JP2003170050A - Exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst enhanced in its cleaning capacity by further enhancing the support concentration of a carrier. <P>SOLUTION: In manufacturing the occlusion and reduction type exhaust gas cleaning catalyst by supporting an occlusion material for occluding a substance to be cleaned and a reducing catalyst for reducing and cleaning the substance to be cleaned discharged from the occlusion material on the carrier, a nitrogen oxide occluding element being the occlusion material and a noble metal being the reducing catalyst are supported on the carrier comprising manganese oxide having a hollandite type crystal structure. Typically, the nitrogen oxide occluding element is selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals and the noble metal is selected from the group consisting of Pt, Pd and Rh. <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 an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst having an enhanced ability to store and purify nitrogen oxides (NOx) in oxygen-excess exhaust gas during lean operation of an internal combustion engine.

[0002]

2. Description of the Related Art The performance of an exhaust gas purifying catalyst is greatly influenced by the set air-fuel ratio of the engine, and the lean air-fuel mixture, that is, the lean side where the air-fuel ratio is large, has a large amount of oxygen in the exhaust gas after combustion, resulting in oxidation. Actively, the reducing action becomes inactive. On the other hand, on the rich side where the air-fuel ratio is small, the amount of oxygen in the exhaust gas after combustion becomes small, and the oxidizing action becomes inactive and the reducing action becomes active.

In recent years, in response to the demand for low fuel consumption of automobiles, lean side operation in which the air-fuel mixture is burned with an oxygen-rich mixture as much as possible during normal driving has been performed, and the lean side is sufficiently operated.
A catalyst that can purify Ox has been desired.

The applicant of the present invention has disclosed in Japanese Patent Application Laid-Open No. 6-31139 that carbon monoxide (C
We have proposed an exhaust gas purification catalyst for simultaneously purifying O), hydrocarbons (HC), and nitrogen oxides (NOx). This is because the exhaust gas in excess of oxygen is brought into contact with an exhaust gas purifying catalyst in which a porous carrier carries an alkali metal oxide and platinum, whereby the above-mentioned NOx generated during lean operation.
Is a storage-reduction catalyst in which a substance to be purified such as a substance to be purified is stored in an alkali metal oxide and the substance to be purified released during rich operation is reduced and purified by platinum.

[0005] This can exhibit a far higher purification performance than conventional general-purpose catalysts such as a three-way catalyst in which palladium, platinum and rhodium are supported on an alumina carrier.

However, the social demand for environmental purification is increasing, and there has been a demand for an exhaust gas purification catalyst having further improved purification performance.

As a major factor for improving the purification performance,
It is important to improve the supported concentration of the carrier that supports the storage material and the reduction catalyst.

From this viewpoint, the exhaust gas purifying catalyst proposed above has a limit. That is, in the porous structure of alumina, zeolite, zirconia, silica-alumina, silica, etc. used as a carrier, the supporting concentration was already saturated with respect to its supporting ability, and the supporting concentration could not be further increased.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an exhaust gas purifying catalyst whose purifying performance is improved by further increasing the supported concentration of the carrier.

[0010]

In order to achieve the above object, the exhaust gas purifying catalyst of the present invention has, on a carrier, an occlusion material that occludes a substance to be purified and a substance to be purified released from the substance. In an exhaust reduction type exhaust gas purification catalyst supporting a reduction catalyst for reduction and purification, a carrier made of manganese oxide having a hollandite type crystal structure, a nitrogen oxide storage element as an absorption material, and a reduction catalyst It is characterized by supporting a noble metal.

The nitrogen oxide storage element is typically selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals.

The noble metal is typically selected from the group consisting of Pt, Pd and Rh.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention, the storage material and the carrier for the reduction catalyst are made of hollandite.
It is characterized in that it is composed of manganese oxide having a dite) type crystal structure. This hollandite type crystal structure is Mn
O ₆ octahedron is the basic unit, and this is a structure in which 2 units in the vertical direction × 2 units in the horizontal direction are arranged and a pore is formed in the center, and the entire carrier has a three-dimensional pore structure.

This three-dimensional pore structure enables the exhaust gas to sufficiently diffuse in the carrier, and at the same time, the storage material and the reduction catalyst are supported in a highly dispersed manner in atomic units to dramatically increase the supported concentration, The purification ability as an exhaust gas purification catalyst is significantly improved.

Alumina, zirconia, silica-alumina, silica, etc., which have been used as carriers in conventional exhaust gas purification catalysts, do not have the three-dimensional pore structure of the present invention, and are therefore called "porous structures". Also, compared with the highly dispersed carrier at the atomic level according to the present invention, the carrier form was planar.

