JPH1099691A - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

Abstract

[PROBLEMS] To purify exhaust gas having low temperature, high flow rate and high oxygen concentration, such as exhaust gas of a direct injection gasoline engine. SOLUTION: Exhaust gas is mixed with a catalyst obtained by mixing β-type zeolite having pores having different vertical and horizontal pore diameters with activated alumina, and carrying Pt and Pd as a catalyst metal on the mixture. Purify exhaust gas.

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 an exhaust gas purifying method, and more particularly, to a low temperature, high flow rate exhaust gas during a lean combustion operation of a diesel engine or a direct injection gasoline engine. This is an invention particularly suitable for purifying exhaust gas having a high oxygen concentration.

[0002]

2. Description of the Related Art With respect to a catalyst for purifying exhaust gas of an automobile engine, a β-type zeolite is loaded with a catalytic metal such as Cu by ion exchange to reduce NOx (nitrogen oxide) contained in the exhaust gas. There is a proposal to enhance the decomposition and purification performance (JP-A-5-220403).
Reference). This proposal aims to enhance the high-temperature durability of a NOx catalyst by utilizing the fact that β-zeolite has higher heat resistance than other zeolites (ZSM-5).

[0003]

However, when the catalyst metal is simply supported on the β-type zeolite, particularly when the exhaust gas temperature is as low as 100 to 140 ° C. and the oxygen concentration is high, HC in the exhaust gas is particularly low. (Hydrocarbons) and CO (carbon monoxide) cannot be sufficiently oxidized and purified. SV
When the (space velocity) is high, the tendency becomes remarkable. That is, in the case of the exhaust gas of the direct injection gasoline engine operated at A / F = 40 to 100, the temperature is λ.
= 200 ° C lower than the exhaust gas of a gasoline engine operated at = 1, and even when the catalyst is directly connected to the exhaust manifold, the exhaust gas temperature at the catalyst inlet is only about 130 ° C during idling operation. Such low-temperature exhaust gas cannot be purified. In addition, the exhaust gas of the direct injection gasoline engine has a problem that it is difficult to oxidatively decompose because the chemically stable aroma component and paraffin are increased.

[0004]

According to the present inventors, the pores of the above-mentioned β-type zeolite have different vertical and horizontal diameters, and the zeolite having such pores adsorbs a relatively large amount of HC in exhaust gas even at a low temperature. The present inventors have found that if the adsorption phenomenon is used well, the low-temperature activity of the catalyst is dramatically improved, and the present invention has been completed.

That is, the exhaust gas purifying catalyst according to the invention of the present application comprises a zeolite having pores having different vertical and horizontal pore sizes, activated alumina, and a noble metal, and the noble metal is used as a catalyst metal in the above-mentioned zeolite and active catalyst. It is supported on at least one of alumina.

[0006] In the case of this catalyst, the activity at a low temperature is remarkably superior to that of a catalyst comprising only a combination of activated alumina and a noble metal. The reason is not clear, but is considered as follows.

In the case of a combination of activated alumina and a noble metal, the activity of the noble metal itself is low when the exhaust gas temperature is low, and the low-temperature activity as a catalyst is low because the activated alumina itself does not originally have a high ability to adsorb HC and the like. It can be said that it is low. On the other hand, zeolite having different vertical and horizontal diameters of the pores efficiently absorbs relatively large molecules to small molecules in the exhaust gas even when the exhaust gas temperature is low due to the pore shape. Although the amount of adsorption can be increased and the amount of adsorption is large, the once adsorbed molecule is hardly desorbed due to the short diameter portion. Therefore, in the case of the catalyst of the present invention, at a low temperature, the zeolite adsorbs HC and the like in the exhaust gas and provides it to the oxidation catalytic reaction by the noble metal. It is considered that the purification efficiency of the exhaust gas is relatively high even if is not so high.

However, simply supporting a noble metal on such a zeolite does not exhibit the expected low-temperature activity. That is, in the present invention, the activated alumina has a function of increasing the low-temperature activity of the noble metal by the interaction with the noble metal.
It is considered that this has an effect on promoting the catalytic reaction by the noble metal in terms of adsorption and desorption such as.

In preparing the catalyst, the noble metal may be supported on a mixture of the zeolite and activated alumina, and the noble metal may be supported on the zeolite and activated alumina. May be mixed with activated alumina carrying a noble metal, or the above zeolite may be mixed, or activated zeolite carrying a noble metal may be mixed with activated alumina.

In order to obtain the above-mentioned effect, the zeolite preferably has a minor axis of pores of 0.5 nm or more, more preferably 0.6 nm or less, and further has a major axis of less than 0.6 nm. The ratio to the minor axis (major axis / minor axis) is preferably larger than 1.10. Preferred as such zeolite is β-type zeolite.
In the case of the zeolite, the minor axis is 0.55 nm and the major axis is 0.5 mm.
It has pores of 7 nm, and the ratio of the major axis / minor axis of the pores is 1.
25 or more. In the combination of activated alumina and β-type zeolite, the ratio is preferably (active alumina) :( β-type zeolite) = 1: 1.

As the noble metal, a combination of Pt and Pd excellent in low-temperature activity as an oxidation catalyst is preferable, and Pt / Pd = 5/3 is particularly preferable. When Pt and Pd are combined as a noble metal and supported on a mixture of the β-type zeolite and activated alumina, the interaction between the Pt and Pd and the β-type zeolite,
In addition, the interaction between Pt and Pd and activated alumina is strengthened, which is advantageous for improving the low-temperature activity of the catalyst.

