JPH0531367A - Catalyst for exhaust gas purification - Google Patents

Abstract

PURPOSE:To obtain a ternary catalyst having a catalytic activity even at 900 deg.C or higher and high purifying function against NOx. CONSTITUTION:A composite oxide with a perovskite-type structure having a general formula Ln1-xAxMO3 (Ln denotes rare earth metals except for Ce, A denotes Ce or alkaline earth metals, M a transition metal, either one denotes one or two kinds of them, respectively, 0<x<1), a heat-resistant oxide which contains Ce and Zr and/or rare earth metals except for Ce and at least a part of which becomes a composite oxide or a solid solution, and a noble metal are made to coexist. Due to the coexistence of the heat-resistant oxide, the heat-resistance is heightened and due to the coexistence of the noble metal, purifying function against NOx is heightened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying three-way catalyst excellent in purifying carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) that are used even at 900 ° C. or higher. is there.

[0002]

2. Description of the Related Art Complex oxides having a perovskite structure composed of rare earth metals, alkaline earth metals and transition metals are practically used as inexpensive three-way catalysts for purifying CO, HC and NOx. Expected (JP-A-59-87046 and JP-A-60-82138)
(See the official gazette). However, each of these catalysts is 800
It is intended to be used below ℃, and when it is necessary to have high catalytic activity in a high temperature range of 900 ℃ or higher, such as a catalyst for automobile exhaust gas, and it is also necessary to satisfy the durability at high temperatures, it is sufficient. Not a catalyst. That is, the composite oxide having the perovskite structure is sintered at a high temperature of 900 ° C. or higher to reduce the effective surface area, and the catalytic activity is significantly lowered. Further, although this perovskite type composite oxide has an excellent purification ability for CO and HC, NO
The purification capacity of x is slightly inferior and is not sufficient for practical use as a three-way catalyst for automobile exhaust gas.

Therefore, activity deterioration due to sintering, which is a problem when a complex oxide having a perovskite structure is used as a catalyst component at a high temperature of 900 ° C. or higher for a long time, is improved, and the NOx purification ability of the catalyst component is improved. In order to achieve this, an exhaust gas purifying catalyst has been proposed in which a complex oxide having a perovskite structure, a complex oxide having heat resistance, and a noble metal coexist (see Japanese Patent Laid-Open No. 1-168343). The heat-resistant composite oxide used therein has a structure similar in chemical property to the perovskite-type composite oxide of the catalyst component, and contains a rare earth metal and an alkaline earth metal.

[0004]

The present invention, like the invention of the above-cited Japanese Patent Application Laid-Open No. 1-168343, prevents a decrease in the catalytic activity of the complex oxide catalyst having a perovskite structure at 900 ° C. or higher, and It is intended to improve the NOx purification capacity, and an object thereof is to provide a refractory oxide having a structure different from that of the cited invention.

[0005]

The present invention is based on the general formula Ln _1-
xAxMO ₃ (Ln is a rare earth metal except Ce, A is Ce
Or an alkaline earth metal, M is a transition metal, a composite oxide having a perovskite structure represented by 0 <x <1), and Ce and Zr, or a rare earth metal other than Ce, at least part of which is a composite oxide Alternatively, it is an exhaust gas purification catalyst in which a heat-resistant oxide that is a solid solution and a noble metal coexist. Here, Ln, A, and M are not limited to one type each, and include two or more types.

The first feature of the present invention is that a heat-resistant oxide is allowed to coexist with a catalyst component composed of a complex oxide having a perovskite structure. The heat-resistant oxide includes cerium oxide and zirconium oxide, or an oxide of a rare earth metal other than cerium, and at least a part of these oxides exists as a complex oxide or a solid solution, and at a high temperature of 900 ° C. or higher. Can also suppress sintering. The effect of this heat-resistant oxide is effective both in the case of using the catalyst component supported on the heat-resistant oxide and in the case of using the catalyst component powder and the heat-resistant oxide powder as a mixture. The catalyst component is exhibited in a highly dispersed state on the carrier even after being used for a long time at a high temperature of 900 ° C. or higher, or is present in a highly dispersed state between the heat resistant oxide powders. Since this heat-resistant oxide is thermally stable even at a high temperature of 900 ° C. or higher and does not easily react with the catalyst component, it is possible to prevent deterioration of the catalyst activity due to destruction of the perovskite structure of the catalyst component.