Zeolite represented by ZSM-5, which has also been conventionally used as a carrier, has a three-dimensional pore structure equivalent to that of the present invention, but Si which forms a skeleton.
Since O ₂ reacts with the storage element to become vitrified, it cannot be used as a carrier of the storage reduction type exhaust gas purification catalyst.

[0017]

[Example 1] According to the present invention, a carrier comprising manganese oxide having a hollandite type crystal structure
An exhaust gas purification catalyst carrying barium (Ba) as an Ox storage material and platinum (Pt) as a reduction catalyst was produced by the following procedure.

<Support: Preparation of hollandite type manganese oxide> KMnO ₄ 11.
An aqueous solution in which 8 g was dissolved was prepared.

Separately, 60 ml of ion-exchanged water was added to MnSO 4.
An aqueous solution in which 11.6 g of ₄ (dehydrated) was dissolved was prepared.

After mixing the above two aqueous solutions, concentrated nitric acid 6
ml was added.

The obtained mixed solution was placed in an autoclave having a volume of 300 ml and subjected to hydrothermal synthesis at 100 ° C. for 24 hours.

The product obtained by hydrothermal synthesis was filtered to collect solids, washed with ion-exchanged water, and dried at 120 ° C. to obtain a powder sample.

Five batches of sample powder were prepared by the above procedure.

When the obtained sample powder was subjected to X-ray diffraction, it was confirmed to have a hollandite type crystal structure. An example of an X-ray diffraction chart using CuKα rays is shown in FIG.

The specific surface area of the sample powder is 13
It was 5 m < ² > g ^<-1 >.

<Catalyst Preparation: Supporting Barium (Ba) and Platinum (Pt)> 20 g of the hollandite-type manganese oxide powder prepared above was dispersed in water, and 8.5 g of barium acetate was added thereto, followed by concentration. It was dried to obtain barium acetate-supported powder.

This barium acetate-supported powder was heated at 550 ° C. for 2 hours.
Burned for hours.

Ammonium carbonate (NH ₄ HCO ₃ ) 2.
The barium acetate-supporting powder after firing was dispersed in an aqueous solution in which 1 g was dissolved in 300 g of water, and after stirring for 15 minutes, the solid content filtered by suction was dried to obtain a barium carbonate-supporting powder.

This barium carbonate-supporting powder was dispersed in 300 g of water, and the number of moles of platinum (Pt) was 1.71.
A dinitrodiamine platinum complex solution was added so as to have a concentration of × 10 ^-3 mol, the mixture was stirred for 30 minutes, and then concentrated and dried to recover a powder.

After drying this powder at 110 ° C. for 2 hours,
It was baked at 450 ° C. for 2 hours.

As a result, a powdery exhaust gas purifying catalyst in which barium (Ba) as a NOx storage material and platinum (Pt) as a reducing catalyst are supported on a carrier made of hollandite type manganese oxide is obtained. Was given.

The obtained catalyst powder had a supported concentration of platinum (Pt) of 8.54 × 10 ⁻⁵ mol per 1 g of the catalyst powder,
The supported concentration of barium (Ba) was 1.
It is 67 × 10 ⁻³ mol.

The above catalyst powder was molded into pellets having a diameter of 1 to 3 mm and subjected to a test.

Example 2 According to the present invention, a carrier composed of manganese oxide having a hollandite type crystal structure,
An exhaust gas purification catalyst carrying barium (K) as a NOx storage material and platinum (Pt) as a reduction catalyst was produced by the following procedure.

20 g of the hollandite-type manganese oxide powder prepared in Example 1 was dispersed in 300 g of water, and the number of moles of platinum (Pt) was 1.71 × 10 ⁻³ mol.
Add the dinitrodiamine platinum complex solution so that
After stirring for 0 minutes, the powder was collected by concentration and drying.

After drying this powder at 110 ° C. for 2 hours,
It was baked at 450 ° C. for 2 hours to obtain a platinum (Pt) -supported powder.

The platinum-supported powder after firing was dispersed in 200 ml of water, 3.26 g of potassium acetate was added thereto, and the mixture was concentrated and dried to recover the powder.

After drying this powder at 120 ° C. for 2 hours,
It was baked at 550 ° C. for 2 hours.

As a result, a powdery exhaust gas purifying catalyst in which potassium (K) as a NOx storage material and platinum (Pt) as a reducing catalyst are supported on a carrier made of hollandite type manganese oxide is obtained. Was given.

The obtained catalyst powder had a supported concentration of platinum (Pt) of 8.54 × 10 ⁻⁵ mol per 1 g of the catalyst powder,
The supported concentration of potassium (K) was 1.6 per 1 g of catalyst powder.
It is 7 × 10 ⁻³ mol.

The above catalyst powder was molded into pellets having a diameter of 1 to 3 mm and subjected to a test.

