U.S. patent application number 11/819557 was filed with the patent office on 2008-01-10 for cerium oxide-zirconium oxide-based mixed oxide and method for producing the same.
Invention is credited to Masatoshi Maruki, Hiroshi Okamoto.
Application Number | 20080009410 11/819557 |
Document ID | / |
Family ID | 38651253 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080009410 |
Kind Code |
A1 |
Okamoto; Hiroshi ; et
al. |
January 10, 2008 |
Cerium oxide-zirconium oxide-based mixed oxide and method for
producing the same
Abstract
The present invention provides a cerium oxide-zirconium
oxide-based mixed oxide having superior platinum dispersibility and
a suitable OSC, and a simple production process thereof. The cerium
oxide-zirconium oxide-based mixed oxide comprises cerium oxide and
zirconium oxide, wherein (1) the weight ratio of CeO.sub.2:
ZrO.sub.2 is 60:40 to 90:10, and (2) the cerium oxide and the
zirconium oxide are present as a mixture, the zirconium oxide being
composed of a solid solution in which tetragonal or cubic zirconium
oxide contains cerium.
Inventors: |
Okamoto; Hiroshi;
(Osaka-shi, JP) ; Maruki; Masatoshi; (Osaka-shi,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Family ID: |
38651253 |
Appl. No.: |
11/819557 |
Filed: |
June 28, 2007 |
Current U.S.
Class: |
502/304 |
Current CPC
Class: |
C01P 2006/12 20130101;
C01G 25/00 20130101; Y02T 10/12 20130101; B01J 37/0201 20130101;
C01P 2002/72 20130101; B01D 53/945 20130101; B01J 2523/00 20130101;
C01P 2004/03 20130101; B01J 23/42 20130101; B01J 37/038 20130101;
B01J 23/002 20130101; B01D 2255/908 20130101; B01D 2255/407
20130101; Y02T 10/22 20130101; B01J 23/63 20130101; B01J 2523/00
20130101; B01J 2523/3712 20130101; B01J 2523/48 20130101 |
Class at
Publication: |
502/304 |
International
Class: |
B01J 23/10 20060101
B01J023/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2006 |
JP |
2006-207112 |
Claims
1. A cerium oxide-zirconium oxide-based mixed oxide comprising
cerium oxide and zirconium oxide, wherein (1) the weight ratio of
CeO.sub.2:ZrO.sub.2 is 60:40 to 90:10, and (2) the cerium oxide and
the zirconium oxide are present as a mixture, the zirconium oxide
being composed of a solid solution in which tetragonal or cubic
zirconium oxide contains cerium.
2. The cerium oxide-zirconium oxide-based mixed oxide according to
claim 1, wherein the ratio of the cerium oxide as calculated from
the X-ray diffraction peak intensity after heat treatment for 3
hours at 1000.degree. C. in air is 50% by volume or more.
3. The cerium oxide-zirconium oxide-based mixed oxide according to
claim 1 or 2, wherein the mixed oxide contains 1 to 20 wt % of one
type or two or more types of oxides selected from rare earth
elements excluding cerium, transition metal elements, aluminum and
silicon.
4. A method for producing a cerium oxide-zirconium oxide-based
mixed oxide comprising: a first step of adding a sulfating agent to
a cerous salt to obtain a slurry containing a cerous sulfate-alkali
metal mixed salt; a second step of adding a cerium salt and a
zirconium salt to the slurry; a third step of adding an alkali to
the mixture obtained in the second step to obtain a mixed hydroxide
containing cerium hydroxide and zirconium hydroxide; and a fourth
step of heat-treating the mixed hydroxide to obtain a mixed oxide
containing cerium oxide and zirconium oxide.
5. The method for producing a cerium oxide-zirconium oxide-based
mixed oxide according to claim 4, wherein one type or two or more
types of metal salt selected from rare earth elements excluding
cerium, transition metal elements, aluminum and silicon is added in
the second step.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a cerium oxide-zirconium
oxide-based mixed oxide and a production process thereof.
BACKGROUD OF THE INVENTION
[0002] Since the oxidation-reduction potential of Ce.sup.4+ and
Ce.sup.3+ is small at about 1.6 V and the reaction represented by
the following formula proceeds reversibly, cerium oxide has an
oxygen storage capacity (OSC), and is used as a co-catalyst or
catalyst support of automotive three-way catalysts.
