Large Dimensional CeO2 Nanoflakes by Microwave-Assisted Synthesis: Lamellar Nano-Channels and Surface Oxygen Vacancies Promote Catalytic Activity

Article abstract of DOI:10.1002/cctc.201800784

Large nano-structured flakes of CeO₂ (20–80 nm in thickness, up to 5.6 μm in diameter) were synthesized by a combination of microwave (MW), ultraviolet (UV) and ultrasound (US), with or without pressure (P). The CeO₂ structures were systematically examined by XRD, SEM, N₂ sorption, HRTEM, XPS, Raman and H₂-TPR. The synthesized CeO₂ nanoflakes were composed by 3.0–7.5 nm nanoparticles with the (111) surface exposed, and laminated to nanoflakes with 3.42–3.85 nm nano-channels in between. MW-assisting was beneficial to form a higher surface Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio and surface oxygen vacancies during short synthesis procedure. A Raman peak at 480 cm^?1 correlating with bulk Ce³⁺ was detected. H₂-TPR found MW and MW+P had more surface Ce³⁺ (surface oxygen vacancies). CO oxidation and imine conversion proved that MW+P was the optimum condition to produce highly active CeO₂ nanoflakes. The much better catalytic performance than CeO₂ from solvothermal preparation, due to the larger channel gap (3.85 nm),a higher Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio (32 %) and more surface oxygen vacancies on the particles of the organized flake structures.

Full text of DOI:10.1002/cctc.201800784

Accepted Article
Title: Large dimensional CeO2 nanoflakes by microwave-assisting
synthesis: Lamellar nano-channel and surface oxygen vacancies
promoted catalytic activity
Authors: Huihui Ding, Jingxia Yang, Shuyi Ma, Nevzat Yigit, Jingli Xu,
Günther Rupprechter, and Jinjie Wang
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To be cited as: ChemCatChem 10.1002/cctc.201800784
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ChemCatChem
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Large dimensional CeO nanoflakes by microwave-assisting
2
synthesis: Lamellar nano-channel and surface oxygen vacancies
promoted catalytic activity
a
a,b*
a
b
a
b
Huihui Ding , Jingxia Yang , Shuyi Ma , Nevzat Yigit , Jingli Xu , Günther Rupprechter , JinJie
a*
Wang
Abstract: Large nano-structured flakes of CeO
2
(20-80 nm in
gel methods), but also yield products with excellent properties
because more defects existed during short microwave-assisted
thickness, up to 5.6 μm in diameter) were synthesized by a
[
5]
combination of microwave (MW), ultraviolet (UV) and ultrasound
synthesis procedure.
It has been reported that microwave-
with smaller particles (size
around 3.3-4.0 nm), higher Ce ratio, which is more active for
(
US), with or without pressure (P). The CeO
systematically examined by XRD, SEM, N sorption, HRTEM, XPS,
Raman and -TPR. The synthesized CeO nanoflakes were
2
structures were
assisted synthesis can produce CeO
2
3
+
2
[
5a]
H
2
2
methanol oxidation than samples obtained by precipitation.
Different morphologies of CeO can also be obtained by
microwave-assisted routes, such as nanoparticles,
composed by 3.0-7.5 nm nanoparticles with (111) surface exposed,
and laminated to nanoflakes with 3.42-3.85 nm nano-channels in
between. MW-assisting was beneficial to form more surface
2
[
5b-d]
hollow
[
5e]
[6]
[7]
nanospheres,
core-shell
structures,
nanaocubes,
and small nanoflakes (<200 nm in diameter).
3+
3+
4+
[7a, 8]
[9]
Ce /(Ce +Ce ) ratio and surface oxygen vacancies during short
nanowires,
Nanoflake-structured CeO
-
1
synthesis procedure. A Raman peak at 480 cm correlating with bulk
2
has shown excellent properties in
3
+
[10]
[11]
Ce was detected. H
2
-TPR test found MW and MW+P had more
sensors
or catalytic applications
due to more defects
3+
surface Ce amount (surface oxygen vacancies). CO oxidation and
imine conversion proved that MW+P was the optimum condition to
exposed on the surface of layered structure. However, to date
there were only few reports about nanoflakes of CeO , and most
of the sheets had a small size (<200 nm in diameter)
2
[
9-10, 12]
produce highly active CeO
2
nanoflakes, with much better catalytic
or
2
CeO nanoflakes with
[11b, 13]
performance than CeO from solvothermal preparation, due to larger
2
were hybrids with other materials.
