pd to Fe in different bimetallic films were listed in
Table 1.
+
Bimetallic Pd–Fe thin films were prepared by dropping 25 ␮L of
1
0 mM acidic solution containing noble metal precursor (H PdCl )
2 4
onto the Fe films for 30 min. Seeing that the standard redox poten-
2−
2+
tials of PdCl4 /Pd and Fe /Fe relative to the standard hydrogen
electrode (SHE) are +0.951 V and −0.440 V, respectively, the depo-
sition of noble metal Pd onto the surface of Fe thin films occurred
through the following galvanic replacement reaction:
3. Results and discussion
3.1. Preparation of nanostructured Pd–Fe bimetallic thin films
and their surface morphology
0
2−
→ Fe2+ + Pd + 4Cl−
0
Fe + PdCl
(1)
4
After thoroughly rinsing with ultrapure water, the GCEs
modified with the as-prepared Pd–Fe thin film (abbreviated to
Pd–Fe/GCE) were placed in 0.5 M H SO for carrying out the poten-
tial cycling in the designated potential range.
All electrochemical measurements were performed in a con-
ventional three-electrode cell with a CHI 650A electrochemical
Herein nanostructured Fe thin films were first electrodeposited
onto the GCE substrate by a double-potential step electrodeposition
method. From the cyclic voltammograms (CVs) for the voltam-
metric deposition of Fe shown in Fig. 1(a), it is found that the
Fe electrodeposition started at ∼−1.0 V and gave a characteristic
cathodic peak centered on −1.13 V during the first negative-going
2
4

5
8
C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
in the subsequent cathodic scan. The anodic process involves elec-
trooxidation of Fe0 into Fe(II) and Fe(III) species.
Fe + 2OH− → Fe(OH) + 2e
−
(2)
(3)
2
−
−
Fe(OH) + OH → FeOOH + H O + e
2
2
The CVs of the Fe/GCE present similar features to those of the bulk
Fe electrode, thereby confirming the formation of Fe thin films on
the GC substrate. Besides, it is observed that the hydrogen evolu-
tion reaction was decreased obviously on the Fe/GCE as compared
to that on the bulk Fe, because no rapid increase in the cathodic
current took place at ∼−1.32 V on the Fe/GCE.
Fig. 2 shows SEM images (Fig. 2(a)–(e)) and EDS spectrum
(Fig. 2(f)) of the as-prepared Fe thin films fabricated onto the GC
substrate. The obvious peaks for Fe in Fig. 2(f) indicate the existence
of Fe element. On the other hand, the SEM observations demon-
strate that the as-prepared Fe films are not a monolayer film but
probably composed of bilayer films. From Fig. 2(d) and its inset, it
can be seen that the bottom layer with some irregular structures
is a compact thin film composed of quantities of nanoparticles,
whereas the surface layer consists of submicron-scale Fe particles
that adhere to the bottom layer. The surface morphology of the Fe
thin films, especially the shape of large Fe particles in the surface
layer, can be regulated conveniently by changing the concentra-
2+
tion of Fe in the electrolytic solutions. When the concentration
of Fe2+ was relatively low, most of surface particles adopted a
perfect cubic shape. These large particles were scattered on the
bottom layer and their particle size was about 130–170 nm, as indi-
cated by a large-scale SEM image (Fig. 2(a)) and an enlarged SEM
2
+
image (Fig. 2(b)). At a relatively low concentration of Fe ions (e.g.
mM), a small number of nuclei generated by the electrocrystal-
lization tended to homogenously and slowly grow along ꢀ1 0 0ꢁ,
0 1 0ꢁ and ꢀ0 0 1ꢁ directions, thereby forming regular nanocubes.
5
ꢀ
2+
As the Fe ion concentration increased, the surface Fe particles
became uneven in the shape and in size (see Fig. 2(c)–(e)). The vast
majority of large Fe particles in the surface layer were submicron-
scale irregular polyhedra. This phenomenon seemed more evident
in the case of 25 mM. In this case, the nuclei were inclined to quickly
and heterogeneously grow. At the same time, the number of sur-
face Fe particles increased significantly since the density of nuclei
increased with the increasing Fe2+ ion concentration [24]. The high
coverage of surface particles made the bottom layer of Fe thin films
not to be seen, as indicated by Fig. 2(e).
