JPH10272341A - How to remove nitrogen oxides - Google Patents

Abstract

(57) [Abstract] [Objective] In a method for removing NOx contained in exhaust gas discharged from a moving source such as a diesel engine or a gasoline engine by a dilution combustion method using a hydrocarbon as a reducing agent, a low temperature method is used. To provide a catalytic reduction removal method exhibiting high NOx removal activity in a wide temperature range from to high temperature. In a method for reducing and removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides by using hydrocarbons as a reducing agent, an upstream (high-temperature region) and a downstream (low-temperature region) of an exhaust gas flow are respectively provided. Nitrogen oxide removal catalyst (NOx catalyst) capable of mainly causing a hydrocarbon decomposition / oxidation reaction to generate hydrocarbons (active species) that reduce NOx in exhaust gas as a reducing agent in each region
A method for removing nitrogen oxides, comprising:

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst using nitrogen oxides (hereinafter referred to as NOx) contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or a gasoline engine produced by a dilution combustion method using hydrocarbon as a reducing agent. It relates to a method of removing by reduction.

[0002]

2. Description of the Related Art Conventionally, purification of NOx discharged from a fixed generation source (for example, exhaust gas from a power plant boiler) has been made effective by an ammonia selective reduction method. The ammonia selective reduction method has a feature that NOx can be reduced and removed in an atmosphere where oxygen is present, but on the other hand, a moving source (mainly an automobile) in terms of handling ammonia as a reducing agent.
It is said that it is difficult to use NOx discharged from methane for purification. NOx emitted from mobile sources is currently
NOx that is not sufficiently purified and pollutes the environment
Is a major source of

In the case of purifying exhaust gas from a gasoline engine among mobile sources, a three-way catalyst has been put to practical use.
However, gasoline engines are also shifting to a dilution combustion system with a high air-fuel ratio in order to improve fuel efficiency. In this case, the concentration of oxygen in the exhaust gas becomes high, so that the conventional three-way catalyst has a low NOx removal efficiency. It cannot be expected to increase.

[0004] Therefore, the NO in the exhaust gas having a high oxygen concentration is considered.
For x removal, a method of using hydrocarbon as a reducing agent other than ammonia which is difficult to handle and using zeolite carrying a metal as a NOx reduction catalyst has been researched and developed. In the case of removing NOx in the exhaust gas discharged from the moving source, the gas temperature, the gas composition, the gas amount, and the like vary greatly. For example, focusing on the gas temperature that has the greatest influence on NOx removal, the gas temperature has a fluctuation range of 800 ° C. from the outside air temperature. Therefore, it is important to remove NOx in the exhaust gas from the moving source efficiently in a wide temperature range from a low temperature to a high temperature.

Although various studies have been made on catalysts that exhibit sufficient NOx removal performance under the above-mentioned temperature fluctuations, it is difficult to obtain a highly active catalyst over a wide temperature range with a single catalyst. There have been proposed some methods of using the combination of the above. For example, JP-A-2-139
No. 040, focusing on the molecular weight of the hydrocarbon as a reducing agent, a copper ion-exchanged super cage large zeolite catalyst having a large diameter was placed upstream in the flow direction of the exhaust gas, and copper ion exchange was performed. By arranging a zeolite catalyst with a small supercage diameter on the downstream side, a high molecular weight hydrocarbon is captured as a reducing agent by the upstream catalyst and reacts with NOx, and a small molecular weight hydrocarbon is captured by the downstream catalyst. And reacts with NOx, so that it can be efficiently removed. In this case, since the supported metal is Cu, the oxidation of hydrocarbons is promoted from a low reaction temperature, so that hydrocarbons as a reducing agent are insufficient on the downstream side, so that effective NOx removal cannot be expected. Especially,
The tendency is large when the reaction temperature is high.

