JPH0417083B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an ammonia-reducing denitrification catalyst for nitrogen oxides and a denitrification method, and in particular to suppressing the concentration of nitrous oxide (nitrous oxide, N ₂ O), which is a by-product during denitrification, to a low level and reducing nitrogen dioxide (NO _{2 O} ). This invention relates to a denitrification catalyst and a denitrification method suitable for reducing ammonia. The so-called ammonia catalytic reduction denitrification method uses a catalyst to reduce nitrogen oxides (NO _x ) contained in exhaust gas to harmless nitrogen and water.
It has many advantages such as a simple device structure, and has been put into practical use in many applications such as denitration of exhaust gas from combustion devices such as large boilers. The catalysts used in the above method include titanium (Ti), vanadium (V), tungsten (W), and molybdenum (Mo).
Products based on oxides such as oxides, which are excellent in both performance and longevity, are in practical use. However, according to the inventors' study, the denitrification process using the above-mentioned catalyst is effective only when the nitrogen oxide is nitrogen monoxide (NO) or when 50% or more of nitrogen oxide is present.
It is effective when composed of NO and 50% or less of nitrogen dioxide (NO ₂ ), but when it consists mainly of NO ₂ or contains nitrous oxide (N ₂ O), It became clear that it was not very effective. In other words, _{Figure 1} shows the exhaust gas (gas composition:
_NO2 800ppm, NO200ppm, _NH3 1500ppm, _O2 20
%, H ₂ O2 %) using a _conventional catalyst (in the figure, 1 indicates
(TiO ₂ −Co ₂ O ₃ catalyst, 2 is for TiO ₂ −V ₂ O ₅ catalyst), as is clear from this figure, the
It can be seen that although NO ₂ and NO can certainly be reduced, on the other hand, a large amount of N ₂ O is produced as a by-product, and the actual denitrification performance is low. In this way, NO ₂
In the _NH3 reduction denitrification method, which mainly uses nitrogen oxides,
Removal of by-product N ₂ O is an important issue. Figure 2 also shows conventional denitrification catalysts (1, 1,
2 is the same catalyst as above, 3 is a TiO ₂ -WO ₃ catalyst) using N ₂ O (gas composition: N ₂ O 1000 ppm,
This figure shows the results of ammonia reduction of N 2 O (NH ₃ 1000ppm, O ₂ 20%, H ₂ O 2%), but as shown in the figure, conventional denitrification catalysts are inactive for ammonia reduction of N ₂ O. Not only is N ₂ O
It can be seen that there is a tendency to produce as a by-product. Therefore, in addition to NO or N ₂ O, there is N ₂ O as NO _x in exhaust gas.
is contained, N ₂ O will remain as it is, and it can be seen that the removal of N ₂ O is an important issue in this case as well. The properties of the above-mentioned conventional catalysts were unknown until now, but this is because the analytical method for N ₂ O is difficult.
This is thought to be due to the fact that N ₂ O, which is a by-product of the denitrification reaction, has not been studied, and the evaluation of denitrification performance has been mainly based on NO. For this reason, there have been no attempts to reduce N ₂ O to NH ₃ in the past.
