ICC
Original Research Article
Iranian Chemical Communication Payame Noor University
http://icc.journals.pnu.ac.ir
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium
oxide
nanocomposite
by
the
gel
combustion method and application as catalyst for hydrogen revolution Hassan Karamia,b,*, Mohammad Ali Karimib,c, Ehsan Atinc aNano
Research Laboratory (NRL), Payame Noor University (PNU), P.O. BOX 97, Abhar, Iran
bDepartment
of Chemistry, Payame Noor University, 19395-4697, Tehran, Iran
cNanoscience
and Nanotechnology Research Laboratory (NNRL), Department of Chemistry,
Payame Noor University (PNU), Sirjan, Iran Received: 19 September 2015, Accepted: 29 February 2016, Published: 29 February 2016
Abstract This paper presents a new gel combustion method to synthesize palladium nanoparticles, palladium oxide (PdO) nanoparticles and palladium-palladium oxide nanocomposites. In the proposed method, there are some effective parameters including palladium chloride concentration, polyvinyl alcohol (PVA) concentration, acid concentration, solvent composition and combustion temperature that their values are investigated and optimized by the "one at a time" method. The experimental data shows that the combustion temperature is the main factor that controls the sample composition to obtain palladium, palladium oxide or palladium-palladium nanocomposites. Characterization of the synthesized samples is performed by SEM, TEM, XRD and BET specific surface area measurements. The optimized sample consisted of clusters; each cluster is composed of smaller nanoparticles with an average diameter of 25 mm and 10 m2. g-1 specific surface areas. The optimized PdPdO nanocomposite is successfully used as nanocatalyst for the hydrogen revolution. Keywords: Palladium nanoparticles; palladium oxide nanoparticles; palladiumpalladium oxide nanocomposite; gel combustion; catalyst; hydrogen revolution. *Corresponding author: Hassan Karami Tel: +98 (912) 5419727, Fax: +98 (242) 5226932 E-mail:
[email protected]
Iran. Chem. Commun. 4 (2016) …-… Page | 142
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
As a gas-sensing agent, palladium
Introduction Great interest has been focused on the
(Pd) has an interesting characteristic of
noble metal nanoparticles due to their
hydrogen molecules, including the large
interesting size- and shape dependent
capacity of hydrogen occlusion and the
physical and chemical properties. They
catalytic effect of the reaction between
are technologically important in many
hydrogen
fields such as catalysis [1,2], optics [3]
temperature. The Pd characteristics are
and
Raman
very attractive for the hydrogen sensor
spectroscopy [4,5]. Among metals,
for which the sensitivities are enhanced
palladium is widely studied because of
by down-sizing into the nanometer
its optical, spectroscopic, magnetic and
range [10]. Therefore, preparation of
catalytic properties. The development
nano-structured Pd is also an active
of the new methods for the preparation
research field [11-13].
surface
enhanced
and
oxygen
at
room
of uniform palladium nanoparticles
Several synthetic approaches and
becomes a very important issue in
different palladium precursors have
heterogeneous catalysis and hydrogen
been applied to generate palladium
storage materials. Palladium has an
nanoparticles having different shapes
important role in chemical synthesis [6]
and sizes such as chemical reduction of
and
industrial
PdCl2 by NaBH4 [14], arc-discharge
of
[15],
in
many
applications[7,8].
Major
Pd
reduction
of
supercritical
fine Pd particles where both the size
thermally
and shape are critical parameters that
Pd(Fod)2
should be controlled in order to
reduction of PdCl2 [18]. The size and
maximize
For
shape of palladium nanoparticles have
example, the reactivity and selectivity
been discussed as a function of metal
of a nanocatalyst can be tailored by
precursor concentration and of the
controlling the
will
effect of the surfactant molecules.
determine the crystallographic facets
However, the aqueous-phase synthesis
exposed on the surface of a nanocrystal
of
and therefore the number of atoms
environmentally-friendly process is still
located at the edges or corners [9].
challenging.
efficiencies.
shape, as
it
Page | 142
induced [17]
uniform
and
dioxide
in
applications requires very small and
their
carbon
Pd(OAc)2 reduction
[16], of
sonochemical
nanoparticles
by
an
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
Palladium
oxide
is
the
most
Instrumentals
important catalyst that is used for
Morphologies of the samples were
various reactions in the laboratory and
characterized by scanning electron
industry. It has been used for catalytic
microscopy (Model XL-30, Philips,
oxidation of benzyl alcohol [19],
Netherland).
