JPH09117667A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance selectivity to the reduction reaction of NOx by hydrocarbons and thereby obtain a high conversion rate of NOx by allowing an active metal component to be borne by a carrier of mordenite containing titanium with the specified ratio of an outer specific surface area to a total specific surface area, in a catalyst for cleaning exhaust gas. SOLUTION: This catalyst for cleaning exhaust gas is used for the action to clean NOx contained in exhaust gas discharged from an internal combustion engine such as a gasoline engine through catalytic reduction using hydrocarbons as a reducing agent. An active metal component is borne by a carrier of mordenite containing titanium with the occupancy rate of an outer specific surface area for a total specific surface area. The mordenite containing titanium has a titanium atom in the skeletal structure of mordenite, and is of the needle crystal form. In addition, the average aspect ratio of the mordenite is at least, 3 or more, preferably 5-100. The active metal component to be borne by the carrier is copper, manganese or cobalt.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst that uses 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 by the diluted combustion method with hydrocarbon as a reducing agent. The present invention relates to an exhaust gas purifying catalyst used in a method of purifying by reduction.

[0002]

2. Description of the Related Art Conventionally, purification of NOx discharged from a fixed source (for example, a power plant boiler) has been effectively performed by an ammonia selective reduction method. The ammonia selective reduction method has a feature that NOx can be reduced in an atmosphere in which oxygen exists, but on the other hand, it is a mobile generation source (mainly automobiles) from the viewpoint of handling ammonia as a reducing agent.
It is said that it is difficult to use NOx emitted from the exhaust gas for purification. At present, NOx in the exhaust gas discharged from the mobile generation source is not sufficiently purified yet, and it is the main source of NOx polluting the environment.

In the case of purifying exhaust gas from a gasoline engine among mobile sources, a so-called three-way catalyst that oxidizes carbon monoxide and hydrocarbons contained in the exhaust gas into carbon dioxide gas and water and simultaneously reduces NOx to nitrogen. Has been put to practical use. However, even with a gasoline engine, it improves fuel economy,
In order to reduce the total amount of carbon dioxide gas generated by reducing the amount of fuel used as a whole, we are moving to a diluted combustion method with a high air-fuel ratio.In this case, the oxygen concentration in the exhaust gas becomes high, The NOx removal efficiency cannot be expected to be increased with the three-way catalyst. Similarly, the exhaust gas from diesel engines has a high oxygen concentration and cannot use a three-way catalyst.

Recently, NOx in exhaust gas having a high oxygen concentration
A catalytic reduction method for decomposing hydrocarbons with hydrocarbons as a reducing agent has been found, and research has been conducted in many fields. This catalyst is a crystalline aluminosilicate (zeolite) of various active metals including ZSM-5 type and mordenite type.
It is configured to be carried by (for example, Japanese Patent Laid-Open No. 3-5
2644), but a satisfactory catalyst has not yet been obtained.

Although the reaction mechanism of the NOx reduction reaction by hydrocarbons has not been elucidated in detail, it is considered that hydrocarbons are both NOx reducing agents and oxygen reducing agents, and they are a competitive reaction. So in general,
At high temperatures, the reaction between oxygen and hydrocarbons (combustion reaction) takes precedence, and the NOx reduction reaction decreases, so the NOx conversion rate is considered to decrease.

In the hydrocarbon reduction denitration method, the conversion rate of NOx varies depending on the type of zeolite as a carrier and the active metal species supported. In particular, the temperature at which the highest NOx conversion rate is exhibited is greatly influenced by the active metal species. There is a correlation between the temperature at which the maximum NOx conversion is reached and the oxide formation energy of the active metal, and metals with small oxide formation energy such as platinum and rhodium form oxides such as lanthanum and cerium on the low temperature side. Metals with high energy have temperatures on the high temperature side that show the highest conversion rates of NOx.

The exhaust gas discharged from the moving source has a wide variation range of the exhaust gas temperature from the outside air temperature at the engine start to the high temperature at the time of traveling.
Therefore, it is necessary for the NOx purification catalyst to have effective activity in a wide temperature range from low temperature to high temperature.
However, a zeolite-based catalyst supporting one kind of active metal has a temperature range that exhibits an effective NOx conversion rate of at most one hundred and several tens of degrees, so that it cannot cover the practical temperature range. In addition, when a catalyst supporting a metal having a large oxide formation energy and a catalyst supporting a metal having a small oxide formation energy are mixed or in the case of a catalyst supporting two kinds of metals at the same time, the oxide formation energy is small. The effect of metal precedes, and as a result, the conversion rate of NOx at high temperature becomes small.