Furthermore, as a matter of course, no highly active catalyst for this reaction had been found. Exhaust gases from nitric acid plants, metal pickling factories, nitrate pyrolysis furnaces, or, as a special example, nitric acid dissolution processes in nuclear fuel reprocessing, are all highly concentrated nitrogen oxides (most of the NO _x is NO ₂ ), and various methods of removing it are being studied from the viewpoint of pollution control measures or prevention of effects on subsequent equipment. Among these methods, the alkaline cleaning method is currently in the most practical use, but this method has problems such as a low NO _x removal rate and the need for wastewater treatment. For this reason, it is desired to develop a simple dry treatment method such as the ammonia catalytic reduction denitrification method, which has achieved good results in boiler flue gas denitrification methods. However, when the ammonia catalytic reduction denitrification method is applied to the above-mentioned exhaust gas, a problem arises in that a large amount of N ₂ O is produced as a by-product, and the method cannot be used as a practical process. This is because most of the NO _x in the above-mentioned exhaust gas is in the form of NO ₂ , and in this respect its properties are different from boiler exhaust gas, which is mainly composed of nitrogen monoxide (NO). That is, according to the study results of the present inventors, the reaction between NO ₂ and NH ₃ is (1),
It proceeds through the elementary reactions of formulas (1') and (2), and is collectively expressed by formula (3). NO ₂ +2NH ₃ →NO+2NH ^* ₂ +H ₂ O (1) →N ₂ +NH ^* ₂ +2H ₂ O (1') NO ₂ +NH ^* ₂ →N ₂ O+H ₂ O (2) 2NO ₂ +2NH ₃ →N ₂ +N ₂ O + 3H ₂ O (3) NH ^* ₂ : Intermediate adsorbed on the catalyst This reaction mechanism does not depend on the type of catalyst, so no matter what known denitration catalyst is used, NO ₂ to NH ₃ reduction denitrification method ( The by-product of N ₂ O due to equation 2) is considered to be unavoidable. It is also said that a large amount of N ₂ O is generated when highly reducing metals are pickled or dissolved in nitric acid.
In this way, there are a large number of plants that require the removal of _N2O , including potential _N2O sources that produce _N2O as a by-product during ammonia reduction and _{denitrification} . Development of a method is strongly desired. For this reason, the present inventors investigated various measures to reduce N ₂ O, and converted some of the NO ₂ to NO through catalytic pyrolysis prior to the denitrification reaction to reduce NO _x in the exhaust gas.
After adjusting the composition to [NO]/[total NO _x ] 0.5,
A method for carrying out the conventional ammonia reduction and denitrification reaction was proposed. This method uses _NH2 as described below.
The idea is to consume it with NO and prevent the progress of the N ₂ O by-product reaction in formula (2). NO ₂ +2NH ₃ →NO+2NH ^* ₂ +H ₂ O (1) →N ₂ +NH ^* ₂ +2H ₂ O (1') NH ^* ₂ +NO→N ₂ +H ₂ O (4) NO ₂ +NO+2NH ₃ →2N ₂ +3H ₂ O (5) This two-stage denitrification method can suppress the N ₂ O by-product to a low level of several ppm or less, and is an excellent method in principle, but the equipment structure and control are complicated. This poses a practical problem.
Therefore, there is a strong desire to develop a denitrification process that can reduce NO ₂ to ammonia in one step, similar to NO. An object of the present invention is to provide a highly active catalyst for the reduction reaction of N ₂ O with NH ₃ and a method for reducing and denitrating N ₂ O with ammonia using the same. Another object of the present invention is to provide a denitrification method and apparatus that can reduce NO ₂ to NH ₃ in one stage without producing N ₂ O as a by-product. The present inventors investigated the activity of catalysts made of various metal oxides for the _N2O - _NH3 reaction (formula (6) below). 3N ₂ O + 2NH ₃ →4N ₂ +3H ₂ O (6) As a result, transition metal oxides such as vanadium (V), tungsten (W), chromium (Cr), manganese (Mn), and cobalt (Co) are used as active ingredients. None of the known catalysts could promote the ammonia reduction reaction of N ₂ O. In contrast,
Zeolites such as mordenite and Y-type zeolite in which part or all of the alkali metals or alkaline earth metals are replaced with iron (Fe) or hydrogen have high activity specifically in the ammonia reduction reaction of N ₂ O. This is what led to the present invention. In short, _the present invention provides a denitrification catalyst that uses ammonia as a reducing agent to reduce exhaust gas containing nitrogen oxides.
