JP4957176B2 - Nitrogen oxide purification catalyst and nitrogen oxide purification method - Google Patents

Description

  The present invention relates to purification of nitrogen oxides discharged from an internal combustion engine, and provides a nitrogen oxide purification catalyst comprising an iron silicate having a β structure, and a nitrogen oxide purification method using the same. .

  Iron silicate is a kind of metallosilicate and is generally expressed as an aluminum atom contained in the skeleton of an aluminosilicate zeolite substituted with an iron atom. It is characterized by high dispersibility of the substitution element in the silicate skeleton in addition to expressing acid properties different from those of ordinary aluminosilicate zeolites by substitution of the skeleton atoms. Therefore, utilization for various catalytic reactions has been studied.

Various methods have been proposed or put to practical use for purifying nitrogen oxides in exhaust gas, which is one of the applications. As a method for purifying nitrogen oxides emitted from fixed sources such as boilers, a selective catalytic reduction method in which a treatment gas obtained by adding ammonia to exhaust gas is brought into contact with a V ₂ O ₅ —TiO ₂ catalyst is industrially adopted. ing.

  Further, as a method for purifying nitrogen oxides emitted from mobile sources such as automobiles, for example, a method of simultaneously purifying nitrogen oxides, carbon monoxide, and hydrocarbons in a stoichiometric atmosphere using a catalyst in which a noble metal is supported on an alumina carrier (three Original catalyst purification) is widely used for exhaust gas purification of gasoline automobiles. (Patent Document 1)

  Furthermore, with regard to the purification of nitrogen oxides of oxygen-rich exhaust gas represented by lean burn combustion exhaust gas and diesel combustion exhaust gas, selective catalytic reduction method using ammonia and urea (water), similar to the purification of exhaust gas from fixed sources, hydrocarbons The catalytic reduction method using carbon monoxide, hydrogen, alcohol, ether, etc. as a reducing component, the direct decomposition method, and the occlusion reduction method using a noble metal catalyst containing a nitrogen oxide occlusion component are well known. The catalyst is disclosed. In these nitrogen oxide purification technologies, aluminosilicate zeolite having adsorption characteristics is used as a catalyst.

  As utilization of iron silicate in the purification technology of nitrogen oxides, there is a report on iron silicate having an MFI skeleton structure (ZSM-5 structure). For example, Patent Document 2 discloses a catalyst for purifying exhaust gas containing nitrogen oxide in which a coprecipitation complex oxide of copper and gallium is dispersedly supported on a ZSM-5 type iron silicate.

  Further, in Patent Document 3, nitrogen oxide in which an alkali metal exchanger of ZSM-5 type iron silicate is brought into contact with exhaust gas containing nitrogen oxide in the presence of hydrocarbons or oxygen-containing compounds in an atmosphere containing excess oxygen. A purification method is disclosed.

  Patent Document 4 discloses a method for removing nitrogen oxides in which combustion exhaust gas containing nitrogen oxides, oxygen gas and, if necessary, sulfurous acid gas is contact-reacted in the presence of an iron silicate catalyst and a hydrocarbon reducing agent. Yes.

  Further, Patent Document 5 discloses an exhaust gas purification catalyst that mainly removes nitrogen oxides in which at least one of platinum, palladium, rhodium and cobalt is supported on iron silicate. Although the skeleton structure of iron silicates in these patent documents is not specified, since the tetrapropylammonium salt is used as the organic directing agent (SDA) in the synthesis in the examples, the skeleton of the obtained iron silicate is used. It will be apparent to those skilled in the art that the structure is an MFI structure.

Non-Patent Document 1 describes a direct decomposition of N ₂ O using β-type iron silicate or non-selective catalytic reduction using CO as a reducing agent. However, these methods have low NOx purification activity, and a sufficient NOx purification rate cannot be obtained in a practical temperature range.