Japanese Patent Application Laid-Open No. 9-10583 discloses that
A catalyst has been proposed in which three catalyst layers each having a specific component composition for a high temperature range, a medium temperature range, and a low temperature range are arranged in series from the inflow side of exhaust gas. When the exhaust gas is at a high temperature, the catalyst for the high temperature region acts, when the exhaust gas is at a medium temperature, the catalyst for the medium temperature region acts, and when the exhaust gas is at a low temperature, the catalyst for the low temperature region acts. As a result, high NOx activity is maintained over a wide temperature range. However, in these conventional methods, a combination of catalysts in consideration of the action of a hydrocarbon as a reducing agent at a reaction temperature has not been studied.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an N gas contained in exhaust gas discharged from a moving source such as a diesel engine or a gasoline engine by a dilution combustion method.
In a method of removing Ox using a hydrocarbon as a reducing agent, it is an object of the present invention to provide a catalytic reduction removal method exhibiting high NOx removal activity in a wide temperature range from a low temperature to a high temperature.

[0008]

SUMMARY OF THE INVENTION The present inventors have proposed a conventional method for reducing and removing NOx in exhaust gas using a hydrocarbon as a reducing agent under an excess of oxygen using a catalyst. At the same time as the decomposition and oxidation reaction of hydrocarbons that generate hydrocarbons (active species) that react with NOx, other oxidation reactions occur, and the oxidation reaction further proceeds beyond the decomposition and oxidation reaction of hydrocarbons, and the reducing agent NO in exhaust gas as
Insufficient amount of hydrocarbons (active species) to reduce x
It has been found that the reduction reaction of Ox is not sufficiently promoted, and based on this finding, a hydrocarbon (active species) that reacts with NOx as a reducing agent in each temperature region in a high temperature region and a low temperature region in each temperature region. A catalyst for removing nitrogen oxides (NOx catalyst) capable of mainly causing a decomposition and oxidation reaction of generated hydrocarbons (hereinafter, a catalyst used in the high temperature range is a high temperature range catalyst, and a catalyst used in the low temperature range is a low temperature range catalyst. Is also combined.)
The present inventors have found a catalytic reduction removal method which exhibits a high NOx removal activity in a wide temperature range from a low temperature to a high temperature, and have completed the present invention.

The method for removing nitrogen oxides according to the present invention is a method for reducing and removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides by using a hydrocarbon as a reducing agent. And / or a catalyst in which a W metal component is supported on zeolite, and a catalyst in which at least one metal component selected from Cu, Co, Ni, Fe, Mn, and Cr is supported on zeolite is disposed as a low-temperature region catalyst. It is characterized by the following.

[0010]

Embodiments of the present invention will be described below in detail. In the present invention, the high-temperature region catalyst disposed on the upstream side with respect to the exhaust gas flow is a catalyst in which a rare earth element and / or a tungsten (W) metal component is supported on zeolite, and reduces NOx in the exhaust gas as a reducing agent in a high temperature region. It has the ability to decompose and oxidize hydrocarbons that produce hydrocarbons (active species). Particularly, a rare earth metal component is preferable, and specific examples include Ce and La. As the zeolite on which these metal components are supported, generally known zeolites such as mordenite type, ZSM type, faujasite type, β type, A type, chapasite type and erionite type can be used. is there. In particular, mordenite type, Z
SM-5 type zeolites and M in which some of the elements constituting the framework of these zeolites have been replaced by elements such as titanium
FI-type zeolites are preferred.

A zeolite catalyst supporting a rare earth element and / or a W metal component has a small catalytic activity for accelerating the decomposition and oxidation of hydrocarbons. Therefore, when the reaction temperature is low, the decomposition and oxidation of hydrocarbons hardly occur. When the reaction temperature is high (high temperature range), a hydrocarbon is decomposed and oxidized to generate a hydrocarbon (active species) that reacts with NOx as a reducing agent, so that many hydrocarbons (active species) act as a reducing agent. When present, the NOx reduction reaction is promoted. Usually, the reaction temperature at which the reaction occurs varies depending on the type of the high-temperature region catalyst, but is usually 350 ° C. or higher, preferably 35 ° C. or higher.
It is desirable to be in the range of 0 to 600 ° C.

The amount of the rare earth metal and / or tungsten is desirably in the range of 2 to 15% by weight, preferably 4 to 12% by weight as an oxide based on the weight of the catalyst.
The rare earth metal and / or tungsten component can be supported on the zeolite by a well-known method such as an ion exchange method and an inclusive method. The catalyst can be formed into a desired shape such as a pellet, a honeycomb, and a sphere by using an appropriate diluent, a binder and the like.