3+
3+
4+
channel gap (3.85 nm), higher Ce /(Ce +Ce ) ratio (32 %) and
more surface oxygen vacancies on the particles of organized flake
structures.
large dimension and thin layers is still waiting for development,
which probably exhibit outstanding properties due to more
defects exposing on the particle surface of thin layers and easy
transfer of active oxygen atoms (O*) within the organized layer
particles. Similar advantages also exhibited in other nanoflake-
[14]
like catalyst systems. Besides, lamellar channel like structure
may be formed in such nanoflake structures, which can be
benefit to catalytic performance similar as the honeycomb
structure of three-way catalyst.
Introduction
Although CeO
to its unique and excellent oxygen storage (-release) capacity
OSC), studies of CeO and ceria-based materials are still
continuing, with efforts to improve the Ce /(Ce +Ce ) ratio
2
has been used widely as catalyst (support) due
Microwave-aided synthesis can be carried out in different
conditions, such as with or without pressure (pressure in the
range of 3-5 atm; equal to microwave assistant solvothermal
method). Theoretically, ultraviolet light (UV) and ultrasonic (US)
(
2
3
+
3+
4+
3+
(
shorted as “Ce ratio”) and specific surface area and thus the
catalytic performance for both high temperature reactions(e.g.
can have effects on CeO
surface structures of CeO
2
too. UV radiation can cause different
because UV energy can irradiate
[
1]
[2]
CO oxidation, NOx reductions. etc) and low temperature
2
[3]
[4]
conversions (e.g. imine synthesis,
other reactions). Microwave-assisted synthesis is an efficient
route towards CeO , as it can not only dramatically shorten the
reaction time (as compared to conventional solvothermal or sol-
dye degradation
and
different form of organic compounds during microwave assisting
synthesis, similar to UV induced organic reactions. US
application can avoid agglomeration of the products during
synthesis. From the economic and scientific view, it is important
2
to figure out how these parameters can influence the CeO
structures during microwave assisted synthesis, and how they
can further affect the catalytic abilities of CeO materials.
In this study, a series of CeO samples with large
nanoflake structures were synthesized by a high-temperature
solution method, assisted by microwave (MW), ultrasonic (US)
or ultraviolet light (UV). A solvothermal method was also used to
2
a
b
H. Ding, Dr. J. Yang, S. Ma, Prof. J. Xu, Dr. J. Wang
College of Chemistry and Chemical Engineering, Shanghai University
of Engineering Science, LongTeng Road 333, 201620 Shanghai, P.
R. China
Email:J.Yang( yjx09tj@foxmail.com,jxyang@sues.edu.cn)
JinJie Wang( jinjiewang0331@163.com)
Dr. J. Yang, N. Yigit, Prof. G. Rupprechter
Institute of Materials Chemistry, Technische Universität Wien,
Getreidemarkt 9/BC/01,1060 Vienna, Austria
2
2
synthesize CeO
nanoflakes were characterized by XRD, SEM, BET, HRTEM,
XPS, Raman and H -TPR, to figure out the structure and defect
2
as a reference. The synthesized CeO
2
2
Supporting information for this article is given via a link at the end of
the document.
distributions. The obtained samples were used as catalysts for
CO oxidation and imine synthesis (a kind of important
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ChemCatChem
10.1002/cctc.201800784
FULL PAPER
intermediate in biological and pharmaceutical applications) to
check the catalytic performances. Finally, relationships between
CeO catalytic properties and material structures were proposed.
2
0~10 nm, and the average pore size was around 3.42 nm,
except for sample MW+P, having a little larger average pore size
of 3.85 nm. This can be attributed to the PVP structure directing.
2
The SEM morphologies of the calcined CeO are shown in
Figure 3 (a)-(f). All samples exhibited a flake structure with
thickness of 20-80 nm, while the diameter can reach to more
than 5 μm. When ultraviolet light was used as assistance
Results and Discussion
(
MW+UV, MW+UV+US), the layer became thinner, which may
2
Structure of CeO synthesized by microwave
be related to the chemical changes caused by UV light. Similar
thin layers can also be observed in MW+P sample. This agrees
The as-prepared samples were characterized by XRD and
crystalline structures were observed. However, the crystalline
structures were mixtures as indicated by the change of the
relative peak intensities. Previous literature reported that
microwave-assisted as-prepared samples had phases like
with the hysteresis loops of N
slit-shaped pores. Flake-like CeO
before, but the flake sizes were much smaller (< 200 nm in
2
sorption isotherms, resulting in
2
structures were reported
[
9, 17]
[
15]
diameter) than those in this study.