The second feature of the present invention is that a noble metal such as palladium is added in addition to the perovskite type complex oxide as a catalyst component. The complex oxide of perovskite type structure itself can be used as a three-way catalyst, but NOx
The purifying ability against is slightly inferior to HC and CO. Therefore, a small amount of noble metal was added to increase the activity against NOx. The noble metal is supported on the complex oxide having a perovskite structure and the heat-resistant oxide. The supported noble metal undergoes solid solution and precipitation on the reactive perovskite type complex oxide, while on the other hand, it is supported in a highly dispersed state on the heat-resistant oxide having a high specific surface area that does not react, so that the exhaust gas is exhausted. 900 ° C under fluctuating atmosphere
Sintering of noble metals does not easily occur even at the above high temperatures, and purification activity is highly maintained. Thus, the catalyst according to the present invention is an inexpensive exhaust gas purification three-way catalyst that is durable even at high temperatures of 900 ° C. or higher.

One of the catalyst components used in the catalyst of the present invention
A complex oxide having a perovskite structure is a general formula Ln ₁ -xAxMO ₃ . The amount of this composite oxide is preferably 1 to 80% by weight of the total amount of the catalyst. If it is less than 1% by weight, the catalytic activity is low, and conversely if it is more than 80% by weight, the effect of the heat-resistant oxide is difficult to appear, which is not preferable.
The shape, particle size, purity, specific surface area, etc. of the composite oxide of the catalyst component may be in a state usually used as the catalyst component.

The heat-resistant oxide may be used in the same state (shape, particle size, purity, specific surface area) as Al ₂ O ₃ which is widely used as a catalyst component carrier. For example, the specific surface area of the catalyst component is 20 m ² /
g or more is desirable. The ratio of Ce, Zr, and rare earth metal in the heat-resistant oxide is not particularly limited, but Zr is 5 to 100 atoms, preferably 5 to 50 atoms, and rare earth metal other than Ce is 0 to 100 with respect to 100 atoms of Ce. The atomic ratio is preferably 5 to 30 atoms.

The precious metal which is another catalyst component is R of the platinum group.
One or more selected from u, Rh, Pd, Os, Ir and Pt are used. P out of these precious metals
The NOx purification characteristics are most improved when d is used. The amount of the noble metal is 0.01 to 5% by weight, preferably 0.1 to 2% by weight, based on the total amount of the catalyst. If the amount of precious metal is less than 0.01% by weight, the NOx purification capacity is insufficiently improved,
Conversely, the NOx purification capacity is saturated even if it exceeds 5% by weight. These noble metals are used in the state (shape, particle size, purity, specific surface area) usually used in conventional three-way catalysts.

The catalyst according to the present invention can be produced by a method generally used for producing a catalyst. Next, an example of the manufacturing method will be described. When the catalyst component is produced in a state of being supported on a carrier, first, a composite oxide of a heat-resistant oxide or a solid solution serving as a carrier is mixed with an aqueous solution containing a cerium salt and a zirconium salt or a rare earth metal salt.
It is obtained by pyrolyzing at a temperature of 0 ° C. or higher and firing, or by adding an aqueous solution obtained by mixing a commercially available ceria powder (100 m ² / g or more) with a zirconium salt or a rare earth metal salt at a predetermined stoichiometric ratio. It is obtained by drying in air at about 100 ° C. for 5 to 12 hours, and then calcining in air at 600 ° C. for 3 hours. An aqueous solution prepared by mixing a nitrate of a metal constituting a complex oxide having a perovskite structure, which is a catalyst component, in a predetermined stoichiometric ratio was added to a carrier powder made of the heat-resistant oxide, and the mixture was heated at about 100 ° C. for 5 to 12 After being dried in the air for 3 hours, it is further baked in the air at 700 to 800 ° C. for 3 to 10 hours. By this heat treatment, the nitrate is thermally decomposed, and the complex oxide having the perovskite structure is supported on the carrier powder.

