JP2003251150A - Exhaust gas cleaning device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning device exhibiting a high NO<SB>x</SB>cleaning ratio in which a temperature range capable of cleaning NO<SB>x</SB>suitable for cleaning of the exhaust gas of a lean burn engine requiring cleaning of NO<SB>x</SB>is outstandingly enhanced as compared with the conventional one. <P>SOLUTION: In the exhaust gas cleaning device in which a monolith carrier is coated with a catalyst. On the upstream side exhaust gas, the monolith carrier is further coated with a coat which is obtained by carrying a noble metal on a perovskite type composite oxide represented by the chemical formula: A<SB>1-x</SB>A '<SB>x</SB>M<SB>1-y</SB>Ti<SB>y</SB>O<SB>3</SB>(A represents at least one kind of La, Nd, Sm and Gd, A' represents at least one kind of K, Rb and Cs, M represents at least one kind of Mn, Co, Fe and Ni, 0<x<1 and 0<y<1) and on the downstream side, a cleaning catalyst capable of cleaning NO<SB>x</SB>is coated thereon. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine, and more particularly to an exhaust gas purification device having high NO _x purification performance.

[0002]

2. Description of the Related Art In recent years, in order to protect the environment, it is possible to suppress the total amount of carbon dioxide (CO ₂ ) emitted from an internal combustion engine such as an automobile engine, and to suppress the amount of nitrogen oxides (NO _x ). It is a serious issue. As a countermeasure against this, a lean burn engine was developed for the purpose of improving fuel economy, and a storage-reduction type NO _x in which a function of storing NO _x in a lean atmosphere was added to a conventional three-way catalyst for the purpose of purifying the exhaust gas thereof. Purification catalysts have been developed with some success in addressing the above issues.

In this lean burn engine, fuel is always burned under the condition that the air-fuel ratio (A / F) is lean (excess air), and temporarily stoichiometric (theoretical air-fuel ratio) to rich (excess fuel). Burn under conditions. Hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas are efficiently burned and removed by the action of the oxidizing atmosphere and the catalyst on the lean side, while NO _x is trapped by the storage material on the lean side. It is released under a temporary stoichiometric-rich condition, and is reduced and purified by the action of the temporary reducing atmosphere and the catalyst.

Conventionally, storage reduction NO _x NO _x purification catalyst
As the storage material, an alkali metal or an alkaline earth metal is used, and a catalyst component such as platinum and a NO _x storage material are carried on a carrier such as γ-alumina to form an exhaust gas purification catalyst. . Such an exhaust gas purifying catalyst is disclosed, for example, in Japanese Unexamined Patent Publication No.
It is described in JP-A-248458, JP-A-10-33984, and JP-A-10-128114.

On the other hand, it is known that a specific perovskite type complex oxide has a function of decomposing NO _x into N ₂ and O ₂ , for example, JP-A-5-261289 and JP-A-5-245372. Japanese Patent Laid-Open No. 6-315634
The publication describes a catalyst for NO _x catalytic reduction in which a specific perovskite complex oxide is supported on a carrier.

[0006]

However, an occlusion reduction type N carrying an alkali metal or an alkaline earth metal, etc.
With the O _x purification catalyst, it was necessary to further improve the NO _x purification performance at a catalyst temperature above about 500 ° C.

In addition, the method of purifying NO _x by such storage reduction requires temporary combustion under stoichiometric to rich conditions for reducing NO _x , but such combustion is one of fuel. It is generated by using some parts and sacrificing some fuel efficiency. Therefore, in order to further improve fuel economy, it is desirable to reduce the frequency of combustion under temporary stoichiometric-rich conditions as much as possible.

On the other hand, the conventional catalyst containing a perovskite type complex oxide needs to have a high NO _x purification performance in order to be used for purification of exhaust gas discharged from an internal combustion engine such as an automobile engine. Therefore, it is an object of the present invention to provide an exhaust gas purification device that has a particularly high temperature range in which NO _x can be purified, which is significantly higher than in the past, and that exhibits a high NO _x purification rate.

[0009]

The above object is an exhaust gas purifying apparatus in which a catalyst is coated on a monolith carrier, wherein the chemical formula:
A _{1-x A'x} M _1-y Ti _y O ₃ (A is La, Nd, Sm,
And at least one of Gd, A ′ is K, Rb, and C
at least one of s, M is Mn, Co, Fe, and N
At least one of i, 0 <x <1, 0 <y <1) is coated with a catalyst in which a noble metal is supported on a perovskite type composite oxide, and NO _x is provided on the exhaust gas downstream side on the monolith carrier. It is achieved by an exhaust gas purifying device characterized in that a catalyst for purifying is coated.

