Different morphologies of silver nanoparticles as catalysts for the
selective oxidation of styrene in the gas phase
R. J. Chimentão,a I. Kirm,a F. Medina,*a X. Rodríguez,a Y. Cesteros,b P. Salagreb and J. E. Sueirasa
a Departament d’Enginyeria Química, Universitat Rovira i Virgili, 43007 Tarragona, Spain.
E-mail: fmedina@etse.urv.es; Fax: (+34) 977559667; Tel: (+34) 977559787
b Departament de Química Inorgánica, Universitat Rovira i Virgili, 43005 Tarragona, Spain
Received (in Cambridge, UK) 16th January 2004, Accepted 11th February 2004
First published as an Advance Article on the web 26th February 2004
Silver nanoparticles of different morphologies were prepared
using the polyol process and then dispersed on a-alumina.
Catalysts were tested for the selective oxidation of styrene in the
gas phase. Activity and selectivity were strongly dependent on
the morphology of the silver nanoparticles.
mass flow controllers and the styrene was introduced into the
reactor by a pump. The products were analysed by a Shimadzu GC
2010 gas chromatograph equipped with a capillary column and FID
detector.
Fig. 1 shows the morphologies of the catalysts observed by SEM
with a JEOL JSM-35C scanning microscope operated at an
acceleration voltage of 15 kV.
Silver catalysts have become increasingly important in the selective
oxidation of olefins for the synthesis of industrially interesting
products such as epoxides and aldehydes.1 Metal nanoparticles
have attracted considerable attention because of their novel
physical properties and their potential applications in areas such as
catalysis.2 Recently, silver nanoparticles have been synthesised by
reducing silver nitrate with ethylene glycol in the presence of
poly(vinylpyrrolidone) (PVP) via a polyol process.3–5 It is well
known that the activity and selectivity of catalyst nanoparticles are
strongly dependent on their size, shape and surface structure, as
well as on their bulk and surface composition.6 The shape-
controlled synthesis of metal nanoparticles can open up a new
world of heterogeneous catalysis. This approach may help to
understand the effect of crystal planes on chemical reactivity.7
Oriented nanoparticles could also be extended to industrial
applications to obtain many useful chemicals. In these regards,
catalysts obtained from silver nanoparticles seem to be particularly
interesting for studying the selective oxidation of olefins with
oxygen as oxidant because it has been demonstrated that silver is a
selective catalyst for olefin epoxidation.8
The morphologies of the silver nanoparticles prepared by the
polyol process were found to depend heavily on the experimental
conditions such as temperature and the molar ratio between PVP
and AgNO3. Previous studies have suggested that the degree of
polymerization of PVP ( the average number of repeating units in
one PVP molecule) also plays an important role in determining the
morphology of the silver nanoparticles.9 We obtained silver
nanowires (Fig. 1a) when the molar ratio of PVP and AgNO3 was
1.5. These nanowires had a mean diameter of 150 nm. This was
consistent with TEM results. When the molar ratio was increased
from 1.5 to 3, nanopolyhedra were the major product (Fig. 1b). The
SEM image of the 40% Ag/MgO catalyst (Fig. 1c) prepared by
impregnation shows the presence of silver nanowires and other
silver particles dominated by irregular shapes with diameters
between 100 nm and 500 nm. Irregularly shaped particles with
diameters between 200 nm and 1000 nm (Fig. 1d) were also
observed for the 15% Ag/a-Al2O3 catalyst prepared by wetness
impregnation using a silver nitrate solution.
The X-ray diffraction of the nanowires and nanopolyhedra
synthesized using the polyol process suggested that silver existed
purely in a face-centered cubic structure (Fig. 2). The X-ray powder
diffraction (XRD) patterns were recorded using a Siemens D5000
diffractometer using nickel filtered Cu Ka radiation (l = 1.54056
Å) in 2q ranging from 30° to 80°.
This study investigates how different morphologies of silver
nanoparticles supported on a-Al2O3 and MgO affect the selective
oxidation of styrene in the gas phase using oxygen as oxidant. The
promotion effect of potassium hydroxide on the catalytic activity
has also been investigated.
The catalysts were prepared by two procedures. In the first, the
wetness impregnation method was used to impregnate a-Al2O3 and
MgO supports with an appropriate amount of an aqueous solution
of silver nitrate to obtain 15 and 40 wt% of silver, respectively. In
the second, silver nanoparticles (11 wt%) were dispersed on a-
Al2O3 with an acetone solution. The silver nanoparticles were
synthesized via a polyol process. In a typical synthesis of silver
nanoparticles, 30 ml ethylene glycol solution of AgNO3 (0.25 M,
Aldrich) and 30 ml ethylene glycol solution of PVP (0.375 M in
repeating unit weight-average molecular weight ≈ 40 000, Al-
drich) were simultaneously added in 50 ml ethylene glycol at 433
K under vigorous stirring. The reaction mixture was then refluxed
for 45 minutes at this temperature. The nanoparticles obtained were
diluted with acetone (about 10 times by volume) and separated
from ethylene glycol by centrifugation at 4000 rpm for 20 minutes.
Silver nanoparticles were also prepared with a PVP/AgNO3 molar
ratio of 3. The catalysts were dried in an oven at 393 K for 24 hours
and reduced in H2 at 623 K for 3 hours before the characterization
and the activity tests.
The diffraction did not suggest the presence of possible
impurities such as Ag2O and AgNO3. The three peaks detected for
the silver nanoparticles were assigned to diffraction from the (111),
(200) and (220) planes of fcc silver, respectively. The lattice
constants calculated by XRD for the nanowires and nanopolyhedra
were 4.0839 and 4.0872 Å, respectively, which are very close to the
report data (a = 4.0862 Å, Joint Committee on Powder Diffraction
Standards file 04–0783). The ratio of intensity between the (111)
The samples were structurally characterized using X-ray diffrac-
tion (XRD), temperature programmed reduction (TPR), scanning
electron microscopy (SEM), transmission electron microscopy
(TEM) and UV–vis absorption spectroscopy. The selective oxida-
tion of styrene in gas phase at 573 K was carried out in a continuous
fixed-bed reactor over 1.0 g catalyst at atmospheric pressure. The
catalytic tests were operated at different styrene : O2 : Ar molar
ratios. The feed gas mixture (O2, Ar) was delivered by means of
Fig. 1 SEM images of silver catalysts.
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Chimentao
