M. Qi et al.
Molecular Catalysis 510 (2021) 111714
◦
important class of commercial catalysts. The term "multimetallic" is very
broad. Here, "multimetallic" is equal to an alloy, including intermetallic
compounds. For example, Pd-Te catalysts were well known catalysts for
oxidative esterification reactions [12]. Among them, Pb, Bi and Ag were
the most commonly used auxiliaries [5,13,14]. One or more alkali
metals or alkaline earth metals were also added to some catalysts to
increase the activity of the catalyst.
and then calcined at 600 C for another 3 h in air atmosphere to obtain
MgO-Al support.
2 3
O
2.1.2. Pd and Pb deposition on mgo-Al
method
2 3
O by impregnation-reduction
Catalysts prepared by impregnation-reduction method was reported
by Diao et al. [27]. PdCl (Pd: 5 wt%) and NaCl (n(PdCl
): n(NaCl) = 1:
2) was mixed in deionized water for 10 min. And then the brown
2
2
Intermetallic compound nanocrystals had attracted extensive atten-
tion because of their clear crystal structure, controllable atomic distri-
bution and excellent structural stability, which enabled them to have the
best catalytic activity, stability and high selectivity. Pb was often used as
an auxiliary agent of Pd based catalyst because it was easy to prepare
Na
2
PdCl
4
solution was added to water with appropriate amount of
◦
MgO-Al
2
O
3
support at 80 C and stirred for 1 h. After the first step
impregnation, the obtained brownish red solid was washed with
ꢀ
deionized water until no Cl was detected by AgNO
3
. The solid was
Pd
3
Pb alloy crystals to regulate the reaction performance. Compared
3 2
added to the Pb(OOCCH ) (Pb: 5 wt%) aqueous solution for 1 h im-
with Pd nano catalysts, the lattice size and electron density of Pd were
changed by Pb doping. The adsorption capacity of the catalyst for re-
actants or the desorption capacity of the product also changed [15–17].
In the electrocatalytic oxidation of ethanol, formic acid, semi-
hydrogenation of phenylacetylene, aerobic oxidation of amines and so
mersion, and then the hydrazine hydrate solution was gradually dripped
into the above solution to reduce the catalyst for 3 h. After reduction,
◦
black catalyst was washed with water and dried at 45 C under vacuum.
The as-prepared Pd
MgO-Al
(denoted as Pd
5
Pb
5
/MgO-Al
2
O
3
catalyst (denoted as Pd
5
Pb
5
/
◦
◦
2
O
3
) was annealed at 600 C and 800 C in Ar stream for 12 h
on, an appropriate amount of Pb and Pd were added to form Pd
3
Pb alloy
5
Pb
5
/MgO-Al
2
O
3
–600 and Pd
5
Pb
5
/MgO-Al
2
O
3
–800
◦
catalysts. These catalysts had better reaction performance than single Pd
catalysts [15–19]. The order of crystal structure means that the particles
inside the crystal can selectively occupy different positions and are ar-
ranged regularly with each other. In the alloy catalyst, Pd and the
auxiliary metal are randomly distributed at the Wycoff atomic position.
The active site of catalysts varies according to their structure and surface
composition. Different from the disordered alloys, the ordered inter-
metallic phases can better control the structure and electronic effects,
resulting in the order of composition and position, thus forming uniform
active sites on the same surface [20–22]. At present, it has been reported
that an ordered intermetallic compound had been used as an electro-
chemical catalyst for an oxygen reduction reaction, ethanol oxidation
and ammonia synthesis [23–25]. Ordered intermetallic compounds
exhibited superior performance compared to disordered structures. In
such intermetallic phases, strong interaction between the second tran-
sition metal and Pd provided long-term stability and avoided deactiva-
tion [26]. Ordered-crystal structure catalysts have also obtained good
results in the non-electrocatalytic oxidation of CO and dibromomethane
respectively). Also annealed at 800 C, Pd (5 wt%) and Pb (3.2 wt%)
loaded on MgO-Al
2 3 5
O was also prepared (denoted as Pd Pb3.2/MgO-
Al
2 3
O –800).
