1. science
2. physics
3. nanotechnology

Nano Research
Nano Res
DOI
10.1007/s01274-015-0704-3
Mass production of Co3O4@CeO2 core@shell
nanowires for catalytic CO oxidation
Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (), Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b,
Yinghui Wanga and Hongjie Zhanga ()
Nano Res., Just Accepted Manuscript • DOI: 1 10.1007/s01274-015-0704-3
http://www.thenanoresearch.com on December 24 2014
© Tsinghua University Press 2014
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1
TABLE OF CONTENTS (TOC)
Mass Production of Co3O4@CeO2 Core@Shell
Nanowires for Catalytic CO Oxidation
Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (),
Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b, Yinghui Wanga
and Hongjie Zhanga ()
aState
Key Laboratory of Rare Earth Resource Utilization,
Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun, Jilin, China.
bSchool
of the Chinese Academy of Sciences, Beijing
Co3O4@CeO2 core@shell nanowires have been successfully prepared
100039 (China)
in
# The two authors contribute equally to this work.
Co(CO3)0.5(OH)·0.11H2O@CeO2
mass
production
by
thermal
precursors.
decomposition
The
of
successful
fabrication of the core@shell structures leads to remarkably improved
catalytic activity and stability of Co3O4. The best sample can catalyze
100 % CO conversion at a temperature as low as 160 oC. Detailed
study reveals that CO oxidation possibly takes place at the interface of
Co3O4 and CeO2, demonstrating obvious synergistic effects between
the two components.
Provide the authors’ webside if possible.
Hongjie Zhang, http://lab.datatang.com/2007DA173041/AreaIndex.aspx?ItemID=68553
Mass Production of Co3O4@CeO2
Nanowires for Catalytic CO Oxidation
Core@Shell
Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (), Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b, Yinghui
Wanga and Hongjie Zhanga ()
Received: day month year
ABSTRACT
Revised: day month year
In this paper, Co3O4@CeO2 core@shell nanowires were successfully prepared via
thermal decomposition of Co(CO3)0.5(OH)·0.11H2O@CeO2 core@shell nanowire
precursors. As the CO oxidation catalyst, Co3O4@CeO2 shows remarkably
enhanced catalytic performance compared to Co3O4 nanowires and CeO2 NPs,
demonstrating obvious synergistic effects between the two components. It also
suggests that the CeO2 shell coating can effectively keep Co 3O4 nanowires from
agglomeration, and hence remarkably improve the structure stability of Co3O4
catalyst. And the fabrication of the well dispersed core@shell structure results in
a maximized interface area between Co3O4 and CeO2 as well as a smaller Co3O4
size, which might be responsible for the enhanced catalytic activity of
Co3O4@CeO2. Further study reveals that CO oxidation possibly takes place at
the interface of Co3O4 and CeO2. The influence of calcination temperatures and
component ratio between Co3O4 and CeO2 have been then investigated in detail
on the catalytic performance of Co3O4@CeO2 core@shell nanowires, the best of
which obtained by calcination at 250 oC for 3 h with a Ce molar content about
38.5 % can catalyze 100 % CO conversion at a lower temperature of 160 oC.
More importantly more than 2.5 g of the Co3O4@CeO2 core@shell nanowires can
be produced in one pot by this simple process, which would be beneficial to
their practical applications as automobile exhaust gas treatment catalysts.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Co3O4@CeO2, core@shell,
nanowires, CO oxidation,
synergistic effects
Nano Res.
2
1 Introduction
Catalytic oxidation of carbon monoxide (CO) has
components.
drawn continuous attention because of the serious
exhibited
health effects associated with exposure to CO.
stability as oxidation reaction catalysts[12,22-25]. Hence
Co3O4, a typical spinel-structure transition metal
it is reasonably considered that the activity as well as
oxide, has been subjected to intense interest recently
the stability of Co3O4 catalyst could be optimized
due to its excellent ability for catalytic CO oxidation,
through
and has been regarded as an alternative to noble
core@shell structures.
metal catalysts
For
instance,
Ag@CeO2,
Pt@CeO2,
Au@CeO2 and Pd@CeO2 core@shell catalysts have
good
the
activity
facile
and
high-temperature
fabrication
of
Co 3O4@CeO2
. Co3O4 nanorods synthesized by
Generally, core@shell structures are synthesized
Xie’s group have shown good catalytic performance,
through hydrolysis of the precursors to deposit the
which can catalyze CO oxidation at a low
shell component onto a preformed core[25]. However,
temperature of –77 oC in a trace moist stream of
it is necessary to do some surface modification on the
normal feed gas[2]. They attributed this to the
core in advance so as to avoid independent
abundance of active Co3+ species on {110} planes of
nucleation
the Co3O4 nanorods. Besides, the size of Co3O4
layer-by-layer technology is a multistep process that
nanostructure is also thought to be important in
requires
determining its catalytic activity
. However, there
modification, so it is not conducive to large-scale
is still few reports concerning optimizing the
synthesis and has seriously limited the practical
stability of Co3O4 catalysts, because for practical
applications of such catalysts. Besides, the reverse
needs, catalysts are often required to be working at
micelle method can be used to prepare core@shell
relatively high temperatures without removing a
structures[6,9]. However, the synthetic procedure is
mass
conditions,
also multistep and consumes considerable time and
nanomaterials are apt to aggregate or deform,
energy. Meanwhile, to get the specific core@shell
resulting in heavy loss of catalytic active centers
structures, organic species such as surfactants have
and
even
been largely used[25,26], some of which are hard to be
inactivation. Therefore, the synthesis of Co3O4
removed completely, and hence the catalytic active
catalysts with high activity as well as stability has
centers of nanocatalysts might be contaminated,
become an area of great focus in material science.
