JPH10202064A - Method for catalytically reducing nitrogen oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain stable and efficient catalytic reduction of NOx in waste gas at a low temp. without using a large amt. of a reducing agent even under coexistence of O2 , SOx and moisture when NOx is catalytically reduced with hydrocarbon as a reducing agent. SOLUTION: When NOx contained in waste gas is catalytically reduced with a reducing agent in the presence of a catalyst, the waste gas contg. NOx and hydrocarbons as a reducing agent is brought into contact with a 1st catalyst obtd. by carrying phosphoric acid and at least one of phosphates, chlorides and sulfates of groups Ib, VIIa and VIII elements of the Periodic table on a carrier in the 1st stage and the waste gas is brought into contact with a 2nd catalyst selected from among silver, silver oxide and silver aluminate in the 2nd stage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for catalytically reducing nitrogen oxides contained in exhaust gas using a reducing agent in the presence of a catalyst. Using hydrocarbon as a reducing agent for harmful nitrogen oxides in exhaust gas discharged from diesel engine vehicles, reducing and removing the ratio (molar ratio) of the reducing agent to nitrogen oxides stably and efficiently. To a method for catalytic reduction of nitrogen oxides.

[0002]

2. Description of the Related Art Conventionally, nitrogen oxides contained in exhaust gas are obtained by oxidizing the nitrogen oxides and then absorbing them into an alkali, or by using a reducing agent such as ammonia, hydrogen, carbon monoxide, or a hydrocarbon. It has been removed by a method of converting to nitrogen.

However, according to the former method, it is necessary to take measures for treating the generated alkaline waste liquid to prevent the occurrence of pollution. On the other hand, according to the latter method, when ammonia is used as the reducing agent, it reacts with the sulfur oxide in the exhaust gas to form salts, and as a result, the reduction activity of the catalyst is reduced. Also, hydrogen, carbon monoxide,
Even when hydrocarbons and the like are used as reducing agents, they react with oxygen present at a higher concentration than nitrogen oxides present at a lower concentration, so a large amount of reducing agent is required to reduce nitrogen oxides. There is a problem that.

[0004] For this reason, recently, a method of directly decomposing nitrogen oxides with a catalyst in the absence of a reducing agent has been proposed. However, such a conventionally known catalyst has been proposed.
There is a problem that it is difficult to put into practical use due to low decomposition activity of nitrogen oxides.

Further, as a new catalyst for catalytic reduction of nitrogen oxides using a hydrocarbon or an oxygen-containing compound as a reducing agent, H
Zeolite and Cu ion exchange ZSM-5 have been proposed. Among them, H-type ZSM-5 (SiO ₂ / Al ₂
O ₃ molar ratio = 30 to 40) is considered to be optimal.
However, even with such H-type ZSM-5, it is still difficult to say that the H-type ZSM-5 has a sufficient reducing activity. In particular, when moisture is contained in the gas, the aluminum in the zeolite structure is dealuminated and the performance is reduced. There is a demand for a catalyst for catalytic reduction of nitrogen oxides which has a higher reduction activity because of a rapid decrease and has excellent durability even when the gas contains moisture.

[0006]

Accordingly, a catalyst comprising silver or silver oxide supported on an inorganic oxide has been proposed. However, such a catalyst has a high oxidation activity and a selective reaction to nitrogen oxide. The removal rate of nitrogen oxides is low because of low properties. Further, since the temperature range in which the catalyst has a nitrogen oxide decomposition activity is as high as 450 to 600 ° C., it is necessary to heat the exhaust gas in advance in order to effectively decompose the nitrogen oxide in the exhaust gas. However, there is a problem in practical use. Further, a catalyst in which silver or silver oxide is supported on an inorganic oxide has a problem that the catalytic activity is remarkably deteriorated in the presence of sulfur oxide (JP-A-5-317647). In addition, conventional catalysts for catalytic reduction of nitrogen oxides generally do not have sufficient heat resistance, and there is a strong demand for even higher heat resistance depending on the application.

The present invention has been made in view of the above circumstances, and has as its object to catalytically reduce nitrogen oxides in the presence of a catalyst using a hydrocarbon as a reducing agent. A method for stably and efficiently reducing nitrogen oxides in an exhaust gas while reducing the ratio (molar ratio) of a reducing agent to nitrogen oxides in the presence of oxygen, sulfur oxides, and moisture. To provide a method for catalytic reduction of nitrogen oxides.

[0008]

According to the present invention, there is provided a method for catalytically reducing nitrogen oxides contained in exhaust gas using a reducing agent in the presence of a catalyst. Phosphoric acid on the carrier with exhaust gas containing oxides,
Contact with a first catalyst carrying at least one selected from the group consisting of phosphates, chlorides and sulfates of Group Ib, VIIa and VIII elements, and as a second step, silver, silver oxide and It is characterized by being brought into contact with a second catalyst selected from silver aluminate.

[0009]

According to the method of the present invention, as a first step, an exhaust gas containing a reducing agent consisting of hydrocarbons and nitrogen oxides is transferred to a carrier from phosphoric acid, phosphate, chloride and sulfate. By bringing the reducing agent into contact with a first catalyst supporting at least one selected member (which may be simply referred to as phosphoric acid or the like), the reducing agent has excellent reactivity with nitrogen oxides, for example, Oxygen-containing organic compounds and lower molecular weight hydrocarbons are converted into
The nitrogen oxide acts as a reducing agent in the presence of the second catalyst.

The above-mentioned phosphates, chlorides and sulfates are phosphates, chlorides and sulfates of Group Ib, VIIa and VIII elements, respectively. Therefore, specific examples of phosphates include, for example, silver phosphate, copper phosphate, manganese phosphate,
Examples thereof include iron phosphate, cobalt phosphate, and nickel phosphate. Specific examples of chlorides include, for example, silver chloride, copper chloride, manganese chloride, iron chloride, cobalt chloride, nickel chloride and the like. Further, specific examples of the sulfate include, for example, silver sulfate, copper sulfate, manganese sulfate, iron sulfate, cobalt sulfate, nickel sulfate and the like.

The first catalyst is obtained by supporting an appropriate carrier such as γ-alumina with phosphoric acid or the like by an appropriate method such as an impregnation method. For example, after γ-alumina is impregnated with phosphoric acid or the like, it is calcined in air at a temperature of about 200 to 500 ° C. to obtain a catalyst in which γ-alumina supports phosphoric acid or the like.

The loading ratio of phosphoric acid or the like on the carrier, that is, the ratio of phosphoric acid or the like to the total weight of the phosphoric acid or the like and the carrier depends on the reaction conditions in which the catalyst is placed, but is usually 0.05 to 5%. 5
%, Preferably in the range of 0.1 to 3% by weight. When the supporting ratio of phosphoric acid or the like on a carrier is less than 0.05% by weight, the ability to convert a reducing agent comprising a hydrocarbon into an oxygen-containing organic compound or a low-molecular-weight hydrocarbon is insufficient. In the stage, the nitrogen oxides cannot be efficiently catalytically reduced. However, even when the loading is more than 5% by weight, the catalytic reduction of nitrogen oxides cannot be efficiently performed in the second stage.

In the first stage, the space velocity when the exhaust gas containing the reducing agent composed of hydrocarbons and nitrogen oxide is brought into contact with such a first catalyst is usually 10,000 to 10
It is in the range of 00000 hr ^-1 .

According to the method of the present invention, a hydrocarbon is used as the reducing agent. As specific examples, for example, methane,
Alkanes such as ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane; alkenes such as ethylene, propylene, isobutylene, 1-butene and 2-butene; and aromatic carbonization such as benzene, toluene and xylene Examples thereof include hydrogen, mineral oil-based hydrocarbons such as gasoline, kerosene, light oil, and heavy oil, and mixtures of two or more of these. Particularly, in the present invention, among the above, lower alkanes such as propane and butane, lower alkenes such as ethylene, propylene, isobutylene, 1-butene and 2-butene, and light oil are preferably used. These hydrocarbons may be used alone or in combination of two or more as needed.

Further, according to the present invention, the lower aliphatic hydrocarbons composed of lower alkenes, lower alkanes or a mixture thereof having up to 6 carbon atoms contained in the exhaust gas of a lean-burn gasoline vehicle are converted to carbonized reducing agents. It is preferably used as a hydrogen component. Among them, ethylene, propylene,
Alkenes such as butylene, alkanes such as propane and butane, and hydrocarbons mainly containing a mixture thereof are preferably used.

The hydrocarbon as the reducing agent varies depending on the specific hydrocarbon used, but is usually used in a molar ratio to nitrogen oxides in the exhaust gas in a range of about 0.1 to 5. When the amount of the hydrocarbon used is less than 0.1 in terms of the molar ratio to the nitrogen oxides, sufficient reducing activity cannot be obtained for the nitrogen oxides. On the other hand, when the molar ratio exceeds 5, Since a large amount of unreacted hydrocarbons is emitted, a post-treatment for recovering the nitrogen oxides after the catalytic reduction treatment of the nitrogen oxides is required.

According to the method of the present invention, preferably, the reducing agent comprising a hydrocarbon having a predetermined molar ratio with respect to nitrogen oxides in the exhaust gas is brought into contact with the first catalyst, thereby obtaining The reducing agent is efficiently converted into an oxygen-containing organic compound or a low molecular weight hydrocarbon having excellent selective reactivity with nitrogen oxides, and in the second stage, these are used as reducing agents to form a second catalyst together with nitrogen oxides. Thus, the nitrogen oxide can be reduced and removed stably and efficiently while reducing the amount of the reducing agent used for the nitrogen oxide.

In the method of the present invention, the second catalyst is
It is selected from silver, silver oxide or silver aluminate, and among them, silver aluminate having high catalytic activity is preferably used. Among these second catalysts, silver and silver oxide are usually used by being supported on a solid acid carrier such as an oxide having a large specific surface area, for example, alumina, silica, silica-alumina, zirconia, titania, and zeolite. . Among these carriers, alumina having an excellent supporting effect is particularly preferably used.

Among aluminas, Japanese Patent Application Laid-Open No. 7-1713
As described in Japanese Patent No. 47, the content of alkali metals and alkaline earth metals is 0.5% by weight or less, and the volume of pores formed from pores having a diameter of 60 Å or less is 0.06 cm ^3. / G and alumina having a pore volume of 0.1 cm ³ / g or more formed from pores having a diameter of 80 Å or less are particularly preferably used. Porous alumina having such a pore volume promotes appropriate oxidation of the reducing agent and cooperates with silver or silver oxide carried thereon to effectively catalytically reduce nitrogen oxides. Can be.

As described above, the second catalyst composed of silver or silver oxide can be used in various forms such as honeycomb, spherical, etc. by itself or after being supported on a carrier by a conventionally known molding method. It can be formed into a shape. At the time of this molding, a molding aid, a molded body reinforcement, an inorganic fiber, an organic binder, and the like may be appropriately compounded. Further, the second catalyst can be coated and supported on a preformed inert substrate by a wash coat method or the like. As the substrate, for example, it can be supported on a honeycomb structure made of clay such as cordierite.

The amount of silver or silver oxide supported on the carrier is, in terms of silver, in the range of 0.1 to 5% by weight, preferably 0.5 to 3% by weight. The loading amount is 0.1% by weight
When the amount is less than the above, the reducing activity of the nitrogen oxide is not sufficient. On the other hand, when the amount is more than 5% by weight, the oxidizing activity is too high and the selectivity is poor.

In the present invention, as the second catalyst,
A catalyst comprising silver aluminate is preferably used. This second catalyst composed of silver aluminate is also usually used by being supported on a solid acid carrier such as an oxide having a large specific surface area, for example, alumina, silica, silica-alumina, zirconia, titania, and zeolite. Such a catalyst can be prepared, for example, according to any one of the following methods (1) to (4).

(1) A water-soluble silver salt such as silver nitrate is charged into a chiller in which a solid acid carrier is dispersed, and the pH of the slurry is maintained at around 8.0 where no silver hydroxide is formed, and the pH of the solid acid is reduced. The silver ions are fixed on the ion exchange site. Here, when alumina is used as the solid acid, the solid acid in which silver ions are fixed in this way is converted into an aqueous solution containing sufficient chloride ions to fix the silver ions, for example, an aqueous hydrochloric acid solution. After generating silver chloride by immersion,
First, a solid acid catalyst supporting silver chloride is prepared by removing excess chlorine ions by washing with water or the like.

Next, this is heated at 600 to 900 ° C. in an oxidizing atmosphere such as air or the like, preferably in the presence of steam.
If silver aluminate is produced by heating and baking at a temperature of about 700 ° C., preferably about 700 ° C. to 800 ° C., a solid acid catalyst carrying silver aluminate can be obtained.

(2) For example, an aqueous solution is prepared by uniformly mixing a water-soluble salt which is a precursor of a solid acid such as aluminum nitrate and the like with a water-soluble silver salt such as silver nitrate and the like. A precipitate is formed by a method such as neutralization in the presence, and then the precipitate is repeatedly subjected to filtration, washing with water, and repulping. Silver is supported on the solid acid.

Then, in the same manner as described above, in an oxidizing atmosphere, preferably in the presence of water vapor,
If silver aluminate is generated by heating and baking at a temperature of about 900 ° C., preferably about 700 to 800 ° C., a solid acid catalyst carrying silver aluminate can be obtained.

(3) Hydrated alumina was immersed in an aqueous solution of a water-soluble aluminum salt such as aluminum nitrate and a water-soluble silver salt such as silver nitrate to impregnate the pores of the alumina with the aluminum salt and silver salt. Thereafter, it is dried by a suitable means such as a spray dryer, and thereafter, as described above,
In an oxidizing atmosphere, preferably in the presence of steam, 600
If silver aluminate is produced by heating and baking at a temperature of about 900 ° C., preferably about 700 ° C. to 800 ° C., a solid acid catalyst carrying silver aluminate can be obtained.

(4) As another method, an alkali metal aluminate such as sodium aluminate and the like
By mixing and drying an aqueous solution of silver nitrate equivalent to ４4 times equivalent by spray drying and drying, and eutecticizing the obtained granules at a temperature of 300 to 800 ° C. in the absence of moisture, silver aluminate is obtained. The resultant is washed with water to remove excess silver nitrate and sodium nitrate, whereby a highly pure silver aluminate can be obtained. This silver aluminate and a solid acid such as alumina are uniformly mixed and pulverized by a wet method using a ball mill or the like, and then dried, whereby alumina carrying silver aluminate can be obtained.

Also in the preparation of the second catalyst, alumina is particularly preferably used as the solid acid carrier, and among these aluminas, as described above, the content of alkali metal and alkaline earth metal is 0.1%. 5% by weight or less, and the pore volume formed from pores having a diameter of 60 Å or less is 0.06 cm ³ / g or more, and the pore volume formed from pores having a diameter of 80 Å or less is 0.1 cm ³ / g. g or more of alumina is particularly preferably used. Porous alumina having such a pore volume promotes appropriate oxidation of the reducing agent and cooperates with silver aluminate carried thereon to effectively catalytically reduce nitrogen oxides. it can.

In the second catalyst, the loading amount of silver aluminate on the solid acid carrier is 0.01 to 10% by weight in terms of silver.
Is preferably within the range. When the supported amount of silver aluminate exceeds 10% by weight in terms of silver, the oxidizing power of the obtained catalyst is too high and the selectivity is poor. On the other hand, the supported amount is less than 0.01% by weight in terms of silver. When the amount is too small, the catalytic activity is not sufficient. In particular, in the present invention, the loading amount of silver aluminate is preferably in the range of 0.1 to 5% by weight.
When the supported amount is in this range, an excellent characteristic that the space velocity dependence of the catalytic reduction reaction of nitrogen oxide is extremely small can be obtained.

In the second stage, the space velocity when the exhaust gas containing the reducing agent together with the nitrogen oxide is brought into contact with such a second catalyst is usually in the range of 5,000 to 100,000 hr ^-1 . The second catalyst used in the second step has a lower oxidizing activity and is superior in selectivity to nitrogen oxides compared to the first catalyst used in the first step. The speed is preferably low, but
Usually, in practical use, the space velocity in the above range is adopted.

According to the method of the present invention, the first step and the second step
The reaction temperature in the step is in the range of 150 to 600C, preferably in the range of 250 to 550C. If necessary, the reaction temperature may be changed in the first stage and the second stage.

According to the present invention, as described above, in the first stage, the exhaust gas containing a reducing agent composed of a hydrocarbon and nitrogen oxides has a relatively high partial oxidation activity, but has a low complete oxidation ability. Contacting the reducing agent with an oxygen-containing organic compound or a low-molecular-weight hydrocarbon by efficient partial oxidation to form a reducing agent having excellent selective reactivity with nitrogen oxides;
In the second stage, the exhaust gas is brought into contact with a second catalyst having excellent selective reduction activity of nitrogen oxides in the presence of these reducing agents, thereby reducing the usage ratio (molar ratio) of the reducing agents to nitrogen oxides. However, even in the presence of oxygen, sulfur oxides and moisture, nitrogen oxides can be stably and efficiently reduced and decomposed.

[0034]

EXAMPLES The present invention will be described below with reference to examples along with examples of catalyst preparation for each step, but the present invention is not limited to these examples.

(1) Preparation of First Catalyst Preparation Example 1 60 g of alumina powder obtained by pulverizing pellets of γ-alumina (KHA-24 manufactured by Sumitomo Chemical Co., Ltd.) and alumina sol (Nissan Chemical Industry Co., Ltd.) 520) 6 g and an appropriate amount of water are mixed, and the obtained mixture is mixed with 100 g of zirconia balls.
Was wet-ground with a planetary mill for 5 minutes to prepare a wash coat slurry. The slurry was applied to a cordierite substrate having a cell number of 400 cells / square inch, and γ-alumina was supported at a rate of about 170 g / L. This is fired in air at 500 ° C. for 3 hours to obtain γ
-A carrier supporting alumina (cordierite) was prepared.

On the other hand, a phosphoric acid aqueous solution (85% by weight concentration) 3.
6 g is dissolved in ion-exchanged water to a liquid volume of 30 mL,
The cordierite carrier supporting the above alumina was immersed in this, and the γ-alumina carrier was sufficiently impregnated with phosphoric acid. Next, the alumina-supported carrier was separated from the aqueous phosphoric acid solution, and after removing the excess aqueous solution attached to the surface, the carrier was dried at 100 ° C. for 12 hours.
It was calcined at 0 ° C. to obtain a catalyst (A-1) in which phosphoric acid was supported on γ-alumina at a supporting amount of 2.0% by weight.

Preparation Example 2 2.10 g of nickel chloride (NiCl ₃ ) was dissolved in ion-exchanged water to prepare an aqueous solution having a volume of 30 mL. Hereinafter, in the same manner as in Preparation Example 1, a catalyst (A-2) obtained by loading nickel chloride on γ-alumina at a loading of 2.0% by weight as nickel was obtained.

[0038] Preparation Example 3 was dissolved nickel sulfate _{(NiSO 4 · 6H 2 O)} 4.33g of ion-exchanged water to prepare an aqueous solution to a liquid volume of a 30 mL. Hereinafter, in the same manner as in Preparation Example 1, a catalyst (A-3) in which nickel sulfate was supported on γ-alumina in the amount of 2.0% by weight as nickel was obtained.

Preparation Example 4 Nickel sulfate (NiSO ₄ .6H ₂ O) (1.083 g) was dissolved in ion-exchanged water to prepare an aqueous solution having a volume of 30 mL. Hereinafter, in the same manner as in Preparation Example 1, a catalyst (A-4) obtained by loading nickel sulfate on γ-alumina at a loading of 5.0% by weight as nickel was obtained.

[0040] Preparation Example 5 was dissolved nickel sulfate _{(NiSO 4 · 6H 2 O)} 0.43g of ion-exchanged water to prepare an aqueous solution to a liquid volume of a 30 mL. Hereinafter, in the same manner as in Preparation Example 1, a catalyst (A-5) obtained by loading nickel sulfate on γ-alumina at a loading of 0.2% by weight as nickel was obtained.

Preparation Example 6 4.24 g of manganese sulfate (MnSO ₄ .5H ₂ O) was dissolved in ion-exchanged water to prepare an aqueous solution having a volume of 30 mL. Hereinafter, in the same manner as in Preparation Example 1, a catalyst (A-6) in which manganese sulfate was supported as manganese on γ-alumina at a supporting amount of 2.0% by weight was obtained.

Preparation Example 7 1.52 g of silver nitrate (AgNO ₃ ) was dissolved in ion-exchanged water to prepare an aqueous solution having a volume of 30 mL. Hereinafter, in the same manner as in Preparation Example 1, silver nitrate is used as silver in γ-alumina.
It was loaded at a loading of 2.0% by weight. Thereafter, the catalyst was immersed in 1% by weight hydrochloric acid to convert silver nitrate into silver chloride.
There was obtained a catalyst (A-7) in which silver chloride was supported as silver in a supporting amount of 2.0% by weight.

Preparation Example 8 3.06 g of silver phosphate (Ag ₃ PO ₄ ) was dissolved in a 10% by weight aqueous phosphoric acid solution to make the volume 30 mL. The cordierite carrier supporting the same alumina as in Preparation Example 1 was immersed in the aqueous solution of phosphoric acid containing silver phosphate to sufficiently impregnate the γ-alumina carrier with silver phosphate. Then
The alumina-supported carrier was separated from the aqueous silver phosphate solution, and excess aqueous solution attached to the surface was removed.
Dried for 2 hours, and further fired in air at 500 ° C.
A catalyst (A-8) was obtained in which silver phosphate was supported on γ-alumina in a loading amount of 2.0% by weight as silver.

[0044] (2) a second catalyst Preparation Example 9 of aluminum nitrate _{(Al (NO 3) 3 ·} 9H 2 O) 8.6
9 g, 1.58 g of silver nitrate and 100 g of hydrated alumina (manufactured by Mizusawa Chemical Industry Co., Ltd.) were mixed with an appropriate amount of water to prepare a paste. This was kneaded and dried using a heating kneader, and then heated and calcined at 500 ° C. for 3 hours to obtain a supported amount of 2.5.
An alumina powder catalyst supporting silver by weight was obtained.

60 g of this alumina powder catalyst and 6 g of silica sol (Snowtex N manufactured by Nissan Chemical Industries, Ltd.) are mixed with an appropriate amount of water, and wet-ground with a planetary mill for 5 minutes using 100 g of zirconia balls as a grinding medium. Thus, a wash coat slurry was prepared. The slurry was applied to a cordierite substrate having a cell number of 200 cells / square inch, and the catalyst was supported at a rate of about 150 g / L. This catalyst is called B-1.

Preparation Example 10 Aluminum nitrate (Al (NO ₃ ) ₃ .9H ₂ O) 8.6
9 g, 3.94 g of silver nitrate and 100 g of hydrated alumina (manufactured by Mizusawa Chemical Industry Co., Ltd.) were mixed with an appropriate amount of water to prepare a paste. This was kneaded and dried using a heating kneader, and then dried under an air atmosphere containing 10% by weight of water.
C. for 3 hours to obtain an alumina powder catalyst having silver aluminate supported thereon at a loading of 2.5% by weight in terms of silver weight.

60 g of this alumina powder catalyst and 6 g of silica sol (Snowtex N manufactured by Nissan Chemical Industries, Ltd.) are mixed with an appropriate amount of water, and wet-ground with a planetary mill for 5 minutes using 100 g of zirconia balls as a grinding medium. Thus, a wash coat slurry was prepared. The slurry was applied to a cordierite substrate having a cell number of 200 cells / square inch, and the catalyst was supported at a rate of about 150 g / L. This catalyst is called B-2.

Examples 1 to 10 (Evaluation Test) The first catalyst (A-1 to 8) prepared as described above was used in the first stage, and the second catalyst (B-1 to 2) was used. In the second step, the nitrogen oxide-containing gas was subjected to nitrogen oxide catalytic reduction under the following test conditions, and the removal rate of nitrogen oxide was determined by a chemical luminescence method. The results are shown in Tables 1 and 2.

(Test Conditions) (1) Gas Composition NO 300 ppm CO 1000 ppm H ₂ 1000 ppm Hydrocarbon 500 ppm O ₂ 10 vol% SO ₂ 200 ppm Water 6 vol% Nitrogen balance (2) Space velocity First stage 50000 (hr ⁻¹ ) Second stage 50000 (hr ^-1 ) (3) Reaction temperature 300 ° C, 350 ° C, 400 ° C, 450 ° C or 500 ° C

Comparative Examples 1 and 2 (Evaluation Test) In the same manner as in Example except that only one of the first catalyst and the second catalyst was used as the catalyst, Catalytic reduction was performed, and the removal rate of nitrogen oxides was determined by a chemical luminescence method. Table 1 shows the results. When only the first catalyst is used, the nitrogen oxide removal rate is very low. When only the second catalyst is used,
Compared with the method of the present invention, the nitrogen oxide removal rate is low over the entire reaction temperature range.

[0051]

[Table 1]

[0052]

[Table 2]

As is clear from the results shown in Tables 1 and 2, according to the method of the present invention, compared with the conventional method, the reduction of nitrogen oxides even in the coexistence of oxygen, sulfur oxides and moisture. By reducing the use ratio (molar ratio) of the agent, nitrogen oxides in exhaust gas can be reduced and removed stably at a high removal rate.

[0054]

As described above, according to the method of the present invention,
Even in the coexistence of oxygen, sulfur oxides and moisture, the nitrogen oxides in the exhaust gas can be stably and efficiently reduced catalytically without using a large amount of reducing agent.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B01D 53/36 102H (72) Inventor Keiichi Tabata 5-1-1 Ebisushimacho, Sakai-shi, Osaka Sakai Chemical Industry Co., Ltd. (72 ) Inventor Noriyuki Ueda 5-1-1 Ebisshima-cho, Sakai City, Osaka Sakai Chemical Industry Co., Ltd.

Claims (4)

[Claims] In a method for catalytically reducing nitrogen oxides contained in an exhaust gas using a reducing agent in the presence of a catalyst, as a first step, an exhaust gas containing a reducing agent composed of a hydrocarbon and nitrogen oxides is used as a carrier. Phosphoric acid, Periodic Table Ib, VIIa and VIII
Contact with a first catalyst supporting at least one selected from the group consisting of phosphates, chlorides and sulfates of a group element, and as a second step, a second catalyst selected from silver, silver oxide and silver aluminate A catalytic reduction method for contacting nitrogen oxides, which comprises contacting the catalyst with a catalyst. 2. The method according to claim 1, wherein the first catalyst comprises phosphoric acid,
b, the amount of at least one selected from the group consisting of phosphates, chlorides and sulfates of Group VIIa and VIII elements is in the range of 0.05 to 5% by weight; 2. The method according to claim 1, wherein the amount is in the range of 0.1 to 5% by weight and the supported amount of silver aluminate is in the range of 0.01 to 10% by weight. 3. The method according to claim 1, wherein the reducing agent is based on light oil. 4. The method according to claim 1, wherein in the first and second stages,
4. The method according to claim 1, wherein the exhaust gas is contacted with the catalyst at a temperature in the range of 600.degree.