Nanometric colloidal sols of CeO2ZrO2 solid solution as catalyst modifiers. I. Preparation and structure

Article abstract of DOI:10.1246/bcsj.20130159

Stable nanometric colloidal sols consisting of fluorite-type metal oxides were prepared for use as chloride-free precursors for automotive catalysts. The aqueous solution containing metal carbonates and tetramethylammonium (TMA) ions was degassed and hydr

Full text of DOI:10.1246/bcsj.20130159

1
210 Bull. Chem. Soc. Jpn. Vol. 86, No. 10, 1210­1215 (2013)
© 2013 The Chemical Society of Japan
Nanometric Colloidal Sols of CeO ­ZrO Solid Solution
2
2
as Catalyst Modifiers. I. Preparation and Structure
Kenji Tanimoto,1,2 Hirokazu Kato, Miyuki Hidaka, Satoshi Hinokuma,
2
1
3
Keita Ikeue, and Masato Machida*1
1
1Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology,
Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555
2Specialty Materials Research Laboratories, Nissan Chemical Industries, Ltd., 11-1 Kitasode, Sodegaura, Chiba 299-0266
3Unit of Elements Strategy Initiative for Catalysts and Batteries, Kyoto University,
Kyoto Daigaku Katsura, Sakyo-ku, Kyoto 615-8520
Received June 6, 2013; E-mail: machida@kumamoto-u.ac.jp
Stable nanometric colloidal sols consisting of fluorite-type metal oxides were prepared for use as chloride-free
precursors for automotive catalysts. The aqueous solution containing metal carbonates and tetramethylammonium (TMA)
ions was degassed and hydrothermally treated at 140­160 °C. Colloidal suspensions thus formed were purified by
ultrafiltration to yield very stable and condensed sol solutions of CeO2 (CE), ZrO2 (ZR) and their solid solutions, CeO2­
ZrO2 (CZ), which were stabilized by TMA. Each oxide sol contained 6­8 nm primary particles, but they formed
aggregates larger than 40 nm. Local structural analysis by means of XAFS suggested that CZ sol exhibited two types of
solid solution domains containing more or less Ce. The dried CZ sol exhibited higher thermal stability and oxygen storage
capacity than CE and ZR sols.
In the current automotive three-way catalyst (TWC) formu-
lation, metal oxides with fluorite-type structure play extremely
expected to play critical roles as catalyst precursors because
their nanoparticles of narrow size distribution and high reac-
tivity are able to promote the catalytic performance. Easy
handling of sols is another benefit suitable for the present
catalyst preparation routes. Although traditional oxide sols
such as Al O and SiO have been used as a binder in the slurry
for preparation of monolithic honeycomb catalysts, CeO2-
based oxides in the form of sol have not been recognized as
main catalyst precursors.
1­5
important roles.
CeO -based oxides with varying oxygen
2
stoichiometries are able to store a large amount of oxygen
under an oxidizing atmosphere and release it under a reducing
atmosphere and thus achieve a buffering effect on fluctuating
2
3
2
6
­14
29
air-to-fuel ratio.
The uptake of oxygen allows NO con-
version to continue during rich-to-lean transients, whereas the
release of oxygen promotes oxidation of CO and hydrocarbons
during lean-to-rich transients. CeO2­ZrO2 solid solutions are
most widely used for this purpose because of large oxygen
The present work aims to study preparation and structure
of colloidal sols of fluorite-type metal oxides, CeO , ZrO , and
2
2
storage capacity (OSC) and high thermal stability.15 CeO is
their composite system (CeO ­ZrO ) as precursors of automo-
2 2
2
also known as a useful support material to effectively suppress
tive catalysts. The three types of nanometric and chloride-free
colloidal suspensions were prepared by hydrothermal reactions
followed by ultrafiltration. The local structure of as-prepared
CeO ­ZrO sols and their thermal behavior are especially
sintering for Pt and Pd.1
6,17
The interactions between these
metals and CeO2 under oxidizing atmosphere promote the
redispersion of metal crystallites and thus contribute to the
2
2
sintering inhibition mechanism. Furthermore, CeO shows cata-
highlighted, because of the practical importance as the most
efficient oxygen storage material. The dynamic oxygen release­
storage cycles were carried out to evaluate the oxygen storage
property of dried sols.
