JP3254092B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, purification of NOx emitted 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 in an atmosphere in which oxygen is present, but it is difficult to use the purification of NOx discharged from a mobile source due to the handling of ammonia as a reducing agent. It has been. N in exhaust gas emitted from mobile sources (mainly automobiles)
At present, Ox is not sufficiently purified, and is a major source of NOx polluting the environment.

In the case of purifying exhaust gas from a gasoline engine among mobile sources, carbon monoxide and hydrocarbons contained in the exhaust gas are oxidized to carbon dioxide gas and water, and simultaneously, NOx is removed.
So-called three-way catalysts for reducing nitrogen to nitrogen have 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 and reduce the amount of carbon dioxide generated by reducing gasoline consumption. Since the concentration becomes high, it cannot be expected that the conventional three-way catalyst increases the NOx removal efficiency. Similarly, exhaust gas from a diesel engine also has a high oxygen concentration and cannot use a three-way catalyst.

Recently, NOx in exhaust gas with high oxygen concentration
A catalytic reduction method has been found which decomposes methane with a hydrocarbon as a reducing agent, and has been studied in many fields. This catalyst
Various active metals such as copper are supported on a crystalline aluminosilicate (zeolite) such as a ZSM-5 type or a mordenite type (see, for example,
-52644). However, a satisfactory catalyst has not yet been obtained.

[0005] In the case of denitration by the hydrocarbon reduction method, the reaction mechanism has not been elucidated in detail, but hydrocarbons are N
At the same time as a reducing agent for Ox, it is also a reducing agent for oxygen,
It becomes a competitive reaction. Therefore, generally, at high temperatures, the reaction between oxygen and hydrocarbons (combustion reaction) takes precedence, and N
Since the reduction reaction of Ox decreases, the NOx conversion rate decreases.

[0006] In the hydrocarbon reduction denitration method, the conversion rate of NOx varies depending on the type of zeolite used as a carrier and the type of active metal carried. In particular, the temperature at which the maximum conversion of NOx is exhibited is greatly affected by the active metal species. NOx
There is a correlation between the temperature at which the maximum conversion rate of the active metal and the oxide formation energy of the active metal are formed, such as platinum and rhodium.
Metals having a low oxide generation energy have temperatures on the low temperature side, and metals having a high oxide generation energy such as lanthanum and cerium have a temperature on the high temperature side that indicate the highest NOx conversion.

[0007] One of the features of the exhaust gas exhausted from the moving source is that the temperature of the exhaust gas varies widely from the outside air temperature when the engine is started to the high temperature during traveling. Therefore, it is necessary for the NOx purification catalyst to have an effective activity in a wide temperature range from a low temperature to a high temperature. But,
With a zeolite-based catalyst supporting one type of active metal, the temperature range at which an effective NOx conversion is exhibited is at most one hundred and several tens of degrees, so that it cannot cover a practical temperature range.
In addition, when a catalyst supporting a metal having a high oxide generation energy and a catalyst supporting a metal having a low oxide generation energy are mixed, or in the case of a catalyst simultaneously supporting two kinds of metals, the oxide generation energy is low. The effect of the metal precedes, and as a result, the conversion of NOx at high temperatures decreases.

Under these circumstances, the present inventors have arranged a zeolite supporting a metal having a low oxide generation energy on the center side and a zeolite supporting a metal having a high oxide generation energy on the outside (gas side). Have proposed a layered catalyst having a structure in which the oxide formation energy of the active metal gradually decreases from the outside to the inside (Japanese Patent Application No. Hei 6-181).
882). This layered catalyst is an effective NOx in a wide temperature range by a hydrocarbon reduction method in an atmosphere where oxygen is present.
Conversion can be obtained, but the N
In order to increase the Ox conversion, it is desirable that each of the catalysts in each layer has a higher NOx conversion.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a reaction between hydrocarbon and oxygen when reducing NOx contained in exhaust gas discharged from a diesel engine or a gasoline engine by a dilution combustion method with the hydrocarbon. It is an object of the present invention to provide a novel catalyst exhibiting a high NOx conversion rate by slowing down (combustion reaction), thereby increasing selectivity to a reduction reaction of NOx by a hydrocarbon which is competing with the reaction.