CeO.sub.2<=>CeO.sub.2-x+X/2O.sub.2 (X=0 to 0.5)
[0003] However, the OSC of pure cerium oxide is well known to be
extremely low at about X=0.005.
[0004] Thus, in order to improve this, there have been numerous
reports describing (1) improving the heat resistance of the
specific surface area of cerium oxide, and (2) improving the OSC by
inserting Zr.sup.4+ having a small ionic radius into a Ce backbone
to alleviate the increase in volume during the above-mentioned
reaction, by putting zirconium oxide into a solid solution with
cerium oxide.
[0005] On the other hand, in the case of using pure cerium oxide as
a co-catalyst or catalyst support of an automotive three-way
catalyst, the dispersibility thereof is extremely good in the case
of loading with a precious metal, and particularly platinum.
Namely, it is a known fact that the cerium oxide has the superior
characteristic of being able to inhibit aggregation of platinum
particles at high temperatures.
[0006] Therefore, there has recently been a demand for a cerium
oxide-zirconium oxide-based mixed oxide having both of these
characteristics.
[0007] Japanese Examined Patent Publication No. H06-74145 describes
a "composition mainly consisting of ceric oxide having a stabilized
specific surface area, comprising ceric oxide and at least one type
of additive at a ratio of 1 to 20% by weight, wherein the additive
is an oxide of one or more types of other metal elements A selected
from the group consisting of silicon, zirconium and thorium".
[0008] However, only the heat resistance of ceric oxide containing
2.5% zirconia is described in the examples.
[0009] In addition, Japanese Patent No. 3623517 describes a
"composition composed of cerium oxide and zirconium oxide having an
atomic ratio of cerium/zirconium of at least 1 and demonstrating a
specific surface area of at least 35 m.sup.2/g after being calcined
for 6 hours at 900.degree. C. and an oxygen storage capacity of at
least 1.5 ml/g O.sub.2 at 400.degree. C.".
[0010] However, there is no description regarding the
dispersibility of platinum.
[0011] On the other hand, Japanese Patent Application Publication
No. 2002-177781 describes an "exhaust gas purification catalyst
comprising a mixed oxide (B) comprised of cerium oxide loaded onto
a solid solution oxide (A) containing zirconium and cerium".
[0012] However, this catalyst has a unique structure in which
cerium oxide is loaded around a solid solution oxide (A) containing
zirconium and cerium, and what is more, there are no descriptions
whatsoever regarding the OSC and platinum dispersibility
thereof.
[0013] Moreover, Japanese Patent Application Publication No.
2005-314134 describes "metal oxide particles having a core portion
containing a comparatively large amount of a ceria-zirconia solid
solution and a shell portion containing a comparatively large
amount of a second metal oxide", and a "production process of metal
oxide particles having a core portion containing a comparatively
large amount of a ceria-zirconia solid solution and a shell portion
containing a comparatively large amount of a second metal oxide
that provides a sol containing colloidal particles of a
ceria-zirconia solid solution and colloidal particles of a second
metal oxide each having different isoelectric points, comprising
making the pH of the sol closer to the isoelectric point of the
colloidal particles of the ceria-zirconia solid solution than the
isoelectric point of the colloidal particles of the second metal
oxide, aggregating the colloidal particles of the ceria-zirconia
solid solution, making the pH of the sol closer to the isoelectric
point of the colloidal particles of the second metal oxide than the
isoelectric point of the colloidal particles of the ceria-zirconia
solid solution, aggregating the colloidal particles of the second
metal oxide around the aggregated colloidal particles of the
ceria-zirconia solid solution, and drying and firing the resulting
aggregate".
[0014] However, although there are descriptions relating to OSC and
platinum dispersibility, the shell portion and core portion are
characterized by both being in the form of ceria-zirconia solid
solutions, and the production process is extremely unique.
SUMMARY OF THE INVENTION
[0015] With the foregoing in view, an object of the present
invention is to provide a cerium oxide-zirconium oxide-based mixed
oxide having superior platinum dispersibility and a suitable OSC,
and a simple production process thereof.
