JP3143936B2 - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

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 discharged from an internal combustion engine, and more particularly to a catalyst and a method for removing nitrogen oxides, carbon monoxide and hydrocarbons in an oxygen-excess exhaust gas. It is about the method.

[0002]

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

At present, a three-way catalyst having Pt, Rh, Pd, etc. supported on a carrier is used as a catalyst for purifying exhaust gas discharged from an internal combustion engine. Since oxides cannot be purified, they are used together with a system that controls 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 the lean burn exhaust gas becomes excessive in oxygen, the three-way catalyst removes nitrogen oxides. Can not do it.

[0005] As a method for removing nitrogen oxides from an oxygen-excess exhaust gas, reductive denitration is performed by adding ammonia. However, its use range is limited due to an increase in the size of the apparatus and the danger of ammonia.

Recently, a zeolite catalyst capable of purifying nitrogen oxides in an oxygen-excess exhaust gas without adding a special reducing agent such as ammonia has been proposed. For example, JP-A-63-283727 and JP-A-1-13073.
No. 5 proposes that a zeolite catalyst ion-exchanged with a transition metal can purify nitrogen oxides by using unburned hydrocarbons, which are contained in a trace amount in an exhaust gas containing excess oxygen, as a reducing agent.

[0007]

However, the conventional zeolite-based catalysts proposed in JP-A-63-283727 and JP-A-1-130735 have a low nitrogen oxide purification rate and are still in practical use. Is not reached.

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 mainly hydrocarbons having 1 carbon atom, and the conventionally proposed zeolite-based hydrocarbons are used. The catalyst had low nitrogen oxide purification performance.

An object of the present invention is to provide a catalyst and a method for efficiently removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. It is in.

Another object of the present invention is to provide a method for purifying exhaust gas using such a catalyst.

[0011]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, by adding zeolite to cobalt and silver, nitrogen oxides and monoxides were removed from exhaust gas containing excess oxygen. The present inventors have found that carbon and hydrocarbons can be efficiently removed, and that the purification rate of nitrogen oxides can be further increased by adding hydrocarbons to the exhaust gas using the catalyst, thereby completing the present invention. Was.

That is, the present invention contains cobalt and silver as a catalyst for efficiently removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. The present invention provides an exhaust gas purifying catalyst comprising a zeolite obtained in the above manner, and further provides an exhaust gas purifying method comprising adding hydrocarbon to the exhaust gas using the catalyst. .

Hereinafter, the present invention will be described in more detail.

[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 of 2 or more, and z is a number of 0 or more), and many types are known as natural products and synthetic products. The type of zeolite used in the present invention is not particularly limited, but the silica / alumina molar ratio is desirably 10 or more. Typically, ferrierite, Y, mordenite, ZSM-5, ZSM-11 and the like can be mentioned. Of these, ZSM-5 is most preferred. Further, these zeolites may be used as they are,
It may be used as an NH ₄ type or an H type ion-exchanged with H ₄ Cl, NH ₄ NO ₃ , (NH ₄ ) ₂ SO ₄ or the like. Further, even if it contains a cation such as an alkali metal or an alkaline earth metal, it does not matter at all.

[0015] The exhaust gas purifying catalyst of the present invention is characterized by containing cobalt and silver. The method of incorporating cobalt and silver into the zeolite is not particularly limited, and may be performed by an ion exchange method, an impregnation-supporting method, or the like. The cobalt-containing method is preferably an ion-exchange method, and the silver-containing method is an impregnation-supporting method. Is preferred.

In the case of ion exchange, there is no particular limitation. For example, zeolite may be charged into 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 for containing silver is not particularly limited.
Zeolite may be immersed in a solution containing silver, and heated with stirring to remove water. Silver salts include nitrates and acetates.

The contents of cobalt and silver are not particularly limited. Cobalt is represented by CoO / Al ₂ O ₃ molar ratio of 0.5 to 1.5, and silver is represented by Ag ₂ O / Al ₂ O ₃ molar ratio. Is preferably 0.01 to 5.0. That is, (A
_{g 2 O + CoO) / Al} 2 O 3 molar in ratio 0.5-6.
5 is desirable. Co has a CoO / Al ₂ O ₃ molar ratio of 0.1.
If it is lower than 5, sufficient activity cannot be obtained. CoO /
Even if the Al ₂ O ₃ molar ratio is higher than 1.5, it is difficult to obtain the effect of increasing cobalt. Ag ₂ O / Al ₂
If the O ₃ molar ratio is lower than 0.01, sufficient activity cannot be obtained. Even if the Ag ₂ O / Al ₂ O ₃ molar ratio is higher than 5.0, the effect of increasing silver cannot be obtained.

When the sample containing cobalt and silver is used as a catalyst, it may be used after pretreatment such as drying and baking.

