JP3306147B2 - Method for producing catalyst composition for purifying engine exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention, carbon monoxide contained in the exhaust gas such as automobile engine (CO), hydrocarbons (HC), and nitrogen oxides efficiently purifying catalyzing the (NO _x) The present invention relates to a method for producing a catalyst composition for purifying an engine exhaust gas used in the production.

[0002]

2. Description of the Related Art Conventionally, an exhaust gas purifying catalyst for an automobile generally comprises a carrier substrate, a catalyst supporting layer, and a catalyst metal supported on the catalyst supporting layer. For the purpose of efficient purification, various exhaust gas purification catalysts have been developed.

For example, Japanese Patent Publication No. 59-41775, Japanese Patent Application Laid-Open No. 59-90695, Japanese Patent Publication No. 58-20307, etc. disclose techniques using cerium. Cerium in these exhaust gas purifying catalyst present as oxides, oxygen release or uptake (oxygen storage capacity) by the reaction shown in equation (1), to adjust the reduction reaction of the oxidation reaction and NO _x CO and HC Thus, the purification efficiency is improved.

[0004]

Embedded image

It is known that the reaction of the above formula (1) occurs on the surface of cerium oxide particles. However, in the conventional exhaust gas purifying catalyst, 800 ° C.
When the cerium oxide is used at the above-mentioned high temperature, the cerium oxide may grow and the specific surface area may decrease. Therefore, there is a problem that the purification performance is reduced due to a decrease in the oxygen storage capacity.

As exhaust gas purifying catalysts, JP-B-60-7537, JP-A-48-18180, JP-A-61-3531, USP3003020, USP39
No. 51860, US Pat. No. 4,170,573, etc. disclose techniques for simultaneously using cerium and other rare earths and transition metals. For example, Japanese Patent Publication No. 60-7537 discloses an apparatus in which cerium and lanthanum are used simultaneously to form a composite oxide represented by the formula (2).

[0007]

Embedded image

Further, it has been proposed to add components such as zirconium and lanthanum (Japanese Patent Laid-Open No. 61-26).
No. 2521, JP-A-61-262522). Although the addition of these components such as zirconium is effective in suppressing the growth of cerium grains, it tends to segregate on cerium oxide particles, causing a problem that the original properties of cerium are lost. Therefore, it is desirable that cerium and other components are completely and uniformly mixed, but it has been conventionally difficult to maintain the uniformity of these compositions in a thermally stable state.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to prevent cerium oxide grains from growing at a high temperature and to prevent the cerium oxide grains from growing. An object of the present invention is to provide a method for producing a catalyst composition for an engine exhaust gas purifying catalyst in which an agent does not segregate on cerium oxide particles and a reduction in purification performance is prevented.

[0010]

According to the present invention, there is provided a method for producing a catalyst composition for purifying an engine exhaust gas, comprising the steps of preparing crystalline particles of cerium oxide having a particle size of 0.1 μm or less; Adding the particles to a solution containing zirconium and a rare earth metal (excluding cerium) and then drying the solution to form a dried product; and heating the dried product at 750 ° C. or higher to obtain cerium oxide and zirconium oxide. And forming a solid solution comprising a rare earth metal (excluding cerium) oxide.

[0011]

In the method for producing a catalyst composition for purifying an engine exhaust gas according to the present invention, the crystalline particles of cerium oxide to be used are composed of fine particles having a size of 0.1 μm or less, and thus have high reactivity.
For this reason, the crystalline particles of cerium oxide are added to an aqueous solution of a salt of zirconium and a rare earth metal (excluding cerium), and a small amount of rare earth metal (excluding cerium) is further added to a relatively high temperature of 750 ° C. or higher. , A uniform solid solution composed of cerium oxide, zirconium oxide, and rare earth metal (excluding cerium) oxide can be formed. At this time, the addition of a small amount of a rare earth metal (except for cerium) has an effect of widening the composition composition region of the solid solution.

[0012]

According to the method for producing the catalyst composition for purifying engine exhaust gas of the present invention, each oxide constituting the composition can be obtained in a solid solution state, and fine particles having a high specific surface area can be produced. You. That is, it is possible to provide an engine exhaust gas purifying catalyst composition having both a high specific surface area and excellent durability.

[0013]

【Example】

Hereinafter, the present invention will be described based on examples.

The catalyst composition for purifying engine exhaust gas of the present invention is a solid solution of cerium oxide, zirconium oxide and rare earth metal (excluding cerium) oxide, wherein zirconium and rare earth metal (excluding cerium) are in atomic ratio. And zirconium and rare earth metals (excluding cerium) are configured to be 0.25 to 0.55 in total in atomic ratio.