resulting
[1-5]
of
streams.
serious
Under
catalytic
[4,5]
such
deterioration
and
of
the
precise
in
shell
control
and
unsatisfactory
component.
complex
catalytic
This
surface
activity.
Fabrication of core@shell structures has been
Consequently, it seems more meaningful to develop
identified as an efficient way to inhibit agglomeration
an effective way to realize the facile, clean and mass
so as to improve the stability of nanomaterials[6-15]. In
production of Co3O4@CeO2 core@shell structures.
this consideration numerous kinds of oxides, such as
Here, we report the synthesis of high-quality
CeO2, SiO2, and ZrO2, have been adopted as the
Co3O4@CeO2 core@shell structures in gram level.
stable shell components[6,7,8,12,15]. In particular, CeO2, a
First,
typical kind of multifunctional rare earth oxide,
prepared as precursors
receives intense attention due to its wide applications
coated by a CeO2 shell followed by the previously
in catalysis[16-25]. It possesses strong oxygen storage
reported strategy[28]. After calcination in air, the
capacity that makes it highly active in oxidation
as-obtained
reaction. More importantly, it can also show excellent
core@shell nanowires can be thermally decomposed
synergistic
and transformed into the final monodisperse
effects
with
other
catalytic
active
Co(CO3)0.5(OH)·0.11H2O
[27]
nanowires
were
, and then they were
Co(CO3)0.5(OH)·0.11H2O@CeO2
Address correspondence to Dapeng Liu, [email protected]; Hongjie Zhang, [email protected]
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3Nano Res.
Co3O4@CeO2 core@shell nanowires built up by
were purified by centrifugation and washed with
Co3O4 and CeO2 nanoparticles (NPs). In order to
deionized water and ethanol for three times, and
investigate the transformation process, thermal
then
gravimetric
of
Co(CO3)0.5(OH)·0.11H2O@CeO2-1. By tuning the
Co(CO3)0.5(OH)·0.11H2O@CeO2 has been done in
amount of Ce(NO3)3 and HMT, another three
combination with the CO catalytic test, X-ray
Co(CO3)0.5(OH)·0.11H2O@CeO2
diffraction (XRD), scanning electron microscope
synthesized followed by the above procedure, and
(SEM) and transmission electron microscopic (TEM)
the
analyses.
Co(CO3)0.5(OH)·0.11H2O@CeO2-2
analysis
Then
the
(TGA)
influence
of
calcination
dried
at
60
as-obtained
o
C
and
precursors
products
(1.3
Co(CO3)0.5(OH)·0.11H2O@CeO2-3
(0.325
systematically investigated to study the optimal
Ce(NO3)3,
solution)
condition for catalytic CO oxidation.
Co(CO3)0.5(OH)·0.11H2O@CeO2-4
10
mL
of
HMT
HMT
as
mmol
Co3O4@CeO2
was
of
named
Ce(NO3)3,
nanowires
mL
were
as
were
temperatures on the catalytic performance of
core@shell
30
named
solution),
mmol
(0.16
and
mmol
Ce(NO3)3, 5 mL of HMT solution), respectively.
2 Experimental
Preparation of Co(CO3)0.5(OH)·0.11H2O nanowires
nanowires:
(Co precursor): Co(CO3)0.5(OH)·0.11H2O nanowires
Co(CO3)0.5(OH)·0.11H2O@CeO2-1 were calcined at
were
250, 350 and 500
synthesized
by
hydrothermal procedure
a
[27]
previously
reported
. Typically, 0.56 g of
Preparation
corresponding
of
Co3O4@CeO2
The
o
core@shell
precursor
of
C for 3 h in air, and the
products
are
named
as
CoSO4·7H2O was dissolved in 40 mL of a mixture
Co3O4@CeO2-1-250,
containing 7 mL of glycerol and 33 mL of deionized
Co3O4@CeO2-1-500,
water. After stirred for about 10 min, a transparent
Co(CO3)0.5(OH)·0.11H2O@CeO2-2, -3, and –4 were all
solution was obtained, into which 0.10 g of urea
calcined at 250 oC for 3 h in air as well, and the
was then added. 30 min later, the solution was
corresponding
transferred into a 50 mL Teflon-lined stainless steel
Co3O4@CeO2-2-250,
autoclave, followed by heating at 170
Co3O4@CeO2-4-250, respectively.
o
C for a
period of 24 h in an electric oven. Afterwards the
autoclave
was
cooled
naturally
to
room
temperature. The products were collected and
Co3O4@CeO2-1-350
respectively.
