JPH05220403A - Exhaust gas purifying catalyst - Google Patents

Abstract

PURPOSE:To provide an exhaust gas purifying catalyst excellent in NOx purifying activity after durable treatment and effective for purifying exhaust gas of an oxygen excessive atmosphere. CONSTITUTION:At least one kind of a metal selected from copper, cobalt, nickel, iron and platinum is supported on zeolite beta or copper and alkaline earth metal and/or rare earth metal are supported on zeolite beta.

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 for purifying nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly to a lean burn engine. Exhaust gas purification capable of efficiently decomposing NOx in exhaust gas (so-called lean exhaust gas) in an oxygen excess atmosphere such as exhaust gas into nitrogen (N ₂ ) and oxygen (O ₂ ) for purification Regarding catalysts. In the present invention, the term "excess oxygen atmosphere" means that an amount of oxygen that is in excess of that required to completely oxidize reducing substances such as carbon monoxide, hydrogen and hydrocarbons contained in exhaust gas. The atmosphere that is included.

[0002]

2. Description of the Related Art Nitrogen oxides (NOx) and carbon monoxide (C) which are harmful substances in exhaust gas discharged from an internal combustion engine.
O) and hydrocarbons (HC) are, for example, platinum, rhodium,
It is known to remove palladium and the like with a three-way catalyst supported on a carrier. However, as for diesel engine exhaust gas, the three-way catalyst is not effective for purifying nitrogen oxides because the exhaust gas contains a large amount of oxygen.

In recent gasoline engines,
It has become necessary to perform lean combustion for the purpose of improving fuel efficiency and reducing carbon dioxide emissions. However, since the exhaust gas of this lean burn gasoline engine is in an oxygen excess atmosphere, the above-mentioned conventional three-way catalyst has not been effective.

Under such circumstances, various catalysts for purifying exhaust gas of automobiles have been proposed which simultaneously oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx). .. As such a catalyst, for example, a copper-supported zeolite catalyst (Cu / ZSM-5) in which copper is ion-exchanged with zeolite is known as a catalyst for catalytically decomposing NOx (see, for example, JP-A-60-125250).

US Pat. No. 3,308,069 (1967) (Mob
il Oil) shows the manufacturing method of zeolite β,
No mention is made of any NOx purification catalyst using zeolite β.

[0006]

However, the conventionally known copper ion exchanged zeolite (ZSM-5) catalyst is
Since it has a problem in catalyst durability, it has not been put to practical use. That is, the copper / zeolite (ZSM-5) catalyst is
Although the Ox purification rate is high, there is a problem that the NOx purification rate is low after endurance treatment at a high temperature of, for example, about 600 ° C. Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst that has high heat resistance and a high NOx purification rate even after endurance treatment.

[0007]

According to the present invention, there is provided an exhaust gas purifying catalyst comprising zeolite β carrying at least one metal selected from copper, cobalt, nickel, iron and platinum. It

According to the invention, the zeolite β also comprises
Provided is an exhaust gas purifying catalyst which co-supports copper and at least one metal selected from alkaline earth metals and rare earth metals.

The inventors of the present invention have heretofore known zeolite (ZS
M-5) While conducting various investigations to solve the problem of durability of the supported exhaust gas purification catalyst, copper, cobalt, nickel, iron and / or platinum metal was added to conventional zeolite (ZSM-5). We succeeded in obtaining an exhaust gas purifying catalyst with high heat resistance by loading it on zeolite β different from the above.

As a result of further study by the present inventors, the catalyst β is further improved in purification performance and durability by coexisting copper and alkaline earth metal and / or rare earth metal on zeolite β. I found that I can do it.

Zeolite β itself is known, and its production method is described in, for example, the above-mentioned US Pat. No. 3,308,069. In the present invention, zeolite β produced by such a method can be used as a carrier.

The method for producing zeolite β will be further described. As described in the above-mentioned US patent, silica source, for example, silica sol, alumina source, for example, sodium aluminate, and crystallization type agent, for example, TEA.
(Tetraethylammonium hydroxide)
Zeolite β can be synthesized by hydrothermal synthesis.
For hydrothermal synthesis, for example, a mixed gel body is placed in a Teflon-lined stainless steel autoclave under natural pressure (up to 20 kg / cm).
² ) By holding at 150 ° C for 2 to 10 days, the desired zeolite β crystal can be obtained. The obtained crystal is washed with water, dried, and heated at 550 ° C. to remove the residual organic content, whereby a catalyst carrier can be obtained. At this time, the SiO ₂ / Al ₂ O ₃ molar ratio is 10
-100 are preferable, and 30-40 are more preferable.

In the first aspect of the present invention, the above catalyst carrier is subjected to a conventional method such as ion exchange method or impregnation method to obtain Cu or C.
At least one metal selected from o, Ni, Fe, Pt, and Pd is supported. Ion exchange can be performed by immersing in an aqueous solution of a suitable water-soluble salt of the above-mentioned metal (eg, acetate, nitrate, etc.). The amount of the metal supported is not particularly limited, but the atomic ratio of Al to the metal in the zeolite β is preferably 1: 0.5 to 1: 2.