Ce(HCOO)
3
16]
(JCPDS No. 49-1245) or CeCO
3
OH (JCPDS No.
[
41-0013),
here only part of the peaks belong to CeCO OH.
3
The strongest peak at low diffraction angle was probably caused
by the ordered lamellar structure, which was self-assembled by
organic residues and PVP surfactant, as shown by the following
SEM and TEM results. The layer distance was about 1.0 nm
based on the diffraction peak 2 theta=8.801°. After the samples
were calcined at 500 °C in the air for 1 h, the organic residues
2
were removed and the typical CeO crystalline phases (JCPDF
No: 34-0394) appeared. The crystalline degrees of these
samples are different. UV and pressure (P) were benefit to get
larger crystalline particles, while US suppressed the particles
growth and resulted in larger BET surface area because of
ultrasonical dispersion. When the particle sizes are calculated by
Scherrer equations based on the strongest peak at 28.5º, the
particle sizes were in the range of 3.0 nm – 7.5 nm.
Figure 2. N
2 2
sorption isotherm (left) and pore size distribution (right) of CeO
prepared under different conditions and calcined at 500 °C in air for 1 h
2
Figure 1. XRD of CeO prepared using different conditions, (a) as-prepared, (b)
calcined at 500 °C in air for 1h. The particle sizes are calculated by Scherrer
equations based on the strongest peak at 28.5º
The calcined samples were further characterized by N
2
sorption isotherms (Figure 2 left) and the pore size distribution
was calculated from the desorption branch of the isotherm
(
Figure 2 right). The hysteresis loops of N
2
sorption isotherm
[
16]
belong to H3 or H4 according to IUPAC classification,
associated with plate-like particles giving rise to slit-shaped
pores. This is consistent with the samples morphologies
observed by SEM and TEM. The MW+US sample possessed
2
the largest specific surface area, SBET 104.6 m /g, while sample
2
Figure 3. SEM morphologies of CeO prepared using different conditions and
2
MW+P had the lowest SBET of 47.9 m /g. The ultrasound
calcined at 500 °C in air for 1 h.
reduced the crystalline particles’ size and increased the surface
area. The widest pore size distributions were in the range of
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Sample MW+UV was further characterized by TEM
because of the thinner layer structure. It was found that the plate
structure was composed of very thin layers before calcination
in 1 nm thin layers, and laminated to thicker nanoflakes with 3
nm gap in between from TEM (Figure 5 (a)). After calcination,
the Ce containing layers were decomposed and transformed to
(
Figure 4 (a-a1). In some areas fence like structure was
crystalline CeO
preserved, and the gap in between was about 3.42 - 3.85 nm
from pore size distribution of N sorption (Figure 5 (b)). These
gaps can serve as reaction channels of catalytic reactions to
improve the CeO performance, which is similar to the
2
(3.0 -7.5 nm in diameter), the lamellar structure
observed (Figure 4 (a2)), probably corresponding to the cross-
section of the layer structure. These layers may yield the strong
XRD diffraction at 2 theta=8.801°. The thickness of one thin
layer and distances in between were about 1 and 3 nm,
respectively. This is almost consistent with 3.42 nm pore size of
2
2
honeycomb channels of three-way catalyst.
slit-shaped pores in N
2
adsorption results. After calcinations, the
XP spectra were used to characterize the valence states of
large layer structure was still existed (b), but small crystalline
particles with a size of about 5-6 nm were observed (b1), in
which the lattice distance was 0.31 nm (b2), corresponding to
the calcined CeO
examined. Normally, the Ce 3d region is formed by five doublets
2
(Figure 6) and the Ce 3d regions were
0
0
(V denotes Ce 3d5/2, U denotes Ce 3d3/2). V , V´, U , U´ belong
3
+
4+
(
111) facet.
to Ce species, while V, V´´, V´´´, U, U´´ and U´´´ belong to Ce
3
+
0
species. To determine the Ce ratio, the peak areas of the U
0
(
V ) and U´ (V´) peaks were related to the area of the entire Ce
[
1c]
3+
3
d region. The autoclaved CeO
2
(A) exhibited a Ce ratio of
22.5%, lower than the value of most microwave assisted
samples (except MW sample). For the microwave assisted
3
+
samples, the highest Ce ratio of 32.5% belonged to MW+P,
3
+
while sample MW showed the lowest Ce value of 21.4%. The
3
+
ultrasound assistance produced a higher Ce ratio (MW+US:
3
+
28.9%) than the UV aided sample (MW+UV: 23.6%). The Ce
ratio of MW+US+UV was 27.7%, a bit lower than that of MW+US,
3
+
which may be due to UV irradiation. The higher Ce ratio of
microwave-aided samples can be attributed to the shorter
[5]
synthesizing time, causing more defects in the samples.