[0013] In the present invention, the low-temperature region catalyst disposed downstream of the exhaust gas flow includes Cu, Co, Ni, Fe, Mn,
A catalyst in which at least one metal component selected from Cr is supported on zeolite, and has the ability to decompose and oxidize hydrocarbons and reduce NOx at low temperatures. The catalyst may contain another active metal component as long as it contains at least one of the above-mentioned metal components. For example, a zeolite catalyst containing both metal components of Cu and Ce is exemplified. As the zeolite on which these metal components are supported, generally known zeolites such as mordenite type, ZSM type, faujasite type, β type, A type, chabazite type and erionite type can be used. In particular, β-type, mordenite-type, and ZSM-5-type zeolites are preferred.

Metals such as Cu, Co, Ni, Fe, Mn, and Cr have oxide generation energies of 60 to 350 (KJ).
/ Mole O atom), and the zeolite catalyst supporting the metal has a large catalytic action to accelerate the decomposition and oxidation of hydrocarbons. Therefore, when used in the high temperature range where the reaction temperature is high, undesired carbonization is undesirable. The oxidation reaction of hydrogen is promoted, and the oxidation reaction further proceeds beyond the decomposition oxidation reaction in which a large amount of hydrocarbons (active species) acting as a NOx reducing agent in the above-mentioned catalyst are present. Hydrocarbons (active species)
The reduction reaction of is reduced. A reaction temperature range in which a large amount of hydrocarbons (active species) acting as a reducing agent in the above-mentioned catalyst are present,
That is, the reaction temperature range in which the NOx reduction reaction is promoted is usually in the range of 200 to 450 ° C, preferably 250 to 400 ° C. However, it is preferable to use an appropriate low temperature region catalyst in accordance with the type of the high temperature region catalyst.

It is desirable that the amount of the metal component of the above-mentioned low-temperature region catalyst is in the range of 1 to 15% by weight, preferably 3 to 11% by weight as an oxide based on the catalyst. The metal component can be supported on zeolite by a known method such as an ion exchange method and an impregnation method. The catalyst is in the form of a pellet, a honeycomb, using an appropriate diluent, a binder, or the like.
A desired shape such as a spherical shape can be obtained.

In the method of the present invention, the high-temperature region catalyst disposed upstream and the low-temperature region catalyst disposed downstream with respect to the exhaust gas flow may be disposed in a single reactor, Further, each catalyst may be arranged in a plurality of reactors. Further, the ratio (volume ratio) of the high-temperature region catalyst to the low-temperature region catalyst is 10:90 to 90:10, preferably 1: 1.
It is desirable to be in the range of 5:85 to 60:40. In the middle between the upstream side and the downstream side, the upstream side catalyst and the downstream side catalyst may be partially mixed with each other. In the present invention, if the proportion of the catalyst to be arranged is out of the above range, the desired effect may not be obtained.

In the method of the present invention, since the exhaust gas discharged from an internal combustion engine such as a diesel engine is initially at a high temperature, the rare-earth and / or W metal which causes the decomposition and oxidation reaction of the hydrocarbon together with the hydrocarbon only in a high-temperature region. The component is brought into contact with the supported zeolite catalyst to promote the NOx reduction reaction. However, when the temperature of the exhaust gas decreases, the catalyst used in the above-mentioned high temperature range does not promote the decomposition and oxidation reaction of hydrocarbons, so that the hydrocarbon (active species) as a reducing agent is insufficient, and the reduction reaction of NOx is reduced. descend. Therefore, the reducing agent is brought into contact with a zeolite catalyst supporting at least one metal component selected from Cu, Co, Ni, Fe, Mn, and Cr, which mainly causes a decomposition and oxidation reaction of the hydrocarbon even in a low temperature range. Is supplied to promote the reduction reaction of NOx, so that a high NOx removal can be realized in a wide temperature range.

In the method of the present invention, conditions generally used for removing NOx discharged from a mobile source can be adopted, for example, 150 to 800 ° C., preferably 200 to 6 ° C.
Exhaust gas temperature of 00 ° C, space velocity 5,000-300,
A condition such as 000 hr ^-1 is adopted.

