JP3362401B2 - Exhaust gas purification catalyst - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying method for removing nitrogen oxides, carbon monoxide and hydrocarbons from exhaust gas discharged from an internal combustion engine such as an automobile engine. The present invention relates to a catalyst, and more particularly to a catalyst for purifying nitrogen oxides of an exhaust gas containing excess oxygen and a method for using the same. 2. Description of the Related Art Nitrogen oxides, carbon monoxide and hydrocarbons, which are harmful substances in exhaust gas discharged from an internal combustion engine, are, for example, three-way catalysts in which Pt, Rh, Pd and the like are supported on a carrier. Has been removed. However, since exhaust gas contains a large amount of oxygen, there is no effective catalyst for removing nitrogen oxides, and exhaust gas purification by the catalyst has not been performed. Further, in recent gasoline engines, it has become necessary to perform lean combustion for the purpose of lowering fuel consumption and reducing carbon dioxide emissions. However, since the exhaust gas of a lean burn gasoline engine is in an oxygen-excess atmosphere, the above-described conventional three-way catalyst cannot be used, and a method for removing harmful components, particularly nitrogen oxides, has not been put to practical use. [0004] As a method for removing nitrogen oxides in the exhaust gas containing excess oxygen, a method of adding a reducing agent such as ammonia and a method of absorbing and removing nitrogen oxides by an alkali are known. It is not an effective method for use in a vehicle that is a mobile source and its application is limited. In recent years, it has been reported that a zeolite catalyst obtained by ion-exchanging a transition metal can remove nitrogen oxides in an oxygen-excess exhaust gas without adding a special reducing agent such as ammonia. For example, JP-A-63-283727 and JP-A-1-130735 disclose that nitrogen oxides are contained even in an oxygen-excess exhaust gas containing a trace amount of a reducing agent such as unburned carbon monoxide and hydrocarbon. Zeolite-based catalysts that can be selectively reduced have been proposed. JP-A-1-135541 and JP-A-3-232533 disclose a zeolite catalyst obtained by ion-exchanging a noble metal, and JP-A-2-102740 discloses a zeolite-type silicate containing a rare earth element. Catalysts each supporting a noble metal on a salt have been proposed. [0007] However, the noble metal-containing zeolite-based catalysts proposed so far have high nitrogen oxide purifying ability, but are not sufficiently durable, and are not suitable for internal combustion engines, especially automobile exhaust gas. As a purification catalyst, higher durability performance is required. An object of the present invention is to solve the above-mentioned problems of the prior art, and to simultaneously remove nitrogen oxides, carbon monoxide and hydrocarbons from exhaust gas discharged from an internal combustion engine such as an automobile. An object of the present invention is to provide an exhaust gas purifying catalyst having high durability performance, which is a catalyst. Another object of the present invention is to provide a method for purifying exhaust gas using such a catalyst. The present inventors have conducted intensive studies on the above problems, and as a result, have found that a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 15 to 10 ⁵ contains Pt and K, Cs or At least one element selected from alkaline earth metals
It has been found that a catalyst containing hydrogen by an ion exchange method has a high nitrogen oxide purifying ability and a high durability performance, and has completed the present invention. [0011] The present invention, nitrogen oxides, oxygen excess exhaust gas containing carbon monoxide and hydrocarbons, nitrogen oxides, a zeolite catalyst for removing carbon monoxide and hydrocarbons, SiO ₂ / Al ₂ O ₃ molar ratio is 15 to 10 ⁵ zeolite and Pt and K, ions of at least one or more of elemental selected from Cs or alkaline earth metal
Exhaust gas purifying catalyst, wherein Rukoto is contained by an exchange method, and the exhaust gas purifying catalyst, nitrogen oxides, nitrogen oxides in the exhaust gas which comprises contacting a combustion exhaust gas containing carbon monoxide and hydrocarbons , Carbon monoxide and hydrocarbons. Hereinafter, the present invention will be described in detail. Exhaust gas purifying catalyst according to the [0013] present invention, SiO ₂ / Al ₂ O ₃ molar ratio which contains at least one or more elements selected from Pt and K, Cs or alkaline earth metal is at 15 to 10 ⁵ It is a zeolite. [0014] The zeolite is generally _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where n is the valence of the cation M, x is 0.8 to 1.2
, Y is a number of 2 or more, and z is a number of 0 or more.)
