JPH08206499A - Catalyst for removing nitrogen oxide - Google Patents

Abstract

PURPOSE: To efficiently remove nitrogen oxide contained in exhaust gas, which is in an oxidizing atmosphere of a high oxygen concentration and which contains sulfur oxide, continuously even at a high temperature, in the presence of hydrocarbons or organic compounds containing oxygen. CONSTITUTION: In the catalyst for removing nitrogen oxide for use in the presence of hydrocarbons or organic compounds containing oxygen, alumina prepared by hydrolyzing organic aluminum compound is used as the porous heat resistant carrier, and at least one kind of active component selected from among silver or silver compounds is carried on the alumina.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying nitrogen oxides, more specifically, a lean burn gasoline engine automobile,
The present invention relates to a catalyst for purifying nitrogen oxides for efficiently removing and detoxifying nitrogen oxides contained in exhaust gas from diesel engine automobiles or exhaust gas with high oxygen concentration such as boiler exhaust gas.

[0002]

2. Description of the Related Art In recent years, purifying nitrogen oxides in exhaust gas having a high oxygen concentration such as exhaust gas from a lean burn gasoline engine vehicle, a diesel engine vehicle, a cogeneration generator or a boiler for the purpose of preventing air pollution. Is an urgent task.

Generally, V ₂ O ₅ / TiO ₂ is used as a method for removing nitrogen oxides in exhaust gas having a high oxygen concentration.
Ammonia reduction method using a _two- system catalyst is well known, and nitrogen oxide is selectively reduced by ammonia to convert into harmless nitrogen. However, since this method handles highly dangerous ammonia, it is difficult to adapt to exhaust gas from mobile sources such as automobiles. Further, this method is not suitable as a method for removing nitrogen oxides generated from a limited space such as an automobile because the equipment becomes large.

Therefore, it has been expected to develop a nitrogen oxide removing method which uses a safe reducing agent instead of the ammonia reducing method and saves space. For example, JP-A-4-28184
No. 4 discloses that propylene is added to exhaust gas containing nitrogen oxides to form a reducing agent, and mixed gas of these and Ag / Al ₂
A method of converting nitrogen oxides to nitrogen by contacting with an O ₃ -based catalyst is disclosed, and JP-A-4-366740 discloses a ZSM- having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to 60. A method has been proposed in which a catalyst containing Fe in 5 is brought into contact with a gas containing nitrogen oxide, oxygen and ethylene as a hydrocarbon to remove nitrogen oxide in the gas.

However, the above-mentioned Japanese Patent Laid-Open No. 4-28184.
In conventional nitrogen oxide removal technology by adding hydrocarbons or oxygen-containing organic compounds as a reducing agent as disclosed in Japanese Patent Laid-Open No. 4 and Japanese Patent Laid-Open No. 4-377740, comparisons at temperatures of 400 ° C. or lower are made. Although it is effective in removing nitrogen oxides in a relatively low temperature range, there is a problem that nitrogen oxide removing performance is poor in a high temperature range of 500 ° C. or higher.

In the technique of removing nitrogen oxides by adding a reducing agent such as hydrocarbons or oxygen-containing organic compounds, it is important that the added reducing agent is not completely oxidized as a desired characteristic of the catalyst. Is. A catalyst having a high complete oxidation activity, for example, a catalyst supporting silver, silver oxide, a platinum group element, or the like, functions as a reducing agent for nitrogen oxides by promoting the complete oxidation reaction of the added reducing agent. I will lose it. This is the reason why the nitrogen oxide removing activity in the high temperature region in the prior art is low.

Further, in JP-A-6-7641 and JP-A-6-71175, nitrogen oxidation is carried out to reduce a nitrogen oxide in which silver or a silver compound is supported on a porous inorganic oxide such as γ-alumina. Although a substance removal catalyst is disclosed, removal of nitrogen oxides at high temperature cannot be performed efficiently even in this case.

[0008]

The object of the present invention is to prevent nitrogen oxides contained in exhaust gas in an oxidizing atmosphere having a high oxygen concentration from coexisting with hydrocarbons or oxygen-containing organic compounds even at high temperatures. It is to provide a catalyst for purifying nitrogen oxides that can be efficiently removed, and further to provide a catalyst for purifying nitrogen oxides in which the removal efficiency is less reduced even by the coexistence of sulfur oxides that coexist in the exhaust gas. is there.

