JPH11276898A - Exhaust gas purification catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To purify NOx discharged from an internal combustion engine efficiently especially in an oxygen-excess atmosphere by using zeolite containing trivalent or tetravalent manganese as an exhaust gas purification catalyst. SOLUTION: By zeolite containing trivalent or tetravalent manganese, NOx discharged from an internal combustion engine is removed in a low HC concentration in which an HC/NOx ratio is not greater than 5. This catalyst is obtained by a method in which Na-type ZSM5 slurry 23.8 in silica/alumina ratio is added with manganese nitrate, agitated under heating, filtered, washed, dried, and baked. The valence of manganese contained in the catalyst is a mixed valency of trivalent and tetravalent. The catalyst is molded and crushed to obtain pellets, and the activity is measured. In this way, when NOx discharged from an internal combustion engine is removed especially in an oxygen- excess atmosphere, high NOx purification performance is obtained in a low HC quantity of 5 or less.

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 capable of effectively purifying NOx discharged from an internal combustion engine such as an automobile and a generator, particularly in an atmosphere containing excess oxygen.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of effective use of resources and suppression of global warming, an internal combustion engine having low fuel consumption by lean combustion has been desired. Since NOx discharged from the internal combustion engine has an oxygen-excess atmosphere, it cannot be sufficiently purified by a conventionally used three-way catalyst, and the development of a high-performance catalyst has been desired.

For example, Japanese Unexamined Patent Publication No. 60-125250 discloses that NOx can be purified by a zeolite containing copper even in an oxygen-excess atmosphere. Although many catalysts have been reported since then, in order to obtain high purification performance, the amount of HC (hydrocarbon) required as a reducing agent was large in each case. For example, JP-A-4-
363146 describes Mn (manganese) / ZS
When M5 (zeolite) is used as a catalyst, HC / NO
A large amount of HC was added so that x became 6 or more.

[0004]

In the conventional manganese-containing zeolite catalyst described above, when the amount of HC in the exhaust gas from the engine is small, such as in a diesel engine, the fuel efficiency is deteriorated by adding the catalyst. However, there is a problem that the advantage of lean combustion is lost in the first place.

The present invention has been made by paying attention to the above points, and specifically, has a low HC / NOx ratio of 5 or less and NOx.
An object is to provide a catalyst that can be purified.

[0006]

Accordingly, the present inventor has first developed HC required for NOx purification for some representative catalysts.
As a result of obtaining the / NOx ratio and analyzing the reaction mechanism, two points, activation of NO and suppression of HC combustion, were found. Therefore, when examining many catalysts at these two points, it was found that when a catalyst composed of zeolite containing manganese at a valence of III or IV, a high purification performance can be obtained even at a low HC / NOx ratio of 5 or less. I understand. That is, by providing the following configuration, the above-mentioned problem can be solved, and NOx can be removed under an oxygen-excess atmosphere even with a small amount of HC or a small amount of added HC, so that both low fuel consumption and lean NOx purification in a lean burn engine can be achieved. Becomes possible.

(1) NOx exhausted from an internal combustion engine, which is composed of zeolite containing manganese having a valence of III or IV
At a low HC concentration at which the ratio of HC to NOx is 5 or less.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below as Examples 1 and 2.

The zeolites used in Examples and Comparative Examples are substances having a high non-stoichiometric ratio (M ^I , M ^II _1/2 ).
aluminosilicate represented by _m Al _m Si _n O _{2 (m + n)} × H ₂ O, characterized by exchanged cation species (M ^I and M ^II are cations of I valence and II valence, respectively), Si and Al-silica ratio (n / (m / 2)) which is the ratio of Al and A type, Y type,
It is determined by the crystal structure represented by ZSM5 type, mordenite type and the like.

(Example 1) The silica-alumina ratio was 23.8.
Manganese nitrate was added to the slurry of Na type ZSM5 in
The mixture was stirred for 24 hours while being heated to 0 ° C., and washed by filtration.
After drying at 0 ° C. for 10 hours, it was calcined at 500 ° C. for 2 hours to obtain a catalyst 1 (Mn [III, IV] / ZSM5). When the valence of manganese contained in the catalyst was measured by XPS (X-ray photoelectron spectroscopy) or the like, the valence of III and IV was mixed. The catalyst was molded and pulverized to obtain pellets of 0.5 to 1.0 mm, and the activity was measured in evaluation tests 1 and 2. Table 1 shows the results.
It was shown to.