products
Preparation
were
For
control,
named
Co3O4@CeO2-3-250,
of
Co3O4
and
as
and
nanowires:
Co(CO3)0.5(OH)·0.11H2O nanowires were directly
calcined at 250 oC for 3 h in air.
washed with deionized water and ethanol for three
Preparation of Co3O4-CeO2 hybrids: 0.03 g of the
times by centrifugation, and then dried at 60 oC
as-prepared Co3O4 nanowires were ultrasonically
overnight.
dispersed in a mixed solution of 12 mL of water and
Preparation
of
Co(CO3)0.5(OH)·0.11H2O@CeO2
12 mL of ethanol, and then 0.24 mmol Ce(NO3)3 and
core@shell nanowires (Co precursor@CeO2): 0.1 g of
10 mL of 0.02 g/mL HMT aqueous solution were
Co(CO3)0.5(OH)·0.11H2O
were
added in turn. Then the temperature of the solution
ultrasonically dispersed in a mixed solution of 50
was increased to 70 oC and refluxed for 2 h before
mL of water and 50 mL of ethanol, and then 0.65
being cooled to room temperature. The products
mmol
g/mL
were purified by centrifugation and washed with
hexamethylenetetramine (HMT) aqueous solution
deionized water and ethanol for three times, and
were added in turn. Then the temperature of the
then dried at 60 oC.
Ce(NO3)3
and
nanowires
20
mL
of
0.02
solution was increased to 70 oC and refluxed for 2 h
Preparation of pure CeO2 NPs: 1 mmol Ce(NO3)3
before cooled to room temperature. The products
was dissolved in a mixed solution of 20 mL of
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Nano Res.
4
deionized water and 20 mL of ethanol. Then 25 mL
CO2. For each time, 30 mg of the sample was heated
of 0.02 g/mL HMT aqueous solution was added.
from room temperature to 900 °C at a rate of
Then the temperature of the mixture was increased
10 °C/min. A gaseous mixture of 5 vol. % H 2 in N2
to 70 C and refluxed for 2 h before being cooled to
was used as reductant at a flow rate of 20 mL/min.
o
room temperature. The products were purified by
Catalytic tests: 25 mg of catalysts were put in a
centrifugation and washed with deionized water
stainless steel reaction tube. The CO oxidation
and ethanol for three times, and then dried at 60 oC.
catalytic tests were performed under an atmosphere
Finally, the products were calcined in air at 250 oC
of 1 % CO and 20 % O2 in N2 at a fixed space
for 3 h in air.
velocity of 50 mL/min. The composition of the gas
Preparation of Co3O4-CeO2 mixtures: 0.058 g of
was monitored on-line by gas chromatography.
the above mentioned Co3O4 nanowires and 0.042 g
of CeO2 NPs were physical mixed by grinding in an
3 Results and discussion
agate mortar for half an hour.
Characterization: The XRD data of the products
were collected on a Rigaku-D/max 2500 V X-ray
diffractometer with Cu-K radiation ( = 1.5418 Å ),
with an operation voltage and current maintained
at 40 kV and 40 mA. TEM images were obtained
with a TECNAI G2 high-resolution transmission
electron microscope operating at 200 kV. A
HITACHI S-4800 field emission scanning electron
microscope (FE-SEM) was used to characterize the
morphology of the samples. X-ray photoelectron
spectroscopy (XPS) measurement was performed
on
an
ESCALAB-MKII
250
photoelectron
spectrometer (VG Co.) with Al-K X-ray radiation
Figure 1. (A) SEM and (B and C) TEM images of Co precursor;
(D) SEM and (E and F) TEM images of Co precursor@CeO2
(Inset: HRTEM of CeO2).
as the X-ray source for excitation. TGA curves of the
sample was acquired by using a SDT 2960 thermal
analyzer at a heating rate of 10 oC min-1 in air
atmosphere within a temperature range between 20
and 700 oC. A GC 9800 gas chromatography tester
was employed to obtain the CO conversion curves
of the samples. N2 sorption isotherms were
obtained at 77 K on an Auto-sorb-1 apparatus.
Scheme 1. Schematic process for preparation of Co3O4@CeO2
Inductively coupled plasma (ICP) analyses were
core@shell nanowires.
performed
with
a
Varian
Liberty
200
spectrophotometer to determine the Ce content.
H2-temperature-programmed reduction (TPR) was
conducted on a TPDRO 1100 apparatus supplied by
Thermo-Finnigan Company. Before detection by the
TCD, the gas was purified by a trap containing CaO
+ NaOH materials in order to remove the H2O and
The as-obtained samples were characterized by
SEM and TEM. From the SEM and TEM images
(Figure 1A to 1C), it can be clearly seen that Co
precursor is composed by uniform and well
dispersed nanowires with several micrometres in
length and tens of nanometers in width. After
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5Nano Res.
