JPH04260441A - Catalyst and method for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To provide a catalyst removing nitrogen oxide, carbon monoxide and hydrocarbon from oxygen excessive exhaust gas containing nitrogen oxide, carbon monoxide and hydrocarbon with high purifying efficiency. CONSTITUTION:A catalyst is obtained by adding cobalt and silver to zeolite and has purifying function removing nitrogen oxide, carbon monoxide and hydrocarbon from oxygen excessive exhaust gas containing nitrogen oxide, carbon monoxide and hydrocarbon.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a catalyst and method for purifying exhaust gas emitted from an internal combustion engine, and in particular to a catalyst and method for removing nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-excess exhaust gas. It is about the method.

0002

BACKGROUND OF THE INVENTION Currently, purification of nitrogen oxides, carbon monoxide, hydrocarbons, etc. is becoming more important as environmental problems become more serious. Nitrogen oxides are generated from various mobile sources such as internal combustion engines such as automobile gasoline engines, boilers in factory plants, gas engines in cogeneration systems,
Large amounts are also emitted from stationary sources, such as internal combustion engines such as gas turbines, and their purification is an urgent and serious social issue.

Currently, a three-way catalyst in which Pt, Rh, Pd, etc. are supported on a carrier is used as a purification catalyst for exhaust gas emitted from an internal combustion engine. Since it is not possible to purify oxides, it is used in conjunction 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 improving fuel efficiency and reducing carbon dioxide emissions, but the lean burn exhaust gas has an excess of oxygen, so the three-way catalyst described above removes nitrogen oxides. Can not do it.

[0005] As a method for removing nitrogen oxides from oxygen-excess exhaust gas, reduction denitrification by adding ammonia has been carried out, but its range of use is limited due to the large size of the equipment and the danger of ammonia.

[0006]Recently, a zeolite catalyst has been proposed that can purify nitrogen oxides in oxygen-excess exhaust gas without adding a special reducing agent such as ammonia. For example, JP-A-63-283727 and JP-A-1-13073.
Publication No. 5 proposes that a zeolite catalyst in which transition metals are ion-exchanged can purify nitrogen oxides by using unburned hydrocarbons, which are present in small amounts even in oxygen-excessive exhaust gas, as a reducing agent.

[0007]

[Problems to be Solved by the Invention] However, the conventional zeolite catalysts proposed in JP-A-63-283727 and JP-A-1-130735 have a low nitrogen oxide purification rate and are still not in practical use. It has not reached the level of

Furthermore, in the case of internal combustion engines such as gas engines and gas turbines that use gaseous fuel, the trace hydrocarbons contained in the exhaust gas are mainly 1-carbon hydrocarbons, and the zeolite-based The catalyst had low nitrogen oxide purification performance.

An object of the present invention is to provide a catalyst and method for efficiently removing nitrogen oxides, carbon monoxide, and hydrocarbons from 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 present inventors have made extensive studies to solve the above problems, and have found that by incorporating cobalt and silver into zeolite, nitrogen oxides and monoxide can be removed from oxygen-excess exhaust gas. They discovered that carbon and hydrocarbons can be efficiently removed, and that by adding hydrocarbons to the exhaust gas using the catalyst, the purification rate of nitrogen oxides can be further increased, leading to the completion of the present invention. Ta.

That is, the present invention uses cobalt and silver-containing catalysts to efficiently remove nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons. The present invention provides an exhaust gas purification catalyst characterized in that it is made of zeolite, and further provides an exhaust gas purification method characterized in that hydrocarbons are added to the exhaust gas using the catalyst. .

The present invention will be explained in more detail below.

The zeolite used in the present invention generally has the following formula: xM2/nO.Al2O3.ySiO2.zH2O (where n is the valence of the cation M, x is a number in the range of 0.8 to 1.2, and y is 2 It is a crystalline aluminosilicate having a composition of the above number, where z is a number of 0 or more), and many types are known as natural products and synthetic products. Although the type of zeolite used in the present invention is not particularly limited, it is desirable that the silica/alumina molar ratio is 10 or more. Typical examples include ferrierite, Y, mordenite, ZSM-5, and ZSM-11. Among these, ZSM-5 is most preferred. In addition, these zeolites may be used as they are, but they are
There is no problem in using it as NH4 type or H type which is ion-exchanged with Cl, NH4NO3, (NH4)2SO4, etc. Further, there is no problem even if cations such as alkali metals and alkaline earth metals are included.

The exhaust gas purification catalyst of the present invention is characterized by containing cobalt and silver. The method for incorporating cobalt and silver into zeolite is not particularly limited, and may be carried out by an ion exchange method, an impregnation support method, etc., but an ion exchange method is preferable as a method for containing cobalt, and an impregnation support method is a method for containing silver. is preferred.