[0010]

In the case of a denitration method in which a hydrocarbon is used as a reducing agent in an atmosphere where oxygen is present, the elementary reaction mechanism is complicated. This is a competitive reaction with the NOx reduction reaction. On the high temperature side, the combustion reaction of hydrocarbons takes precedence, and the reduction reaction of NOx is suppressed. On the other hand, on the low-temperature side, the decomposition reaction of hydrocarbons is suppressed, and a shortage of reducing agent components such as CH radicals, which are decomposition products, is caused. As a result, the reduction reaction of NOx is also suppressed, and the conversion rate of NOx is similarly reduced.

As an example of using zeolite as a catalyst for cracking hydrocarbons, catalytic cracking catalysts (FCC catalysts) in the process of cracking heavy oil to produce gasoline, etc., are widely known. It is known that the decomposition rate of is affected by the type of zeolite, its crystallinity, the type of acid sites and their distribution. In particular, the influence of the strength and distribution of acid sites is significant, and greatly affects the yield and properties of gasoline. Therefore, in the case of hydrocarbon reduction denitration, it is predicted that the surface state such as the nature and distribution of acid sites of zeolite is expected to be affected, and the present inventors have conducted various studies on surface modification of zeolite. The present inventors have found that titanium-containing zeolites delay the combustion reaction of hydrocarbons and improve the selectivity of the NOx reduction reaction, thereby completing the present invention.

The present invention relates to an exhaust gas purifying catalyst for reducing and removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons by a hydrocarbon. A zeolite of mordenite type and (a) a titanium atom incorporated into a part of the framework structure of the zeolite crystal, or
(B) An exhaust gas purifying catalyst, which is a zeolite surface-modified by strongly binding or depositing titania on the surface of the zeolite, wherein the titanium-containing zeolite carries an active metal.

The zeolite is one or more zeolites selected from faujasite type and mordenite type.

The titanium-containing zeolite according to the present invention is not simply a mixture of zeolite and titanium oxide,
The zeolite is a zeolite in which a titanium atom is incorporated in a part of the framework structure of the zeolite crystal, or a zeolite in which titania is firmly bound or deposited on the surface of the zeolite and the surface is modified.

The zeolite reacts and crystallizes in the presence of a titanium source such as an aqueous titanium salt solution or titania sol in the reaction mixture composition at the time of zeolite synthesis, or
Add a titanium salt or titania sol to zeolite and allow it to react sufficiently, preferably 100 ° C. or higher, for example, 130 to 1
It is obtained by performing an aging reaction at a temperature of 80 ° C. for a long time, for example, for 2 to 4 days.

The method for synthesizing the above-mentioned titanium-containing zeolite will be described more specifically as follows.

In the first method, an aqueous solution of caustic soda, an aqueous solution of titanium salt and a silica sol are mixed with water glass as a silica source, which is a main component of zeolite. Next, an aqueous aluminate solution is added with stirring to form a reaction mixture composition. The reaction mixture composition is aged at a predetermined temperature for a predetermined time and crystallized to obtain a titanium-containing zeolite. The feature of this synthesis method is that zeolite is grown from silica-titanium hydroxy gel, and the obtained zeolite is incorporated into the zeolite skeleton based on the result of nuclear magnetic resonance analysis, and exists as a skeleton-forming element. It is that you are.

In the second method, zeolite is dispersed in water, and a separately prepared titanium salt aqueous solution is added and mixed. The mixed solution is aged at a predetermined temperature of 130 to 180 ° C. for a predetermined time in an autoclave to sufficiently react titanium and zeolite to obtain a titanium-containing zeolite. The feature of this method is
Although the obtained zeolite contains titanium by chemical analysis, the diffraction of titanium oxide is not obtained by X-ray diffraction, and unlike the zeolite obtained by the first method, the zeolite is obtained from the result of nuclear magnetic resonance analysis. There is no evidence of titanium in the skeleton, and most of the titanium is strongly bonded or deposited on the zeolite surface in an amorphous state.

As described above, an exhaust gas purifying catalyst in which titanium is incorporated in its skeleton or an active metal is supported on a zeolite bonded and deposited on a molecular level, performs a combustion reaction of hydrocarbons in an atmosphere in which oxygen exists. It has been found that it retards and promotes the NOx reduction reaction by hydrocarbons, and improves the selectivity of the NOx reduction reaction particularly at high temperatures. The zeolite used in the catalyst of the present invention is a zeolite whose surface is modified by reacting titanium with zeolite or titania sol arranged as an element forming a zeolite skeleton, and has a titanium content of 1 to 30 wt% as titanium oxide, preferably. Is desirably in the range of 3 to 20 wt%.