coated with a CeO2 shell, the smooth surface of Co
of
precursor nanowire becomes obviously rough
Co(CO3)0.5(OH)·0.11H2O@CeO2-1. Figure 3 shows
(Figure 1D to 1F), indicating the success of CeO2
that the major weight loss (about 10 %) takes place
shell coating process. The as-prepared products
in the temperature range of 230 to 300 °C, which is
well keep the wire-like morphology as Co precursor
in consistence with its corresponding DSC analysis
and each nanowire is completely wrapped by a
(Figure S4). This part of loss should be attributed to
shell built up by hundreds of 6 nm sized CeO2 NPs
the decomposition of carbonates and hydroxide
self-assembled together. The inset in Figure 1F
groups
shows the lattice spacing of 0.31 nm which
Coincidentally, the first cycling curve of CO
corresponds well to the characteristic (111) plane of
conversion fully supports the TGA-DSC results that
fluorite-phase CeO2. Combining with the XRD
above 230 °C the sample can totally catalyze CO
results (Figure S1) it firmly demonstrates the
oxidation
core@shell
Co(CO3)0.5(OH)·0.11H2O into Co3O4.
structure
formation
of
the
decomposition
of
process
of
Co(CO3)0.5(OH)·0.11H2O[27].
due
to
the
transformation
of
Co(CO3)0.5(OH)·0.11H2O@CeO2. More than 3 g of
Co(CO3)0.5(OH)·0.11H2O@CeO2 can be obtained in
250
one pot (see Figure S2), and its schematic
o
process as described in Scheme 1.
Catalytic oxidation of CO is chosen here as the
model
reaction
to
evaluate
200
T100 ( C)
fabrication has been summarized to a two-step
the
about
the
transformation
Decrease
Stable
100
catalytic
performance of the samples. In order to study the
details
150
50
of
Co(CO3)0.5(OH)·0.11H2O as well as its influence on
0
1
the catalytic performance, the CO oxidation cycling
2
3
4
5
6
7
8
9
10
Cycle
tests of Co(CO3)0.5(OH)·0.11H2O@CeO2-1 have been
done in the temperature range from 50 to 250 oC. As
Figure 2. Cycling tests of Co(CO3)0.5(OH)·0.11H2O@CeO2-1
shown in Figure 2 and S3, it can be found that
for CO conversion.
during the tests the value of T100 (the temperature
o
C in the following cycles. In general, catalysts often
degrade more or less under long-term and
high-temperature
catalytic
conditions
due
to
aggregation, growth or some other reasons like
poisoning. The abnormal enhancement of the
catalytic activity aroused our great interests to
investigate this phenomenon in depth.
100
120
80
110
60
100
40
90
20
80
0
70
50
As reported by Lou, et al[27], the transformation of
100
150
200
250
300
Weight Loss (%)
fifth cycle to 160 oC and then remained stable at 160
CO Conversion (%)
for 100 % CO conversion) kept decreasing until the
350
Temperature ( C)
o
Co(CO3)0.5(OH)·0.11H2O to Co3O4 starts at about 200
o
C, so it is considered that such transformation
would proceed during the catalytic process. Firstly,
Figure 3. TGA curve and the first cycling curve of CO
conversion of Co(CO3)0.5(OH)·0.11H2O@CeO2-1.
TGA was employed to get the detailed information
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Nano Res.
Further
6
insight
into
the
transformation
Co(CO3)0.5(OH)·0.11H2O@CeO2-1
requires
of
XRD
analysis. After ten cycling tests for CO oxidation,
the
sample
was
collected
and
labeled
as
Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after 10. In Figure
4, it can be clearly seen that the heat treatment
during
the
cycling
tests
transformation
results
of
in
obvious
orthorhombic-phase
Co(CO3)0.5(OH)·0.11H2O into spinel-phase Co3O4, as
the
intensity
of
the
corresponding
peak
of
Co(CO3)0.5(OH)·0.11H2O decreased quickly, but that
of Co3O4 was gradually enhanced. Based on the
above analysis of CO catalysis associated with TGA
and XRD curves, it confirms that the transformation
of Co(CO3)0.5(OH)·0.11H2O into Co3O4 happened
Figure
4.
XRD
patterns
Co(CO3)0.5(OH)·0.11H2O@CeO2-1,
Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after
10
Co3O4@CeO2-1-250.
of
and
(A)
(B)
(C)
during the whole cycling tests process, which might
start
from
the
surface
parts
of
Co(CO3)0.5(OH)·0.11H2O nanowires to the inner. As
the cycling tests continued, more and more
Co(CO3)0.5(OH)·0.11H2O
component
was
decomposed, and after five cycles, its surface parts
that do work for catalytic CO oxidation transformed
completely into Co3O4. That is why at this stage
their catalytic activity became better and the T100
was decreased continuously. For the last five cycles,
the conversion possibly proceeded in the inner
parts of the Co(CO3)0.5(OH)·0.11H2O nanowires,
therefore the T100 remained unchanged any more.
These results suggest that only surface Co3O4
Figure 5. (A to C) TEM images of Co3O4@CeO2-1-250, (D
and E) the corresponding EDX mapping analysis.
components adjacent to CeO2 worked well for
while the peak intensity of Co3O4 becomes more
catalytic CO oxidation. That is to say CO oxidation
intense. TEM (Figure 5) and SEM images (Figure S5)
possibly takes place at the interface of Co 3O4 and
show that the wire-like structure maintained well
CeO2 components
after either CO catalytic test or calcination.