Next, P is added onto the complex oxide having the perovskite structure supported on the heat-resistant complex oxide carrier as described above.
It carries a noble metal such as t, Pd, or Rh. For example, the above powder is impregnated with an aqueous solution of palladium nitrate, dried in the atmosphere at about 100 ° C. for 5 to 12 hours, and then baked at 600 ° C. for 3 hours in the atmosphere to support Pd. Alternatively, the noble metal may be supported after the fine powder of the complex oxide having the perovskite structure and the fine powder of the heat-resistant complex oxide are mixed. The fine powder of perovskite type complex oxide is manufactured by first adding a predetermined amount of a neutralizing agent such as sodium carbonate or ammonium carbonate to an aqueous solution in which nitrates of respective metals constituting the perovskite structure, oxalate and the like are mixed. Coprecipitate. The mixture is then washed with water, filtered, dried and dried at 500-600 ° C for 3-
After firing in the air for 5 hours, crush and further 700-8
Bake in the atmosphere at 00 ° C for 3 to 5 hours to obtain fine powder. The specific surface area of this powder is preferably at least a few m ² / g or more. A heat-resistant oxide powder is mixed with the composite oxide fine powder having a perovskite structure produced as described above, and a noble metal is supported on the mixed powder by the same method as described above to obtain a target catalyst.

The catalyst may be used by adding a binder to the catalyst and molding it into a predetermined shape, or by adding water to the base material in the form of a slurry to apply the slurry. It is also possible to coat the base material with a heat-resistant oxide, and then support the complex component oxide having a perovskite structure and a noble metal which are catalyst components.

[0014]

【Example】

(Example 1) A heat-resistant complex oxide used as a carrier was a commercially available cerium oxide powder having a high specific surface area (CeO ₂ specific surface area 1).
30 m ² / g, purity 99.9% / TREO (all rare earth oxides) 111.9 g were prepared, and zirconium oxynitrate (ZrO (NO ₃ ) ₂ ) aqueous solution (liquid specific gravity 1.51, 25.0% by weight in terms of ZrO ₂ is contained) 147.
9 g, and 26.0 g of an yttrium nitrate (Y (NO ₃ ) ₃ ) aqueous solution (liquid specific gravity: 1.62, containing 21.7% by weight in terms of Y ₂ O _{3 in} the liquid) were added, and mixed well by mixing. While 11
It was dried in the air at 0 ° C. for 10 hours. Then 6 in the atmosphere
00 ° C. conducted for 3 hours _{at, (Ce 0. 65 Zr 0} . 30 Y 0.
₀₅ ) About 150 g of O ₂ composite oxide was obtained.

Next, 50 g of this powder was prepared, and 70.4 g of lanthanum nitrate (La (NO ₃ ) ₃ .6H ₂ O) and 17.7 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) were prepared. Then, 100 ml of an aqueous solution in which 59.2 g of cobalt nitrate (Co (NO ₃ ) ₂ .6H ₂ O) was dissolved was added, followed by drying at 110 ° C. for 10 hours in the atmosphere. Thereafter, 800 ° C. in air, subjected to 3 hours baking, the nitrate is thermally _{decomposed, (Ce 0. 65 Zr 0} .
₃₀ Y _{0. 05)} a composite oxide having a perovskite structure containing Co on _{O 2 (La 0. 8 Ce} 0. 2) to obtain a powder carrying the CoO _3.

Thereafter, this powder was impregnated with an aqueous palladium nitrate solution, dried at 110 ° C. for 10 hours in the air, and further calcined at 600 ° C. for 3 hours in the air to obtain a catalyst carrying 0.5 g of Pd. (Sample No. 1) was prepared.
This component of the catalyst in a weight ratio _{(.. La 0 8 Ce 0} 2) CoO 3:
_{(.. Ce 0 65 Zr 0} 30 Y 0 05.) O 2: Pd = 50: 50:
It was 0.5.

