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76
Chemistry Letters Vol.38, No.2 (2009)
Preparation of Ultrathin Palladium Membrane Using Electrophoresis
of Metallic Nanoparticles
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2
Ã1
Aki Tominaga, Osamu Nakagoe, and Shuji Tanabe
Graduate School of Science and Technology, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521
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2
Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University,
1-14 Bunkyo-machi, Nagasaki 852-8521
(Received October 14, 2008; CL-080981; E-mail: s-tanabe@nagasaki-u.ac.jp)
A new procedure to prepare ultrathin membrane composed
of nanoparticles was investigated. The nanoparticles were pre-
pared in aqueous solution containing Pd ions by sonochemical
_
+
II
reduction. Electrophoresis was used to fix the nanoparticles on a
porous substrate disc. The ultrathin membrane was successfully
prepared with the combination procedure, and the obtained Pd
membrane showed high perm-selectivity for H2 and 3.85 of
the separation factor of H2/N2 at room temperature.
H
2
SO
4
cathode
anode
To supply large amount of hydrogen, production and purifi-
cation are the most significant issues for widespread adoption
fuel cells. Improving the activity of hydrogen perm-selective
porous
substrate
1
Pt
nanoparticle
membrane is crucial for realization of a hydrogen economy. In
general, thinner and denser membranes show high performance
for hydrogen permeation due to decreasing time through the
membrane and increasing collision frequency between gas and
the walls of micropores in the membrane. Pd and its alloy mem-
branes are commercially used because they have higher perme-
Figure 1. Schematic illustration of the experimental setup for
the electrophoretic deposition.
images the average particle size was calculated to be 4.04 nm,
and very narrow size distribution was also obtained. Using sono-
chemical reduction monodisperse ultrafine Pd nanoparticles
were obtained.
2
ability of hydrogen than that of the other gases. However, they
are expensive and difficult to make thin films less than 2 mm in
thickness by conventional methods.
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,3
The obtained Pd colloidal solution was very stable because
no agglomeration was observed over long storage at room tem-
perature. An electrophoresis method was applied to prepare thin
layer membrane which consisted of Pd nanoparticles. Prior to the
electrophoresis process, the zeta potential of Pd colloid was
measured at room temperature and was À11:0 mV at pH 2.9.
Figure 1 shows schematic representation of how the membrane
was modified. Pd colloidal solution was put in the cathodal side
in the electrophoretic bath because the Pd nanoparticles moved
to the anodic side owing to negative zeta potential when the
DC electric potential was applied to the solution. A commercial-
ly supplied porous alumina disc (prepared by anodic oxidation:
AAO disc) was used as substrate to support Pd membrane. Be-
fore electrophoresis, many micropores on the surface of AAO
disc were observed by field emission SEM observation as shown
in Figure 2a. Those micropores completely disappeared after the
process (Figure 2b), and the white color of the original AAO disc
changed to black, meaning the Pd nanoparticles moved and
In this letter, we introduce a new method for the assembly of
hydrogen perm-selective membrane by electrophoretic bottom-
up assembly of palladium nanoparticles on a porous substrate.
The perm-selectivity of the membrane was evaluated at room
temperature.
Monodisperse Pd nanoparticles were prepared from aqueous
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Pd solution with sonochemcal reduction. Designated amounts
.
of Na2[PdCl4] 3H2O were added to ultrapure water (0.1–
.4 mM) and stirred for a couple of hours. 100 mL of the
0
solution was put in a glass vessel, polyethylene(40)glycol mono-
stearate (PEG40-MS) was added, and the solution was purged
with pure Ar for 20 min. Ultrasonic irradiation was carried out
at room temperature, using a multiwave ultrasonic generator
2
(
200 kHz, 6.0 W/cm ) equipped with a barium titanate oscillator
of 65 mm in diameter. Formation of nanoparticles was moni-
tored during ultrasonic irradiation by a UV–vis spectrophotom-
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eter. The absorption peak around 400 nm due to Pd complex de-
7
creased with increasing irradiation time, while a broad absorp-
tion band corresponding to Pd0 nanoparticles increased. The
color of the solution also changed from yellow to dark gray after
stacked on the surface of the disc.
The gas permeance properties of obtained Pd membranes
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strongly depend on the initial concentration of Pd . The perme-
À4
2
0 min of irradiation. Therefore, it was decided that the reduc-
II
ance of hydrogen (nitrogen) decreased from 1:53 Â 10
À5
À6
À4
tion of Pd had finished at 20 min of irradiation because the
00 nm absorption peak completely disappeared and that no
(6:09 Â 10 ) to 7:03 Â 10 (1:83 Â 10 ) with increasing ini-
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tial Pd concentrations ranging from 0 to 0.4 mM. Especially, in
more change of the broad absorption band occurred after further
irradiation.
The prepared Pd nanoparticles were observed by high-reso-
lution transmission electron microscopy (HR-TEM). From TEM
the case of 1.0 mM, no permeances for hydrogen and nitrogen
were observed. On the contrary, the separation factor increased
from 2.51 to 3.85 with increasing concentrations. The reason
that the properties depended on the Pd concentration was the
Copyright ꢀ 2009 The Chemical Society of Japan
Tominaga, Aki
