JPH05277376A - Nox removing catalyst and nox removing method utilizing the catalyst - Google Patents

Abstract

PURPOSE:To obtain a catalyst capable of efficiently removing NOx and having excellent high-temp. heat resistance by incorporating at least one kind selected from among fluorides of manganese, lead, lithium, rare-earth element and alkaline-earth metal into the catalyst. CONSTITUTION:At least one kind selected from among fluorides of manganese, lead, lithium, rare-earth element and alkaline-earth metal is incorporated into the NOx removing catalyst. At least one kind of metal selected from a group consisting of platinum, palladium, copper, cobalt, nickel and iron is preferably deposited on the fluoride. Meanwhile, a refractory inorg. oxide can be added to the component.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a catalyst for removing nitrogen oxides. For details, internal combustion engines such as automobile engines,
For example, the present invention relates to a catalyst that removes nitrogen oxides in exhaust gas discharged from internal combustion engines such as gasoline engines, diesel engines, boilers, and industrial plants.

[0002]

2. Description of the Related Art In recent years, exhaust gas emitted from internal combustion engines such as automobiles, boilers, and industrial plants contains harmful components of nitrogen oxides (hereinafter, nitrogen oxides may be collectively referred to as NOx). , Causing air pollution. Therefore, removal of NOx in the exhaust gas has been studied from various viewpoints.

Conventionally, for example, in the case of automobile exhaust gas, a method has been used in which the exhaust gas is treated using a three-way catalyst to remove hydrocarbons (HC) and carbon monoxide (CO) as well as NOx. This method is carried out under the condition that the air (the ratio of air and fuel is referred to as "A / F") is introduced so that the fuel can be completely burned. However, when the ratio of air to fuel increases (hereinafter referred to as "oxidizing atmosphere state"),
Excessive amount of oxygen exists than the amount of oxygen required to completely burn unburned components such as hydrocarbons and carbon monoxide in the exhaust gas, and in such an oxidizing atmosphere state,
It is difficult to reduce and remove NOx with a normal three-way catalyst.

Further, when removing nitrogen oxides in a diesel engine or a boiler of an internal combustion engine, it is common to use a reducing agent such as ammonia, hydrogen or carbon monoxide. However, this method has a problem that a special device is required for collecting and treating unreacted reducing agent.

Recently, as a method for removing NOx, a method using a NOx decomposition catalyst composed of a crystalline aluminosilicate containing copper ions has been proposed (Japanese Patent Laid-Open No. 60-12).
5250, U.S. Pat. No. 4,297,328), which simply states that nitric oxide (NO) can be decomposed into nitrogen (N ₂ ) and oxygen (O ₂ ). However, it is difficult to effectively remove nitrogen oxides under actual exhaust gas conditions.

Further, in Japanese Patent Laid-Open No. 63-100919, when exhaust gas is treated with a copper-containing catalyst in the presence of hydrocarbons in an oxidizing atmosphere, the reaction between NOx and hydrocarbons is preferentially promoted. It is described that NOx can be efficiently removed. The hydrocarbons used in this method are:
It is said that it may be a hydrocarbon contained in the exhaust gas or a hydrocarbon added from the outside as needed. As a specific embodiment, the exhaust gas is first contacted with a copper-containing catalyst to remove NOx, and then oxidized. A method for removing hydrocarbons, carbon monoxide and the like by contacting with a catalyst is also disclosed.
This method has a high temperature at which nitrogen oxides can be removed, and its effect is low at low temperatures.

Further, the above catalyst has poor heat resistance and its NOx decomposing performance deteriorates when exposed to high temperature exhaust gas. Therefore, as a countermeasure against this, the above catalysts are arranged in parallel, and when the exhaust gas becomes high temperature, it is oxidized. A method of bypassing the catalyst or the three-way catalyst has been disclosed (JP-A-1-171625).

As described above, the present situation is that no catalyst for decomposing nitrogen oxides which efficiently decomposes and removes NOx in exhaust gas and has excellent high temperature heat resistance has been developed.

[0009]

The object of the present invention is NOx.
It is intended to provide a catalyst for removing nitrogen oxides, which efficiently removes nitrogen oxides and has excellent high temperature heat resistance.