JP 7-289905 A JP-A-5-305240 Japanese Patent No. 2691643 JP-A-5-154349 Japanese Patent No. 2605956 Journal of Catalysis, 232 (2005) 318-334

While the efficient purification of nitrogen oxides is desired, the catalysts proposed so far have not been sufficient in terms of purification activity over a wide temperature range.
An object of the present invention is to provide an iron silicate catalyst having catalytic performance for efficiently purifying nitrogen oxide in a wide temperature range from low temperature to high temperature, and a method for purifying nitrogen oxide using the same.

  In view of these circumstances, the present inventors have conducted extensive studies on the denitration performance of the zeolite catalyst and the relationship between the type of zeolite and the catalytically active metal and the state of existence thereof. As a result, in selective reduction using ammonia or the like as a reducing agent, The present inventors have found that iron silicates having a β structure have excellent nitrogen oxide purification performance and have completed the present invention.

Hereinafter, the present invention will be described in detail.
Iron silicate is generally
M _{((2x + 2y) / n)} O.xFe ₂ O ₃ .yAl ₂ O ₃ .zSiO ₂ .wH ₂ O
(Where n is the valence of the cation M, x, y, and z are mole fractions of Fe ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ , respectively, and x + y + z = 1. W is 0 or more. Number)
And can be classified and specified by a crystal structure confirmed by X-ray diffraction or the like.

  β-type iron silicate is a metallosilicate having three-dimensional pores in which 0.76 × 0.64 nm and 0.55 × 0.55 nm pores composed of oxygen 12-membered rings intersect. The X-ray diffraction pattern of β-type iron silicate is characterized by the lattice spacing d (angstrom) shown in Table 1 and its diffraction intensity.

  Iron silicate has a structure in which trivalent aluminum of aluminosilicate zeolite is substituted with trivalent iron, and has solid acid properties derived from insufficient charge of the silicate skeleton similarly to aluminosilicate zeolite. At the same time, compared to a catalyst in which iron is supported on a conventional aluminosilicate zeolite, iron silicate is in a state in which iron as an active metal of the catalyst is very highly dispersed on the support, which is the iron silicate in the catalytic reaction. It is a high activity expression factor.

The β-type iron silicate of the present invention has an S iO ₂ / Fe ₂ O ₃ molar ratio of ₃₀ to 300, whereby nitrogen oxide purification activity is obtained, preferably 50 to 180.

In the purification of nitrogen oxides by iron silicate, the direct reaction active site is iron, but it is known that aluminum in the skeleton also contributes to the activity improvement. Although the mechanism for improving the activity is unknown, it is considered that the solid acidity derived from skeletal aluminum has an influence. In consideration of the durability of the catalyst and the contribution to the reaction of skeletal aluminum, the SiO ₂ / Al ₂ O ₃ molar ratio of the β-type iron silicate of the present invention is 30 to 180, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 60 to More preferably, it is 150.

Next, a method for producing the β-type iron silicate of the present invention will be described.
The raw material for synthesis is basically composed of a silica source, an aluminum source, an iron source, an SDA raw material, and water. Colloidal silica, amorphous silica, sodium silicate, tetraethylorthosilicate, aluminosilicate gel, etc. as the silica source, aluminum sulfate, sodium aluminate, aluminum hydroxide, aluminum chloride, aluminosilicate gel, metallic aluminum, etc. as the iron source, iron As a source, iron nitrate, iron chloride, iron sulfate or the like can be used, and it is desirable to have a form that can be mixed with other components sufficiently uniformly. As SDA raw materials, tetraethylammonium hydroxide having tetraethylammonium cation, tetraethylammonium bromide, tetraethylammonium fluoride, octamethylenebiskinucridium, α, α′-diquinuclidium-p-xylene, α, α′-diquinuclidium- m-xylene, α, α′-diquinuclidium-o-xylene, 1,4-diazabicyclo [2,2,2] octane, 1,3,3, N, N-pentamethyl-6-azonium bicyclo [3,2 , 1] octane or at least one of a group of compounds containing N, N-diethyl-1,3,3-trimethyl-6-azonium bicyclo [3,2,1] octane cation can be used.