[29-30]
.
To confirm the above judgement on the catalytic
CO
oxidation
process,
constant
exists in the core position of the nanowires, and
calcination temperature of 250 oC and prolonged
element Ce distributes wider which is a typical shell
the calcination time to 3 h to realize the complete
feature. The absence of Co peaks and the presence
transformation
of
of
we
kept
a
Elemental mapping indicates element Co only
Co(CO3)0.5(OH)·0.11H2O
into
Ce
peaks
in
the
XPS
spectrum
of
Co3O4. The as-obtained product is labeled as
Co3O4@CeO2-1-250 (see Figure S6) further verifies
Co3O4@CeO2-1-250. Its corresponding XRD pattern
the thick shell coating of CeO2. The catalytic test
(Figure
shows that Co3O4@CeO2-1-250 can also catalyze
4)
shows
that
the
peaks
of
Co(CO3)0.5(OH)·0.11H2O completely disappear,
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7Nano Res.
100 % CO conversion at 160 oC (see Figure 6) which
reveals that the CeO2 shell coating efficiently
is
prevented these nanowires from aggregation when
the
same
with
that
of
Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after 10. All these
proofs
point
to
the
fact
calcined.
that
Co(CO3)0.5(OH)·0.11H2O@CeO2 should go through a
intermediate
core@shell@shell
state
of
Co(CO3)0.5(OH)·0.11H2O@Co3O4@CeO2, and finally
turns into Co3O4@CeO2, as described in Scheme 1.
Despite the difference of the core components
between Co(CO3)0.5(OH)·0.11H2O@Co3O4-1-after 10
and Co3O4@CeO2-1-250, they show much similar
catalytic activities, firmly indicating that only those
surface Co3O4 components interfaced with CeO2 do
work
well
for
catalytic
CO
oxidation.
Conversion of CO (%)
100
80
Co3O4 @CeO2-1-250
60
Figure 7. (A and B) TEM images of Co3O4 nanowires, and (C
40
and D) Co3O4-CeO2 hybrids.
20
Co3O4 nanowires
50
100
150
200
250 o 300
Temperature ( C)
350
Figure 6. CO conversion curve of Co3O4@CeO2-1-250.
However, until now it is still inconclusive
whether coating a CeO2 shell could improve the
Conversion of CO (%)
100
0
Co3O4-CeO2 mixtures
Co3O4-CeO2 hybrids
80
CeO2 NPs
60
40
20
0
catalytic activity and stability against calcination of
the Co3O4 nanowires or not, so comparative studies
50
100
150
200
250
o
300
350
Temperature ( C)
have been performed towards the following four
samples of Co3O4 nanowires, Co3O4-CeO2 mixtures,
Co3O4-CeO2 hybrids and pure CeO2 NPs (details see
Figure 8. CO conversion curves of Co3O4 nanowires,
Experimental Section). As shown in Figure 7 and S7,
Co3O4-CeO2 mixtures, Co3O4-CeO2 hybrids and pure CeO2
all of the as-obtained Co3O4 nanowire samples lost
NPs.
their original wire-like morphology, and most of
them
aggregated
comparison,
together
into
Co3O4@CeO2-1-250
bundles.
In
showed
well-dispersed wire-like core@shell structure. It
For Co3O4-CeO2 hybrids, the CeO2 shell coating
was
fabricated
after
the
calcination
process,
resulting in irregular CeO2 coated Co3O4 bundles. In
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Nano Res.
8
other words, the CeO2 shell coating should proceed
physical and chemical properties of materials[3,33].
before calcined, so Co3O4 nanowires could be
Calcination time, calcination atmosphere, especially
efficiently prevented from aggregation during the
calcination temperature could greatly affect the
calcination process, resulting in the remarkably
catalytic performance of the catalysts[12,26]. So the
improved structure stability of Co3O4 catalysts. The
effects of calcination temperature need to be further
sizes of Co3O4 NPs in Co3O4 nanowires, Co3O4-CeO2
investigated towards our core@shell catalysts.
mixtures and Co3O4-CeO2 hybrids are 9.9, 10.0 and
Co(CO3)0.5(OH)·0.11H2O@CeO2-1 precursors were
10.1 nm, respectively (XRD patterns see Figure S8),
then calcined at 350 and 500 oC, and thus obtained
calculated by the Scherrer equation. Whereas the
products are named as Co3O4@CeO2-1-350 and
Co3O4 NPs in Co3O4@CeO2-1-250 show a much
Co3O4@CeO2-1-500, respectively.
smaller size of about 5.9 nm. Obviously, the coating
Figure
of CeO2 shell leads to much smaller Co3O4 NPs,
Co3O4@CeO2-1-500
which might be responsible for the optimization of
core@shell
the catalytic activity.
Co3O4@CeO2-1-250.