Fig. 1. CVs of (a) GCE in 250 mM NaCl + 10 M FeCl2 mixed solution and (b) Fe/GCE
−1
and bulk Fe in 1.0 M KOH solution. Scan rate: 50 mV s
.
potential scan. However, both the onset potential and cathodic peak
potential of the Fe deposition shifted positively by about 100 mV
from the second cycle onwards and remained unchanged there-
after. The similar results were reported previously [23,24]. The
present voltammetric behavior indicates that Fe deposition pro-
cess tended to nucleate at a more negative potential on a foreign
substrate. Once a monolayer of Fe film formed on the GC substrate,
the subsequent deposition on the Fe film became easier, which can
be reflected from the significant positive-shift of reduction peak
potential. With the increase of potential cyclic number, more Fe
nuclei formed on the GC substrate or on the previously formed Fe
monolayer, which led to the progressive growth of Fe particles.
The deposition of Fe on GCE is a heterogeneous nucleation pro-
cess, and the Fe “island” forms through a Volmer–Weber growth
mode [23,25]. As expected, when selecting −1.15 V and −1.05 V
as nucleation potential and nuclear growth potential, respectively,
the nano/submicron Fe thin films with hierarchical architectures
on the GC substrate were obtained.
CVs of the Fe film modified GCE (Fe/GCE) electrode and the bulk
Fe one in 1.0 M KOH solution are shown in Fig. 1(b). As is well
known, the oxidation and passivation processes of Fe electrode
will take place in alkaline solutions under applied anodic poten-
tials, resulting in the formation of Fe oxide multilayers. The CVs of
the bulk Fe electrode display two pairs of current peaks in poten-
tial range between −1.4 V and 0 V. Two oxidation peaks appeared
near −1.1 V and −0.73 V in the anodic scan, and their corresponding
reduction peaks were observed respectively at ∼−1.3 V and −1.13 V
The nanostructured Pd–Fe bimetallic thin films could be fab-
ricated on the GC substrate by using the nanostructured Fe thin
films as the sacrificial templates to react with the H PdCl solutions.
2
4
Fig. 3(a)–(d) show typical SEM images for a series of Pd–Fe thin films
obtained by galvanic replacement of Fe films with Pd. The surface
morphology of the bimetallic thin films strongly depended on that
of the sacrificial Fe templates, more exactly on the concentration of
2+
Fe ions used to prepare the Fe film templates. When the concen-
2+
tration of Fe ions increased from 5 mM to 25 mM, the morphology
of the bimetallic Pd–Fe films changed from individual particles to
submicron-scale flower-shaped particles depending on the signifi-
cant change in the topography of Fe film templates. The higher the
concentration of Fe2+, the rougher the surface morphology of Pd–Fe
bimetallic films seemed. The rougher surface structure can provide
larger electrochemical active surface area (EASA) of Pd components
in the bimetallic thin films (see Table 1), which were determined
by means of the oxygen adsorption method [26]. The appearance
of both Pd and Fe peaks in the EDS spectrum (Fig. 3(e)) has evi-
dence that the as-prepared films were bimetallic Pd–Fe thin films.
The present bimetallic thin films are expected to be in favor of the
dechlorination reaction of COCs. The combination of electrodepo-
sition and galvanic replacement enables us to design and fabricate
the bimetallic Pd–Fe thin films with controllable morphologies,

C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
59
Fig. 2. SEM images and EDS spectrum of the Fe thin films electrodeposited onto GCE substrates in electrolyte solutions with different Fe2+ concentrations: (a) 5 mM, (b)
mM, (c) 10 mM, (d) 15 mM, (e) 25 mM, and (f) EDS of Fe thin films.
5
which is more suited to acting as the cathode catalyst materials for
electrochemical dechlorination reactions than the Pd–Fe nanopar-
ticles. The microstructure and morphology of the Pd–Fe thin films
can be tailored by choosing the Fe film templates or changing the
replacement reaction time. The same is true of the composition of
the Pd–Fe thin films.
difference in the dechlorination reactivity between absorbed and
adsorbed hydrogen. It is observed that, the hydrogen evolution and
the hydrogen adsorption were inhibited considerably due to the
reaction between adsorbed hydrogen and the adsorbed CT. And the
redox peaks associated with the hydrogen adsorption/desorption
completely disappeared in the CT-containing solution. Accordingly,
the use of Pd–Fe/GCE provides the great possibility to study the
effect of different types of hydrogen on electrochemical reductive
dechlorination of CT.
