JP2001246249A - Exhaust gas purification catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst capable of reliably avoiding a decrease in durability due to cracks while ensuring a sufficient NOx storage capacity by optimally setting the amount of potassium added to a catalyst layer. provide. SOLUTION: In an exhaust gas purifying catalyst including a cordierite carrier 1 and a catalyst layer 2 and at least potassium added to the catalyst layer 2 as an occluding agent, the amount of potassium added to the catalyst layer 2 is determined by NOx storage. The range was set from 8.5 g / l, which is the lower limit capable of satisfying the required amount of capacity, to 20 g / l, which is the upper limit capable of preventing damage exceeding an allowable limit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst excellent in durability and exhaust gas purifying performance.

[0002]

[Related Background Art] Lean-burn engines such as lean-burn engines and in-cylinder injection engines use a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio in a predetermined operating range in order to improve fuel efficiency and exhaust gas characteristics. Be driven. NOx (nitrogen oxide) in exhaust gas during lean air-fuel ratio operation
Is not sufficiently purified by a three-way catalyst. Therefore, a NOx catalyst for storing NOx in exhaust gas in an oxidizing atmosphere is provided, and the NOx stored in the catalyst is reduced to N2 (nitrogen) in a reducing atmosphere. Is known to reduce NOx emissions to the atmosphere. In this type of lean NOx storage catalyst, potassium (K) is added to the catalyst layer as a NOx storage agent, as described in, for example, JP-A-9-85093.
There is one that improves Ox storage performance.

[0003]

Although the NOx storage capacity increases with an increase in the amount of potassium added, cracks may occur in the catalyst under conditions where the NOx catalyst is exposed to a high temperature for a long period of time. This causes a decrease in durability. In order to investigate the cause of the crack generation in the NOx catalyst, the present inventors manufactured a NOx catalyst in which potassium was added as a storage agent to a catalyst layer supported on a honeycomb-type cordierite carrier.
A bench test of an engine equipped with a catalyst and a running test of a vehicle equipped with this type of engine were performed. In the bench test and the actual vehicle running test, the engine and the vehicle were operated under the condition that the NOx catalyst was exposed to a predetermined or higher temperature for a considerable time. After completion of the operation, element analysis on the cut surface of the NOx catalyst was performed by the EPMA method (electron probe micropartial analysis method), and cordierite (Mg ₂
In the Al ₄ Si ₅ O ₁₈ ) layer, a compound of potassium, magnesium, aluminum, silicon and oxygen (KMg ₄ Al ₉ Si ₉₎
O ₃₆ ) and the compound of potassium, aluminum, silicon and oxygen (KAlSiO ₄ ) were confirmed to be present.

According to the above experiment, when the NOx catalyst is exposed to a high temperature, potassium added to the catalyst layer (wash coat) penetrates into the cordierite carrier, and potassium reacts with the cordierite under a high temperature atmosphere. To form the above compounds. Here, it is understood that the potassium compound easily penetrates into the cordierite carrier because of its high water solubility and low melting point. Then, when a compound having a different coefficient of thermal expansion from cordierite is formed in the cordierite carrier, cracks are generated in the cordierite carrier with a change in the catalyst temperature during and before and after the use of the catalyst, and the NOx catalyst The durability will be reduced.

Accordingly, the present invention provides an exhaust gas purifying method capable of properly setting the amount of potassium to be added to a catalyst layer to ensure sufficient NOx storage capacity and reliably preventing deterioration in durability due to cracks. It is intended to provide a catalyst for use.

[0006]

To achieve the above object, the present invention comprises a cordierite carrier and a catalyst layer,
In the exhaust gas purifying catalyst obtained by adding at least potassium as an occluding agent to the catalyst layer, the amount of potassium added is 8.5 to 20 g per unit volume of the cordierite carrier.
/ L.

As the amount of potassium added increases, the catalyst NO
x Storage capacity is improved. However, under conditions where the catalyst is exposed to high temperatures, potassium added to the catalyst layer penetrates into the cordierite carrier and has a coefficient of thermal expansion different from cordierite (potassium, magnesium, aluminum, silicon and oxygen compounds). Compound KMs of compound KMg ₄ Al ₉ Si ₉ O ₃₆ , potassium, aluminum, silicon and oxygen
iO ₄ ), which may cause cracks in the cordierite carrier as the catalyst temperature changes. Therefore, it is necessary to limit the amount of added potassium from this point.

[0008] It was determined how the amount of potassium added affects the NOx storage capacity of these catalysts and the amount of damage caused by cracks (see FIG. 2). It can be seen that both the NOx storage capacity and the damage amount increase with an increase in the amount of potassium added. In order to satisfy the required NOx storage capacity, the potassium addition amount of 8.5 g / l or more was secured and the allowable limit was set. It is understood that the amount of potassium added must be suppressed to 20 g / l or less in order to prevent excessive damage.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying catalyst embodying the present invention will be described below. The exhaust gas purifying catalyst of the present embodiment is configured as a NOx catalyst having a honeycomb (monolith) type cordierite carrier including a large number of cells. FIG. 1 shows one cell of the cordierite carrier 1. A catalyst layer 2 is supported on the surface of the cordierite carrier 1, and potassium (K) is added to the catalyst layer 2 as a NOx storage agent.

The cordierite carrier 1 is made of, for example, alumina source powder, silica source powder and magnesia source powder.
A mixture obtained by mixing alumina, silica, and magnesia in a cordierite composition is dispersed in water, the solid content is formed into a honeycomb shape, and the formed honeycomb body is fired. The catalyst layer 2 is prepared, for example, by preparing a slurry composed of a noble metal such as platinum or an alkali metal such as potassium, immersing the cordierite carrier 1 in this slurry, and drying and firing it to remove water. , Formed on the surface of the cordierite carrier 1.

