JP3087321B2 - 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. Things.

[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 amounts from various mobile sources such as internal combustion engines such as gasoline engines of automobiles and fixed sources such as internal combustion engines such as boilers of plant plants, gas engines of 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 not yet reached practical use.

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 composed of hydrocarbons having one carbon atom, and have been conventionally proposed. The purification performance of nitrogen oxides was particularly low with the zeolite-based catalyst.

[0009] It is an object of the present invention to remove nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons.
An object of the present invention is to provide a catalyst for efficiently removing substances, carbon monoxide and hydrocarbons.

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 conducted intensive studies to solve the above-mentioned problems, and as a result, by adding cobalt and palladium to zeolite, nitrogen oxides and monoxide can be removed from exhaust gas containing excess oxygen. It has been found that carbon and hydrocarbons can be efficiently removed, and that the purification rate of nitrogen oxides can be further increased by using the catalyst and adding hydrocarbons to the exhaust gas, thereby completing the present invention. Was.

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

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.)
Is a crystalline aluminosilicate having the following composition, 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 palladium. The method for containing cobalt and palladium in the zeolite is not particularly limited, and may be performed by an ion exchange method, an impregnation-supporting method, or the like. 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 palladium is not particularly limited.
For example, zeolite may be immersed in a solution containing palladium and heated with stirring to remove water. Examples of the palladium salt include chlorides and ammine complexes.

The contents of cobalt and palladium are not particularly limited, but cobalt is 0.2 to 2.5 in terms of CoO / Al ₂ O ₃ molar ratio, and palladium is PdO / Al
_It is preferably from 0.01 to 1.0 in terms of a molar ratio of ₂ O ₃ .
_{That, (PdO + CoO) / Al} 2 O 3 in a molar ratio 0.2 to 3.5 is desirable. Co is CoO / Al ₂ O ₃
If the molar ratio is lower than 0.2, sufficient activity cannot be obtained. Even if the CoO / Al ₂ O ₃ molar ratio is higher than 2.5, it is difficult to obtain the effect of increasing cobalt. Also, P
If the dO / Al ₂ O ₃ molar ratio is lower than 0.01, sufficient activity cannot be obtained. Even if the PdO / Al ₂ O ₃ molar ratio is higher than 1.0, the effect of increasing palladium cannot be obtained.

The sample containing cobalt and palladium may be used after being subjected to pretreatment such as drying and calcination when used as a catalyst.

The catalyst containing cobalt and palladium according to the present invention may have any shape, 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. 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. 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 mixed with 0.25 M Co (CH ₃ COO) _2.
The solution was charged into 1800 ml of a 4H ₂ O aqueous solution and stirred at 80 ° C. for 20 hours 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 and dried at 110 ° C. for 10 hours to obtain Comparative Catalyst 1. As a result of elemental analysis, CoO
The / Al ₂ O ₃ molar ratio was 1.39 .

[0027] Example 1 20g of <Preparation Catalyst 1> Comparative Catalyst 1, 0.05 times the number of moles of alumina in the zeolite _{[Pd (NH 3) 4]} Cl 2 · H 2
It was poured into 100 cc of an aqueous solution in which O was dissolved, dried under reduced pressure at 80 ° C. to support Pd, and then dried at 110 ° C. for 10 hours to obtain Catalyst 1. As a result of elemental analysis, CoO / Al ₂ O ₃
The molar ratio was 1.39, and the PdO / Al ₂ O ₃ molar ratio was 0.05.
Met.

Example 2 <Preparation of catalyst 2> Same as Example 1 except that [Pd (NH ₃ ) ₄ ] Cl ₂ .H ₂ O was 0.1 times the number of moles of alumina in the zeolite. Prepared in the following manner to obtain Catalyst 2. As a result of elemental analysis,
CoO / Al ₂ O ₃ molar ratio is 1.39, PdO / Al ₂ O ₃
The molar ratio was 0.1 .

Example 3 <Preparation of catalyst 3> [Pd (NH ₃ ) ₄ ] Same as Example 1 except that Cl ₂ · H ₂ O was 0.2 times the number of moles of alumina in the zeolite. The catalyst was prepared by the following method. As a result of elemental analysis,
CoO / Al ₂ O ₃ molar ratio is 1.39, PdO / Al ₂ O ₃
The molar ratio was 0.2 .

[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.

[0032] Comparative Example 4 <Comparative Catalyst Preparation of 4> silica / alumina ratio of 40 NH ₄ type ZSM-5: 20
g of an aqueous solution 1 containing [Pd (NH ₃ ) ₄ ] Cl ₂ .H ₂ O at 0.1 times the number of moles of alumina in the zeolite.
The mixture was charged into 80 ml and stirred 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. As a result of the chemical analysis, PdO / Al
_{The 2} O ₃ molar ratio was 0.1.

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> To the reaction gas shown in Table 1, 5,000 ppm of methane was further added, and the catalytic activity was measured in the same manner as in Example 4. Table 5 shows the purification rates of NO and methane when the temperature reached a steady state at each temperature. The CO purification rate was almost 100% for each of the catalysts.

[0039]

[Table 5] Example 7 <Catalyst evaluation 4> 5000 ppm of a mixed gas containing methane as a main component shown in Table 6 was further added to the reaction gas shown in Table 1.
The catalytic activity was measured in the same manner as described above. Table 7 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.

[0040]

[Table 6]

[0041]

[Table 7] Example 8 <Catalyst evaluation 5> Propane was further added to the reaction gas shown in Table 1 by 500 ppm.
After addition, the catalytic activity was measured in the same manner as in Example 4.
Table 8 shows the purification rates of NO and methane when the temperature reached a steady state at each temperature.

[0042]

[Table 8]

[0043]

According to the results of Tables 2, 4, 5, 7, and 8, the zeolite catalyst containing cobalt and palladium of the present invention shows an excess of oxygen containing nitrogen oxides, carbon monoxide and hydrocarbons. It is possible to remove nitrogen oxides, carbon monoxide and hydrocarbons at a high purification rate compared to a comparative catalyst even if the exhaust gas is a natural exhaust gas and the main component of the hydrocarbon is a carbon gas having a carbon number of 1. . Furthermore, nitrogen oxides can be purified at a very high purification rate by adding a hydrocarbon using the catalyst of the present invention. Therefore, the present invention is extremely significant in 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 palladium. An exhaust gas purifying catalyst characterized by the above-mentioned. 2. The use of the exhaust gas purifying catalyst according to claim 1 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 the above-mentioned. 3. The exhaust gas purifying method according to claim 2, wherein the main component of the hydrocarbon has 1 carbon atom. 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 hydrocarbons are further added to the exhaust gas. The exhaust gas purification method according to claim 2, wherein 5. The exhaust gas purifying 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.