The catalyst containing cobalt and silver according to the present invention may be in any form, structure, etc., such as a powder, a pellet, and a honeycomb. The introduction of the metal element can also be performed after molding.

The exhaust gas purifying catalyst of the present invention may be formed into a predetermined shape by adding a binder such as alumina sol, silica sol or clay, or may be formed into a slurry by adding water, and may be formed into a honeycomb or other alumina, magnesia, cordierite or the like. May be used after being applied on a refractory substrate.

The exhaust gas targeted by the catalyst of the present invention is an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. The oxygen-excess exhaust gas refers to an exhaust gas containing oxygen in excess of the amount of oxygen necessary 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 can efficiently purify the exhaust gas even when the main component of the hydrocarbon is the exhaust gas having 1 carbon atom. Generally, most of hydrocarbons contained in 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 hydrocarbon contained in exhaust gas discharged from an engine using gaseous fuel such as a gas engine is carbon number 1.
It is. Normally, the reactivity of hydrocarbons tends to increase as the number of carbon atoms increases, and particularly when the number of carbon atoms is 1, the reactivity is low. Here, the exhaust gas in which the main component of the hydrocarbon has 1 carbon atom refers to an exhaust gas in which 80% or more of the hydrocarbons contained in the exhaust gas have 1 carbon atom. Examples of such exhaust gas include exhaust gas discharged from a lean-burn gas engine using city gas as fuel.

The hydrocarbon to be added is not particularly limited, but the catalyst of the present invention can efficiently purify exhaust gas even if the hydrocarbon is methane or a mixed gas of hydrocarbons containing methane as a main component. can do. A mixed gas of a hydrocarbon containing methane as a main component refers to a mixed gas in which 80% or more of the hydrocarbons in the mixed gas are methane. Examples of such a mixed gas include various city gases.

The concentration of the hydrocarbon to be added is not particularly limited, and may be added so as to have a concentration of about 50 ppm to 1% with respect to the entire exhaust gas. Although the addition amount may be further increased, it is not preferable because it causes a reduction in economic efficiency and a reduction rate of hydrocarbon purification.

[0025]

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.

Comparative Example 1 <Preparation of Comparative Catalyst 1> 200 g of NH ₄ type ZSM-5 zeolite having a silica / alumina ratio of 40 was added to 0.25 M Co (CH ₃ COO)
_It is poured into 1800 ml of a 2.4 H ₂ O aqueous solution,
The mixture was stirred for 0 hour to perform ion exchange. After the slurry was subjected to solid-liquid separation, the zeolite cake was again charged into an aqueous solution having the same composition as above, and the ion exchange operation was performed again. After solid-liquid separation, the solid was washed with 20 liters of pure water.
After drying for 0 hour, Comparative Catalyst 1 was obtained. As a result of elemental analysis, cobalt was 1.39 times that of alumina.

Example 1 <Preparation of Catalyst 1> 20 g of Comparative Catalyst 1 was added to an aqueous solution 10 in which AgNO ₃ was dissolved 0.1 times the mole number of alumina in zeolite.
0 cc, dried under reduced pressure at 80 ° C. to support Ag, and then dried at 110 ° C. for 10 hours to obtain Catalyst 1. As a result of elemental analysis, cobalt was 1.39 times that of alumina and silver was 0.1 times.

Example 2 <Preparation of Catalyst 2> Catalyst 2 was prepared in the same manner as in Example 1 except that the amount of AgNO ₃ was 2.0 times the number of moles of alumina in the zeolite. As a result of elemental analysis, cobalt was 1.39 times that of alumina and silver was 2.0 times.

Example 3 <Preparation of Catalyst 3> Catalyst 3 was prepared in the same manner as in Example 1 except that the amount of AgNO ₃ was 3.0 times the number of moles of alumina in the zeolite. As a result of elemental analysis, cobalt was 1.39 times that of alumina and silver was 3.0 times.

[0030] Comparative Example 2 <Comparative Preparation of Catalyst 2> silica / alumina NH ₄ form the ratio 40 ZSM-5: 1K
g was added to an aqueous solution of 0.1 M copper acetate so that the number of copper atoms was 1 times the number of Al atoms in the zeolite. Thereafter, 2.5% aqueous ammonia was added to adjust the pH to 10.
The mixture was adjusted to 5 and stirred at room temperature for 20 hours to perform an ion exchange treatment. After repeating this operation twice, washing, 110
C. for 12 hours to prepare Comparative Catalyst 2. Chemical analysis, CuO / _Al 2 _{O 3} molar ratio was 1.05.

[0031] Comparative Example 3 <Comparative Catalyst Preparation of 3> silica / alumina ratio of 40 NH ₄ type ZSM-5: 20
g was added to 180 ml of an aqueous solution of nickel acetate tetrahydrate having a concentration of 0.23 M, and the mixture was stirred at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, the same operation was carried out by adding the zeolite cake to an aqueous solution having the above composition again prepared. After the solid-liquid separation, the mixture was sufficiently washed with water and dried at 110 ° C. for 10 hours to obtain Comparative Catalyst 3. Chemical analysis, NiO / _Al 2 _{O 3} molar ratio was 1.40.

