JPH06198191A - Exhaust gas purification catalyst - Google Patents

Abstract

PURPOSE:To provide a catalyst having higher exhaust gas purification ability than a conventional catalyst as a method for efficiently removing NOx from exhaust gas contg. excess oxygen as well as NOx and hydrocarbon. CONSTITUTION:This exhaust gas purification catalyst is a catalyst for removal of NOx from exhaust gas contg. excess oxygen as well as NOx and hydrocarbon and contains Co, Pd and Ag in zeolite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a catalyst for removing nitrogen oxides in exhaust gas with excess oxygen.

[0002]

2. Description of the Related Art At present, purification of nitrogen oxides, carbon monoxide, hydrocarbons and the like is regarded as important because of serious environmental problems. Nitrogen oxides are also found in large quantities from various mobile sources such as internal combustion engines such as automobile gasoline engines, and fixed sources such as internal combustion engines such as boilers in plant plants, gas engines in cogeneration systems, and gas turbines. It is discharged to the United States and its purification is an urgent and serious social issue.

Currently, a three-way catalyst in which Pt, Rh, Pd, etc. are carried on a carrier is used as a purification catalyst for exhaust gas discharged from an internal combustion engine. The three-way catalyst is nitrogen in exhaust gas with excess oxygen. Since it cannot purify oxides, it is used together with a system for controlling the ratio of air to fuel (so-called air-fuel ratio).

On the other hand, a lean burn system has been developed for the purpose of reducing fuel consumption and reducing carbon dioxide emissions. However, since lean burn exhaust gas contains excess oxygen, the three-way catalyst removes nitrogen oxides. Can not do it.

As a method for removing nitrogen oxides from an oxygen-excess exhaust gas, reductive denitration by adding ammonia has been carried out, but its range of use is limited due to the large size of the apparatus and the danger of ammonia.

Recently, a zeolite-based catalyst has been proposed which can purify nitrogen oxides in exhaust gas with excess oxygen without adding a special reducing agent such as ammonia. For example, JP-A-63-283727 and JP-A-1-13073.
Japanese Patent Laid-Open No. 5 proposes that a zeolite catalyst ion-exchanged with a transition metal can purify nitrogen oxides by using unburned hydrocarbons contained in a small amount in oxygen-rich exhaust gas as a reducing agent.

However, JP-A-63-283727
The conventional zeolite-based catalysts proposed in JP-A No. 1-130735 and JP-A No. 1-130735 have not reached the practical range.

Further, in the case of an internal combustion engine using a gaseous fuel such as a gas engine or a gas turbine, the trace hydrocarbons contained in the exhaust gas are hydrocarbons having a carbon number of 1 as a main component, and zeolites that have been conventionally proposed. Nitrogen oxide purification performance was particularly poor with the system catalyst. Therefore, Japanese Patent Application No. 3-42
No. 311 is a catalyst for efficiently removing nitrogen oxides, carbon monoxide, and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons even if the main component of the hydrocarbons is 1. As these, Co- and Pd-containing zeolites have been proposed.

[0009]

[Problems to be Solved by the Invention] Japanese Patent Application No. 3-42311
Although the Co and Pd-containing zeolite catalysts shown in No. 3 can certainly remove nitrogen oxides from oxygen-excess exhaust gas, they are required to have higher nitrogen oxide purification ability as exhaust gas purification catalysts.

The object of the present invention is a method for efficiently removing nitrogen oxides from exhaust gas in excess of oxygen containing nitrogen oxides and hydrocarbons, which is higher than the catalyst disclosed in Japanese Patent Application No. 3-42311. It is to provide a catalyst having an exhaust gas purifying ability.

[0011]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that nitrogen oxides and hydrocarbons can be removed by using zeolite containing cobalt, palladium and silver as a catalyst. The inventors have found that nitrogen oxides can be efficiently removed from the exhaust gas containing excess oxygen, and have completed the present invention.

[0012] That is, the present invention is an exhaust gas purification characterized in that zeolite contains cobalt, palladium and silver as a catalyst for removing nitrogen oxides from exhaust gas in excess of oxygen containing nitrogen oxides and hydrocarbons. It provides a catalyst.

The present invention will be described in more detail below.

