Development of reactive Pd/Fe bimetallic nanotubes for dechlorination reactions

Article abstract of DOI:10.1039/c1jm11435b

We described the synthesis and characterization of a new class of bimetallic nanotubes based on Pd/Fe and demonstrated their efficacy in the dechlorination of PCB 77, a polychlorinated biphenyl. One-dimensional iron metal nanotubes of different diameters were prepared by electroless deposition within the pores of PVP-coated polycarbonate membranes using a simple technique under ambient conditions. The longitudinal nucleation of the nanotubes along the pore walls was achieved by mounting the PC membrane between two halves of a U-shape reaction tube. The composition, morphology, and structure of the Pd/Fe nanotubes were characterized by transmission electron microscopy, scanning electron microscopy, inductively coupled plasma-atomic emission spectroscopy, and X-ray powder diffraction spectroscopy. The as-prepared Pd/Fe bimetallic nanotubes were used in dechlorination of 3,3′,4,4′-tetrachlorobiphenyl (PCB 77). In comparison with Pd/Fe nanoparticles, the Pd/Fe nanotubes demonstrated higher efficiency and faster dechlorination of the PCB.

Full text of DOI:10.1039/c1jm11435b

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PAPER
Development of reactive Pd/Fe bimetallic nanotubes for dechlorination
reactions†
a
Elsayed M. Zahran, Dibakar Bhattacharyya and Leonidas G. Bachas
b
ac
*
Received 5th April 2011, Accepted 16th April 2011
DOI: 10.1039/c1jm11435b
We described the synthesis and characterization of a new class of bimetallic nanotubes based on Pd/Fe
and demonstrated their efficacy in the dechlorination of PCB 77, a polychlorinated biphenyl. One-
dimensional iron metal nanotubes of different diameters were prepared by electroless deposition within
the pores of PVP-coated polycarbonate membranes using a simple technique under ambient conditions.
The longitudinal nucleation of the nanotubes along the pore walls was achieved by mounting the PC
membrane between two halves of a U-shape reaction tube. The composition, morphology, and
structure of the Pd/Fe nanotubes were characterized by transmission electron microscopy, scanning
electron microscopy, inductively coupled plasma-atomic emission spectroscopy, and X-ray powder
diffraction spectroscopy. The as-prepared Pd/Fe bimetallic nanotubes were used in dechlorination of
3,30,4,40-tetrachlorobiphenyl (PCB 77). In comparison with Pd/Fe nanoparticles, the Pd/Fe nanotubes
demonstrated higher efficiency and faster dechlorination of the PCB.
Suzuki coupling, and Heck cross-coupling reactions.18,26–30
Introduction
Interestingly, palladium nanoparticles could form core/shell
bimetallic nanoparticles with silver, gold, platinum, and iron.31–36
Pd/Fe bimetallic nanoparticles have been an effective tech-
nology in dechlorination reactions involving chlorinated organic
hydrocarbons,36–40 in which the Fe core of the nanoparticles
reacts (in the aqueous phase) with water to produce hydrogen.
This hydrogen along with the chlorohydrocarbon adsorbs on the
surface of the palladium nanoparticles, which facilitate the
dechlorination reaction. The smaller the size of Pd/Fe nano-
particles, the higher the reactivity and catalytic surface area per
unit mass. However, owing to the high reactivity of the Fe
nanoparticles and their ferromagnetic properties, Fe nano-
particles tend to aggregate during preparation.41 There is also
evidence of aggregation during the palladization step.14 In
addition, it has been recognized that these particles aggregate
during remediation and adhere to the soil.39 Consequently, the
surface-to-volume ratio of the particles is decreased, which
impacts the rate of the dechlorination reaction. This presents
a major challenge for investigators to stabilize the dispersed form
of the Pd/Fe nanoparticles. Several surfactants have been used to
stabilize the nanoparticles including poly(acrylic acid),11 starch,42
cetyltrimethyl-ammonium bromide,43 carboxylmethylcellulose,13
and Triton X-100.44 In another approach, the aggregation of the
particles is prevented by in situ synthesis of nanoparticles in
porous support such as a polyacrylic acid/polyethersulfone
composite membrane,45 and a poly(vinylidene fluoride) micro-
filtration membrane.12
Nanomaterials have been used extensively over the last two
decades in biosensors, fuel cells, photonics, electronics, and
catalysis,1–6 because of their distinct difference in physical and
chemical properties from bulk materials.7–10 Because of the
tendency of nanoparticles to aggregate, the particles are usually