S9
and
S10,
As shown in
Co3O4@CeO2-1-350
are
in
structures
similar
and
wire-like
compared
with
In the following, the influence of the CeO2 shell
was discussed on the catalytic activity of Co 3O4
catalysts. As shown in Figure 8, Co3O4 nanowires
can catalyze 100 % CO conversion at 360 oC. More
worse, the CO conversion for pure CeO2 NPs was
only 40 % at 350 oC. While Co3O4-CeO2 mixtures
and Co3O4-CeO2 hybrids can catalyze 100 % CO
conversion at lower temperatures of about 320 and
300
o
C, respectively. The enhancement of their
catalytic
activities
could
be
ascribed
to
the
synergistic effects between Co3O4 and CeO .
2 29-31
However, Co3O4@CeO2-1-250 can catalyze 100 % CO
conversion at a much lower temperature of 160 oC.
Figure 9. XRD patterns of (A) Co3O4@CeO2-1-250, (B)
The optimal catalytic activity of Co 3O4@CeO2-1-250
Co3O4@CeO2-1-350, and (C) Co3O4@CeO2-1-500.
compared to Co3O4-CeO2 mixtures and Co3O4-CeO2
hybrids could be ascribed to the fabrication of the
XRD patterns in Figure 9 present that the peaks of
well-dispersed core@shell structures explained by
CeO2 show no difference among Co3O4@CeO2-1-250,
the following points: (1) the maximized interface
Co3O4@CeO2-1-350
area resulting from the well-dispersed core@shell
However, there are some obvious differences of the
structure, which is beneficial for CO oxidation; (2)
Co3O4 peaks among the three samples. As the
the smaller Co3O4 size resulted from the effective
calcination temperature was increased from 250 oC
CeO2 shell coating of the core@shell structure. This
to 350 oC to 500 oC, the intensity of the Co3O4 peaks
comparative
become stronger, and the peaks become sharper
test
well
supports
the
above
indicating
core@shell structures is efficient to optimize the
Co3O4@CeO2-1-500 than Co3O4@CeO2-1-350 than
catalytic activity and stability of Co3O4 catalysts.
Co3O4@CeO2-1-250. The size of the Co3O4 NPs are
important to exert a significant impact on the
better
Co3O4@CeO2-1-500.
hypothesis that the fabrication of Co3O4@CeO2
As known, calcination process is fundamentally
its
and
crystallinity
of
5.9, 10.1 and 12.7 nm for Co3O4@CeO2-1-250,
Co3O4@CeO2-1-350
and
Co3O4@CeO2-1-500,
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9Nano Res.
respectively. The XPS spectra of Co3O4@CeO2-1-350
and Co3O4@CeO2-1-500 in Figure S11 both show five
Ce peaks and no obvious Co peaks, which are
o
403 C
similar to Co3O4@CeO2-1-250. It suggests both
similar
shell
coating
of
CeO2
compared
to
Co3O4@CeO2-1-250. The N2 adsorption–desorption
isotherm of the three samples is depicted in Figure
S12, indicating Type IV behavior of nanoporous
Co3O4@CeO2-1-250,
Co3O4@CeO2-1-350
Intensity (a.u.)
Co3O4@CeO2-1-350 and Co3O4@CeO2-1-500 have the
o
308 C
o
410 C
E
o
× 3 750 C
o
316 C
o
420 C
o
×3
D
o
357 C
750 C
o
487 C
o
×3
C
750 C
o
374 C
and
B
Co3O4@CeO2-1-500 with high surface area of 144.9,
A
o
o
380 C
121.4, 64.0 m2g-1 and average pore width of 6.14,
200
750 C
400
600
800
o
Temperature ( C)
8.11, 9.79 nm, respectively.
The test of catalytic CO oxidation (Figure 10) was
then
conducted
performance
to
of
evaluate
the
Co3O4@CeO2-1-350
catalytic
Figure 11. H2-TPR profiles: (A) pure CeO2; (B) Co3O4
and
nanowires; (C) Co3O4@CeO2-1-500; (D) Co3O4@CeO2-1-350
Co3O4@CeO2-1-500 compared with Co3O4@CeO2-250.
and (E) Co3O4@CeO2-1-250.
The T100 of the three Co3O4@CeO2 samples follows
such an order: Co3O4@CeO2-1-250 (160 °C) <
for CeO2 can be attributed to the reduction of
Co3O4@CeO2-1-350 (250 °C) < Co3O4@CeO2-1-500 (>
surface capping oxygen and bulk oxygen of CeO2,
380 °C). Co3O4@CeO2-1-250, which is obtained by
respectively[12,32]. The two peaks at around 374 °C
calcination at the lowest temperature shows the
and 487 °C in Figure 11B could be attributed to the
highest catalytic activity. In order to study the
two reduction steps of Co3O4 species[1]. It can be
synergetic effects of Co3O4 and CeO2, the catalysts
seen from Figure 11C-E that the Co3O4 reduction
were investigated by H2-TPR. Two broad TPR
peaks of Co3O4@CeO2-1-250, Co3O4@CeO2-1-350 and
peaks (Figure 11A) observed at 380 °C and 750 °C
Co3O4@CeO2-1-500
all
shifted
towards
lower
temperature to about 308, 316, 357 oC for the first
peak and 403, 410, 420 oC for the second peak,
Conversion of CO (%)
100
80
Co3O4@CeO2-1-350
respectively, indicating a typical synergistic effect
Co3O4@CeO2-1-500
between Co3O4 and CeO2. The previous work
reported that the a lower calcination temperature
60
favors reducing the degree of Co3O4@CeO2 interface
breakage that improves the oxidizability of Co3O4.26
40
That is why the oxidizability of Co3O4 in these
20
samples
0
follows
Co3O4@CeO2-1-250
50
100
150
200
250o 300
Temperature ( C)
350
such
>
a
sequence
Co3O4@CeO2-1-350
that
>
Co3O4@CeO2-1-500, which is in agreement with the
changing trends of their catalytic activities. If we
Figure 10. CO conversion curves of Co3O4@CeO2-1-350 and
enlarge the curve in the temperature range of
Co3O4@CeO2-1-500.