[0009]

The above object of the present invention is to support silver in a high oxygen concentration exhaust gas containing sulfur oxides by supporting silver as a compound stable even at high temperature on a specific porous heat-resistant carrier. By contacting nitrogen oxides in the presence of hydrocarbons or oxygen-containing organic compounds,
The present invention has been achieved by finding that nitrogen oxides can be removed efficiently even in a high temperature region.

That is, the catalyst for removing nitrogen oxides of the present invention is a catalyst for purifying nitrogen oxides which is used in the coexistence of hydrocarbons or oxygen-containing organic compounds and which hydrolyzes an organoaluminum compound as a porous heat-resistant carrier. Alumina prepared by decomposing is used, which is loaded with at least one active ingredient selected from silver or a silver compound.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, at least one selected from silver and silver compounds is used as an active component of a catalyst. As the silver compound, silver halides such as silver chloride and silver bromide, silver sulfate, silver phosphate, silver oxide and the like are used. By using such silver or silver compound, high temperature,
That is, nitrogen oxides can be efficiently removed even at a temperature of 500 ° C. or higher.

The amount of silver or silver compound (active ingredient) supported on the porous heat-resistant carrier is preferably 0.01 to 20.0% by weight, more preferably 0.1% by weight in terms of silver based on the total amount of the catalyst. It is 05 to 15.0% by weight. When the amount of silver or a silver compound converted to silver is less than 0.01% by weight based on the total amount of the catalyst, the removal efficiency of nitrogen oxides is low, and 2
If it exceeds 0.0% by weight, the nitrogen oxide removal efficiency at high temperatures becomes poor.

The porous heat-resistant carrier used in the present invention is
An organoaluminum compound, an alumina prepared by hydrolyzing an aluminum alcoholate. Examples of the aluminum alcoholate include aluminum isopropoxide (Al [OCH (C
H _{3) 3] 3),} aluminum ethoxide (Al (OC ₂ H
₅ ) ₃ ), aluminum-t-butoxide (Al [OC
(CH ₃ ) ₃ ] ₃ ) and the like. These aluminum alcoholates are dissolved in water, and a large amount of water is added to hydrolyze them, and the resulting hydrated aluminum oxide is dried, and then at 300 ° C or higher Calcination gives alumina. By using such alumina as a porous heat-resistant carrier,
The removal efficiency of nitrogen oxides at high temperature becomes excellent, and further, the removal efficiency decreases less even in the presence of high sulfur oxides of several ppm to 500 ppm in the exhaust gas. When ordinary alumina made of aluminum sulfate or sodium aluminate, for example, γ-alumina is used as the porous heat-resistant carrier, high nitrogen oxide removal efficiency at high temperature cannot be obtained. By including a certain amount or less of silica in the porous heat-resistant carrier made of alumina prepared by hydrolyzing the organoaluminum compound used in the present invention, the nitrogen oxide removal efficiency at high temperature is further improved. The content is 15% by weight or less. When the content of silica exceeds 15% by weight, the nitrogen oxide removal efficiency at high temperature of the above-mentioned specific alumina is impaired. This porous heat-resistant carrier usually carries silver or a silver compound in the state of an oxide, but is not particularly limited to an oxide, and the above-mentioned silver or silver compound is not formed on a hydrated oxide that has not undergone thermal decomposition. Can also be supported and finally heat treated to form a catalyst.

The method for supporting the above-mentioned silver or silver compound on the porous heat-resistant carrier is not particularly limited and may be any method. For example, ammonium chloride is prepared after suspending the above-mentioned carrier powder in an aqueous solution of silver nitrate. , An aqueous solution of a water-soluble chlorine compound such as sodium chloride may be added, and after being supported as silver chloride on a carrier, heat treatment may be performed to obtain a catalyst.

Such a catalyst for purifying nitrogen oxides of the present invention is usually used as a honeycomb structure catalyst obtained by forming an aqueous slurry, fixing it to the catalyst by washcoating it on a honeycomb, and then firing it. To be

Removal of nitrogen oxides using the catalyst for purifying nitrogen oxides of the present invention is carried out in the presence of hydrocarbons or oxygen-containing organic compounds. The reason for this is that nitric oxide in the exhaust gas having a high oxygen concentration is oxidized by oxygen to generate nitrogen dioxide. This is because hydrocarbons or oxygen-containing organic compounds are used as a reducing agent for reducing this nitrogen dioxide.