That is, Ce _1-xy Zr _x R _y O
_2-Z (R is a rare earth metal (excluding cerium) oxide, z
Represents the amount of oxygen vacancies), wherein the ranges of x and y are 0.1 to 0.3 and the range of x + y is 0.2.
5 to 0.55.

The action of the catalyst composition for purifying engine exhaust gas according to the present invention to show an excellent purifying ability is not clear, but is considered as follows. Cerium oxide as described above by an oxygen storage capacity as shown in equation (1), by adjusting the reduction reaction of the oxidation reaction and NO _X CO and HC for purifying exhaust gas, but the grain growth in the oxide alone According to the study of the present inventors, it has been found that when heated at 1000 ° C., grains grow to a diameter of about 100 μm and become coarse. Then, even if it is attempted to purify CO using, for example, the reaction of the formula (3) after heating at 1000 ° C., the reaction rate becomes almost zero, and the oxygen storage capacity decreases.

[0018]

Embedded image

However, zirconium oxide suppresses the diffusion of cations in the cerium oxide and prevents the cerium oxide from growing at high temperatures. In particular, in the catalyst composition for purifying engine exhaust gas according to the present invention, zirconium oxide, rare earth metal (excluding cerium) oxide and cerium oxide are in a solid solution state (solid solution), and this solid solution is thermally stable. Therefore, a change such as phase separation can be prevented, and the film is stable even when exposed to a temperature for a long time under a condition where a temperature change from room temperature to around 1000 ° C. is repeated.

In this case, if the atomic ratio of each of zirconium and the rare earth metal (excluding cerium) is less than 0.1, it is not preferable because a solid solution is hardly formed. On the other hand, if it exceeds 0.3, other compounds rich in zirconium or rare earth metals (excluding cerium) are respectively formed, and the oxygen storage ability is undesirably reduced. Further, when the combined ratio of zirconium and rare earth metal (excluding cerium) is less than 0.25, the specific surface area tends to decrease at high temperatures, and the solid solution becomes thermally unstable, which is not preferable. If it exceeds 55, segregation on the surface of zirconium tends to occur, which is not preferable.

According to the catalyst composition for purifying engine exhaust gas of the present invention, cerium oxide grains can be prevented from growing at a high temperature, and an additive added for preventing grain growth can be added to the cerium oxide particles. Since segregation can be prevented, it is possible to provide an engine exhaust gas purifying catalyst composition having high purification performance.

The rare earth metal oxide in the catalyst composition for purifying engine exhaust gas of the present invention includes yttrium, scandium, lanthanum, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, and erbium, excluding cerium. , Thulium, ytterbium, and lutetium oxides.
Use more than species. Of these, yttrium or ytterbium, which is the easiest to form a solid solution, is preferable.

The addition of the rare earth metal (excluding cerium) oxide promotes the formation of a solid solution. In order to investigate this effect, the following experiment was performed when the rare earth metal (excluding cerium) oxide was yttrium.

Cerium nitrate is thermally decomposed at 600 ° C., and an aqueous solution of zirconyl nitrate and a mixed aqueous solution of yttrium nitrate and zirconyl nitrate are added to the produced cerium oxide powder.
After drying at 120 ° C., heating at 800 ° C.,
Heated at ° C. The lattice constant was measured from the diffraction line of cerium oxide by a powder X-ray diffractometer. (Ce _1-x Zr
_{x) 1-y} Y y in _{O 2-z / 2, y} = 0 and y = 0.
In the case of 2, the change of the lattice constant when x is changed is shown in FIG.
Shown in

[0025]

FIG.

When Y is not added (y = 0), the solid solution formation of cerium oxide and zirconium oxide is limited by the amount of Zr, and is formed only in the range of x <0.2 (indicated by ● in the figure). At this time, Zr added extra
Is presumed to be present as precipitated on cerium oxide or as a single precipitate. On the other hand, Y
When x was added, x was present in the entire range of 0 to 1 while maintaining the cubic fluorite structure (shown by ○ in the figure). That is, there is no segregation of Zr, and the solid solution exists stably. Such a solid solution can exhibit an oxygen storage capacity that does not change even under various use conditions used as an automobile catalyst.

Hereinafter, specific examples of the present invention will be described. The particle size of the prepared crystalline particles of cerium oxide is
0.1 μm or less. When the particle size exceeds 0.1 μm, the specific surface area is small, and sufficient catalytic action cannot be exhibited. Next, a method of adding an aqueous solution containing zirconium and a rare earth metal (excluding cerium) to the crystalline particles of cerium oxide and drying the aqueous solution is not particularly limited. Next, in the step of heating the dried product to form a solid solution composed of cerium oxide, zirconium oxide, and rare earth metal (excluding cerium) oxide, the heating temperature is set to 7%.
50 ° C. or higher. When the heating temperature is lower than 750 ° C., a solid solution having a sufficient catalytic action is not formed.