The hydrocarbon used as a reducing agent in the method for removing nitrogen oxides of the present invention is usually sufficient to be contained in the exhaust gas, but if necessary, a part of light oil or the like may be bypassed. May be supplied as a hydrocarbon component.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Reference Example 1 Catalyst Production Example 1 No. 3 water glass [SiO ₂ concentration 2 manufactured by Dokai Chemical Industry Co., Ltd.]
4wt%], silica water (diluted by adding 1046 g of pure water to 1392 g) and silica sol [SiO 2 (catalyst Chemical Co., Ltd.)
₂ concentration 30 wt%], 1886 g, and 464 g of an aqueous sodium aluminate solution (Al ₂ O ₃ concentration 22 wt%) were sequentially added with stirring. Next, this was aged at room temperature for 70 hours, and further aged at 175 ° C. for 96 hours for crystallization.
Thereafter, filtration, washing and drying are performed, and further, sodium ion of zeolite is exchanged with ammonium ion, and
Calcination was performed at 2 ° C. for 2 hours to obtain hydrogen mordenite. The cavern ratio of this mordenite was 10.0.

Next, 0.883 g of cerium nitrate [Reagent 1st grade, manufactured by Kanto Chemical Co., Ltd.] was weighed and dissolved in 4.4 g of pure water. 5.5 g of the above-mentioned hydrogen-type mordenite was added to the cerium nitrate aqueous solution, mixed well, and impregnated. The impregnated product was dried at 120 ° C. for 2 hours and calcined at 600 ° C. for 2 hours to prepare a cerium-supported mordenite powder. The powder contained 7.1 wt% of cerium as CeO ₂ . After crushing this powder lightly in a mortar, 100 kg /
Pressure molding was performed at a pressure of cm ² . Next, this molded product was roughly pulverized and then sieved to collect particles having a diameter of 2 to 4 mm to obtain a cerium-supported mordenite catalyst (Ce-M). Similarly, a lanthanum-supported mordenite catalyst (La-M),
A tungsten-supported mordenite catalyst (WM); a copper-supported mordenite catalyst (Cu-M), and a cobalt-supported mordenite catalyst (Co-M) were prepared.

Reference Example 2 Catalyst Production Example 2 No. 3 water glass [SiO ₂ concentration 2 manufactured by Dokai Chemical Industry Co., Ltd.]
[4 wt%] 2987 g of pure water 1188 g was added to dilute water glass, and 347 g of titanium trichloride aqueous solution [TiCl ₃ concentration 20 wt% manufactured by Tokai Chemical Co., Ltd.] and silica sol [SiO ₂ concentration 30 wt% of Catalyst Chemical Industry Co., Ltd.] 601g,
Sodium aluminate aqueous solution (Al ₂ O ₃ concentration 22 wt%) 4
63 g were added sequentially with stirring. Next, this was aged at room temperature for 70 hours, and further aged at 175 ° C. for 120 hours for crystallization. Thereafter, filtration, washing, and drying were performed, and further, sodium ion of zeolite was exchanged with ammonium ion, and calcined at 600 ° C. for 2 hours to obtain hydrogen-containing titanium-containing mordenite. The zeolite is TiO ₂
3.5 wt%, A1 ₂ O ₃ 10.5 wt%, SiO ₂ 8
It contained 6.0 wt%, and it was confirmed from the nuclear magnetic resonance spectrum that titanium was incorporated in the zeolite skeleton as a skeleton-forming element of the zeolite. Using this zeolite, 7.0 w of CeO ₂ was obtained in the same manner as in Reference Example 1.
A cerium-supported titanium-containing mordenite catalyst containing t% (Ce-TM) was prepared.