produced in the presence of dispersing agents that introduce
surface functionalities (e.g., carboxylates) or are immobilized
within polymer matrices.11–14 The unique catalytic properties of
the nanoscale materials are attributed to their high surface-to-
volume ratio15,16 and the chemical valence unsaturation of the
surface atoms,17 which enable binding of reactants. Furthermore,
the electronic effects of the almost filled d-shell of group 11 and
12 transition metals make the corresponding nanoparticles effi-
cient catalysts in many reactions.18–20 Specifically, palladium
nanoparticles have received much attention not only because of
their high catalytic activity, which is similar to platinum,21–23 but
also because they are cost-effective in comparison to plat-
inum.24,25 Palladium nanoparticles have been used for several
reactions such as hydrogenation, dechlorination, oxidation,
aDepartment of Chemistry, University of Kentucky, Lexington, KY, 40506,
USA
bDepartment of Chemical and Material Engineering, University of
Kentucky, Lexington, KY, 40506, USA
cDepartment of Chemistry, University of Miami, Coral Gables, FL, 33146,
USA. E-mail: bachas@miami.edu; Fax: +1 859 323-1069; Tel: +1 859
257-6350
† Electronic supplementary information available: Optical image of the
U-shape reaction tube, TEM images of Pd/Fe nanotubes and
nanoparticles and analysis of the SAED pattern of the Pd/Fe
nanotubes. See DOI: 10.1039/c1jm11435b
In recent years,16,46–51 attention has been devoted to the
nanotube architecture because of several superior properties. For
instance, the hollow interior of nanotubes provides a high ratio
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of surface-to-core atoms,52 which enhances surface-dependent
characteristics such as surface plasmon resonance,53 conduc-
tivity,54 reactivity and catalytic ability.55 Moreover, metal
nanotubes are of special interest because of the quantum
confinement along the tube and the excess charge density on the
interior wall.56,57 The nanotube structure exhibits higher exposed
surface area to mass ratio than nanoparticles, even when
aggregates are formed; this is because the internal nanoparticles
in an aggregate have their surface covered by the next layer of
particles, whereas aggregates of nanotubes tend to stack in
a fashion similar to the ‘‘Mikado game’’ pieces. Although contact
is made among the nanotubes, the majority of the surface is
unobstructed (Chart 1). All these interesting characteristics make
the Pd/Fe nanotubes a potential route to increase the efficiency of
the environmentally important dechlorination reactions.
(PC)79–81 or anodized alumina membranes (AAM).80,82–84
However, it was difficult under this approach to prevent the
oxidation of the Fe metal nanotubes to iron oxide nanotubes,
which could not be used for hydrogenation or dechlorination
reactions. We describe here the synthesis and characterization of
a new class of bimetallic nanotubes based on Pd/Fe (Pd is post-
coated of Fe nanotubes) using a modified-template based
method. The efficacy of the as-prepared Pd/Fe nanotubes toward
PCB dechlorination is also demonstrated.
Experimental
1. Synthesis of palladium/iron (Pd/Fe) nanoparticles
The Pd/Fe nanoparticles were prepared according to the proce-
dure published elsewhere36–40 with some modifications. Briefly,
the reaction was carried out in a 500 mL three-neck round
bottom flask, where a solution of 0.1 M FeSO4 stabilized with 0.5
M ascorbic acid was added. A fast stream of argon was injected
from one neck, which is used for degassing and stirring the
reaction mixture and a vacuum was applied on the other neck. A
solution of 0.2 M NaBH4 was injected from the vertical neck
using a peristaltic pump at a rate of 0.1 mL sꢀ1. The addition of
sodium borohydride was stopped when the reaction mixture
turned black and bubbles ceased to evaporate. A strong magnet
was used to collect the as-prepared Fe nanoparticles to the
bottom of the flask, while the solution was decanted. The last
step was repeated three times for washing with oxygen-free water
and two times with ethanol. A shell of palladium nanoparticles
was deposited on the surface of the iron nanoparticles by soaking
the freshly prepared iron nanoparticles in 0.01 wt% ethanolic
solution of palladium acetate (Alfa Aesar, Ward Hill, MA 01835)
for 2–24 h. At the specified time, the Pd/Fe nanoparticles were
separated and washed two times with absolute ethanol using the
strong magnet protocol mentioned previously. The as-prepared
Pd/Fe nanoparticles were dispersed in ethanol or dried under
vacuum for use in dechlorination or further characterizations.
