JPH1147605A - Catalyst and method for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purifying exhaust gas, which exhibits excellent purifying activity against NOx under such exhaust gas conditions that temp. and HC/NOx ratio are relatively low by incorporating a zeolite catalyst containing platinum and an alkali metal, etc., with a zeolite catalyst containing copper and a catalyst containing a noble metal in a specific arrangement. SOLUTION: The exhaust gas which is obtained from the internal combustion engine operated at such lean area that the ratio of air to fuel is above 14.7:1, is purified by using a multi-stage purifying catalysts which comprises at least two catalysts and installed in series with such arrangement that a honeycomb monolithic catalyst A coated with β-type zeolite containing Pt-component and a component of alkali metal, etc., is installed at the upstream side in the exhaust line and a honeycomb monolithic catalyst B containing Pt and/or Pd is installed at the downstream side. A layer comprising β-zeolite and/or MFI-zeolite containing Cu-component may be laminated on the β-zeolite layer of the catalyst A. The purifying conditions during the lean-burn engine operation are such that the oxygen concentration is kept above 5% and HC/NOx ratio is adjusted to at most 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst and a method for purifying exhaust gas from an internal combustion engine operated in a lean region, in particular, an automobile engine, and more particularly, to a series multistage type which realizes high purification efficiency. The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method using the same.

[0002]

2. Description of the Related Art Conventionally, three-way catalysts have been widely used as catalysts for purifying exhaust gas from automobile engines. In such a three-way catalyst, a catalyst mainly containing alumina containing various components such as a noble metal component such as Pt, Pd and Rh and a ceria (CeO ₂ ) component is mainly used, and an engine having a vicinity of the stoichiometric air-fuel ratio is used. It shows high purification efficiency for exhaust gas when operated at.

[0003] On the other hand, in recent years, from the viewpoint of improving fuel efficiency and reducing carbon dioxide emissions, a lean burn engine that operates even at an air-fuel ratio higher than the stoichiometric air-fuel ratio has attracted attention. The exhaust gas of such an engine (hereinafter, referred to as "lean exhaust gas") has an oxygen content lower than that of a conventional engine that operates only near the stoichiometric air-fuel ratio (hereinafter, referred to as "stoichiometric exhaust gas"). Highly, the three-way catalyst is insufficient in purifying nitrogen oxides (NOx). Therefore, a new catalyst applicable to a lean burn engine operating at a wide air-fuel ratio has been desired.

Under such circumstances, various metal components are converted to Y
It is known that zeolite-based catalysts supported on zeolites such as L-type, L-type, mordenite, and MFI have the ability to purify NOx relatively efficiently in lean exhaust gas in the presence of hydrocarbons (HC). . The metal component includes copper (Cu), cobalt (Co), silver (A
g), transition metal components such as nickel (Ni) and iron (Fe) are used, and platinum (Pt) has been recognized to be effective as a noble metal component.
Zeolite-based catalysts show relatively excellent NOx purification performance even under high flow gas conditions, and are expected to be applied to exhaust gas purification of small mobile sources such as automobiles and stationary type private power generation engines. I have.

[0005]

However, zeolite-based catalysts supporting the above-mentioned metal components have the following problems, and have not been put to practical use as catalysts for purifying automobile exhaust gas operated under lean conditions. .

First, the temperature range in which NOx can be purified relatively efficiently is narrow, and in particular, a sufficient NOx purification capability cannot be obtained in a relatively low temperature range of 150 ° C. to 300 ° C. To solve this problem, for example, by providing a noble metal catalyst layer below the Cu-zeolite catalyst, the reaction heat in the noble metal catalyst layer is used to operate the upper Cu-zeolite catalyst from a lower temperature. Have already been proposed (Japanese Patent Laid-Open No. 1-127044, Japanese Patent Laid-Open No. 5-6888).
No. 8). However, in this case, there is a problem that deterioration is increased due to heat of oxidation reaction in the lower noble metal catalyst layer. Furthermore, since HCs are oxidized and consumed in the noble metal catalyst layer, the NOx purification rate is reduced. The effect is C
When a noble metal component is allowed to coexist in the u-zeolite catalyst layer (JP-A-1-31074, JP-A-5-16893)
No. 9).