The zeolite used in the present invention has a SiO ₂ / Al ₂ O ₃ molar ratio of
15 to 10 is of ^5. If the SiO ₂ / Al ₂ O ₃ molar ratio is less than 15, the heat resistance and durability of the zeolite itself are low, so that sufficient heat resistance and durability of the catalyst cannot be obtained. Generally, those having a SiO ₂ / Al ₂ O ₃ molar ratio of about 15 to 2,000 are used. The zeolite constituting the catalyst of the present invention may be either a natural product or a synthetic product. The method for producing these zeolites is not particularly limited, but typically, ferrierite, Y, ZSM -5, ZSM-11, ZS
Zeolites such as M-12 and ZSM-20 can be used. These zeolites usually have the cation M
In general, when used as a catalyst, it is used as it is or after being subjected to ion exchange to NH ₄ type or H type after being treated with ammonium salt, mineral acid or the like or calcined. . The zeolite catalyst of the present invention contains Pt. The method for incorporating Pt into the zeolite is not particularly limited, and it can be generally contained by ion exchange using a water-soluble salt, impregnation-supporting method, evaporation to dryness method, or the like. The concentration of Pt ions in the aqueous solution when Pt is contained can be arbitrarily set depending on the Pt content of the target catalyst. The Pt ion can be used in the form of a soluble salt, and as the soluble salt, nitrate, chloride, ammine complex salt and the like can be suitably used. The content of Pt is preferably 0.1 to 5% by weight, more preferably 0.5 to 4% by weight. In the present invention, the zeolite catalyst further K, characterized in that it contains at least one or more of elemental selected from Cs or alkaline earth metals.
At least one element selected from K, Cs or alkaline earth metal may be contained after Pt is contained, but at least one element selected from K, Cs or alkaline earth metal is first used. Is desirably used after being contained. K, in order to incorporate the Cs or alkaline earth metals can be more contained in the ion exchange process. Ion exchange may be performed by a usual method using a water-soluble salt. As K, Cs or the alkaline earth metal, a soluble salt may be used, and as the soluble salt, nitrate, acetate, oxalate, chloride and the like can be suitably used. The total content of at least one or more elements selected from K, Cs and alkaline earth metals is represented by a molar ratio of each oxide to alumina in the zeolite,
0.1-1.2 is preferable, and 0.2-1.0 is more preferable. In addition, as the alkaline earth metal, Mg, Ca,
Sr, Ba can be used. A sample containing at least one element selected from Pt and K, Cs or alkaline earth metal is generally used after solid-liquid separation, washing and drying.
Further, if necessary, it can be used after firing or reduction treatment. The exhaust gas purifying catalyst of the present invention can be used after being mixed with a binder such as a clay mineral and molded. Alternatively, zeolite may be molded in advance, and the molded body may contain at least one element selected from Pt and K, Cs or alkaline earth metals. The binder used in forming the zeolite is not particularly limited, but clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane, sepiolite, silica, alumina and the like can be used. Or,
A binderless zeolite molded body obtained by directly synthesizing a molded body without using a binder may be used. Alternatively, zeolite may be wash-coated on a cordierite or metal honeycomb substrate and used. The removal of nitrogen oxides, carbon monoxide and hydrocarbons from the oxygen-excess exhaust gas can be carried out by bringing the exhaust gas purifying catalyst of the present invention into contact with the exhaust gas. The term "excess oxygen-exhaust gas" as used in the present invention refers to an exhaust gas containing oxygen in excess of the amount required to completely oxidize carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas. Specific examples of such exhaust gas include exhaust gas emitted from an internal combustion engine of an automobile or the like, particularly exhaust gas in a state where air-fuel efficiency is large (a so-called lean region). The conditions for using the catalyst are not particularly limited, but the temperature is 100 ° C. to 900 ° C., more preferably 150 ° C. to 800 ° C.
Is preferred. The space velocity is 1,000-500,
000 hr- ¹ . The space velocity is a value obtained by dividing a gas flow rate (cc / hr) by a catalyst volume (cc). The performance of the exhaust gas catalyst does not change at all even if it is applied to an exhaust gas containing carbon monoxide, hydrocarbons and hydrogen and not containing excess oxygen. EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Example 1 <Preparation of Catalyst 1> A zeolite similar to ZSM-5 was synthesized according to the method of Example 5 in JP-A-59-54,620. It had the following chemical composition, expressed as a molar ratio of oxides on an anhydrous basis. 1.1 Na ₂ O, Al ₂ O ₃ , 40 SiO _{2 The} obtained Na-type ZSM-5;
The solution was added to 1800 ml of an aqueous solution of cesium chloride at ol / liter and stirred at 80 ° C. for 20 hours. After solid-liquid separation, wash thoroughly with water, dry at 110 ° C for 20 hours, and use Cs-exchanged ZSM-5.
