Three dimensionally ordered macroporous Ce1-xZr xO2 solid solutions for diesel soot combustion

Article abstract of DOI:10.1039/b915027g

The microstructure with open, interconnected macropores of 3DOM Ce _1-xZr_xO₂, successfully prepared using PMMA colloidal crystal as template and cerium nitrate and zirconium oxide chloride as raw materials, facilitates the

Full text of DOI:10.1039/b915027g

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Three dimensionally ordered macroporous Ce Zr O solid solutions
1
Àx x 2
for diesel soot combustionw
a
a
a
a
a
a
Guizhen Zhang, Zhen Zhao,* Jian Liu, Guiyuan Jiang, Aijun Duan, Jianxiong Zheng,
a
b
Shengli Chen and Renxian Zhou
Received (in Cambridge, UK) 23rd July 2009, Accepted 11th November 2009
First published as an Advance Article on the web 26th November 2009
DOI: 10.1039/b915027g
The microstructure with open, interconnected macropores of
DOM Ce1 Zr O , successfully prepared using PMMA
controlling the porosity of catalysts is highly desirable.
8
However, only a limited number of studies have been directed
3
Àx
x
2
colloidal crystal as template and cerium nitrate and zirconium
oxide chloride as raw materials, facilitates the contact between
soot and catalysts and results in much higher catalytic activity
for diesel soot combustion than the corresponding disordered
macroporous catalysts.
towards the macroporous catalysts for diesel soot combustion.
In recent years, three-dimensionally ordered macroporous
(3DOM) materials with uniform pore size and well-defined
periodic structure have become a focus research topic due to
their potential applications, such as catalysts, catalytic supports,
9
photonic crystals, chemical sensors and absorbents. Pure
Particulate matter (PM, mainly containing soot) emitted from
diesel engines has caused acute health problems to human
beings. The combination of traps and oxidation catalysts
ceria and zirconia oxides have been synthesized as macro-
porous structures with ordered pores. However, to the best of
our knowledge, there are very few reports on the synthesis of
1
10
appears to be one of the most efficient after-treatment techniques.
2
Several kinds of catalysts such as precious metal, perovskite
3DOM Ce1ÀxZr
Ce0.5Zr0.5O inverse opals and demonstrated that 3DOM
2
x
O
2
solid solutions. Haile
synthesized
3
4
or perovskite-like type oxides and ceria-based materials have
been widely studied for diesel soot combustion. Recently,
ceria-based materials are of intensive interest because they
are commercially employed as the components of automotive
three-way catalysts (TWCs), solid oxide fuel cells, and glass-
polishing material, and so forth. Especially Ce1 Zr O -type
materials have much better catalytic activity for propane
oxidation than corresponding samples with different archi-
tectures. Here, we extend the application of 3DOM ceria-
zirconia systems for diesel soot combustion and explore the
relationship between the inherent properties and pore channel
structure of 3DOM structures and their catalytic performance
for diesel soot combustion.
Àx
x
2
solid solutions are one of the key components in TWCs due to
their redox properties, oxygen storage and releasing capacity
5
and thermal stability. Either Ce-based composites with
The present work also reports synthesis of 3DOM solid
x 2
solutions of Ce1ÀxZr O by colloidal crystal templating
6
diameter in the nanometre region or those with ordered
method. To obtain 3DOM metal oxides, metal nitrates were
used as raw materials, which have a low melting point. The
solidification of these salts is necessary before the bursts of
template polymer, polymethyl methacrylate (PMMA) or
polystyrene (PS). One of the most effective methods to solidify
the salts with low melting point is the ligation method. The
chelators used most commonly are citric acid, EG, and so on.
7
mesoporous structure usually have high specific surface areas,
which may greatly improve their catalytic performance.
However, only the outer surfaces of these catalysts are avail-
able for soot combustion because their pore diameters are
much smaller than the particle diameter of soot (usually 425 nm).
If the pore sizes of these catalysts are big enough to permit
diesel soot to enter their inner pores where catalytic soot
combustion can proceed, the number of contact points
between soot and catalysts will be increased dramatically.
Then the catalytic activity for soot oxidation will be enhanced
remarkably. In view of the catalytic combustion of diesel soot,
1
1
12
2 2
Wu and Li successfully obtained 3DOM CeO and ZrO ,
using citric acid as ligand to produce a highly thermally-stable
chelate complex to meet the need of solidification. However,
x 2
the citric acid is unfit to fabricate Ce1ÀxZr O solid solutions
because the formation of the highly thermally-stable complex
needs a long reflux time. During the refluxing process the
mixed solutions containing both Ce and Zr ions will become a
gel which cannot infiltrate the voids of the colloidal crystal.
a
State Key Laboratory of Heavy Oil Processing, China University of
Petroleum, Beijing, China. E-mail: zhenzhao@cup.edu.cn;
Fax: +86-10 69724721; Tel: +86-10 8973 1586
Institute of Catalysis, Zhejiang University, Hangzhou, China.
8
Sadakane’s EG in situ nitrate oxidation method could not
b
only avoid the long reflux time, but also could ensure the
desired metal ratio throughout the whole preparation procedure.
E-mail: zhourenxian@zju.edu.cn; Fax: +86-5718273283;
Tel: +86-5718273283
w Electronic supplementary information (ESI) available: Detailed
description of experimental procedure; SEM images of products with
different Ce/Zr ratios; representative SEM images of disordered
Unfortunately, it is impossible to obtain 3DOM CeO
2 2
or ZrO
using this method, because the oxidation temperature of
1
3
cerium nitrate was relatively high
nitrate is insoluble in EG or methanol. Therefore, 3DOM
Ce1ÀxZr couldn’t be obtained using EG as chelator,
and the zirconium
macroporous Ce1ÀxZr
and soot embedded into the macropores of Ce0.7Zr0.3
TEM images of 3DOM Ce0.7Zr0.3 after soot combustion
reaction; catalytic activity data for diesel soot combustion. See DOI:
0.1039/b915027g
x
O
2
(x = 0.2, 0.3); TEM images of Printex U
2
O ; SEM and
x
O
2
O
2
cerium nitrate and zirconium nitrate as raw materials.
1
However, zirconium oxide chloride (ZrOCl Á8H O) is soluble
2
2
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 457–459 | 457