Figure 4. TEM and HRTEM of CeO
2
prepared by MW+UV. (a-a2) are as-
prepared CeO , (b-b2) are samples further calcined 500 °C in air for 1 h.
2
2
Figure 5. Structure sketch of CeO nanoflakes before and after calcination
The structure sketches were proposed in Figure 5.
According to XRD, SEM and TEM results, the as-prepared
2
Figure 6. the region of Ce 3d XP spectra of CeO prepared using different
conditions and calcined at 500 °C in air for 1 h
nanoflakes were composed by CeCO
3 2
OH, CeO and other
unknown crystalline materials. These materials were organized
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-
1
3+
cm here can be recognized as the response of Ce which
located in the second coordination sphere of the oxygen
vacancy in the CeO2-x bulk. After peak fitting, the area ratios of
-1
peak 480 cm were calculated and the sequence is as follows:
MW+P (9.57%) > MW+US (5.33%) > A (5.09%) > MW+US+UV
(
4.84%) > MW+UV (3.79 %) > MW (3.51%).
Figure 8. H
00 °C in air for 1 h. (a) Full spectra from 50-900°C; (b) Peak fitting of surface
oxygen at 460°C
2 2
-TPR of CeO prepared using different conditions and calcined at
5
2
Figure 7. Raman spectra of CeO prepared using different conditions and
2
H -TPR profiles were shown in Figure 8 in order to detect
calcined at 500 °C in air for 1 h. (a) Raman spectra in the region of 200-1800
the relative amount of surface oxygen vacancies, because
surface oxygen vacancies have been proved very important both
-
1
-1
cm ; (b) Peak fitting in the region of 400-600 cm
[
3a, 3e, 3f]
in CO oxidation and imine conversions.
Two obvious
Raman was used to detect the defects in the synthesized
, as shown in Figure 7. Unlike most of the literature that the
regions were shown at around 460 °C (region I) and 730 °C
CeO
2
(region II) (Figure 8(a)), corresponding to surface and bulk
-1
[22]
oxygen defects caused a shoulder peak at 500-600 cm in
oxygen respectively.
2
H consumption amount were marked in
[
18]
Raman spectra , all the samples we synthesized had a sharp
each sample curve. Except sample MW+UV and MW+US+UV,
sample MW+P exhibited the most H consumption amount
-
1
-1
peak at 480 cm besides the peak at 460 cm (lattice oxygen
2
-1
3
3
motion (F2g mode)) and the shoulder peak at 500-600 cm
defects). Although no literature discussed this peak for
experimental data, the peaks at 480 cm were also observed in
some nanorods researches (eg. Ref
). When the 480 cm sharp peak appeared, the intensity of
(10.32 cm /g) while sample A cost the least H
2
(7.89 cm /g).
(
Sample MW+UV and MW+US+UV showed higher
H
2
-1
3
consumption amount (about 11.20 cm /g), which may be caused
by the thinner layer and more oxygen on the surface (see Figure
[
19]
[20]
Figure 10, Ref
Figure
-1
3+
7
3). The increase of Ce or oxygen vacancies on the surface
[
19-20]
[19]
shoulder peak became dramatically weak
. Similar situation
caused the shift of starting point to lower temperature. The
can also be found in this research, all shoulder peaks were not
very large and it’s very difficult to evaluate the difference of
samples defects basing on this.
starting points of MW+UV was 325 °C while the temperature of
sample MW and MW+P shifted to 250 °C, indicating the latter
3
+
two has more surface Ce or surface oxygen vacancies, and
these two also showed good catalytic activity in the following
catalytic reactions. Region I can be separated into two peaks at
390°C (α) and 460 °C (β) (Figure 8(b)), and the former one
While CeO
Functional Theory(DFT),
vibrational bands in the 480-600 cm region associated with the
2
Raman spectra was calculated by a Density
[
21]
the CeO2-x displayed signature
-1
3
+
3+
4+
presence of oxygen defects and reduced Ce ions. They found
corresponded to active oxygen (Ce -O-Ce ) which had close
3
+
3+
[23]
that bulk CeO2-x Raman bands arising from Ce in direct
proximity to the oxygen defect characterized by a calculated
relationship with surface Ce and surface oxygen defects.