When a Pt-zeolite catalyst is used, a NOx conversion reaction can be realized in a relatively low temperature range of 200 to 250 ° C., but in addition to conversion to N ₂ , conversion to N ₂ O can be neglected. However, it is not environmentally friendly and cannot be used alone. Furthermore, in either the Cu-zeolite-based catalyst or the Pt-zeolite-based catalyst, the NOx purification ability becomes insufficient under the exhaust gas condition where the HC / NOx ratio is low. Therefore, it becomes necessary to supply HCs, alcohols, and the like as reducing agents in a secondary manner. In this case, it is conceivable to separately mount the reducing agent tank on the vehicle or to directly use the fuel as the reducing agent. However, in any case, there is a problem that the fuel efficiency of the engine is sacrificed.

The present invention has been made in view of such problems of the prior art, and can be suitably applied to a lean burn engine, particularly in a low temperature range and low HC / NOx.
It is an object of the present invention to provide an exhaust gas purifying catalyst and an exhaust gas purifying method that exhibit excellent NOx purifying performance even under specific exhaust gas conditions.

[0009]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that a zeolite catalyst containing platinum and an alkali metal, a zeolite catalyst containing copper, and a noble metal component are contained. By combining the catalysts in a specific arrangement, it has been found that extremely excellent NOx purifying ability can be exhibited, and the above-mentioned problems can be solved. Thus, the present invention has been completed.

That is, the exhaust gas purifying catalyst of the present invention is a multistage exhaust gas purifying catalyst comprising two or more catalysts arranged in series in an exhaust system of an internal combustion engine operated in a lean region where the air-fuel ratio exceeds 14.7. A Pt catalyst upstream of the exhaust system
Component, and a honeycomb-shaped monolith catalyst A coated with β zeolite containing at least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, is disposed, and on the downstream side thereof, Pt and And / or a honeycomb-shaped monolith catalyst B containing Pd is provided, and a catalyst A ′ is formed by providing a layer containing a β zeolite containing a Cu component and / or an MFI zeolite on an upper layer of the β zeolite layer in the catalyst A. It is characterized by comprising.

In the exhaust gas purifying catalyst of the present invention, a layer containing Cu-containing β zeolite and / or MFI zeolite is provided on a honeycomb-shaped monolithic carrier, and a catalyst A ″ separate from the catalyst A is provided. Forming this catalyst A "
May be disposed between the catalyst A and the catalyst B to form a three-stage catalyst arranged in series.

Further, an exhaust gas purifying method of the present invention is an exhaust gas purifying method using the above-mentioned exhaust gas purifying catalyst, wherein the series-arranged multi-stage catalyst has an oxygen concentration of 5% or more and HC / N
It is characterized in that exhaust gas is circulated and brought into contact under conditions of an Ox ratio of 10 or less.

[0013]

In a typical example of the catalyst of the present invention, a zeolite-based catalyst containing platinum and an alkali metal or the like and further containing copper (catalyst A ') and a zeolite-based catalyst containing a noble metal (catalyst B) are arranged upstream of the exhaust system. And arranged in series. Alternatively, a zeolite catalyst containing platinum and an alkali metal (catalyst A), a zeolite catalyst containing copper (catalyst A ″), and a catalyst containing a noble metal (catalyst B) are sequentially connected in series from the upstream side of the exhaust system. Placed.

Here, platinum and alkali metal in the catalyst A or the catalyst A 'are used to concentrate NOx and HCs in combination with a zeolite-based catalyst containing copper, and to form NOx-HC
Has the function of increasing the frequency of reaction. Further, the combination of platinum and an alkali metal or the like also realizes NOx adsorption from a low temperature. In this case, the component such as the alkali metal suppresses the oxidizing ability of Pt and NOx required for NOx reduction purification. It also plays a role in accelerating the above-mentioned reaction by concentrating the compound.

