JPH0538420A - Treatment method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To remove NOx in an exhaust gas efficiently. CONSTITUTION:From a viewpoint that in the reduction reaction between NOx and hydrocarbons on a reduction catalyst in an oxidative atmosphere, NO2, is reduced more easily than NO, an exhaust gas is previously contacted with an oxidation catalyst to oxidize NO in the exhaust gas into NO2. The treated exhaust gas, in an oxidative atmosphere containing excessive oxygen and in the presence of hydrocarbons, is contacted with a reduction catalyst selected from zeolite of a proton type, zeolite of an alkali metal type, and acid metal oxide (metal oxide treated with a compound having sulfate group to reduce NO2, in the exhaust gas into nitrogen.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to the presence of hydrocarbons to which nitrogen oxides in exhaust gas have been previously converted into nitrogen dioxide, and the exhaust gas has been added in a small amount under oxidizing conditions as a whole in the presence of excess oxygen. The following relates to a method for removing nitrogen oxides in exhaust gas by contacting with a specific catalyst.

[0002]

2. Description of the Related Art Nitrogen oxides in various exhaust gases (hereinafter referred to as
Since "NOx") is harmful to health and can cause photochemical smog and acid rain, it is desired to develop an effective treatment means.

Conventionally, several methods for reducing NOx in exhaust gas using a catalyst have been put into practical use as methods for removing NOx. For example, (a) a three-way catalyst method for gasoline automobiles, and (b) a selective catalytic reduction method using ammonia for exhaust gas from a large facility emission source such as a boiler. Further, as another proposed method, (c) NOx in exhaust gas using hydrocarbon
There is a method for removing zeolite containing various metals with a gas containing NOx in the presence of hydrocarbons (Japanese Patent Laid-Open No. 63-283727, etc.), and various metals are added to zeolite as a catalyst. Things have been proposed.

Further, the present inventors have proposed (d) a method for removing NOx in exhaust gas using hydrocarbons, in the presence of hydrocarbons, a specific zeolite or an exhaust gas containing NOx and a metal oxide having acidity. There has been proposed a method of contacting with each other (Japanese Patent Application No. 2-139340).

[0005]

The above method (a) is
The hydrocarbon components and carbon monoxide contained in the exhaust gas of automobiles are converted into water and carbon dioxide by a catalyst, and at the same time NOx
Is reduced to nitrogen, but is burned so that the amount of oxygen contained in NOx and the amount of oxygen required to oxidize hydrocarbon components and carbon monoxide are chemically equivalent. Must be adjusted, and cannot be applied in principle in a system in which excess oxygen exists, such as a diesel engine.

In addition, since the method (b) uses ammonia, which is a toxic and combustible high-pressure gas, a huge facility is required for safety, and it is technically applicable to apply when the exhaust gas source moves. It's extremely difficult.

On the other hand, the method (c) is mainly applied to gasoline automobiles, and it is difficult to apply it under the exhaust gas conditions of a diesel engine and its activity is insufficient. That is, since various metals are contained as components of the catalyst, not only are they poisoned by sulfur oxides discharged from the diesel engine, but also the activity of the added metal is reduced due to aggregation of the added metal. In NOx exhaust gas
It is not suitable for the removal of pesticides and has not been put to practical use.

In the method (d), NOx can be decomposed and removed more efficiently than in the conventional method in the presence of hydrocarbons, but a method capable of removing NOx at a higher rate using these catalysts is available. Coveted.

The present invention has been made as a result of studying various problems existing in the above (a) to (d). In the oxidizing atmosphere, exhaust gas from various facilities including diesel engine exhaust gas is generated. It is an object of the present invention to propose a method for efficiently removing NOx of.

[0010]

Means and Actions for Solving the Problems The present inventors have
In order to solve the above problems existing in the prior art, as a result of intensive studies, after converting NOx in exhaust gas into nitrogen dioxide (hereinafter, “NO ₂ ”) in advance, in the presence of hydrocarbons, It was found that NOx can be removed at a higher rate than before by contacting with a specific catalyst, and the present invention has been completed.