As the titanium-containing zeolite used in the catalyst of the present invention, the types of zeolites usually used for reducing and denitrifying hydrocarbons can be used. In particular, faujasite-type, mordenite-type, and ZSM-5-type zeolites are preferable, and faujasite-type and mordenite-type zeolites are preferable because of high denitration activity at high temperatures. The active metal to be supported on the zeolite is not particularly limited, and may be any active metal used for the denitration catalyst, for example, conventionally known copper, manganese, cobalt, nickel, chromium, iron, cerium, Examples thereof include metals such as lanthanum, protonium, platinum, rhodium, and palladium, or oxides thereof. The active metal can be supported on the zeolite by a known method, for example, an impregnation method. The loading amount of the active metal may be in the range of the conventional usage of the active metal, and is, for example, in the range of 0.01 to 80 wt% as an oxide.

The catalyst of the present invention can be formed into a desired shape such as a spherical shape, a pellet shape, a honeycomb shape, and the like using other carrier components and a commonly used forming aid. But,
When the amount of the titanium-containing zeolite is small, the catalytic activity is reduced.
It is desirably at least t%, preferably at least 70 wt%.

Further, the exhaust gas purifying catalyst of the present invention can be used under the conditions usually used for purifying NOx discharged from a mobile source, and is used at 150 ° C. to 800 ° C., preferably 200 ° C. to 800 ° C. Use at an exhaust gas temperature of 600 ° C. and a space velocity of 5,000 to 300,000 hr ⁻¹ is preferred.

[0023]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Production Example 1 of Carrier No. 3 water glass [containing 24 wt% of SiO ₂ manufactured by Dokai Chemical Industry Co., Ltd.] To a diluted water glass obtained by adding 487 g of pure water to 1231 g, a caustic soda aqueous solution (48 wt%) was added.
NaOH reagent primary grade) 225 g, titanium trichloride aqueous solution [containing 20 wt% of TiCl ₃ manufactured by Tokai Chemical Co., Ltd.] 34
7 g of silica sol (containing 63 wt% of SiO _2, manufactured by Sekiyu Kasei Kogyo KK) were added in an order of 331 g with stirring. Lastly, sodium aluminate (manufactured by Catalyst Chemical Industry Co., Ltd.)
463 g was added and dissolved, and the mixture was aged at room temperature for 70 hours and further aged at 95 ° C. for 60 hours to be crystallized. Thereafter, filtration, washing and drying (120 ° C. for 24 hours) were performed, and ammonium ion exchange was further performed to obtain an ammonium-exchanged faujasite type zeolite (zeolite A). Table 1 shows the chemical analysis values of the zeolite A. From the nuclear magnetic resonance spectrum of zeolite A, it was confirmed that titanium was incorporated in the zeolite skeleton as a skeleton-forming element of zeolite.

Production Example 2 of Carrier No. 3 water glass [containing 24 wt% of SiO ₂ manufactured by Dokai Chemical Industry Co., Ltd.] To a diluted water glass obtained by adding 1188 g of pure water to 2987 g, an aqueous solution of titanium trichloride [TiC
l ₃ 20 wt% containing Tokai Chemical Co., Ltd.] 347 g, silica sol [SiO ₂ 30 wt% containing Shokubai Kasei Kogyo (KK) 601g, aqueous solution of sodium aluminate (Al ₂
463 g (containing 22 wt% of O ₃ ) were sequentially added with stirring, aged at room temperature for 70 hours, and further aged at 175 ° C. for 120 hours for crystallization. Thereafter, filtration, washing, and drying (120 ° C. for 24 hours) are performed, and ammonia exchange is further performed.
A type titanium-containing mordenite (zeolite C) was obtained. Table 1 shows the chemical analysis values of the zeolite C. From the nuclear magnetic resonance spectrum of zeolite C, it was confirmed that titanium was incorporated in the zeolite skeleton.

[0026] Production Example 3 silica-alumina ratio of the support (SiO ₂ / Al ₂ O ₃ molar ratio) 11 H- type mordenite [manufactured by Tosoh Corporation] titania sol solution was separately prepared 315g [TiO ₂ 1 wt% containing Catalysts & Chemicals Industries (Trade name) 3500 g. This dispersion was aged at 150 ° C. for 72 hours. afterwards,
Filtration, washing, and drying (120 ° C. for 24 hours) were performed, followed by calcination at 600 ° C. for 2 hours to obtain H-type titanium-containing mordenite (zeolite D). Table 1 shows the chemical analysis values of zeolite D. As a result of the nuclear magnetic resonance spectrum of zeolite D, titanium was not incorporated in the zeolite framework.