As described above, one of the features of the present invention resides in the combination of β zeolite, which has an extremely strong adsorptive power, with platinum, an alkali metal, etc., and realizes a specifically high purification effect by this combination. Is what you do. Further, the catalyst B is composed of untreated HC in the catalyst A and the catalyst A '
It has the function of oxidizing and purifying substances and CO.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the in-line multistage catalyst of the present invention will be described in detail. First, various components and elements will be described. As described above, the present catalyst has A / F = 14.
A catalyst applicable to an engine exhaust operated in a lean region of more than 7, comprising platinum and a β zeolite containing at least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals. A honeycomb monolith catalyst A ′ coated and further provided with a layer containing β zeolite containing copper and / or MFI zeolite as an upper layer, and a honeycomb monolith catalyst B containing platinum and / or palladium, or containing platinum Monolithic catalyst A coated with β zeolite containing at least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, β zeolite containing copper and / or
Alternatively, there is provided a catalyst A ″ in which a layer containing MFI zeolite is formed on a honeycomb monolith carrier, and a honeycomb monolith catalyst B containing platinum and / or palladium.

Here, at least one component selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth metal contained in the catalyst A or the catalyst A 'is calcium (Ca), potassium (K), Barium (Ba),
It is at least one selected from the group consisting of cesium (Cs) and lanthanum (La), and its content is preferably 0.2 to 0.6 mol per liter of the honeycomb-shaped monolith catalyst. If it is less than 0.2 mol, the effect of adsorbing and concentrating NOx becomes insufficient and the oxidizing power of platinum is strong, so that not only the effect of improving the low-temperature activity is not obtained,
NOx reduction ability may be adversely affected. On the other hand, 0.
If it exceeds 6 mol, the properties of platinum may be lost due to the coating effect, and the effect of adsorbing and concentrating NOx may not be obtained.

The β zeolite containing platinum and containing at least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals is a silica / alumina ratio (SiO ₂ / Al ₂ O). ₍₃ mole ratio) 20-1
Preferably, the β-zeolite and / or MFI zeolite supporting copper has a silica / alumina ratio of 25 to 80. By controlling the silica / alumina ratio in this manner, particularly NOx Can be effectively purified.

The former condition is necessary to ensure a stable zeolite structure, and the latter condition is that the silica / alumina ratio is 2
If it is less than 5, the thermal stability of the zeolite will be insufficient, and the heat resistance and durability of the catalyst may decrease. Conversely, if the silica / alumina ratio exceeds 80, the amount of the active metal component carried is reduced, and the active sites are reduced, so that the desired catalytic activity may not be obtained.

In the present invention, the above-mentioned zeolite can be used without treatment. However, if the crystallinity is increased by hydrothermal treatment or resynthesis, the catalyst becomes more stable, and a catalyst having high heat resistance and high durability can be obtained. It is preferable to use a zeolite that has been subjected to such treatment because it can be obtained.

Further, the catalyst of the present invention must be used in a honeycomb shape for multilayering. In this case, the above-mentioned catalyst is applied by coating the above-mentioned zeolite-based catalyst or a noble metal component on a honeycomb-shaped monolithic carrier. A, A ', A "and B are formed. As the honeycomb carrier, cordierite-based one is generally widely used, but is not limited thereto, and a honeycomb carrier made of a metal material is also effective. However, the catalyst component powder itself can be formed into a honeycomb shape.

As described above, by making the shape of the catalyst into a honeycomb shape, the contact area between the catalyst and the exhaust gas can be increased and the pressure loss can be suppressed, so that a large amount of vibration occurs in a limited space. This is particularly effective for automotive catalysts that require exhaust gas treatment.