Two general approaches have been followed for the synthesis
of metallic nanotubes. The template-free methods include
hydrothermal synthesis, surfactant-assisted solvothermal
synthesis, polyol, and electrodeposition from ionic liquid.58–61
The template-based method, popularized by Martin’s group,62,63
is the most successful and generic approach for the preparation
of 1-D nanostructures with hollow interior. In this approach
a nanoporous membrane is utilized for nucleation of 1-D
nanoscale materials inside the membrane pores. This is followed
by wet etching or calcination of the membrane in order to
liberate the nanotubes. The preparation of the nanotubes inside
the membrane pores is achieved by several methods including
electrochemical deposition, electroless deposition, polymer-
assisted deposition, sol–gel deposition, and chemical vapor
deposition (CVD).52,64–69 Most of these methods required pre-
functionalization of the membrane pores to allow for the
nucleation of the 1-D nanotubes. Alternatively, a simple
approach of the conventional template-based method has been
introduced.70–72 In this approach, the template is placed between
two compartments of a U-shape reaction tube in a way that
separates two different electrolyte solutions. This design allows
for the diffusion of the electrolytes inside the membrane pores.
Consequently, a local reaction results in the deposition of the
nanomaterials on the pore-walls.
2. Synthesis of (Pd/Fe) nanotubes
To date, several metallic nanotubes have been prepared, such
as those of Au, Pd, Ni, Zn, Ag, Pt, and Pb.73–78 However, the
preparation of zero-valent iron (ZVI) nanotubes remains a chal-
lenge because of the high reactivity of iron metal nanostructures
in the aqueous chemical etching processes of the membrane
template or the oxidation to iron oxides during template calci-
nations. Indeed, few reports have described the synthesis of Fe
nanotubes. In most of these reports, electrodeposition is used to
grow the Fe nanotubes in the nanopores of polycarbonate
Track-etched polycarbonate (PC) nanoporous membranes of
400, 200, and 100 nm pore diameters and 47 mm diameter
(Millipore, Billerica, MA 01821) were used as a template for the
preparation of the iron nanotubes. These membranes are sold in
hydrophilic or hydrophobic form with a thickness of 7–22 mm
with 5–20% pore density. In a typical synthesis of one-dimen-
sional Fe0 nanotubes, a PC membrane was mounted between the
two compartments of a custom made U-tube cell in a way that
the membrane separates two equal volumes of electrolyte solu-
tions (see ESI†, Fig. S1). These two electrolyte solutions were
prepared as follows: the oxidant solution was composed of 1 ꢁ
10ꢀ3 M ferrous sulfate stabilized by 1 ꢁ 10ꢀ2 M ascorbic acid; the
reductant solution was composed of 1 ꢁ 10ꢀ2 M sodium boro-
hydride. The U-tube cell was leveled horizontally. The oxidant
solution was degassed for 20 min by argon prior to pouring in the
U-tube. Both solutions were poured in each half of the U-tube
simultaneously. Different reaction spans were investigated for
optimum yield of nanotubes. At a specific time, the solutions
were poured off the U-tube and the membrane was instantly
sonicated for 2 min in oxygen free water to remove the unwanted
Chart 1 Schematic diagram indicating the higher exposed surface area
of nanotube aggregates as compared to nanoparticle aggregates.
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nanoparticles if any was formed on the membrane surface.
Subsequently, the membrane was washed thoroughly with water
and ethanol and dried with argon gas. The as-prepared Fe
nanotubes were extracted by dissolving the PC membrane in 10
mL methylene chloride and washing two times with methylene
chloride and two times with ethanol. Palladium nanoparticles
were deposited on the as-prepared Fe nanotube by adding 1 mg
of palladium acetate to 10 mL Fe nanotubes colloidal solution.
The palladization reaction was run overnight, and the Pd/Fe
nanotubes were washed two times with ethanol and dispersed in
ethanol.
and nanotubes. The samples were prepared by dissolving 1 mg of
dry Fe and Pd/Fe nanoparticles and nanotubes in 1 mL nitric
acid and further diluted to 25 mL.
5. Dechlorination experiment
Batch reactions were conducted to study the reactivity of the as-
prepared Pd/Fe nanotubes in dechlorination of 3,30,4,40-tetra-
chlorobiphenyl (PCB 77), a tetrachlorinated PCB congener.
These reactions were performed in two different sizes glass vials
with PTFE septa caps. For 1 g Lꢀ1 Pd/Fe nanoparticles or 0.1 g
Lꢀ1 Pd/Fe nanotubes metal loading, the reactions were done in
a 65 mL glass vial in which 60 mL of 25 mM 50/50 (v/v) ethanol/
water solution of PCB 77 was added. The vials were placed on
a wrest shaker at maximum speed. All experiments were per-
formed under identical agitation conditions to ensure the absence
of bias from external diffusion constrains on the interpretation of
the experiments. At specific times, the nanotubes or nano-
particles were collected using a strong magnet, and 1 mL of the
solution was withdrawn and transferred to a 2 mL glass vial
containing 1 mL hexanes for extraction of the PCBs. Because of
the mass confinement of the Pd/Fe nanotubes, the 1 g Lꢀ1 metal
loading was done in 2 mL glass vials. In this case, at the specific
time, the vial was sacrificed for extraction of the PCBs. The
extraction vials were set on a shaker overnight to attain extrac-
tion equilibrium. A volume of 100 mL of the hexane layer was
combined with 10 mL of PCB 209 as internal standard before
GC-MS (Agilent, Santa Clara CA 95051) analysis.