The H consumption amounts of the first fitting peak α were
calculated basing on the area ratio: MW+P (7.01 cm /g) > MW
2
-
1
3+
3
band at 500 cm , and those originating from Ce located in the
second coordination sphere of the oxygen vacancy, giving rise
3
3
(6.48 cm /g) ≈ MW+US (6.47 cm /g) > MW+UV+US (6.31
-1
[21]
3
3
3
to a sharp band at 480 cm (Table 4 & Figure 6 in Ref. ). In
this research, the nanoflakes were composed by small CeO
particles of 3.0 - 7.5 nm, which was comparable to the Ce32
cm /g) >A (4.96 cm /g) > MW+UV (4.62 cm /g), suggesting
MW+P possessed the most Ce (oxygen vacancies) on the
surface, while MW+UV had the least surface Ce amount
(surface oxygen vacancies). It should be pointed out that sample
3
+
2
3
+
O
64
[
21]
cell model (c.a. 2-3 nm) in Ref. . Thus the sharp peak at 480
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3
+
-6
MW had the lowest Ce ratio from XPS and Raman spectra, but
the H consumption of peak α of sample MW was much more
mol/s∙m², nevertheless higher than the values (6.42×10 mol/s∙g,
-8
2
2.32×10 mol/s∙m²) of the best sample CeB/AO/F127-STE we
[
1c]
[19, 24]
than sample A. This indicated that the sample MW had more
oxygen vacancies on the top surface, resulting in the following
good catalytic performances.
reported before and other CO oxidation rates
. The best
sample MW+P has an even better catalytic activity, with r250°C
=
-5
-7
1.90×10 mol/s∙g and R250°C= 3.97×10 mol/s∙m². The r250°C of
MW+P was twice than that of sample A, and about 3 times as
the value of sample CeB/AO/F127-STE, while R250°C of MW+P
was about 16 times higher of sample CeB/AO/F127-STE. The
Detection of catalytic activity
improved catalytic performance of MW-assisting CeO
2
can be
3
+
contributed to high Ce
ratio and the organized lamellar
CO oxidation was used to test the catalytic activities of the
channels, although the surface areas are low.
synthesized samples (Figure 9). The ignition temperature (T10%
,
the reaction temperature required for 10% CO conversion), and
the light-off temperature T90% (the temperature at which 90 %
conversion of CO is achieved) were checked, as listed in Table
1.
The best sample for CO oxidation was MW+P, with
T10%=216 °C and T90%=318 °C , while the worst performance was
due to sample A (T10%=243 °C , T90%=405 °C ). The catalytic
activity of the least active sample A is comparable with our
previously
reported
CeO
2
sample
CeB/AO/F127-STE
2
[1c]
(
T10%=253 °C , T90%=398 °C , SBET=277 m /g)
, indicating
sample MW+P had a much higher catalytic activity. The
sequence of T10% is the same as that of T90%: MW+P (216 °C ,
3
18 °C ) < MW+US (220 °C , 322 °C ) ≤ MW (220°C , 333°C ) <
MW+US+UV (224°C , 349°C ) < MW+UV (227°C , 373°C ) < A
243°C , 405°C ).
The reaction rates at 250°C normalized per gram (r250°C
(
)
Figure 9. CO oxidation (5 vol% CO in 50 mL/min flow) at different
and per unit surface area (R250°C) are calculated to enable the
comparison of the catalytic activity of the different samples using
the conversions below 20%, as listed in Table 1. The least active
2
temperatures for CeO samples (20 mg)
-6
-7
sample A is with r250°C= 9.08×10 mol/s∙g and R250°C= 1.24×10
Table 1. Summary of CO oxidation results by using different CeO
2
as catalysts
a
b
c
d
T
10%
T
90%
r
250°C
R
250°C
Condition of CO
Sample
(
°C)
(°C)
405
333
318
322
373
349
398
(mol/s∙g)
(mol/s∙m²)
oxidation reaction
-
-
-
-
-
-
6
-7
A
MW
MW+P
MW+US
MW+UV
MW+US+UV
CeB/AO/F127-STE
243
220
216
220
227
224
253
9.08×10
1.64×10
1.90×10
1.65×10
1.32×10
1.58×10
6.42×10
1.24×10
5
5
5
5
5
-7
2.16×10
-7
3.97×10
50 mL/min
(5 vol% CO)
0.02 g catalyst
-7
1.57×10
1.47×10
-7
-7
1.69×10
[
1c]
-6
-8
2.32×10
[
24c]
-7
-9
160 mL/min (0.4vol % CO)
2
CeO
CeO
CeO
2
2
2
2
-180
/
/
/
/
5.15×10
6.44×10
8.37×10
1.03×10