Under such circumstances, the present inventors have arranged a zeolite carrying a metal having a small oxide formation energy on the center side and a zeolite carrying a metal having a high oxide formation energy on the outside (gas side). , A layered structure catalyst having a structure in which the oxide formation energy decreases sequentially from the outside to the inside (Japanese Patent Application No. 6-18188).
2). This layered structure catalyst can obtain an effective NOx conversion rate in a wide temperature range by a hydrocarbon reduction method in an atmosphere in which oxygen is present. The proportion must be reduced. Therefore, it is desired to provide a catalyst in which each catalyst in each layer has a higher NOx conversion rate.

On the other hand, as an example of using zeolite as a catalyst for decomposing hydrocarbons, a catalytic cracking catalyst (FCC catalyst) in the process of decomposing heavy oil to produce gasoline or the like.
Is widely known, and there are many studies on it, and it is known that the decomposition rate of heavy oil is affected by the type of zeolite, the degree of crystallinity, the type of acid sites and their distribution. According to the research conducted by the present inventors, it has been found that the influence of the strength and distribution of the acid point is large, and that it is greatly related to the yield and properties of gasoline.

[0010]

OBJECT OF THE INVENTION The object of the present invention is to include N contained in exhaust gas discharged from a fixed source or a mobile source such as a diesel engine or a gasoline engine by the diluted combustion method.
By slowing the reaction (combustion reaction) between hydrocarbon and oxygen when reducing and removing Ox with hydrocarbon, the selectivity for NOx reduction reaction by the hydrocarbon which is a competitive reaction with this is increased. However, a new catalyst showing a high NOx conversion rate is provided.

[0011]

Means for Solving the Problems From the experience of the researches of FCC catalysts, the present inventors have found that even in the case of hydrocarbon reduction denitration, the surface condition such as the nature and distribution of acid sites of zeolite influences. As a result of various investigations on the surface modification of zeolite, it was found that a catalyst using mordenite modified with titanium having a large external specific surface area as a carrier delays the combustion of hydrocarbons and enhances the selectivity of NOx reduction reaction. The present invention has been found to be improved, and the present invention has been completed.

That is, the present invention relates to an exhaust gas purifying catalyst for reducing and removing nitrogen oxides from hydrocarbon-rich exhaust gas containing nitrogen oxides and hydrocarbons by using hydrocarbons. Ratio is 7
% Of titanium-containing mordenite as a carrier, and an active metal component supported on the carrier, to an exhaust gas purifying catalyst.

The titanium-containing mordenite is characterized in that the ratio of the external specific surface area to the total specific surface area is 7% or more. Such a large external specific surface area is characterized by a scanning electric field as shown in FIG. It is presumed that it is derived from the shape of an aggregate of needle-like crystals of mordenite as seen from the micrograph.

A catalyst having a high NOx conversion cannot be obtained with a catalyst using a carrier whose external specific surface area occupies less than 7%. Although the reason why the conversion rate of NOx is improved by increasing the external specific surface area of titanium-containing mordenite is not clear, it is needle-shaped crystals (shape) that the external specific surface area works effectively in a reaction with a very high space velocity. Therefore, it is presumed that the gas diffusion into the crystal is facilitated and the reduction reaction of NOx and hydrocarbon is promoted. The proportion of the external specific surface area is preferably 9% or more, and the upper limit thereof is about 20%.

The total surface area is measured by the BET method, and the external surface area is measured according to J. H. De BOER
et al Journal of Catalys
It is measured by the Va-t plotting method described in is4, P319-323 (1965).

Further, the titanium-containing mordenite in the present invention contains titanium (Ti) atoms in the mordenite skeleton structure. The presence of Ti atoms in the zeolite skeleton structure was confirmed by infrared absorption spectrum, and it has been reported that an absorption peak appears near 970 cm ^-1 [eg,
B. Kausharr. et al. Catalysis
[Letter, 1, p-81 (1988)], the infrared absorption spectrum of the titanium-containing mordenite used in the present invention shows an absorption peak at 960 cm ⁻¹ as shown in FIG. It can be seen that it exists in the structure. In the titanium-containing mordenite used in the present invention, the Ti atom is used as an oxide in an amount of 0.01 to 2
It is desirable to contain 0% by weight, preferably 0.01 to 10% by weight.

