JPH06246162A - Catalyst for purifying waste gas and method for purifying nitrogen oxide - Google Patents

Abstract

PURPOSE:To realize a sufficient rate of nitrogen oxide purification and to effectively remove nitrogen oxide from oxygen contg. waste combustion gas even when oxygen concentration is at a high level which is the practical one by using hydrocarbon in the presence of a catalyst for purifying waste gas consisting of zeolite contg. indium and rare earth metal. CONSTITUTION:A catalyst for purifying waste gas is constituted of zeolite contg. indium and rare earth metal and having >=10 SiO2/Al2O3 mole ratios. Nitrogen oxide is purified from oxygen contg. waste combustion gas in the presence of the catalyst by using hydrocarbon. Thus, even when the oxygen concentration is at a high level that is the practical one, sufficient rate of nitrogen oxide purification is realized to effectively remove the nitrogen oxide from the oxygen contg. waste combustion gas. Even at a gas hourly space velocity(GHSV) of >=100,000/h, a high rate of nitrogen oxide purification is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and a method for purifying nitrogen oxides, and more particularly to a method for efficiently purifying nitrogen oxides, which are air pollutants, from combustion exhaust gas containing oxygen. Is.

[0002]

2. Description of the Related Art From the viewpoint of environmental protection, purification of air pollutants is a major social issue. In particular, purification of combustion exhaust gas accompanying the expansion of industrial activities is an urgent issue at present.

Nitrogen oxides contained in combustion exhaust gas discharged from a factory which is a fixed generation source and an automobile which is a mobile generation source is a gas which is said to be a cause of photochemical smog and which is harmful to the human body, particularly nitric oxide. Since (NO) is difficult to purify, it is the most important subject for consideration.

Several methods have been proposed so far for purifying nitrogen oxides in combustion exhaust gas. For example, a method called a catalytic reduction method is a method of purifying NO into N ₂ and H ₂ O on a catalyst by using a reducing agent such as ammonia or hydrogen. However, since this method uses a dangerous reducing agent, it is necessary to take measures for recovery and leakage of the reducing agent, which is effective for large-scale fixed sources, but is not suitable for mobile sources such as automobiles.

On the other hand, many catalysts have been developed and are generally used for purification of exhaust gas of a gasoline engine whose exhaust gas is a reducing gas. However, these catalysts cannot be used because they cannot purify nitrogen oxides in the presence of oxygen.

By the way, catalytic decomposition of NO, that is, NO
The method of directly decomposing Pd into N ₂ and O ₂ only needs to pass the exhaust gas to the catalyst layer and is extremely simple, so that it has a wide range of applications. Various catalysts have heretofore been found in this regard. Although Pt, Cu, and Co-based catalysts have an effect on NO decomposing activity, they all have a problem that they are poisoned by generated oxygen. Since exhaust gas from a diesel engine and exhaust gas from a lean-burn gasoline engine usually contain oxygen, conventional catalysts cannot handle it, and development of a new method is desired.

Several catalysts have been proposed for such problems. For example, (A) US Pat.
No. 28 and Japanese Patent Application Laid-Open No. 63-283727 propose a method of purifying nitrogen oxides in a combustion gas containing oxygen in the presence of hydrocarbons with a zeoliite catalyst containing copper, cobalt or the like. On the other hand, recently (B) Chemis
Try Letters P.1025-1026 (1992) shows that ZSM-5 type zeolite ion-exchanged with gallium or indium has a high purification rate of nitrogen oxides under a high condition of 10% oxygen.

[0008]

However, in the known catalysts such as the above (A), when the oxygen concentration becomes high, the combustion reaction of hydrocarbons with oxygen increases, and the ability to purify nitrogen oxides remarkably deteriorates. There are a lot of problems to be realized. Further, in the above (B), since the hydrocarbon concentration in the automobile exhaust gas is not sufficiently present with respect to the nitrogen oxides, it is necessary to efficiently purify the nitrogen oxides with a slight amount of hydrocarbons. However, these catalysts have a problem that the ability to purify nitrogen oxides decreases as the hydrocarbon concentration decreases. Therefore, in order to put it into practical use, a catalyst having a further improved nitrogen oxide purification capability is required.

It is an object of the present invention to purify nitrogen oxides from combustion exhaust gas containing oxygen with hydrocarbons at a practical level.