The present invention has the chemical formula: A _{1-x A'x} M _1-y Ti _y
A catalyst in which a precious metal such as platinum is directly supported on a specific perovskite type composite oxide represented by O ₃ is coated on the exhaust gas upstream side of the monolith carrier, and on the downstream side of the monolith carrier. The exhaust gas purifying apparatus has a structure in which a NO _x purifying catalyst is coated.

In the structure of the exhaust gas purifying apparatus of the present invention, the catalyst on the upstream side can decompose NO _x into N ₂ and O ₂ at a high temperature, whereby the exhaust gas purifying apparatus as a whole has a high temperature. The NO _x purification rate below is significantly increased. The reason for this is considered as follows.

When a part of the crystal lattice of the perovskite type complex oxide of the general formula ABO ₃ is replaced by a specific element, the crystal structure becomes more unstable in addition to the strain of the crystal due to the difference in atomic radius. As a result, the oxygen atom changes its valence, and oxygen can be absorbed and released. Also, this B
The elements of the site can play the role of redox,
It is considered that this redox action becomes remarkable especially when a part of the B site element is replaced with titanium.

Due to the action of absorbing and releasing oxygen and the action of redox, the perovskite type complex oxide specified in the present invention has the action of decomposing NO _x into N ₂ and O ₂ , and this action is essentially It is considered that it is promoted at a high temperature and also by supporting a noble metal.

Further, in the structure of the exhaust gas purifying apparatus of the present invention, the catalyst on the upstream side can function as a NO _x storage material at a temperature significantly higher than the conventional one, whereby NO at a high temperature can be obtained. _x Purification rate is increased. The reason for this is considered as follows. It is considered that the alkali metal or alkaline earth metal as the conventional NO _x storage material is changed to nitrate under lean conditions and to carbonate under rich conditions. It is considered that these salts are decomposed at a temperature higher than about 500 ° C., and thus the NO _x storage performance at a high temperature is deteriorated.

On the other hand, the perovskite type complex oxide specified in the present invention is stabilized even at high temperature due to the compounding effect derived from the specific composition, while at high temperature, A
At least part of potassium, rubidium, or cesium at the site can be liberated from the perovskite-type composite oxide, and the remaining negative ions act as a base point for NO _x , and the negatively ionized NO _x is converted into positive ions. Is considered to be captured by potassium, rubidium, or cesium. Therefore, the perovskite-type composite oxide occludes NO _x even at high temperatures, and the captured NO _x can be reduced and purified under rich conditions.

In the exhaust gas purifying apparatus of the present invention, a catalyst for purifying NO _x is coated on the downstream side of the perovskite type composite oxide carrying such a noble metal,
This catalyst is preferably a storage reduction type NO _x purification catalyst or a three-way catalyst. As described above, the catalyst on the upstream side has a function of decomposing NO _x into N ₂ and O ₂ at a high temperature and can function as a NO _x storage material. Therefore, by coating the downstream side with a catalyst for purifying NO _x such as a storage reduction type NO _x purifying catalyst or a three-way catalyst, NO _x purifying performance in a low temperature range is secured, and both catalysts have a wide temperature range. A high NOx purification rate can be exhibited in the range.

Further, the decomposing action of NO _x on the upstream side of the catalyst to N ₂ and O _2, it is possible to reduce the frequency of combustion in transient stoichiometric-rich conditions, that leads to improvement in fuel consumption You can

The upstream and downstream catalysts can be coated on one monolith support, or each catalyst can be coated on a separate monolith support and the monolith supports can be substantially contacted. It can also be arranged. In either configuration, a compact exhaust gas purification device can be constructed.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION
Suitable for purification of exhaust gas of a lean burn engine that requires purification of O _x , a perovskite-type composite oxide catalyst in which a noble metal is supported on a specific perovskite-type composite oxide is coated on the exhaust gas upstream side of a monolith carrier, A NO _x purification catalyst is coated on the monolithic carrier downstream of this catalyst.

Examples of the lean burn engine include a cylinder injection type spark ignition internal combustion engine which burns a lean air-fuel mixture in the cylinder as a whole to realize stratified combustion, and also includes a diesel internal combustion engine. Since these internal combustion engines burn fuel under lean conditions, a relatively large amount of NO _x is generated and it is necessary to purify NO _x .

The perovskite complex oxide is A
_{1-x A'x} M _1-y Ti _y O ₃ (A is at least one of La, Nd, Sm, and Gd, A'is K, Rb, and Cs
At least one of M, M is Mn, Co, Fe, and Ni
A specific perovskite-type composite oxide containing Ti having a chemical formula of at least one of 0 <x <1, more preferably 0.05 <x <0.45, and still more preferably 0.2 < x <0.4, 0 <y <1, more preferably 0.05 <y <0.45, and still more preferably 0.2 <y <0.4.