3.2. Electrochemical behavior of Pd–Fe thin films in acidic
solutions
3
.2.1. Voltammetric behavior of Pd–Fe thin films in acidic
solutions
CVs of a GCE covered with the Pd–Fe thin films (Pd–Fe/GCE for
3.2.2. Galvanic corrosion of Pd–Fe bimetallic films in acidic
solutions
It is known that a galvanic cell is formed when two dissimilar
metals are connected electrically while both are immersed in an
electrolyte solution and the corrosion rate depends on the surface
reaction of both metals. In the Pd–Fe bimetallic system, Pd serves
as the cathode in the galvanic cell and the H2 produced from the
Fe corrosion is transferred to the Pd surface, since Pd0 has the
short) in 0.5 M H SO4 solutions with and without CT are shown in
Fig. 4(a). The CVs obtained in the CT-free solution are basically sim-
ilar to those of the nanostructured Pd thin films in H SO4 solution
2
2
[
27]. In the hydrogen region, the CVs give two pairs of redox peaks:
0
the two large peaks at the more negative potentials are attributed
to the hydrogen evolution and the desorption of absorbed hydro-
gen respectively, whereas the two small ones at the more positive
potentials are associated with the hydrogen adsorption/desorption.
Theadsorbedhydrogenexistsintheformofatomichydrogenonthe
Pd surface, which is the stronger reductant for the dechlorination
of COCs than the absorbed hydrogen embedded in the Pd lattice.
The great change of CV profile in the presence of CT exemplifies the
low hydrogen overpotential and the high exchange current density
+
for H /H . The galvanic effect can accelerate the electron transfer
2
0
0
from the corroding Fe to the Pd , which is helpful to reduce the
accumulation of corrosion intermediates over the Fe surface and
further increase the efficiency of hydrogen utilization. It is obvious
that the galvanic corrosion of Pd–Fe bimetallic system can enhance
0
0
the dechlorination rate of COCs as compared with single Fe or Pd

6
0
C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
Fig. 3. SEM images and EDS spectrum of the Pd–Fe bimetallic thin films. The Fe film templates were prepared in electrolyte solutions containing different concentrations of
2+
Fe ions: (a) 5 mM, (b) 10 mM, (c) 15 mM, and (d) 25 mM.
catalyst. This is the main reason that many researchers incorporate
a noble metal with low hydrogen overpotential, such as Pd, into
Fe nanoparticles [14,28]. The dechlorination mechanism of COCs
decreased significantly, accompanied by the great change in anodic
−
1
Tafel slope (63.4 mV dec ) and little change in the cathodic Tafel
0
−1
slope (116.3 mV dec ). Besides, in CT-containing solution, a new
on the Pd–Fe can be expressed as follows [12,29]:
redox couple was observed around 0.38 V from the measured polar-
ization curve. It is inferred that the oxidation–reduction reaction
is related to the dechlorination reaction between CCl4 and Fe(II)
oxides.
Fe + 2H → Fe2+ + H
+
2
(I)
Fe + RCl + H → RH + Fe + Cl−
+
2+
+
+ 2e− ↔ 2Had ↔ H2
Pd + H2 → Pd · H2
2
H
3
.3. Catalytic activity of Pd–Fe thin films towards the
(II)
dechlorination of CT
Pd + RCl → Pd· · ·Cl· · ·R
Pd · H + Pd· · ·Cl· · ·R → RH + H+ + Cl− + 2Pd
3.3.1. Effect of applied potentials
(III)
2
Now we focused on investigating catalytic activity of Pd–Fe
thin films towards the CT dechlorination under different cathodic
potentials.
Fig. 4(b) shows the steady-state curves for the freshly prepared
Pd–Fe/GCE in H SO solutions with and without CT. Firstly, it can be
2
4
estimatedfromthesteady-statecurveof thePd–Fe/GCEin theblank
solution that the open-circuit corrosion potential of the Pd–Fe thin
films is ∼0.54 V (vs. SCE), about 1.0 V more positive than that of bulk
Fe electrode (<−0.5 V vs. SCE) under the identical conditions. Theo-
retically, the Fe components of Pd–Fe films have been passivated at
such a high positive potential [30]. The driving force for the strong
Electrochemical reductive dechlorination of CT was performed
in potentiostatic mode. The reaction was controlled at 303 K and
was kept for 30 min. Several representative cathodic potentials,
−0.05 V (the adsorption of hydrogen occurred), −0.08 V (a mono-
layer of adsorbed hydrogen formed), −0.1 V (hydrogen started to
enter into Pd lattices), −0.15 V (the Pd ␣-hydride phase formed),
−0.2 V (the ␤-hydride phase formed and hydrogen started to
evolve on the electrode surface) [31], were selected to study the
effect of different types of hydrogen on removal efficiency of CT.
