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K. Aoki et al. / Electrochimica Acta 52 (2007) 2485–2491
values of the number ratio are 0.48, 0.22, 0.10 and 0.05 for
N = 103, 104, 105 and 106 (corresponding to r = 1.6, 3.4, 7.3,
15.8 nm for the density 10 g cm−3), respectively [7]. This cal-
culation has been demonstrated to be nearly valid for the silver
stearate nanoparticles for r = 2.5 nm. For palladium nanopar-
ticles often investigated, the diameters range 2–8 nm [28–37],
which corresponds to N = 500–2000 and (S/a2)/N = 0.6–0.18.
The non-negligible values of the ratio indicate a significant
contribution of shells. In contrast, a submicron particle, e.g.,
r = 0.1 m, has the ratio, 0.0056, and hence the amount of the
surfactant is as little as impurities. This value seems to be too
small for conventional synthetic conditions of the amount of a
surfactant. Either case may be realized: only the small amount
of stabilizers participates in forming particles; or a particle has
no core-shell structure but takes a mixture. Submicron particles
may be different from conventional nanoparticles in structure.
Our concern here is directed to the structure as well as the
functionality of submicron palladium particles. This report deals
with synthesis of submicron palladium particles stabilized with
palladium acetate, determination of the number ratio of the sta-
bilizer to the metal, and catalytic efficiency for methylene blue.
a balance and a thermo-controlled oven through which nitrogen
3. Results and discussion
3.1. Formation of Pd particles
The mixture of palladium acetate, PVP and hydrazine showed
pale yellow color. The reflux changed the color of the solution
into almost transparent black. The UV spectra (Fig. 1) show not
only consumption of palladium acetate at 400 nm after the reduc-
tion but also replacement of the ultraviolet band by the broad
visible band [38]. The appearance of the visible band supports
formation of an electronic band structure of palladium particles
[39,40]. In contrast, the disappearance of the band at 400 nm is
due to making the spectrum featureless by light scattering. The
reduction of palladium acetate may be obeyed by [41,42]
N2H4·H2O → N2H5+ + OH−
(1)
Pd2+ + 2N2H5+ + 10OH− → Pd + 2N2 + 10H2O + 6e−
(2)
2. Experimental
If k palladium atoms and m palladium acetate molecules form
a particle with the help of PVP, the reaction is given by
2.1. Chemicals
kPd + mPd(CH3COO)2 → Pdk[Pd(CH3COO)2]m
(3)
Palladium acetate (99.9%, Aldrich), poly-(vinyl pyrroli-
done), abbreviated as PVP, with 40,000 molecular weight,
hydrazine (98.0%, Kanto), tetra-n-butylammonium hexafluo-
rophosphate abbreviated as TBAPF6 (TCI), dichloromethane
(99.9%, Wako) were used as received. Acetonitrile (99.5%,
Wako) was treated with molecular sieves, 4A 1/8 (Wako) before
electrochemical measurements. Water was distilled and ion-
exchanged.
Fig. 2 shows the SEM photographs of the palladium parti-
cles before (A) and after (B) the centrifugation and the filtra-
tion through the 0.2 m micropore filter. Two sizes of spheri-
cal particles 0.2 and 2 m in diameter are appreciable before
the treatment (A). The appearance of the uniform diameter,
0.23 0.04 m, after the treatment is obviously due to the
removal of small particles and residues by the centrifugation
and of large particles by the filtration. The size distribution after
the treatment is shown in Fig. 2(C).
The treated particles did not dissolve in aqueous solutions,
acetonitrile or dichloromethane, but were dispersed in a form of
a turbid suspension. The suspension in acetonitrile was stable
for more than one day without sedimentation or aggregation.
The particles were reduced neither with ferrous ion nor ascor-
Palladium particles were synthesized as follows: 100 cm3
ethanol solution including 20 mg palladium acetate was mixed
with 100 cm3 water containing 80 mg PVP, exhibiting yellow
color. Five milligrams of hydrazine was added to the mixture
that was deaerated by N2 for 20 min in advance. The mixture
was refluxed for 5 h, and re-dispersed in a mixture of water and
ethanol (1:1, v/v), water, ethanol, and ethanol in turn. The rinsed
material was centrifuged, re-dispersed in water, centrifuged
again, and re-dispersed in ethanol. The purified suspension in
ethanol was turbid. It was filtrated with microfilter with 0.2 m
pores and then the filtrated solution was black transparent. The
product was dried at room temperature and got in powder form.
−
bic acid. In contrast, they were oxidized with MnO4 in acidic
2.2. Electrochemical and other measurements
The Pt and Pd disks 1.6 mm in diameter (BAS) were used
as working electrodes. The Ag|AgxO was used as a reference
electrode, of which potential was more positive by 0.053 V than
an Ag|AgCl reference electrode in 3 M NaCl.
UV- and vis-spectrometry was carried out with UV-570
(JASCO). Scanning electron microscopy (SEM) was made with
SH-600 (Hitachi) for the palladium Pd particles at 13 kV. The
instrument of thermogravimetry was home made by combining
Fig. 1. Spectra of (a) 0.05 mM Pd(COO(CH3))2 and (b) 0.01 mg cm−3 Pd par-
ticles in dichloromethane.