JPH0796194A - Waste gas purification catalyst and method for removing nox - Google Patents

Abstract

PURPOSE:To efficiently remove NOx from waste combustion gas contg. oxygen and to enable efficient removal even in the presence of SOx. CONSTITUTION:This waste gas purification catalyst is made of a zeolite contg. indium and a group Ib metal of the Periodic Table. The molar ratio of SiO2 to Al2O3 in the zeolite is >=10. In the presence of this catalyst, NOx is removed from waste combustion gas contg. oxygen by using a hydrocarbon.

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. .

[0002]

2. Description of the Related Art Purification of air pollutants is a major social issue from the viewpoint of environmental protection. 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 factories, which are fixed sources, and automobiles, which are mobile sources, are said to be the cause of photochemical smog and are gases harmful to the human body. In particular, nitric oxide (NO) is difficult to purify, and has become the most important subject for study. Several methods for purifying nitrogen oxides in combustion exhaust gas have been proposed so far. 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 operated near the stoichiometric air-fuel ratio. However, these catalysts cannot be used because they cannot purify nitrogen oxides in the presence of excess 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 for this as well. 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 JP-A-63-283727 propose a method of purifying nitrogen oxides in a combustion gas containing oxygen in the presence of hydrocarbons by a zeolite catalyst containing copper, cobalt or the like. On the other hand, recently (B) Chemistr
In Y Letters P.1025-1026 (1992), ZSM-5 type zeolite ion-exchanged for gallium and indium is oxygen 1
It has been shown that the purification rate of nitrogen oxides is high under conditions as high as 0%.

[0007]

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. In addition, it is known that sulfur oxides are present in actual exhaust gas, which poisons the catalyst and greatly reduces the activity. Therefore, for practical use, it is also an important issue that the catalyst has sulfur oxide resistance.

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

[0009]

Means for Solving the Problems The present invention is intended to solve the above-mentioned problems, and the present inventors have proposed that in the presence of a catalyst composed of a zeolite containing indium and a metal of Group Ib of the periodic table, oxygen It has been found that nitrogen oxides in the combustion exhaust gas containing can be efficiently purified by hydrocarbons. Further, even 50,000 h ^-1 or more 100,000 h ^-1 or more high gas space velocities required for automobile exhaust gas purifying catalyst according to the catalyst of the present invention (GHSV), yet sufficiently even in the presence of sulfur oxides It has been found that it exhibits a high nitrogen oxide purification capacity.

That is, the present invention provides a purification catalyst comprising indium and a metal of Group Ib of the periodic table and having a SiO ₂ / Al ₂ O ₃ ratio of 10 or more in a molar ratio, and a hydrocarbon in the presence of such a catalyst. Is used to purify nitrogen oxides from combustion exhaust gas containing oxygen.

The zeolite referred to in the present invention is a crystalline aluminosilicate, and its composition is generally represented by the formula (1). _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ...... (1) ( wherein, n valence of the cation M, 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 zeolite is one in which Si, Al, and O are regularly bonded in a three-dimensional manner, and has various crystal structures depending on the difference in structural units. 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, mordenite, erionite, ferrierite,
There are chabazite and the like. As synthetic products, synthetic products of these natural products, X-type, Y-type, MFI, beta-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, ferrierite and beta 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, the zeolite synthesis method is 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 metals of Group Ib of the periodic table used in the present invention include copper, silver and gold.

In the present invention, the method of introducing indium and a metal of Group Ib of the periodic table is not particularly limited. Examples of the introduction method include an ion exchange method for exchanging cations in the zeolite with indium cations and metal cations of Group Ib of the periodic table, 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 metal salt of Group Ib of the periodic table and an alkaline solution such as aqueous ammonia is added to adjust the pH is also preferably used.

In the present invention, the catalyst preferably further contains hydrogen ions. As a method for introducing hydrogen ions into zeolite, there may be mentioned a method in which ion exchange is carried out directly with an aqueous acid solution or exchange with ammonium ions and then calcination is carried out. When the ion at the cation site is an organic nitrogen-containing cation, it is decomposed by firing and converted into a hydrogen ion.

