JPH06126187A - Removing method for nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a method for decreasing nitrogen oxide in exhaust gas containing excess oxygen which is discharged from an internal combustion engine such as an automobile. CONSTITUTION:Rare earth elements are so carried on zeolite having the mol ratio of silica to alumina of >=10 that the content of rare earth elements is regulated to two times or more for number of moles of alumina contained in zeolite. Thereafter a catalyst incorporating copper ions into the said zeolite is brought into contact with exhaust gas containing nitrogen oxide, hydrocarbon, carbon monoxide and excess oxygen.

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 for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas discharged from an internal combustion engine of an automobile or the like, and further to nitrogen from exhaust gas under excess oxygen. The present invention relates to a method for reducing oxides.

[0002]

2. Description of the Related Art At present, nitrogen oxides, carbon monoxide and hydrocarbons, which are harmful to the human body in exhaust gas emitted from gasoline engines, are ternary in which platinum, rhodium and palladium are supported on a carrier. It has been removed by the catalyst. However, with respect to exhaust gas from a diesel engine, oxygen concentration in the exhaust gas is higher than that from a gasoline engine, and therefore reduction denitration by a three-way catalyst is difficult.

Further, in recent years, a lean-burn gasoline engine has been developed in order to reduce fuel consumption due to reduction of carbon dioxide emission. However, since the exhaust gas of this lean burn gasoline engine is in an oxygen excess atmosphere, it is difficult to denitrate with the conventional three-way catalyst as described above, and the method for removing nitrogen oxides has not been put to practical use.

As a method for removing nitrogen oxides from exhaust gas having excess oxygen, a selective catalytic reduction method on V ₂ O ₅ / TiO ₂ using ammonia as a reducing agent and an absorption method to an alkaline solution have been used so far. It is known, but in any case, the range of use is limited, and it is difficult to apply it to automobiles and the like which are mobile sources.

Recently, it has been reported that a zeolite catalyst ion-exchanged with a transition metal can reduce and remove nitrogen oxides in exhaust gas in the presence of excess oxygen without adding a selective reducing agent such as ammonia. JP-A-63-283727 and JP-A-1-130735).

However, transition metal-containing, especially copper-containing zeolite catalysts are
There is a problem that the activity is significantly deteriorated, and it is necessary to improve the catalyst performance and durability.

As problems with the copper-containing zeolite catalyst, it is presumed that agglomeration of copper ions as an active metal and formation of coke as a catalyst poison are considered. To solve these problems, copper ions and cerium are added to zeolite. A catalyst in which one of rare earth elements such as lanthanum and neodymium is ion-exchanged has been proposed (JP-A-3-89942). In addition, it has been reported that a catalyst having an alkaline earth metal impregnated and supported in addition to copper and a rare earth element improves durability (Japanese Patent Laid-Open No. 3-202).
157).

On the other hand, an exhaust gas purifying catalyst containing only rare earth elements is also known. For example, a zeolite catalyst carrying ion-exchanged rare earth elements such as cerium and praseodymium as active metal species has been proposed (Misono, et al., Che.
m. Lett. , P1001, 1991). However, this catalyst has not been put into practical use because its activity is lowered by treating it at a high temperature and it does not have sufficient durability.

In the catalyst proposed in Japanese Patent Laid-Open No. 3-89942, the rare earth element is made to coexist at the ion exchange point together with the copper ion which is the active point, so that the supported amount is less than the exchange capacity of zeolite. Especially SiO ₂ / Al ₂ O ₃
When zeolite with a high molar ratio is used, the exchange capacity of cations decreases, so that it becomes impossible to carry a sufficient amount of rare earth element by ion exchange, resulting in insufficient durability. On the other hand, a loading method such as impregnation loading is easy to control the loading amount, but there is a risk that the dispersibility of the loaded metal species may be reduced, and pores may be clogged. Therefore, it is necessary to adopt an optimal supporting method. Generally, there is a contradictory relationship between the performance improvement due to an increase in the carried amount and the state of the supported metal, particularly the performance improvement due to the dispersibility. Japanese Patent Laid-Open No. 3-20
The catalyst proposed in Japanese Patent No. 2157 contains copper and a rare earth element by an impregnation method, and it is considered that the dispersibility of the rare earth element is low. Further, in the impregnation method, the ion exchange of copper ions is difficult to proceed and it exists as an oxide or an aggregate, so that it does not have sufficient activity and durability. For this reason, conventional zeolite catalysts containing rare earth elements and copper ions have not been put into practical use because their durability is insufficient.