2
18,19
lytic activities for water­gas shift reactions,
ing, and combustion of soot emitted from diesel engines,
steam reform-
2
0
21­27
Another type of fluorite-type oxide, ZrO , is utilized as a
2
support oxide capable of anchoring highly-dispersed Rh parti-
cles, and thus recognized as an irreplaceable material to lessen
the amount of this scarcest precious metal.28
The common procedure for preparing TWCs is by im-
pregnating an aqueous solution containing salts of precious
metals onto oxide support powders. The catalyst powders thus
obtained are made into slurry for the monolith coating. During
these procedures, Ce and/or Zr are added in forms of fine oxide
powders or water-soluble salts. In the present study, however,
we are interested in the use of colloidal oxide sols, which are
Experimental
Preparation of Fluorite-Type Oxide Sols. Nanometric
colloidal sols of three fluorite-type oxides, CeO , ZrO , and
CeO ­ZrO , were prepared as shown in Figure 1 using ZrO-
2
2
2
2
(CO3) (AMR Technologies, Inc.), Ce2(CO3)3 (AMR Technol-
ogies, Inc.), [(CH ) N] (CO ) (TMAC, Tama Chemical Co.,
3
4
2
3
Ltd.), and H O (Kanto Chemical Co., Inc.) as starting mate-
rials. Tetramethylammonium (TMA) ions ((CH ) N ) act as a
stabilizer for colloidal dispersion of oxide nanoparticles. The
2
2
+
3
4
Published on the web July 27, 2013; doi:10.1246/bcsj.20130159

K. Tanimoto et al.
TMAC
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1211
degas CO . The resulting slurry containing 6.0 mass % of CeO
2
2
(
pH 11.2) was hydrothermally treated at 140 °C for 6 h under
stirring to convert into a sol containing 6.0 mass % CeO2
pH 10.9). As in the case of ZR sol above described, ultra-
2
H O
Metal Carbonate
(
(60 °C)
(Hydrogen peroxide)
filtration of the product gave a stable yellowish CE sol
1
05 °C
containing 40 mass % CeO (pH 9.4) as shown in Figure 2.
2
CeO2­ZrO2 (CZ) Sol: In a 3-L glass vessel, 819.8 g of an
aqueous solution of TMAC (44.5 mass % as [(CH ) N](OH))
Degas (CO
Autoclave
2
↑)
3
4
2
(H O)
and 435.5 g of water were mixed by stirring and heated up to
0 °C, where 172.1 g of Ce (CO ) and 183.9 g of ZrO(CO )
6
2
3 3
3
3
+
140~150 °C
~6 h
was slowly added (Ce/Zr molar ratio: 0.8). To oxidize Ce
4+
to Ce , 388.7 g of 35% H O was added to the solution at a
2
2
3
constant rate by use of a peristaltic tube pump. The mixture was
then heated at 105 °C for 6 h under reflux to completely dissolve
the carbonate and degas CO2. The resulting slurry containing
Ultrafiltration
8.0 mass % of CZ (pH 10.5) was hydrothermally treated at
140 °C for 6 h under stirring to convert into a sol containing
8.0 mass % CZ (pH 10.3). After ultrafiltration, a stable orange
Fluorite-type oxide sol
Figure 1. Preparation flow of the fluorite-type oxide sol.
CZ sol containing 30 mass % CeO ­ZrO (pH 9.5) was obtained
2
2
as shown in Figure 2. Because the Ce/Zr molar ratio of as-
prepared colloidal particles was confirmed to be unity, a part of
Zr was lost by the ultrafiltration step.
Characterization. The particles size in as-prepared colloi-
dal suspensions was determined by dynamic light scattering
(
DLS) measurements (Submicron Particle Size Analyzer N5,
Beckman Coulter) using a He-Ne laser (25 mW, 632.8 nm) at
a scattering angle of 90°. The sols were dried by evaporation
to dryness under reduced pressures at ambient temperature for
the following characterization. The powder X-ray diffraction
(XRD) measurement was performed using monochromated
Cu K¡ radiation (30 kV, 20 mA, Rigaku Multiflex). The size of
crystallite was calculated by line-broadening analysis of XRD
profiles using the (111) reflection. The microstructure of dried
sols was observed by transmission electron microscopy (TEM,
JEM1010, JEOL Ltd.) operating at 100 kV. The colloidal
particles were mounted by immersing a Cu grid mesh covered
with a microgrid carbon into sol solutions. TG/DTA mea-
CE
CZ
ZR
Figure 2. Photographs of as-prepared CE, CZ, and ZR sols
in bottles.
detailed preparation procedures for each sol are described in
the following.
