JPH0490826A - Exhaust gas purification method - Google Patents

Abstract

PURPOSE:To efficiently remove NOx from the exhaust gases emitting from various facilities and prevent the other harmful components from emitting by a method wherein NOx containing exhaust gases are brought into a step-wise contact with a reducing and an oxidizing catalyst in the presence of hydrocarbon under an oxidative atmosphere where an excessive oxygen is present. CONSTITUTION:Proton type zeolite, alkali metal type zeolite or an acidic metal oxide treated and adjusted with the compounds having sulfuric acid radical is used as a reductive catalyst on the upstream side of an exhaust gaseous stream. On the downstream side of the exhaust gaseous stream, the catalytic component such as platinum, palladium, copper and iron supported alone or in combination on a porous carrier such as active alumina and silica is used as an oxidizing catalyst. The reducing catalyst disposed on the upstream side of the exhaust gaseous stream in the presence of hydrocarbon under an oxidative atmosphere where an excessive oxygen is present accelerates a reaction between hydrocarbon and NOx in preference to that between hydrocarbon and oxygen to reductively decompose NOx in the exhaust gases. Moreover, the oxidizing catalyst disposed on the downstream side of the exhaust gaseous stream removes oxidatively harmful components such as hydrocarbon and carbon monoxide.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention provides a method for reducing nitrogen oxides in exhaust gases by removing nitrogen oxides from a small amount of added hydrocarbons or remaining in the exhaust gases under generally oxidizing conditions in the presence of excess oxygen. The present invention relates to an exhaust gas purification method that removes nitrogen oxides from exhaust gas by bringing it into contact with a specific catalyst in the presence of hydrocarbons.

[Conventional technology]

Nitrogen oxides (hereinafter referred to as rNOx+) in various exhaust gases are
Since it is harmful to health and can also cause photochemical smong and acid rain, there is a desire to develop effective treatment methods.

Conventionally, as methods for removing NOx, several methods have already been put into practical use that use catalysts to reduce NOx in exhaust gas.

For example, (a) three-way catalyst method in gasoline cars,
Port C) A selective Ml reduction method using ammonia for exhaust gas from large equipment sources such as boilers can be mentioned.

Other methods that have been proposed include (c) a method for removing NOx from exhaust gas using hydrocarbons, in which zeolite containing various metals is brought into contact with a gas containing NOx in the presence of hydrocarbons (particularly Kaisho 63-283727
No. 2, etc.), and catalysts prepared by adding various metals to Zeolide have been proposed.

Furthermore, the present inventors have proposed (d) a method for removing NOx in exhaust gas using hydrocarbons, in the presence of hydrocarbons,
A method has been proposed in which a specific zeolite or acidic metal oxide is brought into contact with exhaust gas containing NOx (Japanese Patent Application No. 139340/1999).

[Problem to be solved by the invention]

In the method (a) above, hydrocarbon components and carbon monoxide contained in combustion exhaust gas from automobiles are converted to water and carbon dioxide using a catalyst, and at the same time, NOx is reduced to nitrogen. Combustion must be adjusted so that the amount of oxygen is chemically equal to the amount of oxygen required to oxidize hydrocarbon components and carbon monoxide. In existing systems, application is impossible in principle.

In addition, method (b) uses ammonia, which is a toxic and flammable high-pressure gas, and requires huge equipment for safety reasons, making it technically extremely difficult to apply when the exhaust gas source is moving. be.

On the other hand, method (c) is mainly targeted at gasoline-powered vehicles, and is difficult to apply under the exhaust gas conditions of diesel engines, and its activity is insufficient. In other words, since the catalyst contains various metals, it is not only poisoned by sulfur oxides discharged from diesel engines, but also
Since the added metal aggregates and the activity decreases, it is not suitable for removing NOx from exhaust gas from a diesel engine, and has not been put into practical use.

In addition, method (d) can decompose and remove NOx more efficiently in the presence of hydrocarbons than conventional methods, but depending on the reaction conditions such as the catalyst 2 reaction salinity used, the hydrocarbon added as a reducing agent and the oxidation Carbon monoxide, an intermediate product, may leak out.

