JP3482662B2 - 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 method for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a method 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.

A method for removing nitrogen oxides using a nickel-containing zeolite catalyst is disclosed in JP-A-4-21024.
No. 4 publication.

However, 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 methane, which is a hydrocarbon having 1 carbon atom, and have been conventionally proposed. The existing zeolite-based catalysts had a particularly low nitrogen oxide purification performance.

[0009] US Pat. No. 5,698,698 discloses that nitrogen oxide can be purified even in exhaust gas with excess oxygen by using a zeolite catalyst containing methane as the hydrocarbon species and containing nickel.
149512.

Further, recently, according to Misono, et al., Using a zeolite catalyst in which palladium is ion-exchanged, nitrogen oxides,
A method of removing nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-rich exhaust gas containing carbon monoxide and hydrocarbons even if the hydrocarbon is methane has been reported (The 65th Spring Annual Meeting of the Chemical Society of Japan). Meeting, Proceedings 420th page).

[0011]

However, the conventionally proposed purification methods are insufficient in exhaust gas purification performance, and higher nitrogen oxide purification performance is required.

It is an object of the present invention to remove nitrogen oxides, carbon monoxide and hydrocarbons from oxygen-rich exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons.
An exhaust gas purification method having a higher exhaust gas purification rate is provided.

[0013]

Means for Solving the Problems As a result of intensive studies by the present inventors, a zeolite containing palladium and nickel efficiently removes nitrogen oxides from exhaust gas containing excess oxygen containing nitrogen oxides and hydrocarbons. The inventors have found that the catalyst can be used and have completed the present invention.

That is, the present invention is characterized by using zeolite containing palladium and nickel as a catalyst for removing nitrogen oxides from exhaust gas containing excess oxygen containing nitrogen oxides and hydrocarbons. It provides a method.

The present invention will be described in more detail below.

The zeolite 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 ion-exchanged with NH ₄ Cl, NH ₄ NO ₃ , (NH ₄ ) ₂ SO ₄ or the like to form NH ₄ or they 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.

The exhaust gas purifying catalyst of the present invention is characterized in that zeolite contains palladium and nickel. The method of incorporating palladium into zeolite is not particularly limited, and it may be performed by an ion exchange method, an impregnation supporting method, or the like. When the palladium is ion-exchanged with the zeolite, the zeolite is put into a solution containing palladium ions,
It suffices to stir at -100 ° C for several hours to several tens of hours. Examples of the palladium salt used include acetate, nitrate, ammine complex salt, chloride and the like.

Further, the method of containing nickel is not particularly limited, and it may be performed by an ion exchange method, an impregnation supporting method, or the like. When nickel is ion-exchanged with zeolite, the zeolite is added to a solution containing nickel ions, and 20 to 1
The stirring may be performed at 00 ° C for several hours to several tens of hours. Examples of the nickel salt used include nitrates, acetates, sulfates and the like.

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

The content of palladium and nickel is not particularly limited, but the content of palladium is PdO / Al ₂
O ₃ molar ratio is preferably 0.01 to 1.0,
0.05 to 0.4 is more preferable. Although the content of nickel is not particularly limited, it is preferably 0.2 to 2.5, more preferably 0.8 to 2.0 in terms of NiO / Al ₂ O ₃ molar ratio. When the PdO / Al ₂ O ₃ molar ratio is lower than 0.01, sufficient activity cannot be obtained. In addition, PdO / Al ₂ O ₃
Even if the molar ratio is higher than 1.0, the effect of increasing the amount of palladium cannot be obtained. When the NiO / Al ₂ O ₃ molar ratio is lower than 0.2, sufficient activity cannot be obtained. Also, NiO / A
Even if the l ₂ O ₃ molar ratio is higher than 2.5, it is difficult to obtain the effect of increasing the amount of nickel.

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

The zeolite catalyst containing palladium and nickel according to the present invention includes powder, pellets,
The shape and structure of the honeycomb-shaped body etc. are not limited. Further, the introduction of the metal element can be performed after the molding.

The exhaust gas purifying catalyst of the present invention is formed 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 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 can efficiently purify the exhaust gas even for the exhaust gas whose main component of the hydrocarbon is methane having 1 carbon atom. You can Generally, most of the hydrocarbons contained in the exhaust gas discharged from an engine using liquid fuel such as an automobile are hydrocarbons having 2 or more carbon atoms. On the other hand, the main component of the hydrocarbon contained in the exhaust gas discharged from an engine using a gaseous fuel such as a gas engine is methane. Normal,
The reactivity of hydrocarbons tends to increase as the number of carbon atoms increases, and the reactivity is particularly low in the case of methane having 1 carbon atom. Here, the exhaust gas containing methane as a main component of hydrocarbon means an exhaust gas in which 80% or more of the hydrocarbon contained in the exhaust gas is methane. 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. for removing nitrogen oxides are not particularly limited, but the space velocity is 100 to 50,000.
It is preferable that the temperature is 0 hr ⁻¹ and the temperature is 200 to 800 ° C.