That is, in the method for removing nitrogen oxides of the present invention, the nitrogen oxides in the exhaust gas are converted into nitrogen dioxide in advance, and then the exhaust gas is subjected to the presence of hydrocarbons in an oxidizing atmosphere containing excess oxygen. It is characterized in that it is brought into contact with at least one catalyst selected from proton-type zeolite, alkali metal-type zeolite, and acidic metal oxides. Further, the exhaust gas purification method of the present invention is also characterized in that a metal oxide treated with a compound having a sulfate group is used as the above-mentioned acidic metal oxide.

The details of the method of the present invention will be described below together with the operation. Diesel engine, including gasoline engine, NOx generated from various combustion equipment, almost all the nitric oxide (hereinafter, "NO"), and some is oxidized to NO ₂ while being discharged to the outside, NOx N depending on the concentration measurement location
Although the proportion of O ₂ changes, about 90% or more is NO in the exhaust gas of ordinary combustion equipment.

Conventionally, when NOx is reduced to nitrogen and removed, NO ₂ has a higher valence of nitrogen than NO, and it has been generally considered that reduction is difficult. However, according to the study by the present inventors, N by the hydrocarbons on the reduction catalyst is reduced.
With respect to the selective reduction of Ox, a surprising result that NO ₂ is more easily reduced to nitrogen than NO, which is completely unexpected, was initially obtained, and the present invention has been completed based on this finding.

In the method of the present invention, in order to convert NOx in the exhaust gas into NO ₂ in advance, a known method such as contacting the target gas with an oxidation catalyst or an oxidizing substance such as ozone or chlorine dioxide is used. be able to.

Here, the conversion of NO in the exhaust gas to NO ₂ is about 70% or more, preferably about 90% or more of NO,
Particularly preferably, NOx can be efficiently removed by setting almost all the amount to NO ₂ .

That is, in the method of the present invention, the exhaust gas to be treated is brought into contact with the oxidation catalyst in the first step, and then hydrocarbons as a reducing agent are added, and in the second step, it is brought into contact with the reducing catalyst to remove NOx. To do.

As an oxidation catalyst used to oxidize NO in exhaust gas to NO ₂ , a porous carrier such as activated alumina, silica, zirconia or zeolite can be used.
Noble metals such as palladium, rhodium, ruthenium and iridium, base metals such as lanthanum, cerium, copper and iron; perovskite type crystal structures such as cobalt lanthanum trioxide, lanthanum trioxide and cobalt strontium trioxide; The thing which carried in combination of 2 or more types is mentioned. The loading amount of these catalyst components is about 0.01 to 2 wt% for the noble metal and about 5 to 70 wt% for the base metal. If the supported amount is too small, the effect as an oxidation catalyst cannot be expected so much, and even if the supported amount exceeds the appropriate amount, the effect corresponding to it cannot be improved.

It is also possible to use ozone as another method for oxidizing NOx in exhaust gas to NO ₂ . In this case, NO ₂ can be obtained by adding ozone generated from the ozone generator according to the amount of NOx in the exhaust gas. The amount of ozone added is almost equimolar to NO in the exhaust gas so that most of NOx is N
Although it becomes O ₂ , it is preferable to add it in a slight excess in order to make it completely NO ₂ .

In the method of the present invention, N
After oxidizing O to NO ₂ , reduction catalysts that can be used to reduce NO ₂ in the presence of hydrocarbons are proton-type zeolite, alkali metal-type zeolite, or alumina (aluminum oxide), silica-alumina. One or more reducing catalysts selected from metal oxides having acidity as described above.

The above-mentioned zeolite in the method of the present invention is
Specifically, pentasil-type zeolite, mordenite,
Various zeolites such as Y-type zeolite, X-type zeolite, L-type zeolite and silicalite can be used. The Cavern ratio of these zeolites (the formula weight ratio of SiO ₂ to Al ₂ O ₃ ) is not particularly limited, but is preferably relatively high in view of stability against heat and steam, and more preferably about 5 to 5. 200, more preferably about 10-100
Is.

These zeolites can be produced by a known method. Silica, silica sol, silica source such as sodium silicate, alumina gel, alumina sol, alumina source such as sodium aluminate, sodium hydroxide, sodium silicate, etc. The alkali metal type zeolite can be obtained by hydrothermally synthesizing a raw material mixture containing an alkali source, water, and optionally an organic base such as amine, separating the product, washing with water and drying. To make it completely Na type,
It becomes possible by ion exchange.