[0014] Zeolite to be used in the present invention, _{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 greater than or equal to 2, and z is a number greater than or equal to 0)
It is a crystalline aluminosilicate having the following composition, and many kinds are known as natural products and synthetic products. The type of zeolite used in the present invention is not particularly limited, but it is desirable that the SiO ₂ / Al ₂ O ₃ molar ratio is 10 or more. Typically, ferrierite, Y, mordenite, ZSM-5, ZSM-11 and the like can be mentioned. Of these, ZSM-5 is most preferred. These zeolites may be used as they are, but they are ion-exchanged with NH ₄ Cl, NH ₄ NO ₃ , (NH ₄ ) ₂ SO ₄ or the like to form NH ₄ or burned or converted into protons by a mineral acid or the like. It can be exchanged with ions and used as the H type. Further, it does not matter even if it contains cations such as alkali metals and alkaline earth metals.

The exhaust gas purifying catalyst of the present invention is characterized by containing cobalt, palladium and silver.
The method of incorporating cobalt into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation-supporting method, or the like, but an ion exchange method is preferred as the cobalt content method. In the case of carrying out ion exchange of cobalt, zeolite may be added to a solution containing cobalt ions and stirred at 20 to 100 ° C. for several hours. Examples of the cobalt salt used include acetate, nitrate, oxalate, chloride and the like.

The method of incorporating palladium into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation supporting method or the like. When the zeolite is impregnated with and loaded with palladium, the zeolite may be added to a solution containing palladium ions and the solvent may be removed by heating. Examples of the palladium salt used include acetate, nitrate, ammine complex salt, chloride and the like.

The method of incorporating silver into zeolite is not particularly limited, and it may be carried out by an ion exchange method, an impregnation supporting method or the like. When impregnating and supporting silver, zeolite may be added to a solution containing silver ions and the solvent may be removed by heating. Examples of the silver salt used include acetate and nitrate.

The order in which the zeolite contains cobalt, palladium and silver is not particularly limited, and finally the zeolite may contain cobalt, palladium and silver.

The content of cobalt, palladium and silver is not particularly limited, but the content of cobalt is CoO /
The Al ₂ O ₃ molar ratio is preferably 0.2 to 2.5, more preferably 0.8 to 2.0. The content of palladium is 0.01 to 50 in terms of PdO / Al ₂ O ₃ molar ratio.
1.0 is preferable, and 0.05 to 0.4 is more preferable.
The silver content is expressed by Ag ₂ O / Al ₂ O ₃ molar ratio of 0.
005 to 0.5 is preferable, and 0.01 to 0.4 is more preferable. If the content of each metal is lower than the above range, sufficient activity cannot be obtained, and if it exceeds the above range, the effect corresponding to it cannot be obtained.

When the zeolite containing cobalt, palladium and silver is used as a catalyst, it may be used after being subjected to pretreatment such as drying and firing.

The zeolite catalyst containing cobalt, palladium and silver according to the present invention may have any shape, structure, etc. such as powder, pellets and honeycombs. Furthermore, the introduction of the metal element can be performed after the zeolite is molded.

The exhaust gas purifying catalyst of the present invention is molded into a predetermined shape by adding a binder such as alumina sol, silica sol or clay, or is made into a slurry by adding water, and alumina, magnesia, cordierite or the like in the shape of a honeycomb or the like is formed. It may be used after being coated on the refractory base material.

The exhaust gas targeted by the catalyst of the present invention is an oxygen-rich exhaust gas containing nitrogen oxides. Exhaust gas with excess oxygen means exhaust gas containing oxygen in excess of the amount of oxygen required to completely oxidize reducing components such as carbon monoxide and hydrocarbons contained in the exhaust gas. Further, the hydrocarbon contained in the exhaust gas is not particularly limited, but the catalyst of the present invention is also applicable to the exhaust gas in which the main component of the hydrocarbon is C1.
Exhaust gas can be efficiently purified. Typically,
Most of the hydrocarbons contained in the exhaust gas discharged from engines using liquid fuels such as automobiles are hydrocarbons having 2 or more carbon atoms. On the other hand, the main component of hydrocarbons contained in exhaust gas discharged from an engine using a gaseous fuel such as a gas engine has 1 carbon atom. Usually, the reactivity of hydrocarbon tends to increase as the carbon number increases, and the reactivity is particularly low when the carbon number is 1. Here, the exhaust gas having a carbon number of 1 as the main component of hydrocarbons means an exhaust gas having 80% or more of the hydrocarbons contained in the exhaust gas having a carbon number of 1. Examples of such exhaust gas include exhaust gas discharged from a lean-burn gas engine using city gas as fuel.