is an exhaust gas containing
xM _2/o O・yAl ₂ O ₃ .zSiO ₂ .mH ₂ O (where x, y,
z is an integer, M is an arbitrary element, and n indicates the valence of M) A compound having a zeolite structure, characterized in that part or all of the M element has been replaced with hydrogen or/and iron atoms. This is an ammonia reduction and denitrification catalyst. In addition, the present invention utilizes the fact that the ammonia reduction denitrification catalyst for N ₂ O is also active in the reaction between NO ₂ and NH ₃ (formula (3) above), making it possible to denitrate NO ₂ in one step. It is something. That is, by using the above catalyst and selecting reaction conditions to specific values,
The reduction reaction of NO ₂ with ammonia and the ammonia reduction reaction of N ₂ O produced by the reaction are carried out sequentially in the same catalyst layer, and NO ₂ is reduced to ammonia in one stage without producing N ₂ O as a by-product. It has been made possible. It is important that the catalyst used in the present invention has Fe or H ions incorporated into the zeolite framework. In other words, the substituted ions in the hydrogen- and/or iron-substituted zeolite catalyst of the present invention have unique properties and are chemically very different from the ions in the metal oxide alone or in the oxide supported on a carrier. The behavior and thus the catalyst are essentially different from iron oxide catalysts supported on alumina, silica, or silica-alumina. In general, zeolites are classified into
It is an aluminosilicate with a complex framework structure, generally xM _2/o O・yAl ₂ O ₃ , zSiO ₂ , mH ₂ O (where M is any metal element such as Na, K, Ca, is indicated by its valence). The metal element M is usually an alkali metal or an alkaline earth metal, which can be replaced with hydrogen or other transition metal cations. The zeolite that can be used in the present invention may be any zeolite as long as it has the above properties, but in terms of the heat resistance and activity of the catalyst, the SiO ₂ /Al ₂ O ₃ ratio is 3 or more. So-called high-silica zeolites are suitable, and specifically mordenite, clinoptilite, haujasite, zeolite Y, etc. are suitable. Further, the method for replacing hydrogen and iron in the above zeolite may be any method as long as the purpose can be achieved, as long as a hydrogen- and/or iron-substituted zeolite that retains the zeolite framework structure can be obtained. , within the scope of the present invention. The catalyst of the present invention typically comprises a natural or synthetic zeolite such as mordenite, clinoptilite, or haujasite, which is mixed with hydrochloric acid, an aqueous ammonium chloride (NH ₄ Cl) solution, or ferric chloride (FeCl ₃ ).
The zeolite is immersed in an aqueous solution or an aqueous solution of ferric nitrate (Fe(NO ₃ ) ₃ ) to replace the alkali metal or alkaline earth metal ions in the zeolite with hydrogen and/or iron ions, and then calcined at approximately 500°C. It can be manufactured using In this case, if a mixed aqueous solution of ammonium chloride and ferric chloride is used, a catalyst in which more hydrogen than iron is substituted can be obtained, and if an aqueous ferric nitrate solution is used, a catalyst in which almost all iron has been substituted can be obtained. Furthermore, if an aqueous ammonium chloride solution and an aqueous ferric nitrate solution are used in combination, a catalyst in which more iron than hydrogen can be substituted can be obtained. Next, FIG. 3 shows a system diagram when the catalyst according to the present invention is used for denitration of N ₂ O-containing gas. The gas led through the pipe 4 is injected with ammonia through the pipe 5 and enters the catalyst layer 7 in the reactor 6, where the N ₂ O in the exhaust gas is easily reduced to harmless nitrogen and water with ammonia, and the gas is injected with ammonia through the pipe 5. It is discharged from 8. Furthermore, using the above catalyst, NO ₂ is produced in a one-step process.