Got. Next, 15 g of Cs-exchanged ZSM-5 was added to 135 ml of an aqueous solution of tetraamminedichloroplatinum salt having a concentration of 0.013 mol / liter, and the mixture was stirred at 40 ° C for 20 hours. After solid-liquid separation, the mixture was thoroughly washed with water and dried at 110 ° C. for 20 hours to obtain Catalyst 1. As a result of the chemical analysis, Cs ₂ O /
The Al ₂ O ₃ molar ratio was 0.73, and the Pt content was 1.9% by weight. Example 2 <Preparation of Catalyst 2> 200 g of Na-type ZSM-5 synthesized in Example 1 was added to a 1.0 mol / L potassium chloride aqueous solution 180
The mixture was added to 0 ml and stirred at 80 ° C. for 20 hours. After solid-liquid separation, wash thoroughly with water and dry at 110 ° C for 20 hours.
SM-5 was obtained. Next, 15 g of K exchange ZSM-5;
The solution was added to 135 ml of an aqueous solution of tetraamminedichloroplatinum salt having a concentration of 0.013 mol / liter,
Stirred for hours. After solid-liquid separation, wash thoroughly with water and
After drying for 0 hour, catalyst 2 was obtained. As a result of chemical analysis, K ₂ O
The / Al ₂ O ₃ molar ratio was 0.71, and the Pt content was 2.1% by weight. Example 3 <Preparation of Catalyst 3> 200 g of Na-type ZSM-5 synthesized in Example 1 was added to an aqueous solution of potassium chloride 900 having a concentration of 1.0 mol / liter.
and 900 ml of a 1.0 mol / l barium chloride aqueous solution, and stirred at 80 ° C. for 20 hours. After the solid-liquid separation, it was thoroughly washed with water and dried at 110 ° C. for 20 hours to obtain K, Ba exchanged ZSM-5. Next, K, Ba
Exchange ZSM-5; 15 g of an aqueous solution 135 of a tetraamminedichloroplatinum salt having a concentration of 0.013 mol / liter.
Then, the mixture was stirred at 40 ° C. for 20 hours. After solid-liquid separation, the mixture was sufficiently washed with water and dried at 110 ° C. for 20 hours to obtain Catalyst 3. As a result of chemical analysis, the K ₂ O / Al ₂ O ₃ molar ratio was 0.31, the Ba ₂ O / Al ₂ O ₃ molar ratio was 0.38, and P
The t content was 2.3% by weight. Comparative Example 1 <Preparation of Comparative Catalyst 1> 15 g of Na-type ZSM-5 synthesized in Example 1 was put into 135 ml of an aqueous solution of tetraamminedichloroplatinum salt having a concentration of 0.013 mol / liter, and the solution was added at 20 ° C. Stirred for hours. After solid-liquid separation, the mixture was sufficiently washed with water and dried at 110 ° C. for 20 hours to obtain Comparative Catalyst 1. As a result of chemical analysis, Na ₂
The O / Al ₂ O ₃ molar ratio is 0.75, and the Pt content is 1.8.
% By weight. Comparative Example 2 <Preparation of Comparative Catalyst 2> 200 g of Na-type ZSM-5 synthesized in Example 1 was added to an aqueous ammonium chloride solution 1 having a concentration of 1.0 mol / liter.
The mixture was added to 800 ml, and stirred at 80 ° C. for 20 hours. After solid-liquid separation, the mixture was sufficiently washed with water and dried at 110 ° C. for 20 hours to obtain ammonium-exchanged ZSM-5. Next, 15 g of ammonium-exchanged ZSM-5 was added to an aqueous solution of 135 ml of an aqueous solution of tetraamminedichloroplatinum salt having a concentration of 0.013 mol / liter.
and stirred at 40 ° C. for 20 hours. After solid-liquid separation,
After sufficiently washing with water and drying at 110 ° C. for 20 hours, Comparative Catalyst 2 was obtained. As a result of chemical analysis, the Pt content was 2.1% by weight. Example 4 <Preparation of Catalyst 4> The concentration of the aqueous solution of tetraamminedichloroplatinum was adjusted to 0.0
The same operation as in Example 1 was performed except that the amount was 22 mol / L, and a catalyst 4 was obtained. As a result of chemical analysis, Cs ₂
O / Al ₂ O ₃ molar ratio is 0.48, Pt content is 3.7
% By weight. Example 5 <Preparation of Catalyst 5> The concentration of the aqueous solution of tetraamminedichloroplatinum was adjusted to 0.0
The same operation as in Example 2 was performed except that the amount was 22 mol / L, to obtain a catalyst 5. As a result of chemical analysis, K ₂ O
The / Al ₂ O ₃ molar ratio was 0.53, and the Pt content was 3.5% by weight. Example 6 <Preparation of Catalyst 6> 200 g of Na-type ZSM-5 synthesized in Example 1 was added to a barium chloride aqueous solution 180 having a concentration of 1.0 mol / liter.