What is necessary is just to set the composition of a raw material mixture arbitrarily in the following range. Moreover, you may add the component which has crystallization promotion effects, such as a seed crystal.
SiO ₂ / _Al 2 _{O 3} molar ratio from 15 to 30,000
SiO ₂ / _Fe 2 _{O 3} molar ratio from 30 to 300
Molar H ₂ O / SiO ₂ ratio 5-50
SDA / SiO ₂ molar ratio of 0.1 to 5

  The β-type iron silicate according to the present invention is crystallized by allowing a raw material mixture of water, silica, alumina, iron and SDA to crystallize in a sealed pressure vessel at an arbitrary temperature of 100 to 180 ° C. for a sufficient time. Obtainable.

  At the time of crystallization, the raw material mixture may be mixed and stirred or may be left standing. After completion of crystallization, the mixture is allowed to cool, solid-liquid separation, washed with a sufficient amount of pure water, and dried at an arbitrary temperature of 100 to 150 ° C. to obtain the β-type iron silicate according to the present invention.

  The β-type iron silicate of the present invention can be used as it is as a purification catalyst for nitrogen oxides. The β-type iron silicate immediately after synthesis may contain SDA (organic directing agent in synthesis) in the pores, and can be used as a catalyst for purifying nitrogen oxides after removing these if necessary. .

As the SDA removal treatment, a liquid phase treatment using a chemical solution containing an acidic solution or an SDA decomposition component, an exchange treatment using a resin or the like, or a thermal decomposition treatment may be employed, and these treatments may be combined. Furthermore, it can also be used by converting to H type or NH ₄ type using the ion exchange ability of iron silicate.

  A catalytically active metal species may be supported on the β-type iron silicate of the present invention. The metal species to be supported is not particularly limited, but is, for example, one or more elements selected from the group consisting of Group 8, 9, 10 and 11 elements, particularly iron, cobalt, palladium, iridium, platinum, copper, silver, and gold. Preferably, it is at least one of iron, palladium, platinum, copper and silver.

  In addition, a promoter component such as rare earth metal, titanium or zirconia can be additionally added. The loading method for loading the active metal species is not particularly limited. As the loading method, methods such as an ion exchange method, an impregnation loading method, an evaporation to dryness method, a precipitation loading method, and a physical mixing method can be employed. The raw materials used for supporting the metal may be any of soluble / insoluble materials such as nitrates, sulfates, acetates, chlorides, complex salts, oxides and complex oxides.

The amount of metal supported is not limited, but a range of 0.1 to 10% by weight is particularly preferable.
The catalyst of the present invention can be used after being mixed with a binder such as silica, alumina and clay mineral. Examples of clay minerals used for molding include kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite. Moreover, it can also be used by wash-coating on a cordierite or metal honeycomb substrate.

  The catalyst of the present invention can selectively purify nitrogen oxides in the exhaust gas by contacting the exhaust gas containing nitrogen oxides in the presence of ammonia, urea, and organic amines as a reducing agent. Examples of nitrogen oxides purified by the present invention include nitric oxide, nitrogen dioxide, dinitrogen trioxide, dinitrogen tetroxide, dinitrogen monoxide, and mixtures thereof. Nitric oxide, nitrogen dioxide, and dinitrogen monoxide are preferred. Here, the nitrogen oxide concentration of the exhaust gas that can be treated by the present invention is not limited.

  The method of adding the reducing agent is not particularly limited, and a method of directly adding the reducing component in a gaseous state, a method of spraying and vaporizing a liquid such as an aqueous solution, a method of spraying pyrolysis, and the like can be employed. The addition amount of these reducing agents can be arbitrarily set so that nitrogen oxides can be sufficiently purified.