Indium used in the present invention, Ib of the periodic table
The raw material compound of the metal or ammonium salt of Group I can be used in any form as long as it is a water-soluble salt. For example, sulfate,
Hydrochloride, nitrate, etc. can be mentioned. Examples of the acid aqueous solution include aqueous solutions of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and the like.

As the order of introduction, a method of first introducing a specific ion and a method of simultaneously introducing a specific ion are conceivable, but are not particularly limited. The indium content of the zeolite used in the present invention is preferably 0.4 to 12% by weight, more preferably 1 to 8% by weight.
The metal content of group b is preferably 0.6 to 20% by weight, more preferably 3 to 12% by weight. When hydrogen ions are contained, the hydrogen ions are preferably 0.1 to 1.0 equivalent, and more preferably 0.1 to 0.8 equivalent, relative to 1 equivalent of aluminum constituting the zeolite structure.

The catalyst of the present invention is prepared by introducing indium and a metal or hydrogen ion of Group Ib of the periodic table into zeolite, and then using, for example, an inorganic oxide such as silica or alumina or clay as a binder, spherical, columnar, honeycomb-shaped, etc. No. 1 may be molded into a suitable shape, or may be molded into a molded body made of alumina, cordierite or the like, such as a honeycomb. Also, a binder may be added to the zeolite before introduction of indium and metal of the Periodic Table Ib and / or hydrogen ion, and thereafter, indium and metal of the Periodic Table Ib and / or hydrogen ion may be introduced. good. In any case, it is not particularly limited.

The hydrocarbons used in the present invention are compounds composed of carbon and hydrogen, and are 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 about 1 to 10 carbon atoms, paraffins, naphthenes and cyclic unsaturated hydrocarbons. Specific examples of preferable hydrocarbons include ethylene,
Propylene, butylene, pentene, hexene, heptene, octene, nonene, methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclohexene and the like can be mentioned. 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 7 in terms of methane with respect to nitrogen oxides contained in the combustion exhaust gas,
More preferably, it is present in a 0.4 to 5 molar ratio. If the molar ratio is less than 0.2, the purification rate of nitrogen oxides will be low. On the other hand, if the molar ratio is more than 7, excess hydrocarbons 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 the operating conditions, but is typically 5 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. Further, the sulfur oxides contained in the exhaust gas markedly reduced the ability of the catalyst to purify nitrogen oxides. Therefore, according to the method of the present invention,
It exhibits sufficient nitrogen oxide purification capacity even at high gas space velocities such as automobile exhaust gas, and even in the presence of sulfur oxides.

Of course, the hydrocarbons contained in the combustion exhaust gas can be used to carry out the removal method of the present invention, but in order to increase the concentration of hydrocarbons in the combustion exhaust gas, existing fuels as hydrocarbons are used. It is possible to use a part of fuel oil such as light oil or 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 light oil or 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 in 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.

[0027]

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

(Catalyst Preparation) Catalyst 1 Na-type MFI having a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 4 times. 1.0% containing 17.5 g of indium nitrate trihydrate
Dispersed in 1 liter of aqueous solution, stirred overnight at room temperature and then filtered. After washing twice with 250 ml of water, it was dried at 110 ° C. overnight. The loaded amount of indium ion-exchanged with the zeolite was 3.8% by weight as a metal. Ammonium ion was 0.72 equivalent to 1 equivalent of aluminum constituting the zeolite. In addition to this 7.9
20 ml of a weight% silver nitrate aqueous solution was dropped from a buret,
Impregnated. After drying this at 110 ° C overnight, 30 ml / m
It was treated in an in-helium gas stream at 550 ° C. for 4 hours to decompose ammonium ions into hydrogen ions and then subjected to reaction.