Next, the arrangement of the catalyst of the present invention will be described. As described above, the catalyst of the present invention includes the catalyst A, the catalyst A ′, the catalyst A ″, and the catalyst B as components.
Alternatively, there may be mentioned a configuration in which catalyst A + catalyst A ′ and catalyst B are arranged in series in this order.

In the above-mentioned catalyst A ', it is preferable that a β zeolite layer containing platinum, an alkali metal and the like be a lower layer, and a β zeolite and / or MFI zeolite layer containing copper be disposed as an upper layer. When this layer structure is reversed, the amount of NOx that is discharged without purification increases,
Inefficient.

In the above arrangement, the catalyst A
Is NOx and HC by the combination of catalyst A 'or its active component Cu-β and / or MFI zeolite.
And condensate them to increase the frequency of the NOx-HC reaction. In this case, the β zeolite has a large adsorption capacity for HCs and a high adsorption speed, which is considered to be derived from a relatively large pore diameter.

As described above, one of the features of the present invention is that β-zeolite is combined with Pt, an alkali metal and an alkaline earth metal, and it has been found that this combination can achieve a specifically high purification effect. It is in.
The catalyst B has a function of oxidizing and purifying unpurified HCs and CO in the catalyst A, A ′ or A ″. Therefore, it is necessary to carry a catalyst component for realizing such oxidative purification. In this case, Pt and / or Pd is supported, but when Rh is further supported, NOx purification is assisted, which is effective.

Next, the catalyst A in the catalyst of the present invention,
A method for producing A ′, A ″ and the catalyst B will be described.
As a raw material of various metal components supported on zeolite or MFI zeolite, various compounds such as inorganic acid salts, oxides, organic acid salts, chlorides, carbonates, sodium salts, ammonium salts and ammine complex compounds may be used. These can be supported on the zeolite by a commonly used method such as an ion exchange method and an impregnation method.

In the case of loading by a usual ion exchange method or impregnation method, the metal raw material is often used in a solution, and the pH can be appropriately adjusted by adding an acid or a base to the solution. Adjustment may give better results, but the invention is not limited by such loading methods. The zeolite-based catalyst obtained as described above is applied to a honeycomb carrier using a binder such as alumina sol according to a conventional method, whereby honeycomb monolith catalysts A, A ′, A ″ and B are obtained. Can be

Next, the exhaust gas purifying method of the present invention will be described in detail. The purification method of the present invention uses the above-mentioned catalyst of the present invention. The catalyst of the present invention having the above-described multi-stage structure of the series arrangement is provided with an oxygen concentration of 5% or more and HC / NO.
Under the stoichiometric conditions where the ratio of x is 10 or less (but not including 0), high efficiency NOx purification is achieved by flowing and contacting exhaust gas.

According to the purification method of the present invention, HC in exhaust gas can be concentrated on the surface of the zeolite catalyst and can be used with high efficiency. However, when HC is excessive, NOx is covered by coating with HC.
Purification efficiency may decrease. Further, when the amount of oxygen in the exhaust gas is small, HC remains accumulated on the zeolite, and the performance may be deteriorated. That is, the oxygen content is 5%
As described above, when the HC amount is not too large and the HC / NOx ratio is lower than the level required for a normal catalyst, highly efficient purification performance can be maintained during long-term catalytic action. However,
If no HC is present, the NOx reduction reaction does not proceed, so the lower limit of the HC / NOx ratio must exceed 0,
One or more is preferable and 10 or less is effective.

[0031]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto. (Example _{1) (1) SiO 2 /} Al 2 O 3 in the preparation dinitrodiamineplatinum solution of honeycomb catalyst A
After adding NH ₄ type β zeolite powder having a molar ratio of about 28 and sufficiently stirring, the mixture was dried in a drier at 120 ° C. for 8 hours, and further calcined in an air stream at 500 ° C. for 2 hours to obtain a Pt of 1 A Pt-β zeolite powder loaded with 0.2 wt% was obtained.

