JPH0326343A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst which removes NOx at a high rate even in an atmosphere contg. excess oxygen shifting air-fuel ratio to the lean side by coating a refractory carrier with zeolite having a supported rare earth element. CONSTITUTION:A rare earth element whose atomic number is between 57 and 71, especially La or Ce is supported on zeolite having about 5-10Angstrom pore diameter which is slightly larger than the diameter of NOx molecules and 10-200 ratio of silica to alumina by ion exchange or other method. A refractory carrier such as cordierite is coated with the zeolite. Since NOx is selectively taken in the pores in the zeolite and the rare earth element itself also has superior adsorbing property, NOx is adsorbed on the resulting catalyst at a high rate and is satisfactorily removed even in a lean atmosphere.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a catalyst for purifying exhaust gas from automobiles, and particularly to a catalyst that can purify NOx at a high rate even in an oxygen-rich atmosphere where the air-fuel ratio is on the lean side. It is.

(Prior art) Carbon monoxide (Co) is used as a catalyst for purifying automobile exhaust gas.
) and hydrocarbons (HC), and nitrogen oxides (NO
Catalysts that simultaneously reduce X) are widely used. Basically, such catalysts are made by coating a γ-alumina slurry on a refractory support such as cordierite, burning it out, and then adding Pd, Pd.
It supports metals such as T, Rh, etc., or a mixture thereof. In addition, many proposals have been made to increase the catalytic activity. For example, Japanese Patent Application Laid-open No. 11147/1983 includes rare earth oxide particles, alumina particles, γ-alumina particles stabilized with rare earth oxides, and Disclosed is a catalyst of a type in which a noble metal or the like is dispersed on particles selected from the group consisting of a mixture thereof, which is characterized in that Rh is dispersed on particles that do not substantially contain rare earth oxides.

By the way, the catalysts that have been used or proposed so far are:
The purification characteristics are greatly influenced by the air-fuel ratio setting of the engine, and in lean air-fuel mixtures, that is, on the lean side where the air-fuel ratio is large, the amount of oxygen (02) increases even after combustion, and the oxidation effect becomes active.
The reducing action becomes inactive. On the contrary, on the rich side where the air-fuel ratio is small, the oxidizing action becomes inactive and the reducing action becomes active. The stoichiometric air-fuel ratio (A
/ F = 14.6), the conventional three-way catalyst works most effectively.

Therefore, in automobiles equipped with exhaust gas purification devices using three-way catalysts, feedback control is performed to detect the oxygen concentration in the exhaust system and maintain the air-fuel mixture near the stoichiometric air-fuel ratio.

(Problems to be Solved by the Invention) On the other hand, there is a demand for lower fuel consumption in automobiles, and it is known that for this purpose, it is sufficient to combust as much oxygen-excessive mixture as possible during normal driving. However, if this is done, the air-fuel ratio will be on the lean side, resulting in an oxygen-excess atmosphere, and although the harmful elements {Ic, Co} in the exhaust gas can be removed by oxidation, NO
Because X is prevented from contacting the active metal by 02 adsorbed on the catalyst bed, it is difficult to remove it by reduction. There was a problem in that the purification efficiency was significantly reduced due to the bonding. For this reason, in the past, it has not been possible to make the air-fuel mixture leaner in automobiles that use catalysts to purify exhaust gas to a high degree.

The present invention has been made to solve the above problems, and its purpose is to provide an exhaust gas purifying catalyst that can reduce and remove NO and 1 even on the lean side.

<Means for Solving the Problems> The exhaust gas purifying catalyst of the present invention is characterized in that zeolite carrying a rare earth element is coated on a refractory carrier.

As the above-mentioned rare earth element, one or more elements selected from rare earth elements having an atomic number of 57 to 71 can be used, and it is particularly preferable to use La, Ce, Nd, and Gd.

The zeolite for the exhaust gas purifying catalyst of the present invention preferably has a pore diameter of about 5 to 10 pores, which is slightly larger than the molecular diameter of NOx. Moreover, zeolite having a silica/alumina ratio of 10 to 200 is preferable.

Rare earth elements can be supported on zeolite by, for example, ion exchange. As is well known, zeolite has the general formula: xMz/n = Alz 03 ・y S i Oz
Crystalline aluminosilicate represented by M (n-valent metal)
, x, y, a tunnel structure (
Many types with different pore sizes are commercially available. In addition, since a part of Si4''' is replaced with Affi'', there is a lack of positive charge, and to compensate for this deficiency, cations such as Na'' and K+ are retained within the crystal, resulting in high cation exchange. Therefore, rare earth elements can be supported by ion exchange.