From the data listed in Table 2, it can be found that the adsorbed
hydrogen at −0.08 V display a high dechlorination activity, and the
positive-shift of the corrosion potential is the quite high exchange
+
current density of H /H on Pd micro-cathodes. The cathodic Tafel
2
1
−
−1
slope of 116.1 mV dec and anodic Tafel slope of 101 mV dec ,
while in CT-containing solution, the open-circuit potential (OCP)
shifted about 20 mV to negative direction and the corrosion current

C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
61
surface at −0.15 V. At the moment, there were few active sites on
the surface of bimetallic catalysts available for chemisorption of
CT. Considering that the surface concentration of metal chloride
formed through the chemisorption of CT was quite low, the
removal efficiency of CT was naturally relatively low in this case.
Nevertheless, this situation was changed at the more negative
potentials (e.g. −0.2 V). The adsorbed hydrogen began to desorb
from the Pd sites because of the onset of the HER; CT molecules
were therefore able to adsorb on the electrode surface and form
enough metal chloride. In addition, the evolving hydrogen was able
to react with the adsorbed CT molecules at the electrode/solution
interface. Consequently, the electrocatalytic hydrogenation (ECH)
of CT was promoted significantly, and correspondingly the removal
efficiency of CT increased again and even reached a higher value.
3.3.2. Effect of the Pd–Fe ratios in bimetallic thin films
For galvanic corrosion of as-prepared Pd–Fe thin film electrode
in acidic solutions, the larger the Pd/Fe area ratios, the faster the
corrosion rate of Fe components. The large Pd/Fe area ratio is
favorable for increasing dechlorination efficiency of COCs on the
Pd-Fe bimetallic catalysts from the point of view of accelerating
iron corrosion. But on the other hand, the galvanic corrosion can
also cause the passivation of Fe components, which is disadvanta-
geous to the dechlorination of COCs. Therefore, both positive and
negative effects of galvanic corrosion on catalytic dechlorination
activity of the Pd–Fe bimetallic catalysts have to be considered
when designing and fabricating high performance bimetallic cata-
lysts. The experimental results confirmed this inference.
According to dechlorination efficiencies of the as-prepared
Pd–Fe bimetallic catalysts listed in Table 1, dechlorination activ-
ity of the Pd–Fe catalysts first increases and then decreases with
the decrease of Pd/Fe weight ratio. At the weight ratio of 5.9,
the bimetallic catalyst exhibits the highest dechlorination activity
(97.4%) under otherwise identical conditions (temperature: 303 K;
electrolysis time at −0.2 V: 30 min). When the Pd/Fe weight ratio is
much larger than 5.9 (e.g. 399), Fe components are either passivated
due to serious galvanic corrosion or prevented by the Pd compo-
nents from contacting aqueous solutions. The aforementioned two
factors decrease the amount of atomic hydrogen adsorbed on the
Pd components, thereby leading to the relatively low dechlorina-
tion efficiency (84%). On the contrary, when the Pd/Fe weight ratio
is lower than 5.9 (e.g. 4.9 and 0.2), the catalytic active sites avail-
able for chemisorption of CT and hydrogen are evidently reduced.
Moreover, the corrosion rate of Fe components is decreased as the
area of cathodic metal (Pd) is decreased. It is unsurprising that the
dechlorination efficiency of CT over the bimetallic catalysts with
low Pd/Fe weight ratios become much lower (51.9% for Pd/Fe ratio:
Fig. 4. CVs (a) and Tafel polarization curves (b) of Pd–Fe/GCE in 0.5 M H2SO4 with
and without 30 mg L CT. Scan rate of CVs: 50 mV s .