Catalyst 2 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
20 g of type zeolite was dispersed in 1.0 liter of an aqueous solution containing 8.4 g of silver nitrate, stirred at room temperature overnight, and then filtered. This was washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated twice.
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. This was washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated twice. Then, it was dried at 110 ° C. overnight. The loaded amount of indium ion-exchanged with zeolite was 3.3% by weight as a metal, and the loaded amount of silver was 4.6% by weight.

Catalyst 3 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 35
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 4 times. 1.0% containing 17.5 g of indium nitrate trihydrate
Dispersed in 1 liter of aqueous solution, stirred overnight at room temperature and then filtered. After washing twice with 250 ml of water, it was dried at 110 ° C. overnight. The loaded amount of indium ion-exchanged with the zeolite was 2.9% by weight as a metal. Ammonium ion was 0.65 equivalent to 1 equivalent of aluminum constituting the zeolite. In addition to this 7.9
20 ml of a weight% silver nitrate aqueous solution was dropped from a buret,
Impregnated. After drying this at 110 ° C overnight, 30 ml / m
It was treated in an in-helium gas stream at 550 ° C. for 4 hours to decompose ammonium ions into hydrogen ions and then subjected to reaction.

Catalyst 4 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
Type zeolite 20g to 17.5g indium nitrate.3
It was dispersed in 1.0 liter of an aqueous solution containing a hydrate, stirred at room temperature overnight, and then filtered. Then, it was washed twice with 250 ml of water and then dried at 110 ° C. overnight. The amount of indium ion-exchanged with the zeolite was 6.5% by weight as metal. Further, 20 ml of a 10.5 wt% aqueous solution of chloroauric acid was added dropwise from a buret to impregnate it.

Catalyst 5 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 35
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 4 times. 1.0% containing 17.5 g of indium nitrate trihydrate
Dispersed in 1 liter of aqueous solution, stirred overnight at room temperature and then filtered. After washing twice with 250 ml of water, it was dried at 110 ° C. overnight. The loaded amount of indium ion-exchanged with the zeolite was 2.9% by weight as a metal. Ammonium ion was 0.65 equivalent to 1 equivalent of aluminum constituting the zeolite. In addition to this, 10.
20 ml of 5 wt% chloroauric acid aqueous solution was dropped from the buret to impregnate. After drying this at 110 ℃ overnight, 30ml
/ Min in helium gas flow at 550 ° C. for 4 hours to decompose ammonium ions into hydrogen ions, which were then subjected to reaction.

Catalyst 6 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
Type zeolite 20g to 17.5g indium nitrate.3
It was dispersed in 1.0 liter of an aqueous solution containing a hydrate, stirred at room temperature overnight, and then filtered. Then, it was washed twice with 250 ml of water and then dried at 110 ° C. overnight. The amount of indium ion-exchanged with the zeolite was 6.5% by weight as metal.

Catalyst 7 Na ₂ MFI with SiO ₂ / Al ₂ O ₃ molar ratio of about 35
Type zeolite 20g to 17.5g indium nitrate.3
It was dispersed in 1.0 liter of an aqueous solution containing a hydrate, stirred at room temperature overnight, and then filtered. Then, it was washed twice with 250 ml of water and then dried at 110 ° C. overnight. The supported amount of indium ion-exchanged with the zeolite was 4.1% by weight as a metal.

Catalyst 8 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
20 g of type zeolite was dispersed in 1.0 liter of an aqueous solution containing 8.4 g of silver nitrate, stirred at room temperature overnight, and then filtered. Then, it was washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated twice. It was dried at 110 ° C. overnight. The amount of silver ion-exchanged with the zeolite was 9.6% by weight as metal.

Catalyst 9 Na-type MFI with SiO ₂ / Al ₂ O ₃ molar ratio of about 25
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 4 times. Ammonium ion is the aluminum component of zeolite 1
It was 1.0 equivalent to the equivalent. 7.9% by weight to this
20 ml of an aqueous solution of silver nitrate was dropped from a buret to impregnate. After drying this at 110 ° C. overnight, it was treated at 550 ° C. for 4 hours in a 30 ml / min helium stream to decompose ammonium ions into hydrogen ions, and then subjected to reaction.