Here, the alkali metal type zeolite may be treated with an ammonium chloride or ammonium nitrate aqueous solution to form an ammonium type zeolite, and then calcined at a temperature range of about 400 to 700 ° C. to form a proton type zeolite. it can. By using the proton type zeolite, NO ₂ can be decomposed and removed more efficiently.

Further, in the method of the present invention, an acidic metal oxide can be used, and examples thereof include metals such as alumina (Al ₂ O ₃ ), titanium oxide (TiO), zirconium oxide (ZrO ₂ ). Oxide, silica alumina (S
iO ₂ · Al ₂ O ₃ ), silica magnesia (SiO ₂ · Mg)
O), silica zirconia (SiO ₂ .ZrO ₂ ), alumina titania (Al ₂ O ₃ .TiO ₂ ), and other complex oxides.

Further, even those composed of three or more metal oxides can be effectively used as long as they show solid acidity, and those having a strong acidity and a large amount of acid are particularly preferable.

As another example of the metal oxide, a metal oxide prepared by treating with a compound having a sulfate group can be used, and NO ₂ is decomposed more efficiently than an untreated metal oxide. can do. Specific examples of the compound having a sulfate group include sulfuric acid, ammonium sulfate, and the like, and any other compound that produces a sulfate group on the metal oxide by dry baking after the treatment can be used. A compound having a sulfate group, for example, a metal oxide catalyst to be treated with sulfuric acid is prepared by using alumina (Al ₂ O ₃ ),
It can be obtained by bringing a metal oxide such as titanium oxide (TiO ₂ ) into contact with sulfuric acid of a specific concentration at room temperature, drying and then calcining in air at a specific temperature. By similarly treating the hydroxides with sulfuric acid, an even more highly active catalyst is obtained. The concentration of sulfuric acid used for the treatment varies depending on the type of oxide, but is usually about 0.01 to 10 mol / l, preferably about 0.1.
˜5 mol / l, about 5 to 20 times the sulfuric acid concentration is used based on the weight of the catalyst, and it is contacted with the metal oxide. Here, when the concentration of sulfuric acid is less than about 0.01 mol / l, the effect on NO ₂ decomposition activity due to contact with sulfuric acid cannot be expected so much, and when it exceeds about 10 mol / l, the formation of metal sulfate or the catalyst is promoted. There is a possibility that the structure may be broken, which is not preferable.

When ammonium sulfate is used as a compound having a sulfate group for the treatment, the treatment can be carried out in the same manner as above. The optimum temperature for air calcination also varies depending on the type of oxide, but is usually about 300 to 850.
C., preferably about 400-700.degree. If the air calcination temperature is less than about 300 ° C., the compounds such as sulfuric acid used for the treatment may not be removed, and the catalytic active sites may not be formed.
If it exceeds 50 ° C, the surface area of the catalyst may be reduced or the active sites of the catalyst may be destroyed.

As a specific example of removing NOx using these catalysts, a reactor in which an oxidation catalyst is arranged is arranged in an exhaust gas introducing section (first stage), and a reactor in which a reducing catalyst is arranged is arranged in an exhaust gas discharging section (second stage). However, there is a method of providing an inlet for hydrocarbons in the middle. It is also possible to arrange each catalyst in one reactor at a ratio according to the required characteristics, and to provide a hydrocarbons inlet at the boundary between the oxidation catalyst and the reduction catalyst.

The above catalyst may have any shape and structure such as powder, granules, pellets and honeycombs. Also,
It is also possible to mix non-acidic oxides such as silica used for the purpose of molding the catalyst, silica sol, or molding agents such as carbon wax and fats and oils.

The NOx-containing gas to be treated by the method of the present invention includes, for example, diesel engine exhaust gas from diesel automobiles and stationary diesel engines, gasoline engine exhaust gas from gasoline automobiles, nitric acid production plants, various combustion facilities, etc. Exhaust gas can be mentioned.

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

Here, the oxidizing atmosphere is used to completely oxidize carbon monoxide, hydrogen and hydrocarbons contained in the exhaust gas and reducing substances of hydrocarbons added as necessary in the method of the present invention. It means an atmosphere containing an excess of oxygen in excess of the amount of oxygen necessary to convert it into water and carbon dioxide.For example, in the case of exhaust gas discharged from an internal combustion engine of an automobile, the air ratio is large ( The atmosphere of the lean area)
Usually, the excess oxygen ratio is about 20 to 200%.