Another invention of this application is an exhaust gas purification method using the above-described exhaust gas purification catalyst, which contains HC or CO and has a temperature of 100 ° C. to 140 ° C.
By contacting the exhaust gas at a temperature of ° C. with the exhaust gas purifying catalyst, the HC or CO is oxidized and purified. That is, the exhaust gas temperature is 100 ° C.
Even at a low temperature of 140 ° C., the oxidation reaction of HC or CO can efficiently proceed.

[0013]

Thus, according to the present invention, there is provided a zeolite having pores having different vertical and horizontal pore sizes, activated alumina, and a noble metal, and the noble metal serves as a catalyst metal in at least one of the zeolite and the activated alumina. Since the zeolite is supported, the excellent adsorption capacity of the zeolite and the excellent oxidation catalyst function of the combination of the noble metal and activated alumina work together to provide excellent low-temperature activity in purifying HC and the like in exhaust gas. Is obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst of the present invention is supported on a honeycomb carrier, and this carrier is disposed in an exhaust passage near an exhaust manifold of a direct injection gasoline engine. In a direct injection gasoline engine, the air-fuel ratio is increased from λ = 1 to A / F = 40.
It can be adjusted from about 10 to about 10.

<Preparation of Catalyst> -Catalyst of Example- γ-alumina as active alumina and β-type zeolite (SiO ₂ / Al ₂ O ₃ = 25) were mixed at a weight ratio of 1: 1. A slurry was prepared by adding 10 wt% of hydrated alumina, wash-coated on a cordierite honeycomb carrier, and fired at 500 ° C. for 2 hours in the air. Then, a predetermined amount of dinitrodiamineplatinum (II) nitric acid acidic solution (Pt is 3.6 wt% of γ-alumina)
Amount) and a predetermined amount of a dinitrodiaminepalladium (II) nitric acid acidic solution (palladium is 2.2 wt% of γ-alumina).
%), Impregnated into the above-mentioned wash coat layer of the honeycomb carrier, dried at 150 ° C. for 1 hour, and then baked at 500 ° C. for 2 hours in the air to obtain the objective. An exhaust gas purifying catalyst was obtained.

Comparative Example 1 A catalyst was prepared according to the same conditions and method as in the above Examples, except that γ-alumina was used as a wash coat for the honeycomb carrier. The wash coat amount of γ-alumina was the same as the combined coat amount of γ-alumina and β-type zeolite in the examples.

Comparative Example 2 A catalyst was prepared according to the same conditions and method as in the above Examples except that the wash coat for the honeycomb carrier was only β-type zeolite. The wash coat amount of β-type zeolite was the same as the total coat amount of γ-alumina and β-type zeolite in the examples.

<Evaluation of Catalyst> -Low Temperature Activity- For each of the catalysts of the above Examples and Comparative Examples, the light-off performance, that is, T-50 relating to HC purification (exhaust gas temperature at which a 50% purification rate is obtained ( Temperature at the catalyst inlet))
Was evaluated by a rig test. The composition of the test gas is as follows and simulates the exhaust gas of a direct injection gasoline engine (corresponding to A / F = 40).

HC: 4000 ppm C, CO: 0.14
%, NO; 250 ppm, O ₂ ; 17%, CO ₂ ;
%, H ₂ ; 0.07%, balance N ₂ test gas was passed through the catalyst at SV = 110,000 / h.

The test results are shown in FIG. According to the figure, the temperature of T-50 is lower by about 20 ° C. than that of Comparative Example 1 in the catalyst of the example. From this, Pt
It can be seen that when γ-alumina and β-type zeolite are combined with Pd and Pd as catalyst metals, the low-temperature activity of the catalyst is dramatically improved. In Comparative Example 2, the β-type zeolite was used as the supporting base material as in the Example, but the temperature of T-50 was slightly higher than that in Comparative Example 1 using γ-alumina. Therefore, the expected effect cannot be obtained even if only β-type zeolite is used as the supporting base material, and γ-alumina and β
It can be said that the expected low-temperature activity can be obtained only when there is an interaction with the zeolite or an interaction between these and Pt and Pd.

-Regarding NOx purification rate-Further, when the NOx purification rates of the respective catalysts of the above-mentioned Examples and Comparative Examples were measured by a rig test under the same conditions as in the evaluation of the low-temperature activity, the results shown in FIG. 2 were obtained. Was. The exhaust gas temperature immediately before the catalyst in this test is 130 ° C.
According to the figure, the embodiment using a mixture of γ-alumina and β-type zeolite as the supporting base material has a higher oxygen concentration of the test gas (A / F = 40), but has a higher efficiency than the comparative example. It shows a very high purification rate. This is because, as described above, the catalysts of the examples have excellent low-temperature activity and the HC in the exhaust gas
It is considered that, since the HC has a high ability to decompose, when the HC is oxidatively decomposed, the HC works efficiently as a reducing agent for the decomposition reaction of NO to accelerate the decomposition reaction.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of a rig test for evaluating low-temperature activities of Examples and Comparative Examples.

FIG. 2 is a graph showing the results of a rig test relating to the NOx purification rates of Examples and Comparative Examples.

[Explanation of symbols]

 None.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Makoto Kyogoku 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd.

Claims (4)

[Claims] 1. A zeolite having pores having different vertical and horizontal pore sizes, activated alumina, and a noble metal, wherein the noble metal is supported on at least one of the zeolite and the activated alumina as a catalyst metal. Exhaust gas purification catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the zeolite is a β-type zeolite. 3. The exhaust gas purifying catalyst according to claim 2, wherein Pt and Pd as the noble metals are carried on a mixture of the β-type zeolite and activated alumina. Gas purification catalyst. 4. It contains HC or CO and has a temperature of 100
An exhaust gas at a temperature of from 140 ° C. to 140 ° C. is brought into contact with the catalyst for purifying exhaust gas according to claim 1, whereby the HC is reduced.
Alternatively, an exhaust gas purification method characterized by purifying by oxidizing CO.