Catalyst 10 Na-type MFI with a SiO ₂ / Al ₂ O ₃ molar ratio of about 35
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 4 times. Ammonium ion is the aluminum component of zeolite 1
It was 1.0 equivalent to the equivalent. 20 ml of a 10.5% by weight aqueous solution of chloroauric acid was dropped from the buret and impregnated. After drying this at 110 ° C overnight, 30 ml / mi
It was treated in an n-helium gas stream at 550 ° C. for 4 hours to decompose ammonium ions into hydrogen ions and then subjected to reaction.

(Catalyst Evaluation) Examples 1 to 5 and Comparative Examples 1 to 5 Using the catalysts 1 to 10 prepared by the above method, under the reaction conditions shown in Table 1, oxygen 12 vol.% And nitric oxide 1000 pp.
m, a gas containing 250 ppm of ethylene as a hydrocarbon and 50 ppm of sulfur dioxide was reacted to examine the removal performance of nitric oxide. The conversion rate of NO was calculated from the conversion rate of NO to N ₂ . The results are shown in Table 2.

[0039]

[Table 1]

[0040]

[Table 2]

Examples 6 to 8 and Comparative Examples 6 to 8 Using the catalysts prepared by the above-mentioned method, under the reaction conditions shown in Table 3, oxygen 12% by volume, nitric oxide 1000 ppm, propane as hydrocarbon 167 ppm, sulfur dioxide 50 ppm
The reaction was carried out with a gas containing nitrogen and the removal performance of nitric oxide was examined. The conversion rate of NO was calculated from the conversion rate of NO to N ₂ .
The results are shown in Table 4.

[0042]

[Table 3]

[0043]

[Table 4]

As is clear from the results shown in Tables 2 and 4, a catalyst composed of zeolite containing indium and a metal of Group Ib of the periodic table and / or hydrogen ion and having a SiO ₂ / Al ₂ O ₃ molar ratio of 10 or more was prepared. If used, a small amount of hydrocarbons can be used to efficiently purify nitrogen oxides from combustion exhaust gas containing oxygen. Further, even if sulfur oxide, which is a catalyst poisoning component, is contained in the reaction gas, it works effectively.

[0045]

According to the present invention, nitrogen oxides can be efficiently removed from combustion exhaust gas containing oxygen. Further, it can be efficiently removed even in the presence of sulfur oxide.

Front page continuation (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B01J 20/18 Z 7202-4G B01D 53/36 102 D

Claims (9)

[Claims] 1. An exhaust gas purification catalyst comprising indium and a metal of Group Ib of the periodic table, a zeolite having a SiO ₂ / Al ₂ O ₃ ratio of 10 or more in a molar ratio. 2. The exhaust gas purifying catalyst according to claim 1, wherein the zeolite further contains hydrogen ions. 3. The exhaust gas purifying catalyst according to claim 1, wherein the content of indium is 0.4 to 12% by weight based on the weight of zeolite. 4. The content of Group Ib of the periodic table is 0.6 to 20% by weight based on the weight of the zeolite.
Exhaust gas purification catalyst described. 5. The exhaust gas purifying catalyst according to claim 2, wherein the content of hydrogen ions is 0.1 to 1.0 equivalent with respect to 1 equivalent of aluminum constituting the zeolite. 6. The exhaust gas purifying catalyst according to claim 1, wherein the zeolite is a zeolite having an MFI type structure. 7. In the presence of the catalyst according to claim 1 or 2,
A method for purifying nitrogen oxides, which comprises purifying nitrogen oxides from a combustion exhaust gas containing oxygen using hydrocarbons. 8. The temperature of the catalyst according to claim 7,
The method for purifying nitrogen oxides according to claim 7, wherein the temperature is 800 ° C. 9. The method for purifying nitrogen oxides according to claim 7, wherein hydrocarbons are present at a molar ratio of 0.2 to 7 in terms of methane with respect to nitrogen oxides contained in the exhaust gas.