[0010]

Means for Solving the Problems The present invention is to solve the above-mentioned problems. The present inventors have found that in the presence of a catalyst composed of zeolite containing indium and a rare earth metal, in a combustion exhaust gas containing oxygen. It was found that nitrogen oxides can be efficiently purified by hydrocarbons. Further, according to the catalyst of the present invention, 50,000 h ⁻¹ required for an automobile exhaust gas purification catalyst
More than 100,000 h ^-1 or more high gas space velocity (GH
It has been found that SV) shows a sufficiently high nitrogen oxide purification capacity.

That is, according to the present invention, the SiO ₂ / Al ₂ O ₃ molar ratio containing indium and rare earth metal is 10 or less.
An exhaust gas purifying catalyst comprising the above zeolite, and a nitrogen oxide purifying method characterized by purifying nitrogen oxides from a combustion exhaust gas containing oxygen in the presence of such a catalyst using hydrocarbons.

The zeolite referred to in the present invention is a crystalline aluminosilicate, and its composition is generally represented by the following formula (1).

XM _{2 / n} O.Al ₂ O ₃ .ySiO ₂ .zH ₂ O (1) (wherein, n is the valence of the cation M, and x is 0.8 to 2.0.
, Y is a number of 2.0 or more, and z is a number of 0 or more.) The basic structure of the zeolite is a regular three-dimensional bond of Si, Al and O.
It has various crystal structures. Many kinds of zeolites are known, but they are characterized by X-ray diffraction and have different names depending on their crystal structure. For example, as natural products, there are mordenite, erionite, ferrierite, chabazite, etc., and as synthetic products, synthetic products of these natural products, X
Type, Y type, MFI type and the like are known.

The zeolite used in the present invention has a molar ratio of SiO ₂ / Al ₂ O ₃ of 1 from the viewpoint of heat resistance and steam resistance.
It must be 0 or more. The zeolite structure is not particularly limited, but MFI, mordenite and ferrierite are preferable. Either a natural product or a synthetic product may be used, but in the former case, a synthetic product is preferably used since it contains impurities and requires purification.

Generally speaking, zeolite synthesis methods are as follows:
A suitable silica source, alumina source, alkali source, or optionally a metal compound (eg, F 2) in place of the alumina source.
e, Ga, etc.) and are mixed together and crystallized under hydrothermal conditions of about 100 to 250 ° C. A method of adding an organic substance called a template to the above mixture has also been proposed. Zeolites are generally commercially available, and they may be used.

Examples of the rare earth metal used in the present invention include lanthanoids such as lanthanum, cerium, praseodymium and neodymium, and yttrium.

In the present invention, the method of introducing indium and rare earth metal into zeolite is not particularly limited. Examples of the introduction method include an ion exchange method for exchanging cations in the zeolite with indium cations and rare earth metal cations, and an impregnation method for immersing the zeolite in a solution containing a target metal. In the case of the ion exchange method, a method in which zeolite is dispersed in a solution of indium or a rare earth metal salt and an alkaline solution such as aqueous ammonia is added to adjust the pH is also preferably used.

The raw material compounds of indium and rare earth metal used in the present invention can be used in any form as long as they are water-soluble salts. For example, sulfates, hydrochlorides, nitrates and the like can be mentioned. As the metal species, those that generate cations are preferable. As the order of introduction, a method of first introducing a specific metal and a method of simultaneously introducing a specific metal are conceivable, but are not particularly limited. The indium content of the zeolite used in the present invention is 0.4 to 12% by weight, preferably 1 to 8% by weight, and the rare earth metal content is 0.1.
-14% by weight, preferably 0.3-10% by weight.

The catalyst of the present invention is prepared by introducing indium and a rare earth metal into zeolite, and then molding it into an appropriate shape such as spherical, columnar or honeycomb by using an inorganic oxide such as silica or alumina or clay as a binder. Alternatively, it may be coated with a molded body made of alumina, cordierite, or the like, such as a honeycomb. It is also possible to add a binder before introducing indium and rare earth metal into the zeolite and mold the zeolite, and then introduce indium and rare earth metal. In any case, it is not particularly limited.

The hydrocarbon used in the present invention is a compound composed of carbon and hydrogen, usually so-called olefins, paraffins, cyclic compounds or hydrocarbons containing these compounds. It is preferably volatile and gaseous at the processing temperature of the present invention. More preferably, it is at least one hydrocarbon selected from olefins having 1 to 6 carbon atoms, paraffins, naphthenes and cyclic unsaturated hydrocarbons. Specific examples of preferable hydrocarbons include ethylene, propylene, butylene, pentene, hexene, methane, ethane, propane, butane, pentane, hexane, cyclopropane, cyclobutane, cyclopentane, cyclohexane and cyclohexene. It goes without saying that unburned hydrocarbons contained in the combustion exhaust gas are also preferably used. The hydrocarbons present on the catalyst are in a ratio of 0.2 to 5 in terms of methane with respect to nitrogen oxides contained in the combustion exhaust gas, more preferably 0.4.
It is preferable to exist in a molar ratio of from 4 to 4. If it is less than 0.2 mol ratio, the purification rate of nitrogen oxides will be low, while if it is more than 5 mol ratio, excess hydrocarbon will be present and a new hydrocarbon purification device will be required, which is not preferable.