The present invention was made as a result of studying various problems existing in (a) to (d) above, and is a method for reducing NOx in exhaust gas from various equipment, including diesel engine exhaust gas, in an oxidizing atmosphere. In addition to efficiently removing
The purpose is to propose an exhaust gas purification method that does not emit other harmful components.

[Means and effects for solving the problem] The present inventors,
In order to solve the above-mentioned problems in the conventional technology, we have conducted intensive research and found that using a specific catalyst in a specific combination causes a decrease in activity even in exhaust gas containing sulfur oxides. Also, - without emitting harmful substances such as carbon oxide, it generates N at a higher rate than before.
It was discovered that Ox can be removed, and the present invention was completed.

That is, in order to achieve the above object, the exhaust gas purification method of the present invention uses proton type zeolite, alkali metal type zeolite, or acidic metal oxide in an oxidizing atmosphere containing excess oxygen and in the presence of hydrocarbons. The method is characterized in that one or more catalysts selected from the above are brought into contact with exhaust gas containing NOx, and then the exhaust gas is brought into contact with an oxidation catalyst.

Furthermore, the exhaust gas purification method of the present invention is characterized in that a metal oxide treated with a compound having a sulfate radical is used as the above-mentioned acidic metal oxide.

Hereinafter, the details of the method of the present invention will be explained along with its operation.

In the method of the present invention, the exhaust gas to be treated is brought into contact with a reduction catalyst and an oxidation catalyst in stages.

At this time, the catalyst that can be used on the upper stage side (that is, the upstream side of the exhaust gas) is a proton type zeolite, an alkali metal type zeolite, or a metal oxide with acidity such as alumina (aluminum oxide) or silica alumina. One or more selected reduction catalysts.

Specifically, the above zeolite in the method of the present invention is
Various zeolites can be used, such as pentasil type zeolite, mordenite, Y type zeolite, X type zeolite, L type zeolite, and silicalite.

The Kayhan ratio of these zeolites (S i Oz vs. Alt
Although there are no particular restrictions on the O2 ratio, it is preferably relatively high in terms of stability against heat and water vapor, more preferably about 5 to 200, and even more preferably about 1.
It is 0-100.

These zeolites can be produced by known methods, including silica, silica sol, silica sources such as sodium silicate, alumina gel, and alumina sol.

A raw material mixture containing an alumina source such as sodium aluminate, an alkali source such as sodium hydroxide or sodium silicate, water, and if necessary an organic base such as an amine is hydrothermally synthesized, and after separating the product, washing with water, It can be dried to form alkali metal type theolite.

Complete Na-type conversion can be achieved by ion exchange.

Here, the alkali metal type zeolite is treated with ammonium chloride or ammonium nitrate aqueous solution to form ammonium type zeolite, and then the
Proton type zeolite can be obtained by firing at a temperature range of 0°C.

By using proton type zeolite, it becomes possible to decompose and remove NOx even more efficiently.

Furthermore, in the method of the present invention, acidic metal oxides can also be used, such as alumina (Af!z
O3), titanium oxide (TiO), zirconium oxide (Z
metal oxides such as rOz), silica alumina (SiOz
Aj2zO, +), silica magnesia (S i Oz・
Mg0), silica zirconia (S i O2ZrO
z), alumina titanium (Alz03.Ti0z), and other composite oxides.

Further, even those consisting of three or more metal oxide components can be effectively used as long as they exhibit solid acidity, and those that are highly acidic and have a large amount of acid are particularly preferred.

As an example of other metal oxides, metal oxides prepared by treatment with a compound having a sulfate group can be used,
It can decompose NOx more efficiently than untreated metal oxides.

A specific example of a compound having a sulfate group is sulfuric acid.

Examples include ammonium sulfate, and any other compound that produces a sulfate group on the metal oxide by drying and firing after treatment can be used.

The preparation of metal oxide catalysts by treatment with compounds having sulfate radicals, e.g. sulfuric acid, can be carried out using alumina (AI!).