Comparative Example 1 For comparison, except that the conventional γ-alumina (γ-Al ₂ O ₃ ) was used as a carrier instead of the hollandite-type manganese oxide in Example 1, execution was carried out. An exhaust gas purification catalyst was produced in the same manner as in Example 1, pellets were molded and subjected to the test.

[Comparative Example 2] For comparison, an experiment was conducted except that the conventional γ-alumina (γ-Al ₂ O ₃ ) was used as a carrier instead of the hollandite type manganese oxide in Example 2. An exhaust gas purification catalyst was produced in the same manner as in Example 1, molded into pellets, and subjected to the test.

A model gas evaluation test was conducted on the catalyst pellets produced in Examples 1 and 2 and Comparative Examples 1 and 2 under the following conditions.

<Model Gas Evaluation Test> The catalyst pellets were placed in a reaction vessel, and the model exhaust gas shown in Table 1 was introduced at a gas space velocity of 100,000 h ⁻¹ .

The catalyst bed temperature is variously set in the range of 200 to 600 ° C., the rich gas steady state is switched to the lean gas steady state, and NOx of the gas discharged from the reaction vessel is changed.
The amount of NOx stored by the catalyst until the concentration reached a steady state (NOx saturated storage amount) was measured.

[0047]

[Table 1]

FIG. 2 shows the measurement results of Example 1 and Comparative Example 1 in which the storage material is barium (Ba), and FIG. 3 shows Example 2 and Comparative Example 2 in which the storage material is potassium (K). The measurement result of is shown.

From each figure, no matter which storage material was used, Comparative Example 1 using γ-alumina as a carrier as in the conventional case,
It can be seen that the NOx storage amount is significantly improved in Examples 1 and 2 in which the hollandite-type manganese oxide according to the present invention is used as a carrier, as compared with 2. This is because three-dimensional pores of hollandite-type manganese oxide used as a carrier in the catalyst of the present invention carry barium or potassium as a storage material in a highly dispersed manner at the atomic level, and exhaust gas is sufficiently contained in the catalyst. It is thought to be for the purpose of diffusion. Similarly, it is considered that platinum as a reducing catalyst is also highly dispersed and supported on the three-dimensional pores at the atomic level, and the NOx storage capacity and the exhaust gas reduction capacity are improved, and the exhaust gas with high purification capacity is provided. A purification catalyst is obtained.

[0050]

According to the present invention, since the manganese oxide having a hollandite type crystal structure is used as the carrier, the occluding material and the reducing catalyst can be carried in the three-dimensional pores at a high atomic level dispersion. A high NOx purification capacity that cannot be obtained with conventional catalysts can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing an example of an X-ray diffraction chart of a hollandite type crystal structure manganese oxide according to the present invention.

FIG. 2 is a graph showing the saturated storage amount of NOx at various temperatures in a carrier made of hollandite type crystal structure manganese oxide according to the present invention as barium (B) as a NOx storage material.
a) and a catalyst supporting platinum (Pt) as a reduction catalyst (Example 1) and conventional γ-alumina (γ-Al ₂ O).
₃ is a graph showing comparison with a catalyst (Comparative Example 1) in which the same occlusion material and platinum are supported on a carrier composed of ₃ ).

FIG. 3 is a graph showing the NOx saturated storage amount at various temperatures in a carrier composed of hollandite-type crystal structure manganese oxide according to the present invention, potassium (K) as a NOx storage material, and platinum (Pt) as a reduction catalyst. ) And a conventional catalyst (Example 2) and conventional γ-alumina (γ-Al _2
3 is a graph showing a comparison of a catalyst (Comparative Example 2) in which the same storage material and platinum are supported on a carrier made of O ₃ ).

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4D048 AA06 AA13 AA18 AB05 BA01Y                       BA02Y BA14X BA15X BA18Y                       BA28X BA30X BA31X BA33Y                       BA41X BB01 EA04                 4G069 AA03 BB02A BB02B BB04A                       BB04B BC01A BC03A BC03B                       BC08A BC13A BC13B BC38A                       BC62A BC62B BC71A BC72A                       BC75A BC75B CA02 CA03                       CA08 CA09 CA13 EA02Y                       EB18Y EC22X EC22Y

Claims (3)

[Claims] 1. An occlusion-reduction type exhaust gas purifying catalyst in which a carrier carries an occlusion material for occlusion of a substance to be purified and a reduction catalyst for reducing and purifying the substance to be purified released from the occlusion material, An exhaust gas purifying catalyst, characterized in that a carrier made of manganese oxide having a dite-type crystal structure carries a nitrogen oxide storage element as a storage material and a noble metal as a reduction catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the nitrogen oxide storage element is selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals. 3. The exhaust gas purifying catalyst according to claim 1, wherein the metal is selected from the group consisting of Pt, Pd and Rh.