[0016] As a result of conducting extensive studies to achieve the
above-mentioned object, the inventors of the present invention
unexpectedly found that by adding a cerium salt and a zirconium
salt to a slurry containing a cerous sulfate-alkali metal mixed
salt, adding an alkali thereto to obtain a mixed hydroxide
containing cerium hydroxide and zirconium hydroxide, followed by
subjecting this to heat treatment to obtain a mixed oxide
comprising cerium oxide and tetragonal or cubic zirconium oxide as
a mixture, the zirconium oxide is composed of a solid solution
containing cerium, a cerium oxide-zirconium oxide-based mixed oxide
is obtained having superior platinum dispersibility and a suitable
OSC.
[0017] The present invention provides the following on the basis of
this finding.
[0018] 1. A cerium oxide-zirconium oxide-based mixed oxide
comprising cerium oxide and zirconium oxide, wherein (1) the weight
ratio of CeO.sub.2:ZrO.sub.2 is 60:40 to 90:10, and (2) the cerium
oxide and the zirconium oxide are present as a mixture, the
zirconium oxide being composed of a solid solution in which
tetragonal or cubic zirconium oxide contains cerium.
[0019] 2. The cerium oxide-zirconium oxide-based mixed oxide
according to above 1, wherein the ratio of the cerium oxide as
calculated from the X-ray diffraction peak intensity after heat
treatment for 3 hours at 1000.degree. C. in air is 50% by volume or
more.
[0020] 3. The cerium oxide-zirconium oxide-based mixed oxide
according to above 1 or 2, wherein the mixed oxide contains 1 to 20
wt % of one type or two or more types of oxides selected from rare
earth elements excluding cerium, transition metal elements,
aluminum and silicon.
[0021] 4. A method for producing a cerium oxide-zirconium
oxide-based mixed oxide comprising: a first step of adding a
sulfating agent to a cerous salt to obtain a slurry containing a
cerous sulfate-alkali metal mixed salt; a second step of adding a
cerium salt and a zirconium salt to the slurry; a third step of
adding an alkali to the mixture obtained in the second step to
obtain a mixed hydroxide containing cerium hydroxide and zirconium
hydroxide; and a fourth step of heat-treating the mixed hydroxide
to obtain a mixed oxide containing cerium oxide and zirconium
oxide.
[0022] 5. The method for producing a cerium oxide-zirconium
oxide-based mixed oxide according to above 4, wherein one type or
two or more types of metal salt selected from rare earth elements
excluding cerium, transition metal elements, aluminum and silicon
is added in the second step.
[0023] According to the present invention, a cerium oxide-zirconium
oxide-based mixed oxide, and a simple production process thereof,
having superior platinum dispersibility and a suitable OSC, can be
provided which can be preferably used in the relevant art as a
co-catalyst or catalyst support and the like of an automotive
three-way catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows the X-ray diffraction results of powders
obtained by heat-treating oxides obtained in Example 1 and
Comparative Example 1 for 3 hours at 1000.degree. C.;
[0025] FIG. 2 shows the X-ray diffraction results of powders
obtained by loading the oxides obtained in Example 1 and
Comparative Example 1 with 1 wt % Pt followed by heat treating for
3 hours at 900.degree. C.;
[0026] FIG. 3 shows an SEM image of a powder obtained by loading
the oxide obtained in Example 1 with 1 wt % Pt followed by heat
treating for 3 hours at 900.degree. C.; and
[0027] FIG. 4 shows an SEM image of a powder obtained by loading
the oxide obtained in Comparative Example 1 with 1 wt % Pt followed
by heat-treating for 3 hours at 900.degree. C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The following provides a detailed explanation of the cerium
oxide-zirconium oxide-based mixed oxide and production process
thereof of the present invention.
[0029] Furthermore, the zirconia referred to in the present
invention refers to ordinary zirconia, and includes that containing
impurity metal compounds, including hafnia, up to 10% by
weight.
[0030] In addition, in the present invention, unless specifically
stated otherwise, "%" refers to percent by weight.