In this oxidizing atmosphere, the oxidation catalyst arranged on the upstream side of the exhaust gas oxidizes NO into NO ₂ .
The reduction catalyst arranged on the downstream side of the exhaust gas preferentially promotes the reaction between the hydrocarbons and NO _{2 over} the reaction between the hydrocarbons and oxygen to reduce NO ₂ into nitrogen.

The amount of the hydrocarbons added in the intermediate portion is not particularly limited, but an excessive amount than the amount necessary for the reductive decomposition of NO ₂ promotes the reduction reaction, so it is preferable to add it in excess. Usually, the amount of hydrocarbons used is present in about 20 to 2,000% excess, preferably about 30 to 1,500% excess of the stoichiometric amount required for reductive decomposition of NO ₂ .

Here, the theoretical amount of hydrocarbons required is
In the present invention, it is defined as hydrocarbons necessary for reductive decomposition of nitrogen dioxide (NO ₂ ), for example, 1,000 pp using propane as the hydrocarbons.
The theoretical amount of propane when the nitric oxide (NO) of m is reductively decomposed in the presence of oxygen is 200 ppm. In general,
Although depending on the amount of NOx in the exhaust gas, the amount of hydrocarbons present is about 50 to 10,000 ppm in terms of methane.

The hydrocarbons used in the present invention are not particularly limited, including those in a gaseous state and a liquid state, and any hydrocarbon that can be vaporized at the reaction temperature can be used. As a gas,
Specific examples include hydrocarbons such as methane, ethane, propane, ethylene, propylene and butylene.
Examples of liquids include petroleum hydrocarbons such as gasoline and light oil, alcohols such as methanol and ethanol, ethers such as dimethyl ether and ethyl ether, and oxygen-containing compounds such as ketones such as acetone and methyl ethyl ketone. be able to.

In the reaction, as described above, the oxidation catalyst is prepared on the upstream side of the exhaust gas, the reduction catalyst is prepared on the downstream side of the exhaust gas, the NOx-containing exhaust gas is passed through in the oxidizing atmosphere, and the reducing agent is added from the intermediate portion of both catalysts. This is done by adding certain hydrocarbons.

By the above steps, NOx in the exhaust gas can be reduced and decomposed. Further, by preparing a reactor having an oxidation catalyst at the final stage of exhaust gas and continuously passing the exhaust gas, reaction conditions can be improved. Depending on the situation, it is possible to oxidize and remove unreacted hydrocarbons and harmful components such as carbon monoxide that may be discharged.

The reaction temperature in the method of the present invention varies depending on the type of catalyst and hydrocarbons, but a temperature close to the temperature of the exhaust gas is preferable because exhaust gas heating equipment and the like are not required, and it is preferably about 100 to 800 ° C. About 200-600
Use within the range of ° C is effective. Here, when olefins are used as the added hydrocarbons, the temperature is preferably slightly lower than when paraffins are used. The reaction pressure is not particularly limited, the reaction proceeds at a normal exhaust pressure,
The space velocity (SV) at which the exhaust gas is introduced into the catalyst layer varies depending on other reaction conditions such as reaction temperature and required NOx and harmful component removal rates, and is not particularly limited, but is about 500.
100 to 100,000 Hr ^-1 , preferably about 1,000 to 7
It is in the range of 10,000 Hr ^-1 . In the method of the present invention, when treating the exhaust gas from the internal combustion engine, it is preferable that the catalyst be arranged downstream of the exhaust manifold.

[0039]

EXAMPLES Next, examples of the method of the present invention will be given, but the method of the present invention is not limited to these examples. Example 1 (Preparation of pentasil-type zeolite) In an aqueous solution prepared by dissolving 957 g of sodium silicate in 1,200 g of water, 96 g of aluminum sulfate and 80 g of concentrated sulfuric acid in 1,600 g of water.
g, an aqueous solution in which 360 g of sodium chloride is dissolved is added to 3
The mixture was gradually added with stirring for 0 minutes and mixed. Further, 120 g of tetrapropylammonium bromide was added to adjust the pH to 10. Charge this mixture into an autoclave and
Upon stirring at 5 ° C for 16 hours, crystallization occurred. After separating the product, it is washed with water and dried to form the base SiO ₂ / Al ₂ O ₃
= 34.5 pentasil type ZSM-5 zeolite was obtained.