Example 2 Prepared in Example 1 (Ce ₀ ..
₆₅ Zr _{0. 30} Y _{0 .05} ) O ₂ powder (50 g) was prepared, and the addition amount of the metal salt for obtaining the complex oxide having a perovskite structure supported on the powder was 70.4 g of lanthanum nitrate and 17% of cerium nitrate. 0.7 g, cobalt nitrate 23.7 g and iron nitrate
_{(Fe (NO 3) 3 ·} 9H 2 O) in addition to the 49.6g is the same manner as in Example 1 to prepare a catalyst (sample No.2) according to the second embodiment. This component of the catalyst in a weight ratio _{(La 0 8 Ce 0 2.} .) (Co 0 4 Fe 0 6..) O 3:. (Ce 0 65 Z
r _{0 .30} Y _{0 .05} ) O ₂ : Pd = 50: 50: 0.5.

(Example 3) 132.6 g of the cerium oxide powder used in Example 1 was mixed with 9 parts of an aqueous solution of zirconium oxynitrate.
Added 8.6g and yttrium nitrate solution 15.7 g, in the same manner as in Example _{1 (Ce 0. 77 Zr 0} .
₂₀ Y _{0. 03)} was obtained O ₂ refractory composite oxide. Next, 50 g of this powder was prepared, and the catalyst (Sample No. 3) according to this Example 3 was prepared with the same mixing ratio and operation as in Example 2. Component of the catalyst in a weight ratio _{(La 0. 8 Ce 0.} 2)
_{(. Co 0 4 Fe 0 6} .) O 3: (... Ce 0 77 Zr 0 20 Y 0 03) O 2:
It was Pd = 50: 50: 0.5.

Example 4 Prepared in Example 3 (Ce ₀ ..
_{77 Zr 0. 20 Y 0.} 03) O 2 refractory composite oxide powder 50g was prepared, to which lanthanum nitrate 79.2 g, cerium nitrate 8.9 g, cobalt nitrate 23.7 g, iron nitrate 49.6g
The aqueous solution of was added and subjected to baking and similar drying as in Example 1, on _{(CeZrY) O 2 (La 0} . 9 Ce 0. 1) (C
_{o 0. 4 Fe 0. 6} ) to give the O ₃ carrying the powder. Then, dinitrodiamine platinum ((NH ₃ ) ₂ (NO ₂ ) ₂ Pt) was added to the powder.
By impregnating with a nitric acid aqueous solution and by the same operation as in Example 1,
A catalyst (Sample No. 4) according to this Example 4 was prepared. This component of the catalyst in a weight ratio _{(La 0. 9 Ce 0.} 1) (C
_{. o 0 4 Fe 0 6)} O 3:... (Ce 0 77 Zr 0 20 Y 0 03) O 2:. P
It was t = 50: 50: 0.5.

(Embodiment 5) 111.9 g of the cerium nitrate powder used in Embodiment 1 was added to an aqueous solution of zirconium oxynitrate 1
47.9g and lanthanum nitrate _{(La (NO 3) 3 ·} 6H 2 O)
50 ml of an aqueous solution in which 22.1 g was dissolved was added, and
In the same operation as _{(Ce 0. 65 Zr 0.} 30 La 0. 05) was obtained O ₂ refractory composite oxide. Next, prepare 50 g of this powder,
The addition amount of the metal salt for obtaining the complex oxide having a perovskite structure supported on the lanthanum nitrate 74.1
g, strontium nitrate (Sr (NO ₃ ) ₂ ) 9.1 g, cobalt nitrate 31.1 g, and iron nitrate 43.2 g by the same operation as in Example 1 (catalyst No. 5). .5) was prepared. This component of the catalyst in a weight ratio _{(La 0 8 Sr 0 2.} .) (Co 0 5 Fe 0 5..) O 3:. (Ce 0 65 Z
r _{0 .30} La _{0 .05} ) O ₂ : Pd = 50: 50: 0.5.