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in methanol, so it can be used to replace zirconium nitrate as
the source of zirconium ions. Therefore, ZrOCl O was
Á8H
added into an EG–methanol solution of cerium nitrate. Then
2
2
1
4
the heteropolynuclear complex containing ceric and zirconic
ions came into being at low temperature during the solvation
process, meaning that the solidification of the two salts
occurred before the decomposition of the template polymer.
At the same time, the long time refluxing process was avoided
by using EG as chelator and solvent. So the liquid precursor
containing both Ce and Zr ions with the desired Ce/Zr ratio
were obtained and can infiltrate smoothly into the voids of the
template spheres. Under the optimized conditions, a series of
Fig. 1 (a) SEM and (b) TEM images of 3DOM Ce0.7Zr0.3
inset in (a) shows an enlarged area of the 3DOM Ce0.7Zr0.3 , and the
one in (b) is the corresponding SAED pattern. The SEM images for
the 3DOM Ce1ÀxZr with other Ce/Zr ratios are shown in Fig. S1w.
2
O . The
O
2
3
DOM Ce1ÀxZr
obtained successfully. The above facile procedure to produce
DOM Ce–Zr solid solutions can be further used to fabricate
other 3DOM Zr-based solid solution materials, such as the
x 2
O with different Ce/Zr ratios can be
x 2
O
2
presence of ZrO monoclinic phase in all the XRD profiles of
3
solid solutions. The results indicate the formation of homo-
geneous Ce Zr O solid solutions. With decreasing ceria
3
+
3+
third kind of reducible cations (Pr , Y ) modified Ce–Zr
solid solutions. These 3DOM solid solution materials with
highly ordered macroporous structures have been successfully
synthesized and will be reported in other papers.
1Àx
x
2
content, the diffraction peaks of 2y show a slight upshift
compared with pure CeO . This effect is primarily due to
2
4
+
Zr
entering into the unit cell of ceria, leading to the
4+
shrinkage of the lattice, since the ion radius of Zr (0.086 nm)
Fig. 1 shows the field emission scanning electron microscope
4
+
is smaller than that of Ce (0.101 nm). The peaks are sharp
for the sample with low ceria, getting broader as the ceria
content is increased. This phenomenon is ascribable to the
presence of small crystallites. It is in agreement with crystallite
size data determined using the Scherrer equation from XRD
data (Table S1w).
(
FESEM) and transmission electron microscope (TEM)
images of the obtained Ce0.7Zr0.3O . The images showed that
2
the material was macroporous, containing periodic voids with
average diameter of 275 nm, which was smaller than 444 nm of
the PMMA sphere size, meaning that shrinkage of the solid
solution took place during the process of heat treatment. The
next layer was visible clearly in the enlarged SEM image
The UV-Raman spectra of 3DOM Ce Zr O (0 r x r 1)
1Àx
were presented in Fig. 2b. Six Raman-active modes
x
2
(
inset in Fig. 1a). The voids were interconnected through open
1
A1g + 2B1g + 3E ) are for t-ZrO , while for the cubic
7
(
windows, ca. 120 nm in diameter. The walls of the macro-
porous samples were crystalline, which was confirmed by
selected area electron diffraction (SAED) (inset in Fig. 1b).
Structural parameters of all 3DOM samples are summarized
in Table S1w. The Brunauer–Emmett–Teller (BET) surface
g
2
fluorite-type structure of CeO
8
2
, there is one Raman-active
À1
1
mode (F2g
)
centered at around 455 cm . A weak band
À1
À1
at B300 cm and a shoulder peak at B600 cm are also
observed, which could be attributed to the Raman inactive
19
2
À1
2
À1
transverse and longitudinal optical phonon modes, respectively.
areas were between 21.4 m g and 121.1 m g , which are in
1
6
For each solid solution sample, there are three Raman bands
À1
the range of typical macroporous oxides. The porosity of
DOM samples are around 90%, higher than those of corres-
located at B320, 455, and 630 cm . The band observed
at around 455 cm could be attributed to the symmetric
3
À1
ponding disordered macroporous (DM) samples (Fig. S2w).
Moreover, the macropore size distribution is narrow, while for
the corresponding DM samples the pore size distribution is
not monodispersed (Fig. S3w), indicating that the micro-
structures of 3DOM samples are more ordered, open and
interconnected than those of DM samples. It is suggested that
stretching mode (F ) of the oxygen atoms around cerium
2g
2
ions. The band at ca. 630 cm corresponded to a doubly
0
À1
degenerate longitudinal optical mode of CeO , and this band
2
could be linked to oxygen vacancies due to the substitution of
4
+
21
Zr into the ceria lattice. The appearance of a very weak
À1
band at 320 cm could be related to the displacement of the
2
3
DOM materials can have an advantage over DM samples
2
oxygen atoms from their ideal fluorite lattice positions.
À1
because of the lower diffusion barriers, which benefit the
contact efficiency between soot and catalytic active sites.
Furthermore, the peak at B630 cm
is relatively more
15
pronounced for the samples with x 4 0.3, while for x o 0.3
Another significant result was the absence of ‘template effect’.
À1
the peak at B455 cm is relatively dominant. These results
In general, the template effects are prominent, but using either
PMMA or PS as template, the 3DOM Ce1 Zr O could be
Àx
x
2
prepared under the same calcination conditions (Fig. S1w).
X-Ray diffraction patterns of 3DOM Ce1ÀxZr solid
solutions with different Ce/Zr ratios, pure CeO and ZrO
after calcination at 650 1C for 5 h are showed in Fig. 2a. For
pure ZrO , the XRD pattern shows a monoclinic symmetry.
The XRD peaks of Ce0.2Zr0.8 can be attributed to a tetra-
x 2
O
2
2
2
O
2
gonal phase (JCPDS 80-0785). With decreasing zirconium
content, the crystal phase gradually turns to fluorite cubic
phase of CeO , while the tetragonal phase and cubic phase
2
Fig. 2 (Left) XRD patterns, (Right) UV-Raman spectra of 3DOM
might coexist in Ce0.7Zr0.3O . There is no indication of the
2
Ce1ÀxZr
O
x 2
(0 r x r 1) samples calcined at 650 1C.
4
58 | Chem. Commun., 2010, 46, 457–459
This journal is ꢀc The Royal Society of Chemistry 2010