The flue gas used in the present invention contains oxygen, and preferably contains 0.1% by volume or more of oxygen. This combustion exhaust gas is emitted from a normal internal combustion engine, boiler, or the like. The present invention is particularly effective for exhaust gas containing a large amount of oxygen, such as combustion exhaust gas from a diesel engine or a lean-burn gasoline engine. The oxygen concentration in the combustion exhaust gas of the diesel engine varies depending on operating conditions, but is typically 8 to 16%, and is 3 to 8% in the lean-burn gasoline engine.

The nitrogen oxide purification temperature according to the present invention is a catalyst layer temperature of preferably 200 to 800 ° C, more preferably 250 to 600 ° C. If this purification temperature is low, the purification of nitrogen oxides will be insufficient, and if the purification temperature is too high, the coexisting hydrocarbons will burn and the purification of nitrogen oxides will decrease, which is not preferable.

In the conventional method, when the combustion exhaust gas treatment rate per catalyst volume, that is, the gas space velocity is increased, for example, 5
Ten thousand h ^-1 or more, there is even more far from the practical level for reduces the purification performance of nitrogen oxides when 100,000 h ^-1 or more mobile sources such as automobiles. Therefore, according to the method of the present invention, a sufficient nitrogen oxide purifying ability is exhibited even at a high gas space velocity such as automobile exhaust gas.

Of course, the hydrocarbons contained in the combustion exhaust gas can be used to carry out the purification method of the present invention, but in order to increase the concentration of hydrocarbons in the combustion exhaust gas, existing fuels as hydrocarbons can be used. It is possible to use a part of fuel oil such as light oil and gasoline in a tank and directly add it to the catalyst layer provided on the exhaust gas outlet side through the bypass line without sending it to the engine. A reforming section may be provided, and a part of fuel oil such as gasoline may be subjected to a reforming treatment or the like and then added to the catalyst layer.

Further, in the case of a diesel engine, by delaying the fuel injection timing within the engine, it is possible to change the explosive combustion conditions and increase the hydrocarbon concentration in the exhaust gas. Further, in order to maintain the temperature of the catalyst layer in an appropriate range, combustion exhaust gas that has been brought to a predetermined temperature by a cooler or the like may be introduced, and if the combustion exhaust gas does not reach the predetermined temperature, the catalyst layer may be heated. Good.

[0026]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 (Catalyst preparation) 20 g of Na-type MFI-type zeolite having about 25 SiO ₂ / Al ₂ O ₃ was dispersed in 1.0 liter of an aqueous solution containing 32.0 g of cerium nitrate hexahydrate. Then
Stir overnight at room temperature, then filter. Then, water 250
Washed twice with ml. Next, this was dispersed in 1.0 liter of an aqueous solution containing 17.5 g of indium nitrate trihydrate, stirred at room temperature overnight, and then filtered. 250m of water
It was washed twice with 1 and dried at 110 ° C. overnight. The loaded amount of indium ion-exchanged with zeolite was 5.2% by weight as a metal, and the loaded amount of cerium was 3.4% by weight as a metal.

Example 2 20 g of Na-type MFI-type zeolite having about 35 SiO ₂ / Al ₂ O ₃ was dispersed in 1.0 liter of an aqueous solution containing 32.0 g of cerium nitrate hexahydrate, and the mixture was stirred at room temperature. Stir overnight and then filter. Then, it was washed twice with 250 ml of water. Next, this was dispersed in 1.0 liter of an aqueous solution containing 17.5 g of indium nitrate trihydrate, stirred at room temperature overnight, and then filtered. After washing twice with 250 ml of water, it was dried at 110 ° C. overnight. The amount of indium ion-exchanged with zeolite was 3.7% by weight as a metal, and the amount of cerium was 2.0% by weight as a metal.
Met.