Metal oxides such as titanium oxide (T i O
Although obtained by air calcination at specific temperatures, even higher activity catalysts can be obtained by similar treatment of amorphous oxides or the corresponding hydroxides with sulfuric acid.

The concentration of sulfuric acid used in the treatment varies depending on the type of oxide, but is usually about 0.01 to 10 moff/2, preferably about 0.1 to 5 moj2/f! About 5 to 20 times the amount of sulfuric acid based on the weight of the catalyst is used at the same concentration and brought into contact with the metal oxide.

Here, if the concentration of g acid is less than about 0.01 mof/f,
Contact with sulfuric acid cannot be expected to have much of an effect on NOx decomposition activity, and if it exceeds about ]Omoffi/ffi, there is a risk of metal sulfate formation or destruction of the catalyst structure, which is not preferable.

Furthermore, when ammonium g-acid is used as a compound having a sulfate group in the treatment, the treatment can be carried out in the same manner as above.

The optimum air firing temperature also varies depending on the type of oxide, but is usually about 300 to 850°C, preferably about 400°C.
~700°C.

If the air calcination temperature is less than about 300°C, compounds such as sulfuric acid used in the treatment may not be removed and catalyst active sites may not be formed; if it exceeds 850°C, the catalyst surface area may decrease or the catalyst active sites may not be formed. Destruction may occur.

On the other hand, in the method of the present invention, the oxidation catalyst that can be used on the lower stage side (that is, the downstream side of the exhaust gas) includes, for example, platinum, palladium, rhodium, ruthenium, Catalyst components such as noble metals such as iridium, base metals such as lanthanum, cerium, copper, and iron, and perovskite crystal structures such as lanthanum cobalt trioxide, lanthanum iron trioxide, and cobalt strontium trioxide are supported singly or in combination. Can be mentioned.

In this case, the supported amount of the catalyst component is about 0.01 to 2 wt% for noble metals, and about 5 wt% for base metals.
It is about 70 wt%.

If the amount of each supported is small, the effect as an oxidation catalyst cannot be expected, and even if the amount of each supported exceeds the appropriate amount, the effect cannot be improved commensurately.

The usage ratio of the above-mentioned reduction catalyst and oxidation catalyst, the amount of catalyst components supported on the oxidation catalyst, etc. can be selected as appropriate depending on the required performance. If the catalyst is an intermediate oxide, it is possible to use a mixture of the reduction catalyst and the oxidation catalyst, but generally the reduction catalyst is placed on the upstream side of the exhaust gas, and the oxidation catalyst is placed on the downstream side of the exhaust gas.

As a specific example of purifying exhaust gas using these catalysts, a reactor with a reduction catalyst placed in the exhaust gas introduction section (first stage) and a reactor with an oxidation catalyst placed in the exhaust gas discharge part (second stage) are used. Alternatively, it is also possible to arrange and use each catalyst in a single reactor at a ratio depending on the required performance.

Generally, the ratio of reduction catalyst (A) and oxidation catalyst (B) is (
A)/(B) approximately 0.5/9°5-9. 51
0. Used in the range of 5.

The above-mentioned catalyst may have any shape or structure, such as powder, single grain, pellet, or honeycomb shape.

It is also possible to mix a non-acidic oxide such as silica, silica sol, or a molding agent such as carbon wax, oil or fat used for the purpose of catalyst molding.

The NOx-containing gas to be treated by the method of the present invention includes:
Exemplary exhaust gases include diesel engine exhaust gas from diesel cars and stationary diesel engines, gasoline engine exhaust gas from gasoline cars, and exhaust gas from nitric acid manufacturing plants and various combustion equipment.

The NOx in these exhaust gases is removed by using the catalyst and bringing the exhaust gas into contact with the catalyst in an oxidizing atmosphere in the presence of hydrocarbons.

Here, the oxidizing atmosphere means to completely oxidize the carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas, as well as the reducing substances of the hydrocarbons added as necessary in the method of the present invention, to create water and carbon dioxide. This refers to an atmosphere that contains excess oxygen than the amount of oxygen required to convert it into oxygen. For example, in the case of exhaust gas emitted from internal combustion engines such as automobiles, an atmosphere with a large air ratio (lean region) The excess oxygen rate is usually about 20 to 200%.