ZrO (ZR) Sol: In a 3-L glass vessel, 1306.1 g of an
2
¹
1
aqueous solution of TMAC (44.5 mass % as [(CH ) N](OH))
surement in a flow of air at the heating rate of 10 °C min
3
4
and 995.7 g of water were mixed by stirring and 976.5 g of
ZrO(CO ) (40.2 mass % as ZrO ) was slowly added. After
was carried out with a Brucker TG-DTA2000SA instrument.
The size of primary particle for each sol was calculated from
3
2
stirring for 1 h, the mixture was heated at 105 °C for 6 h under
BET surface area, which was determined from N adsorption
2
reflux to completely dissolve the carbonate and degas CO . The
isotherms measured at 77 K (Macsorb Model-1201, Mountec
Co., Ltd.). Prior to the measurement, each sol was dried at
105 °C in a drying oven and heated at 300 °C for 1 h. The
surface areas of dried sols after calcination up to 1000 °C for
5 h in air were also measured.
Local structures around Zr and Ce atoms were studied by
X-ray absorption fine structure (XAFS) of Zr and Ce K-edges,
which were recorded on the NW10A station at Photon
Factory for Advanced Ring (PF-AR), High Energy Acceler-
ator Research Organization (KEK), Tsukuba (Proposal No.
2011G619). A Si(311) double-crystal monochromator was
used. The spectra were recorded at ambient temperature in
a transmission mode. The incident and transmitted X-rays in
Ce K-edge EXAFS measurement were monitored in 17- and
31-cm-long ionization chambers filled with Ar and Kr, respec-
tively. The ionization chambers for the incident and transmitted
X-rays in Zr K-edge EXAFS measurement were filled with Ar
2
resulting slurry containing 12.5 mass % ZrO (pH 10.6) was
2
transferred to a stainless steel autoclave vessel, where hydro-
thermal treatment was carried out at 140 °C for 3 h under
stirring. The product was a stable and homogeneous colloidal
suspension containing 12.5 mass % ZrO2 (pH 9.8) containing
no precipitation. Finally, ultrafiltration using a membrane filter
5
(
differential molecular weight 2 © 10 ) was applied to remove
an excess amount of electrolytes to obtain a white ZR sol
containing 40 mass % ZrO (pH 9.6) as shown in Figure 2.
2
CeO (CE) Sol: In a 3-L glass vessel, 1004.2 g of an
2
aqueous solution of TMAC (44.5 mass % as [(CH ) N](OH))
3
4
and 354.4 g of water were mixed by stirring and heated up
to 80 °C, where 240.9 g of Ce2(CO3)3 (50.0 mass % as CeO2)
was slowly added. To oxidize Ce3 to Ce , 408.0 g of 35%
+
4+
H O was added to the solution at a constant rate by use of a
2
2
peristaltic tube pump. The mixture was then heated at 105 °C
for 6 h under reflux to completely dissolve the carbonate and
50% + N 50%. The powder sample was pressed into a disk
2

1
212 Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013)
Colloidal Sols of CeO2­ZrO2 Solid Solution
of 10 mm in diameter after its volume was adjusted with boron
nitride powders to give an appropriate absorbance at the edge
energy for XAFS measurement. XAFS data were processed
using a REX 2000 program (Rigaku). EXAFS oscillation was
extracted by fitting a cubic spline function through the post edge
in size consist of aggregates of primary particles smaller than
10 nm. The size of particle aggregates of CZ sol was smallest
(¯20 nm), whereas those of CE and ZR sols were more than
30 nm. Table 1 shows physicochemical properties of three
types of sols. The BET surface areas of dried sols were in a
range of 130­160 m g . The particle sizes calculated from the
surface area are in the range of 6­8 nm, which is in accordance
with TEM images. Because these values are close to crystallite
sizes estimated by line-broadening analysis of XRD patterns,
one primary particle should be composed of a single crystallite.
The particle sizes in the colloidal suspensions were determined
by dynamic light scattering. They ranged from about 50 to
more than 100 nm in diameter, depending on the oxide com-
position. These values were larger than those observed in the
TEM images in Figure 4, because of the occurrence of further
aggregation in the sol solution.