The principle of denitration of nitrogen into _N2 will be explained. As shown in Figure 4, using a reactor filled with a large amount of catalyst that is active in both the NO ₂ and NH ₃ reaction in equation (3) and the ammonia reduction reaction of N ₂ O, When ammonia reduction of NO ₂ is carried out under conditions where a large excess of the amount of ammonia required for the reaction is injected, two reaction zones as shown in the figure are formed in the catalyst layer. That is, near the inlet of the catalyst layer, equation (3), which has a faster reaction rate, proceeds preferentially, converting NO ₂ and NH ₃ to N ₂ ,
A first reaction zone is formed that produces _N2O and water, a _NO2 - _NH3 reaction zone. Since the catalyst layer is long enough, the N ₂ O generated here and the excess NH ₃ injected and remaining unused in the reaction of equation (3) are combined.
reacts as shown in equation (6) due to the NH ₃ reduction activity of N ₂ O of the catalyst, becoming N ₂ and H ₂ O, and the second
reaction zone, i.e., a N ₂ O ammonia reduction zone is formed. 3N ₂ O + 2NH ₃ →4N ₂ +3H ₂ O (6) In this way, if the catalyst species and reaction conditions are appropriately selected, NO ₂ can be reduced to N ₂ and H ₂ O with ammonia using a single catalyst layer. can. In the denitrification method of the present invention, the catalyst and reaction conditions are selected as follows. First, the catalyst used needs to be active in both the NO ₂ -NH ₃ reaction of formula (3) and the N ₂ O reduction reaction of formula (6). Such catalysts include the aforementioned H and/or Fe discovered by the present inventors.
Examples include substituted zeolite catalysts, and high-silica zeolite catalysts such as mordenite and zeolite Y are particularly suitable. Second, the amount of NH ₃ to be injected is determined by formula (3) and (5).
It is necessary to choose an amount of NH ₃ sufficient to complete the two reactions in the equation. For this purpose, all of the gas
Stoichiometrically, it is preferable to select the ratio of NO _x to injected NH ₃ (denoted as NH ₃ /NO _x ) to be 1.34 or more, particularly 1.6 or more. Thirdly, it is necessary to fill the catalyst in such a way that the aforementioned two reaction zones are formed within the catalyst layer, and for this reason, in the case of the aforementioned catalyst, the areal velocity AV (processing gas amount/total external surface area of the catalyst ) value is 5 or less, especially 3
It is preferable to select the following. By satisfying the above three conditions,
NO ₂ can be reduced to N ₂ and H ₂ O with ammonia while suppressing the amount of N ₂ O produced as a by-product. The reactor used in the present invention may be one filled with a single catalyst that is active for both the NO ₂ -NH ₃ reaction and the N ₂ O - NH ₃ reaction as shown in Figure 4, but as shown in Figure 5. As shown in (3) on the upstream side of reactor 9,
The total amount of catalyst used using a catalyst layer 10 having an excellent activity for the NO ₂ - NH ₃ reaction of the formula and a catalyst layer 11 having an excellent activity for the N ₂ O - NH ₃ reaction on the downstream side. can be reduced. In addition,
4 is a gas pipe to be treated, 5 is an ammonia injection pipe 8
is the processing gas piping. Next, FIG. 6 shows another system diagram when the catalyst of the present invention is used for denitration of NO ₂ -containing gas. The NO ₂ -containing gas led through the pipe 4 is injected with ammonia through the ammonia injection pipe 5 and then introduced into the reactor 12 filled with a known denitrification catalyst, where it is converted into nitrogen and N by the reaction of equation (3) described above. ₂ Ammonia is reduced to O and water. Here, the exhaust gas is further injected with ammonia from the ammonia injection pipe 7, and then led to the reactor 13 filled with the catalyst 11 according to the present invention, where N ₂ O is reduced to ammonia as shown in the above-mentioned equation (6). . This results in
Even if there is a gas mainly composed of NO ₂ , denitration processing can be performed without producing N ₂ O as a by-product. Hereinafter, the present invention will be explained in more detail with reference to Examples. Examples 1 to 3 (Catalyst production example) 10 g each of powders of mordenite, horujasite, and zeolite Y were added to a mixed aqueous solution of ammonium chloride and ferric chloride (NH ₄ Cl 50 g/,
After soaking in 100ml of FeCl ₃ (10g/) for 24 hours,
It was washed and filtered with 1 water, and then dried at 150°C.