Alumina sol and water were mixed with the zeolite catalyst powder, put into a magnetic ball mill pot, mixed and pulverized for about 20 minutes to obtain a Pt-β zeolite slurry.
The amount of the alumina sol added was 8 wt% as Al ₂ O _{3 based on} the Pt-β zeolite catalyst powder from which the adsorbed water had been removed.

The slurry obtained as described above was mixed with 1
A 0.5 L cordierite honeycomb carrier having about 400 flow channels per square inch cross section is applied, dried with hot air at 150 ° C., and then baked at 500 ° C. for 1 hour to obtain a honeycomb catalyst having a catalyst coating amount of about 120 g / L. I got After immersing the honeycomb catalyst in a mixed aqueous solution of calcium acetate and barium acetate, the honeycomb catalyst was dried at 120 ° C., calcined at 500 ° C. for 1 hour, and Ca
And Ba are each added in an amount of 0.1 per liter of the honeycomb catalyst.
A honeycomb catalyst A containing 8 mol and 0.21 mol was obtained.

[0034] (2) was 8.5 and the pH of the solution by adding ammonia water to the aqueous copper nitrate solution preparation concentration of 0.1M honeycomb catalyst A ". SiO ₂ / A into the solution
A powder of H-type MFI zeolite having a molar ratio of l ₂ O ₃ of about 45 was added thereto, stirred well, and then filtered to separate a solid and a liquid. By repeating the above stirring / filtration operation twice,
An MFI zeolite catalyst cake carrying Cu ion exchange was obtained. The cake is dried in a dryer at 120 ° C. for 24 hours or more, and then dried in an electric furnace under atmospheric air for 600 hours.
By calcining at 4 ° C. for 4 hours, an MFI zeolite catalyst powder carrying 5.8 wt% of Cu was obtained.

The obtained catalyst powder was mixed with alumina sol and water, and pulverized in a magnetic ball mill pot for 20 minutes to form a slurry. This slurry was coated on a 1.0-liter honeycomb carrier by performing the same operation as the above-mentioned zeolite-based catalyst A to obtain catalyst A ″. The coating amount of the catalyst was about 250 g / L.

(3) Preparation of Honeycomb Catalyst B An aqueous solution of dinitrodiammine palladium was added to activated alumina containing γ-alumina as a main component, and Pd was supported by impregnation, followed by drying in a drier at 120 ° C. for 8 hours and in an air stream. 45
Baking at 0 ° C. for 2 hours, P loaded with 2.8 wt% of Pd
d-Activated alumina was obtained. Activated alumina and nitric acid acidic boehmite sol were added to and mixed with this Pd-activated alumina,
A slurry was obtained by grinding for 1 hour in a magnetic ball mill pot. This slurry was coated on a 1.2 L honeycomb carrier in the same manner as in the above zeolite-based catalyst. The cord amount of the catalyst was about 80 g / L.

Next, an aqueous solution of dinitrodiammine platinum is added to the activated alumina in the same manner as above, and the Pt component is supported by the impregnation method. And Pt-activated alumina powder carrying 1.2 wt% of Pt. A nitric acid boehmite sol and water were added to and mixed with the Pt-activated alumina powder, and the mixture was pulverized with a magnetic ball mill pot for 1 hour to obtain a slurry. This slurry was coated on the above-mentioned honeycomb coated with Pd-activated alumina, and similarly, a honeycomb catalyst B was obtained through the steps of drying and firing. The coating amount of the Pt-activated alumina on the honeycomb carrier was 60 g / L.
Met.

The honeycomb catalysts A, A ″ and B of the above (1), (2) and (3) are arranged in series and assembled into a catalytic converter, and [0.5L + 1.0L + 1.2L] = 2.
7 L of a three-stage catalyst [1] was obtained.