Example 1 Cerium nitrate [first grade reagent, manufactured by Kanto Chemical Co., Ltd.] 0.88
3 g was precisely weighed and dissolved in 4.4 g of pure water. In this cerium nitrate aqueous solution, the zeolite A5.
5 g was added and mixed well. This mixture is heated at 120 ° C. for 2 hours.
After drying for 2 hours, the mixture was calcined at 600 ° C. for 2 hours to obtain a catalyst powder. After crushing this powder lightly in a mortar, 100kg
/ Cm ² and coarsely pulverized. Intermediate particles having a diameter of 1.0 to 1.4 mm were obtained by a sieve to obtain a Ce-supported titanium-containing faujasite catalyst. This catalyst is referred to as Catalyst A. Catalyst A contained 6.9 wt% of CeO ₂ .

Comparative Example 1 A Ce-supported faujasite catalyst B was prepared in the same manner as in Example 1 using a commercially available H-type faujasite-type zeolite (referred to as zeolite B manufactured by Catalyst Chemical Industry Co., Ltd.). This catalyst B contained 7.0 wt% of CeO ₂ . Table 1 shows the chemical analysis values of zeolite B.

Example 2 0.851 g of copper nitrate (first grade reagent, manufactured by Kanto Chemical Co., Ltd.) was precisely weighed and dissolved in 4.4 g of pure water. 5.5 g of zeolite C obtained in Carrier Production Example 2 was added to the aqueous copper nitrate solution and mixed well. The mixture is dried at 120 ° C. for 2 hours,
It was further calcined at 600 ° C. for 2 hours to obtain a catalyst powder. After this powder is lightly ground in a mortar, it is pressed under the condition of 100 kg / cm ² , coarsely ground, and sieved with a sieve at 1.0 to 1.0.
The intermediate particles having a diameter of 4 mm were taken to obtain a Cu-containing titanium-containing mordenite catalyst (catalyst C-1). In addition, C of catalyst C-1
The uO content is 5.6%.

Example 3 Example 2 was carried out using zeolite D obtained in carrier production example 3.
A Cu-supported titanium-containing mordenite catalyst was obtained in the same manner as in (Catalyst D-1). In addition, the CuO content of the catalyst D-1 is 5.6%.

Example 4 Cerium nitrate [reagent first grade, manufactured by Kanto Chemical Co., Ltd.] 0.88
3 g was precisely weighed and dissolved in 4.4 g of pure water. The zeolite C5. Obtained in Carrier Production Example 2 was added to this cerium nitrate aqueous solution.
5 g was added and mixed well. This mixture is heated at 120 ° C. for 2 hours.
After drying for 2 hours, the mixture was calcined at 600 ° C. for 2 hours to obtain a catalyst powder. After crushing this powder lightly in a mortar, 100kg
/ Cm ² and coarsely pulverized. 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 C-2). The CeO ₂ content of the catalyst C-2 was 7.0%.

Example 5 Example 4 was carried out using zeolite D obtained in carrier production example 3.
A Ce-supported titanium-containing mordenite catalyst was obtained in the same manner as in (Catalyst D-2). Note that CeO of the catalyst D-2 was used.
_{2 The} content is 7.0%.

Comparative Example 2 Titanium oxide (anatase type, manufactured by Ishihara Sangyo Co., Ltd.) may be blended with H-type mordenite (used as zeolite E, manufactured by Tosoh Corporation) so as to have a weight ratio of 5.5%. Mixing was performed to prepare a titanium oxide / mordenite mixture. 0.851 g of copper nitrate was precisely weighed, and 5.5 g of the above-mentioned titanium oxide / mordenite mixture was added to an aqueous solution of copper nitrate obtained by dissolving in 4.4 g of pure water and mixed well. Then, 12
It was dried at 0 ° C. for 2 hours and calcined at 600 ° C. for 2 hours to obtain a catalyst powder. After the obtained catalyst powder is lightly pulverized in a mortar, it is pressed under a condition of 100 kg / cm ² and coarsely pulverized. Was obtained (catalyst E-1). The CuO content of the catalyst E-1 was 5.6%.
It is. Table 1 shows the chemical analysis values of zeolite E.

Comparative Example 3 0.883 g of cerium nitrate was dissolved in 4.4 g of pure water, and the titanium oxide / mordenite mixture obtained in Comparative Example 2 was used.
5 g was added and mixed well. Thereafter, a Ce-supported titanium oxide / mordenite catalyst was obtained in the same manner as in Comparative Example 2 (catalyst E-2). In addition, the CeO ₂ content of the catalyst E-2 is 7.0%.