4.29×10
0
.20 g catalyst (SBET=120 m /g)
40 mL/min (1 vol% CO)
.02 g catalyst (SBET=31 m /g)
30 mL/min (1 vol% CO)
[
24a]
-6
-7
-6
-7
/
2.08×10
2
0
[
19]
320
(
380
(
-8
-500
(CeCu0)
1.12×10
2
T
50%
)
)
0.03 g catalyst (SBET=74.5 m /g)
[
24b]
-9
80 mL/min (1 vol% CO)
0.50 g catalyst (SBET=202 m /g)
CeO
5.10×10
2
T
50%
a
Reaction temperature for 10% CO conversion
Reaction temperature for 90% CO conversion
b
c
d
Reaction rate (mol/s∙g) of CO oxidation at 250 °C per gram , by using the converted mole amount of CO at 250 °C and normalized it to per gram
Normalized specific reaction rates (mol/s∙m²) of CO oxidation on a unit surface area at 250 °C, by using the converted mole amount of CO at 250 °C and
normalized it to per specific area
-
1
-1
-1 -1
Imine conversion was also tested by using different CeO
2
conversion is MW+P (9.52 mmol∙g h )> MW (9.13 mmol∙g h )>
-1
-1
-1 -1
catalysts (Figure 10), as a model of low temperature catalyst. It
has been reported earlier that CeO can be used as an easily
separable catalyst for imine synthesis at low temperature (<
MW+US (7.05 mmol∙g h ) > MW+US+UV (6.31 mmol∙g h )> A
-1
-1
-1 -1
2
(4.87 mmol∙g h )
consistent with the ranking of surface active oxygen in H
fitted peak α (Figure 8 (b)). The best imine conversion rate (9.52
>
MW+UV (2.84 mmol∙g h ). This is
2
-TPR
[
5e]
1
00 °C)
and oxygen vacancies played an important role
-1 -1
during this reaction (Figure 10 (b)). The order of imine
mmol∙g h ) appeared for the system with MW+P as catalyst
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-
1 -1
(
higher than the reported 0.46 mmol∙g h of sample at 30 °C in
temperature reaction) were chosen as two models. Basically,
[
16]
-1 -1
[3e]
3+
3+
4+
Ref
and 4.9 mmol∙g h of sample at 50°C in Ref ), while
Ce /(Ce +Ce ) ratio had close relationship with the catalytic
performance of CeO nanoflake except sample MW (Figure 11
(a1) & (b1)). MW showed good catalytic performance although
-1 -1
the lowest one (2.84 mmol∙g h ) belonged to sample MW+UV.
This generally fits to the rank of the CO oxidation, except
MW+UV and A. As the imine conversion worked at 50 °C, the
Ce migration was not as fast as that at high temperature of CO
oxidation (> 200 °C), which may contribute to the inverse
performance of MW + UV and A during CO oxidation (> 200 °C).
Similar situation can be observed in Fenton-like dye degradation
2
3
+
3+
4+
the Ce /(Ce +Ce ) ratio was the worst one during all samples.
Nevertheless, from TPR results, the surface oxygen vacancies
3
+
3+
of sample MW was more than the others, indicating more Ce
and more oxygen vacancies stayed on the top surface layer,
rather than on sub-surface layers. This compensated the inferior
3
+
[3e, 25]
(
Figure S4).
total Ce amount, as reported by other literatures.
Here,
-1
the area ratios of 480 cm peak in Raman spectra were also
displayed (Figure 11 (a2 & b2)), and the rank was consistent
3
+
with Ce ratio calculated by XPS Ce 3d fitting results, proving
3
+
that it was caused by Ce . When H
was concerned, the reaction rate of both catalytic reactions fitted
quite good with the H consumption amount of peak α (Figure 11
a4) & (b4)), rather than the H consumption amount of region I
Figure 11 (a3) & (b3)), indicating surface Ce amount (surface
2 2
consumption of H -TPR
2
(
(
2
3
+
oxygen vacancies) governed the catalytic activities of CeO
2
nanoflakes, and sample MW had more surface oxygen
vacancies. However, the surface area showed no close
relationship with the catalytic performance of CeO nanoflakes,
2
which may be because channel gap were similar (3.4 nm) and
the differences of surface areas were not large enough during
2
these nanoflake samples (48-105 m /g) (Figure 11 (a5) & (b5)).
The only exception was MW+P, with the channel gap of 3.8 nm.