  The exhaust gas may contain components other than nitrogen oxides, and may contain, for example, hydrocarbons, carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, sulfur oxides, and water. Specifically, the method of the present invention can purify nitrogen oxides from a wide variety of exhaust gases such as diesel vehicles, gasoline vehicles, boilers, gas turbines and the like.

In the method for purifying nitrogen oxides of the present invention, the space velocity when contacting the exhaust gas with the catalyst comprising the β-type iron silicate of the present invention is not particularly limited, but the preferred space velocity is 500 to 500,000 hr ^{−1 on a} volume basis. More preferably, it is 2000 to 300,000 hr ⁻¹ .

  The β-type iron silicate of the present invention has high nitrogen oxide purification performance and can purify nitrogen oxide over a wide temperature range.

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

<Example 1>
To 235 g of tetraethylammonium hydroxide 35% aqueous solution (hereinafter TEAOH), 1.26 g of aluminum hydroxide, 8.36 g of iron nitrate, 62.8 g of amorphous silica powder (trade name: Nipseal VN-3) made by Tosoh silica and 172 g of water Was added and mixed thoroughly with stirring. The composition of the reaction mixture was 90SiO ₂ : Al ₂ O ₃ : Fe ₂ O ₃ : 54TEAOH: 1800H ₂ O. The reaction mixture was sealed in a stainless steel autoclave and heated at 150 ° C. for 96 hours for crystallization. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was baked at 600 ° C. under air flow to obtain β-type iron silicate-1. From the X-ray diffraction, β-type iron silicate-1 has the X-ray diffraction pattern shown in Table 1, the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis is 64, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 69. there were.
β-type iron silicate-1 was used as a catalyst 1 for a catalytic reaction test.

<Example 2>
A reaction mixture was prepared in the same manner as β-type iron silicate-1, except that the composition ratio of the reaction mixture to be crystallized was changed to 60SiO ₂ : Al ₂ O ₃ : 0.33Fe ₂ O ₃ : 36TEAOH: 1200H ₂ O. did. The reaction mixture was sealed in a stainless steel autoclave and heated at 150 ° C. for 96 hours for crystallization. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was fired at 600 ° C. under air flow to obtain β-type iron silicate-2. From the X-ray diffraction, β-type iron silicate-2 has the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis is 45, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 126. there were.
β-type iron silicate-2 was used as a catalyst 2 for a catalytic reaction test.

<Example 3>
A reaction mixture was prepared in the same manner as β-type iron silicate-1, except that the composition ratio of the reaction mixture to be crystallized was changed to 90SiO ₂ : Al ₂ O ₃ : 1.8Fe ₂ O ₃ : 63TEAOH: 1800H ₂ O. did. This reaction mixture was sealed in a stainless steel autoclave and crystallized by heating at 150 ° C. for 192 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was fired at 600 ° C. under air flow to obtain β-type iron silicate-3. From the X-ray diffraction, β-type iron silicate-3 has the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis is 69, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 46. there were.
β-type iron silicate-3 was used as a catalyst 3 for a catalytic reaction test.

<Example 4>
A reaction mixture was prepared in the same manner as β-type iron silicate-1, except that the composition ratio of the reaction mixture to be crystallized was changed to 40SiO ₂ : Al ₂ O ₃ : 0.13Fe ₂ O ₃ : 12TEAOH: 640H ₂ O. did. The reaction mixture was sealed in a stainless steel autoclave and crystallized by heating at 150 ° C. for 72 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was fired at 600 ° C. under air flow to obtain β-type iron silicate-4. From X-ray diffraction, β-type iron silicate-4 has the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in ICP emission analysis is 38, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 213. there were.
β-type iron silicate-4 was used as a catalyst 4 for a catalytic reaction test.