(Preparation of Proton-Type Pentacyl-Type Zeolite) Ammonium nitrate 1 mol / l solution 500 ml
Then, 20 g of the above pentasil-type zeolite is charged into
The mixture was refluxed while stirring day and night, and then centrifuged. This was washed 5 times with pure water, dried overnight at 110 ° C., and air-baked at 500 ° C. for 3 hours to prepare a proton-type ZSM-5 zeolite.

(NOx removal reaction) 0.1 g of commercially available 0.5% platinum alumina as an oxidation catalyst, and H / ZSM-5 zeolite 0.1% prepared as described above as a reduction catalyst.
1 g of a normal pressure fixed bed flow reactor was charged (platinum alumina on the upstream side, H / ZSM-5 was placed on the downstream side, and a hydrocarbons injection port was provided in the middle part), and the reaction temperature shown in Table 1 Then, helium gas containing 1,000 ppm NO and 10% oxygen was flowed at a flow rate of 60 ml / min, and helium gas containing propane was injected from a hydrocarbons inlet at a flow rate of 2 ml / min. However, the injection amount of propane was changed so that the propane concentration in the mixed gas after the injection of propane was as shown in Table 1. The analysis of the reaction gas was performed using a gas chromatograph, and the reduction decomposition rate of NO was determined from the yield of generated nitrogen. Table 1 shows the reductive decomposition rate of NO.

COMPARATIVE EXAMPLE 1 The reaction was carried out in the same manner as in Example 1 except that platinum alumina as the upstream oxidation catalyst was not used and only 0.1 g of H / ZSM-5 was used as the catalyst. The results are also shown in Table 1. As is clear from Table 1, Comparative Example 1 has a lower reductive decomposition rate of NO than Example 1, and an oxidation catalyst (platinum alumina) is arranged on the exhaust upstream side to oxidize NO to NO ₂ in advance. It can be seen that this has the effect of increasing the reductive decomposition rate.

Example 2 The reaction was carried out in the same manner as in Example 1 except that the amount of H / ZSM-5 of the reducing catalyst used was 0.02 g, and the results are also shown in Table 1.

Comparative Example 2 The reaction was carried out in the same manner as in Example 2 except that platinum oxide as the oxidation catalyst on the upstream side was not used and only 0.02 g of H / ZSM-5 was used as the catalyst. The results are also shown in Table 1. As is clear from Table 1, Comparative Example 2 has a lower reductive decomposition rate of NO than Example 2, and an oxidation catalyst (platinum alumina) is arranged on the upstream side of exhaust gas to oxidize NO to NO ₂ in advance. It can be seen that this has the effect of increasing the reductive decomposition rate.

[0045]

[Table 1]

[0046] Example 3 oxidation catalyst, nitrate 38g ferric _{(Fe (NO 3) 3 ·} 9
H ₂ O) was dissolved in 300 ml of distilled water, 35 g of commercially available γ-alumina was added thereto with stirring, and 14% aqueous ammonia was added dropwise until pH 8 was reached to deposit iron hydroxide on the alumina. 50 in the air after filtration, washing with water and drying
It was prepared by firing at 0 ° C. for 3 hours. 1 g of this oxidation catalyst and 1 g of commercially available γ-alumina as a reduction catalyst,
In the same manner as in Example 1, the atmospheric pressure fixed bed flow reactor was charged,
At the reaction temperatures shown in Table 2, 1,000 ppm NO and 10
Helium gas containing 60% oxygen was flowed at a flow rate of 60 ml / min, and helium gas containing propane was injected from the hydrocarbon inlet at a flow rate of 2 ml / min. However, the injection amount of propane was such that the propane concentration in the mixed gas after the injection of propane was 640 ppm. Analysis of exhaust gas,
The reduction decomposition rate of NO was calculated in the same manner as in Example 1, and the results are shown in Table 2.

