X. Yang et al.
Molecular Catalysis 457 (2018) 1–7
the other types of oxygen species (lattice oxygen or surface oxygen)
on the preparation method of pure CeO
2
. The samples with the parti-
[
17].
In addition to gaseous oxygen, it is necessary to verify the role of
cular molar ratios of Mn to Ce of 1/3, 1/1 and 3/1 in the shape of either
cubes or rods were labeled as Mn Ce O-cube, Mn Ce O-cube,
Mn Ce O-cube, Mn Ce O-rod, Mn Ce O-rod, and Mn Ce O-rod, re-
spectively.
The detailed palladium-loaded procedure of the precipitation
method is as follows: PdCl (0.42 g) was firstly dissolved in 50.0 ml of
aqueous solution and the pH was subsequently adjusted to 1.0 with
concentrated hydrochloric acid. The CeO and MnO –CeO supports
(5.0 g) were then impregnated with the above PdCl solution. Following
1
3
1
1
other oxygen species in the catalytic reaction. Oxygen species are
proven to play a vital catalytic role in low-temperature CO oxidation,
the oxidative coupling of methane, the removal of volatile organic
compounds and the purification automotive exhaust gas, etc. [18]. As
for oxidation carbonylation of phenol, we proposed in our previous
3
1
1
3
1
1
3
1
2
−
study that 1) the formation of more OH group, a type of oxygen
species, is more beneficial to the redox cycle between active Pd species
2
x
2
2
and supports, and 2) the nature of oxygen species on the surface of a
that, the precipitating agent, NaOH solution (3 mol/L), was added
dropwise into the above slurry until the pH value was 9–10. Finally, the
supported palladium catalysts were separated by filtration and washed
with distilled water several times. They were then dried at 333 K and
calcined at 573 K for 3 h. The palladium supported catalysts are de-
2
+
catalyst strongly depends on the metal cation (Pb ) [19]. To our best
knowledge, metal doping is a typical method to alter the oxygen species
in oxides. Recently, another strategy of tuning the oxygen species of
catalytic material via the morphology effect in microscopic scale was
reported [20–22]. Many researches have been done on the effect of
morphology on metal oxide catalysts in catalytic performance, such as
noted as Pd/Mn
x y
Ce O(x,y = 1 or 3) corresponding to the supports.
2 x 2
CeO , MnO -CeO mixed oxides, etc. [23,24]. The catalytic properties
2.2. Characterization
of metal oxide catalysts were attributed to specific crystal planes ex-
posed on nanocrystals with different morphology, such as the (100)
X-ray Diffraction (XRD) patterns were recorded on a D8 Advance
(Bruker, Germany) diffractometer using Cu Kα radiation operated at
40 kV and 200 mA, with 2θ from 10° to 80°.
planes and (110) planes exposed in cubic and rod-like CeO
tively. This is because the lattice oxygen species on the (100) planes
have a higher mobility than that on the (110) plane in CeO [23].
Furthermore, the strong morphology-induced effect of CeO on the
active Pt species was observed during CO oxidation, where three
morphological types of CeO (cube, rod and octahedral) were employed
2
, respec-
2
Transmission Electron Microscope (TEM) images were conducted on
a JEM-2100 (JEOL, Japan) operated at 200 kV, and high-resolution
TEM (HRTEM) images were taken on a FEI Tecnai G2 F30 microscope
operated at 300 kV. The specimen was prepared by ultrasonically dis-
persing the sample powder in ethanol, and droplets of the suspension
were deposited on a carbon-coated copper grid and dried in air. Energy
dispersive X-ray spectroscopy (EDX/EDS) was used to determine the
chemical composition of the samples.
2
2
as supports [25]. Therefore, it is essential to investigate the morpho-
logical effect of nanocatalysts and verify the role of oxygen species in
the catalytic cycle of active palladium species, which would give an
insight into the development of catalysts with high and stable perfor-
mance.
