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E.V. Ramos-Fernandez et al. / Applied Catalysis A: General 374 (2010) 221–227
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Many studies have been developed during the last years on the
preparation and characterization of Pt based catalysts for the
selective hydrogenation of unsaturated aldehydes. Ru has been
also studied as the active phase, but to a lesser extent, and some
questions remain unsolved. Thus, the use of different precursors
(chlorinated or chlorine-free) and its effect on the catalytic
The extent of hydrogen spillover was studied by H2-TPD, and
the analysis of the products was carried out by on-line mass
spectrometry. The samples were reduced (623 K, 1 h) in flowing
a,b
hydrogen (50 cm3 minꢁ1
) and then cooled down to room
temperature in the same atmosphere. Then, H2 was replaced by
He and, when the H2 signal at the mass spectrometer was
equilibrated, a temperature-programmed desorption experiment
was carried out (heating rate, 10 K minꢁ1).
performance in the selective hydrogenation of
aldehydes have not been studied.
a,b unsaturated
The presence of chlorine in the catalysts can strongly modify
their catalytic properties. Systematic comparisons have been
carried out between Cl-containing and Cl-free metal catalysts, and
the effect of the reduction temperature at which the catalysts have
been submitted and of the nature of the metal precursors have
been shown. Residual chlorine, which remains in the catalysts after
the reduction pre-treatment, can have several effects on the
properties and performance of the catalysts, one of them being the
decrease of the metal dispersion [31]. The formation of alloy
phases between the active metal and the partially reduced support
can also be affected by the presence of chlorine. Additionally, the
product distribution can change depending on amount of residual
chlorine. The effect of chlorine in chemoselective hydrogenations
is complex. The selectivity of crotonaldehyde hydrogenation into
crotyl alcohol has been shown to be affected in a detrimental way
on Pt/CeO2 [27,32] and Pt/SnO2 [22], whereas the effect is
beneficial on Pt/ZnO [17] and Pt/TiO2 [9].
X-ray photoelectron spectra (XPS) were acquired with a VG-
Microtech Multilab 3000 spectrometer equipped with a hemi-
spherical electron analyzer and
a Mg Ka (h = 1253.6 eV,
1 eV = 1.6302 ꢂ 10ꢁ19 J) 300-W X-ray source. The powder sample
was pressed into small Inox cylinders and then mounted on a
sample rod placed in a pre-treatment chamber and reduced in
flowing H2 for 1 h at 473 and 623 K before being transferred to the
analysis chamber. Before recording the spectra, the sample was
maintained in the analysis chamber until a residual pressure of ca.
5 ꢂ 10ꢁ7 N mꢁ2 was reached. The spectra were collected at pass
energy of 50 eV. The intensities were estimated by calculating the
integral of each peak, after subtraction of the S-shaped back-
ground, and by fitting the experimental curve to a combination of
Lorentzian (30%) and Gaussian (70%) lines. All binding energies
(BE) were referenced to the C 1s line at 284.6 eV, which provided
binding energy values with an accuracy of ꢃ0.2 eV. The surface
Ru/Zn and Cl/Zn atomic ratios were estimated from the integrated
intensities corrected by the atomic sensitivity factors [33].
The effects of the metal precursor in Ru/ZnO catalysts have been
studied in this work. Two catalysts have been prepared, one of
them using a chlorine-containing precursor and the other one with
a chlorine-free precursor. The effect of the reduction temperature
has been also studied.
The catalytic behaviour of the samples in the vapour-phase
hydrogenation of crotonaldehyde (2-butenal) was tested in a U-
shaped quartz micro-flow reactor at atmospheric pressure under
differential conditions. Before each reaction run, the catalyst
(around 0.1 g) was reduced in situ at 473 and at 623 K under
flowing hydrogen (50 cm3 minꢁ1) for 1 h and then cooled under
hydrogen to the reaction temperature, 353 K. Then, it was
contacted with a reaction mixture (total flow, 50 cm3 minꢁ1; H2/
CROALD ratio, 26) containing purified hydrogen and crotonalde-
hyde (Fluka, 99.5%) prepared by passing a hydrogen flow through a
thermostabilized saturator (293 K) containing the unsaturated
aldehyde. The concentration of the reactants and products at the
outlet of the reactor was determined by on line gas chromatogra-
phy with a Carbowax 20M 58/90 semicapillary column.
2. Experimental
The ZnO support was prepared by a homogeneous precipita-
tion method. An aqueous solution (pH 9) of Zn(NO3)2ꢀ6H2O and
CO(NH2)2 was gently heated at 370 K, and it was kept under
stirring for 24 h. Then, the precipitate formed was filtered,
washed three times with ultra-pure water, dried at 383 K
overnight and calcined in air at 773 K for 2 h. Two Ru/ZnO
catalysts were prepared using different Ru precursors. Thus,
Ru(Cl)/ZnO was prepared by impregnation of the ZnO support
with and aqueous solution of RuCl3ꢀxH2O, whereas Ru(A)/ZnO
was prepared by impregnation with an acetone solution of
Ru(C5H7O2)3. In both cases, the amount of the ruthenium
precursor was the appropriate to obtain a 1 wt.% Ru loading.
After 24 h under stirring at room temperature, the excess of
solvent was removed by gently heating. Then, the samples were
calcined in air at 673 K for 2 h.
3. Results and discussion
3.1. Support characterization
Fig. 1 shows the XRD pattern of the ZnO support. All the
diffraction peaks in the pattern can be assigned to the hexagonal
˚
(wurtzite-type) ZnO structure, with lattice constants a = 3.25 A
˚
The X-ray diffraction pattern of the support was obtained with a
JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode
and a Ni filter, and using a 28 minꢁ1 scanning rate.
Nitrogen adsorption isotherms were obtained at 77 K in a
Coulter Omnisorb-610 equipment. Before the experiment, the
samples were out-gassed at 523 K for 4 h under vacuum
(10ꢁ6 kPa). The specific surface area (SBET) was obtained using
the BET method.
and c = 5.20 A. Fig. 1 also reports the crystallographic planes which
produce each diffraction peak. The comparison of the observed and
standard intensities [34] of the diffraction peaks allows to conclude
that there are not preferred orientations. It can be seen that the
Temperature-programmed reduction (TPR) measurements
were carried out in a U-shaped quartz reactor, using a 5%H2/He
gas flow of 50 cm3 minꢁ1 and about 100 mg of catalyst. Hydrogen
consumption was monitored by online mass spectrometry.
Conventional TEM analysis was carried out with a JOEL model
JEM-210 electron microscope working at 200 kV and equipped
with a INCA Energy TEM 100 analytical system and a SIS MegaView
II camera. The samples, previously reduced at 623 K, were
suspended in methanol and placed on copper grids with a
holey-carbon film support.
Fig. 1. XRD patterns of support.