3
2 ¹1
region. The k -weighted EXAFS oscillation in the 3.0­16.0 ¡
region was Fourier transformed for curve-fitting analysis. Phase
shifts and backscattering amplitude functions for Ce­O, Ce­O­
Ce, Zr­O, and Zr­O­Zr shells were extracted from the EXAFS
data of CeO and ZrO (Rare Metallic Co., Ltd.). The curve-
2
2
fitting analysis for Ce­O­Zr shell was performed using
parameters calculated by FEFF code.
The powdered CZ sol was impregnated with an aqueous
solution of [Pt(NH ) (NO ) ] (Tanaka Precious Metals) fol-
3
2
2 2
lowed by drying and calcination at 600 °C for 3 h in air
1 mass % Pt) before evaluation of OSC. The measurement was
(
done by use of microbalance (TG, Rigaku 8120), which was
connected to a dual-gas supplying system. The catalyst (10 mg)
Local Structure of CZ Sol. Although the XRD result in
Figure 3 suggests the difference of lattice constants in accord-
ance with the formation of CeO ­ZrO , several different types
was first heated in a stream of N up to 800 °C, where a con-
2
2
2
stant weight was attained within 30 min. The gas feed was then
switched between 1.4% H (120 min) and 0.7% O (40 min)
CE
CZ
ZR
2
2
balanced by N2 with recording the catalyst weight at this
temperature.
Results and Discussion
Structure and Microstructure of Oxide Sols. As-prepared
three types of alkaline sols maintained stable colloid dispersion
without sedimentation or coagulation in spite of their high
solid fractions (30­40 mass %). Figure 3 shows powder XRD
patterns of oxide sols dried in vacuo at ambient temperature.
The diffraction peaks could be assigned to fluorite-type struc-
ture with cubic (CE and CZ) and monoclinic (ZR) symmetries.
As can be seen from broad diffraction peaks, their crystallinity
was very low, but it is sufficient to detect the shift of diffraction
peaks of CZ toward higher angles compared to CE. Because of
1
00 nm
the smaller ionic radius of Zr4 (0.087 nm) than that of Ce
0.102 nm), the shift is a clear indication of the formation of
CeO ­ZrO solid solutions having smaller lattice constants.
+
4+
1
0 nm
(
2
2
Figure 4. TEM photographs of as-prepared fluorite-type
oxide sols with low and high magnifications.
The microstructure of dried sols was observed by TEM. As
shown in Figure 4, the secondary particles of several ten nm
Table 1. Physicochemical Properties of Fluorite-Type
Oxide Sols
CeO2
CeO -ZrO2
2
Sol
CE
CZ
ZR
ZrO2
Oxide
Oxide content/mass %
Stabilizer
Specific gravity/g mL
Viscosity /mPa s
CeO2
40
TMAH
1.56
3
CeO2­ZrO2a) ZrO2
30
TMAH
1.36
7
CE
40
b)
TMAH
1.53
4
¹1
c)
¹1
CZ
ZR
pH
SBET/m g
Crystallite size /nm
Particle size /nm
9.4
130
9
9.5
157
7
9.6
160
6
2
¹1
d)
e)
8
6
7
f)
Particle size /nm
110
49
90
a) Ce:Zr = 1:1. b) Tetramethylammonium hydroxide.
c) Measured at 25 °C. d) Calculated from XRD line-broad-
ening analysis using the (111) reflection. e) Calculated from
BET surface area (SBET) of dried sols. f) Determined by dy-
namic light scattering measurement of colloidal suspensions.
0
10
20
30
40
50
60
70
2
θ / degree
Figure 3. XRD patterns of fluorite-type oxide sols after
drying in vacuo at ambient temperature.

K. Tanimoto et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1213
3
2
1
0
0
0
0
a)
b)
2
1
0
0
0
CZ
CZ
ZrO2
CeO2
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Distance / Å
Distance / Å
Figure 5. a) Fourier-transformed Zr K-edge EXAFS for CZ and ZrO2. b) Fourier-transformed Ce K-edge EXAFS for CZ and
cubic CeO2.
of solid solutions with different local structures were known in
Table 2. Fitting Results Obtained from Zr K-edge and
Ce K-edge EXAFS Analysis
3
0
a)
the system. The local environment around Ce and Zr atoms
was therefore studied using XAFS. Figure 5a shows Fourier
transforms of Zr K-edge EXAFS of dried CZ sol and mono-
rc)
/¡
2 d)
·
CNb)
(«0.3)
R factor
/%
Oxide sol Shell
/10 ¡2
¹2
clinic ZrO as a reference without correction for phase shift.