In the second embodiment of the present invention, Cu is supported on the zeolite β carrier by an ion exchange method or an impregnation method according to a conventional method. The amount of Cu supported is not particularly limited, but is preferably 50 to 50 relative to Al in zeolite β.
80 mol% is good. If the supported amount is less than 50 mol%, it becomes difficult to obtain the desired catalytic effect. On the contrary, if it exceeds 80 mol%, C
u tends to move and aggregate, and also tends to make it difficult to support rare earth elements and alkaline earth elements.

As the rare earth element used in the second aspect of the present invention, La, Ce, Nd, Y, Pr and Sm are desirable,
Among the alkaline earth elements, Mg, Ca, S
r and Ba are desirable. The catalyst of the present invention supports one or more of the above elements in addition to Cu. The amount of the rare earth element and / or the alkaline earth element supported is not particularly limited, but is preferably 20 to 50 mol% as a ratio to Al in the zeolite β, and the supported amount including Cu is Al in the zeolite β. On the other hand, it is preferably 80 to 120 mol%, and most preferably 100 mol%.

In the second embodiment of the present invention, when Cu is carried on the zeolite β by ion exchange, it is preferable to add ammonia or the like in an aqueous solution made basic so that the exchangeability is good. The pH at that time is most preferably in the range of 10 to 12.

The rare earth metal and alkaline earth metal can be loaded in accordance with a general method, for example, acetic acid, nitrate, etc., and the order of loading is preferably Cu ion exchange, for the purpose of suppressing re-elution of Cu ions. It is preferable that the pH is supported in the range of 10 to 12 by adding ammonia or the like.

The catalyst according to the present invention can be used in any form such as powder, pellets, and honeycombs regardless of shape and structure.

The exhaust gas purifying catalyst according to the present invention is NO
Contacting with exhaust gas containing x, CO and HC, especially oxygen-excess exhaust gas (that is, exhaust gas in a state with a large air-fuel ratio (so-called lean region) exhausted from an internal combustion engine of an automobile, etc.) by a normal method. Thus, the exhaust gas can be purified.

In the purification method using the catalyst according to the present invention, the space velocity (SV) for introducing the exhaust gas into the catalyst layer is not particularly limited. The temperature of the catalyst layer is preferably 300 to 500 ° C.

[0021]

According to the first aspect of the present invention, since zeolite β, which has a higher heat resistance than conventional zeolite (ZSM-5), is used as a carrier, it is used in oxidizing exhaust gas containing hydrocarbons and nitrogen oxides. It is possible to effectively remove the nitrogen oxides, and it is possible to obtain a catalyst having a durability higher than that of the conventionally known Cu / ZSM-5 catalyst.

In the second embodiment of the present invention, since zeolite β supports copper and rare earth and / or alkaline earth metal, Cu has an ability to selectively adsorb NO, and is superior to other ones, It has excellent NO adsorption capacity, while rare earth and alkaline earth elements themselves also adsorb NO and NO ₂ and have catalytic activity. Therefore, Cu and rare earth and / or alkaline earth elements are converted to zeolite β. By carrying it, it exhibits excellent performance. Especially, Cu ion is 600
At a temperature of up to 800 ° C., it tended to be reduced to metallic Cu and migrated and agglomerated on the zeolite, and the durability tended to be lowered. However, according to the present invention, this Cu agglomerated is rare earth and / or alkaline earth element. Also has an effect of suppressing by interposing Cu between Cu ions, and the durability performance can be improved.

[0023]

EXAMPLES The present invention will be specifically described below with reference to examples, but it goes without saying that the present invention is not limited to the following examples.

Example 1 and Comparative Example 1 Method for Producing Zeolite β 17.4 g of sodium aluminate, silica sol (Snowtex N SiO ₂ manufactured by Nissan Kagaku KK, 40% by weight) 581.0
g, TEAOH (made by Aldrich tetraethylammonium hydroxide, 40% by weight) and a mixed gel body consisting of 48.0 g of water were prepared, and the mixed gel body was placed in a Teflon-lined stainless steel autoclave and kept at 150 ° C for 6 days. , Hydrothermal synthesis was performed. The obtained crystal was washed with water and dried, and the residual organic components were removed at 550 ° C to obtain the target zeolite β. The Si / Al ratio at this time was 20.

The obtained zeolite β was immersed in an aqueous solution of copper acetate (pH = 11 by adding ammonia) as a catalyst species to carry out copper ion exchange. Ion exchange rate is 107
%Met. After ion exchange, dry the powder (100 ° C) and
It was calcined at 00 ° C to obtain a Cu / zeolite β catalyst (Example 1). As a comparative catalyst, Cu / ZSM-5 (Comparative Example 1) was obtained by copper ion exchange (the ion exchange method was the same as above).
The ion exchange rate was 109%.