[0021] (Example 6) composite oxide having a perovskite structure _{(La 0. 8 Ce 0.} 2) (Co 0. 4 Fe 0. 6) O 3 powder preparation will be described. Lanthanum nitrate 103.9 g, cerium nitrate 26.1 g, cobalt nitrate 34.9 g, iron nitrate 7
0.3 liter of an aqueous solution prepared by dissolving 2.7 g in pure water was prepared. Next, 0.5 liter of an aqueous solution in which 50 g of sodium carbonate was dissolved was prepared as a neutralizing coprecipitant. The neutralizing coprecipitant was added dropwise to the above aqueous solution to obtain a coprecipitate. The coprecipitate was thoroughly washed with water, filtered, and dried under vacuum. 600 ° C
In after calcination for 3 hours in the air, ground and then subjected to calcination for 3 hours in air at 800 ° C., further ground, (La _{0. 8
Ce 0. 2) (Co 0} . 4 Fe 0. 6) to create a powder of O _3. Prepared in Example _{1 (Ce 0. 65 Zr 0} . 30 Y 0. 05) O 2 powder 80
created by g and the _{(La 0. 8 Ce 0.} 2) (Co 0. 4 Fe
_0.6) was O ₃ powder 20g were mixed well. Next, this mixed powder was impregnated with palladium using an aqueous solution of palladium nitrate, dried at 110 ° C. for 10 hours, and then at 600 ° C. for 3 hours.
A catalyst (Sample No. 6) carrying 0.5 g of Pd was prepared by firing in air for a period of time.

Example 7 Instead of the method of preparing the perovskite type complex oxide of Example 6, lanthanum nitrate 76.7 was used.
g, neodymium nitrate _{(Nd (NO 3) 3 ·} 6H 2 O) 38.2
g, cobalt nitrate 43.7 g, in the same operation using the iron nitrate _{60.6g (La 0. 59 Nd 0} . 29 Ce 0. 12) (Co 0. 5
Fe _{0. 5)} to create a powder of O _3. The powder 80g and prepared in Example _{1 (Ce 0. 65 Zr 0} . 30 Y 0. 05) O 2 powder 20
After mixing well, Pd was adjusted to 0.
A catalyst supporting 5 g (Sample No. 7) was prepared.

(Example 8) 137.70 g of the cerium oxide powder used in Example 1 was mixed with 98.
6g was added, in the same manner as in Example 1 (Ce _{0. 8} Zr
_0.2) to give the O ₂ refractory composite oxide powder. According to this heat composite oxide powder 50g to Example _{6 (La 0. 8 Ce 0} . 2) (C
_{o 0. 4 Fe 0. 6} ) O 3 after a perovskite powder 50g were mixed well, the Pd and 0.5g supported catalyst (Sample No.
8) was prepared.

[0024] (Example 9) according to Example 6 (La _{0. 8} Ce
_0.2) (by _{Co 0. 4 Fe 0. 6} ) O 3 powder 80g Example 8
_{(Ce 0. 8 Zr 0.} 2) O 2 after heat composite oxide powder 20g were mixed well, the Pd and 0.5g supported catalyst (Sample No.
9) was prepared.

[0025] (Example 10) in place of cerium nitrate in Example 6 in the same manner by using strontium nitrate _{12.7g (La 0. 8 Sr 0} . 2) (Co 0. 4 Fe 0. 6) O _Three perovskite type powders were prepared. 80 g of this powder and Example 8
In accordance _{(Ce 0. 8 Zr 0.} 2) O 2 refractory composite oxide powder 20g
Was thoroughly mixed, and then 0.5 g of Pd was carried to prepare a catalyst (Sample No. 10).

[0027] (Comparative Example 1) composite oxide having a perovskite structure produced in Example _{6 (La 0. 8 Ce 0} . 2) (Co 0. 4 F
e _{0. 6)} using O ₃ powder 100 g, which in the Pd in the same manner as in Example 1 using the aqueous palladium nitrate solution 0.5g
A supported catalyst (Sample No. C1) was prepared.