2.1.3. Pd and Pb deposition on MgO-Al
The preparation of ordered Pd Pb crystal by hydrothermal method
was reported in literature [28]. 0.3 mmol (0.0883 g) Na PdCl and
0.1 mmol (0.0379 g) Pb(OAc) were mixed in 18 mL water and taken in
2 3
O by hydrothermal method
3
2
4
2
a tetrafluoroethylene lined autoclave. The autoclave was ultrasonicated
for 30 min to obtain a homogenous solution. 3 mL hydrazine hydrate
◦
was added to the solution and the autoclave was kept at 150 C for 16 h.
Then the autoclave was cooled and the contents was washed with
ethanol and water three times respectively to obtain pure Pd
3
Pb crystal.
◦
The product was dried overnight at 45 C in vacuum oven. The prepa-
ration of catalyst by hydrothermal method is similar to that of pure
crystal. The difference is that in addition to adding Pd (5 wt%) and Pb
2 3
(5 wt%) precursors to the hydrothermal reactor, the MgO-Al O support
is also added (denoted as Pd
Pb
5 5
2
/MgO-Al O
3
–
H). The obtained black
ꢀ
[
27,28]. In view of the crystal structure of the ordered catalyst and the
solid was washed with deionized water until no Cl was detected by
◦
excellent activity produced, we have developed the idea of applying it to
the direct oxidation esterification reaction. However, to date, reports on
ordered Pd-based intermetallic chemicals remained very limited. The
most important reason was that the particle size during high tempera-
ture annealing was difficult to control, and there was no effective
method [29,30].
AgNO
3
. The product was also dried overnight at 45 C in vacuum oven.
2.2. Characterization
The phase structure analysis of the catalyst was carried out by a
Rigaku D8MAX-2400 X-ray diffraction (XRD) from Japan. Analysis
In this study, we use Al
2
O
3
as support to synthesize ordered or
conditions: tube voltage 40 kV, tube current 30 mA, Cu target K
α
ray
disordered Pd Pb intermetallic by impregnation-reduction method. It
3
(incident wavelength 0.15406 nm), continuous scanning, scanning
◦
ꢀ 1
◦
◦
mainly includes the mixing of Pd and Pb precursors and the co-reduction
of Pd and Pb precursors as well as high temperature heat treatment. The
synthesized ordered intermetallic nano catalyst is then used for direct
oxidative esterification of aldehyde with methanol. The formation rate
of intermetallic compounds in the alloy is controlled by solid-diffusion
and increases exponentially with annealing temperature [31].
Compared with low temperature preparation, high temperature
quenching helps to form ordered crystals, and low temperature prepa-
ration tends to form alloys or disordered structures.
speed 10 min , scanning range (2θ) 5 ꢀ 90
Transmission electron microscopy (TEM) images were performed
using a JEOL JEM 2100F TEM at an accelerating voltage of 200 kV.
Energy dispersive spectroscopy (EDS) analysis was performed by using a
Sirion field emission scanning electron.
X-ray photoelectron spectroscopy (XPS) analysis was performed by
using a Thermo Fisher Scientific ESCALAB 250Xi, with a system using a
focused monochromatic Al Ka X-ray (1486.7 eV) source for excitation
and a spherical section analyzer.
2
. Experimention
2.3. Catalyst activity testing
2
2
.1. Catalyst preparation
.1.1. Synthesis of MgO-Al
The reaction was carried out in a 100 mL stainless steel jacketed
high-pressure batch reactor. 2.5 g of the prepared catalyst was added to
the reaction vessel, followed by the addition of 75 mL of a mixed solu-
tion of methanol and methacrolein, and the molar ratio of methanol to
aldehyde was 8:1. The mass flow meter controls a stable oxygen flow
2 3
O
2 3
Bar γ-Al O was bought from Tianjin Chemical Research Institute.
The support was crushed and the particles with the size of 150 - 180
μ
m
◦
ꢀ 1
were selected. After calcination at 600 C for 3 h, γ-Al
in the solution of Mg(CH
2
O
3
was immersed
rate of 30 mL• min , and the reaction was controlled at a temperature
◦
◦
◦
3
COO)
2
•4H
2
O (Mg: 2 wt%) at 80 C until the
of 80 C and a pressure of 0.3 MPa. The magnetic stirrer is heated and
solution dried. The obtained solid was dried in vacuum at 45 C for 12 h,
agitated to maintain good contact of the gas-liquid-solid three-phase.
2