Example 2 (1) Preparation of Honeycomb Catalyst A 'A slurry of the Pt-β zeolite catalyst powder obtained in the same manner as in Example 1 was prepared using a cordier having about 400 channels per square inch cross section. The honeycomb was coated on 1.5 L of the light honeycomb carrier and subjected to the same processes as in Example 1 to obtain a honeycomb catalyst having a catalyst coating amount of about 45 g / L. Carrying of Ca, Ba and K on the honeycomb catalyst was carried out in the same manner.
And K are each 0.2 L per 1 L of honeycomb catalyst capacity.
A honeycomb catalyst A containing 1 mol, 0.19 mol and 0.04 mol was obtained.

Further, on this honeycomb catalyst A, a slurry containing the Cu-MFI zeolite catalyst powder obtained in Example 1 was coated, and the same process was repeated for about 210
g / L of a coat was formed to obtain a multilayer honeycomb catalyst A '. The multilayer honeycomb catalyst A ′ and the honeycomb catalyst B of Example 1 are arranged in series in a catalytic converter and incorporated.
[1.5 L + 1.2 L] = 2.7 L of a two-stage catalyst [2] was obtained.

(Example 3) Ca, B of honeycomb catalyst A '
The same operation as in Example 2 was repeated except that the supported amounts of a and K were 0.02 mol, 0.18 mol, and 0.01 mol, respectively, per liter of the honeycomb catalyst, and a two-stage catalyst [3] was obtained. Obtained.

Example 4 Ca, B of honeycomb catalyst A '
The same operation as in Example 2 was repeated except that the supported amounts of a and K were 0.20 mol, 0.32 mol, and 0.05 mol, respectively, per liter of the honeycomb catalyst, and the two-stage catalyst [4] I got

Example 5 β-zeolite in Pt-β zeolite catalyst powder was prepared by converting SiO ₂ / Al ₂ O ₃ molar ratio to about 2
A two-stage catalyst [5] was obtained by repeating the same operation as in Example 2 except that β zeolite of 2.5 was used.

Example 6 β-zeolite in Pt-β zeolite catalyst powder was prepared by converting SiO ₂ / Al ₂ O ₃ molar ratio to about 1
A two-stage catalyst [6] was obtained by repeating the same operation as in Example 2 except that 25 was replaced with β zeolite.

Example 7 In the preparation of the Cu-MFI zeolite catalyst powder of Example 1, the H-type MFI zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of about 45 was converted to SiO ₂ /
A two-stage catalyst [7] was obtained in the same manner as in Example 2 except that the powder of NH ₄ type zeolite having an Al ₂ O ₃ molar ratio of about 28 was used. However, the amount of supported Cu of the Cu-β zeolite was 6.6 wt%.

Example 8 In the preparation of the Cu-MFI zeolite catalyst powder of Example 1, the H-type MFI zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of about 45 was converted to SiO ₂ /
A two-stage catalyst [8] was obtained in the same manner as in Example 2 except that the powder was changed to NH ₄ type MFI zeolite having an Al ₂ O ₃ molar ratio of about 76. However, the amount of supported Cu of this Cu-MFI zeolite was 4.9 wt%.

Example 9 Ca, B of honeycomb catalyst A '
a and K are replaced with a combination of Ba, La, and Cs, and the supported amount of each is per 1 L of the honeycomb catalyst capacity.
The same operation as in Example 2 was repeated except that the amounts were 0.2 mol, 0.1 mol and 0.02 mol, and the two-stage catalyst [9]
I got

(Comparative Example 1) In Example 1, honeycomb catalyst A "+ honeycomb catalyst B was incorporated into a catalytic converter except for honeycomb catalyst A, and [1.0 L + 1.2 L]
= 2.2 L of a two-stage catalyst Ref [1].

(Comparative Example 2) The SiO ₂ /
An NH ₄ type zeolite having an Al ₂ O ₃ molar ratio of about 28 was converted to S
A two-stage catalyst Ref [2] was obtained in the same manner as in Example 2 except that the NH ₄ type MFI zeolite having an iO ₂ / Al ₂ O ₃ molar ratio of about 32 was used.