[0010]

Means for Solving the Problems As a result of intensive studies for solving the above problems, the present inventors have found that a group consisting of manganese (Mn), lead (Pb), lithium (Li), rare earths and alkaline earth metals. The present invention has been completed by using a catalyst containing at least one kind of fluoride selected from the above as a catalyst for purifying a nitrogen oxide-containing gas.

More specifically, nitrogen oxide removal characterized by comprising a catalytically active substance containing at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earths and alkaline earth metals. For catalyst.

This catalytically active substance preferably contains at least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron. Further, the catalytically active substance may contain a refractory inorganic oxide.

At least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and at least one selected from the group consisting of manganese, lead, lithium, rare earth and alkaline earth metals. When using one kind of fluoride as a catalytically active substance, at least one kind of metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron is added to manganese, lead, lithium, a rare earth element and an alkali. It is preferably carried on at least one fluoride selected from the group consisting of earth metals.

When using at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals, a refractory inorganic oxide and platinum, a refractory inorganic material is used. Platinum as an oxide, powder supporting platinum on a refractory inorganic oxide, manganese, lead, lithium,
It is preferably mixed with at least one fluoride selected from the group consisting of rare earth elements and alkaline earth metals.

The method for removing nitrogen oxides using the catalyst of the present invention includes (a) at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earths and alkaline earth metals. A catalytically active substance containing
(B) at least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and at least one selected from the group consisting of manganese, lead, lithium, rare earths and alkaline earth metals A catalytically active substance containing the fluoride of, or (iii) platinum, palladium,
At least one metal selected from the group consisting of copper, cobalt, nickel and iron, and at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earth and alkaline earth metals, and fire resistance A catalyst in which a monolithic structure is coated with a catalytically active substance containing an inorganic oxide,
It is preferable to pass exhaust gas in an oxidizing atmosphere in the presence of hydrocarbons. The present invention will be described in detail below.

The fluoride according to the present invention comprises Mn, In and P
It is at least one kind of fluoride (hereinafter, also referred to as fluoride) selected from the group consisting of b, Sn, rare earth elements and alkaline earth metals, and preferably lithium fluoride.

These fluorides can be molded into a predetermined shape, for example, a spherical shape or a cylindrical shape, and used.
It can be coated on an inert carrier such as a silicone carbide, and an integrated structure for exhaust gas purification such as an open honeycomb, a plug honeycomb, a ceramic foam, a metal mesh and a metal honeycomb, and is preferably used as an integrated structure. It is used by coating.

When the inactive carrier or monolithic structure is coated with a fluoride, it is preferably coated in an amount of 100 to 500 g, and more preferably 150 to 300 g, per liter of the inert carrier or monolithic structure. Is.
When it is less than 100 g, the activity of removing NOx is lowered, and when it exceeds 500 g, the activity corresponding to the supported amount cannot be obtained.

Further, in the present invention, Pt,
It is also possible to support at least one metal selected from the group consisting of Pd, Cu, Co, Ni and Fe (hereinafter referred to as additive component) to serve as a catalyst, and a preferred combination of this fluoride and additive component is The fluoride is an alkaline earth metal and the additive component is Pt.

The loading ratio of the added component is
0.1% by weight to 10% by weight, preferably 0.5
% To 5% by weight. When it is less than 0.1% by weight, the NOx removal rate in the low temperature range is lowered,
When it exceeds 10% by weight, the NOx removal efficiency commensurate with the supported amount cannot be obtained.

As the additive component source, nitrates, sulfates, acetates, oxalates and the like are used.

In addition to the fluoride and the additive component, a refractory inorganic oxide can be further added. This refractory inorganic oxide may be any as long as it is usually used for a catalyst, for example, α-alumina, or γ, δ,
Activated alumina such as η and θ, titania, zirconia or their composite oxides, alumina-titania, alumina-
Zirconia, titania-zirconia, etc. can be used. Activated alumina is particularly preferable. Further, these refractory inorganic oxides have a BET surface area of 50 to 200 m.
It is preferably a refractory inorganic oxide having ² / g.