3. Electron microscopy
A JEOL 2010F field emission electron microscope equipped with
an energy-dispersive X-ray detector and running at an acceler-
ating voltage of 200 kV was used for the nanoscale character-
ization of Pd/Fe nanoparticles, Fe nanotubes and Pd/Fe
nanotubes. The size, structure, morphology, and elemental
composition were studied using bright field transmission electron
microscopy, energy dispersive spectroscopy (EDS), selected area
electron diffraction (SAED), and dark field scanning trans-
mission electron microscopy. EDS line profile at STEM mode
was used to verify the hollow structure of the nanotube. To
obtain this profile a 1 nm in diameter electron probe was scanned
transversally across a Fe nanotube and a Fe nanowire while the
Fe Ka X-ray was detected at discrete positions along the line
with a dwell time of 1 s. EDS-mapping at STEM mode was used
to study the distribution and the ensemble of Pd nanoparticles on
the as-prepared Fe nanotubes. A 0.5 nm electron probe was used
to scan an area of 48 ꢁ 40 pixels of the Pd/Fe nanotube with
a dwell time of 0.5 s. High-resolution transmission electron
microscopy (HRTEM) measurements along with SAED were
performed to investigate the crystal structure of the as-prepared
Fe and Pd/Fe nanotubes. The TEM samples were prepared by
dispersing the as-prepared nanotubes or nanoparticles in ethanol
and dropping few drops to a copper grid with a lacy carbon layer.
A Hitachi 4300 scanning electron microscope (SEM) was also
used for characterization of the morphology of the as-prepared
Pd/Fe nanotubes. The sample was prepared by dropping and air
drying a small volume of the Pd/Fe ethanol colloidal solution
onto a silicon wafer. The silicon wafer was then sputter-coated
with thin layer of gold–palladium alloy in order to increase the
electrical conductivity of the sample and to prevent charging of
the specimen surface during electron irradiation.
Results and discussion
Different diameter Fe nanotubes were synthesized by a simple
and cost-effective procedure at ambient conditions. Addition-
ally, this procedure provides control of the size, morphology, and
crystal structure of the 1-D nanostructures via altering the pore
size of the template and the concentrations of the reactants.
Specifically,
a track-etched polycarbonate membrane was
mounted between two compartments of a custom made U-shape
reaction tube. The polycarbonate membrane was chosen instead
of other templates, such as porous alumina, because the latter
requires etching of the template in harsh aqueous media, which
lead to oxidation of the ZVI. The longitudinal nucleation of the
nanotubes along the pore walls was achieved by the localized
electroless reduction of ferrous sulfate, stabilized by ascorbic
acid, placed in one side of the U-tube cell, upon diffusion of
sodium borohydride from the other side of the cell. Different
mechanisms have been proposed to describe the growth of 1-D
nanostructures leading to solid nanowires or hollow nanotubes
based on the concentration of the reactants, the interactions
between the template pores and reagents, and the reaction
duration and conditions.72,85,86 On the account of these mecha-
nisms, the concentration of the reactants and the reaction
duration were selected to direct the system toward the formation
of nanotube-like structures. In a typical procedure that resulted
in formation of hollow nanotubes, the oxidant solution was
composed of 1 ꢁ 10ꢀ3 M iron(II) sulfate and 1 ꢁ 10ꢀ2 M ascorbic
acid, while 1 ꢁ 10ꢀ2 M NaBH4 was used as the reductant.
Ascorbic acid was used in the oxidant compartment to prevent
the formation of iron oxides, which block the template pores, and
4. XRD, BET, and ICP analysis
The purity and crystalline structure of the as-synthesized Fe
nanoparticles were studied using a Bruker AXS D8 Discover
powder X-ray diffractometer (XRD) operating in Bragg config-
ꢀ
uration and equipped with a Cu Ka radiation (l ¼ 1.54056 A)
source. The diffraction patterns were acquired over the range of
2q 25–90ꢂ at a scanning rate of 1ꢂ minꢀ1 with a step size of 0.02ꢂ.