The obtained powder was molded to a diameter of 10 mm and a thickness of 5 mm under a total pressure of 3 tons, and then calcined at 500°C for 2 hours to form the catalyst A of the present invention.
B and C were obtained. Examples 4 to 6 (Catalyst production example) In Example 1, ferric nitrate (Fe(NO ₃ ) ₂ ) was used instead of the ammonium chloride-ferric chloride mixed solution.
20g/, 5g/and 1 aqueous solution, respectively.
Catalysts D, E, and F of the present invention were obtained in the same manner as in Example 1, except that 10 g of each mordenite powder was immersed in these solutions using 10 g/g/). Examples 7 to 9 (Catalyst production example) A slurry obtained by immersing 10 g each of powders of mordenite, horujasite, and zeolite Y in an aqueous ammonium chloride solution (50 g/) for 24 hours, washing with water in step 1, and filtering. Another 1g/
Catalysts G, H and I of the present invention were obtained in the same manner as in Example 1. Comparative Examples 1 to 6 (Manufacturing Example of Comparative Catalysts) Mordenite, haujasite, and zeolite Y used in Examples 1 to 9 were used as Comparative Catalysts 1 to 3, and α-Al ₂ O ₃ powder, silica alumina (SiO ₂ 70 wt.
%, _{Al2O3 30wt} %) and titanium oxide ( _TiO2 )
To 10 g of each powder, 1 g of ferric nitrate and 3 ml of water were added and kneaded in a mortar, followed by drying, molding, and calcining in the same manner as in Example 1, and these were used as Comparative Catalysts 4, 5, and 6. The catalysts obtained in the following Examples and Comparative Examples were
The powder was crushed into ~20 meshes and used to examine the activity of N ₂ O in the ammonia reduction reaction under the following conditions. (1) Gas composition N ₂ O 1000ppm NH ₃ 1000ppm O ₂ 20% H ₂ O 2% N ₂ remaining (2) Space velocity (SV) 10000h ^-1 (3) Reaction temperature 300-500℃ (4) N ₂ O Analysis method: Infrared absorption spectrophotometry Figure 7 shows the N ₂ O ammonia reduction performance of catalysts A, B, and C of Examples 1 to 3 and comparative catalysts 1 to 3. In the figure, A, B, and C are catalyst A of the present invention,
B, C, ratio 1, ratio 2, ratio 3 are comparative catalysts 1, 2, 3
Each is shown below. From the results shown in the figure, the comparative catalysts containing iron oxide as an active component and the zeolite catalysts without hydrogen and iron substitution (Comparative Examples 1 to 3) have no ammonia reduction activity of N ₂ O, whereas the present invention Hydrogen and iron substituted zeolite catalysts A, B, C
It can be seen that both have high N ₂ O reduction activity. Further, Table 1 shows the N ₂ O removal rate at 450° C. and the hydrogen or iron substitution rate based on the elemental analysis results of the catalysts of Examples 1 to 9 and Comparative Catalysts 1 to 6. As is clear from this table, by replacing alkali metals or alkaline earth metals in zeolite with hydrogen or iron,
It can be seen that the activity for the ammonia reduction reaction of N ₂ O is significantly improved. Furthermore, it can be seen that high activity is shown when not all exchangeable cations are replaced by iron, but by both iron and hydrogen.

[Table] According to Examples 1 to 9 above, it becomes possible to denitrify N ₂ O, which conventionally could not be reduced with ammonia, by reducing it with ammonia. Furthermore, N ₂ O generated when NO ₂ is contained in a high proportion, such as in nitric acid plant exhaust gas, can be removed by the catalyst of the present invention. Next, an example of ammonia reduction denitrification of _NO2- containing gas using the catalyst of the present invention will be described. Example 10 42 equivalent % of Na, K and Ca in mordenite
After crushing the H,Fe-mordenite catalyst tablet molded body in which hydrogen (H) was substituted and 35 equivalent% iron (Fe), the inner diameter
A 30 mm glass reaction tube was filled and _an ammonia reduction test of NO ₂ was performed under the following reaction conditions.