−1
−1
removal efficiency of CT calculated at the potential reached 91.3%,
only slightly lower than that (97.4%) obtained at −0.2 V in the case
of hydrogen evolution. However, the removal efficiency of CT at
−0.15 V was relatively low (39%). The similar phenomenon was
observed for the electrochemical reductive dechlorination of CT
on nanostructured Pd thin films [27]. The Pd surface was covered
by adsorbed-H atoms at −0.08 V, which was advantageous to the
surface reaction between H atoms and adsorbed CT molecules,
thereby leading to the relatively high removal efficiency. However,
as the potential became more negative, H atoms started to perme-
ate into the Pd lattices, forming the Pd ␣-hydride phase. Because
of lack of active adsorbed-H, the CT removal efficiency was lower
than that at −0.08 V. The change of removal efficiency with the
applied potentials can be interpreted based on the voltammetric
4
.9 and 40.7% for Pd/Fe ratio: 0.2).
3.3.3. Effect of temperature
Taking electrochemical reductive dechlorination of CT on the
Pd–Fe thin films (Pd/Fe ratio: 5.9) as an example, we studied the
influence of temperature on the rate of CT dechlorination at a con-
stant potential of −0.2 V. Increasing temperature has a beneficial
influence on the CT dechlorination. The removal efficiency of CT
increased from 92.4% to 97.4%, 98.1% and nearly complete removal
behavior of the Pd–Fe thin films. In CT-free H SO4 solutions, a
2
monolayer of hydrogen atoms already occupied the whole Pd
Table 2
(99.9%) as the temperature gradually increased from 298 K to 303 K,
The RE and Ea for electrochemical reductive dechlorination of CT at different applied
cathodic potentials on the Pd–Fe thin films in H2SO4 solutions. (Pd/Fe weight ratio:
3
13 K and 323 K.
5
.9; for removal efficiency test, time: 30 min; temperature: 303 K.)
The effect of temperature was also used to evaluate the activa-
tion energy of CT dechlorination reaction on the Pd–Fe thin films.
The electrode reaction rate can be defined in term of current density
and activation energy, as shown in Eq. (4).
Potential (V)
RE (%)
Ea (KJ mol−1)
−
−
−
−
−
0.05
0.08
0.1
0.15
0.2
52.1
91.3
59.4
39.0
97.4
49.1
24.4
39.0
51.0
23.7
ꢀ
ꢁ
∂
ln I
Ea
RT2
= −
(4)
∂T
Á

6
2
C. Qiu et al. / Journal of Molecular Catalysis A: Chemical 350 (2011) 56–63
kobs. The apparent first-order rate constant k
for CT dechlori-
obs
−
2
−1
nation reaction are 8.15 × 10 min . The lager kobs (compared
to the previous reported values [34]) demonstrates that our as-
prepared Pd–Fe bimetallic film electrode is an effective catalyst for
CT dechlorination reaction.
4. Conclusions
Nanostructured Pd–Fe bimetallic thin films were directly fab-
ricated on glassy carbon substrates by means of double-potential
step electrodeposition and galvanic replacement methods. The
Pd/Fe weight ratio in the bimetallic films is an important parameter
that affects the dechlorination activity of the bimetallic catalysts
and can be adjusted by selecting nanostructured Fe film tem-
plates with different thickness and surface morphology, which
are dependent on the concentration of Fe2+ ions in electrolyte
solutions. When the weight ratio of Pd/Fe is 5.9, the bimetal-
lic thin film catalyst exhibits the highest dechlorination activity.
The activation energy Ea for dechlorination by different types of
hydrogen is in accord with the removal efficiency of CT, and a
minimum Ea and the highest removal efficiency were obtained at
Fig. 5. Relationship between ln(C0/C) and reaction time (t).
−
0.2 V. Besides, the removal efficiency of CT increased consider-
ably with increasing temperature. The reaction was found to be
pseudo-first-order.
where Á is overpotential, Ea is activation energy, R is the gas con-
stant, and T is the absolute temperature. Integrating Eq. (4) results
in:
Ea
Acknowledgements
ln I = −
+ ln A
(5)
RT
This work was supported by National Natural Science Foun-
dation of China (21073111), Beijing National Laboratory for
Molecular Science, and the National Basic Research Program of
China (2009CB930103).
where A is an integral constant. Thus, plotting lnI against 1/T creates
a set of Arrhenius plots for the CT dechlorination reaction. Based on
the relationship between lnI and 1/T, the Ea values under different
cathodic potentials were calculated and listed in Table 2. It is found
that the Ea value is the lowest at −0.2 V, and the highest at −0.15 V.
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