Formation of palladium nanoparticles
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 4, April, 2006
651
alumina (specific BET surface area 155 m2 g–1), and silica (speꢀ
cific BET surface area 550 m2 g–1) (Aldrich) were used. A soluꢀ
tion of the PdII tetraaquacomplex was prepared according to a
previously described procedure.28 The source of FeIII aqua ions
was a solution of the Fe2(SO4)3•9H2O salt in perchloric acid.
The concentration of palladium(II) was determined spectroꢀ
photometrically in a greenꢀcolored solution of the complex,
which was obtained by the addition of excess tin(II) chloride to
the analyzed sample.29 The concentration of iron(III) as a comꢀ
plex with sulfosalicylic acid was determined spectrophotoꢀ
metrically.30
TEM image. Three conforming diagrams (at least with two hunꢀ
dreds of nanoparticles) were analyzed to attain good statistics.
Xꢀray diffraction (XRD) studies were carried out with a
Bruker D5000 instrument equipped with a rotating anode.
The specific surface area of samples was determined by the
Brunauer—Emmett—Teller (BET) method with a Micromeritics
Gemenie surface analyzer.
Results and Discussion
It has been reported previously6,32,33 that lower olefins
are rapidly oxidized with the PdII tetraaquacomplex to
form the corresponding carbonyl compounds and pallaꢀ
dium black. The introduction of FeIII, regardless of its
excess over PdII, does not result in the catalytic reaction
with respect to palladium. However, in the present work
we found the catalytic oxidation of ethylene, propene,
and butꢀ1ꢀene in the presence of the PdII tetraaquaꢀ
complex and FeIII aqua ions in an aqueous medium to the
corresponding carbonyl compounds.
On contact of ethylene, propene, or butꢀ1ꢀene with
the catalytic solution, palladium black precipitates rapꢀ
idly within several minutes. At the same time, FeIII is
quantitatively reduced to FeII, and the corresponding carꢀ
bonyl compound (acetaldehyde, acetone, or methyl ethyl
ketone) is formed according to the reaction
The preparation of the Pdꢀsupported metallic catalyst
Pd/ZrO2/SO4 was based on the reduction of the PdII tetraꢀ
aquacomplex with the FeII aquacomplex.31
.
(1)
The calculated amount of sulfated zirconium oxide
ZrO2/SO4 (0.45 g) was introduced into a temperatureꢀcontrolled
reactor, and a solution (25 mL) of the PdII tetraaquacomplex
(concentration 2 mmol L–1) in 0.5 M perchloric acid was added.
Then the calculated amount of Mohr´s salt (0.2 g) correspondꢀ
ing to an iron(II) concentration in the resulting solution of
0.02 mol L–1 was added. The mixture was vigorously stirred
magnetically for 1 h under argon at 60 °C. Then the solution was
filtered, and the precipitate was multiply washed with water and
dried above P2O5. The prepared Pd/ZrO2/SO4 catalyst (dark
gray powder) contained 1 wt.% Pd. A similar procedure was
used for the preparation of Pd/Al2O3 and Pd/SiO2.
The kinetic experiment was carried out as follows. A weighed
sample of Pd/ZrO2/SO4 (40 mg) or a solution of the PdII
tetraaquacomplex was added to a solution of iron(III) in perꢀ
chloric acid with a specified concentration. In all experiments,
the volume of the reaction solution was 10 mL. The reaction was
carried out in an volumetric setup in a temperatureꢀcontrolled
shaken reactor of the "catalytic duck" type in the 40—80 °C
temperature interval, and the amount of consumed olefin was
directly determined. The frequency of shaking, above which the
reaction rate remained unchanged, i.e., when the kinetic regime
was established, was determined in preliminary experiments.
Testing reactions were carried out for comparison, where a magꢀ
netic stirrer with a rotation rate of 1200 ppm was used as a
stirring device, and adequate values of the reaction rate were
obtained.
The products of olefin oxidation were extracted from aqueꢀ
ous solutions with chlorobenzene and analyzed by chromatoꢀ
graphy coupled with mass spectrometry (HP 5890 GCD, colꢀ
umn HPꢀ5 Crosslinked 5% PH Siloxane, column length 30 m).
The concentration of a carbonyl compound was determined by
the gravimetric method by measuring the amount of hydrazone,
which is formed by the interaction with a solution of 2,4ꢀdiꢀ
nitrophenylhydrazine.
,
(2)
R = H, Me, Et.
The material balance of reaction (2) is presented in
Table 1. The data in Table 1 show that the amount of the
formed carbonyl compound exceeds the initial concenꢀ
tration of PdII by several times. This indicates the cataꢀ
lytic character of reaction (2) in which the turnover numꢀ
ber reached 250 under certain conditions.
The conversion of FeIII in reaction (2) is close to 100%.
As can be seen from the data in Table 1, the change in the
initial palladium concentration ([Pd2+aq]0) by more than
order of magnitude changes insignificantly the reaction
rate and depth. It was found that the solution retains
catalytic activity after palladium black was separated. It is
most likely that during the reaction the palladium parꢀ
ticles responsible for catalysis are reversibly transferred to
solution from the palladium black surface under the acꢀ
tion of the medium.
Studies by transmission electron microscopy (TEM) were
carried out with a JEOL 1200 EXII microscope (100 kV). The
solvent was removed in vacuo from 1 mL of the analyzed soluꢀ
tion (sulfuric acid was used instead of perchloric acid), and
ethanol (5 mL) was added. The resulting solution was ultraꢀ
sonicated, filtered, and supported on copper. The particle size
distribution was determined using an amplified (by 100 times)
It can be assumed that the concentration of these parꢀ
ticles in solution is characterized by the limiting value in
all cases.