The reduction rate of N 2 O and the by-product rate of N ₂ O were investigated. Test conditions (1) Gas composition NO ₂ 1000ppm NH ₃ 1800ppm (NH ₃ /NO ₂ = 1.8) O ₂ 20% H ₂ O 2% N ₂ remaining (2) Reaction temperature 450℃ (3) Area velocity AV10, 7, 5, 3, 1 m/h Note that for the analysis of NO ₂ and N ₂ O, a Chemilumi method NO _x meter and infrared spectrophotometry were used. Here, the reduction rate of NO ₂ and the by-product rate of N ₂ O are defined by the following formula. NO ₂ reduction rate (%) = NO ₂ concentration at catalyst layer inlet - NO _x concentration at catalyst layer outlet / NO ₂ concentration at catalyst layer inlet × 100 N ₂ O by-product rate (%) = N ₂ O concentration at catalyst layer outlet / catalyst Bed inlet NO ₂ concentration x 100 Comparative example 7 Among the test conditions of Example 10, NH ₃ concentration
Example except that the concentration was 1000ppm (NH ₃ /NO ₂ = 1.0).
The test was conducted in the same manner as in 10. FIG. 8 shows the test results of Example 10 and Comparative Example 7. In the figure, D indicates Example 10, and ratio 7 indicates Comparative Example 7. From this figure, the method of the present invention (Example 10)
It can be seen that this method is extremely effective in reducing the amount of by-product N ₂ O during ammonia reduction and denitrification of NO ₂ . moreover
In Example 10, where the NH ₃ concentration is high, as the AV decreases (increases the contact time), NO ₂ first decreases and the amount of N ₂ O by-product increases, and then N ₂ decreases due to the NH ₃ reduction reaction of N ₂ O. Although a decrease in O occurs, in Comparative Example 7 where the NH ₃ concentration is low, the NH ₃ reduction reaction of N ₂ O cannot proceed.
It can be seen that no matter how much the AV value is reduced, the effect of reducing N ₂ O does not occur. Furthermore, from the results shown in this figure, it can be seen that the AV value that defines the present invention is preferably 5 or less, preferably 3 or less. Example 11 Under the test conditions of Example 10, AV=1.0m/
The test was conducted in the same manner as in Example 10, except that the NH ₃ concentration was 500 to 2000 ppm. FIG. 9 shows the results of Example 11. From this figure, in order to prevent the by-product of N ₂ O,
It can be seen that it is preferable to set the NH ₃ /NO ₂ ratio to a stoichiometric value of 4/3 or more, particularly 1.6 or more. Comparative Example 8 Example 10 was carried out in the same manner as in Example 10, except that a titanium oxide-vanadium pentoxide catalyst (TiO ₂ -V ₂ O ₅ catalyst), which is commonly used in the ammonia reduction denitration method of NO, was used as the catalyst. I conducted a test. FIG. 10 shows a comparison of the results of Example 10 and Comparative Example 8. In the figure, D is for Example 10, and ratio 8 is for Comparative Example 8. This figure also shows that the method of the present invention, which uses a catalyst with a high N ₂ O decomposition rate under specific conditions, is an effective method. Examples 12-14 Under the test conditions of Example 10, AV was set at 3 m/h.
A test was conducted in the same manner as in Example 10, except that mordenite, haujasite, and zeolite Y substituted with hydrogen and iron were used as catalysts. Note that the hydrogen and iron substitution rates of each catalyst used at this time are based on Na, K or
It is shown in Table 2 when expressed in equivalent % with respect to the total amount of Ca.

Table 3 also summarizes the performance of the catalysts of Examples 12 to 14, and it can be seen from the results that hydrogen and iron substituted zeolites give good results when implementing the present invention.