In this oxidizing atmosphere, the catalyst disposed upstream of the exhaust gas preferentially promotes the reaction between hydrocarbons and NOx rather than the reaction between hydrocarbons and oxygen, and decomposes NOx.

The hydrocarbon to be present may be the hydrocarbon remaining in the exhaust gas, but if the amount is insufficient than the amount necessary to promote the above reaction, it is necessary to add the hydrocarbon from the outside.

The amount of hydrocarbon is not particularly limited, but if it is in excess of the amount required for reductive decomposition of NOx, the reduction reaction will progress more.
It is preferable to add the hydrocarbon in excess, and the amount of hydrocarbon used is usually about 20 to 2,000 times the theoretical amount required for reductive decomposition of NOx.
0%, preferably about 30-1.500% excess.

Here, the theoretical amount of necessary hydrocarbons refers to the amount of nitrogen dioxide (N
Ox ) is defined as a hydrocarbon necessary for reductive decomposition. For example, using propane as the hydrocarbon, 1. The theoretical amount of propane when OO'Oppm nitrogen monoxide (No) is reductively decomposed in the presence of oxygen is 200 ppm.

Generally, depending on the amount of NOx in the exhaust gas, the amount of hydrocarbons to be present is approximately 50 to 10,000 p when replaced with methane.
It is about pm.

The hydrocarbon used in the present invention is not particularly limited, including gaseous and liquid hydrocarbons, and any hydrocarbon that vaporizes at the reaction temperature can be used.

Specific examples of gaseous ones include hydrocarbons such as methane, ethane, propane, ethylene, propylene, and butylene, and examples of liquid ones include petroleum hydrocarbons such as gasoline and light oil, and alcohols. , ethers, ketones, and other oxygen-containing compounds.

The reaction is carried out by preparing a reactor in which the zeolite or metal oxide is placed upstream of the exhaust gas, allowing hydrocarbons to exist in an oxidizing atmosphere, and passing the NOx-containing exhaust gas.

Through the above process, NOx in the exhaust gas can be reductively decomposed, but by preparing a reactor equipped with an oxidation catalyst downstream of the exhaust gas and passing the exhaust gas continuously, NOx can be removed and hydrocarbons can be decomposed. and - Harmful components such as carbon oxide can be removed by oxidation.

The optimum reaction temperature at this time differs depending on the type of catalyst and hydrocarbon, but a temperature close to the exhaust gas temperature is preferable because it does not require exhaust gas heating equipment, and is approximately 200 to 80℃.
It is effective to use the temperature at 00°C, especially in the range of about 300 to 600"C.

Here, when olefins are used as the added hydrocarbon, the temperature is preferably slightly lower than when paraffins are used.

The reaction pressure is not particularly limited; the reaction proceeds at normal exhaust pressure, and the space velocity (SV) at which exhaust gas is introduced into the catalyst layer
) varies depending on other reaction conditions such as reaction temperature and the required removal rate of NOx and harmful components, and is not particularly limited.
about 500 to 100°000 Hr-', preferably about 1,
The range is from 000 to 70°000 Hr-'.

In addition, in the method of the present invention, when treating exhaust gas from an internal combustion engine, the catalyst is preferably placed downstream of the exhaust manifold.

[Example] Next, examples of the method of the present invention will be given. Two examples of the method of the present invention are as follows.
The invention is not limited to these examples.

Example 1 (Preparation of pentasil type zeolite) In an aqueous solution in which 957 g of sodium silicate was dissolved in 1,200 g of water, 41 g of aluminum sulfate was dissolved in 1,600 g of water.
G,! An aqueous solution containing 80 g of sulfuric acid and 360 g of sodium chloride was gradually added and mixed over 30 minutes with stirring.

Furthermore, 120 g of tetrapropylammonium bromide was added to adjust the pH to 10.

Pour this mixture into an autoclave and heat it at 165°C for 1
After stirring for 6 hours, crystallization occurred.

