Catalyst for exhaust gas purification

Details Catalyst for exhaust gas purification Catalyst for exhaust gas purification

2002-10-25

2004-09-28

Stoner, Kliey (Department: 1725)

Catalyst, solid sorbent, or support therefor: product or process

Zeolite or clay, including gallium analogs

And additional al or si containing component

C502S064000, C502S074000, C502S305000, C502S313000, C502S325000, C502S339000, C502S349000, C502S353000, C502S240000, C502S262000, C502S261000

Reexamination Certificate

active

06797663

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an exhaust gas clean-up catalyst, especially an exhaust gas clean-up catalyst capable of keeping a clean-up ability high.
BACKGROUND ART
In order to improve fuel consumption characteristic and exhaust gas characteristic, lean-burn engines such as lean-burn engines, in-cylinder fuel injection engines, etc. are operated at a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio, in predetermined operating regions. While the engine is operated at a lean air-fuel ratio, NO
x
(nitrogen oxide) contained in exhaust gas cannot sufficiently be removed by a three-way catalyst. Thus, it is known to provide an NO
x
catalyst which occludes NO
x
contained in exhaust gas in an oxidative atmosphere, and reduce the NO
x
occluded by the NO
x
catalyst to N
2
(nitrogen) in a reducing atmosphere, to thereby reduce the emission of NO
x
into the atmosphere. As an occlusion-type lean NO
x
catalyst of this kind, there is one whose NO
x
occluding ability is improved by adding potassium (K), which is an alkaline metal, as an NO
x
occluding agent, as disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 9-85093.
However, when the NO
x
catalyst having potassium added is placed in high temperature for a long time, cracks form in the catalyst in some cases. Such cracks lower the durability of the NO
x
catalyst.
In order to find out causes of the lowering of the durability of the NO
x
catalyst, the inventors of the present invention produced an NO
x
catalyst in which the above-mentioned potassium (K), which is an alkaline metal, was added as an NO
x
occluding agent to a catalytic layer supported by a honeycomb-type cordierite support (porous support), and carried out a bench test of an engine equipped with the NO
x
catalyst, and a driving test of a vehicle having such an engine installed. In the bench test and the practical vehicle driving test, the engine and the vehicle were operated and driven in the conditions in which the NO
x
catalyst was exposed to high temperature of 650° C. and above for quite a long time. After the engine and the vehicle were thus operated and driven, elemental analysis of the NO
x
catalyst was carried out on its cutting plane by EPMA (Electron Probe Microanalysis). By this, it was found that in the cordierite (Mg
2
Al
4
Si
5
O
18
) layer of the catalyst existed KMg
4
Al
9
Si
9
O
36
, a compound of potassium, magnesium, aluminum, silicon and oxygen, and KAlSiO
4
, a compound of potassium, aluminum, silicon and oxygen.
From the above experiment, it is thought that when the NO
x
catalyst is exposed to high temperature, potassium which has been added to the catalytic layer (wash coat) penetrates into the cordierite support and reacts with cordierite in a high-temperature atmosphere to form the above-mentioned compounds. Here, it is thought that since potassium compounds are high in water solubility and low in melting point, potassium easily penetrates into the cordierite support. When compounds having a thermal expansion coefficient different from that of cordierite is formed in the cordierite support, cracks form in the cordierite support due to change in the catalyst temperature during, before and after the use of the catalyst, and thus the strength of the catalyst lowers.
As mentioned above, the NO
x
catalyst which contains potassium or the like as an occluding agent is used in an oxidative atmosphere. In the oxidative atmosphere, nitrate (—NO
3
) of the occluding agent is formed by chemical reaction of the occluding agent with nitrogen contained in exhaust gas, and hence, the NO
x
occluding ability lowers. Here, the occluding ability can be restored by forming a reducing atmosphere around the NO
x
catalyst and thereby degrading the nitrate. However, even when such measures are taken, the clean-up ability lowers in some cases in which the NO
x
catalyst is used in high temperature for a long time.
From the results of an experiment carried out by the inventors, it is thought that one of the causes of the clean-up ability lowering lies in the occluding agent gradually vaporizing and escaping from the NO
x
catalyst in high temperature, thus a large amount of the occluding agent in the catalyst getting lost. Specifically, the inventors produced an NO
x
catalyst in which a cordierite support supported a catalytic layer which contained potassium as an occluding agent, and obtained the amount of potassium contained in the unused NO
x
catalyst by XRF (X-ray Fluorescence Spectrochemical Analysis). Then, after using the catalyst in high temperature for a long time (for example, in 850° C. for 32 hours), the amount of potassium contained in the catalyst was obtained. Then, the ratio of the lost potassium was obtained by dividing the difference between the amount of potassium contained in the unused catalyst and the amount of potassium contained in the after-use catalyst by the amount of potassium contained in the unused catalyst. It was found that the ratio of the lost potassium was several ten % to 50%.
In this connection, Japanese Unexamined Patent Publication No. 2000-279810 discloses a technique in which, for example, when alkaline metal such as potassium is used as an NO
x
occluding agent, silicon which has an affinity to alkaline metal is provided in a catalytic layer to restrain the movement of the alkaline metal into a base (cordierite support) and thereby retain the alkaline metal in the catalytic layer.
However, an occluding agent made of alkaline metal has, as its property, a large electron-donating function. Thus, there is a problem that by emitting electrons, alkaline metal lowers NO
x
oxidation performance by precious metal. Specifically, the NO
x
occluding catalyst makes NO react with oxygen and also with alkaline metal and occludes NO
x
in the form of the above-mentioned nitrate (—NO
3
), but the electron-donating function of alkaline metal hinders the reaction of NO with oxygen. Occluding agents which are higher in occluding ability such as potassium tend to be higher in electron-donating function.
Thus, when a larger amount of alkaline metal such as potassium is retained in the catalytic layer as in the technique disclosed in the above-mentioned publication, occlusion ability which depends on the reaction of NO with alkaline metal improves, while the oxidation reaction of NO does not go on well. Thus, balance between the reactions is lost, which leads to a problem that NO
x
removing ability lowers.
Another problem with the occluding agent made of alkaline metal such as potassium is that alkaline metal reacts with S (sulfur) contained in exhaust gas to form sulfate (—SO
4
), which deteriorates the NO
x
occluding ability. (This is called sulfur-poisoning.) Regarding this problem, it is known that sulfur in exhaust gas can be removed by making the catalyst high in temperature and making the exhaust gas rich in air-fuel ratio to thereby form a reducing atmosphere. (This is called sulfur-purging.) However, sulfur-purging has a problem that if the occluding agent moves and is retained deep below the catalyst surface with which exhaust gas flow comes in contact, the occluding agent which has turned into sulfate is not easily exposed to the high-temperature reducing atmosphere, and hence it takes time to remove sulfur.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide an exhaust gas clean-up catalyst in which movement of an occluding agent is restrained without lowering the oxidation ability, and degree of lowering of the exhaust-gas clean-up ability is much reduced, and which has high durability.
In order to achieve the above object, the present invention provides an exhaust gas clean-up catalyst comprising a support and a catalytic layer, said catalytic layer containing, as an occluding agent, at least one chosen from a group consisting of alkaline metals and alkaline earth metals, wherein said catalytic layer comprises a first catalytic layer which contains said occluding agent and an acid material having a high affinity to said occl

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