The larger gap allowed more reactant into the channel,
beneficial to the catalytic performance.
Figure 10. (a) Imine conversion rate (per catalyst amount per hour) by using
different CeO (50 mg); all values were calculated for conversion below 20%.
2
The experiments were repeated for 3 times, and the average conversion
values were given with error bars. (b) Reaction mechanism of imine
[
3e]
conversion by using CeO
2
as catalyst (taken from authors’ previous work.
)
Figure 11. Correlation of catalytic activity with different parameters. (a) CO
Parameters influencing catalytic activity
oxidation rate vs. parameters, (b) imine conversion rate vs. parameters.
As the previous reaction rates were calculated based on the
initial conversion values (lower than 20%) for both CO oxidation
and imine conversion, and the lamellar gaps of the CeO flakes
2
were almost the same (3.42 nm) except sample MW+P (3.85
nm), the two catalytic reactions can be recognized as chemical
control processing with negligible mass transportation difference.
Thus it’s important to figure out that which property of the
sample has close relationship with the active sites. Besides the
From the above analysis, it can be seen clearly that the
3
+
relative amount of surface oxygen vacancies (surface Ce )
governed the catalytic activity of CeO nanoflakes, rather than
the surface area and the bulk oxygen vacancies (bulk Ce ), as
2
3
+
3
+
sketched in Figure 12. Generally, the total Ce ratio had close
relationship with the amount of surface oxygen vacancies (eg.
sample MW+P, MW+US), but sample MW existed as an
3
+
exception. It had the lowest Ce ratio (21%), but most of them
stayed on the surface, resulting in high catalytic performance in
both CO oxidation and imine synthesis. Sample MW+P exhibited
benefit of lamellar reaction channel structure of CeO
2
nanoflakes,
the intrinsic properties of CeO nanoparticles can also influence
2
the catalytic ability. Figure 11 shows the parameters influencing
the catalytic activity. Depending on the reaction temperature, CO
oxidation (high temperature reaction) and imine conversion (low
3
+
the highest catalytic activity due to abundant Ce in both
3
+
surface and bulk lattices (highest Ce ratio : 32%). The nano-
layer gap of CeO nanoflakes can serve as reaction channel, in
2
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10.1002/cctc.201800784
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3
+
3+
4+
which the reactant had more chance to get access to active
oxygen atom during going through the channel, than just on the
surface. Thus the lager channel gap of sample MW+P (3.85 nm)
was also better than smaller channel gap (3.42 nm) of other
samples, because larger channel gap can allow reactant getting
into the flakes much easier.
the laminal channel gap, Ce /(Ce +Ce ) ratio and especially
surface oxygen vacancies, rather than on the specific surface
area.
Experimental Section
Cerium
(III)
nitrate
hexahydrate
(Ce(NO
3
)
3 2
·6H O),
urea,
polyvinylpyrrolidone (PVP, K10-14), are used as starting materials with
ethylene glycol (EG) being used as solvent. All materials were AR grade
and used as received. The microwave, ultrasonic and ultraviolet light
2
were used to assist the synthesis of CeO .
Preparation of CeO
2
For all synthesis routes, the solution content and the reaction
temperature were the same, and as well as the preparation procedure.
First, 5 mmol Ce(NO ) ·6H O and 0.5 g PVP were dissolved in 20 mL EG,
3 3 2
2
Figure 12. Mechanism sketch of CeO nanoflakes as catalyst for CO oxidation
and imine synthesis
named as solution A; 7.5 mmol urea and 0.5 g PVP were dissolved in 15
mL EG to form solution B. Then solution B was slowly added into solution
A, and further stirred for 20 min to form a stable and uniform reaction
solution.
Conclusions
For CeO
carried on
2
synthesis, the microwave assisted reactions were
Multifunctional Microwave Synthesis Extraction
2
A series of CeO samples with nanoflake structure (20-80 nm in
thickness, up to 5.6 μm in diameter) were synthesized by the
a
&
Workstation, Model Uwave-2000 from SINEO Microwave Chemistry
Technology (China) Co., Ltd. This equipment can combine microwave,
ultrasonic and ultraviolet light together during the synthesis. It can also
be used to perform reactions under pressure (3 atm) with microwave
irradiation, equal to microwave assisted solvothermal reaction. The
power of microwave can be adjusted in the range of 250-1000 W by
variable frequency. The ultrasonic amplitude was 70%. The wavelength
of ultraviolet light was 365 nm and the power was 300 W. For all
synthesis with microwave assistance, the heating programs were the
same. The temperature was risen to 180 °C at 15 °C/min ramping rate,
and then kept at 180°C for 10 min. The synthesis conditions for all
samples are summarized in Table 2. For the synthesis without pressure,
the reaction solution was poured into a four-necked quartz flask, which
was connected with a condenser to prevent solvent evaporation at high
temperature.