< Comparative Example 1 >
To 237 g of tetraethylammonium hydroxide 35% aqueous solution (hereinafter TEAOH), 2.53 g of iron nitrate, 63.3 g of amorphous silica powder (trade name: Nipsil VN-3) made by Tosoh silica and 177 g of water were added and mixed thoroughly. did. The composition of the reaction mixture _{400SiO 2: Al 2 O 3:} 1.33Fe 2 O 3: 280TEAOH: was 8000H ₂ O. This reaction mixture was sealed in a stainless steel autoclave and crystallized by heating at 150 ° C. for 192 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was fired at 600 ° C. under air flow to obtain β-type iron silicate-5. From X-ray diffraction, β-type iron silicate-5 has the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in ICP emission analysis is 298, and the SiO ₂ / Fe ₂ O ₃ molar ratio is 205. there were.
β-type iron silicate-5 was used as a comparative catalyst 1 for a catalytic reaction test.

<Comparative example 2 >
A β-type zeolite was synthesized with reference to the method disclosed in JP-A-2-2933021. A sodium silicate aqueous solution (SiO ₂ ; 130 g / l, Na ₂ O; 41.8 g / l, Al ₂ O ₃ ; 0.05 g / l) in an overflow type reaction tank (actual volume 4.8 liters) in a stirring state And an aqueous solution of aluminum sulfate (Al ₂ O ₃ ; 21.3 g / l, SO ₄ ; 240 g / l) were simultaneously supplied at a flow rate of 18.2 liter / Hr and 4.5 liter / Hr, respectively, and reacted with stirring. A slurry product was obtained. At this time, the average residence time of the slurry was 12.5 minutes. Moreover, the supply amount of the sodium silicate aqueous solution was adjusted so that the pH of the reaction tank during the reaction was 6-8. The slurry product overflowed from the reaction tank was dehydrated with Nutsche and washed with water to obtain a granular amorphous aluminosilicate.

The granular amorphous aluminosilicate; 189 g, solid sodium hydroxide; 1.4 g, solid potassium hydroxide; 3.5 g and 20 wt% tetraethylammonium aqueous solution; . The raw slurry was transferred to a 1 liter closed pressure vessel and crystallized at 150 ° C. for 96 hours while stirring at a peripheral speed of 0.8 m / s. The slurry mixture after crystallization was subjected to solid-liquid separation, washed with a sufficient amount of pure water, and dried at 110 ° C. The obtained dry powder was calcined at 600 ° C. in an air stream to obtain β-type zeolite-1. From X-ray diffraction, β-type zeolite-1 had the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis was 36.
β-type zeolite-1 was subjected to a catalytic reaction test as a comparative catalyst 2 .

<Comparative Example 3 >
Tosoh β-type zeolite (trade name: HSZ-940NHA) having a SiO ₂ / Al ₂ O ₃ molar ratio of 40 was calcined at 600 ° C. in a dry air stream to obtain β-type zeolite-2. From X-ray diffraction, β-type zeolite-2 had the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis was 40.
β-type zeolite-2 was used as a comparative catalyst 3 for a catalytic reaction test.

<Comparative example 4 >
The supported amount of iron is accurately weighed so as to be 3 _{wt% Fe (NO 3) 3 ·} 9 with an aqueous solution of hydrate and impregnation of iron β-type zeolite-2 obtained in Comparative Example 3 . Iron-supported β-type zeolite-1 calcined at 500 ° C. was used as a comparative catalyst 4 for a catalytic reaction test.

<Comparative Example 5 >
The supported amount of iron is accurately weighed to be 1 wt% Fe (NO 3) using an aqueous solution of _{3 nonahydrate} was impregnated supporting iron on β-type zeolite-2 obtained in Comparative Example 3 . Iron-supported β-type zeolite-2 air calcined at 500 ° C. was subjected to a catalytic reaction test as a comparative catalyst 5 .