Comparative Example 4 Using zeolite E, the same operation as in Example 3 was carried out to obtain CuO.
Was obtained (catalyst E-3) containing 5.6 wt% of Cu.

Comparative Example 5 The same operation as in Example 4 was performed using zeolite E and CeO
_Thus, a Ce-supported mordenite catalyst containing 7.0 wt% of ₂ was obtained (catalyst E-4).

Example 6 (Evaluation of Activity of Catalysts of Examples and Comparative Examples) The activity test apparatus used for evaluating the conversion of hydrocarbons and the conversion of NOx of a catalyst is a normal flow-type glass reaction tube, an automatic control electric furnace. And a gas mixing device. 0.3 g of the catalyst obtained in each of the examples and comparative examples was charged into a reaction tube, and the composition of the gas was NO = 400 ppm, hexane (C ₆ H ₁₄ ) = 400 ppm, O ₂ = 5%, and H ₂ O = 5.
%, N ₂ = balanced mixed gas SV = 10,000 h
The mixture was passed through a reaction tube under the condition of r ⁻¹ , and the hexane conversion and the NOx conversion at each predetermined temperature were determined.

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

Table 2 shows the hexane conversion at 400 ° C. for each catalyst. Generally, NO from hydrocarbons
In the reduction reaction of x, although the detailed reason is unknown, when the conversion of hydrocarbons is high, the conversion of NOx tends to decrease. As shown in Table 2, the hexane conversion of each catalyst using titanium-containing faujasite-type zeolite and titanium-containing mordenite-type zeolite as carriers was smaller than that of the comparative example, and the decomposition rate was slow. This tendency is obvious, and the tendency is particularly remarkable in faujasite (zeolite A) and mordenite (zeolite C) in which titanium is substituted in the skeleton.

FIG. 1 shows the NOx conversion rates of the catalyst A of the example and the catalyst B of the comparative example, and FIG. 2 shows the catalysts C-2 and C-2 of the example.
The result of NOx conversion of D-2 and the catalysts E-2 and E-4 of a comparative example is shown. The catalysts A, C-2 and D-2 of the present invention are:
All were NO for the catalysts B, E-2, and E-4 of the comparative examples.
It can be seen that the x conversion is greatly improved. The effect increases as the temperature increases, and a remarkable effect is seen in the catalyst of the mordenite-type zeolite whose skeleton is substituted. Although the reason for the effect is not clear, it is considered that the selectivity of the reduction reaction between NOx and the hydrocarbon was improved by reducing the decomposition rate of the hydrocarbon.

[0041]

[Table 1] * 1: Commercially available H-type faujasite type zeolite manufactured by Kasei Kagaku Co., Ltd. * 2: Commercially available H-type mordenite manufactured by Tosoh Corporation

[0042]

[Table 2]

[0043]

The catalyst of the present invention exhibits a high NOx conversion over a wide temperature range, so that the catalyst for the exhaust gas such as a diesel engine or a gasoline engine by a dilution combustion method can be used.
It is useful as a catalyst for removing Ox.

[Brief description of the drawings]

FIG. 1 is a graph showing the denitration ratio at each reaction temperature of a catalyst A of Example 1 and a catalyst B of Comparative Example 1 according to the present invention.

FIG. 2 shows catalysts C-2 and D of Examples 4 and 5 according to the present invention.
4 is a graph showing the denitration ratio at each reaction temperature of catalysts E-2 and E-4 of Comparative Examples 3 and 5.

Continuation of the front page (72) Inventor Shiro Nakamoto 13-2 Kitaminato-cho, Wakamatsu-ku, Kitakyushu-shi, Fukuoka Prefecture Inside the Wakamatsu Plant of Catalyst Chemicals Co., Ltd. (56) References JP-A-4-334527 (JP, A) JP-A-4-193348 (JP, A) JP-A-4-29747 (JP, A) JP-A-6-15179 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21 / 00-38/74

Claims (1)

(57) [Claims] 1. An exhaust gas purifying catalyst for reducing and removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons, wherein the titanium-containing zeolite is of faujasite type or mordenite type. And (a) a zeolite in which a titanium atom is incorporated in a part of the skeleton structure of the zeolite crystal, or (b) a zeolite in which titania is strongly bonded or deposited on the surface of the zeolite and the surface is modified. And an active metal supported on the titanium-containing zeolite.