2
(
«0.02) («0.02)
For CZ, one peak at around 1.7 ¡ can be assigned to Zr­O,
whereas the other peak at around 3.1 ¡ is contributions from
Zr­O­Zr and/or Zr­O­Ce shells, which is less intense than that
Zr K-edge
CZ
Zr­O
6.9
6.1
3.3
2.0
3.0
2.0
7.0
4.0
2.15
3.56
3.59
2.05
2.15
2.25
3.46
3.98
0.53
0.46
0.49
0.14
0.10
0.10
0.64
0.69
1.4
0.23
0.23
3.3
3.3
3.3
1.0
2.4
of monoclinic ZrO . The second shell peak shifted to a longer
Zr­O­Zr
Zr­O­Ce
Zr­O
Zr­O
Zr­O
2
distance than ZrO implies the formation of solid solution and/
2
ZrO2
or ZrO phases with different symmetries. We also measured
2
Ce K-edge EXAFS of CZ and CeO2 as a reference (Figure 5b).
CZ showed a weak and broad second shell peak at around
Zr­O­Zr
Zr­O­Zr
3
.5 ¡, which was assigned to Ce­O­Ce and Ce­O­Zr.
To obtain detailed structural parameters, we performed
curve-fitting analysis of first and second shells of Zr and
Ce K-edge EXAFS. Structural parameters obtained from
the curve-fitting are summarized in Table 2. The second shell
of Zr K-edge EXAFS for CZ yielded the best fit when two
shells of Zr­O­Zr (3.56 ¡) and Zr­O­Ce (3.59 ¡) were taken
into consideration. Similarly, Ce K-edge EXAFS for CZ also
suggests the presence of Ce­O­Zr with the very close atomic
distance (3.60 ¡) in addition to Ce­O­Ce (3.81 ¡). The theo-
retical coordination number (CN) of the Ce­O­Ce in cubic
fluorite-type structure is 12, but the sum of CN of the second
shell in the present analysis is less than 12, probably because of
the very small size of colloidal particles (<10 nm) as shown by
TEM (Figure 4). Moreover, it should be noted that the ratios of
CNs for Zr­O­Ce to Zr­O­Zr or Ce­O­Zr to Ce­O­Ce were
less than unity. It was reported that Ce and Zr K-edge EXAFS
for completely homogenous equimolar CeO ­ZrO solid solu-
Ce K-edge
CZ
Ce­O
4.9
5.6
2.9
8.0
2.34
3.81
3.60
2.34
3.82
0.40
0.15
0.49
0.40
0.12
1.7
Ce­O­Ce
Ce­O­Zr
Ce­O
0.49
0.49
3.5
CeO2
Ce­O­Ce 12.0
2.3
a) Interval of k-space to r-space of FT is 3.0­16.0 ¡¹1.
b) Coordination number. c) Atomic distance. d) Debye­Waller
factor.
Local inhomogeneities of the CeO2­ZrO2 solid solutions lead
to Ce-rich and Zr-rich domains, which are undetectable by
conventional XRD. The two-domain structure of CZ sol
in the present work is therefore very similar to those of the
equimolar solid solutions.
3
2
2
2
tion demonstrated the CN ratios equal to unity.30 The present
solid solution with lower CN ratios (ca. 0.5) suggests the
formation of two different domains containing more or less
Ce with the size of around the unit cell. Taking these results
into consideration, the present CZ sol can be identified as a
solid solution, but its local structure consists of two different
domains. It is well-known that the CeO ­ZrO phase diagram
Thermal Stability and OSC of Dried Sols.
Figure 6
shows the TG/DTA profile of CZ sol dried at 110 °C in air.
The sol exhibited three steps of weight losses at <150, 150­
400, and >400 °C, which would be assigned to desorption of
physisorbed water, decomposition of organic stabilizer (TMA)
and dehydroxylation, respectively. A DTA profile shows a
strong exothermic peak in the second step of weight loss as
a result of combustion of organic moiety. Therefore, thermal
2
2
has unresolved and complex problems due to the presence of
3
1
metastable phases arising from compositional fluctuation.

1
214 Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013)
Colloidal Sols of CeO2­ZrO2 Solid Solution
6
4
2
0
0
0
0
0
5
TG
1
mass%
a)
O2 H2
-
b)
-
-
-
10
15
20
c)
d)
DTA
0
200
400
600
800
1000
0
5
10
15
20
Temperature / °C
Time / h
Figure 6. A TG/DTA profile of CZ sol measured in a flow
of air.