When using the catalyst of the present invention, hydrocarbon may be added to the exhaust gas. The hydrocarbon to be added is not particularly limited, but the catalyst of the present invention can efficiently purify the exhaust gas even if the hydrocarbon is methane or a mixed gas of hydrocarbons containing methane as a main component. The mixed gas of hydrocarbons containing methane as a main component means a mixed gas in which 80% or more of the hydrocarbons in the mixed gas are methane. The concentration of the added hydrocarbon is not particularly limited and may be about 50 ppm to 1%. Although the addition amount may be increased, it is not preferable because it causes a decrease in economic efficiency and a reduction rate of hydrocarbons. Further, if the hydrocarbon concentration in the exhaust gas is sufficiently high, the hydrocarbon need not be added.

The space velocity, temperature, etc. when removing nitrogen oxides are not particularly limited, but the space velocity is 100 to 50000.
It is preferable that the temperature is 0 hr ⁻¹ and the temperature is 200 to 800 ° C.

[0026]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1 <Preparation of catalyst 1> NH ₄ type ZSM-having a SiO ₂ / Al ₂ O ₃ molar ratio of 40.
200 g of 5 Zeolite was added to 0.25 M Co (CH ₃ CO
_O) was added to 2 · 4H ₂ O aqueous solution 1800 ml, 80 ° C.
Ion exchange was carried out with stirring for 20 hours. After the solid-liquid separation of the slurry, the zeolite cake was again put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, wash with 20 liters of pure water,
It dried at 10 degreeC for 10 hours, and obtained Co-ZSM-5. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39. 0.1 times of Pd thus with respect to the number of moles of alumina in the zeolite resulting _{Co-ZSM-5 (NO 3} )
Aqueous solution 100 in which ₂ and 0.1 times more AgNO ₃ was dissolved
It was put into a ml, dried under reduced pressure at 80 ° C. to support Pd and silver, and then dried at 110 ° C. for 10 hours to obtain a catalyst 1. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39,
PdO / Al ₂ O ₃ molar ratio is 0.1, Ag ₂ O / Al ₂
O ₃ was 0.05.

Example 2 <Preparation of catalyst 2> Catalyst 2 was prepared in the same manner as in Example 1 except that AgNO ₃ was 0.2 times the number of moles of alumina in the zeolite. As a result of elemental analysis, CoO / Al ₂ O
₃ molar ratio is 1.39, PdO / Al ₂ O ₃ molar ratio is 0.
1, Ag ₂ O / Al ₂ O ₃ was 0.1.

Example 3 <Preparation of catalyst 3> Catalyst 3 was prepared in the same manner as in Example 1 except that AgNO ₃ was 0.5 times the number of moles of alumina in the zeolite. As a result of elemental analysis, CoO / Al ₂ O
₃ molar ratio is 1.39, PdO / Al ₂ O ₃ molar ratio is 0.
1, Ag ₂ O / Al ₂ O ₃ was 0.25.

Example 4 <Preparation of catalyst 4> Catalyst 4 was prepared in the same manner as in Example 1 except that AgNO ₃ was 0.8 times the number of moles of alumina in the zeolite. As a result of elemental analysis, CoO / Al ₂ O
₃ molar ratio is 1.39, PdO / Al ₂ O ₃ molar ratio is 0.
1, Ag ₂ O / Al ₂ O ₃ was 0.4.

Comparative Example 1 <Preparation of Comparative Catalyst 1> NH ₄ type ZSM- having a SiO ₂ / Al ₂ O ₃ molar ratio of 40.
200 g of 5 zeolite was mixed with 0.25 M Co (CH ₃ C
_OO) was charged into 2 · 4H ₂ O aqueous solution 1800 ml, 80
Ion exchange was performed by stirring at 0 ° C for 20 hours. After the solid-liquid separation of the slurry, the zeolite cake was again put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, wash with 20 liters of pure water,
It dried at 10 degreeC for 10 hours, and obtained Co-ZSM-5. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39. 0.1 times the Pd (N) of the number of moles of alumina in the Co-ZSM-5 zeolite thus obtained.
Pour into 100 ml of an aqueous solution in which O ₃ ) ₂ is dissolved,
After being dried under reduced pressure at 10 ° C., it was dried at 110 ° C. for 10 hours to obtain Comparative Catalyst 1. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39 and the PdO / Al ₂ O ₃ molar ratio was 0.1.

Comparative Example 2 <Preparation of Comparative Catalyst 2> NH ₄ type ZSM-having a SiO ₂ / Al ₂ O ₃ molar ratio of 40.
200 g of 5 zeolite was mixed with 0.25 M Co (CH ₃ C
_OO) was charged into 2 · 4H ₂ O aqueous solution 1800 ml, 80
Ion exchange was performed by stirring at 0 ° C for 20 hours. After the solid-liquid separation of the slurry, the zeolite cake was again put into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, wash with 20 liters of pure water,
It dried at 10 degreeC for 10 hours, and obtained Co-ZSM-5. As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.39. Thus obtained was dissolved Co-ZSM-5 of AgNO ₃ 0.5 times the number of moles of alumina in the zeolite was placed in an aqueous solution 100 ml, after dried under reduced pressure at 80 ° C., 10 hours at 110 ° C. It was dried and designated as Comparative Catalyst 2.
As a result of elemental analysis, the CoO / Al ₂ O ₃ molar ratio was 1.3.
9, the Ag ₂ O / Al ₂ O ₃ molar ratio was 0.25.