Preparation of such a perovskite type complex oxide is carried out, for example, by mixing powders of the elemental nitrates, acetates, chlorides, etc. contained in a predetermined ratio, or after making them into an aqueous solution and mixing and drying. , 400-1000 in air
It can be performed by heating to ℃.

Platinum (Pt), palladium (Pd), rhodium (R
h), a noble metal selected from at least one of gold (Au) and iridium (Ir), more preferably Pt,
At least one of Pd and Rh is supported to prepare the upstream catalyst. This loading can be performed by any method capable of supporting the metal particles on the carrier, for example, a precipitation method, an adsorption method, an ion exchange method, a reduction precipitation method, an evaporation dryness method, or the like. .

Preferably, the noble metal is 0.1 to 5 based on 100 g of the perovskite type complex oxide.
g, more preferably 0.3 to 4 g. Preferably, the perovskite-type composite oxide is adjusted to have a particle size of 0.05 to 20 μm by pulverization or the like, and then a noble metal is supported. This is because the noble metal is more evenly dispersed on the perovskite complex oxide.

N coated on the downstream monolith carrier
The O _x purification catalyst can be any catalyst capable of purifying NO _x , but preferably the NO _x storage material such as an alkali metal or an alkaline earth metal and the catalyst component such as platinum are γ -A storage-reduction type NO _x purification catalyst that is supported on a carrier such as alumina, and a three-way catalyst that does not support a NO _x storage material. Alternatively, the NO _x purification catalyst may be an adsorbent or an adsorbent such as a zeolite, an alkaline substance, or various complex oxides that temporarily or long-term adsorbs or absorbs NO _x by the basic points or pores. it can.

The perovskite-type composite oxide carrying these noble metals and the NO _x purification catalyst are coated on an ordinary honeycomb-shaped monolith carrier with a wash coat or the like to form the exhaust gas purification apparatus of the present invention. can do. FIG. 1 schematically illustrates the configuration of an exhaust gas purification device of the present invention. Hereinafter, the present invention will be described more specifically with reference to Examples.

[0027]

Example 1 Pt was added to the perovskite type composite oxide as follows.
Was prepared, and the upstream catalyst was coated on the upstream monolith support. 770g of Nd (NO _{3) 3} ·
6H ₂ O, 111 g of RbNO ₃ , 504 g of Mn (N
_{O 3) 2 · 6H 2 O} , and after dissolution, stirring to deionized water 3000cc titania sol containing solid 60 g, water is evaporated, 120 ° C. × 2 hours drying and 480 ° C. × 5
Calcination is performed for 900 hours and then firing is performed at 900 ° C. for 10 hours to obtain a perovskite complex oxide Nd _0.7 Rb _0.3 Mn.
A powder of _0.7 Ti _0.3 O ₃ was obtained.

200 g of the obtained composite oxide powder was added to 200 g.
The mixture was dispersed in 0 cc of ion-exchanged water and stirred, and a dinitrodiammine platinum nitric acid aqueous solution was added in an amount corresponding to 10 g Pt, and then water was evaporated, followed by drying at 120 ° C for 2 hours and 500
Firing at 2 ° C. for 2 hours gave a Pt-supported perovskite-type composite oxide powder.

Next, to this Pt-supported perovskite-type composite oxide powder, zirconia sol corresponding to a solid content of 20 mass% based on the total solid content and an appropriate amount of ion-exchanged water were added,
A slurry was prepared by stirring. This slurry has a diameter of 3
A monolith carrier having a length of 0 mm and a length of 20 mm was coated with 4 g as a solid content, dried at 120 ° C. for 2 hours, and dried at 500 ° C. × 1.
The monolith carrier was coated with the upstream catalyst by calcination for a period of time.

Separately, a downstream catalyst consisting of a storage reduction type NO _x purification catalyst was prepared as follows and was coated on the downstream monolith carrier. γ-alumina powder, an alumina sol corresponding to a solid content of 20 mass% based on the total solid content of this powder, and an appropriate amount of ion-exchanged water were added,
A slurry was prepared by stirring. This slurry has a diameter of 3
A monolith carrier having a length of 0 mm and a length of 30 mm was coated with 6 g as a solid content, dried at 120 ° C for 2 hours, and dried at 500 ° C for 2 hours.
Baking for hours.

Then, P was added to the above-mentioned coating layer using an aqueous solution of dinitrodiammine platinum nitric acid equivalent to 0.042 g Pt.
After carrying t and baking at 250 ° C. for 2 hours,
2 g of potassium acetate and 1.07 g of barium acetate were adsorbed and supported by water to prepare a downstream catalyst. The upstream and downstream monolith carriers thus obtained are arranged in contact with each other,
An exhaust gas purifying device of the present invention was constructed.