650 °C to 900 °C for three times, the signal at 750 °C
for CeO2 could be still clearly seen, indicating the
existence of bulk oxygen of CeO2. Based on the
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Nano Res.
10
above results and discussions, it is concluded that
Co3O4@CeO2-1-250, whereas the intensity ratio of
the optimal catalytic activity of Co3O4@CeO2-1-250
CeO2 to Co3O4 peaks is decreased with the decreased
can be ascribed to the following reasons. (1) better
thickness of CeO2 shell.
oxidizability of Co3O4, which might be caused by
the lower degree of Co3O4@CeO2 interface breakage
100
Conversion of CO (%)
resulting from the lower calcination temperature[26];
(2) the smaller sized Co3O4 NPs than the other two
samples; (3) the bigger BET surface area than the
other two samples[34]; (4) worse crystallinity of
Co3O4@CeO2-1-250 that might bring more surface
defects and thus higher surface energy, which are in
80
Co3O4@CeO2-2-250
60
Co3O4@CeO2-3-250
Co3O4@CeO2-4-250
40
20
favor of the CO adsorption, resulting in the optimal
0
catalytic activity for CO oxidation .
[3]
50
100
150
Besides calcination temperatures, the component
ratio of hetero-catalysts also play a significant role in
the catalytic performance[32]. So by simply varying
200
250
300
o
350
Temperature ( C)
Figure 12. CO conversion curves of Co3O4@CeO2-2-250,
Co3O4@CeO2-3-250, and Co3O4@CeO2-4-250.
the amount of Ce(NO3)3, a series of Co3O4@CeO2
core@shell nanowires have been synthesized to
investigate the effects of component ratio between
270
Co3O4 and CeO2 on their catalytic activities. The
Co3O4@CeO2-2-250,
Co3O4@CeO2-4-250
are
named
Co3O4@CeO2-3-250,
(experimental
details
as
240
and
o
samples
T ( C)
corresponding
see
Experimental Section). As shown in Figure S13 and
210
180
S14, the three comparative samples show similar
core@shell wire-like structure to Co3O4@CeO2-1-250,
150
except for the CeO2 shell thickness. The average
0
10
diameters of Co3O4@CeO2 core@shell nanowires,
estimated by the size distribution data, are 120, 95, 65,
and 53 nm for Co3O4@CeO2-2-250, Co3O4@CeO2-1-250,
Co3O4@CeO2-3-250,
respectively,
and
indicating
that
the
ICP-MS. As shown in Table S1, the Ce molar contents
are 50.3 %, 38.5 %, 18.2 % and 8.9 % for
Co3O4@CeO2-2-250,
Co3O4@CeO2-1-250,
and
Co3O4@CeO2-4-250,
respectively. Figure S15 presents the XRD patterns of
Co3O4@CeO2-3-250,
and
Co3O4@CeO2-4-250 that he peak positions and shapes
of
the
three
samples
40
50
60
Figure 13. The relationship of Ce molar contents and the
catalytic activities of the Co3O4@CeO2 samples.
corresponding
thinner. The Co and Ce contents were determined by
Co3O4@CeO2-2-250,
30
Co3O4@CeO2-4-250
average CeO2 shell thicknesses become thinner and
Co3O4@CeO2-3-250,
20
Ce Content (mol %)
are
the
same
with
The catalytic performance on CO oxidation of the
three comparative samples were evaluated and the
results are shown in Figure 12. The catalytic activity
of
the
samples
follows
Co3O4@CeO2-1-250 (160
(170
o
C)
>
such
a
order:
C) > Co3O4@CeO2-3-250
o
Co3O4@CeO2-2-250
(240
o
C)
>
Co3O4@CeO2-4-250 (270 C). Figure 13 presents the
o
relationship between T100 and the Ce molar content
of the Co3O4@CeO2 core@shell samples. First, from
Co3O4@CeO2-4-250 to Co3O4@CeO2-3-250 and then
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Nano Res.
11
to Co3O4@CeO2-1-250, the catalytic activities were
Table 1. Characteristics of the as-obtained samples and their
enhanced with increasing Ce contents. However the
catalytic performance for CO oxidation.
catalytic
activity
decreased
in
the
case
of
Co3O4@CeO2-2-250 while further increasing the Ce
Sample
content. This suggests that the catalytic activities of
T
o
( C)
Size
[a]
(nm)
T100
[b]
(oC)
our Co3O4@CeO2 samples are highly dependent on
Co3O4@CeO2-1-250
250
5.9
160
the Ce molar content, and tuning component ratio
Co3O4@CeO2-1-350
350
10.1
250
of this kind of hetero-catalysts would be an efficient
Co3O4@CeO2-1-500
500
12.7
>380
way to optimize the catalytic performance.
