Mendeleev
Communications
Mendeleev Commun., 2007, 17, 268–270
An XPS study of the synergetic effect of gold and nickel
supported on SiO2 in the catalytic isomerization of allylbenzene
Alexander Yu. Vasil’kov,*a Sergey A. Nikolaev,b Vladimir V. Smirnov,b
Alexander V. Naumkin,a Ilija O. Volkova and Vladislav L. Podshibikhina
a
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 495 135 5080; e-mail: alexvas@ineos.ac.ru
b Department of Chemistry, M. V. Lomonosov Moscow State University, 119992 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2007.09.006
A synergetic effect and catalytic activity in allylbenzene isomerization have been found for the Au–Ni/SiO2 system prepared by
metal-vapour synthesis.
The high catalytic activity and selectivity of gold nanoparticles
are attractive.1 The metal-vapour synthesis (MVS) is widely
used for the preparation of colloidal solutions,2 polymers3
and inorganic nanomaterials4 containing Au nanoparticles. For
example,5 gold-containing nanoparticles obtained by MVS and
supported on SiO2 and γ-Al2O3 manifested a high catalytic
activity in CCl4 addition to multiple bonds.
The aim of this work was to study the catalytic activity of a
gold-containing nanocomposite towards the allylbenzene iso-
merization to trans- and cis-methylstyrene.
The dispersion of supported particles was estimated by X-ray
analysis using an approach described elsewhere.7 All the pre-
paration steps were done under argon purified by the usual
Schlenk technique.
The XPS spectra were recorded on a Kratos XSAM-800
spectrometer using MgKα radiation (90 W) at about 10–8 Torr.
Gold and nickel foils used as reference samples were cleaned
by Ar+ ion bombardment (partial pressure of 5×10–5 Torr) at a
kinetic energy of 2 keV at an incidence angle of 45° in an
ultrahigh vacuum (below 10–8 Torr). The XPS spectra were
background subtracted (assuming linear and Shirley background
due to the secondary electrons for the insulating catalysts and
metal foils, respectively) and fitted with Gaussian line profiles.
The Si 2p (103.9 eV) peak for the SiO2 substrate was used
for the charge compensation and calibration of binding energies.
The quantitative analysis was based on the atomic sensitivity
factors.8 The samples were placed into the spectrometer onto a Ti
sample holder and catalytical reactor in atmospheric environment.
The catalytic activity towards the allylbenzene isomerization
A (mole of product mole of metal–1 h–1) was measured by usual
experiments: 0.1 g catalyst at metal content in the sample of
10–5–10–6 mol, 7.5×10–4 mol of allylbenzene, 170 °C. The
as-measured reaction rate was normalised to the metal content
at a surface of gold nanoparticles, which was estimated using
relations obtained in hemisphere approximation of particle
shape.9,10
H
H
H
H
H
H
Me
H
∆
+
CH2
Me
Au or Ni/SiO2
The catalysis of the process with gold-containing nanocom-
posites is characterised by a strong size effect and synergism of
the catalytic activity in the case of systems containining gold
and nickel simultaneously.
For the catalyst preparation, metals of 99.8–99.9% purity
were used. A tungsten rod with a diameter of 1.5–2 mm
(99.98% purity) was used as a metal evaporator. Toluene,
triethylamine and allylbenzene (Aldrich) were dried over CaH2
or Na and used as-dephlegmated. Fine silica powder (Aldrich,
500 m2 g–1) was used as a substrate for supporting Au-con-
taining nanoparticles. Before deposition, the substrates were
activated at 10–2 Torr and 300 °C for 6 h. All organic reactants
were outgassed by several consecutive freeze–thaw cycles at
at least 10–2 Torr just before their utilization in the MVS.
Vapours of one or two metals were deposited on a reactor
surface cooled with liquid nitrogen simultaneously with Et3N or
PhMe at 10–4 Torr. At the end of the synthesis, the cooling was
stopped, the reactor was filled with argon and the suspension of
the co-condensate was pressed out from the reactor into a
Schlenk bulb containing SiO2 under vacuum. The excessive
suspension was removed, and the as-prepared composite was
dried at 10–2 Torr and 100 °C. Supported systems were prepared
by the impregnation of the activated substrate with mono- or
bimetallic organosols, which were prepared by MVS using a
stationary set-up with a 5 dm3 glass reactor.6 The metal content
of the samples was determined by atomic adsorption spectro-
metry (AAS) on a Hitachi 180-80 instrument. The metal was
washed off the support with a solution of HCl–HNO3 (4:1). The
sensitivity of this method was 5×10–6–5×10–5 g dm–3.
Catalytic experiments were performed in glass ampoules
under vacuum conditions with intense agitation, under conditions
of the kinetic control of the reaction process. The products were
analysed by gas chromatography and chromatography-mass-
spectrometry.
PhMe and Et3N were used in MVS for the preparation of
Au, Fe, Co and Ni organosoles.5–7 The utilization of Et3N is
preferable for the preparation of the supported Au system since
a particle size distribution with a maximum at about 3 nm can be
obtained in this case. The utilization of an Au–PhMe suspension
gives rise to the formation of larger particles of size 20–40 nm.
For the Ni–Et3N system, a broad particle size distribution from
15 to 95 nm with a maximum at 50 nm was measured.
The supported systems based on gold sols in toluene containing
particles with the average size d(Aun)av > 30 nm are inactive in
allylbenzene isomerization. A decrease in the gold particle size
down to 2–3 nm causes a sharp increase in the catalytic activity.
Data on the catalytic activity of nanocomposites 1–5, their
compositions and properties are presented in Table 1.
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