sensing of OH radicals [20], oxidation
microscope (Zeiss EM900, 80Kev) was
of hydrocarbons [21], photocatalytic
used for determining the exact size and
degradation of bacterial indicators, etc.
shape of particles. The composition and
[22].
particles sizes of the samples were
Transmission
electron
There are many reports about the
determined by XRD (E- Pert- MPD,
synthesis of Palladium nanocomposites
Phillips, Netherland). The effective
such
palladium-polyaniline
surface areas were determined by
nanocomposite [23, 24], palladium-
Brunauer-Emmet-Teller (BET) specific
polycarbonate
surface area measurements based on N2
[26],
as
[25],
palladium-silica
palladium-polypyrole
palladium-carbon
nanotube
palladium-hematite
[29],
[27],
adsorption/desorption (Temcem 902 A
[28],
Z Picc, Germany).
palladium-
Procedure
alumina [30], etc. The large number of
Appropriate
reports
palladium
palladium chloride was dissolved in a
their
mixed solvent ethanol: HCl (0.1 M)
about
nanocomposites
reveals
weight
percent
of
importance in catalysts and sensors.
(60:40). Then 8 g polyvinyl alcohol
In this work, we present a new
(PVA) was added into the obtained
controllable method to synthesize Pd
solution. The mixture was stirred until a
nanoparticles, PdO nanoparticles and
transparent and homogeneous solution
Pd-PdO nanocomposites.
was obtained which called "sol".
Experimental
During
Materials and reagents
temperature should be kept lower than
All reagents and compounds were in
80 ºC. After sol formation, the mixture
analytical grade and obtained from
was
Merck, Fluka and Loba Chimie (India).
homogeneous gel. The obtained gel was
Double-distilled water was used in all
heated until the complete solvent
experiments.
evaporation and the formation of a
Page | 143
sol
slowly
formation,
heated
to
solution
form
a
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
dried gel. The dried gel was pyrrolized
optimization
at 500 ºC for 5 h. At this stage,
presented.
aluminum
Effect of gel making value
and
zinc
salts
were
experimental
set
is
decomposed and converted to metal
Polyvinyl alcohol (PVA), generally
oxides. Aluminum oxide and zinc oxide
used as suspension stabilizer, exhibits
were
a
the effect of reducing particle size of
obtained
the sample by decreasing the surface
connected
nanocomposite.
to
form
The
nanocomposites
were
studied
by
tension and by improving the dispersion
scanning electron microscopy (SEM),
of
transmission
microscopy
polymerization reaction [31]. Polyvinyl
(TEM), EDAX analysis, XRD patterns
alcohol (PVA) is a hydrophilic polymer
and
(BET)
contributing hydroxyl group on each of
specific surface area measurements.
its repeating units, which permits the
SEM and TEM instruments were used
development of hydrogen bonds with
to define the morphology of the
the hydroxyl and carboxyl groups of
samples. XRD, TEM, SEM and BET
reactants
were used for determining particles
sedimentation
sizes. The compositions of the samples
particles to trammel in gel pores while
were measured by XRD and EDX
the solvent evaporates silently and thus,
analyses.
not
Results and discussion
combustion method, the gel network
In the present method, there are some
rigidity controls the morphology and
effective parameters which can control
the particle sizes of the synthesized
the morphology, the particle sizes and
sample
the composition of the synthesized
nanostructured [33, 34]. In the gel
samples. The parameters include gel-
structure, palladium chloride molecules
making
solvent
were homogeneously dispersed through
composition, pH, palladium salt value,
the polymeric network. Because of gel
and combustion temperature. The effect
network
of each parameter was investigated and
molecules in the gel network cannot
optimized by the one factor at a time
alter their positions. Therefore, during
method. In the following sections, the
the combustion of the gel’s layers, the
electron
Brunauer-Emmet-Teller
value,
mixed
Page | 144
the
reactants
[32].
impacting
to
during
the
PVA
prolongs
reaction,
causing
again.
make
rigidity,
In
a
the
the
gel
uniform
dispersed
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
palladium chloride molecules of the
agent (PVA) has not any effect on the
burnt layers combine with each other to
sample
create PdCl2 particles. The formed
temperature, all samples were in Pd-
particles are calcined to yield Pd, PdO
PdO nanocomposite form, including 50
or Pd-PdO nanocomposites during the
wt% Pd and 50 %wt PdO. The PVA
pyrrolysis
these
amount affects on the morphology and
were
particle sizes of the samples. Figure 1
paralyzed at a constant combustion
shows the SEM images of the samples
temperature (500 °C). XRD patterns
that were synthesized by different
showed that the amount of gel- making
content of PVA.
process.
experiments,
all
In samples
composition.