The BET surface area was obtained by N2 adsorption at 77 K
using a Quantachrome NOVA 1200 at the University of Florida,
Particle Engineering Research Center. A Thermo iCAP 6500 duo
ICP-AES system (Thermo Fisher Scientific) was used to deter-
mine the metallic composition of the Fe and Pd/Fe nanoparticles
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to maintain diffusion of the Fe(II) ions inside the membrane
pores. These conditions enhance the longitudinal diffusion of the
reagents over rapid inward nucleation.
membranes with 400, 200, 100 nm pore sizes, respectively. The
corresponding inner diameters were found to be 400 ꢃ 40, 200 ꢃ
20, and 105 ꢃ 10 nm. Although the length of the as-synthesized
Fe nanotubes is controlled by the thickness of the PC membrane,
the smaller diameter nanotubes were longer than the larger ones,
which might be attributed to the increase in mechanical strength
as the size decreases. As shown in Fig. 1, the length of the as-
synthesized nanotubes is in the range of 2–5 mm. For simplifi-
cation, 400, 200, 100 nm are used throughout this article to
designate the different as-prepared Fe nanotubes. In order to
maintain the rough texture of the nanotubes surface, which is
desirable for higher reactive surface area; no further annealing
was done (see ESI†, Fig. S2).
The diffusion of ions was confirmed by continuous measure-
ments of the pH during the reaction along with ICP analysis for
Fe, Na, and B at different reaction times. It was found that the
pH increased by 2 units in the oxidant side and 1 unit in the
reductant side. Decreasing the template pore size resulted in less
change in the pH at the oxidant side; however, no effect was
noticed on the pH change at the reductant side. This can be
ꢀ
explained by hydrolysis of BH4 which causes a pH increase at
ꢀ
both sides of the U-tube. Migration of Na+ and unreacted BH4
ions to the oxidant side was proven by ICP-AES analysis, which
indicated that 12% (w/v) of the total Na and B was found in the
oxidant side after carrying out the reaction for 2 h. No iron was
found in the reductant side after 2 h. This indicates that as the Fe2+
diffuses into the membrane pores, it is being reduced to Fe0
nanoparticles, which nucleate on the pore walls to form the iron
nanotubes architecture. It is noteworthy that the pores of the
commercially available hydrophilic PC membranes are covered
with a layer of polyvinylpyrrolidone (PVP) which is prone to
bind and stabilize the Fe nanoparticles.14 This interaction made
the formation of iron nanotube motifs more remarkable, which is
consistent with prior described mechanisms.72,85,86 The optimum
reaction duration was found to be 120, 75, and 45 min for PC
membranes having pore sizes of 400, 200, and 100 nm,
respectively.
The crystal structure of the Fe nanotubes was controlled by the
concentration of the reactants. At higher concentrations of
NaBH4 and FeSO4, organized amorphous structures were
observed (see ESI†, Fig. S3). On contrary, at lower concentra-
tions, the as-synthesized Fe nanotubes had a polycrystalline
structure. The HRTEM image, Fig. 2B, shows the different
crystalline domains, with the values between the planes matching
that of the cubic crystal structure. Further, the selected area
electron diffraction (SAED) pattern shown in Fig. 2A is consis-
tent with polycrystalline nature of the Fe nanotubes. The SAED
pattern was analyzed with the CHT diffraction analysis script87
on the digital micrograph software (see ESI†, Fig. S4) to deter-
mine the lattice spacing of various crystalline domains. The
analysis showed six different crystalline domains (110), (200),
(211), (220), (310), and (222) with lattice spacings of 2.0268,
The synthesized Fe nanotubes were liberated from the
template by dissolving the PC membrane in 5 mL methylene
chloride. The morphology and aspect ratio of the Fe nanotubes
were identified using TEM. Various sizes Fe nanotubes were
successfully synthesized using the above-mentioned procedure
(Fig. 1). The outer diameters of the tubes were found to be 411
ꢃ 44 nm, 211 ꢃ 26 nm, and 130 ꢃ 19 nm (N $ 50 tubes) for PC
ꢀ
1.4332, 1.1702, 1.0134, 0.9064, and 0.8275 A, respectively. The
SAED pattern matches exactly that of the Fe polycrystalline
body centered cube (BCC), JCPDS (006-0696), which indicates
the high purity of the as-prepared Fe nanotubes. To further
elucidate the crystallinity and purity of the Fe nanotubes, powder
XRD measurements were acquired for the dry powder (dried
Fig. 1 TEM images of Fe nanotubes synthesized using PC membrane with (A) 400 nm, (B) 200 nm, (C) 100 nm pore sizes, and (D) EDS analysis of 400
nm Fe nanotubes.