Examples of the hydrocarbons include olefinic hydrocarbons such as ethylene and propylene, paraffinic hydrocarbons such as propane, aromatic hydrocarbons such as toluene and xylene. Examples of the oxygen-containing organic compounds include alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, ethylene glycol and propylene glycol, and ketones such as acetone, ethyl methyl ketone and methyl propyl ketone.

[0018]

The catalyst of the present invention has a stable heat-resistant silver or silver compound that does not decompose even at high temperatures, supported on a porous heat-resistant carrier made of a specific alumina. In this catalyst, the specific alumina that is the porous heat-resistant carrier has extremely high purity, and the structure is stable even when used in an atmosphere in which water vapor coexists. For this reason, the catalyst of the present invention is particularly effective in removing nitrogen oxides even at high temperatures and is stable for a long period of time in an atmosphere containing about 10% by volume of water vapor and about several ppm to about 500 ppm of sulfur oxides. Retains removal characteristics.

[0019]

EXAMPLES The present invention will now be described in more detail with reference to Examples and the like. Example 1 To 250 g of pure water, 4.14 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C., 100 g of powder of hydrated alumina oxide obtained by hydrolysis of aluminum isopropoxide was added and stirred and suspended. An aqueous solution prepared by dissolving 1.43 g of separately prepared ammonium chloride in 50 g of pure water was added over 15 minutes to support silver chloride on the particles of hydrated aluminum oxide. After aging for 60 minutes, it was filtered, washed, dried at 120 ° C., calcined at 450 ° C. for 60 minutes and pulverized to obtain a silver chloride / alumina catalyst containing 3.0% by weight in terms of silver. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 45 g of this catalyst and 100 g of pure water,
Acetic acid 2.25 g and alumina sol (alumina content 1
0.5 wt%, the same applies hereinafter) 21.4 g was added, stirred and mixed, and the median diameter of the catalyst was 8.5 μm using a wet pulverizer.
To prepare an aqueous slurry having a solid content concentration of 28.0% by weight. When the viscosity of this slurry was measured, it was 15 cps.
Met.

This slurry has a diameter of 20 mm and a height of 16 m.
A cordierite honeycomb structure having m and the number of cells of 400 cells / inch ² was dipped and pulled up, and then excess slurry in the cells was removed by pressurized air. 190-2
Dry at 00 ° C and repeat this operation twice, finally
The honeycomb structure catalyst for purifying nitrogen oxides was obtained by firing at 00 ° C. for 90 minutes.

Using this honeycomb structure catalyst, a nitrogen oxide purification test was conducted according to the following procedure. In this test method, the prepared honeycomb structure catalyst was filled in a stainless steel reaction tube, and the temperature was raised to a predetermined temperature while circulating a gas having the following composition. The nitrogen oxide removal rate was measured by maintaining the temperature at a predetermined reaction temperature for 60 minutes to perform a comparative test of temperature dependency on the nitrogen oxide removal activity of the catalyst. Table 2 shows the results. Regarding the life of this catalyst, a catalyst performance life comparison test was carried out for 200 hours while maintaining the reaction temperature at 450 ° C. under the following reaction gas composition and reaction conditions. The results are shown in Table 3.

(Reaction gas composition) NO 1,000 ppm SO ₂ 80 ppm O ₂ 10 vol% H ₂ O 10 vol% C ₂ H ₅ OH 1,563 ppm N ₂ balance (reaction conditions) C ₂ H ₅ OH / NO weight ratio 3 GHSV 20,000 Hr ^-1

The reaction gas passing through the catalyst layer at a predetermined reaction temperature was sampled at the outlet of the reaction tube, and the nitrogen oxide concentration was measured by a chemiluminescence nitrogen oxide analyzer. The nitrogen oxide removal rate was calculated by the following formula. Removal rate (%) = [(1,000 ppm-nitrogen oxide concentration ppm at reaction tube outlet) / nitrogen oxide concentration in raw material reaction gas 1,000 ppm] × 100