1. Cerium Oxide-Zirconium Oxide-Based Mixed Oxide
[0031] The cerium oxide-zirconium oxide-based mixed oxide of the
present invention is a cerium oxide-zirconium oxide-based mixed
oxide comprising cerium oxide and zirconium oxide, wherein (1) the
weight ratio of CeO.sub.2:ZrO.sub.2 is 60:40 to 90:10, and (2) the
cerium oxide and the zirconium oxide are present as a mixture, the
zirconium oxide being composed of a solid solution in which
tetragonal or cubic zirconium oxide contains cerium.
[0032] First, the weight ratio of the cerium oxide and the
zirconium oxide is CeO.sub.2 to ZrO.sub.2=60:40.about.90:10,
preferably 63:35.about.85:15, and particularly preferably
70:30.about.80:20. If the ratio of cerium oxide is less than 60%,
the ratio of the presence of cerium oxide decreases resulting in a
high potential for a decrease in the dispersibility of Pt, while if
the composite ratio exceeds 90%, OSC decreases thereby making this
undesirable.
[0033] Next, "the cerium oxide and the zirconium oxide are present
as a mixture, the zirconium oxide being composed of a solid
solution in which tetragonal or cubic zirconium oxide contains
cerium" indicates that, in terms of crystal chemistry, (1) cerium
oxide phase and (2) tetragonal or cubic zirconium oxide phase which
forms a solid solution containing cerium, namely the two peaks of
cerium oxide and tetragonal or cubic zirconium oxide, are clearly
recognized.
[0034] In terms of powder engineering, (1) cerium oxide phase and
(2) tetragonal or cubic zirconium oxide phase which is composed of
a solid solution containing cerium are equally mixed each other (as
defined in the original Japanese in the Kojien, 2nd revised
edition, Oct. 15, 1979, 4th printing). In other words, the
zirconium oxide phase is dispersed in the cerium oxide phase
basically in the present invention.
[0035] Thus, the "mixed oxide (B) comprised of cerium oxide merely
loaded onto a solid solution oxide (A) containing zirconium and
cerium" described in Japanese Patent Application Publication No.
2002-177781, and the "metal oxide particles having a core portion
containing a comparatively large amount of a ceria-zirconia solid
solution and a shell portion containing a comparatively large
amount of a second metal oxide" described in Japanese Patent
Application Publication No. 2005-314.134 are not included in the
present invention.
[0036] Furthermore, for reference purposes, the results of X-ray
diffraction of a powder obtained in Example 1 of the present
invention are shown in FIG. 1.
[0037] According to this, (1) cerium oxide and (2) tetragonal or
cubic zirconium oxide which is composed of a solid solution
containing cerium, namely the two peaks of cerium oxide and
tetragonal or cubic zirconium oxide can be seen to be clearly
observed.
[0038] In this manner, since the cerium oxide-zirconium oxide-based
mixed oxide of the present invention has the characteristics
described above, it has the superior platinum dispersibility and
suitable OSC described below.
[0039] The OSC of the cerium oxide-zirconium oxide-based mixed
oxide of the present invention is 0.20 mmol-O.sub.2/g or more and
preferably 0.25 mmol-O.sub.2/g or more. If the OSC is less than
0.20 mmol-O.sub.2/g, the OSC of ceria actually functioning as an
automotive catalyst decreases, thereby making this undesirable.
[0040] On the other hand, in the case of having loaded platinum
onto the cerium oxide-zirconium oxide-based mixed oxide of the
present invention following heat treatment for 3 hours at
900.degree. C., the particle size of platinum is 200 nm or less and
preferably 170 nm or less. If the particle size exceeds 200 nm, the
number of active sites as a catalyst decreases, thus resulting in
the possibility of a decrease in catalyst activity and making this
undesirable.
[0041] Moreover, the content of cerium oxide as calculated from
X-ray diffraction peak intensity of the cerium oxide-zirconium
oxide-based mixed oxide of the present invention following heat
treatment for 3 hours at 1000.degree. C. in air is preferably 50%
by volume or more, more preferably 60% by volume or more and
particularly preferably 70% by volume or more. If this ratio of
cerium oxide is less than 50% by volume, Pt dispersibility
decreases, thereby making this undesirable.