Ion exchange can be carried out, for example, by immersing zeolite powder in an acetic acid or nitric acid aqueous solution of rare earth elements for several tens of hours, filtering the aqueous solution, washing the zeolite 1 powder, and then
This can be done by baking at ~600°C for several hours. At this time, if aqueous ammonia is added dropwise to the rare earth element aqueous solution and adjusted to an appropriate pH, the ion exchange rate can be improved depending on the type of rare earth element.

The zeolite can be coated on the carrier by, for example, mixing zeolite powder supporting a rare earth element with a binder made of alumina sol or silica sol, wash-coating the carrier with the resulting slurry, and then firing. Further, the zeolite may be subjected to ion exchange after being wash coated.

(Function) The exhaust gas purification catalyst of the present invention is coated with zeolite on which rare earth elements are supported, and since NoX is selectively taken into the pores of the zeolite, the rare earth elements themselves also have excellent adsorption properties. Therefore, the adsorption rate of NO and l onto the catalyst is high. Furthermore, since rare earth elements have the effect of increasing the expression of solid acid sites in zeolite, the NOx purification performance is significantly improved. In addition, rare earth elements, especially La, C
e, The presence of NdSGd improves the heat resistance of zeolite.

(Example) Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 Zeolite (Y-type zeolite, mordenite or Z
SM-5-zeolite) powder was immersed for 24 hours in a 0.1 M rare earth element aqueous acetic acid solution (lanthanum acetate, cerium acetate or neodymium acetate aqueous solution), the aqueous solution was filtered off, washed, and then heated at 500'C. Each rare earth element was supported on each zeolite in the amount shown in Table 1 by an ion exchange method consisting of burning out for 2 hours.

Table 1 Each zeolite powder alumina obtained by the above method is coated. However, no calcining is performed, and the calcining temperature is 8.
Let it be 00°C. The coated carrier was added to 0.053 g of pt
The sample was immersed in a platinum solution containing dinitrodia ξ for 2 hours, pulled out, dried at 100°C for 1 hour, and calcined at 200°C for 2 hours.

Next, it was immersed in a rhodium chloride solution containing 0.011 g of rhodium for 2 hours, taken out, and heated at 100°C x 1.
Let dry for an hour. As a result, catalyst No. 10 is obtained.

Test Example The catalysts (No. 1 to No. IO) manufactured above had the power shown in Table 2 and the air-fuel ratio (^/F) was 18
A durability test of 700"c x 3 hours is carried out under the conditions of a space velocity <SV> - about 8 and a time arc of too long under the flow of a model gas corresponding to the above. Regarding the catalyst after durability and the catalyst before durability, Using the same model gas, the inlet gas temperature was 40
Exhaust gas purification performance was investigated under conditions of 0°C and SV=87,000 hours. As a result, HC,
The maximum purification rates (%) of CO and NoX are shown in Table 3.

? 100g, 70g of silica sol (20% by weight SiO■)
, and 70 g of pure water are mixed and stirred to produce a coating slurry. Next, a cordierite honeycomb carrier l having a diameter of 30 mm and a length of 50 marks as shown in Fig. l is immersed in pure water.
After blowing off excess moisture, the sample is immersed in the slurry, and after blowing off the excess slurry, it is dried at 100°C for 2 hours, and calcined at 300°C for 1.5 hours. The process of dipping into the slurry, blowing off, and calcining is repeated one more time. After that, perform the burning precepts at 500°C for 3 hours. As a result, a catalyst (
No. 1 to 9) are obtained.

Comparative Example Al ξNasol (10% by weight A1■0+) 70g, activated AlξNa powder 100g, C'eO■ powder 10g, 2
15g of 3% by weight aluminum nitrate aqueous solution and 30g of pure water
Mix and stir to obtain a slurry. Using this slurry, a cordierite honeycomb carrier with a diameter of 30 mm and a length of 50 mm was coated with the same coating method as in the example. Comparative example of exhaust gas purification catalyst (
No. 10) hardly purifies NOx under lean atmosphere both before and after durability.
The exhaust gas purifying catalyst of the example purified NOx by 50% or more both before and after the durability test. From this, it is clear that the exhaust gas purifying catalyst of the present invention can satisfactorily purify NOX even in a lean atmosphere.

(Effects of the Invention) Since the exhaust gas purifying catalyst of the present invention is coated with zeolite supporting rare earth elements, the amount of NO.1 adsorbed onto the catalyst and the NO. The purification rate is significantly improved, the expression of solid acidity is further increased, NoX can be effectively purified even in a lean atmosphere, and it also has excellent heat resistance.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a monolithic carrier used in manufacturing an exhaust gas purifying catalyst according to an embodiment of the present invention.

Claims (1)

[Claims] An exhaust gas purification catalyst characterized by a zeolite carrying rare earth elements coated on a refractory carrier.