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effect and the improvement in activity/selectivity, for examples,
2. Experimental
in hydrodesulfurization, CO oxidation, and acetylene trimerization
[13–16].
2.1. Catalyst preparation
The partial hydrogenation of unsaturated organic compounds
for the synthesis of fine organic chemicals, e.g., selective hydro-
genation of alkyne to the corresponding alkene is both of industrial
tion are useful in the synthesis of biological active compounds, the
monometallic Pd and bimetallic Pd–Au catalysts has been focused
ensemble by Au atoms has shown to result in the improvement of
hydrogenation activity [26,27]. A number of studies also reported
the modification of activity and selectivity of supported Au–Pd cat-
alysts due to electron transfer between Au and Pd [28–31]. For
example, Chen and Lee [32] suggested that Pd could donate some
electrons to Au, resulting in the hydrogen adsorption ability of Au,
alloying effect appeared to depend on several factors such as the
[24,25,33–35].
Despite the variety of supports being used for preparation of
supported Au-Pd catalysts such as ␥-Al2O3 [26,36], SiO2 [29], SiO2-
Al2O3 [37], CeO2 [38], carbon [39], and TiO2 [4,25,36,40], only a few
studies systematically reported the influence of support nature on
the interaction of Au–Pd species and their corresponding catalytic
behavior. Smolentseva et al. [38] showed that Pd–Au/Al2O3 exerted
than Pd–Au/CeO2 but the Pd–Au/CeO2 manifested higher activity
and selectivity in the selective oxidation of arabinose to arabinonic
with the mutual interaction between these metals. On the other
hand, Kolli et al. [36] reported that the nature of supports between
Al2O3 and TiO2 had negligible effect on the composition of AuPd
metal particles and did not participate in the selective hydrogena-
tion of butadiene. In our recent study [40], the exertion of electronic
modification by Au–Pd alloy was found to depend on the TiO2 crys-
tallite size in which the modification was more pronounced on the
larger TiO2 (15 nm) compared to the smaller one (9 nm), resulting in
higher activity and lower selectivity in the selective hydrogenation
of 1-heptyne.
The monometallic 1 wt.% Pd/TiO2 and bimetallic 1 wt.%
Au-1 wt.% Pd/TiO2 catalysts were prepared by one-step FSP
as described elsewhere [1,4]. Gold (III) chloride trihydrate
(Sigma–Aldrich), palladium (II) acetylacetonate (Aldrich), and tita-
nium (IV) butoxide (Aldrich) were chosen as Au, Pd, and TiO2
precursors, respectively. The liquid precursor solution was pre-
pared by dissolving all precursors into the mixture solution of
xylene/acetonitrile (70/30 vol%) at total concentration of 0.5 M,
then feeding into the flame reactor by using a syringe pump at a
flow rate of 5 mL/min and dispersing with oxygen at a flow rate
of 5 L/min to form the fine spray droplet after that. The pressure
drop at the capillary tip was constantly adjusted at 1.5 bar by tun-
ing the orifice gap area at the nozzle. To ignite the spray flame,
oxygen and methane were provided as the supporting flame feed
gases through a ring around the nozzle outlet at a flow rate of 3
and 1.5 L/min, respectively. The additional sheath oxygen was sup-
plied through a sintered metal plate ring at flow rate of 25 L/min.
A glass fiber filter with the aid of a vacuum pump was used to
collect the product particles formed. The monometallic Pd/TiO2
catalyst and TiO2 support were also prepared by FSP method
under similar conditions. The monometallic Pd/TiO2 and bimetallic
AuPd/TiO2 catalysts were pretreated under hydrogen atmosphere
(50 cm3/min) at 40 and 500 ◦C for 2 h before being tested for
catalytic performance. The monometallic Pd/TiO2 and bimetal-
lic AuPd/TiO2 catalysts reduced at 40 and 500 ◦C are denoted as
Pd/TiO2 R40, Pd/TiO2 R500, AuPd/TiO2 R40, and AuPd/TiO2 R500,
respectively.
2.2. Catalyst characterization
The X-ray diffraction analysis was conducted on XRD D8
Advance of Bruker AXS with Ni-filter CuK␣ (ꢀ = 1.54056 A◦) radia-
tion from 20◦ to 80◦ 2ꢁ and step size of 0.020563 (step time = 88.5 s).
The nitrogen physisorption technique was used to determine the
BET specific surface area, pore volume, and pore diameter by using
a Micromeritics ASAP 2020 automated system. The sample was
degassed at 200 ◦C for 2 h (heating rate of 2.0 ◦C /min) under vac-
uum prior to N2 adsorption analysis, which was carried out at
liquid nitrogen temperature (−196 ◦C). XPS was performed using
a Kratos Ultra DLD X-ray photoelectron spectrometer. The in situ
high-energy XPS analysis was performed under reducing condi-
tion to ensure that the catalysts are in the active form. The C1s
peak was used as reference at binding energy of 285.0 eV to cal-
ibrate for all XPS spectra. The TEM observations were performed
in a JEOL JEM 2010 transmission electron microscope equipped
with a LaB6 electron beam source, a UHR polepiece (point resolu-
tion: 0.196 nm) and a Pentafet-Link Energy-Dispersive X-ray (EDX)
spectrometer (and INCA software) from Oxford Instruments. The
reducibility of catalyst as a function of temperature (TPR profile)
was investigated by temperature programmed reduction technique
using a MicromeriticsChemiSorb 2750 with ChemiSoftTPx soft-
ware. Approximately 0.1 g of catalyst was pretreated under N2
(25 mL/min) at 400 ◦C for 1 h before the TPR analysis in order to
remove possible impurities contained in the samples. After cooling
down to room temperature under N2, the sample was exposed to
mixture of 10% H2 in Ar flowing at 25 mL/min with the temper-
ature ramped of 10 ◦C/min from 35 ◦C to 700 ◦C. The temperature
was held at 700 ◦C for 1 h and then cooled to room temperature.
The actual amounts of Au and Pd were determined by the ICP-OES
using the Optima 2100 DV spectrometer.
High temperature reduction (i.e., at 500 ◦C) usually manifests
the strong metal-support interaction (SMSI) of TiO2 and group VIII
transition metals (e.g., Pd, Pt, Ni, and Ir). The beneficial effect of
SMSI on the catalyst performances has been reported for supported
Pd/TiO2 in the selective hydrogenation of acetylenic compounds
[41–43]. However, the effect of reduction temperature on the char-
acteristics and catalytic performances of AuPd alloy particles has
not been investigated to much of a degree.
In the present work, the effect of reduction temperature on the
catalytic performances of bimetallic Au-Pd/TiO2 was investigated
and compared to the monometallic Pd/TiO2 in the liquid-phase
selective hydrogenation of 1-heptyne under mild reaction con-
ditions. Prior to the reaction tests, the catalysts were pretreated
under hydrogen reduction temperature of 40 or 500 ◦C. The char-
acteristics of catalysts were also investigated by X-ray diffraction
(XRD), N2 physisorption, temperature-programmed reduction (H2-
TPR), X-ray photoelectron spectroscopy (XPS), and transmission
electron spectroscopy-energy dispersive X-ray spectroscopy (TEM-
EDX).