In
this
Figure 1. SEM images of Pd/PdO nanocomposites synthesized at different initial concentrations of PVA; 0 (a), 4 (b), 6 (c), 8 (d), 10 (e) and 12 %wt (f).
As it is visible in Figure 1, the PVA content
changes
considerably
complete and suitable to control the
the
mechanism and kinetics of Pd-PdO
morphology and particle sizes of the
nanocomposite particles. At lower PVA
samples. The compact nanoparticles in
content, the gel network is not rigid
the absence of PVA is converted to
enough. Because of the low solubility of
nanoporous rice shape clusters. The gel
PVA in ethanol-HCl solution, at higher
network is completed when the PVA
content (12 %wt, Figure 1f); it is not
content is increased from 0 to 10 %wt.
obtained a uniform gel. Therefore, the
At 10 %wt PVA, the gel network is
Page | 145
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
sample
doesn’t
have
uniform
solubility of palladium chloride is low
morphology.
in ethanol-HCl mixed solution.
Effect of initial palladium salt content
Solvent composition
on the morphology and particle sizes
Based on our previous experiences [33],
It is expected that the content of
ethanol-water is a suitable solvent to
palladium
have
make PVA sol and PVA gel for
considerable effects on the morphology
synthesizing metal oxide nanoparticles.
and
Initial
chloride
particle
sizes
will
of
Pd-PdO
experiments
showed
that
nanocomposites. For evaluation of this
palladium chloride does not have
parameter, there was an attempt to
enough
synthesize some sample at different
According to chemistry facts, palladium
amounts of palladium chloride. Figure 2
chloride only has enough solubility in
shows the SEM images of the samples
acidic media. Therefore, we used HCl
that were synthesized at 1, 2 and 3 %wt
solution mixed with ethanol as suitable
palladium chloride.
solvent that both PVA and palladium
solubility
in
this
solvent.
At the contents lower than 1 %wt,
chloride have enough solubility to make
the amount of the final sample with
a uniform sol and consequently a
respect to initial sol volume is very low
uniform gel.
so the sample weight is not enough for
Figure 3 shows the effect of
collecting and analysis. In addition, as
different concentrations of HCl on the
Figure 2 shows, at low (1 %wt) and
morphology of the samples. At low
high (3 %wt) concentrations, the sample
concentration, palladium chloride does
does not have uniform morphology. At
not have enough solubility. At 0.1 M
high concentration (3 %wt), there are
HCl, the sample includes uniform
two reasons for decreasing of sample
nanoclusters
homogeneity.
nanoseeds.
made
from
uniform
At higher concentrations,
Firstly, the ratio of salt to PVA is
the hydroxyl groups in PVA chains are
high, causing the gel network to be too
protonated. The protonated PVA cannot
weak to control the pyrrolysis and
make a polymeric gel to control the
calcination processes. Secondly, the
synthesis process.
Page | 143
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
Figure 2. Effect of palladium chloride weight percentage (%wt) on the morphology and the particles sizes of the samples. The SEM images of samples with different palladium salt percentages of 1 %wt (a), 2 %wt (b) and 3 %wt (c).
Page | 143
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
Figure 3. SEM images of different samples synthesized at different concentrations of hydrochloric acid; 0.05 M (a), 0.1 M (b), 0.5 M (c), 1 M (d) and 2 M (e). All SEM images are in same magnifications of 15000.
In the next step, the effect of
compositions of mixed solvents. The
ethanol/HCl solution ratio in a mixed
effect of solvent composition was
solvent
shown in Figure 4 using SEM images.
was
investigated.
Several
samples were synthesized at different
Page | 144
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
Figure 4. Effect of solvent composition on the SEM images of nanocomposite samples synthesized at 60:40 ethanol: HCl solution (a), 70:30 ethanol: HCl solution (b) and 80:20 ethanol: HCl solution (c) as solvent in the synthesis process.