The amount of these components added to the catalyst layer 2 varies depending on the amount of potassium contained in the slurry.
When preparing the slurry, the amount of each component is determined. In the present embodiment, potassium is 8.5 to 8.5 per unit volume (per liter) of the cordierite carrier 1.
The blending amount in the slurry is adjusted so that it is added to the catalyst layer 2 in the range of 20 g / l.

As described above, a NOx catalyst obtained by coating the cordierite carrier 1 with the catalyst layer 2 is obtained. As is conventionally known, the NOx catalyst is housed in a case and arranged in an exhaust pipe of a lean burn internal combustion engine. This N
According to the Ox catalyst, NOx in the exhaust gas is stored in the form of nitrate under the action of the catalyst species dispersed in the catalyst layer 2 during operation of the engine at a lean air-fuel ratio. Also, during operation of the engine at a rich air-fuel ratio, nitrate is decomposed, and the stored NOx
Is reduced to nitrogen and released from the NOx catalyst to the atmosphere.

When the internal combustion engine equipped with such a NOx catalyst is operated for a long time, the NOx catalyst is exposed to a high temperature for a long time. In this case, potassium added to the catalyst layer penetrates into the cordierite carrier 1 and has a coefficient of thermal expansion different from that of cordierite (a compound of potassium, magnesium, aluminum, silicon and oxygen KMg ₄ Al ₉ Si).
₉ O _{3 6} , a compound of potassium, aluminum, silicon and oxygen, such as KAlSiO ₄ ), which becomes a factor of causing cracks in the cordierite carrier 1 with a change in catalyst temperature accompanying engine stop and operation. Therefore, it is necessary to limit the amount of added potassium from this point. On the other hand, since the NOx storage capacity of the catalyst tends to increase with an increase in the amount of potassium added, there is a trade-off between the NOx storage capacity and the damage amount.

The present inventor has determined through tests how the amount of potassium added affects the NOx storage capacity of these catalysts and the amount of damage caused by cracks. As shown in FIG. 2 showing the test results, it was found that both the NOx storage capacity and the damage amount increased with an increase in the amount of potassium added. In order to satisfy the required NOx storage capacity, the unit volume of the cordierite carrier 1 was required. It is necessary to secure a potassium addition amount of 8.5 g / l or more per unit. On the other hand, in order to prevent damage exceeding an allowable limit, the potassium addition amount is suppressed to 20 g / l or less per unit volume of the cordierite carrier 1. It turns out that it is necessary.

As described above, in this embodiment, the amount of potassium to be added to the catalyst layer 2 is set within a range derived from the test results. Therefore, it is possible to reliably prevent a decrease in durability due to cracks while ensuring a sufficient NOx storage capacity. The description of the embodiment is finished above, but aspects of the present invention are not limited to this embodiment. For example, in the above embodiment, the cordierite carrier 1 is immersed in a slurry containing these components in order to add potassium and the like to the catalyst layer 2, but any method capable of realizing the desired amount of added potassium is used. For example, the present invention is not limited to this. For example, the constituent particles of the catalyst layer 2 may be impregnated with potassium or the like, or may be impregnated by reacting a wash-coated catalyst with a salt.

[0016]

As described above, according to the exhaust gas purifying catalyst of the present invention, the amount of potassium added to the catalyst layer is reduced by NO.
8.5 g / which is the lower limit that can satisfy the required amount of x storage capacity.
1 to 20 g / l, which is the upper limit capable of preventing damage exceeding the allowable limit, it is possible to ensure sufficient NOx storage capacity and to reliably prevent a decrease in durability due to cracks. .

[Brief description of the drawings]

FIG. 1 is a sectional view showing cells of a cordierite carrier in an exhaust gas purifying catalyst according to an embodiment.

FIG. 2 is an explanatory diagram showing the relationship between NOx storage capacity, the amount of damage due to cracks, and the amount of potassium added.

[Explanation of symbols]

 1 cordierite carrier 2 catalyst layer

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/10 B01D 53/36 104A 3/28 301 (72) Inventor Naoto Yamada Shibago, Minato-ku, Tokyo No. 33-8, Mitsubishi Motors Industry Co., Ltd. (72) Inventor Iwachimichi Uniform 5-33-8, Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Industry Co., Ltd. No. 33-8 Mitsubishi Motors Corporation F-term (reference) 3G091 AA12 AB06 AB08 BA10 BA14 BA39 CB02 FA06 FB02 FB03 FB10 FC04 FC07 FC08 GA06 GB01X GB02W GB02Y GB06W GB10X GB17X HA18 4D002 AA12 AB01 AC10 BA04 DA03 GB08 AA18A06 AB03 BA01X BA01Y BA03X BA03Y BA06X BA06Y BA12X BA12Y BA14X BA14Y BB02 EA04 4G069 AA03 AA08 BA13A BA13B BC03A BC03B CA03 CA09 DA06 EA19 FA03 FB15 FB30 FB31 FC08

Claims (1)

[Claims] 1. An exhaust gas purifying catalyst comprising a cordierite carrier and a catalyst layer, wherein at least potassium is added to the catalyst layer as an occluding agent, wherein the amount of potassium added is per unit volume of the cordierite carrier. An exhaust gas purifying catalyst, wherein the catalyst is set to 8.5 to 20 g / l.