After separating the product, it is washed with water and dried to form the base 5i02/
ZSM-, which is a pentasil type of Al-,03-62,7
5 zeolite was obtained.

(Preparation of proton type pentasil type zeolite) To 500 ml of i-liquid containing 1 mol/l ammonium nitrate,
20 g of the above-mentioned pentasil type zeolite was added, and the mixture was refluxed and centrifuged while stirring all day and night.

This was washed 5 times with pure water, dried at 110°C overnight, and then
Proton type 23M-5 zeolite was prepared by air calcination at 0°C for 3 hours.

(NOx removal reaction) 1 g of proton type ZSM5 zeolite prepared as described above and 1 g of commercially available 0.5% platinum alumina were packed into an atmospheric pressure fixed bed flow reactor. Platinum alumina is placed on the side), at the reaction temperature shown in Table 1,
The reaction was carried out by flowing helium gas containing 1.000 ppm of nitrogen monoxide (hereinafter referred to as rNOJ), 10% oxygen, and 300 ppm of propane at a flow rate of 60 ml/min.

The reductive decomposition rate of No was determined from the yield of produced nitrogen, and the reaction gas was analyzed using a gas chromatograph.

Table 1 shows the reduction decomposition rate of NO and the amounts of back monoxide, propane, and carbon dioxide discharged as Example 1.

As is clear from Table 1, in this case, propane is almost completely oxidized to carbon dioxide at the same time as the reductive decomposition of NOx occurs.

Comparative Example 1 The reaction was carried out in the same manner as in Example 1 except that the catalyst used was only 1 g of H/ZSM-5. The results are also shown in Table 1 as Comparative Example 1.

As is clear from Table 1, in this case, a large amount of carbon monoxide flows out, and when the reaction temperature is as low as 300°C, a large amount of propane also flows out, indicating that harmful components remain.

Example 2 Commercially available zirconium hydroxide Log was taken on a filter paper and 0.
5mof! //! After flowing sulfuric acid at 150 mA, it was air-dried, and then calcined in an air stream at 600°C for 3 hours to obtain sulfuric acid-treated zirconium oxide.

1 g of the obtained sulfuric acid-treated zirconium oxide and commercially available 0.5
% platinum alumina was charged into an ordinary pressure fixed bed flow reactor in the same manner as in Example 1, and at the reaction temperature shown in Table 2, 1 g of platinum alumina was added.
Helium gas containing 0 ppm NO, 10% oxygen, and 300 ppm propane at 60 mj/min! The calculation of the reductive decomposition rate of 2NO and the analysis of the exhaust gas were carried out in the same manner as in Example 1.

The results are shown in Table 2 as Example 2.

Comparative Example 2 The reaction was carried out in the same manner as in Example 2 except that only 1 g of sulfuric acid-treated zirconium was used as the catalyst. The results are also shown in Table 2 as Comparative Example 2.

Example 3 Commercially available powdered alumina for FCC (surface area 285 m''/g) was used as the NOx reduction catalyst, and 30 m''/g was used as the oxidation catalyst.
% of ferric oxide supported on alumina was used.

The oxidation catalyst was 38 g of ferric nitrate (Fe(NOx)3-
300ml of 9H,O)! 35 g of commercially available γ-alumina was added thereto with stirring, and 14% ammonia water was added dropwise until the pH reached 8 to deposit iron hydroxide on the alumina, followed by filtration, washing with water, and drying. , was obtained by firing in air at 500°C for 3 hours.

1 g each of the above alumina and oxidation catalyst were charged into an atmospheric fixed bed flow reactor as in Example 1, and at a reaction temperature of 500°C, helium gas containing 1,000 ppm of NO and 10% oxygen was added every time. The reaction was carried out by flowing at a flow rate of 60 m2 per minute and changing the amount of propane added as shown in Table 3.

Calculation of NO reduction decomposition rate and analysis of exhaust gas are performed in Example 1.
The results are shown in Table 3 as Example 3.

Comparative Example 3 The catalyst used was 1 g of powdered alumina for FCC (surface area 28
The reaction was carried out in the same manner as in Example 3, except that only 5 m''/g) was used, and the results are also shown in Table 3 as Comparative Example 3.