The experimental results showed
solvent with 60 %V/V ethanol and 40
that in ethanol content lower than 60
%V/V HCl solution is a suitable solvent
%V/V, PVA has no solubility. At
to synthesize uniform nanoclusters.
higher
Effect of pyrrolysis temperature
values,
palladium
chloride
cannot completely dissolve. Based on
In the present method, during the
the SEM images of Figure 4, the mixed
pyrrolysis
Page | 142
(combustion)
process,
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
organic gel network is burnt and the
5a and 5b), the clusters are compact and
trapped palladium chloride molecules in
they don't have enough porosity. At
holes in the polymeric network are
these temperatures, calcination and
calcinated
and
into
particle growth rates are low because
palladium,
palladium
or
the combustion rate is low resulting in
oxide
smaller particles of the samples. But, at
chemical
lower temperature, the samples don’t
converted oxide
palladium-palladium nanocomposites.
The
reactions of the combustion step are as
have uniform nanoclusters.
follows:
At higher temperatures (550 and 600 °C; Figs. 5d and 5e), it can be seen that
Heat
(eq. 1)
PdCl2 Pd Cl 2
the samples consist large crystals. The
Heat
(eq. 2) 2 PdCl2 O2 2 PdO 2Cl 2
presence of large crystals can be related
(eq.
to this fact that the combustion,
3)
(m n) PdCl 2
1 O2 mPd .nPdO 2Cl 2 2 Heat
The pyrrolysis temperature controls the share of each reaction (eqs. 1 to 3) in the final product. In equation 3, m and n can be varied as the synthesis
the
temperature
effect was
high. Therefore, the samples that are synthesized
in
these
temperatures
include agglomerated particles and large crystals. At 500 °C, combustion, calcination, nucleation and particle growth rates are suitable to form
temperature is changed. First,
calcination and particle growth rates are
of
studied
pyrrolysis on
the
morphology of samples (Figure 5). As it can be seen in Figure 5, the morphology of the samples is changed considerably when the pyrrolysis temperature is increased from 400 to 600 °C. At lower
uniform nanoclusters (Figure 5c). Gel network is a control cage for palladium chloride molecules. When a gel is burned, burning rate is a limiting factor for the calcination rate of the salts and also a controller factor to reduce nucleation and particle growth [33].
temperatures (400 and 450 °C; Figures
Page | 143
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
Figure 5. SEM images of the samples synthesized at different paralysis temperatures, including 400 °C (a), 450 °C (b), 500 °C (c), 550 °C (d) and 600 °C (e).
Figure 6 shows the TEM images of the sample synthesized at 500 °C. As it can be seen in Figure 6, the sample includes uniform and spherical nanoparticles with an average diameter of 25 nm.
Figure 6. TEM images of Pd/PdO nanocomposite sample synthesized in the optimized conditions in two magnifications 20000 (left) and 70000 (right).
Page | 142
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
In the next step, the effect of
synthesized at different temperatures.
pyrrolysis temperature was studied on
As Figure 8 shows, Pd content of the
the sample composition. Seven samples
samples decreases when the pyrrolysis
were
different
temperature is increased. At 350 °C, Pd
temperatures under free atmosphere.
content of the sample is more than 98
The composition of the samples was
%wt and, at 700 °C, the Pd content is
characterized by XRD patterns. Figure
lower than 2 %wt. Based on the
7 shows the XRD patterns of three
obtained results, the presented method
samples (as some examples) that were
is a powerful technique to synthesize Pd
synthesized at 400, 500 and 600 °C. As
nanoparticles, PdO nanoparticles and
it can be seen in Figure 7, Pd content of
Pd-PdO nanocomposites. By comparing
the
the
the morphology, particles sizes and the
pyrrolysis temperature is increased. The
composition of the samples that are
reason of this fact is clear. Under free
synthesized
atmosphere conditions, Pd reacts with
temperatures, it can be concluded that
oxygen to produce PdO. At lower
the introduced method is more suitable
temperatures, the oxidation reaction is
for
negligible. On the other hand, PVA can
nanocomposites.
synthesized
sample
at
decreases
when
act as a reducing agent to convert Pd
2+
0
into Pd .
different
preparation
of
pyrrolysis
Pd-PdO
In the optimum conditions, the Pd/PdO nanocomposite sample includes
Figure 8 was drawn based on the XRD
at
patterns
of
the
samples
uniform nanoclusters with uniform nanoseeds.