<Comparative Example 6 >
ZSM-5 type iron silicate was prepared with reference to Journal of Catalyisis, 232 (2005) 318-334. To 131 g of tetrapropylammonium hydroxide 10% aqueous solution (hereinafter TPAOH), 0.87 g of aluminum hydroxide, 5.80 g of iron nitrate, 43.5 g of amorphous silica powder (trade name: Nipsil VN-3) manufactured by Tosoh Silica, 48 10.8 g of% sodium hydroxide and 288 g of water were added and mixed thoroughly with stirring. The composition of the reaction mixture was 90SiO ₂ : Al ₂ O ₃ : Fe ₂ O ₃ : 9TPAOH: 18NaOH: 3240H ₂ O. The reaction mixture was sealed in a stainless steel autoclave and crystallized by heating at 175 ° C. for 120 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was calcined at 600 ° C. under air flow to obtain ZSM-5 type iron silicate (Na type). The obtained ZSM-5 type iron silicate (Na type) was subjected to repeated ion exchange operations three times using a 0.1 M aqueous ammonium chloride solution to form NH ₄ type, and then dried at 110 ° C. ZSM-5 type iron silicate (NH ₄ type) dry powder was calcined at 600 ° C. under air flow to obtain H type ZSM-5 type iron silicate. In the ICP emission analysis, the SiO ₂ / Al ₂ O ₃ molar ratio was 85, and the SiO ₂ / Fe ₂ O ₃ molar ratio was 85.
ZSM-5 type iron silicate was subjected to a catalytic reaction test as comparative catalyst 6 .

<Catalytic reaction test>
The catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were pressed and then crushed and sized to 12 to 20 mesh. 1.5 cc of each sized catalyst was charged into a normal pressure fixed bed flow type reaction tube. While the gas having the composition shown in Table 2 was passed through the catalyst layer at 1500 cc / min, the nitrogen oxide removal activity at a constant temperature of 100 to 500 ° C. was evaluated.

The nitrogen oxide removal activity is represented by the following formula.
(Equation 1)
_XNOx = {([NOx] _in- [NOx] _out ) / [NOx] _in } * 100
Here, X _NOx is the nitrogen oxide purification rate (%), [NOx] _in is the nitrogen oxide concentration of the incoming gas, and [NOx] out is the nitrogen oxide concentration of the outgoing gas.
Tables 3 and 4 show the nitrogen oxide purification rates of the catalysts 1 to 4 and the comparative catalysts 1 to 6 at arbitrary temperatures.

Furthermore, 3 cc of each catalyst was filled in a normal pressure fixed bed flow type reaction tube, and was treated by circulating air containing H ₂ O = 10 vol% at 700 cc for 20 hours at 300 cc / min (endurance treatment). Tables 5 and 6 show the results of evaluating the nitrogen oxide removal activity under the same conditions as in the catalytic reaction test described above for each catalyst after the durability treatment.

  From the above results, the β-type iron silicate of the present invention has nitrogen oxide purification performance and excellent durability in a wide temperature range, and can efficiently purify nitrogen oxide.

Claims (4)

Some of the aluminum of the crystalline aluminosilicate be one that is substituted by iron having a β skeleton structure, _SiO 2 / _Al 2 _{O 3} molar ratio of skeleton is 30 to 180, and _SiO 2 / Fe skeletal A function of selectively reducing nitrogen oxides in exhaust gas by reacting at least one of ammonia, urea, and organic amines as a reducing agent, comprising iron silicate having a ₂ O ₃ molar ratio of 30 to 300. A catalyst for purifying nitrogen oxides, which is characterized by being exerted. The nitrogen oxide purification catalyst according to claim 1, comprising an iron silicate having a SiO ₂ / Fe ₂ O ₃ molar ratio of the skeleton of 50 to 180. The nitrogen oxide purification catalyst according to claim 1 or 2, comprising an iron silicate having a SiO ₂ / Fe ₂ O ₃ molar ratio of the skeleton of 60 to 150. The nitrogen oxide purification catalyst according to any one of claims 1 to 3 is used to selectively reduce nitrogen oxides in exhaust gas by reacting at least one of ammonia, urea, and organic amines as a reducing agent. A method for purifying nitrogen oxides.