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Fig. 3 Hollow interior of the as-synthesized Fe nanotubes: (A) EDS line
mapping of Fe Ka across the as-synthesized Fe nanotube, (B) EDS line
mapping of Fe Ka across Fe nanorod, and (C) the comparable probe
position peak profile.
Fig. 2 Crystalline characterization of the as-synthesized Fe nanotubes:
(A) SAED pattern, (B) HRTEM image, and (C) XRD spectrum of the
dry powder of the nanotubes. The same crystalline structure was noticed
for Fe nanotubes of different sizes prepared from 1 ꢁ 10ꢀ3 M iron (II)
sulfate, 1 ꢁ 10ꢀ2 M ascorbic acid, and 1 ꢁ 10ꢀ2 M NaBH4 as a typical
concentration of the reactants.
Electroless deposition of a palladium layer was achieved by
soaking the freshly prepared Fe nanotube in an alcoholic solution
of 0.5% w/v palladium acetate according to the following
equation:
Fe0 + Pd2+ / Fe2+ + Pd0
under nitrogen atmosphere) of the liberated Fe nanotubes. The
XRD pattern (Fig. 2C) shows three peaks at 2q of 44ꢂ, 65ꢂ, and
80ꢂ which represent the (110), (200), (211) crystalline domains.
This pattern matched with Fe syn, JCPDS (006-0696). No iron
oxide peak was found in the XRD or EDS spectra. This suggests
that any iron oxide that may be present on the metallic iron
nanotubes is below the detection limit of XRD, which is in the
range of 3–5%. We attribute this high purity to the use of
ascorbic acid during the reaction, which maintains the produced
Fe nanotubes in a reducing medium and inhibits the formation of
iron oxide on the surface of the tube. The high purity of the ZVI
nanotubes is desirable for the subsequent deposition of the
palladium shell onto the Fe nanotubes as well as for high reac-
tivity in aqueous solution to produce the hydrogen required in
the hydrodehalogenation reactions.
It is expected that the presence of acetate prevents Fe2+ from re-
precipitating as iron hydroxide on the surface.
The new material was characterized by TEM, SEM, BET and
ICP-AES. As shown in the low magnification TEM images
(Fig. 4), the as-synthesized Fe nanotubes maintain their aspect
ratio after deposition of the palladium layer. This indicates good
mechanical strength of the as-synthesized Fe nanotubes. It is also
evident that the Pd is deposited on the available surface area of
the Fe nanotubes as a uniform layer of bound nanoparticles. The
presence of Pd particles on the Fe nanotube surface is noticeable
by comparing the TEM image of the Fe nanotubes in Fig. 1 to
the Pd/Fe nanotubes in Fig. 4. The Pd/Fe nanotubes maintain
a hollow interior after the deposition of the Pd layer. This fine
structure provides with high exposed surface area for catalytic
reactions. This hypothesis was further elucidated by multi-point
N2-BET surface area analysis. The BET surface area of the as-
synthesized Pd/Fe nanotubes was found to be 74.6 ꢃ 8.8 m2 gꢀ1
(average ꢃ one standard deviation from triplicate analysis of
a sample) as measured by nitrogen adsorption analysis. The
surface area of the nanotube was also calculated geometrically
(as that of a hollow cylinder) based on TEM measurements of the
inner and outer diameter of the nanotube, and it was found to be
51 m2 gꢀ1. This geometrically calculated area does not take into
consideration the surface roughness due to the deposited Pd,
which may explain its lower value than the surface area obtained
through BET analysis. The BET surface area was used for all
Although, the hollow nature of the as-prepared Fe nanotube is
evident from the low magnification TEM images (Fig. 1), further
elucidation of the hollow structure of the Fe nanotubes was per-
formed using the EDS line mapping in STEM mode. A 1 nm
electron probe was scanned across the as-synthesized Fe nanotube
and a Fe nanorod, while recording the Fe signal at a 1 s dwell time.
The ‘‘bell’’ shapeof the FeKa EDS peak of the Fe nanorod(Fig. 3)
represents the high intensity of the material in the center of the
nanorod (solid interior). On contrary, the ‘‘saddle’’ shape of the Fe
Ka EDS profile of the nanotube indicates the hollow interior of
the as-synthesized Fe nanotubes, which in turn provides a high
surface area for the deposition of palladium nanoparticles.
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Fig. 4 Characterization of the morphology of as-synthesized Pd/Fe nanotubes: (A) TEM image of 400 nm Pd/Fe nanotubes, (B) TEM image of 200 nm
Pd/Fe nanotubes, (C) HRTEM image of 400 nm Pd/Fe nanotubes, (D) SEM of 200 nm Pd/Fe nanotubes, and (E) EDS analysis of 400 nm Pd/Fe
nanotubes.
subsequent calculations of the kinetics of the dechlorination
reaction.
