M. Estrada et al. / Applied Catalysis A: General 473 (2014) 96–103
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the further analysis of the support effect on the transformation of
alcohols over gold catalysts prepared using the commercial MgO
acidic conditions [21]. Usually the hydrolysis of MgO occurs dur-
ing the gold deposition by DP technique from aqueous solutions
[22]. However, thermal treatment of the material at 500 ◦C per-
mits to decompose Mg(OH)2 and re-form MgO [23]. The aim of the
present work was to study the effect of the support transforma-
tion from Mg(OH)2 to MgO on the state of gold and its activity in
the oxidation of benzyl alcohol with molecular oxygen in methanol
solutions. We have also studied the interaction of key components
of the reaction media with the Au/MgO and Au/Mg(OH)2 catalysts
by IR spectroscopy.
particles, more than 200 particles were chosen. The mean diameter
(dm) of particles was calculated using the following formula:
ꢀ
i(xidi)
ꢀ
dm
=
,
ixi
where xi is the number of particles with diameter di.
A spectrometer Nexus-760 with a DTGS detector and in situ
cell from Harrick was used in the diffuse reflectance Fourier trans-
form (DRIFT) mode for analysis of benzyl alcohol and benzaldehyde
adsorbed at 30 ◦C. Before the benzyl alcohol or benzaldehyde
adsorption the samples were heated in situ in Ar (a flow rate of
20 mL/min, ultra high purity grade from Infra) with a temperature
increase up to 350 ◦C at a ramp rate of 20 ◦C/min. Then temperature
was decreased to 30 ◦C. The spectra of the samples before the ben-
zyl alcohol or benzaldehyde adsorption were recorded using the
spectrum of mirror as a reference with a resolution of 4 cm−1. The
adsorption was carried out until the sample saturation in a flowing
mixture of benzyl alcohol (0.01 vol.%) or benzaldehyde (0.16 vol.%)
and Ar (a total flow rate of 20 mL/min) for about 1 h. After the sta-
bilization of FTIR spectra the samples were flushed by Ar at 30 ◦C
for 30 min to remove the gas phase or weakly adsorbed benzyl
alcohol or bezaldehyde. In order to study a thermal stability of the
adsorbed benzyl alcohol and benzaldehyde the catalyst sample was
heated from 30 ◦C up to 90 ◦C at a ramp rate of 5 ◦C/min in the
Ar flow (20 mL/min). Before the entrance to the FTIR cell, Ar was
dried by passing through a trap cooled at −20 ◦C by the mixture
of n-isopropanol and liquid nitrogen. During the adsorption and
thermal desorption of benzyl alcohol or benzaldehyde the spectra
were recorded in the Kubelka-Munk unites (32 scans) using a spec-
trum of the catalyst sample before adsorption as a reference. The
temperature of the sample within a DRIFT cell during the thermal
desorption was precisely measured using a thin thermocouple with
a fast response. In preliminary experiments, no significant temper-
ature gradient through a catalyst bed was found at the same ramp
rate of 5 ◦C/min.
2. Experimental
Gold (2.6 wt.%) was supported by deposition–precipitation
using HAuCl4 as a gold precursor and urea as a precipitation agent
as in [24]. MgO (4.0 g, Mallinckrodt) was added to an aqueous solu-
tion (400 mL) of HAuCl4 (1.6 × 10−3 M) and urea (0.42 M). The initial
pH of the solution was ca. 2. The suspension was vigorously stirred
at 80 ◦C for 4 h. Then, the solid material was filtered and washed
with ammonium hydroxide (25.0 M) for 30 min. The last procedure
developed in [25] is quite effective to stabilize small gold NPs. After
stirring with ammonium hydroxide, the pH of the solution was ca.
10. Finally, the sample was washed with water until the pH of the
solution reached the value of 7, then filtered, and dried at room tem-
perature for 24 h. The freshly prepared sample denoted as Au/Mg-F
was used to obtain gold supported on Mg(OH)2 via the reduction in
the mixture of H2 (5 vol.%) in He (a total flow rate of 50 mL/min) at
a heating rate of 20 ◦C/min up to 350 ◦C. The obtained sample was
denoted as Au/Mg-350. For the preparation of gold supported on
MgO, the Au/Mg-F sample was reduced in the same gas mixture at
500 ◦C at the same heating rate and denoted as Au/Mg-500.
2.3. Catalytic oxidation experiments
The reactions were carried out in a stainless steel reactor
equipped with a magnetic stirrer. In a typical run, a mixture of ben-
zyl alcohol (2.5 mmol), methanol (2 mL), and the catalyst (10 mg;
1.3 mol of Au: 0.05 mol%) was transferred in the reactor. The reac-
tor was pressurized with oxygen to the total pressure of 10 atm
and placed in an oil bath; then, the solution was intensively stirred
at 110 ◦C for the reported time. The reactions were followed by
gas chromatography (GC) (Shimadzu 17 instrument, Carbowax
20M capillary column). At appropriate time intervals, stirring was
stopped and after catalyst settling aliquots were taken and analyzed
by GC. The structures of the products were confirmed by GC/MS
(Shimadzu QP2010-PLUS instrument, 70 eV).
2.2. Catalyst characterization
The specific surface area and pore size distribution were deter-
mined according to the BET method by nitrogen adsorption
measurements in a Tri-Star III device. Before analysis the samples
were treated in a vacuum (10−3 torr) at 300 ◦C for 4 h.
X-ray diffraction (XRD) analysis was carried out with a Philips
X’pert diffractometer equipped with a curved graphite monochro-
mator applying a CuK␣ (ꢀ = 0.154 nm) radiation.
Photoelectron spectroscopy (XPS) data were obtained by a
Kratos AXIS 165 spectrometer using a monochromatic AlK␣ radia-
tion (hꢁ = 1486.58 eV) and fixed analyzer pass energy of 20 eV. All
measured binding energies (BE) were referred to C 1s line of adven-
titious carbon at 284.8 eV. The deconvolution of spectra was carried
out with a background estimation using the Shirley software.
Gold content (2.6 wt.%) was determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) on a Varian Liberty
110 instrument. Solid samples pretreated at 350 ◦C in Ar flow for
30 min were first digested at room temperature in HF for 12 h and
then in a mixture of HCl and HNO3 for 30 min. The solution obtained
was diluted with deionized water and analyzed.
Au/Mg-F sample and Mg(OH)2 reference sample are practically
equal (Fig. 1). Thus, during the gold deposition by the DP proce-
dure the starting MgO was completely transformed into Mg(OH)2
via the interaction with water [21]. The process seemed to be accel-
erated by the presence of the strong HAuCl4 acid at the beginning
of the DP procedure. The thermal treatment of the Au/Mg-F sam-
ple at 350 ◦C in a hydrogen flow did not lead to the decomposition
of Mg(OH)2 into MgO. It should be noted that no reflections corre-
sponding to Au metal nanoparticles (Au NPs) have been detected in
the XRD pattern of both the Au/Mg-F and Au/Mg-350 samples. This
High-resolution transmission electron microscopy (HRTEM)
was performed with a JEOL 2010 microscope. The sample was dis-
persed by ultrasonic in isopropanol and supported on a copper grid
covered with a carbon film. To determine the mean diameter of gold