[0042] In addition, the specific surface area of the cerium
oxide-zirconium oxide-based mixed oxide of the present invention
following heat treatment for 3 hours at 1000.degree. C. in air is
preferably 10 m.sup.2/g or more. If the specific surface area is
less than 10 m.sup.2/g, there is the possibility of promoting
sintering of the precious metal serving as the catalyst, thereby
making this undesirable.
[0043] Furthermore, the cerium oxide-zirconium oxide-based mixed
oxide of the present invention may also contain 1 to 20% by weight
of one type or two or more types of oxides selected from the group
consisting of oxides of rare earth elements excluding cerium,
transition metal elements, aluminum and silicon.
[0044] Examples of rare earth elements other than cerium include
lanthanoid elements such as Sc, Y, La, Pr or Nd. Examples of
transition metal elements include Ti, Cr, Mn, Fe, Co, Ni, Cu, Mo
and W. If these metal oxides are contained at less than 1% by
weight, there is no effect of improving heat resistance, while if
contained in excess of 20% by weight, the ratio of CeO.sub.2
decreases and the cubic crystal ratio of the cerium oxide
decreases, thereby making this undesirable.
2. Method for Producing Cerium Oxide-Zirconium Oxide-Based Mixed
Oxide
[0045] (First Step)
[0046] First, in the present invention, a sulfating agent is added
to a cerous salt to obtain a slurry containing a cerous
sulfate-alkali metal mixed salt.
[0047] The cerous salt may be that which provides cerous ion, and
for example, one type or two or more types of cerous sulfate,
cerous chloride or cerous nitrate can be used. Although the solvent
is suitably selected according to the type of cerium salt used and
the like, it is normally desirable to use water (and preferably
pure water or ion exchange water).
[0048] Although there are no particular limitations on the
concentration of the cerium salt solution, typically it is
preferably 5 to 200 g, and particularly preferably 50 to 100 g, of
cerium oxide (CeO.sub.2) equivalent in 1000 g of solvent.
[0049] A sulfating agent that forms a cerous sulfate-alkali metal
mixed salt by reacting with cerous ion is preferably used for the
sulfating agent, examples of which include sodium sulfate and
potassium sulfate.
[0050] The sulfating agent may be in the form of, for example, a
powder or a solution, and is preferably used in the form of a
solution (and particularly preferably an aqueous solution). The
concentration in the case of using as a solvent can be suitably
set.
[0051] For example, the sulfating agent is added so that the weight
ratio of Na.sub.2SO.sub.4/CeO.sub.2 in the case of using
Na.sub.2SO.sub.4 is 1.5 to 2.5, and the free acid concentration of
the mixture is preferably 0.2 to 2.2 N (normal).
[0052] Although examples of free acids include, but are not
particularly limited to, sulfuric acid, nitric acid and
hydrochloric acid, hydrochloric acid is preferable from the
standpoint of having superior productivity on an industrial
scale.
[0053] After having added the sulfating agent to the cerous salt,
this solution is heated to 85.degree. C. or higher and held (aged)
at that temperature for a fixed period of time to form a cerous
sulfate-alkali metal mixed salt.
[0054] There are no particular limitations on the cerous
sulfate-alkali metal mixed salt, and examples include
Ce.sub.2(SO.sub.4).sub.32M.sub.2SO.sub.4 and
M.sub.3[Ce.sub.2(SO.sub.4).sub.3] (wherein M represents an alkali
metal) .
[0055] In this manner, a slurry can be obtained containing a cerous
sulfate-alkali metal mixed salt.
[0056] Although the resulting cerous sulfate-alkali metal mixed
salt may be filtered or rinsed with water and the like for the
purpose of solid-liquid separation as necessary, since the present
invention comprises a subsequent step, it can normally be used in
the next step without filtering.
[0057] In the present invention, it is clear that a cerous
sulfate-alkali metal mixed salt can be used directly as a starting
substance, and this case is also included in the scope of the
present invention.
[0058] (Second Step)
[0059] Next, a cerium salt and a zirconium salt are added to the
slurry obtained in the first step.
[0060] The cerium salt may be either a cerous salt or a ceric salt
provided it provides cerium ion. For example, one type or two or
more types of cerium salts such as cerium sulfate, cerium chloride
or cerium nitrate can be used.