Dechlorination of PCB 77
For comparison purposes, Pd/Fe nanoparticles (ESI†, Fig. S6)
were prepared according to literature procedures with some
modifications.36,39,88 Specifically, ascorbic acid was used to
stabilize the FeSO4 solution and prevent the oxidation of the
The elemental composition of the as-synthesized Pd/Fe nano-
tubes was determined by EDS analysis. The EDS spectra in Fig. 4
show a peak at 2.81 keV assigned to Pd La, which confirmed the
presence of Pd in the sample. The composition of the as-synthe-
sized Pd/Fe nanotubes was determined by EDS analysis to be
97.8% Fe and 2.2% Pd. No oxygen peak was observed suggesting
minimal oxidation of the metallic iron after the deposition of
palladium particles. These results are consistent with the 97.5% Fe
and 2.5% Pd obtained by ICP-AES analysis. Formation of
a highly dispersed uniform shell of palladium is very desirable
because it leads to greater catalytic surface area. The morphology
of the Pd shell was studied by dark field scanning transmission
electron microscopy (DF-STEM). As evident from Fig. 5, the Pd
nanoparticles are well dispersed as a uniform shell on the core Fe
nanotubes. Further, EDS mapping for Fe and Pd was performed
on STEM mode using a 1 nm electron probe with a dwell time of
0.5 s. The Fe Ka and Pd La profiles were simultaneously detected
over a 48 ꢁ 40 pixel area, the designated square in Fig. 5. The EDS
maps in Fig. 5 represent the 3-D distribution of Fe and Pd in the
as-synthesized Pd/Fe nanotubes. As evident from the images, the
Pd nanoparticles are distributed on the surface of the Fe nano-
tubes with very high dispersion, which provides with very high
catalytic surface area. It is also worth noting that the Pd/Fe
nanotubes maintained large surface area even in aggregation as
evident from (see ESI†, Fig. S5).
Fig. 5 Characterization of the palladium distribution on the as-
synthesized Fe nanotubes: (A) DF-STEM image of 400 nm Pd/Fe
nanotube, (B) color mix image of the EDS mapping of Fe and Pd across
a whole nanotube, (C) EDS map of Fe with high resolution, (D) EDS
map of Pd with high resolution, and (E) color mix image of Fe map and
Pd map.
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produced Fe particles. This condition was used to mimic the
solutions used in the preparation of Fe nanotubes. It was found
by XRD (ESI†, Fig. S7) that the Pd/Fe nanoparticles prepared
with this procedure exhibit metallic iron BCC polycrystalline
structure similar to that of Pd/Fe nanotubes. It is also evident
from the XRD spectrum of Pd/Fe nanoparticles in Fig. S7 (ESI†)
that the amount of oxidized Fe as a result of the surface reaction
of depositing Pd nanoparticles is below the detection limit of
XRD. This condition was desirable for high efficiency of the
dechlorination of PCB 77. The particle size was found by TEM
to be about 25 ꢃ 10 nm with a core of about 20 nm ZVI particles
and a layer of very small palladium particles (see ESI†, Fig. S6).
It has been established that the palladium content has a direct
influence on the efficiency of dechlorination reactions.12,89
Accordingly, Pd/Fe nanoparticles were prepared that had
a palladium content consistent with that of the as-synthesized Pd/
Fe nanotubes as determined by ICP-AES. The Pd/Fe nano-
particles were found to have 15.5 ꢃ 2.8 m2 gꢀ1 average BET
surface area. It should be noted that there are other Pd/Fe
nanoparticle systems reported, such as those in which the parti-
cles are supported within membranes.40,90–92 In this study and for
comparison purposes, we use a membrane-free Pd/Fe nano-
particle system that had a similar Pd content to that of the as-
synthesized Pd/Fe nanotubes.
Fig. 6 Dechlorination profile of PCB 77 using 400 nm Pd/Fe nanotubes
with metal loading 1 mg mLꢀ1 and 2.5% Pd-to-Fe ratio: (O) tetra-
chlorobiphenyl, PCB 77, (-) trichlorobiphenyl, PCB 37 + PCB 35, (:)
dichlorobiphenyl, PCB 11 + PCB 12 + PCB 13 + PCB 15, (,) mono-
chlorobiphenyl, PCB 2 + PCB 3, (B) biphenyl, and (+) carbon balance.