Page | 142
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
Figure 7. XRD patterns for the three Pd/PdO nanocomposite samples pyrrolized at different temperatures of 400, 500 and 600 ºC
Brunauer- Emmet- Teller (BET)
Application as catalyst
specific surface area measurements
The catalytic performance of the
were used for the determination of the
palladium-palladium
pore volume, the specific surface area
nanocomposite (50 %wt Pd and 50 %wt
and the nanoparticles sizes. Based on
PdO)
the BET and BJH plots which were
Voltammetry
obtained
gas
hydrogen in acidic solution. For this
adsorption/desorption isotherms, the
purpose, commercial palladium powder
based 2
on -1
sample has 10 m .g
N2
specific surface
area and 19.5 nm average diameter.
pore
was
studied for
oxide
by
Cyclic
revolution
of
and the prepared palladium-palladium oxide nanocomposite were filled in the porosities of the graphite electrodes.
Page | 143
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
The obtained electrodes were used as
palladium-palladium
working
cyclic
nanocomposite were compared with
The
those of graphite and commercial
electrodes
Voltammetry
in
experiments.
obtained cyclic voltammetric results for
oxide
palldium.
Figure 8. Effect of pyrrolysis temperature on the sample composition. Any of the samples does not have any impurity.
Figure
9
shows
cyclic
obtained
results
for
the
catalytic
voltammograms of hydrogen reduction
performance of Pd/PdO nanocomposite,
in sulfuric acid solution (1 M) on the
the prepared nanocomposite can be
different substrates (pure
used as catalyst in fuel cells to facilitate
graphite,
commercial palladium intercalated in
and
graphite porosities and the synthesized
hydrogen reactions
palladium-palladium
increase
current
densities
of
oxide
An interesting phenomenon is
nanocomposite intercalated in graphite
considered on CV diagram taken by
porosities). As it can be seen in Figure
palladium-palladium
9,
oxide
nanocomposite substrate; an oxidation
reduces
peak was observed at 0.14 V versus
palladium-palladium
nanocomposite
not
only
oxide
reduction potential of hydrogen ions
Ag/AgCl
reference
(positive shift), but also increases
Commercial
palladium
hydrogen revolution rate with respect to
graphite substrates did not show this
commercial palladium. Based on the
peak. The observed peak is related to
Page | 143
electrode. and
also
Synthesis of nanostructured palladium, palladium oxide and palladium-palladium …
oxidation of the adsorbed hydrogen gas.
At oxidation sweep (from -0.3 to 0.3
This peak reveals that palladium-
V), the adsorbed hydrogen gas is
palladium oxide nanocomposite has
oxidized to hydrogen ion. There is not
more capacity to adsorb hydrogen gas.
any adsorption of realized hydrogen on
Hydrogen ions are reduced during
the surface of graphite and commercial
potential sweeping from 0.0 to -0.85 V.
palladium or it is very weak. The strong
The realized hydrogen was strongly
adsorption of hydrogen gas on the
adsorbed by palladium-palladium oxide
surface of the nanocomposite helps to
nanocomposite.
improve its catalytic performance.
Figure 9. Cyclic voltammograms for hydrogen revolution in 1 M sulfuric acid on the surface of palladium-palladium nanocomposite (a), commercial palladium power and pure graphite (c). All potential were recorded versus Ag/AgCl reference electrode. Commercial palladium and palladium-palladium nanocomposite powders were intercalated (filled) on the graphite pores. The filled graphite electrodes were used as working electrodes.
Conclusion
synthesized with the more specific
A gel combustion method based on
surface area. They were expected to
PVA as a gel-masking agent can be
have some improved ability in gas
used
sensing and photocatalytic applications.
as
synthesize
a
powerful PdO,
Pd
method and
to their
nanocomposites. In this procedure, Pd,
Palladium
nanoparticles
can
be
successfully used as nanocatalyst to
PdO and their nanocomposites were
Page | 143
H. Karami et al. / Iranian Chemical Communication 4 (2016) …-…
catalyze hydrogen generation in acid
[9] A. Kolmakov, D.O. Klenov, Y.
media.
Lilach, S. Stemmer, M. Moskovits,
Acknowledgments
Nano Lett. 2005, 5, 667-673.
We gratefully acknowledge the support
[10] J. Chen, B. Wiley, J. McLellan, Y.
of this research by Payame Noor
Xiong, Z. Li, Y. Xia, Nano Lett. 2005,
University Research Council.
5, 2058-2062.
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