[0061] In addition, the zirconium salt may be any zirconium salt
provided it provides zirconium ion, and for example, one type or
two or more types of zirconium salts such as zirconium oxynitrate,
zirconium oxychloride or zirconium nitrate can be used.
[0062] Although there are no particular limitations on the
concentration of the cerium salt, typically it is preferably 5 to
200 g, and particularly preferably 50 to 100 g, as cerium oxide
(CeO.sub.2) in 1000 g of solvent.
[0063] In addition, this applies similarly to the concentration of
the zirconium salt.
[0064] In order to maintain a certain constant OSC at 600.degree.
C. for the cerium oxide-zirconium oxide-based mixed oxide of the
present invention, it is necessary for tetragonal or cubic
zirconium oxide composed of a solid solution containing cerium to
be present in addition to the cerium oxide.
[0065] Consequently, the ratio of the cerium salt and zirconium
salt is preferably, as oxide equivalent, such that CeO.sub.2:
ZrO.sub.2=40 to 60:60 to 40, and particularly preferably CeO.sub.2:
ZrO.sub.2=50:50.
[0066] Furthermore, although there are no particular limitations
thereon, in the case of adding cerium salt and zirconium salt at a
ratio of CeO.sub.2:ZrO.sub.2=40:60 of oxide equivalent when the
amount of CeO.sub.2 contained in the slurry containing a cerous
sulfate-alkali metal salt is defined as X and the amount of oxide
(CeO.sub.2+ZrO.sub.2) in the added cerium salt and zirconium salt
is defined as Y, in order to make the ratio of CeO.sub.2 to
ZrO.sub.2 (weight ratio) in the cerium oxide-zirconium oxide-based
mixed oxide of the present invention 60:40 to 90:10, the weight
ratio of X and Y is such that X:Y is within the range of 1:2 to
5:1.
[0067] Furthermore, although the cerium oxide-zirconium oxide-based
mixed metal oxide of the present invention can contain 1 to 20% of
one type or two or more types of oxides selected from oxides of
rare earth elements excluding cerium, transition metal elements,
aluminum and silicon, in this case, this can be accommodated in the
present step by adding a prescribed amount of one type or two or
more types of a metal salt selected from the group consisting of
metal salts of rare earth elements excluding cerium, transition
metal elements, aluminum and silicon.
[0068] (Third Step)
[0069] An alkali is added to the slurry containing a cerous
sulfate-alkali metal mixed salt to which was added a cerium salt
and a zirconium salt prepared in the second step to obtain a mixed
hydroxide containing cerium hydroxide and zirconium hydroxide.
[0070] There are no particular limitations on the alkali. Examples
of alkalis that can be used include ammonium hydroxide, ammonium
bicarbonate, sodium hydroxide and potassium hydroxide.
[0071] Among these, sodium hydroxide is preferable for the reason
that it can be inexpensively used industrially.
[0072] There are no particular limitations on the amount of alkali
added provided it is able to cause the formation of a precipitate
from the aforementioned solution, and is normally such that the pH
of the solution is 11 or higher and preferably 12 or higher.
[0073] This neutralization step, namely the neutralization of a
solution in which three components consisting of solid cerium salt,
ionic cerium salt (cerium ion) and ionic zirconium salt (zirconium
ion) are all present, is the major characteristic of the present
invention, and the cerium oxide-zirconium oxide-based mixed metal
oxide of the present invention, comprising "a mixture of cerium
oxide and tetragonal or cubic zirconium oxide which is a solid
solution comprising cerium", can be produced by heat treating the
mixed hydroxide containing cerium hydroxide and zirconium hydroxide
obtained in this step.
[0074] Although this mechanism is not completely understood, it is
presumed to be as described below.
[0075] Namely, by respectively using (1) a solid cerium salt and
(2) an ionic cerium salt, the degree of the mixing of the cerium
salts and zirconium can be controlled. In other words, the (2)
ionic cerium salt coprecipitates with ionic zirconium salt in the
step comprising addition of alkali resulting in a hydroxide in
which Ce and Zr are dispersed well. On the other hand, since the
(1) solid cerium salt is already in the form of aggregated
particles and not in an ionic state, dispersion with zirconium ion
is not adequate, resulting in a hydroxide which is difficult to
mixed in the neutralization step.