Example 2 To 250 g of pure water, 23.3 g of silver nitrate was added and dissolved. Alumina powder 100 obtained by baking hydrated aluminum oxide produced by hydrolysis of aluminum isopropoxide at 550 ° C. for 3 hours in this liquid maintained at 40 ° C.
g was added, and the mixture was stirred and suspended. An aqueous solution prepared by dissolving 8.0 g of separately prepared sodium carbonate in 50 g of pure water was added over 15 minutes to support silver carbonate on the alumina particles. After aging for 60 minutes, it is filtered, washed, dried at 120 ° C, and further 45 minutes.
A silver carbonate / alumina catalyst containing 12.5% by weight in terms of silver was obtained by firing at 0 ° C. and pulverizing. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 65 g of this catalyst and 100 g of pure water,
Add 2.0 g of acetic acid and 18.6 g of alumina sol, stir and mix, and use a wet pulverizer to obtain a median diameter of 1
Adjust the particle size to 0.5 μm and adjust the solid content to 3
A 6.1 wt% aqueous slurry was prepared. When the viscosity of this slurry was measured, it was 58 cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

Example 3 170 g of hydrated aluminum oxide powder produced by hydrolysis of aluminum ethoxide was used as SiO ₂ .
It was suspended and stirred in 530 g of an aqueous solution of silica sol containing 5 g. This slurry is spray dried and then 55
It was calcined at 0 ° C. for 3 hours to prepare a silica / alumina powder carrier containing 15% by weight of silica.

To 500 g of pure water, 4.93 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C., 100 g of the silica / alumina powder carrier containing 15% by weight of silica prepared above was added, and the mixture was stirred and suspended. An aqueous solution prepared by dissolving 2.26 g of separately prepared sodium sulfate in 50 g of pure water was added over 15 minutes to support silver sulfate on the silica / alumina particles. After aging for 60 minutes, filter, wash, and
It was dried at 0 ° C., further baked at 450 ° C., pulverized to obtain a silver sulfate / alumina catalyst containing 3.0% by weight in terms of silver. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 50 g of this catalyst and 100 g of pure water,
Acetic acid (2.5 g) and alumina sol (23.8 g) were added, and the mixture was stirred and mixed, and further, a median diameter of 3.
The particle size was adjusted so as to be 5 μm, and the solid content concentration was 29.
An 8 wt% aqueous slurry was prepared. The viscosity of this slurry was measured and found to be 21 cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

Example 4 4.14 g of silver nitrate was added and dissolved in 500 g of pure water. To this solution maintained at 40 ° C, 100 g of powder of hydrated aluminum oxide produced by hydrolysis of aluminum-t-butoxide was added and stirred and suspended. It was evaporated and solidified while being heated, dried at 120 ° C., and further baked at 450 ° C. Crushed to obtain a silver oxide / alumina catalyst containing 3.4% by weight in terms of silver. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 65 g of this catalyst and 100 g of pure water,
Add 2.0 g of acetic acid and 18.6 g of alumina sol, stir and mix, and use a wet pulverizer to obtain a median diameter of 1
Adjust the particle size to 0.5 μm and adjust the solid content to 3
A 6.1 wt% aqueous slurry was prepared. When the viscosity of this slurry was measured, it was 58 cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

Comparative Example 1 An aqueous solution of sulfuric acid was added to an aqueous solution of sodium aluminate for neutralization, and the obtained precipitate of hydrated aluminum oxide was washed, dried and pulverized, and then calcined at 550 ° C. for 4 hours to obtain an alumina powder carrier. Got

To 500 g of pure water, 4.93 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C, 100 g of an alumina powder carrier was added and stirred and suspended. An aqueous solution prepared by dissolving 1.72 g of separately prepared ammonium chloride in 50 g of pure water was added over 15 minutes to support silver chloride on the alumina particles. After aging for 60 minutes, filter, wash, and
A silver chloride / alumina catalyst containing 3.0% by weight in terms of silver was obtained by drying at 0 ° C., calcining at 450 ° C., crushing. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 50 g of this catalyst and 100 g of pure water,
Add 2.5 g of acetic acid and 23.8 g of alumina sol, stir and mix, and use a wet pulverizer to obtain a median diameter of the catalyst of 6.
An aqueous slurry having a solid content of 27.8% by weight was prepared with a thickness of 1 μm. The viscosity of this slurry was measured and found to be 36
It was cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

Comparative Example 2 Ammonia water was added to an aqueous solution of aluminum sulfate for neutralization, and the obtained hydrated aluminum oxide precipitate was washed, dried and pulverized, and then calcined at 550 ° C. for 4 hours to obtain an alumina powder carrier. Obtained.