When performing ion exchange, there are no particular restrictions; for example, zeolite may be added to a solution containing cobalt ions and stirred at 20 to 100° C. for several hours. Cobalt salts used include acetate, nitrate, oxalate,
Examples include chloride.

[0017] There are no particular restrictions on the method of containing silver; for example,
This can be done by soaking zeolite in a solution containing silver and applying heat while stirring to remove water. Examples of silver salts include nitrates and acetates.

The contents of cobalt and silver are not particularly limited, but the content of cobalt is 0.5 to 1.5 in CoO/Al2O3 molar ratio, and the content of silver is 0.01 to 5.0 in Ag2O/Al2O3 molar ratio. is preferred. That is, (A
g2O+CoO)/Al2O3 molar ratio is 0.5 to 6.
5 is desirable. Co has a CoO/Al2O3 molar ratio of 0.
When it is lower than 5, sufficient activity cannot be obtained. CoO/
Even if the Al2O3 molar ratio is higher than 1.5, it is difficult to obtain the effect of increasing cobalt. Also, Ag2O/Al2
When the O3 molar ratio is lower than 0.01, sufficient activity cannot be obtained. Even if the Ag2O/Al2O3 molar ratio is higher than 5.0, the effect of increasing silver cannot be obtained.

The sample containing cobalt and silver may be subjected to pretreatment such as drying or calcination before being used as a catalyst.

The catalyst containing cobalt and silver according to the present invention may have any shape or structure, such as powder, pellet, or honeycomb. Further, the introduction of the metal element can also be performed after molding.

The exhaust gas purification catalyst of the present invention can be formed into a predetermined shape by adding a binder such as alumina sol, silica sol, or clay, or made into a slurry by adding water to form alumina, magnesia, cordierite, etc. in the shape of a honeycomb or the like. It may be used after being coated on a fire-resistant substrate.

[0022] The exhaust gas targeted by the catalyst of the present invention is an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons. Excessive oxygen exhaust gas refers to exhaust gas that contains 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, although there is no particular restriction on the hydrocarbons contained in the exhaust gas, the catalyst of the present invention can efficiently purify the exhaust gas even if the main component of the hydrocarbon is one carbon number. Generally, most of the hydrocarbons contained in exhaust gas discharged from an engine using liquid fuel such as an automobile are hydrocarbons having two or more carbon atoms. on the other hand,
The main component of hydrocarbons contained in the exhaust gas emitted from engines that use gaseous fuel, such as gas engines, has a carbon number of 1.
It is. Generally, the reactivity of hydrocarbons tends to increase as the number of carbon atoms increases, and when the number of carbon atoms is 1, the reactivity is particularly low. Here, the exhaust gas in which the main component of hydrocarbons is 1 carbon number refers to exhaust gas in which 80% or more of the hydrocarbons contained in the exhaust gas have 1 carbon number. Examples of such exhaust gas include exhaust gas discharged from lean-burn gas engines using city gas as fuel.

Although there are no particular restrictions on the hydrocarbon to be added, 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 hydrocarbons 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 it may be added to a concentration of about 50 ppm to 1% of the total exhaust gas. Although it is possible to further increase the amount added, this is not so preferable because it causes a decrease in economic efficiency and a decrease in the hydrocarbon purification rate.

[0025]

EXAMPLES The present invention will be explained in more detail in the following Examples, but the present invention is not limited to these Examples.

Comparative Example 1 <Preparation of Comparative Catalyst 1> NH4 type ZSM-5 zeolite with a silica/alumina ratio of 40: 20
0g to 0.25M Co(CH3COO)2.4H2
The mixture was poured into 1800 ml of an O aqueous solution and stirred at 80° C. for 20 hours to perform ion exchange. After 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 Comparative Catalyst 1. As a result of elemental analysis, the amount of cobalt was 1.39 times that of alumina.

Example 1 <Preparation of catalyst 1> Comparative catalyst 1-2
0g to the number of moles of alumina in the zeolite.
Pour into 100 cc of an aqueous solution containing 1x AgNO3, dry under reduced pressure at 80°C to support Ag, and then heat to 110°C.
The catalyst was dried for 10 hours to obtain catalyst 1. As a result of elemental analysis, the amount of cobalt was 1.39 times that of alumina, and the amount of 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 AgNO3 was increased to 2.0 times the number of moles of alumina in the zeolite. As a result of elemental analysis, the amount of cobalt was 1.39 times that of alumina, and the amount of silver was 2.0 times that of alumina.