[0076] Thus, the hydroxide formed in the neutralization step is a
hydroxide in which cerium hydroxide from (1) and cerium hydroxide
and zirconium hydroxide from (2) are dispersed well, and by then
undergoing firing, a mixed oxide powder containing cerium oxide is
thought to be formed.
[0077] Furthermore, after completion of the neutralization
reaction, it is preferable to maintain the solution containing the
mixed hydroxide containing cerium hydroxide and zirconium hydroxide
at 35 to 60.degree. C. for 1 hour or more from the viewpoint of
aging the resulting precipitate to facilitate separation by
filtration.
[0078] The formed precipitate composed of the mixed hydroxide
containing cerium hydroxide and zirconium hydroxide is then
recovered by a solid-liquid separation method. Solid-liquid
separation may be carried out in accordance with known methods such
as filtration, centrifugal separation and decantation. Following
recovery of the precipitate, the mixed hydroxide containing cerium
hydroxide and zirconium hydroxide is preferably washed with water
to remove any adhered impurities as necessary.
[0079] Furthermore, the resulting mixed hydroxide may be further
dried as necessary. Drying may be carried out in accordance with a
known method such as air drying or heat drying. In addition, the
mixed hydroxide may be subjected to grinding treatment,
classification treatment and the like as necessary following drying
treatment.
[0080] (Fourth Step)
[0081] Finally, a cerium oxide-zirconium oxide-based mixed oxide is
obtained by carrying out heat treatment on the mixed hydroxide
containing cerium hydroxide and zirconium hydroxide.
[0082] There are no particular limitations on the heat treatment,
and it may be normally carried out at about 400 to 900.degree. C.
for 1 to 5 hours.
[0083] There are no particular limitations on the heat treatment
atmosphere, and heat treatment may be normally carried out in air
or an oxidizing atmosphere.
[0084] Furthermore, a mixed oxide obtained in this manner can be
crushed as necessary. There are no particular limitations on the
crushing, and it may be normally carried out with a crushing
machine such as a planetary mill, ball mill or jet mill.
EXAMPLES
[0085] The following provides a further explanation of the
characteristics of the present invention by indicating examples
thereof. Furthermore, the present invention is not limited to these
examples.
[0086] Each of the physical properties was measured using the
methods indicated below in the examples.
(1) Specific Surface Area
[0087] Specific surface area was measured according to the BET
method using a specific surface area measuring instrument (Flowsorb
II, Micromeritics Corp.).
(2) Oxygen Storage Capacity (OSC)
[0088] H.sub.2-TPR was determined according to the Temperature
Programmed Reduction method (Multitask T.P.R., Bel Japan Inc.).
[0089] More specifically, 0.3 g of powder were sufficiently
oxidized by heating to 600.degree. C. and holding for 60 minutes in
highly pure oxygen gas. Next, the powder was heated to from 100 to
900.degree. C. at a heating rate of 10.degree. C./min in a 5%
hydrogen-argon gas flow (100 sccm), and the hydrogen consumed
during this time was measured continuously with a quadrupole mass
spectrometer to obtain a water vapor generation curve accompanying
the rise in temperature. The area under the resulting hydrogen
consumption curve for the amount of hydrogen consumed at
600.degree. C. was taken to be amount of oxygen released at
600.degree. C.
(3) Cerium Oxide Ratio
[0090] The cerium oxide ratio was defined in the manner indicated
below from powder X-ray diffraction data following heat treatment
for 3 hours at 1000.degree. C. when the diffraction intensity of
2.theta.:28.55.degree. equivalent to the (111) plane of cubic
cerium oxide is defined as I.sub.0, and the diffraction intensity
of 2.theta.:30.5.degree. or 30.2.degree. equivalent to the (111)
plane of cubic or tetragonal zirconium oxide is defined as I.sub.1.
Cerium oxide ratio (vol %)=I.sub.0/(I.sub.0+I.sub.1).times.100 (4)
Platinum Particle Size
[0091] Pt particles (white spots in FIGS. 3 and 4) observed by
SEM-EDX were measured by visualizing under a scanning electron
microscope followed by determination of the average value.