[0010]

It is an object of the present invention to use a catalyst which is less likely to undergo thermal deterioration and has excellent durability, and nitrogen oxides from exhaust gas in excess of oxygen discharged from automobiles and the like, especially lean-burn engines. There is a need to provide a more efficient method of removing.

[0011]

Means for Solving the Problems The inventors of the present invention have made earnest studies in view of these circumstances, and as a result, as a result of supporting rare earth elements on a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 10 or more, copper It was found that a catalyst containing ions and having a rare earth element content of at least twice the number of moles of alumina in the zeolite has excellent nitrogen oxide removing ability and durability. Furthermore, the zeolite containing rare earth elements was calcined, and it was found that the catalyst containing copper ions has excellent nitrogen oxide removing ability and durability, and the present invention has been completed. The effect of firing is considered as follows. By calcining the zeolite containing the rare earth element, the supported rare earth can be stabilized. Further, by heat treatment (vacuum firing) in a reducing atmosphere, a low-valence oxide or an oxide phase in an oxygen defect state is partially formed. Therefore, a redox cycle (redox) occurs between the oxide species in the reduced state and the oxide species in the oxidized state, and adsorption and activation of nitrogen oxides and hydrocarbons are promoted. Further, it has been proposed that the denitration reaction of the present invention proceeds with the redox of copper ions, and the redox of rare earth elements facilitates the redox of copper ions as active species.

That is, according to the present invention, a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 10 or more is loaded with copper ions after supporting a rare earth element, and the rare earth element is doubled with respect to the number of moles of alumina in the zeolite. A method for removing nitrogen oxides, which comprises contacting the catalyst containing the above with an exhaust gas in excess of oxygen containing nitrogen oxides, hydrocarbons and carbon monoxide, and further calcining a zeolite containing a rare earth element. It is intended to provide the above-mentioned method using a catalyst, which is characterized by containing copper ions later.

The present invention will be described in detail below.

[0014] Zeolites typically _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where n is the valence of the cation, x is a number in the range of 0.8 to 1.2, y has a composition of 2 or more and z has a composition of 0 or more). Among them, the zeolite used in the present invention has a SiO ₂ / Al ₂ O ₃ molar ratio of 10 or more. The upper limit of the SiO ₂ / Al ₂ O ₃ molar ratio is not particularly limited, but if the SiO ₂ / Al ₂ O ₃ molar ratio is less than 10, the heat resistance and durability of the zeolite itself will be low, so that Sufficient heat resistance and durability cannot be obtained. Generally, a SiO ₂ / Al ₂ O ₃ molar ratio of about 10 to 1000 is used.

The zeolite constituting the catalyst used in the present invention may be either a natural product or a synthetic product, and the production method of these zeolites is not particularly limited, but typically, mordenite, ferrierite, Y, ZSM-
Zeolites such as 5, ZSM-11, ZSM-12 and ZSM-20 can be used. In addition, these zeolites,
N as it is or treated with ammonium salt, mineral acid, etc.
It can also be used as the catalyst of the present invention after ion-exchange with H ₄ type or H type.

The catalyst used in the present invention is prepared by first adding a rare earth element to the above zeolite and then adding copper ions. When the rare earth element is contained after the copper ion is contained, or when the copper ion and the rare earth element are contained at the same time, the copper is supported on the zeolite surface and becomes inactive in the nitrogen oxide removal reaction. ,
Catalytic activity decreases.

In the method of the present invention, the rare earth element can be contained by an acetate decomposition method, an impregnation-supporting method, an evaporation-drying method, or the like. In order to improve the dispersibility of the supported component,
The acetate decomposition method is preferred. In the ion exchange method, there is a risk that the content of the copper ion, which is an active site, decreases due to the exchange of the rare earth element. In the acetate decomposition method, the supporting component metal is supported as an intermediate such as hydroxyacetate or carbonate in a uniform slurry, so that the dispersibility of the supporting component is high. For acetate decomposition, for example, trivalent acetate aqueous solution
It can be performed by heating at 0 to 100 ° C. for a predetermined time.

The rare earth element is not particularly limited, and cerium, lanthanum, yttrium, praseodymium, etc. can be used.