To 500 g of pure water, 4.93 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C, 100 g of an alumina powder carrier was added and stirred and suspended. An aqueous solution prepared by dissolving 1.72 g of separately prepared ammonium chloride in 50 g of pure water was added over 15 minutes to support silver chloride on the alumina particles. After aging for 60 minutes, filter, wash, and
A silver chloride / alumina catalyst containing 3.0% by weight in terms of silver was obtained by drying at 0 ° C., calcining at 450 ° C., crushing. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 50 g of this catalyst and 100 g of pure water,
Add 2.5 g of acetic acid and 23.8 g of alumina sol, stir and mix, and use a wet pulverizer to obtain a median diameter of the catalyst of 6.
An aqueous slurry having a solid content of 27.8% by weight was prepared with a thickness of 1 μm. The viscosity of this slurry was measured and found to be 36
It was cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

Comparative Example 3 Ammonia water was added to an aqueous solution of aluminum sulfate for neutralization, and the obtained precipitate of hydrated aluminum oxide was washed and dried to obtain hydrated aluminum oxide for a carrier.

To 250 g of pure water, 3.70 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C., 100 g of hydrated aluminum oxide powder carrier was added and stirred and suspended.
An aqueous solution prepared by dissolving 1.70 g of separately prepared sodium sulfate in 50 g of pure water was added over 15 minutes to support silver sulfate on the hydrated aluminum oxide particles. After aging for 60 minutes, it is filtered, washed, dried at 120 ° C., further baked at 450 ° C., pulverized to contain 3.0% by weight in terms of silver.
A silver sulfate / alumina catalyst was obtained. Table 1 shows the catalyst composition, silver content, and alumina raw material type of this catalyst.

Next, 45 g of this catalyst and 100 g of pure water,
2.25 g of acetic acid and 21.4 g of alumina sol were added, stirred and mixed, and the median diameter of the catalyst was 7.5 μm using a wet pulverizer to prepare an aqueous slurry having a solid content concentration of 26.5% by weight. When the viscosity of this slurry was measured, it was 28 cps.

Using this slurry, the slurry was applied to the cell surface of the honeycomb structure according to the method of Example 1 and finally baked at 600 ° C. for 90 minutes to prepare a honeycomb structure catalyst. A nitrogen oxide purification test was performed by a method according to. The results are shown in Tables 2-3.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

As described above, by using the catalyst for purifying nitrogen oxides of the present invention, the nitrogen oxides can be efficiently used even under a high oxygen concentration in the presence of sulfur oxides even at a high reaction temperature. Can be removed, and the reducing action can be economically performed by suppressing the complete oxidation reaction of the added hydrocarbon or the oxygen-containing organic compound as a reducing agent. As a result, the reduction of the removal efficiency is suppressed even in the coexistence of the sulfur oxides, and the nitrogen oxides can be efficiently removed even at a high temperature.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location B01J 21/04 ZAB A 21/12 ZAB A 37/02 301 B B01D 53/36 102 A (72) Inventor Kojima Mitsuo 1-26 Takitanihonmachi, Niitsu City, Niigata Prefecture JGC Chemicals Co., Ltd. Niitsu Plant

Claims (3)

[Claims] 1. A nitrogen oxide purifying catalyst used in the coexistence of hydrocarbons or oxygen-containing organic compounds, wherein alumina prepared by hydrolyzing an organoaluminum compound is used as a porous heat-resistant carrier, A catalyst for purifying nitrogen oxides, which carries at least one active component selected from silver or a silver compound. 2. The catalyst for purifying nitrogen oxides according to claim 1, wherein the porous refractory carrier contains 15% by weight or less of silica in addition to alumina. 3. One selected from the silver or silver compounds.
The catalyst for purifying nitrogen oxides according to claim 1 or 2, which contains 0.01 to 20.0% by weight of seeds in terms of silver.