X-ray Photoelectron Spectroscopy (XPS) analysis was carried out on
a K-Alpha XPS instrument (Thermo Fisher Scientific, America), em-
ploying Al- Kα radiation. The binding energy (BE) for the samples was
calibrated by setting the measured BE of C 1s to 284.6 eV.
Our research groups focused on the synthesis of Perovskite and
Cryptomelane-type metal oxides as the supports for palladium catalysts
[
15,17,26,27]. We note that less attention is paid to the morphology of
heterogeneous catalysts in the present study on the oxidative carbo-
nylation of phenol. Herein, CeO and Mn Ce O mixed oxides with
Raman Spectra were measured at room temperature using a DXR
2
x
y
Raman microscope (Thermo Fisher +Scientific, America) with
a
various morphologies were obviously synthesized by the hydrothermal
method as the supports for palladium catalysts. We hope that the re-
search on these morphological effects will provide a comparable
quantitative means of deepening the fundamental understanding of the
role of oxygen species in catalytic oxidative carbonylation reaction.
Moreover, the interaction mechanism between the metal oxide supports
and active palladium species will be explored further. Compared with
morphological tuning, Mn doping was adopted to establish a more clear
relationship among the oxygen species and active Pd species in terms of
catalytic performance.
514.5 nm excitation source from an Ar laser.
Brunaur Emmett Teller (BET) nitrogen adsorption plots were used to
measure the change in the specific surface area of the catalysts on a
Nova 2000e surface area and pore size analyzer (Quantachrome
Instruments, America).
2.3. Catalytic evaluation
The oxidative carbonylation reaction was performed in a 250 ml
stainless steel autoclave equipped with a magnetic stirrer to facilitate
external mass transfer. Phenol 47 g (0.5 mol), tetrabutylammonium
bromide (TBAB) 1 g (3 mmol), and Pd (in the catalyst) 0.47 mmol were
introduced into the autoclave. Then, the autoclave was sealed and he-
2. Experiments
2.1. Sample preparation
ated to 65 °C. Subsequently, the gas mixture of CO and O
2
(CO/
O
2
= 12/1 M ratio) became charged. After the reaction lasted for 4 h at
Both CeO
hydrothermal conditions as supports.
3 3 2
CeO nanocubes were synthesized with a Ce(NO ) ·6H O aqueous
solution(3.00 g, 10 ml) and a NaOH aqueous solution (16.88 g,70 ml).
The above two aqueous solutions were mixed, introduced into an au-
toclave, and kept at 453 K for 24 h. The precipitate from the autoclave
was filtered, washed, and dried at 333 K for 6 h, and finally calcined at
2
and MnO
x
–CeO
2
nanomaterials were prepared under
the pressure of 4.8 MPa, the autoclave was cooled and the products
were taken out. The final products were determined by capillary gas
chromatography. The activity tests were carried out under a kinetic
regime without the impact of mass transfer limitations.
2
3. Results and discussion
6
73 K for 3 h. The solid obtained was denoted as CeO -cubes.
2
3.1. Morphological effects of oxide supports
CeO nanorods were synthesized with a CeCl ·7H O aqueous solu-
2
3
2
tion (2.99 g, 80 ml) and NaOH (38.41 g, 80 ml). The synthetic proce-
dure of nanocubes was similar to that of the nanorods. The temperature
of the autoclave was 423 K for 12 h, and the calcination temperature
The morphology of synthesized metal oxide supports was char-
acterized by TEM, the images of which are shown in Fig. 1. The doping
of Mn did not result in a change in morphology for neither nanocubes
nor nanorods (Fig. 1(a)–(d)). A subtle difference that was present was
was 573 K for 3 h. CeO
Different amounts of Mn(NO
MnO -CeO mixed oxides with the morphology of cubes and rods based
2
nanorods are marked as CeO
2
-rods.
3
)
2
and MnCl were added to prepare
2
that the surface of Mn
1
Ce
3
O-cube (Fig. 1(c)) appeared rougher and
-cube (Fig. 1(a)).
x
2
their corners were rounder than that of pure CeO
2
2