Comparative Example 3 Ca, B of honeycomb catalyst A '
The same operation as in Example 2 was repeated except that the supported amounts of a and K were 0.02 mol, 0.14 mol, and 0.01 mol, respectively, per 1 L of the honeycomb catalyst, and the two-stage catalyst Ref [3]. I got

(Comparative Example 4) Ca, B of honeycomb catalyst A '
The same operation as in Example 2 was repeated except that the supported amounts of a and K were 0.1 mol, 0.35 mol and 0.2 mol, respectively, per 1 L of the honeycomb catalyst volume, and the two-stage catalyst R
ef [4] was obtained.

(Comparative Example 5) A two-stage catalyst Ref [was prepared in the same manner as in Example 2 except that the β zeolite in the Pt-β zeolite catalyst powder was changed to β zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of about 18. 5].

Comparative Example 6 A two-stage catalyst Ref [] was prepared in the same manner as in Example 2 except that β zeolite in the Pt-β zeolite catalyst powder was replaced with β zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of about 135. 6] was obtained.

Comparative Example 7 In the preparation of the Cu-MFI zeolite catalyst powder of Example 1, the H-type MFI zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of about 45 was converted to SiO ₂ /
A two-stage catalyst Ref [7] was obtained in the same manner as in Example 2 except that the powder of NH ₄ type MFI zeolite having an Al ₂ O ₃ molar ratio of about 18 was used. However, the amount of supported Cu of this Cu-β zeolite was 8.6 wt%.

(Comparative Example 8) In the preparation of the Cu-MFI zeolite catalyst powder of Example 1, the H-type MFI zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of about 45 was converted to SiO ₂ /
A two-stage catalyst Ref [8] was obtained in the same manner as in Example 2 except that the powder was changed to NH ₄ type MFI zeolite having an Al ₂ O ₃ molar ratio of about 83. However, the amount of supported Cu of this Cu-MFI zeolite was 3.9 wt%.

(Catalyst Performance Test Example 1) A light-off / light-on repetition test by an engine dynamo device using exhaust gas of an in-line four-cylinder 2L engine was performed on the catalysts of the above embodiment and comparative example, and NOx, HC, The average purification rate was determined for each component of CO. The catalysts of Examples and Comparative Examples were pretreated at 600 ° C. × 100 hr with engine lean exhaust gas before the test. The catalyst inlet temperature during the test was 150 ° C. to 400 ° C., the HC / NOx ratio in the exhaust gas was 2.5, and the gas space velocity was 45.
000h- ¹ . The temperature increase rate of the catalyst inlet temperature during the test was 30 ° C./min, and the temperature decrease rate was 10 ° C./min.
At this time, the average HC / NOx ratio was about 6, and the exhaust gas O ₂ concentration was about 10%. Table 1 shows the average purification rates of HC, CO, and NOx in the light-on / light-off repetition test using the catalysts of Examples and Comparative Examples.

[0057]

[Table 1]

(Catalyst Performance Test Example 2)
Catalyst of Example 2 (Engine lean exhaust gas 600 ° C × 10
(After pretreatment of 0 hr), the HC and NOx concentrations in the exhaust gas of the engine were changed to obtain the average NOx purification ratio with respect to the HC / NOx ratio. Figure 1 shows the results obtained.
Shown in FIG. 1 shows the relationship between the HC / NOx ratio (horizontal axis) and the NO.
x indicates the average purification rate of x (vertical axis). In the catalyst of Example 2 included in the range of the present invention, the NOx purification rate sharply decreases under the condition that the HC / NOx ratio exceeds 10. Rather, it can be seen that a sufficiently high purification rate can be obtained with an appropriate amount of HC.