The amount of rare earth element added during the decomposition of acetate is preferably 5 to 20 with respect to the number of moles of alumina in the zeolite. If the amount of the rare earth element added is less than 5, the rare earth element content of the catalyst used in the present invention may be less than 2 with respect to the number of moles of alumina in the zeolite, in which case sufficient durability can be obtained. Disappear. Also,
Even if it exceeds 20, there is a possibility that the effect corresponding to it may not be obtained.

Copper ions can be generally contained by an ion exchange method, an impregnation-supporting method, an evaporation-drying method, etc.
The ion exchange method is desirable in order to sufficiently introduce copper ions into the ion exchange site.

As the ion exchange method, a commonly used method, for example, a method of ion exchange using an aqueous solution containing copper ions can be adopted. Copper ion can be used in the form of a soluble salt, and as the soluble salt, nitrate, acetate, oxalate, chloride and the like can be used.

The amount of copper ions added during ion exchange is preferably 0.1 to 10 with respect to the number of moles of alumina in the zeolite. If the amount of copper ions added is less than 0.1, the copper content of the catalyst of the present invention may be low, and sufficient catalytic performance may not be obtained. May not be obtained. Regarding the processing conditions, the temperature is usually 10 to 10
A temperature of 0 ° C., a time of 1 hour to 10 days may be sufficient. Further, the ion exchange operation can be repeated if necessary. Further, at the time of ion exchange, an ammonium aqueous solution may be added to the exchange mother liquor for ion exchange.

It is essential that the rare earth element content of the catalyst prepared by the above method is at least twice the number of moles of alumina in the zeolite. The upper limit is not particularly limited, but is preferably 20 and more preferably 10 with respect to the number of moles of alumina in the zeolite. If the rare earth element is less than 2 with respect to the alumina in the zeolite, sufficient catalytic performance and durability cannot be obtained. The content of copper ions is not particularly limited, but is preferably 0.1 to 1.5 times the number of moles of alumina in the zeolite. Copper ion is 0.1 with respect to the number of moles of alumina in the zeolite.
If it is less than the above range, sufficient catalyst performance and durability may not be obtained. Further, even if it exceeds 1.5, there is a possibility that the effect corresponding to it may not be obtained.

Furthermore, the rare earth element can be stabilized by firing the zeolite containing the rare earth element.

The calcination can be carried out by vacuum calcination, air calcination, steam calcination, etc., among which vacuum calcination is preferable. The firing temperature is 300 to 1000 ° C., preferably 400 to 600 ° C. If the firing temperature is 300 ° C. or lower, the carried rare earth element may not be sufficiently stabilized. Further, even if the temperature exceeds 1000 ° C., the rare earth element may be sintered and a sufficient supporting effect may not be obtained. The firing time is preferably 0.5 hours or more in order to sufficiently stabilize the rare earth element. The degree of vacuum in performing vacuum firing is not particularly limited, but preferably 0.1 torr or less.

The exhaust gas purifying catalyst of the present invention can be used by mixing with a binder such as clay mineral and molding. Further, it is also possible to form zeolite in advance and to add copper ions and rare earth elements to the formed body. The binder used for forming the zeolite is not particularly limited, but clay minerals such as carion, attapulgite, montmorillonite, bentonite, allophane and sepiolite, silica, alumina and the like can be used. Alternatively, it may be a binderless zeolite molded product obtained by directly synthesizing a molded product without using a binder. Further, it is also possible to wash coat the zeolite on a honeycomb substrate made of cordierite or metal.

Removal of nitrogen oxides from the oxygen-excess exhaust gas can be carried out by bringing the exhaust gas purification catalyst described above into contact with the exhaust gas. It is essential that the exhaust gas contains nitrogen oxides, hydrocarbons, and carbon monoxide. Excess oxygen excess exhaust gas targeted by the present invention, carbon monoxide contained in the exhaust gas, refers to exhaust gas containing excess oxygen than the amount of oxygen required to completely oxidize hydrogen and hydrocarbons, Specific examples of such exhaust gas include exhaust gas discharged from an internal combustion engine of an automobile or the like, particularly exhaust gas burned in a state where the air-fuel ratio is large.

The concentration of each component gas in the exhaust gas treated in the present invention is not particularly limited, but normally, the nitrogen oxide content is 50%.
To 2000 ppm, hydrocarbons from 10 to 5000 pp
m, oxygen is 0.1 to 20%.