After the reaction processing, the container were cooled to below
50 °C before the suspensions were transferred for separation. All
synthesized samples were centrifuged and washed by ethanol at least 3
times. The final white or yellowish precipitates were collected and dried in
vacuum at 65 °C overnight, named as-prepared samples. Part of the as-
prepared samples were further calcined at 500 °C (2 °C/min ramping
rate) for 1h in air to remove surface organic residues.
assistance of MW, UV and US combinations, with or without
applied pressure (P). The CeO
characterized by XRD, SEM, HRTEM, N
and H -TPR. The synthesized nanoflakes possessed lamellar
channels of 3.42-3.85 nm. Microwave assisting was beneficial to
2
structures were systematically
2
sorption, XPS, Raman
2
3
+
3+
4+
form more surface Ce /(Ce +Ce ) ratio and surface oxygen
-
1
vacancies. Raman spectra showed
a
peak at 480 cm ,
3
+
corresponding to Ce in the bulk, and the fitted value sequence
was almost consistent with the Ce /(Ce +Ce ) ratio of XPS
spectra. The results showed that sample MW+P exhibited the
largest channel gap, the highest Ce /(Ce +Ce ) ratio (32 %)
consumption of peak α 7.01
cm /g), while the MW sample possessed the lowest
3
+
3+
4+
3
+
3+
4+
3
+
and the most surface Ce (H
2
3
3
+
3+
4+
3+
Ce /(Ce +Ce ) ratio (21 %) but more surface Ce
(H
2
3
consumption of peak α 6.48 cm /g). CO oxidation and imine
conversion indicated that MW+P was the optimum condition to
produce high quality CeO
abundant defects on the surface, which exhibited much better
catalytic activity than CeO prepared by solvothermal method.
The performances of CeO nanoflakes were more dependent on
2
nanoflakes catalysts bearing
2
2
2
Table 2. Synthesis conditions of CeO by the microwave-assisted solution method
I
II
III
Condition
Microwave
(MW)
Ultrasonic
(US)
Ultraviolet
(UV)
Pressure
(P)
√
Temperature
program
180 °C , 6 h
No.
Container
Sample
IV
v
1
2
3
4
5
6
A (reference)
MW
MW+P
MW+US
MW+UV
MW+US+UV
Teflon inline
Quartz flask
Teflon inline
Quartz flask
Quartz flask
Quartz flask
√
√
√
√
√
√ (3 atm)
15 °C /min to
180 °C , then kept at
180 °C for 10 min
√
√
√
√
I: Microwave power adjusted in the range of 250-1000 W by variable frequency.
II: The amplitude of US was 70%.
III: The wavelength of ultraviolet light was 365 nm and the power was 300 W.
IV: The Teflon inline was sealed in a steel autoclave, and kept still.
V: The autoclave was put in an oven with 180 °C.
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Acknowledgements
This research was supported by National Natural Science
Foundation of China (Grant number 21601121), the Program for
Professor of Special Appointment (Eastern Scholar) at Shanghai
Institutions of Higher Learning (Grant number QD2016037), and
Young Professor Cultivation Program of Shanghai Municipal
University (Grant number ZZGCD16019). N.Y. and G.R.
acknowledge support by the Austrian Science Fund (FWF, DK+
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2
Keywords: Catalyst • CeO nanoflakes • CO oxidation • Imine
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Entry for the Table of Contents
Layout 2:
FULL PAPER
a
a,b*
Huihui Ding , Jingxia Yang , Shuyi
a
b
a
Ma , Nevzat Yigit , Jingli Xu , Günther
b
a*
Rupprechter , Jinjie Wang
Page No. – Page No.
2
Large dimensional CeO nanoflakes
CeO
2
nanoflakes were obtained by microwave-assisting synthesis, composed by 5-
by microwave-assisting synthesis:
Lamellar nano-channel and surface
oxygen vacancies promoted catalytic
activity
6
nm nanoparticles and laminated with 3.42-3.85 nm channel gaps. MW-assisting
3
+
3+
4+
was beneficial to form more surface Ce /(Ce +Ce ) ratio and surface oxygen
vacancies during short synthesis procedure. Both of them were profitable to the
excellent catalytic performance.
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