Comparative Example 4 <Preparation of Comparative Catalyst 4> Na-type ZSM-5 having a silica / alumina ratio of 40: 20 g
Was added to 180 ml of an aqueous solution containing 0.5 times AgNO ₃ with respect to the number of moles of alumina in the zeolite, followed by stirring at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, it was sufficiently washed with water and dried at 110 ° C. for 10 hours to obtain Comparative Catalyst 4. Chemical _{analysis, Ag 2 O / Al 2 O} 3 molar ratio of 0.39
_{In, Na 2 O / Al 2 O} 3 molar ratio was 0.62.

Example 4 <Catalyst Evaluation 1> Catalysts 1 to 3 and Comparative Catalysts 1 to 4 were each crushed after tablet molding, sized to 12 to 20 mesh, and 1.2 g of the resulting mixture was subjected to a normal pressure fixed bed reactor. Was filled. 1 at 500 ° C under air circulation
After the time pretreatment, a gas having a composition shown in Table 1 (hereinafter referred to as a reaction gas) was passed at 500 ml / min, and 400
The catalyst activity at 500C and 500C was measured. Table 2 shows the purification rates of NO and methane when the temperature reached a steady state at each temperature. The CO purification rate was almost 10 for all catalysts.
It was 0%. Note that the NO purification rate is a value obtained from the following equation, and the methane purification rate is also a value obtained in accordance therewith.

[0034]

(Equation 1)

[0035]

[Table 1]

[0036]

[Table 2] Example 5 <Catalyst evaluation 2> The catalytic activity was measured in the same manner as in Example 4 except that the reaction gas was changed to the reaction gas having the composition shown in Table 3. Table 4 shows the NO and methane purification rates when the temperature reached a steady state at each temperature. In addition, the purification rate of hydrocarbons other than CO and methane was almost 100% in each of the catalysts.

[0037]

[Table 3]

[0038]

[Table 4] Example 6 <Catalyst evaluation 3> 5000 ppm of a mixed gas containing methane as a main component shown in Table 5 was further added to the reaction gas shown in Table 1.
The catalytic activity was measured in the same manner as described above. Table 6 shows the purification rates of NO and methane when the temperature reached a steady state at each temperature. In addition, the purification rate of hydrocarbons other than CO and methane was almost 100% in each of the catalysts.

[0039]

[Table 5]

[0040]

[Table 6] Example 7 <Catalyst evaluation 4> Except that the reaction gas was changed to the reaction gas having the composition shown in Table 7
The catalyst activity was measured in the same manner as in Example 4. Set at each temperature
Table 8 shows the NOx purification rates that were constantly reached. In addition,
The purification rate of CO and propane is
It was almost 100%.

[Table 7]

[Table 8] Example 8 <Evaluation of catalyst 5> The amount of catalyst was changed to a reaction gas having the composition shown in Table 9 for the reaction gas.
In the same manner as in Example 4 except that the catalyst was changed to 2.5 g.
Activity was measured. NOx purification when steady at each temperature
The conversions are shown in Table 10. In addition, CO and propylene
Purification rates were almost 100% for all catalysts.
Was.

[Table 9]

[Table 10]

[0041]

According to the results of Tables 2, 4, 6, 8 and 10 , the zeolite catalyst containing cobalt and silver of the present invention contains nitrogen oxides, carbon monoxide and hydrocarbons.
Nitrogen oxides, carbon monoxide and
It is clear that hydrocarbons can be removed efficiently
It is. Further, the zeolite containing cobalt and silver of the present invention
The main component of the hydrocarbon in the exhaust gas is charcoal.
Even the exhaust gas with prime number 1 is higher than the comparative catalyst
Purification rate removes nitrogen oxides, carbon monoxide and hydrocarbons
can do. Furthermore, it is clear that nitrogen oxides can be purified at a very high purification rate by adding hydrocarbons using the catalyst of the present invention. Therefore, the present invention is extremely significant in terms of environmental conservation. .

Claims (5)

(57) [Claims] 1. A catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, comprising a zeolite containing cobalt and silver. An exhaust gas purifying catalyst, comprising: 2. The exhaust gas purifying catalyst according to claim 1, which is used for removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. An exhaust gas purification method characterized by being used. 3. The main component of the hydrocarbon has 1 carbon atom.
3. The exhaust gas purification method according to item 2 . 4. A method for removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, wherein a hydrocarbon is further added to the exhaust gas. The exhaust gas purification method according to claim 2, wherein: 5. The exhaust gas purification method according to claim 4, wherein the hydrocarbon to be added is methane or a mixed gas of hydrocarbons containing methane as a main component.