The space velocity and temperature of the exhaust gas to be treated are not particularly limited, but preferably space velocity (volume basis) 5
00 to 500000 hr ^-1 , temperature 100 to 800
° C, more preferably space velocity 2000 to 20000
The temperature is ⁰ hr ⁻¹ and the temperature is 200 to 600 ° C.

[0030]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Example 1 <Preparation of catalyst 1> ZS according to Example 5 of JP-A-59-54620.
An M-5-like zeolite was synthesized. It had the following composition, expressed as the molar ratio of oxides in the anhydrous base.

[0032] _{1.0Na 2 O · Al 2 O 3} · 40SiO 2 The sodium-form ZSM-5; concentration is accurately weighed to 50g is 7 times the alumina mole number contained therein to zero. It was added to a 07 mol / L cerium acetate aqueous solution, and cerium was carried by the acetate decomposition method at 80 ° C. for 20 hours. After solid-liquid separation and washing with water, 20 at 110 ℃
Dried for hours. The cerium and sodium contents of this cerium-containing zeolite were examined by chemical analysis, and as a result, they had the following composition expressed in mol of oxide on an anhydrous basis.

[0033] 2.67CeO ₂ · 0.94Na ₂ O · Al
₂ O ₃ · 40SiO ₂ 0.1mo of the cerium-containing zeolite was precisely weighed to double the number of moles of alumina in the zeolite.
The mixture was poured into a 1 / L copper acetate aqueous solution and stirred at 80 ° C. for 20 hours. After solid-liquid separation, it was thoroughly washed and dried at 110 ° C. for 20 hours to obtain catalyst 1.

The contents of copper ion, cerium and sodium in this catalyst were examined by chemical analysis, and as a result, they had the following composition in terms of oxide molar ratio in anhydrous base.

1.06 CuO.2.65 CeO ₂ .0.
_{24Na 2 O · Al 2 O 3} · 40SiO 2 Example 2 <Preparation of Catalyst 2> cerium prepared in Example 1 containing ZSM-5; under vacuum 10g (degree of vacuum 0.01 torr), heating rate 100 ° C.
The temperature was raised to 500 ° C./hr and the temperature was maintained for 1 hour. Then, the cerium-containing zeolite cooled to room temperature under vacuum was put into a 0.1 mol / L copper acetate aqueous solution that was precisely weighed so as to be twice the number of moles of alumina in the zeolite, and the mixture was heated at 50 ° C. for 20 hours. It was stirred. After solid-liquid separation, it was thoroughly washed and dried at 110 ° C. for 20 hours to obtain catalyst 2.

The contents of copper ions, cerium and sodium in this catalyst were examined by chemical analysis, and as a result, they had the following composition in terms of oxide molar ratio on an anhydrous basis.

0.69CuO.2.61CeO ₂ .0.
24Na ₂ O · Al ₂ O ₃ · 40SiO ₂ Example 3 <Preparation of catalysts 3 and 4> Instead of the vacuum calcination treatment of Example 2, dry air or air containing 10% of water vapor in terms of gas volume is 500 ° C.
Catalysts 3 (dry air calcination) and 4 (steam calcination) were prepared in the same manner as in Example 2 except that the calcination treatment was carried out for 1 hour.

The contents of copper ions, cerium and sodium in these catalysts 3 and 4 were examined by chemical analysis, and as a result, they had the following compositions represented by the molar ratio of oxide on an anhydrous basis.

Catalyst 3: 0.68CuO ・ 2.62CeO
₂・ 0.29Na ₂ O ・ Al ₂ O ₃・ 40SiO ₂ catalyst 4: 0.66CuO ・ 2.59CeO ₂・ 0.19
Becomes 15 times the alumina mole number contained a 10g _{therein; Na 2 O · Al 2 O} 3 · 40SiO 2 Example 4 sodium-form ZSM-5 prepared in Example 1 <Preparation of Catalyst 5> Thus, it was added to the cerium acetate aqueous solution having a concentration of 0.07 mol / L precisely weighed, and cerium was carried by the acetate decomposition method at 80 ° C. for 20 hours. After solid-liquid separation, it was washed with water and then dried at 110 ° C. for 20 hours. As a result of examining the cerium and sodium contents of this cerium-containing zeolite by chemical analysis, the cerium-containing zeolite had the following composition represented by an oxide molar ratio on an anhydrous basis.