Comparative Example 3 The reaction was performed in the same manner as in Example 3 except that the iron oxide of the upstream oxidation catalyst was not used and only 1 g of alumina was used as the catalyst, and the results are also shown in Table 2. It was As is clear from Table 2, Comparative Example 3 has a lower reductive decomposition rate of NO than Example 3, and the oxidation catalyst (iron alumina) is provided on the upstream side of the exhaust gas.
It can be seen that arranging and oxidizing NO to NO ₂ in advance has the effect of increasing the reductive decomposition rate.

Example 4 The reaction was performed in the same manner as in Example 3 except that the amount of alumina used as the reducing catalyst was 0.1 g, and the results are also shown in Table 2.

Comparative Example 4 The reaction was carried out in the same manner as in Example 4 except that the iron oxide of the upstream oxidation catalyst was not used and only 0.1 g of alumina was used as the catalyst, and the results are summarized in Table 2. Showed. Table 2
As is clear from the above, Comparative Example 4 has NO compared to Example 4.
It can be seen that the reduction decomposition rate is low and that an oxidation catalyst (iron alumina) is arranged on the upstream side of exhaust gas to oxidize NO into NO ₂ in advance has the effect of increasing the reduction decomposition rate.

[0050]

[Table 2]

Example 5 (Preparation of Sodium-Type Pentacyl-Type Zeolite) A 0.1 mol / l solution of sodium nitrate was prepared, and the base ZSM-5 zeolite prepared in the same manner as in Example 1 was added thereto, The mixture was heated and refluxed with stirring for 1 day and then centrifuged. This was washed 5 times with pure water and dried overnight at 110 ° C. to prepare sodium-type ZSM-5 zeolite.

(NOx removal reaction) 0.5 g of commercially available 0.5% platinum alumina as an oxidation catalyst and 1 g of sodium-type ZSM-5 zeolite prepared as described above as a reduction catalyst are subjected to a reaction under a fixed pressure fixed bed flow system. After filling the device (platinum alumina on the upstream side, sodium type ZSM-5 on the downstream side and providing a hydrocarbons injection port in the middle), 1,000 ppm of NO and 10 were added at the reaction temperature shown in Table 3. Helium gas containing 60% oxygen was flowed at a flow rate of 60 ml / min, and helium gas containing propane was supplied from the hydrocarbon inlet at 2 ml / min.
It was injected at a flow rate of ml. However, the injection amount of propane is 320 pp in the mixed gas after the injection of propane.
It was set to m. The analysis of the reaction gas was performed using a gas chromatograph, and the reduction decomposition rate of NO was determined from the yield of generated nitrogen. Table 3 shows the reductive decomposition rate of NO.

COMPARATIVE EXAMPLE 5 Platinum-alumina as the upstream oxidation catalyst was not used, but only 1 g of sodium type ZSM-5 was used as the catalyst.
The reaction was carried out in the same manner as in Example 5, and the results are also shown in Table 3. As is clear from Table 3, Comparative Example 5 is Example 5
The reduction decomposition rate of NO is lower than that of, and it may be effective to increase the reduction decomposition rate by arranging an oxidation catalyst (platinum alumina) on the upstream side of exhaust gas and oxidizing NO to NO ₂ in advance. I understand.

Example 6 (Preparation of sulfuric acid-treated titanium oxide catalyst) 1 liter of distilled water was placed in a beaker, and 200 ml of commercially available titanium isopropoxide was gradually added with stirring to form a precipitate. 170 ml of concentrated nitric acid was gently added to this to dissolve the precipitate. Next, 200 ml of 25% ammonia water was added dropwise to adjust the pH to 8 to generate a precipitate again, and the precipitate was left standing for one day and night, filtered, washed with water and dried to obtain titanium hydroxide (titanic acid). 10 g of this titanium hydroxide was placed on a filter paper, 150 ml of 0.5 mol / l sulfuric acid was flowed, and then air dried. Finally, it was calcined in an air stream at 500 ° C. for 3 hours to obtain a sulfuric acid treated titanium oxide catalyst.