The titanium-containing mordenite is acicular crystals as shown in FIG. 3, and its average aspect ratio is 3 or more, preferably 5 to 100. If the average aspect ratio is less than 3, the proportion of the external specific surface area of the mordenite may be small, which is not desirable.

The titanium-containing mordenite can be produced by the following method. That is, M ₂ O / Al ₂ O ₃ = 2.0 to 6.0 SiO ₂ / Al ₂ O ₃ = 10 to 50 TiO ₂ / Al ₂ O ₃ = 0.01 to 1.5 in terms of oxide molar composition ratio. A gel-like aqueous reaction mixture with a silica source, an alumina source, a titanium source and an alkali source in the range of H ₂ O / Al ₂ O ₃ = 150 to 500 (where M represents an alkali metal), preferably 0. Pre-aged at a temperature of ~ 60 ° C for 1-72 hours without stirring, then in an autoclave at a temperature of 100-200 ° C for 24-2
The titanium-containing mordenite can be obtained by performing a hydrothermal reaction for crystallization for 00 hours with stirring as necessary.

The active metal component supported on the titanium-containing mordenite of the present invention may be any active metal component usually used in the reductive denitration reaction by hydrocarbon, for example, known copper, manganese, cobalt, nickel, chromium, iron. ,
Examples thereof include metals such as cerium, lanthanum, praseodymium, platinum, rhodium and palladium, and oxides thereof.

The active metal component can be supported on the mordenite carrier by a known method such as an impregnation method. The supported amount of the active metal component may be an amount within the range of use of the usual active metal component, for example, 0.01 to 80 w as an oxide.
t%.

The catalyst of the present invention can be formed into a desired shape such as a spherical shape, a pellet shape, or a honeycomb shape by using other carrier components or a commonly used molding aid. However, when the amount of the titanium-containing mordenite is small, the catalytic activity decreases, so the amount of the titanium-containing mordenite is desired to be 50 wt% or more, preferably 70 wt% or more with respect to the total amount including the molding aid. .

Further, the exhaust gas purifying catalyst of the present invention can be used under the condition that is usually used for purifying NOx discharged from a mobile generation source, and is 150 to 800 ° C., preferably 200 to 600 ° C. Exhaust gas temperature, space velocity 5,000
The use of ⁰ to 300,000 hr ^-1 is preferable.

[0023]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Production Example 1 of Carrier 1 No. 3 water glass 1668.6 g having a silica concentration of 19.5 wt%.
While stirring, 20 wt% titanium trichloride solution 34
7.1 g was added. The resulting gel-like aqueous reaction mixture,
1915.3 g of 30 wt% silica sol, Na ₂ O
923.6 g of a sodium aluminate solution containing 8.5 wt% and Al ₂ O ₃ 11.0 wt% was added. About 10
After stirring for 1 minute until uniform, the mixture was allowed to stand at 30 ° C. for 72 hours for preliminary aging. After the pre-aging, the gel-like aqueous reaction mixture was transferred to an autoclave and aged by heating at 175 ° C. for 100 hours. After completion of aging, the reaction mixture was taken out, filtered, washed and dried after cooling to a temperature of 100 ° C. or lower, then subjected to NH ₄ ion exchange twice, and then 60
The titanium-containing mordenite-1 was obtained by firing at 0 ° C for 2 hours.

For this mordenite-1, the total specific surface area by the BET method and the external surface area by the Va-t plot method were determined, and the aspect ratio was determined by scanning electron micrograph. As a result, the ratio of the external specific surface area to the total specific surface area was 10.5%, and the needle-like mordenite had an average aspect ratio of 7.9. An X-ray diffraction pattern of the titanium-containing mordenite-1 is shown in FIG. 1, an infrared absorption spectrum is shown in FIG. 2, and a scanning electron micrograph is shown in FIG.

Production Example 2 of Carrier In 1916.6 g of No. 3 water glass having a silica concentration of 24 wt%,
20 wt% titanium trichloride solution 771.3 with stirring
g was added. H ₂ O was added to the obtained gel-like aqueous reaction mixture.
1827.1 g, solid silica 444 g, Na ₂ O
923.6 g of a sodium aluminate solution containing 8.5 wt% and Al ₂ O ₃ 11.0 wt% was added, and the mixture was sufficiently stirred until it became uniform. The gel-like aqueous reaction mixture was left to stand, pre-aged at 60 ° C. for 48 hours, transferred to an autoclave, and then heat-aged at 175 ° C. for 120 hours. After completion of the aging, the temperature was cooled to 100 ° C. or lower, the reaction mixture was taken out, filtered, washed, dried, further subjected to ammonium ion exchange, and then calcined to obtain titanium-containing mordenite-2.