The catalyst prepared in Example 1 and the catalyst prepared in Comparative Example 1 were brought into contact with a gas having the following composition and conditions simulating automobile exhaust gas to compare purification performances.

Gas composition (volume%) of model gas: CO;
0.1%, H ₂ ; 0.03%, C ₃ H ₆ ; 0.08%, O ₂ ; 4.3%, N
O; 0.1%, CO ₂ ; 11.9%, H ₂ O; 2.3% and N ₂ ; residual amount SV = 420,000 hr ^-1

The temperature of the catalyst layer is set to 350 ° C., 400 ° C. and 450 ° C.
Table 1 shows the purification rates (initial purification rates) of HC, CO, and NOx obtained as Next, each catalyst was subjected to a durability treatment at 600 ° C. for 20 hours in an air flow containing 10% by volume of water, and the purification performance of this catalyst was evaluated in the same manner as above. Table 2 shows the obtained purification rate (purification rate after endurance).

[0029]

[Table 1]

[0030]

[Table 2]

Example 2 and Comparative Example 2 Catalysts according to the present invention were prepared, and the purification performance for NO was evaluated using a lean model gas with excess oxygen. The same evaluation was performed for the comparative catalyst. Zeolite β, which is the catalyst carrier according to the present invention, was prepared in the same manner as in Example 1. An aqueous solution of copper acetate (pH =
Ion exchange was performed under stirring for 24 hours, filtered, washed, and dried at 110 ° C. The catalyst prepared up to this point is referred to as comparative catalyst C-1. After that, the above-mentioned catalyst supporting Cu is immersed in an aqueous solution of La, Ce, Y, Mg, Ca, Sr or Ba acetate (ammonia is added to adjust pH =
11)), carrying a rare earth or alkaline earth element, filtering, washing, drying at 110 ° C., and then calcining in air at 500 ° C. for 3 hours to obtain the target Example catalysts 2-1 to 2-1. 2-8
Got Comparative catalyst C-2 is a comparative example in which Cu was similarly ion-exchanged with conventional zeolite ZSM-5 instead of zeolite β. Table 3 shows the catalyst compositions and loading amounts (molar ratio to Al in zeolite) of Example 2 and Comparative Example 2.
Shown in.

Table 3 ──────────────────────────────── Catalyst No. Catalyst composition and loading amount (Al in zeolite) To mol ratio /%) ──────────────────────────────── 2-1 Cu (50) + La (30) 2-2 Cu (50) + Ce (30) 2-3 Cu (50) + Y (30) 2-4 Cu (50) + Ca (30) 2-5 Cu (50) + Mg (30) 2- 6 Cu (50) + Sr (30) 2-7 Cu (50) + Ba (30) 2-8 Cu (50) + Mg (30) + La (20) ──────────── ───────────────────── C-1 Cu (50) (Zeolite β) C-2 Cu (50) (ZSM-5) ────── ───────────────────────────

The pellet-form Example 2 (2-
1-2-8) and Comparative Example 2 (C-1 and C-2) were brought into contact with a gas having the following composition and conditions simulating exhaust gas of an automobile to evaluate the purification performance.

Gas composition of the model gas (volume%): CO:
0.1%, H ₂ : 0.03%, C ₃ H ₆ : 0.08%, O ₂ : 4.3%, NO:
0.1%, CO ₂ : 11.9%, H ₂ O: 10% (durability treatment), 3%
(Activity evaluation) and N ₂ : Remaining amount space velocity (SV) = 420,000 hr ^-1 Catalyst activity evaluation: 700 ° C x 5 hours in the above model gas, durable treatment, and then 400 ° C in the above model gas The NOx purification rate was measured at. The results are shown in Table 4.

Table 4 ─────────────── Catalyst No. NOx purification rate (%) ─────────────── 2-1 46 2- 2 44 2-3 42 2-4 48 2-5 50 2-6 45 2-7 41 2-8 51 ─────────────── C-1 30 C-2 12 ─ ────────────── (Note) The initial activity was virtually unchanged from that after endurance treatment.

[0036]

According to the first aspect of the present invention, as is clear from the results of Tables 1 and 2, for example, the initial activity is slightly inferior to that of the comparative catalyst, especially at 400 ° C. and 450 ° C. Later, it has better performance than the comparative catalyst. According to the second aspect of the present invention, as is clear from the results in Table 4, the durability is remarkably superior to that of the comparative catalyst, and the value thereof is superior to that of Example 1.

Claims (2)

[Claims] 1. An exhaust gas purification catalyst comprising zeolite β carrying at least one metal selected from copper, cobalt, nickel, iron and platinum. 2. The exhaust gas purifying catalyst according to claim 1, wherein copper and at least one metal selected from alkaline earth metals and rare earth metals are co-loaded on zeolite β.