Reference Example 3 Catalyst Production Example 3 A sodium aluminate aqueous solution (A1 ₂ O ₃ concentration 1) was placed in a plastic container.
0.2 wt%) 46.4 g, aqueous caustic soda solution (concentration 4
6.3 g, TEAOH (Tetraethyl)
ammonium Hydroxide TEA20
253.6 g of pure water (concentration: 20 wt%) and 48.0 g of pure water were mixed and stirred well. 325 g of silica sol (SiO ₂ concentration of 30 wt%, Catalyst Chemical Industry Co., Ltd.) was gradually added thereto with stirring, and the mixture was further stirred for 30 minutes. Then
This was transferred to an autoclave and crystallized at 160 ° C. for 160 hours. After that, the crystal was taken out from the autoclave, filtered, washed and dried, and further exchanged ammonium ion for sodium ion of zeolite,
Calcination at 600 ° C. for 2 hours gave a hydrogenated β-zeolite. The cavan ratio of this zeolite was 29.3.
Using this zeolite, a copper-supported β-zeolite catalyst (Cu-β) containing 7.2 wt% of CuO was prepared in the same manner as in Reference Example 1 except that copper nitrate was used instead of cerium nitrate in Reference Example 1. did. Similarly, a chromium-supported β-zeolite catalyst (Cr-β), an iron-supported β-zeolite catalyst (Fe-β), and a manganese-supported β-zeolite catalyst (Mn-β) were prepared.

Reference Example 4 Catalyst Preparation Example 4 A copper-supported ZSM-5 catalyst (Cu-ZSM) was prepared according to the catalyst preparation method of Reference Example 1 using ZSM-5 as a zeolite.
A nickel-supported ZSM-5 catalyst (Ni-ZSM) was prepared.

Examples 1 to 12 and Comparative Examples 1 to 4 Using the catalysts prepared in Reference Examples 1 to 4, the NOx conversion was evaluated with the combinations of the catalysts shown in Tables 1 and 2. Test method: The test used an activity test device composed of a usual flow-type reaction tube, an automatically controlled electric furnace, and a gas mixing device. The reaction tube was filled with 20 cc of each combination of catalysts, and the gas composition was as follows: NO = 800 ppm, n-hexane (C ₆ H ₁₄ ) = 800 ppm, O ₂ = 5%, H ₂ O =
5%, N ₂ = balanced mixed gas with SV = 10,000
The reaction solution was allowed to flow through the reaction tube under the conditions of hr ^-1 , and the NO concentration at the outlet of the reaction tube was measured at predetermined temperatures by a chemiluminescence NO analyzer.
Ox conversion was determined. Table 1 shows the reaction results of the examples, and Table 2 shows the reaction results of the comparative examples. As can be seen from Tables 1 and 2, in the method of the present invention, the reaction temperature was from 250 ° C to 500 ° C.
It shows high NOx conversion over a wide range of ° C.

Reaction results of Examples

[Table 1]

[0028]

[Table 2] Reaction results of Comparative Examples

[0029]

According to the present invention, there is provided a method for removing NOx contained in exhaust gas discharged from a moving source such as a diesel engine or a gasoline engine by a dilution combustion method using a hydrocarbon as a reducing agent. A catalytic reduction removal method that exhibits high NOx removal activity over a wide temperature range up to and including:

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B01J 23/74 B01D 53/36 ZAB 102H (72) Inventor Kazuaki Takakura 13-2 Kitaminato-cho, Wakamatsu-ku, Kitakyushu-shi, Fukuoka Inside the Wakamatsu factory

Claims (3)

[Claims] 1. A method for reducing and removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides by using a hydrocarbon as a reducing agent, comprising: an upstream side (high temperature region) and a downstream side (low temperature region) with respect to the exhaust gas flow. In each region, a nitrogen oxide removing catalyst (NOx catalyst) capable of mainly causing a decomposition and oxidation reaction of hydrocarbons that generate hydrocarbons (active species) that reduce NOx in exhaust gas as a reducing agent in each region. A method for removing nitrogen oxides, comprising: 2. The method according to claim 1, wherein the catalyst disposed in the high temperature region is a rare earth and / or
Alternatively, a catalyst in which a W metal component is supported on zeolite, or a catalyst arranged in a low-temperature region catalyst is Cu, Co, Ni, Fe, M
The method for removing nitrogen oxides according to claim 1, wherein the catalyst is a catalyst in which at least one metal component selected from n and Cr is supported on zeolite. 3. The ratio (volume ratio) of the catalyst arranged on the upstream side (high temperature area) to the catalyst arranged on the downstream side (low temperature area) with respect to the exhaust gas flow is 10:90 to 90:10. 3. The method for removing nitrogen oxides according to 2.