[Table] As described above, according to the present invention, NO ₂ is the main
Catalytic ammonia reduction denitrification treatment of exhaust gas containing NO _x becomes possible.

[Brief explanation of drawings]

Figure 1 shows the NO ₂ ammonia reduction performance and N ₂ O by-product rate of the conventional catalyst, Figure 2 shows the N ₂ O ammonia reduction performance of the conventional catalyst, and Figure 3 shows the ammonia reduction performance of the conventional catalyst.
The figure is a schematic diagram of a denitrification device used in the present invention, Figure 4 is a diagram explaining the principle of the present invention, and Figures 5 and 6 are illustrations of a reactor each showing an embodiment of the present invention. 7 is a diagram showing the N ₂ O ammonia reduction performance of the catalyst according to the present invention and a comparative catalyst. FIG. 8 is a diagram showing the comparison results between Example 1 of the present invention and Comparative Example 7. FIG. 9 is a diagram showing the results of Example 11 of the present invention, and FIG. 10 is a diagram showing the results of Example 10 of the present invention. 1... In the case of Example 1, 2... In the case of Comparative Example 1, 3... In the case of Comparative Example 2, 4... To be treated gas piping, 5... Ammonia injection piping, 6, 9, 12,
13... Reactor, 8... Processing gas piping, 10...
NO2 _{- NH3} reaction active catalyst, 11... _N2O - _NH3
Reaction active catalyst.

Claims (1)

[Claims] 1. A denitrification catalyst that uses ammonia as a reducing agent to reduce exhaust gas containing nitrogen oxides,
The exhaust gas is an exhaust gas containing nitrous oxide (N ₂ O), and the above catalyst has the general formula xM _2/o O.
yAl ₂ O ₃ .zSiO ₂ .mH ₂ O (where x, y, z are integers,
Ammonia reduction characterized in that part or all of the M element of a compound having a zeolite structure represented by M is an arbitrary element and n is the valence of M is replaced with hydrogen and/or iron atoms. Denitrification catalyst. 2 A denitrification catalyst that uses ammonia as a reducing agent to reduce exhaust gas containing nitrous oxide (N ₂ O) as a nitrogen oxide, and has the general formula xM _2/o O.
yAl ₂ O ₃・zSiO ₂・mH ₂ O (where x, y, z are integers, M is an arbitrary element, and n indicates the atomic value of M) of the M element in a compound having a zeolite structure. 1. A denitrification apparatus comprising: a packed column filled with hydrogen and/or iron atoms partially or entirely substituted with hydrogen and/or iron atoms; and means for introducing a gas to be treated together with ammonia into the packed column. 3. A denitrification catalyst that uses ammonia as a reducing agent to reduce exhaust gas containing nitrogen oxides,
Zeolite represented by the general formula xM _2/o O・yAl ₂ O ₃・zSiO ₂・mH ₂ O (where x, y, z are integers, M is an arbitrary element, and n indicates the atomic value of M). Part or all of the M element of the compound having the structure is hydrogen or/
The exhaust gas is treated using a catalyst substituted with iron atoms and iron atoms at an areal velocity (gas amount/total outer surface area of the catalyst) of 3 m/h or less and a NH ₃ concentration/NO ₂ concentration ratio of 1.3 or less. (NO ₂ ) and ammonia (NH ₃ ) to produce nitrogen (N ₂ ), nitrous oxide (N ₂ O) and water (H ₂ O);
An ammonia reduction denitrification method characterized by simultaneously performing both reactions to generate N ₂ and H ₂ O from N ₂ O and NH ₃ . 4 A catalyst active in the reaction of producing N ₂ , N ₂ O and H ₂ O from NO _{2 and} NH 3 is packed on the upstream side, and a catalyst described in claim 3 is packed on the downstream side. using a reactor with an area velocity of 3
4. The ammonia reduction denitrification method according to claim 3, wherein the exhaust gas is treated under conditions where the NH ₃ concentration/NO ₂ concentration ratio is 1.3 or more.