Example 4 Aluminum sulfate (Aft(Son)z・16-18H
, O) 300g 1i! was dissolved in water, and 75 g of 24% titanium sulfate was added dropwise while stirring, and then 14% aqueous ammonia was added dropwise until the pH reached 9.

Next, after aging this in a 95°C water bath for several hours,
It is filtered, washed with water until no sulfate radicals are detected, dried, and then calcined in air at 500°C for 3 hours to produce alumina titania (/1.0.Ti) containing 10% titanium oxide.
Oz) was obtained.

1 g of the obtained alumina titania and 0.0 g of commercially available alumina titania. 1 g of 5% palladium alumina was charged into an atmospheric fixed bed flow reactor in the same manner as in Example 1, and at the reaction temperature shown in Table 4, 2. O
The reaction was carried out by flowing helium gas containing Oppm NO, 10% oxygen, and 600 ppm propylene at a flow rate of 60 ml/min.

Calculation of NO reduction decomposition rate and analysis of exhaust gas are performed in Example 1.
The results are shown in Table 4 as Example 4.

Comparative Example 4 The reaction was carried out in the same manner as in Example 4 except that only 1 g of alumina titania was used as the catalyst. The results are also shown in Table 4 as Comparative Example 4.

As is clear from Tables 1 to 4, in the method of the present invention, the reductive decomposition rate of NOx occurs efficiently even in the presence of excess oxygen, and the main harmful components, hydrocarbons and carbon monoxide, are removed. It turns out that it can be oxidized.

〔Effect of the invention〕

As detailed above, according to the method of the present invention, in an oxidizing atmosphere where oxygen is present in excess, N in exhaust gas can be efficiently removed.
Not only can Ox be removed, but also the discharge of harmful components such as -carbon oxide can be effectively prevented.

This is because the proton type zeolite, alkali metal type zeolite, or acidic metal oxide according to the present invention not only preferentially promotes the reaction between NOx and hydrocarbons in the presence of hydrocarbons, but also By using a catalyst, unreacted or generated hydrocarbons, carbon oxides, or other oxidation intermediate products that may flow out depending on the reaction conditions can be completely oxidized into carbon dioxide and water vapor. It is.

As described above, by combining both types of catalysts, the method of the present invention can efficiently remove NOx from diesel engine exhaust gas and other exhaust gases from various equipment, and at the same time prevent the emission of harmful components. It has extremely high industrial value.

Patent applicant Director of the Institute of Industrial Science and Technology Cosmo Research Institute Co., Ltd. Procedural amendment October 1, 1990 1990 Patent Application No. 204102 2 Name of invention Address 1-1 Shibaura-chome, Minato-ku, Tokyo Name: Cosmo Research Institute Co., Ltd. Representative: Kei Iwasa (and 1 other person) 4, Agent address: Chugin 2, 2-10-7 Higashi-Shinbashi, Minato-ku, Tokyo 105
Shinbashi Mansion 302 Place 03 (5472) 4708 7, Detailed explanation of the invention column 8 of the specification subject to amendment, Contents of the amendment (1) “Al2O2” on page 6 lines 18-19 of the specification was replaced with “A
I2O3” is corrected.

(2) Correct "rTiQ" in line 4 of page 8 to "rTiQ2".

(3) On page 14, line 10, "methane substitution" is corrected to "methane conversion."

(4) "Decomposition rate" in line 3 on page 29 is corrected to "decomposition."

(1 other person)

Claims (2)

[Claims] (1) Exhaust gas containing nitrogen oxides and one or more catalysts selected from proton zeolites, alkali metal zeolites, or acidic metal oxides in an oxidizing atmosphere containing excess oxygen and in the presence of hydrocarbons. 1. A method for purifying exhaust gas, which comprises bringing the exhaust gas into contact with an oxidation catalyst and then bringing the exhaust gas into contact with an oxidation catalyst. (2) The exhaust gas purification method according to claim 1, wherein the acidic metal oxide is a metal oxide treated with a compound having a sulfate group.