Example 5 <Catalyst Evaluation 1> Catalysts 1 to 4 and comparative catalysts 1 and 2 were crushed after tableting, respectively, and crushed to 12 to 20 mesh, 1.2 g of which was fixed under a normal pressure fixed bed reactor. Filled. 1 at 500 ° C under air flow
After the pretreatment for an hour, the gas having the composition shown in Table 1 was added to 500
The catalyst activity was measured at 400 ° C. and 500 ° C. at a flow rate of ml / min. NOx when it reaches a steady state at each temperature
Table 2 shows the purification rate of. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value calculated from the following equation.

NOx purification rate (%) = (NOx in-NOx
out) / NOxin × 100 NOxin: Reaction tube inlet NOx concentration NOxout: Reaction tube outlet NOx concentration

[0035]

[Table 1]

[0036]

[Table 2] Example 6 <Catalyst evaluation 2> Catalysts 1 to 4 and comparative catalysts 1 and 2 were crushed after tableting, respectively, sized to 12 to 20 mesh, and 1.2 g thereof was charged into a normal pressure fixed bed reactor. . 1 at 500 ° C under air flow
After the pretreatment for an hour, the gas having the composition shown in Table 3 was added to 500
The catalyst activity was measured at 400 ° C. and 500 ° C. at a flow rate of ml / min. NOx when it reaches a steady state at each temperature
The purification rate is shown in Table 4. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value obtained in the same manner as in Example 5.

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

From the results shown in Tables 2 and 4, the zeolite catalyst containing cobalt, palladium and silver of the present invention efficiently removes nitrogen oxides from exhaust gas containing excess oxygen containing nitrogen oxides and hydrocarbons. It is clear that you can do it. Further, the zeolite catalyst containing cobalt, palladium and silver of the present invention, even if the exhaust gas in which the main component of the hydrocarbon of the exhaust gas is carbon number 1,
Nitrogen oxides can be removed at a higher purification rate than the comparative catalyst. Therefore, the present invention is extremely significant in terms of environmental protection.

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[Procedure amendment]

[Submission date] November 17, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

[0014] Zeolite to be used in the present invention, _{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 greater than or equal to 2, and z is a number greater than or equal to 0)
It is a crystalline aluminosilicate having the following composition, and many kinds are known as natural products and synthetic products. The type of zeolite used in the present invention is not particularly limited, but it is desirable that the SiO ₂ / Al ₂ O ₃ molar ratio is 10 or more. Typically, ferrierite, Y, mordenite, ZSM-5, ZSM-11 and the like can be mentioned. Of these, ZSM-5 is most preferred. Also,
These zeolites may be used as they are, but they are NH ₄ type obtained by ion exchange with NH ₄ Cl , NH ₄ NO ₃ , (NH ₄ ) ₂ SO ₄ or the like, or are ion-exchanged with protons by calcining or mineral acid. However, there is no problem even if it is used as an H type. Further, it does not matter even if it contains cations such as alkali metals and alkaline earth metals.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction content]

[0038]

[Table 4] ─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] January 13, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0036

[Correction method] Change

[Correction content]

[0036]

[Table 2] Example 6 <Catalyst evaluation 2> Catalysts 1 to 4 and comparative catalysts 1 to 2 were crushed after tableting, respectively, and crushed to 12 to 20 mesh, and 5.0 g thereof was filled in an atmospheric fixed bed reactor. did. 1 at 500 ° C under air flow
After the pretreatment for an hour, the gas having the composition shown in Table 3 was added to 500
The catalyst activity was measured at 400 ° C. and 500 ° C. at a flow rate of ml / min. NOx when it reaches a steady state at each temperature
The purification rate is shown in Table 4. The CO purification rate was almost 100% for all the catalysts. The NOx purification rate is a value obtained in the same manner as in Example 5.

Claims (1)

[Claims] 1. A catalyst for removing nitrogen oxides from exhaust gas in excess of oxygen containing nitrogen oxides and hydrocarbons, which comprises:
An exhaust gas purification catalyst comprising zeolite containing cobalt, palladium and silver.