Example 3 <Preparation of Catalyst 3> Catalyst 3 was prepared in the same manner as in Example 1 except that AgNO3 was increased to 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 that of alumina.

Comparative Example 2 <Preparation of Comparative Catalyst 2> 1 kg of NH4 type ZSM-5 with a silica/alumina ratio of 40 was mixed with 0.0 kg of NH4 type ZSM-5 having a silica/alumina ratio of 40 so that the number of copper atoms was 1 times the number of Al atoms in the zeolite. Added to an aqueous solution of 1M copper acetate. After that, 2.
The pH was adjusted to 10.5 by adding 5% ammonia water, and the mixture was stirred at room temperature for 20 hours to perform ion exchange treatment. This operation was repeated twice, followed by washing and drying at 110°C for 12 hours to prepare Comparative Catalyst 2. As a result of chemical analysis, C
The uO/Al2O3 molar ratio was 1.05.

Comparative Example 3 <Preparation of Comparative Catalyst 3> 20 g of NH4 type ZSM-5 with a silica/alumina ratio of 40,
Aqueous solution of nickel acetate tetrahydrate with a concentration of 0.23M 180
ml and stirred at 80°C for 16 hours. After solid-liquid separation of the slurry, the zeolite cake was added to the prepared aqueous solution having the above composition and the same operation was performed. After solid-liquid separation,
Comparative catalyst 3 was obtained by thoroughly washing with water and drying at 110° C. for 10 hours. As a result of chemical analysis, the NiO/Al2O3 molar ratio is 1.
．． It was 40.

Comparative Example 4 <Preparation of Comparative Catalyst 4> 20 g of Na-type ZSM-5 with a silica/alumina ratio of 40 was
gNO3 to the number of moles of alumina in the zeolite
．． The mixture was poured into 180 ml of an aqueous solution containing 5 times the amount and stirred at 80° C. for 16 hours. After solid-liquid separation of the slurry, it was thoroughly washed with water and 11
Comparative catalyst 4 was obtained by drying at 0° C. for 10 hours. As a result of chemical analysis, the Ag2O/Al2O3 molar ratio was 0.39, and Na
The 2O/Al2O3 molar ratio was 0.62.

Example 4 <Catalyst Evaluation 1> Catalysts 1 to 3 and Comparative Catalysts 1 to 4 were each compressed into tablets, crushed, sized to 12 to 20 mesh, and 1.2 g of them was transferred to an atmospheric fixed bed reactor. was filled. 1 at 500℃ under air circulation
After the time pretreatment, a gas having the composition shown in Table 1 (hereinafter referred to as reaction gas) was passed at a rate of 500 ml/min.
Catalytic activity was measured at .degree. C. and 500.degree. Table 2 shows the purification rates of NO and methane when steady state was reached at each temperature. The CO purification rate is approximately 10 for both 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 according to the same.

[0034]

[Math 1]

[0035]

[Table 1]

[0036]

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

[0037]

[Table 3]

[0038]

[Table 4] Example 6 <Catalyst Evaluation 3> Example 4
Catalytic activity was measured in the same manner as above. Table 6 shows the purification rates of NO and methane when steady state was reached at each temperature. Note that the purification rate of hydrocarbons other than CO and methane was approximately 100% in all catalysts.

[0039]

[Table 5]

[0040]

[Table 6]

[0041]

Effects of the Invention From the results in Tables 2, 4, and 6, it is clear that the zeolite catalyst containing cobalt and silver of the present invention is effective against oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons. , Even if the main component of hydrocarbon is exhaust gas with 1 carbon, it has a higher purification rate than the comparative catalyst, and removes nitrogen oxides, nitrogen oxides,
Carbon monoxide and hydrocarbons can be removed, and further,
It is clear that by using the catalyst of the present invention and adding hydrocarbons, nitrogen oxides can be purified at a very high purification rate, and therefore the present invention is extremely significant from the viewpoint of environmental conservation.

Claims (5)

[Claims] Claim 1: A catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons from oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, the catalyst being made from zeolite containing cobalt and silver. An exhaust gas purification catalyst characterized by: 2. The exhaust gas purification catalyst according to claim 1 is used to remove nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons. An exhaust gas purification method characterized by using 3. The exhaust gas purification method according to claim 1, wherein the main component of the hydrocarbon is 1 carbon atom. [Claim 4] In removing nitrogen oxides, carbon monoxide, and hydrocarbons from oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons, hydrocarbons are further added to the exhaust gas. The exhaust gas purification method according to claim 2, characterized in that: 5. The exhaust gas purification method according to claim 4, wherein the hydrocarbon added is methane or a mixed gas of hydrocarbons containing methane as a main component.