Co3O4 nanowires
250
9.9
360
Co3O4-CeO2 mixtures
250
10.0
310
cubes were prepared by the similar self-assembly
Co3O4-CeO2 hybrids
250
10.1
300
process, however, the utilization of Co2+ was as low
Pure CeO2 NPs
250
-
>350
as about 10 %, which limited their practical
Co3O4@CeO2-2-250
250
5.9
240
applications. Here, the utilization of Co2+ in the
Co3O4@CeO2-3-250
250
5.9
170
preparation process of Co3O4@CeO2 core@shell
Co3O4@CeO2-4-250
250
5.9
270
In our previous work, Co3O4@CeO2 core@shell
nanowires has been increased to about 80 %. About
[a] Calcination temperature
2 g of Co(CO3)0.5(OH)·0.11H2O nanowires could be
[b] Average size of Co3O4 NPs calculated from XRD patterns
synthesized by a 500 mL Teflon-lined stainless steel
autoclave.
Then
more
than
3
g
of
Co(CO3)0.5(OH)·0.11H2O@CeO2-1 could be obtained
4 Conclusions
by the self-assembly process by a 1 L flask. After
calcination, more than 2.5 g of Co3O4@CeO2-1-250
In summary, we have successfully realized the
were obtained. In Figure S16, it can be clearly seen
facile, clean and mass production of Co3O4@CeO2
that
produced
core@shell nanowires as catalyst for catalytic CO
Co3O4@CeO2-1-250
oxidation. The catalytic performance of the samples
the
above
mentioned
Co(CO3)0.5(OH)·0.11H2O
mass
and
show no change in their wire-like structures.
has
Meanwhile Co3O4@CeO2-1-250 obtained by mass
experiments suggest that CO oxidation is apt to
production can catalyze 100 % CO conversion at the
take place at the interface between Co3O4 and CeO2
same temperature of 160 oC as well, which is more
components. The high catalytic activity and stability
active
Co3O4
of Co3O4@CeO2 core@shell nanowires should be
CeO2@Cu2O
possibly caused by the optimal synergistic effects of
than
nanowires,
the
previously
CeO2
reported
nanorods,
been
investigated
systematically.
Control
hollow
Co3O4 and CeO2 components resulting from the
nanotubes,
specific core@shell structure. It also suggests that
Co3O4@CeO2 cubes and ZnCo2O4@CeO2 spheres due
the catalytic activity of the Co3O4@CeO2 core@shell
to the lower conversion temperature or with lower
nanowires strongly depends on the calcination
weight of effective catalysts (see Table S1) [32, 34–40].
temperatures and component ratio between Co3O4
Hence
and
nanocomposite,
microsphere,
CeO2-ZnO
Ce-Mn
composite
binary
it is reasonably
oxide
considered
that our
CeO2.
Co3O4@CeO2-1-250
obtained
by
Co3O4@CeO2 core@shell nanowires with good
calcination at 250 oC for 3 h with a Ce molar content
catalytic activity and stability for CO oxidation
about 38.5 % shows the best catalytic activity,
might have great potential for practical application
attaining 100 % CO conversion at a temperature as
as automobile exhaust gas treatment catalysts.
low as 160 oC. It is believed that our Co3O4@CeO2
core@shell nanowires could be promising candidate
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Research
Nano Res.
12
catalysts for CO oxidation as automobile exhaust
gas treatment catalysts. This work supplies a
133, 11279–11288.
[6]
Lu, Z. H.; Jiang, H. L.; Yadav, M.; Aranishi, K.; Xu, Q.
feasible way to fabricate core@shell structures for
Synergistic catalysis of Au-Co@SiO2 nanospheres in
the exploration and optimization of this kind of
hydrolytic dehydrogenation of ammonia borane for
hetero-nanocatalysts.
chemical hydrogen storage. J. Mater. Chem. 2012, 22,
5065–5071.
[7]
Acknowledgements
Arnal,
P.
M.;
Comotti,
M.;
Schüth,
F.
High-temperature-stable catalysts by hollow sphere
encapsulation. Angew. Chem. Int. Ed. 2006, 45,
This work was supported by the financial aid from
the National Natural Science Foundation of China
8224–8227.
[8]
Ge, J. P.; Zhang, Q.; Zhang, T. R.; Yin, Y. D.
(Grant Nos. 91122030, 51272249, 21210001, 21221061
Core-satellite nanocomposite catalysts protected by a
and 21401186), and the National Key Basic Research
porous silica shell; controllable reactivity, high stability,
Program of China (No. 2014CB643802).
and magnetic recyclability. Angew. Chem. Int. Ed. 2008,
Electronic Supplementary Material: Supplementary
material is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
47, 8924–8928.
[9]
Zhang, T. T.; Zhao, H. Y.; He, S. N.; Liu, K.; Liu, H. Y.;
Yin, Y. D.; Gao, C. B. Unconventional route to
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