[0040] 3.83CeO ₂ · 0.96Na ₂ O · Al
₂ O ₃ .40SiO ₂ This cerium-containing zeolite was vacuum-baked in the same manner as in Example 2, and then 0.1 mol / L of copper acetate was precisely weighed to double the number of moles of alumina in the zeolite. It was put into an aqueous solution and stirred at 50 ° C. for 20 hours. After solid-liquid separation, washed thoroughly and dried at 110 ° C. for 20 hours to obtain catalyst 5
Got

The contents of copper ion, cerium and sodium in this catalyst 5 were examined by chemical analysis, and as a result, the catalyst 5 had the following composition expressed in terms of oxide molar ratio on an anhydrous basis.

0.71CuO.3.80CeO ₂ .0.
22Na ₂ O · Al ₂ O ₃ · 40SiO ₂ Example 5 <Preparation of catalyst 6> Lanthanum loading and vacuum at 500 ° C. for 1 hour in the same manner as in Example 2 except that the rare earth element cerium was replaced with lanthanum. After firing (vacuum degree 0.01 torr), copper ion exchange was performed to prepare catalyst 6.

The contents of copper ion, lanthanum and sodium in the catalyst 6 were examined by chemical analysis. As a result, the catalyst had the following composition in terms of oxide molar ratio on an anhydrous basis.

0.70CuO.2.31La ₂ O ₃ .0.
18Na ₂ O · Al ₂ O ₃ · 40SiO ₂ Example 6 <Catalyst activity evaluation test> Catalysts 1 to 6 were each press-molded and then crushed to 12 to 20.
The particles were sized into a mesh. Each sized catalyst (2 cc) was filled in a normal pressure fixed bed flow reaction tube and used for the reaction. As a reaction pretreatment, a gas having the composition shown below (hereinafter referred to as a reaction gas) is used as 400
While circulating at 0 ml / min, raise the temperature to 550 ° C,
Hold for 30 minutes. Then, the NOx removal rate was measured at an arbitrary temperature between 300 and 550 ° C. (reaction 1). The space velocity (volume basis) at this time was 120,000 hr ⁻¹ . Table 2 shows the NOx removal rate in reaction 1 at each temperature. The NOx removal rate is expressed by the following equation.

Further, a durability treatment was carried out at 800 ° C. for 5 hours while circulating a reaction gas through the catalyst 2 cc of which particle size was adjusted to 12 to 20 mesh. Then, the same pretreatment as in Reaction 1 was performed, and the NOx removal rate was measured under the same reaction conditions (Reaction 2). Table 3 shows the NOx removal rate at each temperature in Reaction 2.

X _NOX = {([NOx] in- [NOx] ou
t) / [NOx] in} × 100 X _NOX : NOx removal rate [NOx] in: NOx concentration of inlet gas [NOx] out: NOx concentration of outlet gas

[0047]

[Table 1]

Comparative Example 1 <Preparation of Comparative Catalyst 1> The sodium type ZSM-5 prepared in Example 1 was used; 200
g to 2 with respect to the number of moles of alumina contained therein
It was added to an aqueous solution of copper acetate having a concentration of 0.1 mol / L precisely doubled, and 2.5% aqueous ammonia was immediately added to adjust the pH of the slurry to 10.5, followed by stirring at room temperature for 15 hours. . After solid-liquid separation, it was thoroughly washed and dried at 110 ° C. for 20 hours to obtain comparative catalyst 1. As a result of investigating the copper ion content of this comparative catalyst 1 by chemical analysis, it had the following composition expressed in terms of oxide molar ratio on an anhydrous basis.

1.05CuO.Al ₂ O ₃ .40SiO ₂ Comparative Example 2 <Preparation of Comparative Catalyst 2> Sodium ZSM-5 prepared in Example 1:10 g
Is 3.5 with respect to the number of moles of alumina contained therein.
It was added to a 0.07 mol / L cerium acetate aqueous solution precisely weighed so as to double the amount, and cerium was carried by the acetate decomposition method at 80 ° C. for 20 hours. The cerium and sodium contents of the obtained cerium-containing zeolite were examined by chemical analysis, and as a result, they had the following composition in terms of oxide molar ratio on an anhydrous basis.