(Catalyst Performance Test Example 3) In the purification performance test of Test Example 1, the catalyst of Example 2 was used (after pretreatment of engine lean exhaust gas at 600 ° C. × 100 hours), and the oxygen concentration of the engine exhaust gas composition was 2 to 10%. %, And the average purification rate of the NOx component in the light-off-light-on test after repeating the light-off-light-on 10 times was determined. FIG. 2 shows the obtained results.
FIG. 2 shows the average NOx purification rate (vertical axis) with respect to the oxygen concentration (horizontal axis). When the light-off-light-on test was repeated at an oxygen concentration of less than 5%, the NOx conversion performance was sufficient. It can be clearly understood that it cannot be obtained. That is, the present catalyst exhibits high performance under a relatively high oxygen content of 5% or more.

[0060]

As described above, according to the present invention, a zeolite-based catalyst containing platinum, an alkali metal, and the like,
Since the zeolite-based catalyst containing copper and the HC / CO purification catalyst are arranged in a specific arrangement, the catalyst can be suitably applied to a lean burn engine, particularly in a low temperature range and a low HC / CO ratio.
It is possible to provide an exhaust gas purifying catalyst and an exhaust gas purifying method that exhibit excellent NOx purifying ability even under an exhaust gas condition of a NOx ratio.

Therefore, the use of the exhaust gas purifying method and the catalyst of the present invention makes it possible to purify exhaust gas with high efficiency under conditions including a low temperature of about 150 ° C. and at a low HC / NOx ratio. It is possible to provide an automobile that is low in cost and fuel economy (fuel efficiency).

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the exhaust gas HC / NOx ratio on the NOx purification rate.

FIG. 2 is a graph showing the influence of the exhaust gas O ₂ concentration on the NOx purification rate.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 29/46 F01N 3/24 U F01N 3/24 3/28 301B 3/28 301 B01D 53/36 102H 102B 104A

Claims (6)

[Claims] 1. An air-fuel ratio (Air / fuel ratio = A / F)
Is a multi-stage exhaust gas purifying catalyst in which two or more catalysts are arranged in series in an exhaust system of an internal combustion engine operated in a lean region exceeding 14.7, and platinum (Pt) is provided upstream of the exhaust system. ), A honeycomb-shaped monolith catalyst A coated with β zeolite containing at least one component selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth metal, and a downstream Pt. And / or a honeycomb-shaped monolith catalyst B containing palladium (Pd) was installed, and copper (C
u) A catalyst for purifying exhaust gas, comprising forming a catalyst A ′ having a layer containing β zeolite and / or MFI zeolite containing a component. 2. A catalyst A ″ separate from the catalyst A by applying a layer containing the above β-containing zeolite containing Cu and / or MFI zeolite to a honeycomb monolithic carrier.
The exhaust gas purifying catalyst according to claim 1, wherein the catalyst A "is provided between the catalyst A and the catalyst B. 3. At least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals is calcium (Ca), potassium (K), barium (Ba), cesium (Cs) and 3. The composition according to claim 1, which is at least one component selected from the group consisting of lanthanum (La) and has a content of 0.2 mol to 0.6 mol per 1 L of the honeycomb-shaped monolith catalyst.
The exhaust gas purifying catalyst according to the above. 4. The silica / alumina ratio (SiO ₂ / Al ₂ O _{3) of} β zeolite containing the above Pt and containing at least one component selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals. (Molar ratio) 20-130
The exhaust gas purifying catalyst according to any one of claims 1 to 3, characterized in that: 5. The exhaust gas purifying catalyst according to claim 1, wherein the β-zeolite and / or the MFI zeolite containing Cu has a silica / alumina ratio of 25 to 80. . 6. An exhaust gas purifying method using the exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the exhaust gas purifying method is a series arrangement type according to any one of claims 1 to 5. The ratio between the amount of hydrocarbons and the amount of nitrogen oxides necessary for adding nitrogen oxides to nitrogen by reacting nitrogen oxides with hydrocarbons when the oxygen concentration is 5% or more in the multi-stage catalyst (HC / NOx ratio) Exhaust gas having a flow rate of not more than 10 is brought into contact with the exhaust gas.