(Example 2) The silica-alumina ratio was 18.4.
Manganese nitrate was added to the slurry of Na-type mordenite, and the mixture was stirred at 50 ° C. for 24 hours, filtered, washed, dried at 120 ° C. for 10 hours, and calcined at 500 ° C. for 2 hours to obtain catalyst 2 (Mn [III , IV] / mordenite). When the valence of manganese contained in the catalyst was measured by XPS (X-ray photoelectron spectroscopy) or the like, the valence of III and IV was mixed. The catalyst was molded and pulverized to obtain pellets of 0.5 to 1.0 mm, and the activity was measured in evaluation tests 1 and 2. The results are shown in Table 1.

Next, a catalyst was prepared based on the following Comparative Examples 1 and 2 to be compared with the above-mentioned Example.

Comparative Example 1 A silica-alumina ratio of 23.8
Manganese nitrate was added to a slurry of Na-type ZSM5 with ammonia water while maintaining the pH at about 10, and the mixture was stirred for 24 hours while heating to 50 ° C, and filtered and washed.
After drying for 2 hours, the mixture was calcined at 500 ° C. for 2 hours to obtain catalyst 3 (Mn
[II] / ZSM5) was obtained. The valence of manganese contained in the catalyst was measured by XPS (X-ray photoelectron spectroscopy) and found to be II valence. The catalyst is molded and pulverized to 0.5-1.0
mm pellets were obtained and the activity was measured in evaluation tests 1 and 2. The results are shown in Table 1.

Comparative Example 2 A silica-alumina ratio of 23.8
To a slurry of Na type ZSM5, copper acetate was added while keeping the pH at about 10 with aqueous ammonia, and the mixture was stirred for 24 hours while heating to 50 ° C, washed by filtration, dried at 120 ° C for 10 hours, and then dried at 500 ° C. After firing for 2 hours, catalyst 4 (Cu / ZS
M5) was obtained. The catalyst is molded and pulverized to 0.5-1.0m
m were obtained and the activity was measured in evaluation tests 1 and 2. The results are shown in Table 1.

Evaluation test 1 NO 1000 ppm, C ₃ H ₆ 1000 ppm, O ₂
A reaction gas having a composition of 6%, 10% of CO _2, 10% of H ₂ O, and the balance of N ₂ was used. In this case, the HC / NOx ratio has a low value of 3. The heating temperature of the reaction gas is controlled by using an electric furnace.
b] and after passing [Ca], respectively.
x The purification rate was determined.

NOx purification rate = {(Cb-Ca) / Cb}
× 100 (%) Evaluation test 2 1000 ppm of C ₃ H ₆ was further added to the conditions of the evaluation test 1. In this case, the HC / NOx ratio has a high value of 6.
Under these conditions, a similar method was used to determine the NOx rate.

[0017]

[Table 1]

As shown in Table 1, the catalysts 1 and 2 according to the embodiment of the present invention exhibited a high NOx purification rate even when the HC / NOx ratio was as low as 3, whereas the catalyst 3 of the comparative example did not. , The activity was low even with the same Mn / ZSM5. When the HC / NOx ratio is 3 in the catalyst 4, the activity is low, and the HC / NOx
Even when the ratio was 6, it was still below the purification rate of the present invention.

[0019]

As described above, according to the present invention, when NOx discharged from an internal combustion engine is removed by a catalyst particularly in an oxygen-excess atmosphere, a high N content with a small HC amount of 5 or less is achieved.
Ox purification performance is obtained, and therefore H added to the exhaust
It is possible to reduce the amount of carbon and suppress deterioration of fuel efficiency, and furthermore, it is possible to improve reliability and reduce costs by downsizing and simplifying the HC addition method or its apparatus.

Claims (1)

[Claims] 1. The method according to claim 1, wherein said NOx is composed of a zeolite containing manganese having a valency of III or IV and emitted from an internal combustion engine.
An exhaust gas purifying catalyst, wherein the catalyst is removed at a low HC concentration such that the ratio of HC to NOx is 5 or less.