(NOx removal reaction) 0.5 g of iron-alumina prepared in Example 3 as an oxidation catalyst and 1 g of sulfuric acid-treated titanium oxide prepared as described above as a reduction catalyst were packed in an atmospheric fixed bed flow reactor. (Alumina on the upstream side,
Sulfuric acid-treated titanium oxide is arranged on the downstream side, and a hydrocarbons injection port is provided in the middle part), at a reaction temperature shown in Table 3, 1,0
Helium gas containing 00 ppm NO and 10% oxygen was flowed at a flow rate of 60 ml / min, and helium gas containing propane was injected from the hydrocarbon inlet at a flow rate of 2 ml / min. However, the amount of propane injected was such that the propane concentration in the mixed gas after propane injection was 320 ppm. The analysis of the reaction gas was performed using a gas chromatograph, and the reduction decomposition rate of NO was determined from the yield of generated nitrogen. Table 3 shows the reductive decomposition rate of NO.

Comparative Example 6 The reaction was performed in the same manner as in Example 6 except that iron-alumina of the upstream side oxidation catalyst was not used, and the catalyst used was only 1 g of sulfuric acid-treated titanium oxide, and the results are shown in Table 3. It is also shown. As is clear from Table 3, Comparative Example 6 has a lower reductive decomposition rate of NO than Example 6, and an oxidation catalyst (iron alumina) is arranged on the exhaust upstream side to oxidize NO to NO ₂ in advance. It can be seen that this has the effect of increasing the reductive decomposition rate.

[0057]

[Table 3]

As is clear from Tables 1 to 3, in the method of the present invention, NO which accounts for most of NOx in exhaust gas is NO.
It can be seen that the selective reduction of NOx with hydrocarbons efficiently occurs in the presence of excess oxygen by oxidizing to ₂ and using a specific reduction catalyst.

[0059]

As described in detail above, according to the method of the present invention, NOx in exhaust gas can be efficiently removed in an oxidizing atmosphere in which oxygen is excessively present. this is,
By oxidizing most NO of NOx in the exhaust gas to NO ₂ using an oxidation catalyst on the upstream side of the exhaust gas, the NO ₂ reduction efficiency by the reduction catalyst such as proton-type zeolite and acidic metal oxides is increased. Because.

As described above, according to the method of the present invention, N in exhaust gas is
By previously converting Ox to NO ₂ , NOx can be efficiently removed from exhaust gas from various facilities including diesel engine exhaust gas, which is extremely industrially valuable.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication location F01N 3/08 B 7910-3G 3/10 A 7910-3G (71) Applicant 000105567 Cosmo Oil Co., Ltd. 1-1-1, Shibaura, Minato-ku, Tokyo (74) Attorney-at-law 3 Chiga Kubota (1 outside) (72) Inventor Yoshiaki Kaneda 1-1-1 Higashi, Tsukuba, Ibaraki Prefectural Institute of Technology In-house (72) Hideaki Hamada 1-1, Higashi, Tsukuba-shi, Ibaraki Institute of Chemical Technology, Institute of Industrial Technology (72) Inventor Takehiko Ito 1-1-1-1, East, Tsukuba, Ibaraki Institute of Institute of Chemical Technology (72) Inventor Motoki Sasaki, 1-1, Higashi 1-chome, Tsukuba-shi, Ibaraki Institute of Chemical Technology, Institute of Industrial Technology (72) Inventor Mitsunori Tabata 1134 Gongendo, Satte City, Saitama Prefecture 2 (72) Inventor Fujio Suganuma 228-16 Shinden Shinjuku, Showa-cho, Kita-Katsushika-gun, Saitama (72) Inventor Tadao Nakatsuji 5-1, Ebishima-cho, Sakai-shi, Osaka Sakai Chemical Industry Co., Ltd. (72) Inventor Shimizu Hiromasu 5-1, Ebishima-cho, Sakai City, Osaka Prefecture Sakai Chemical Industry Co., Ltd.

Claims (2)

[Claims] 1. A nitrogen oxide in exhaust gas is converted into nitrogen dioxide in advance, and the exhaust gas is then heated in an oxidizing atmosphere in the presence of excess oxygen in the presence of hydrocarbons to form a proton type zeolite, an alkali metal type zeolite or A method for removing nitrogen oxides, which comprises contacting with one or more catalysts selected from acidic metal oxides. 2. The method for removing nitrogen oxides according to claim 1, wherein the acidic metal oxide is a metal oxide treated with a compound having a sulfate group.