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were synthesized successfully from cerium nitrate and
zirconium oxide chloride by using PMMA and PS colloidal
crystal templates. Experimental results indicate that the open,
interconnected macroporous structure is a desirable feature
for diesel soot combustion. Further work on 3DOM
Ce1 Zr O as catalyst supports is in progress for application
Àx
x
2
in diesel soot combustion.
This work was supported by the National Natural Science
Foundation of China (20833011 and 20803093), and the
8
63 program of China (2006AA06Z346).
Fig. 3 Effect of the Ce–Zr ratio on the catalytic performance of
Ce–Zr macroporous samples for soot combustion.
Notes and references
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4
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O
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0 r x r 1) solid solutions are compared and shown in
Fig. 3 and Fig. S4w. All the catalysts are active in promoting
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CeO and Ce0.7Zr0.3O exhibit the best performance under the
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2
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2
8
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agreement with previous investigations on non-macroporous
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ratios, 3DOM samples show much higher catalytic activity
1
ˇ
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ˆ
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(
i.e., the lower combustion temperature) than DM ones.
1
Compared with that of DM samples, the T50 of 3DOM
samples can be lowered by 51 1C at most, and it can be seen
that the catalytic activities for soot oxidation are related to
surface area (Fig. S6w). However, no proportional relation was
observed between the catalytic activity and surface area
indicating that, besides surface area, there are other factors
affecting the catalytic activity for soot oxidation. For instance,
the intrinsic properties including phase structure, redox
property, and pore channel structure. Furthermore, the
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DOM structure was retained after soot oxidation reaction
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(
Fig. S7w). It indicates that these 3DOM Ce1 Zr O samples
Àx
are thermally stable and can be used repeatedly. Therefore,
x
2
these results strongly suggested that the 3DOM Ce1ÀxZr
should have application potential in various catalytic fields,
especially in diesel soot combustion.
x
O
2
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2
2
x 2
In summary, Ce1ÀxZr O solid solutions with three-
´
´
´
dimensional, periodic arrays of interconnected macropores
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 457–459 | 459