Example 1
[0092] 280 g of a 20% cerous nitrate solution (containing 56 g of
CeO.sub.2 equivalent) were heated to 85.degree. C. followed by the
addition of 624 g of a 25% sodium sulfate solution (containing 156
g of Na.sub.2SO.sub.4 equivalent) and holding for 1 hour at
85.degree. C. to obtain a slurry containing cerous sulfate-sodium
mixed salt.
[0093] 110 g of a 20% zirconium nitrate solution (containing 22 g
of ZrO.sub.2equivalent) and 110 g of a 20% cerous nitrate solution
(containing 22 g of CeO.sub.2 equivalent) were added to this basic
slurry containing cerous sulfate-sodium mixed salt.
[0094] This solution was then neutralized using 500 g of 25% sodium
hydroxide. The pH at this time was 12 or higher.
[0095] Continuing, a hydroxide was obtained by filtering and
rinsing with water.
[0096] The resulting hydroxide was then fired for 5 hours at
650.degree. C. in air to obtain an oxide.
[0097] The X-ray diffraction results for the resulting oxide are
shown in FIG. 1.
[0098] In addition, the specific surface area, specific surface
area following heat treatment for 3 hours at 1000.degree. C., the
cerium oxide ratio and OSC were determined for this oxide.
[0099] On the other hand, Pt was loaded onto this oxide followed by
determination of platinum particle size. In this case, Pt was
loaded by dispensing a dinitrodiamine platinum nitrate solution
(Pt: 4.5%) into the oxide to 1% by weight followed by impregnating
and drying for 5 hours at 500.degree. C.
[0100] Moreover, the X-ray diffraction results after heat treating
for 3 hours at 900.degree. C. in air are shown in FIG. 2.
[0101] These results are shown in Table 1 along with analysis
values.
Comparative Example 1
[0102] 20% zirconium nitrate solution (containing 22 g of ZrO.sub.2
equivalent) and 390 g of a 20% cerium nitrate solution (containing
78 g of CeO.sub.2 equivalent) were mixed.
[0103] This solution was then neutralized using 500 g of 25%
ammonia. The pH at this time was 10.1. Continuing, a hydroxide was
obtained by filtering and rinsing with water.
[0104] The resulting hydroxide was fired for 5 hours at 650.degree.
C. in air to obtain an oxide.
[0105] This oxide was subjected to the same measurements as Example
1. Those results are shown in Table 1 along with analysis values.
Furthermore, X-ray diffraction results for the resulting oxide are
shown in FIG. 1.
[0106] Furthermore, platinum was loaded followed by determination
of platinum particle size in the same manner as Example 1.
[0107] Moreover, the X-ray diffraction results following heat
treatment for 3 hours at 900.degree. C. in air are shown in FIG. 2.
TABLE-US-00001 TABLE 1 Analysis Values and Measurement Results
Comparative Example 1 Example 1 ZrO.sub.2 (wt %) 22.0 22.0
CeO.sub.2 (wt %) 78.0 78.0 SA (m.sup.2/g) 65.7 69.1 Aged SA*.sup.1
(m.sup.2/g) 14.4 20.2 Cerium oxide ratio (vol %) 80.6 5.5 Platinum
particle size after heat Approx. 170 Approx. 350 treatment at
900.degree. C. (nm) OSC (mmol-O.sub.2/g) 0.25 0.31 *.sup.1After
heat treatment for 3 hours at 1000.degree. C.
[0108] According to Table 1, the article of the present invention
was determined to have a suitable OSC of 0.25 mmol-O.sub.2/g, and
the cerium oxide ratio of 80.6 vol % was found to be much higher
than the comparative example.
[0109] The platinum particle size of approximately 170 nm following
heat treatment for 3 hours at 900.degree. C. was not more than half
of that of the comparative example.
[0110] These results indicated that, in the case of assuming an
equal amount of loaded platinum and formation of platinum
crystallites of the same particle size, the product of the present
invention is about 8.7 times greater ((350/170)3) in terms of
quantity(number of particles) and about 2.1 times greater (350/170)
in terms of specific surface area, thus demonstrating extremely
superior dispersibility.
* * * * *