Comparative Example 2 In place of the heat-resistant composite oxide prepared in Example 1, 50 g of commercially available cerium oxide powder having a high specific surface area was prepared, and the same procedure as in Example 1 was performed on the Ce.
O ₂ on a _{(La 0. 8 Ce 0.} 2) carrying the CoO _3, further Pd
Was carried by 0.5 g to prepare a comparative catalyst (Sample No. C2). Component of the catalyst in a weight ratio _{(La 0. 8 Ce 0.} 2)
CoO ₃ : CeO ₂ : Pd = 50: 50: 0.5.

Comparative Example 3 (Ce _{0 .65} Zr ₀ ..
₃₀ Y _{0. 05)} O ₂ commercially available γ-Al ₂ O ₃ (specific surface area 110m
² / g) and a catalyst (Sample No. C3) according to Comparative Example 3 was prepared by the same operation.

[0030] (Comparative Example 4) composite oxide having a perovskite structure produced in Example _{6 (La 0. 8 Ce 0} . 2) (Co 0.
₄ Fe _{0. 6)} O ₃ powder 50g and commercial SrZrO ₃ powder 50
After thoroughly mixing with g, a catalyst (Sample No. C4) carrying 0.5 g of Pd was prepared.

(Test Example) The catalysts prepared in Examples and Comparative Examples were subjected to a durability test of purification activity for 5 hours in exhaust gas at an inlet gas temperature of 930 ° C. The gas composition is carbon monoxide (CO) 1.0%, propylene (C ₃ H ₆ ) 0.1%,
Carbon dioxide (CO ₂ ) 10%, water (H ₂ O) 4%, oxygen (O ₂ ) was used as a variable condition, and the balance was nitrogen (N ₂ ).

(Evaluation) The purification rates of carbon monoxide, propylene and nitric oxide at 450 ° C. were measured for the catalysts subjected to the above heat resistance / durability test. At the time of measurement, the powdery catalyst is pressure-molded into pellets having a diameter of about 2 mm,
The reaction tube was filled. The space velocity at the time of measurement was 30,000 / hour, and the gas composition was 0.7% CO and 0.1 C ₃ H _6.
5%, CO ₂ 10%, H ₂ O 10%, H ₂ 0.23
%, NO is 0.15%, O ₂ is a variable condition, and the balance is N ₂
Is. The results are shown in Table 1 together with the components of the catalyst.
The catalyst of this example is superior in durability to the catalyst of the comparative example.

[0033]

[Table 1]

Continued front page    (72) Inventor Hirohisa Tanaka             3000 Yamanoue, Ryuo Town, Gamo-gun, Shiga Prefecture             Daihatsu Industry Co., Ltd. Shiga Technical Sen             Inside (72) Inventor Tsuboi, English             3000 Yamanoue, Ryuo Town, Gamo-gun, Shiga Prefecture             Daihatsu Industry Co., Ltd. Shiga Technical Sen             Inside (72) Inventor Shinichi Matsumoto             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Norio Kimura             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Hideo Sofukawa             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Masakuni Ozawa             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd.

Claims (3)

[Claims] 1. The general formula Ln ₁ -xAxMO ₃ (Ln is Ce
Rare earth metals other than, A is Ce or alkaline earth metal, M
Is a transition metal, and each is one or more, and 0 <x <
1) a complex oxide having a perovskite structure,
An exhaust gas purifying catalyst in which a noble metal coexists with a heat-resistant oxide containing Ce and Zr or a rare earth metal other than Ce, at least a part of which is a complex oxide or a solid solution. 2. The heat-resistant oxide is configured such that Zr has an atomic ratio of 5 to 100 atoms with respect to 100 atoms of Ce, and a rare earth metal other than Ce has an atomic ratio of 0 to 100 atoms. Exhaust gas purification catalyst described. 3. The exhaust gas purifying catalyst according to claim 1, wherein the noble metal is a platinum group.