Figure 8. Weight changes during reduction­oxidation
cycles of 1 mass % Pt loaded CZ and CE measured at
3
00 and 500 °C. a) CE, 300, b) CE, 500, c) CZ, 300, and d)
1
1
1
1
60
40
20
00
CZ, 500 °C.
ZR
CE
CZ
liquid phase is a key for higher thermal stability at elevated
temperatures.
CZcopre
Finally, the oxygen storage performance was evaluated
under oscillating feed stream conditions, where reducing and
oxidizing gas feeds were cycled. Figure 8 shows the weight
changes of Pt-loaded dried sols of CZ and CE under a cycled
feed stream condition (0.7% O or 1.4% H , He balance) at
8
6
4
2
0
0
0
0
0
2
2
3
00 and 500 °C. CZ exhibited larger weight changes than CE
at each temperature in accordance with the formation of solid
solutions in the CZ sol. The weight change of about 1 mass %
3
00
400
500
600
700
800
900
1000
¹1
at 500 °C corresponds to 92.6 mmol-O ¢(mol-Ce) , which is
2
about 40% of the theoretical OSC according to the stoichio-
metric reaction, CeO ­ZrO = CeO ­ZrO + 0.25O . This is
Calcination temperature / °C
2
2
1.5
2
¹1
2
Figure 7. BET surface areas of dried sols after heating
at elevated temperatures. CZ: CeO2­ZrO2 sol prepared as
shown in Figure 1. CZcopre: CeO2­ZrO2 prepared by con-
ventional coprecipitation method.
quite larger than 14.5 mmol-O ¢(mol-Ce) for CE, which is
2
only 6% of the theoretical OSC of CeO . The measurement was
2
also performed on Pt-unloaded sols at 300 °C, but OSC values
were too small to be evaluated because of very slow kinetics.
Loading Pt or any other precious metals are necessary for
treatment of dried sols at ²400 °C yielded pure oxide phases
without residual contaminants arising from the organic sta-
bilizer. Such a low-temperature decomposition behavior is
suitable for use as catalyst precursors.
oxygen release in a low-concentration H (1.4 vol %).
2
Conclusion
The present study successfully prepared stable and chloride-
free colloidal suspensions of fluorite-type oxides, ZrO , CeO ,
Figure 7 plots the BET surface area of dried sols after
calcination at elevated temperatures in air. The large surface
2
2
and CeO ­ZrO , which can be useful as catalyst precursors. The
2
2
2
¹1
areas of as-prepared sols (>130 m g ) decreased mono-
tonically with an increase of temperature, depending the oxide
composition. The ZR sol steeply lost its surface area at ²500 °C
and CE sol also showed significant sintering at ²700 °C. By
contrast, the CZ sol was found to retain the highest values at
sols consisted of uniform primary particles as large as 6­8 nm,
which form aggregates about 50­110 nm in size. EXAFS ana-
lysis proved that the binary oxide sol of CeO ­ZrO was com-
2
2
posed of solid solutions having Zr-rich and Ce-rich domains.
The organic stabilizer, tetramethylammonium ions, can easily
be removed by heating at 400 °C in air. Large BET surface
²700 °C, suggesting that the formation of solid solution is an
2
¹1
efficient way to prevent sintering. This behavior seems to be
associated with a slow particle growth rate resulting from the
structural complexity due to local inhomogeneities described
areas of dried sol more than 100 m g decreased steeply upon
calcination, but the CeO ­ZrO could retain a larger value than
2
2
CeO and ZrO alone, because the formation of solid solution
2
2
3
1
above. It should be noted that such a thermal stability was
not achieved when CeO ­ZrO was prepared by coprecipitation
achieved tolerance against sintering. The dried sol of CeO2­
ZrO also exhibited a much larger oxygen storage capacity than
2
2
2
from an aqueous solution of nitrates, which yielded a mixture
of CeO and ZrO after calcination. The present preparation
CeO . These features of fluorite-type oxide sols are suitable for
use in catalyst preparation. A further study on the characteristics
as catalyst precursors is now under investigation.
2
2
2
route enabling the formation of CeO ­ZrO solid solution in a
2
2

K. Tanimoto et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1215
This study was partially supported by Elements Science and
Technology Project from the Ministry of Education, Culture,
Sports, Science and Technology. XAFS experiments were
carried out on BL-7C of Photon Factory (PF), High Energy
Accelerator Research Organization (KEK) (Proposal No.
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