For this mordenite-2, the total specific surface area by the BET method and the external specific surface area by the Va-t plot method were determined, and the aspect ratio was determined by scanning electron micrograph. As a result, the ratio of the external specific surface area to the total specific surface area was 9.6%, and the mordenite had an average aspect ratio of 7.3.

Example 1 (Catalyst production example) Cerium nitrate [Reagent grade 1, Kanto Chemical Co., Ltd.] 0.88
3 g was precisely weighed and dissolved in 4.4 g of pure water. To this cerium nitrate aqueous solution, 5.5 g of titanium-containing mordenite-1 obtained in Production Example 1 of carrier was added and mixed well. This mixture was dried at 120 ° C. for 2 hours and then calcined at 600 ° C. for 2 hours to obtain a catalyst powder. This catalyst powder was lightly crushed in a mortar and then pressed under the condition of 100 kg / cm ² , coarsely crushed, and intermediate particles having a diameter of 1.0 to 1.4 mm were collected by a sieve to obtain a Ce-supporting titanium-containing mordenite catalyst. (Catalyst-A). The CeO ₂ content of catalyst-A is 7.0%.

Example 2 (Catalyst production example) 0.851 g of copper nitrate [reagent grade 1, manufactured by Kanto Kagaku Co., Ltd.] was precisely weighed and dissolved in 4.4 g of pure water. The titanium-containing mordenite-1 obtained in Production Example 1 of the carrier was added to this copper nitrate aqueous solution.
5.5 g was added and mixed well. This mixture at 120 ° C
And dried at 600 ° C. for 2 hours to obtain a catalyst powder. Lightly crush the resulting powder in a mortar and then 1
It is pressed under the condition of 00 kg / cm ² , coarsely crushed, and the intermediate particles having a diameter of 1.0 to 1.4 mm are collected by a sieve, and Cu
A supported titanium-containing mordenite catalyst was obtained (Catalyst-B).
The CuO content of catalyst-B is 5.6%.

Example 3 (Catalyst Production Example) Titanium-containing mordenite-2 obtained in Support Production Example 2
Was used to obtain a Ce-supported titanium-containing mordenite catalyst in the same manner as in Example 1 (Catalyst-C). Catalyst-C CeO ₂
The content rate is 6.8%.

Example 4 (Production example of catalyst) Titanium-containing mordenite-2 obtained in Production example 2 of carrier
Was used to obtain a Cu-supported titanium-containing mordenite catalyst in the same manner as in Example 3 (Catalyst-D). The CuO content of catalyst-D is 5.5%.

Comparative Example 1 Mordenite commercially available as mordenite for carrier [Tosoh
A catalyst was prepared using HSZ-640HOA manufactured by K.K. The ratio of the external specific surface area to the total specific surface area of this mordenite was 5.0%, and the result of electron microscope observation was granular crystals. Using this mordenite, a Ce-supporting mordenite catalyst was obtained in the same manner as in Example 1 (Catalyst-E).
The CeO ₂ content of catalyst-E is 7.1%.

Comparative Example 2 Using the same mordenite as in Comparative Example 1, a CuO-supporting mordenite catalyst was obtained in the same manner as in Example 3 (Catalyst-
F). The CuO content of catalyst-F is 5.7%.

Example 5 (Evaluation of Activity of Examples and Comparative Examples) In order to evaluate the activity of the catalyst, hexane (C ₆ H ₁₄ ) was used as a hydrocarbon as a reducing agent to determine the conversion rate of NOx. The activity test device used for evaluation is composed of a normal flow type glass reaction tube, an automatic control type electric furnace and a gas mixing device. The catalysts obtained in Examples and Comparative Examples
The reaction tube was filled with 3 g, and the gas composition was NO = 400 p
pm, hexane (C ₆ H ₁₄ ) = 400 ppm, O ₂ = 5
%, H ₂ O = 5%, N ₂ = balanced mixed gas SV = 1
It was passed through a reaction tube under the condition of 10,000 hr ^-1 , and the hexane conversion rate and the NOx (NO) conversion rate at a predetermined temperature were obtained.

NO was analyzed by a chemiluminescence type NO analyzer and hexane was analyzed by a gas chromatograph using a NB-1 (GL Science) packed column.