[0050] 1.74CeO ₂ · 1.03Na ₂ O · Al
₂ O ₃ .40SiO ₂ This cerium-containing zeolite was subjected to the same copper ion exchange as in Example 1 to obtain Comparative Catalyst 2. The content of copper ions, cerium, and sodium in this comparative catalyst 2 was examined by chemical analysis, and as a result, it had the following composition in terms of oxide molar ratio on an anhydrous basis.

0.86 CuO.1.71 CeO ₂ .0.
31Na by _{2 O · Al 2 O 3 ·} 40SiO 2 Comparative Example 3 ion exchange operation the <Comparative Preparation of Catalyst 3> Copper ions and cerium element were prepared as below using Comparative Catalyst 3 was contained at the same time. 0.1 g of Na-ZSM-5; 10 g prepared in Example 1 was added so that the amount of copper ion and the amount of cerium atom were 5 times the amount of alumina contained therein.
An ion exchange operation was performed in which the mixture was added to a mixed aqueous solution of 1 / L copper acetate and 0.25 mol / L cerium nitrate and stirred at room temperature for 20 hours. The same operation was repeated 3 times, followed by thorough washing and drying to obtain comparative catalyst 3. As a result of investigating the content of copper ions, cerium, and sodium of this comparative catalyst 3 by chemical analysis, the comparative catalyst 3 had the following compositions in terms of oxide molar ratio on an anhydrous basis.

0.65CuO.0.42CeO ₂ .0.
25Na ₂ O · Al ₂ O ₃ · 40SiO ₂ Comparative Example 4 <Preparation of Comparative Catalyst 4> Sodium-type ZSM-5 prepared in Example 1; 10 g of cerium in an amount 2.5 times the amount of alumina in the zeolite. The mixture was impregnated with a mixed aqueous solution of 0.25 mol / L cerium acetate and 0.1 mol / L copper acetate, which was precisely weighed so that the amount of copper ions would be one time the amount of copper ions, for 15 minutes and then dried at 110 ° C. for 20 hours. Then, it was air-calcined at 500 ° C. for 1 hour to obtain a comparative catalyst 4. As a result of investigating the contents of copper ions, cerium and sodium of this comparative catalyst 4 by chemical analysis, the catalyst had the following composition in terms of an oxide molar ratio on an anhydrous basis.

1.01Cu0.2.46CeO ₂ .0.
_{52Na 2 O · Al 2 O 3} · 40SiO 2 Comparative Example 5 <Comparison Preparation of Catalyst 5> Example 1 sodium form ZSM-5 was prepared in; and 7 times the alumina mole number contained a 10g therein It was added to a cerium acetate aqueous solution having a concentration of 0.07 mol / L, which was precisely weighed so as to carry out cerium loading by an acetate decomposition method at 80 ° C. for 20 hours. After solid-liquid separation, washing with water, and drying at 110 ° C. for 20 hours, comparative catalyst 5 was obtained. The cerium and sodium contents of comparative catalyst 5 were examined by chemical analysis and as a result, they had the following composition, expressed in mol of oxide on an anhydrous basis.

[0054] 2.67CeO ₂ · 0.94Na ₂ O · Al
₂ O ₃ .40SiO ₂ Comparative Example 6 <Catalytic activity test> The NOx removal rates of Comparative Catalysts 1 to 5 were measured under the same conditions as in Example 6.

The NOx removal rates in reactions 1 and 2 at each temperature are shown in Tables 2 and 3, respectively.

[0056]

[Table 2]

[0057]

[Table 3]

[0058]

Industrial Applicability According to the method of the present invention, nitrogen oxides can be more efficiently removed from exhaust gas having excess oxygen even after the catalyst is exposed to high temperatures.

Claims (3)

[Claims] 1. A SiO ₂ / Al ₂ O ₃ molar ratio of at least 1.
Nitrogen oxides, hydrocarbons and carbon monoxide were used as catalysts in which 0 or more zeolite is loaded with a rare earth element and then copper ions are added, and the content of the rare earth element is at least twice the number of moles of alumina in the zeolite. A method for removing nitrogen oxides, which comprises contacting the exhaust gas containing oxygen with excess oxygen. 2. The method according to claim 1, wherein the catalyst is prepared by supporting rare earth elements on zeolite, followed by calcination, and further containing copper ions. 3. The method according to claim 2, wherein the calcination of the zeolite is carried out under vacuum.