Example 2 A powder of perovskite type complex oxide Nd _0.7 K _0.3 Mn _0.7 Ti _0.3 O ₃ was obtained in the same manner as in Example 1 except that 76 g of KNO ₃ was used instead of 111 g of RbNO _3. It was
An upstream catalyst was prepared in the same manner as in Example 1 except that 500 g of this powder was used, and was combined with the downstream catalyst prepared in the same manner as in Example 1 to construct an exhaust gas purification apparatus of the present invention. .

Example 3 Example 1 was repeated except that the downstream catalyst was prepared as follows.
An exhaust gas purifying apparatus of the present invention was prepared in the same manner as in.
Zirconia powder supporting 1% by mass of Rh using an aqueous rhodium nitrate solution and γ-alumina powder were mixed at a mass ratio of 0.2: 1, and a solid content of 2 based on the total solid content of the powder.
Alumina sol corresponding to 0 mass% and an appropriate amount of ion-exchanged water were added and stirred to prepare a slurry. Then, in the same manner as in Example 1, the monolith carrier was coated with 6 g as a solid content, and Pt, potassium, and barium were further supported.

Example 4 An exhaust gas purifying apparatus of the present invention was prepared in the same manner as in Example 1 except that potassium and barium were not supported on the downstream catalyst.

Comparative Example 1 Using a Pt-supported perovskite type composite oxide powder prepared in the same manner as in Example 1, the monolithic carrier having a diameter of 30 mm and a length of 50 mm was solidified in the same manner as the upstream catalyst of Example 1. The exhaust gas purifying device of the comparative example was coated with 10 g as the amount.

Comparative Example 2 1 wt% Rh-supported zirconia powder and γ-alumina powder were mixed in a mass ratio of 0.2: 1 using an aqueous rhodium nitrate solution, and the total solid content of the powder was used as a reference. Alumina sol corresponding to a solid content of 20 mass% and an appropriate amount of ion-exchanged water were added and stirred to prepare a slurry. This slurry
A monolith carrier having a diameter of 30 mm and a length of 50 mm is coated with 10 g as a solid content, dried at 120 ° C. for 2 hours, and then dried.
Firing was performed at 0 ° C. for 2 hours.

Then, Pt was added to the above-mentioned coating layer using an aqueous solution of dinitrodiammine platinum nitric acid equivalent to 0.07 g Pt.
Was carried and baked at 250 ° C. for 2 hours, then 0.3
What carried 3 g of potassium acetate and 1.78 g of barium acetate by water absorption was made into the exhaust gas purification apparatus of the comparative example.

-Evaluation of Exhaust Gas Purification Performance- Each of the exhaust gas purification devices of Examples 1 to 4 and Comparative Examples 1 and 2 was
The NO _x purification performance was evaluated using a model gas catalyst performance evaluation device. This evaluation was conducted at the initial stage and after endurance (9
Heat treatment at 00 ° C. for 5 hours) and exposing the following lean and rich model gas atmospheres to the atmosphere that switches every 2 minutes, while NO _{x in a} lean atmosphere.
Purification performance was measured.

Rich gas composition: 500 ppm NO + 20
00ppmHC + 0.6% CO + 10% CO 2 + 0.3%
O ₂ + 5% H ₂ O (residual: N ₂ ) lean gas composition: 500 ppm NO + 2000 ppmH
C + 0.1% CO + 10% CO ₂ + 6.5% O ₂ + 5% H ₂
O (remainder: N ₂ )

The ambient temperature of the model exhaust gas is 700 ° C.
The results of the NO _x purification rate at each temperature are summarized in Table 1. From the results, the exhaust gas purifying apparatus according to the configuration of the present invention can be used in the high temperature range of 700 ° C. and 80 ° C.
It can be seen that at 0 ° C., the NO _x purification rate is significantly improved both in the initial stage and after the endurance test.

[0041]

The temperature range in which NO _x can be purified can be expanded.

[0042]

[Table 1]

[Brief description of drawings]

FIG. 1 is a schematic view illustrating an exhaust gas purification device of the present invention.

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Claims (3)

[Claims] 1. An exhaust gas purification apparatus comprising a monolith carrier coated with a catalyst, wherein the chemical formula: A is provided on the exhaust gas upstream side of the monolith carrier.
_{1-x A'x} M _1-y Ti _y O ₃ (A is at least one of La, Nd, Sm, and Gd, A'is K, Rb, and Cs
At least one of M, M is Mn, Co, Fe, and Ni
At least one, 0 <x <1,0 <catalyst supporting precious metal in the perovskite-type composite oxide represented by y <1) is coated, purifying NO _x in the exhaust gas downstream side on the monolithic support An exhaust gas purifying device, which is characterized by being coated with a catalyst. 2. The exhaust gas purification device according to claim 1, wherein the catalyst for purifying NO _x on the downstream side is a storage reduction type NO _x purification catalyst. 3. The exhaust gas purification device according to claim 1, wherein the noble metal is at least one of Pt, Pd, and Rh.