Example 3 20 g of Na-type MFI-type zeolite having about 25 SiO ₂ / Al ₂ O ₃ was dispersed in 1.0 liter of an aqueous solution containing 12.5 g of lanthanum nitrate hexahydrate, and the mixture was stirred at room temperature. Stir overnight and then filter. Then, it was washed twice with 250 ml of water. This operation was repeated again in the same manner. It is then charged with 17.5 g of indium nitrate trihydrate 1.
Dispersed in 0 liter of aqueous solution, stirred overnight at room temperature and then filtered. After washing twice with 250 ml of water, 110 ℃
Dried overnight. The loaded amount of indium ion-exchanged with zeolite was 4.5% by weight as a metal, and the loaded amount of lanthanum was 0.6% by weight as a metal.

Comparative Example 1 20 g of Na-type MFI-type zeolite having about 25 SiO ₂ / Al ₂ O ₃ was dispersed in 1.0 liter of an aqueous solution containing 17.5 g of indium nitrate trihydrate, and the mixture was allowed to stand at room temperature. Stir overnight at rt and then filter. Then 250 ml of water for 2
After washing twice, it was dried at 110 ° C. overnight. The supported amount of indium ion-exchanged with zeolite is 6.
It was 5% by weight.

Comparative Example 2 20 g of Na-type MFI-type zeolite having SiO ₂ / Al ₂ O ₃ of about 25 was dispersed in 1.0 liter of an aqueous solution containing 32.0 g of cerium nitrate hexahydrate and allowed to stand at room temperature. Stir overnight and then filter. This operation was repeated again in the same manner. Then, wash with 250 ml of water twice, and after filtration 1
It was dried overnight at 10 ° C. The supported amount of cerium ion-exchanged with zeolite was 4.2% by weight as a metal.

Comparative Example 3 20 g of Na-type MFI-type zeolite having SiO ₂ / Al ₂ O ₃ of about 35 was dispersed in 1.0 liter of an aqueous solution containing 32.0 g of cerium nitrate hexahydrate, and the mixture was dispersed at room temperature. Stir overnight and then filter. This operation was repeated again in the same manner. Then, wash with 250 ml of water twice, and after filtration 1
It was dried overnight at 10 ° C. The supported amount of cerium ion-exchanged with zeolite was 2.8% by weight as a metal.

Comparative Example 4 20 g of Na-type MFI-type zeolite having SiO ₂ / Al ₂ O ₃ of about 25 was dispersed in 1.0 liter of an aqueous solution containing 12.5 g of lanthanum nitrate hexahydrate and allowed to stand at room temperature. Stir overnight and then filter. This operation was repeated again in the same manner. Then, wash with 250 ml of water twice, and after filtration 1
It was dried overnight at 10 ° C. The supported amount of lanthanum ion-exchanged with zeolite was 0.8% by weight as metal.

Examples 4 to 6 (Catalyst evaluation) Using the catalysts obtained in Examples 1 to 3, Table 1
12% by volume of oxygen and 1000 of nitric oxide under the reaction conditions shown in
The reaction was carried out with a gas containing 50 ppm of ethylene and 250 ppm of ethylene as a hydrocarbon to examine the removal performance of nitric oxide. NO
The conversion rate of was calculated from the conversion rate of NO to N ₂ . The results are shown in Table 2.

Comparative Examples 5-8 Using the catalysts obtained in Comparative Examples 1 to 4, reaction was carried out under the reaction conditions shown in Table 1 with a gas containing 12% by volume of oxygen, 1000 ppm of nitric oxide, and 250 ppm of ethylene as hydrocarbon. , The removal performance of nitric oxide was investigated. The conversion rate of NO is N
It was determined from the conversion rate of O to N ₂ . The results are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

As is clear from the results shown in Table 2, when a catalyst comprising a zeolite containing indium and a rare earth metal and having a SiO ₂ / Al ₂ O ₃ ratio of 10 or more is used, a trace amount of hydrocarbon is used to generate oxygen. It was found that the nitrogen oxides can be efficiently purified from the combustion exhaust gas containing the nitrogen oxides.

[0039]

EFFECTS OF THE INVENTION According to the present invention, even if the oxygen concentration is at a high level of practical use, a sufficient nitrogen oxide purification rate is exhibited, and nitrogen oxides can be efficiently removed from combustion gas containing oxygen. Also, 100,000 gas space velocity in ^{h -1} or more (G
Even with HSV), a high nitrogen oxide purification rate can be obtained.

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst comprising zeolite containing indium and a rare earth metal and having a SiO ₂ / Al ₂ O ₃ molar ratio of 10 or more. 2. A method for purifying nitrogen oxides, which comprises purifying nitrogen oxides from a combustion exhaust gas containing oxygen using hydrocarbons in the presence of the catalyst according to claim 1.