Table 2 shows the hexane conversion rate and the NO conversion rate at the temperature showing the maximum NOx conversion rate of each catalyst. As shown in Table 2, in each of the examples, the hexane conversion rate is lower and the NO conversion rate is higher than those of the comparative examples. That is, a titanium-containing mordenite catalyst in which the ratio of the external specific surface area to the total specific surface area (O / T in Table 1) is high, the decomposition rate of hydrocarbons is low, the selectivity of NOx reduction reaction is high, and high denitration rate is shown. Is clear.

[0037]

[Table 1]

[0038]

[Table 2]

FIG. 4 shows changes in the hexane conversion rate and the NO conversion rate at each temperature in Example 1 and Comparative Example 1. The catalyst of Example 1 having a high ratio of the external specific surface area to the total specific surface area shows a lower hexane conversion rate than the catalyst of Comparative Example. Therefore, even in the catalyst of the comparative example, the hexane conversion rate can be set to the same value as that of the catalyst of Example 1 by lowering the temperature, but the NO conversion rate decreases as the temperature decreases. The catalysts A, B, C, D thus obtained according to the present invention
It can be seen that in both cases, the NO conversion rate is greatly improved over the catalysts E and F of the comparative examples. The effect is recognized as the effect of titanium-containing mordenite, which has a high ratio of the external specific surface area to the total specific surface area, and the reason why the effect is exhibited is not clear, but slowing the decomposition rate of hydrocarbons causes NO.
This is probably because the selectivity of the reduction reaction between x and hydrocarbons was improved.

The embodiments of the present invention will be listed below. 1. An exhaust gas purifying catalyst for reducing and removing nitrogen oxides from hydrocarbon-containing exhaust gas containing excess nitrogen oxides and hydrocarbons, wherein titanium has a ratio of the external specific surface area to the total specific surface area of 7% or more. An exhaust gas purifying catalyst, characterized in that a mordenite content is used as a carrier, and the carrier carries an active metal component. 2. In an exhaust gas purifying catalyst for reducing and removing nitrogen oxides from hydrocarbon-rich oxygen-containing exhaust gas containing nitrogen oxides and hydrocarbons, the ratio of the external specific surface area to the total specific surface area is 9 to 20%. An exhaust gas purifying catalyst, characterized in that titanium-containing mordenite is used as a carrier, and the carrier carries an active metal component. 3. 3. The exhaust gas purifying catalyst as described in 1 or 2 above, wherein the titanium-containing mordenite is acicular crystals having an average aspect ratio of 3 or more. 4. 3. The exhaust gas purifying catalyst as described in 1 or 2 above, wherein the titanium-containing mordenite is acicular crystals having an average aspect ratio of 5 or more. 5. The exhaust gas purifying catalyst according to the above 1, 2, 3 or 4, wherein the titanium-containing mordenite contains 0.01 to 20% by weight of Ti atoms as an oxide.

[0041]

[Effects] With the catalyst of the present invention, the NOx removal efficiency by the NOx reduction reaction by hydrocarbons could be significantly improved.

[Brief description of the drawings]

FIG. 1 shows an X-ray diffraction diagram of titanium-containing mordenite-1 obtained in Production Example 1 of a carrier according to the present invention.

FIG. 2 shows an infrared absorption spectrum of titanium-containing mordenite-1 obtained in Production Example 1 of the carrier of the present invention.

FIG. 3 shows a scanning electron micrograph of the surface of particles of titanium-containing mordenite-1 obtained in Production Example 1 of the carrier of the present invention.

FIG. 4 is a graph showing a hydrocarbon conversion rate and a NO conversion rate when the catalysts of Example 1 of the present invention and Comparative Example 1 were used.

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location B01D 53/36 102H 102A 104A (72) Inventor Shiro Nakamoto 13-2 Kitaminato-cho, Wakamatsu-ku, Kitakyushu, Fukuoka Prefecture Catalyst formation Industry Wakamatsu Factory

Claims (3)

[Claims] 1. In an exhaust gas purifying catalyst for reducing and removing nitrogen oxides from hydrocarbon-containing excess oxygen containing nitrogen oxides and hydrocarbons by hydrocarbons, the ratio of the external specific surface area to the total specific surface area is 7%. % Of titanium-containing mordenite as a carrier, and an active metal component is supported on the carrier, an exhaust gas purifying catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the titanium-containing mordenite is acicular crystals having an average aspect ratio of 3 or more. 3. The exhaust gas purifying catalyst according to claim 1, wherein the titanium-containing mordenite has a titanium atom in a mordenite skeleton structure.