The mixture was added to 0 ml and stirred at 80 ° C. for 20 hours. After the solid-liquid separation, the same operation was performed by adding the zeolite slurry to an aqueous solution of the above composition prepared again. After solid-liquid separation, wash thoroughly with water, dry at 110 ° C. for 20 hours, and exchange BaSM ZSM-5.
Got. Next, 15 g of Ba-exchanged ZSM-5 was added to 135 ml of an aqueous solution of tetraamminedichloroplatinum salt having a concentration of 0.022 mol / liter, and the mixture was stirred at 40 ° C for 20 hours. After solid-liquid separation, the mixture was sufficiently washed with water and dried at 110 ° C. for 20 hours to obtain Catalyst 6. As a result of the chemical analysis, Ba ₂ O /
The Al ₂ O ₃ molar ratio was 0.58, and the Pt content was 3.0% by weight. Comparative Example 3 <Preparation of Comparative Catalyst 3> The concentration of the aqueous solution of tetraamminedichloroplatinum salt was adjusted to 0.0
The same operation as in Comparative Example 1 was performed except that the amount was 22 mol / L, to obtain Comparative Catalyst 3. As a result of chemical analysis, N
a _{2 O} / _{Al 2} O ₃ molar ratio is 0.54, Pt content was 3.3 wt%. Comparative Example 4 <Preparation of Comparative Catalyst 4> The concentration of the aqueous solution of tetraamminedichloroplatinum salt was adjusted to 0.0
The same operation as in Comparative Example 2 was performed except that the amount was 22 mol / L, to obtain Comparative Catalyst 4. As a result of chemical analysis, P
The t content was 3.6% by weight. Example 7 <Evaluation of activity of catalyst> The catalysts 1 to 6 and the comparative catalysts 1 to 4 were each crushed after press molding, sized to 12 to 20 mesh, and 1 g of the resulting mixture was filled in a normal-pressure fixed bed reaction tube. did. Gas of the composition shown below (hereinafter, referred to as
4000 ml / min. The temperature was raised to 500 ° C. and maintained for 0.5 hour while pre-processing. Thereafter, the catalytic activities at 250 ° C. and 300 ° C. were measured. Table 1 shows the NOx purification rates after reaching the steady state at each temperature. In addition, for each catalyst, a durability treatment was performed at 800 ° C. for 5 hours while flowing a reaction gas, and the NOx purification rate was similarly measured. The NOx purification rate is a value obtained by the following equation. NOx purification rate (%) = (NOxin-NOx
out) / NOxin × 100 NOxin: fixed bed reaction tube inlet NOx concentration NOxout: fixed bed reaction tube exit NOx concentration reactive gas composition _{NO 1200ppm O 2 4.3% H 2} 400ppm CO 1200ppm C 3 H 6 800ppm H 2 O 10% CO ₂ 12% N ₂ balance Incidentally, carbon monoxide, hydrogen and hydrocarbons were hardly detected by any of the catalysts. As shown in Table 1, the catalyst of the present invention is initially equivalent to the conventionally proposed comparative catalyst, and has high nitrogen oxide purifying ability after durability treatment, that is, high durability. It is clear that the exhaust gas purifying ability of the excess oxygen exhaust gas is high. Therefore, by using the exhaust gas purifying catalyst of the present invention, it is possible to purify nitrogen oxides in the oxygen-excess exhaust gas at a high purification rate.

────────────────────────────────────────────────── ─── Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/94

Claims (1)

(57) Claims 1. A zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 15 to 10 ⁵ is added to at least one kind selected from Pt and K, Cs or alkaline earth metals. Ion exchange elements
Characterized in that the inclusion by換法, nitrogen oxides,
From exhaust gas of oxygen excess containing carbon monoxide and hydrocarbons,
An exhaust gas purifying catalyst that removes nitrogen oxides, carbon monoxide and hydrocarbons.