[0027]

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> 20 g of NH ₄ -ZSM-5 zeolite having a silica / alumina ratio of 40 was mixed with 180 m of a 0.25M aqueous nickel acetate solution.
Then, the mixture was put into a reactor and stirred at 80 ° C. for 20 hours to carry out ion exchange. 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, it was washed with 20 liters of pure water and dried at 110 ° C. for 10 hours to obtain Ni-ZSM-5. As a result of elemental analysis, nickel was 1.15 times that of alumina. 10 g of this was added to 90 ml of pure water to obtain 7
% Aqueous ammonia to adjust the pH to 10.0,
[Pd (NH ₃ ) ₄ ] Cl ₂ .H ₂ O was added 0.1 times the number of moles of alumina in the zeolite, and the mixture was stirred at 30 ° C. for 2 hours to perform palladium ion exchange. After this operation, solid-liquid separation, washing and drying at 110 ° C. for 20 hours,
It was designated as catalyst 1. As a result of elemental analysis, nickel was 1.15 times that of alumina, and palladium was 0.1 times that of alumina.

Example 2 <Preparation of Catalyst 2> Catalyst 2 was prepared in the same manner as in Example 1 except that the nickel ion exchange operation was repeated twice. As a result of elemental analysis, nickel was 1.50 times that of alumina and palladium was 0.1 times that of alumina.

Example 3 <Preparation of catalyst 3> Catalyst 3 was prepared in the same manner as in Example 1 except that the nickel ion exchange operation was repeated 3 times. As a result of elemental analysis, nickel was 1.90 times that of alumina, and palladium was 0.1 times that of alumina.

Comparative Example 1 <Preparation of Comparative Catalyst 1> Comparative catalyst 1 was prepared in the same manner as in Example 1 except that palladium ion exchange was not carried out. As a result of elemental analysis,
Nickel was 1.15 times that of alumina.

Comparative Example 2 <Preparation of Comparative Catalyst 2> 10 g of NH ₄ -ZSM-5 zeolite having a silica / alumina ratio of 40 was added to 90 ml of pure water. Then 7%
Ammonia water was added to adjust the pH to 10.0, and the amount of [P was 0.1 times the molar number of alumina in the zeolite].
d (NH ₃ ) ₄ ] Cl ₂ · H ₂ O was added, and the mixture was stirred at 30 ° C. for 2 hours to perform palladium ion exchange. After solid-liquid separation of the slurry, wash with 1 liter of pure water,
It was dried for 0 hour to obtain a palladium-containing zeolite, which was used as a comparative catalyst 2.

As a result of elemental analysis, palladium was 0.1 times that of alumina.

Example 4 <Catalyst evaluation 1> Catalysts 1 to 3 and comparative catalysts 1 and 2 were crushed after tableting,
The particle size was adjusted to 12 to 20 mesh, and 1.2 g of the sized powder was charged into the atmospheric fixed bed reactor. 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
The purification rates of methane and methane are shown in Table 2. The CO purification rate was 100% for all the catalysts. Note that NOx
The purification rate is a value obtained from the following equation.

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

[0036]

[Table 1]

[0037]

[Table 2]

Example 5 <Catalyst evaluation 2> Catalysts 1 to 3 and comparative catalysts 1 and 2 were crushed after tableting, respectively, and crushed to 12 to 20 mesh, 5.0 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 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 4.

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

From the results shown in Tables 2 and 4, the use of the palladium- and nickel-containing zeolite of the present invention as a catalyst efficiently converts nitrogen oxides from exhaust gas containing excess oxygen containing nitrogen oxides and hydrocarbons. Obviously, it can be removed well. Further, according to the method of the present invention, nitrogen oxides can be removed at a higher purification rate than that of the comparative catalyst even if the exhaust gas is methane as a main component of hydrocarbons in the exhaust gas. Therefore, the present invention is extremely significant in terms of environmental protection.

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B01D 53/94 B01J 29/46

Claims (2)

(57) [Claims] 1. A method for purifying an exhaust gas, wherein zeolite containing palladium and nickel is used as a catalyst for removing nitrogen oxides from an oxygen-excessive exhaust gas containing nitrogen oxides and hydrocarbons. 2. The exhaust gas purification method according to claim 1, wherein the main component of the hydrocarbon contained in the exhaust gas is methane.