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples.

Example 1 100 g of cerium oxide powder having a crystal grain size of 0.005 μm
200 cc of water was added thereto to form a slurry, and then a mixed aqueous solution 200 c in which 21.87 g of zirconium nitrate and 76.33 g of yttrium nitrate were dissolved.
c was added, stirred sufficiently, dried under vacuum, and further dried at 120 ° C. This was heated at 800 ° C. for 3 hours and then 900
C. for 3 hours, and the atomic ratio of cerium (Ce), zirconium (Zr) and yttrium (Y) was 0.71:
An engine exhaust gas purifying catalyst composition having a ratio of 0.10: 0.19 (Example sample No. 1) was produced.

Next, instead of the above mixed aqueous solution, an aqueous solution 100c in which only 41.27 g of zirconium nitrate was dissolved was prepared.
Using c, a catalyst composition for purification of engine exhaust gas (Comparative Sample No. 6) having a ratio of Ce: Zr of 0.79: 0.21 was produced in the same procedure as above.

In the same manner, using aqueous solutions having different contents of zirconium nitrate and yttrium nitrate,
e, Zr, Y The catalyst compositions for purifying engine exhaust gas having different atomic ratios (Comparative Sample Nos. 4, 5 and Example Sample N)
o. 2, 3) were produced.

The specific surface area of the catalyst composition for purifying engine exhaust gas thus prepared was measured by BET adsorption of nitrogen. C in the prepared engine exhaust gas purifying catalyst composition
Table 1 shows the measurement results of the atomic ratios of e, Zr, and Y and the specific surface area.

[0033]

[Table 1]

From these results, it can be seen that the oxide containing no Y, which is the catalyst composition for purifying engine exhaust gas of the comparative example, has a relatively high ratio when the Zr content is smaller than that of the cerium oxide (Sample No. 5). Although the surface area is obtained, the effect is not seen when the Zr content is increased (Sample No. 6). On the other hand, Y
It can be seen that the specific surface area of the catalyst composition for purifying engine exhaust gas (Sample Nos. 1, 2, and 3) to which Examples were added was increased.

Sample No. 1 in Table 1 Catalysts A, B, and C were prepared by adding 4, 6, and 2 to the composite oxide of palladium and lanthanum-cobalt-iron. Catalyst A is Ce
Only (corresponding to sample No. 4), catalyst B further added with Zr (corresponding to sample No. 6), and catalyst C with Zr and Y
At the same time (corresponding to sample No. 2).
The former two A and B are comparative examples, and C is an example. A durability test was conducted using these catalysts, and CO in automobile exhaust gas,
The purification characteristics of NO and HC were examined. The test conditions were as follows. The catalyst was mounted on an exhaust path of an engine with a displacement of 1.5 liter, and the vehicle ran at a high speed of 5000 km. Thereafter, a model exhaust gas was flown at a space velocity of 50,000 h ^-1 using a fixed-bed evaluation device for each catalyst, and CO, NO, and HC were heated by a heating method.
Was examined for its purifying ability. 90% C obtained in this test
Table 2 shows the temperatures at which O, NO, and HC are purified.

[0036]

[Table 2]

From these results, it can be seen that the catalyst composition for purifying engine exhaust gas (catalyst C) of this example has excellent durability.

[Brief description of the drawings]

FIG. 1 shows the results of measuring the Zr dependence of the crystal lattice constant of the catalyst composition for purifying engine exhaust gas of the present invention and the catalyst composition for purifying engine exhaust gas of Comparative Example.

Continuing from the front page (72) Inventor Norio Kimura 41 Toyota Chuo R & D Co., Ltd., located at 41, Chuchu-Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture (72) Inventor Hirohisa Tanaka 3000, Yamanoue, Ryuo-cho, Gamo-gun, Shiga Prefecture (72) Inventor Shinichi Matsumoto 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-63-116742 (JP, A) JP-A-1-281144 (JP, A) JP-A-62-168544 (JP, A) JP-A-6-226094 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-38 / 74 B01D 53/94

Claims (1)

(57) [Claims] 1. A step of preparing crystalline particles of cerium oxide having a particle size of 0.1 μm or less, and adding the crystalline particles of cerium oxide to a solution containing zirconium and a rare earth metal (excluding cerium). Drying the dried product to form a solid solution comprising cerium oxide, zirconium oxide and rare earth metal (excluding cerium) oxide by heating the dried product at 750 ° C. or higher; A method for producing a catalyst composition for purifying an exhaust gas of an engine, comprising: