JPH09155163A - Method for catalytic reduction of nitrogen oxides - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably, efficiently and catalytically reduce nitrogen oxides even in the coexistence of oxygen, sulfur oxides or moisture without using a large quantity of a reducing agent and generating a nitrogen-containing compd. such as ammonia, amine or hydrogen cyanide as a byproduct in the catalytic reduction of nitrogen oxides using hydrocarbon as a reducing agent. SOLUTION: In a method for catalytically reducing nitrogen oxides contained in exhaust gas by using hydrocarbon as a reducing agent in the presence of a catalyst, in a first stage, exhaust gas containing hydrocarbon along with nitrogen oxides is brought into contact with a catalyst selected from silver, silver oxide and silver aluminate and, in a second stage, the exhaust gas is brought into contact with a catalyst selected from alumina, silica/alumina, zirconia, and titania.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for catalytic reduction of nitrogen oxides using hydrocarbons as a reducing agent, and more specifically, it removes harmful nitrogen oxides contained in exhaust gas discharged from factories, automobiles and the like. The present invention relates to a method for catalytic reduction of nitrogen oxides which can be stably and efficiently reduced and removed without using a large amount of a reducing agent and without by-products of nitrogen-containing compounds such as ammonia, amine and hydrogen cyanide.

[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. Further, even when hydrogen, carbon monoxide, hydrocarbons, and the like are used as a reducing agent, since they react with oxygen present at a higher concentration than nitrogen oxide present at a lower concentration, it is necessary to reduce nitrogen oxides. However, there is a problem that a large amount of a reducing agent is required.

[0003] For this reason, recently, a method of directly decomposing nitrogen oxides with a catalyst in the absence of a reducing agent has been proposed.
There is a problem that it is difficult to put into practical use due to low decomposition activity of nitrogen oxides.

[0004] 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, it is difficult to say that even such H-type ZSM-5 has sufficient reducing activity, and in particular, when water is contained in the gas, aluminum in the zeolite structure is dealuminated, resulting in poor performance. Falls sharply. Therefore, conventionally, there has been a demand for a catalyst for catalytic reduction of nitrogen oxides, which has higher reduction activity and further has excellent durability even when the gas contains water.

[0005]

Therefore, a catalyst in which silver or a silver oxide is supported on an inorganic oxide has been proposed. Such a catalyst has a high oxidizing activity and a selective reaction with respect to a nitrogen oxide. The removal rate of nitrogen oxides is low due to its poor property. Further, since the temperature range in which the catalyst has a decomposition activity of nitrogen oxides is high, it is necessary to preheat the exhaust gas in order to effectively decompose the nitrogen oxides in the exhaust gas, which poses a problem in practical use. is there. Further, the catalyst in which silver or silver oxide is supported on an inorganic oxide has a problem that the catalytic activity is significantly deteriorated in the presence of sulfur oxide (Japanese Patent Laid-Open No. 5-31).
7647 publication). 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.

Further, according to the catalytic reduction of nitrogen oxides using a catalyst in which silver or silver oxide is supported on an inorganic oxide, when an oxygen-containing organic compound is used as a reducing agent, ammonia as a by-product, Nitrogen-containing compounds such as amine and hydrogen cyanide are produced. In addition to being detrimental, they can form additional nitrogen oxides, which in turn reduces the rate of nitrogen oxide removal. Therefore, conventionally, a method of catalytically reducing nitrogen oxides by combining a tungsten-based catalyst and a vanadium-based catalyst after the silver-based catalyst has been proposed (Japanese Patent Laid-Open No. 7-60119). However, the nitrogen oxide removing performance and the heat resistance of the catalyst are insufficient, and various problems still remain for practical use.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is a catalytic reduction method of nitrogen oxides using hydrocarbon as a reducing agent, such as oxygen and sulfur. Even in the presence of oxides and water, nitrogen oxides in exhaust gas can be stably and efficiently used without using a large amount of reducing agent and without producing nitrogen-containing compounds such as ammonia, amine and hydrogen cyanide as by-products. Another object of the present invention is to provide a method for catalytic reduction of nitrogen oxides, which enables good catalytic reduction.

[0008]

The present invention provides a method for catalytically reducing nitrogen oxides contained in exhaust gas by using hydrocarbon as a reducing agent in the presence of a catalyst.
Exhaust gas containing hydrocarbons along with nitrogen oxides is contacted with a catalyst selected from silver, silver oxide and silver aluminate (hereinafter sometimes referred to as the first catalyst), and as a second step, alumina, silica-alumina, zirconia is used. And a catalyst selected from titania (hereinafter sometimes referred to as a second catalyst).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION According to the method of the present invention, as a first step, exhaust gas containing hydrocarbons together with nitrogen oxides
By contacting with the catalyst, the hydrocarbon selectively reacts with the nitrogen oxide as a reducing agent to be converted into nitrogen or a nitrogen-containing compound, and these nitrogen-containing compounds are converted into nitrogen in the second stage.

The first catalyst is selected from silver, silver oxide or silver aluminate. Among these, silver and silver oxide are usually used by being supported on a solid acid carrier such as a metal oxide having a large specific surface area, for example, alumina, silica, silica-alumina, zirconia, titania or zeolite. Among these carriers, γ-alumina, which has an excellent supporting effect, is particularly preferably used.

Among γ-alumina, JP-A-7-17
As described in Japanese Patent No. 1347, the content of alkali metals and alkaline earth metals is 0.5% by weight or less, and the pore volume formed from pores having a diameter of 60 Å or less is 0.06 cm ^3. Γ-alumina having a pore volume of 0.1 cm ³ / g or more, which is formed from pores having a diameter of not less than / g and a diameter of not more than 80 Å, is particularly preferably used. Porous γ-alumina having such a pore volume promotes appropriate oxidation of the reducing agent, and cooperates with the silver or silver oxide supported on it to effectively catalytically reduce nitrogen oxides. can do.

As described above, the first catalyst made of silver or silver oxide can be formed into various shapes such as a honeycomb shape or a spherical shape by a conventionally known molding method by itself or after being supported on a carrier. Can be molded into. During this molding,
A molding aid, a molded body reinforcing material, an inorganic fiber, an organic binder and the like may be appropriately mixed.

The first catalyst can also be coated and supported on a preformed inert substrate by an appropriate method such as a wash coating method. Examples of the base material include a honeycomb structure made of clay such as cordierite. Further, if necessary, any other known method for preparing a catalyst may be used.

The amount of silver or silver oxide supported on the carrier is from 0.1 to
It is preferably in the range of 5% by weight. When the supported amount is less than 0.1% by weight, the reducing activity of the nitrogen oxide is not sufficient, and when it is more than 5% by weight, the oxidizing activity is too high, resulting in poor selectivity. .

Of the first catalysts, the catalyst composed of silver aluminate 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 silver hydroxide is not generated, thereby obtaining a solid acid ion exchange site. The silver ions are immobilized. 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 immersion to generate silver chloride, excess chloride ions are removed by washing with water or the like to prepare a solid acid catalyst supporting silver chloride. Then, this is placed in an oxidizing atmosphere such as air, preferably in the presence of water vapor, at about 600 to 800 ° C., preferably 700 to 800 ° C.
If silver aluminate is produced by heating and baking at a temperature of about 800 ° C., a solid acid catalyst supporting silver aluminate can be obtained.

(2) For example, an aqueous solution is prepared by homogeneously mixing a water-soluble salt which is a precursor of a solid acid such as aluminum nitrate and a water-soluble silver salt such as silver nitrate. A precipitate is formed by a method such as neutralization in the presence, and then the precipitate is repeatedly filtered, washed with water, and repulped, dried, and calcined to produce a solid acid, and at the same time, chlorided. Silver is supported on the solid acid. Then, in the same manner as described above, in an oxidizing atmosphere, preferably in the presence of steam, 600 to 800
When silver aluminate is produced by heating and baking at a temperature of about [deg.] C., preferably about 700 to 800 [deg.] C., a solid acid catalyst supporting 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 above aluminum salt and silver salt. After that, 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
By producing a silver aluminate by heating and baking it at a temperature of about -800 ° C, preferably about 700-800 ° C, a solid acid catalyst supporting silver aluminate can be obtained.

(4) Furthermore, as another method, an alkali metal aluminate such as sodium aluminate and its 1
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 resulting product is washed with water to remove excess silver nitrate and sodium nitrate, whereby a highly purified product 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.

Alumina is also preferably used as a solid acid carrier for supporting silver aluminate. Further, among the alumina, as described above, the content of the alkali metal and the alkaline earth metal is 0.5% by weight or less, and the pore volume formed by the pores having a diameter of 60 Å or less is 0.06 cm. Those having a pore volume of 0.1 cm ³ / g or more formed from pores having a diameter of ³ / g or more and a diameter of 80 angstroms or less are particularly preferably used. Porous alumina having such a pore volume promotes appropriate oxidation of the reducing agent, and cooperates with the silver aluminate carried on the reducing agent,
Nitrogen oxide can be effectively catalytically reduced.

The amount of silver aluminate supported on the solid acid carrier is
It is preferably in the range of 0.01 to 10% by weight. When the supported amount of silver aluminate exceeds 10% by weight, the oxidizing power of the obtained catalyst is too high and the selectivity is poor, and the supported amount is 0.
If it is less than 01% by weight, the catalytic activity is insufficient. 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 within this range, it is possible to obtain the excellent property that the space velocity dependence of the catalytic reduction reaction of nitrogen oxide is extremely small.

In the first stage, the space velocity when the exhaust gas containing the nitrogen oxide and the reducing agent is brought into contact with such a first catalyst is usually in the range of 5,000 to 50,000 hr ^-1 . In order to obtain a high denitration rate, it is preferable that the space velocity is small, but in practice, the space velocity in the above range is adopted. The reaction temperature in the first stage is usually
It is in the range of 250 to 550 ° C.

In the method of the present invention, examples of the reducing agent composed of hydrocarbon include gaseous ones such as methane, ethane, propane, propylene, butylene and the like, and liquid ones such as pentane and hexane. ,
Single-component hydrocarbons such as octane, heptane, benzene, toluene, xylene, and mineral oil hydrocarbons such as gasoline, kerosene, light oil, and heavy oil can be used. In particular, in the present invention, among the above, lower alkenes such as ethylene, propylene, isobutylene, 1-butene and 2-butene, lower alkanes such as propane and butane, and light oil are preferably used as the reducing agent. These hydrocarbons may be used alone or in combination of two or more as needed.

The hydrocarbon as the reducing agent varies depending on the specific hydrocarbon used, but is usually used in a molar ratio of about 0.1 to 2 with respect to nitrogen oxides in the exhaust gas. When the amount of hydrocarbons used is less than 0.1 in terms of molar ratio to nitrogen oxides, sufficient reducing activity cannot be obtained for nitrogen oxides, while the molar ratio is 2
When it exceeds, the amount of unreacted hydrocarbons is increased, so that after the catalytic reduction treatment of nitrogen oxides, a post-treatment for recovering this is required.

It should be noted that unburned substances or incompletely burned products such as fuel existing in the exhaust gas, that is, hydrocarbons and particulates are also effective as the reducing agent, and these are also included in the hydrocarbon in the present invention. Be done. From this point of view, it can be said that the method of the present invention is also useful as a method for reducing or removing hydrocarbons and particulates in exhaust gas.

According to the method of the present invention, as described above, the exhaust gas containing the hydrocarbon together with the nitrogen oxide is brought into contact with the first catalyst to selectively react the hydrocarbon with the nitrogen oxide. Is converted to nitrogen or a nitrogen-containing compound, and these nitrogen-containing compounds are added in the second step in the presence of the second catalyst,
Convert to nitrogen.

The second catalyst is selected from alumina, silica-alumina, zirconia and titania. As described above, these are called solid acids, and as described above, it is well known that they are used as carriers for various catalysts, but in the present invention, they are used as the second catalyst. Used. This second catalyst is
By an appropriate method that is already well known, for example, a method such as neutralization hydrolysis method, heat hydrolysis method, sol-gel method, etc. to obtain a hydroxide, and then thermally decomposing it to an oxide. It can be prepared. It is preferable that these catalysts have a large specific surface area.

In the present invention, as the second catalyst, alumina or zirconia is particularly preferably used among those mentioned above. Among the alumina, as described above, the alumina which is preferable as the carrier for the first catalyst is preferably used. The second catalyst can also be supported on an inactive base material such as cordierite by a wash coat method, a sol-gel method or the like.

The space velocity in the second stage is usually 50.
The range is from 00 to 100,000 hr ^-1 . Since the decomposition rate of the nitrogen-containing compound in the second stage is high, the space velocity can be increased. The reaction temperature in the second stage is also usually in the range of 250 to 550 ° C.

According to the present invention, as described above, in the first step, the exhaust gas containing the hydrocarbon together with the nitrogen oxide is contacted with the first catalyst to selectively react the hydrocarbon with the nitrogen oxide. , Nitrogen or a nitrogen-containing compound, and this nitrogen-containing compound is converted into nitrogen by the second catalyst in the second step, so that a large amount of reducing agent is not used, and ammonia, amine, hydrogen cyanide, etc. The nitrogen oxides in the exhaust gas can be stably and efficiently reduced and decomposed without producing a harmful harmful nitrogen-containing compound as a by-product.

[0030]

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 3.15 g of silver nitrate (AgNO ₃ ) was added to 100 m of ion-exchanged water.
L. 100 mL of γ-alumina (KHA-24 manufactured by Sumitomo Chemical Co., Ltd.) having an average particle diameter of 3 mm, which was dried in advance at 120 ° C. for 24 hours, was put into the above silver nitrate aqueous solution, and stirred for 30 minutes, and inside the pores of alumina Was thoroughly impregnated with an aqueous silver nitrate solution. Then, γ-alumina was separated from the silver nitrate aqueous solution, and after removing the excess aqueous solution adhering to the surface, it was dried at 100 ° C. for 12 hours, and further in air,
It was calcined at 500 ° C. to obtain a catalyst (A-1) in which silver was supported on γ-alumina in a supported amount of 1% by weight.

[0032] Preparation Example 2 of 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.) were mixed with an appropriate amount of water, and this was wet-ground for 5 minutes with a planetary mill using 100 g of zirconia balls as a grinding medium. To prepare a wash coat slurry. 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 referred to as A-2. (2) Preparation of second catalyst

Preparation Example 3 60 g of silica-alumina (manufactured by FUJI-DAVISON CHEMICAL, specific surface area 320 m ² / g) and 6 g of alumina sol (Snowtex N manufactured by Nissan Chemical Industries, Ltd.) were mixed with an appropriate amount of water, This was wet pulverized for 5 minutes with a planetary mill using 100 g of alumina balls as a pulverizing medium to prepare a wash coat slurry. This slurry was applied to a cordierite substrate having 100 cells / square inch, dried and then calcined at 600 ° C. for 3 hours to support a silica-alumina catalyst at a rate of about 200 g / L. This catalyst is called B-1.

Preparation Example 4 60 g of zirconia (specific surface area 80 m ² / g) obtained by calcining zirconium hydroxide at 500 ° C. for 3 hours and 6 g of alumina sol (Snowtex N manufactured by Nissan Chemical Industries Ltd.) were added in appropriate amounts. This was mixed with water and wet-milled for 5 minutes with a planetary mill using 100 g of alumina balls as a grinding medium to prepare a slurry for wash coat. This slurry was applied to a cordierite substrate having a cell number of 400 cells / square inch, dried, and then calcined at 600 ° C. for 3 hours to carry a zirconia catalyst at a rate of about 200 g / L. This catalyst is called B-2.

Preparation Example 5 The same γ-alumina as used in Example 1 (specific surface area 1
50 m ² / g) 1 kg, polyethylene oxide 1 kg and an appropriate amount of water were sufficiently kneaded, and then extruded into a honeycomb formed article having 400 cells / square inch by an auger screw type extruder. The honeycomb formed article was dried by ventilation at room temperature, then dried by heating at 100 ° C. overnight, and further, 5
It calcined at 00 degreeC for 3 hours, and the alumina catalyst (B-3) was obtained.

Preparation Example 6 Titania (specific surface area 120 m ² / g) 60 obtained by calcining metatitanic acid (TiO ₂ .2H ₂ O) at 500 ° C. for 3 hours
g and titania sol 6 g are mixed with an appropriate amount of water,
About 200 g / L of titania catalyst was prepared in the same manner as in Preparation Example 3.
In the ratio of This catalyst is called B-4.

Examples 1 to 6 (Evaluation Test) The first catalyst (A-1 to 1-2) prepared as described above was used in the first stage, and the second catalyst (B-1 to 4) was used as a second catalyst.
In the stage, the nitrogen oxide-containing gas was subjected to nitrogen oxide catalytic reduction under the following test conditions, and the nitrogen oxide removal rate was determined by gas chromatography with reference to the production of nitrogen. That is, the production rate of nitrogen was determined from (concentration of nitrogen gas in exhaust gas after treatment (ppm) / concentration of nitrogen oxides in exhaust gas before treatment (ppm)) × 100 (%).

For the production rates of ammonia and hydrogen cyanide, the concentrations in the exhaust gas after treatment were detected using the Kitagawa gas detector tube, and the (concentration of ammonia or hydrogen cyanide in the exhaust gas after treatment (( ppm) / concentration of nitrogen oxides in exhaust gas before treatment (ppm)) × 100
Calculated from (%). The results are shown in Tables 1 and 2.

(Test conditions) (However, when light oil was used as the reducing agent, light oil was C12 in terms of C.) (2) Space velocity 1st stage 20000 (hr ^-1 ) 2nd stage 20000 (hr ^-1 ) (3) Reaction temperature 250 ° C, 300 ° C, 350 ° C,
400 ° C, 450 ° C, 500 ° C or 550 ° C

Comparative Examples 1 and 2 (Evaluation Test) Nitrogen oxide catalytic reduction of nitrogen oxide-containing gas was carried out in the same manner as in Examples except that only one of the first catalyst and the second catalyst was used as the catalyst. The removal rate of nitrogen oxides was determined by gas chromatography based on the production of nitrogen. Further, the production rates of ammonia and hydrogen cyanide were determined by the same method as described above. The results are shown in Tables 1 and 2.

Whether light oil or propylene is used as the reducing agent, the catalytic reduction of nitrogen oxides is performed at a higher removal rate than the case where nitrogen oxides are reduced only by the second catalyst. Can be removed.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above, according to the present invention, in the method of catalytic reduction of nitrogen oxides using hydrocarbon as a reducing agent, a large amount of reducing agent can be obtained even in the presence of oxygen, sulfur oxides and water. It is possible to stably and efficiently catalytically reduce nitrogen oxides without using them and without producing nitrogen-containing compounds such as ammonia, amines and hydrogen cyanide as by-products.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 21/16 B01J 23/50 A 23/50 B01D 53/36 ZAB 102C 102H (72) Inventor Tabata Keiichi 5-1, Ebishima-cho, Sakai City, Osaka Prefecture Sakai Chemical Industry Co., Ltd. Central Research Laboratory

Claims (3)

[Claims] 1. A method of catalytically reducing nitrogen oxides contained in exhaust gas by using hydrocarbon as a reducing agent in the presence of a catalyst, wherein as a first step, exhaust gas containing hydrocarbons together with nitrogen oxides is oxidized by silver and oxidation. Contacting with a catalyst selected from silver and silver aluminate, the second step is alumina, silica.
A catalytic reduction method for contacting nitrogen oxides, which comprises contacting with a catalyst selected from alumina, zirconia and titania. 2. The method according to claim 1, wherein the hydrocarbon is light oil. 3. In the first stage and the second stage, 250-
The method according to claim 1 or 2, wherein the exhaust gas is contacted with the catalyst at a temperature in the range of 550 ° C.