It is well established that the rate of metal-promoted dechlo-
rination of chlorinated organic compounds depends not only on
their concentration, but also on the available reactive and cata-
lytic surface area of the metal. Accordingly, a pseudo-first order
kinetic model has been generally used to describe the rate of the
dechlorination reaction.92
3,30,4,40-Tetrachlorobiphenyl (PCB 77),
a
single PCB
congener, was chosen as a model compound for studying the
dechlorination efficiency of the as-synthesized Pd/Fe nanotubes.
It has been found that coplanar PCBs, such as PCB 77, are aryl
hydrocarbon receptor (AhR) agonists that could cause cardio-
vascular diseases.93 It has also been shown that PCB 77 induces
cytochrome P450 1A1 (CYP1A1), which may lead to oxidative
stress.94
dC
ꢀ
¼ kobsC ¼ kSArmaSC
dt
Where C is the concentration of PCB 77 (mol Lꢀ1) at time t, rm is
the metal loading (g Lꢀ1), and aS is the specific surface area of the
metal nanostructures. The surface area normalized constant,
kSA, is widely used to give meaningful comparison between
different systems.
Pd/Fe nanotubes of 400 nm in diameter were used in the
dechlorination of PCB 77 as described in the Experimental
section. Along with PCB 77, the dechlorinated products PCB 37,
PCB 35, PCB 15, PCB 13, PCB 12, PCB 11, PCB 3, PCB 2, and
biphenyl were monitored. The results of the dechlorination of
PCB 77 in a solution containing 25 mM PCB 77 by 1 mg mLꢀ1
metal loading are depicted in Fig. 6. About 95% of PCB 77 was
dechlorinated, mainly to biphenyl and trace concentrations of
lower chlorine-content PCB intermediates, within the first hour
of the reaction. Besides biphenyl, very small amounts of mono-
and di-chlorobiphenyl, and practically no PCB 77 were deter-
mined after two hours of reaction.
To study the kinetics of the dechlorination of PCB 77 using the
Pd/Fe nanotubes, we decreased the metal loading to 0.1 mg
mLꢀ1. This slows down the reaction, so we can follow the rate of
We hypothesize that palladium is deposited on both the exterior
and interior surface of the nanotubes and consequently dechlo-
rination takes place on both surfaces. In order to evaluate the
effect of the internal surface of the Pd/Fe bimetallic nanotubes on
the efficiency of the dechlorination reaction, Fe nanorods of 400
nm diameter were synthesized by carrying out the U-tube
template-based electroless reaction for extended period of time to
attain complete nucleation within the pores of the membrane
template. Subsequently, palladium was deposited on the surface
of the Fe nanorods in a manner similar to the one on Fe nano-
tubes. The Pd/Fe bimetallic nanorods exhibited much slower
dechlorination rate with about 90% dechlorination in 24 h, which,
when compared with the data in Fig. 7, indicate that the internal
surface of the nanotubes contributes to the high efficiency of
dechlorination of the synthesized Pd/Fe bimetallic nanotubes.
Fig. 7 Comparison of the dechlorination efficiency of Pd/Fe nano-
particles and Pd/Fe nanotubes: (A) 0.1 mg mLꢀ1 400 nm Pd/Fe nano-
tubes, (C) 1 mg mLꢀ1 Pd/Fe nanoparticles, and (:) 1 mg mLꢀ1 400 nm
Pd/Fe nanotubes. Inset: linear fit kinetics of the dechlorination of 25 mM
PCB 77 with (C) 1 mg mLꢀ1 Pd/Fe nanoparticles and (A) 0.1 mg mLꢀ1
400 nm Pd/Fe nanotubes.
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the selectivity of the dechlorination reaction.
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Conclusion
We described the synthesis and characterization of a new class of
materials based on Pd/Fe bimetallic nanotubes. The metallic Fe
nanotubes were prepared by a simple approach under ambient
conditions and subsequently coated with Pd to create Pd/Fe
nanotubes. The bimetallic nanotubes are composed of a core of
Fe nanotubes covered with a shell of palladium nanoparticles.
The synthesized Pd/Fe nanotubes have tailored morphology
controlled by the template pore size. The reaction time and the
concentrations of the electrolytes were found to have great effect
on the morphology and crystal structure of the nanotubes. Pd/Fe
nanotubes were used in dechlorination of PCB 77 as halogenated
compound. In comparison with Pd/Fe nanoparticles, the Pd/Fe
bimetallic nanotubes showed higher reactivity in dechlorination
of PCB 77. Pd/Fe nanotubes can be effectively used in remedi-
ation of different other persistent organic pollutants.
Acknowledgements
The authors acknowledge support from the NIEHS-SRP
program (P42ES007380).
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J. Chen, J. Hazard. Mater., 2009, 170, 895–901.
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