When the refractory inorganic oxide is added as described above, when the additive component has a small volume per unit weight such as platinum, the degree of dispersion can be improved, which is preferable.

The content of the fluoride of alkaline earth metal is preferably 1 to 30% by weight based on the refractory inorganic oxide. More preferably, it is 5 to 20% by weight. When it is less than 1% by weight with respect to the refractory inorganic oxide, the NOx removal efficiency in the high temperature range decreases, and 30% by weight
When it exceeds, the NOx removal efficiency in each temperature range decreases.

A method of supporting platinum on the refractory inorganic oxide will be described below by taking refractory inorganic oxide and platinum as examples.
For example, (1) a method in which a refractory inorganic oxide is impregnated with an aqueous solution of a platinum salt, followed by drying and firing, (2) a refractory inorganic oxide is added to the aqueous solution of a platinum salt, and the mixture is mixed with a reducing agent such as hydrazine. It is a method of carrying by reduction.

When the inert carrier or the monolithic structure is coated with a fluoride carrying an additive component and used, 100 g to 500 per 1 liter of the inert carrier or the monolithic structure is used.
g coating, more preferably 150
It is g-300g.

When the amount is less than 100 g, the activity of removing NOx decreases, and when it exceeds 500 g,
The activity corresponding to the loaded amount cannot be obtained.

As a method of supporting the additive component on the fluoride, there can be used a method of adding an aqueous solution of the additive component to the fluoride, mixing, drying and firing.

[0029]

The catalyst for removing nitrogen oxides of the present invention is NOx.
It has the advantages of excellent decomposition performance and high temperature heat resistance. A catalyst composed of at least one fluoride selected from Mn, Pb, Li, rare earth elements and alkaline earth metals can efficiently remove NOx, and the fluoride is stable even when exposed to high temperatures. Being a substance, it has excellent high temperature durability. In addition, Pt, P
By supporting at least one metal selected from the group consisting of d, Cu, Co, Ni and Fe, the efficiency of purifying carbon monoxide and hydrocarbons from the low temperature range is improved, and NO
The x removal efficiency is also improved.

At least one fluoride selected from the group consisting of Mn, Pb, Li, rare earth elements and alkaline earth metals is a usual metal oxide (eg, alumina).
Is superior in NOx adsorption property to N.sub.2, and therefore, due to the reaction of many NOx on the catalyst surface with hydrocarbons,
It is considered that Ox can be decomposed and removed.

[0031]

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

(Example 1) Lithium fluoride [LiF] 1
A 00 g powder was wet pulverized with a ball mill to obtain an aqueous slurry, and a commercially available cordierite honeycomb carrier (made by Nippon Insulator, having a cross section of 1 inch square, 400 gas distribution cells, a diameter of 33 mmφ, Length 76mm,
After immersing a volume of 65 ml), the excess slurry was blown off by compressed air. Then, it was dried at 120 ° C. for 2 hours to obtain a finished catalyst (a). This catalyst was loaded with 200 g / (catalyst liter) of lithium fluoride.

Example 2 Instead of 100 g of lithium fluoride in Example 1, 100 g of lead fluoride [PbF ₂ ] was used.
The same procedure as in Example 1 is carried out except that is used to prepare the finished catalyst (b).
Got This catalyst supported lead fluoride in an amount of 200 g / (catalyst liter).

Example 3 In Example 1, 100 g of lithium fluoride was replaced by manganese fluoride [MnF ₂ ] 1
The same procedure as in Example 1 was carried out except that 100 g was used to obtain a finished catalyst (c). This catalyst contains 200 g of manganese fluoride /
(Catalyst liter) was supported.

(Example 4) In Example 1, 100 g of lithium fluoride was replaced with cerium fluoride [CeF ₃ ] 10
The same procedure as in Example 1 was carried out except that 0 g was used to obtain a finished catalyst (d). This catalyst contains 200 g of cerium fluoride /
(Catalyst liter) was supported.

Example 5 Magnesium fluoride [MgF ₂ ] was used instead of 100 g of lithium fluoride in Example 1.
The same procedure as in Example 1 was carried out except using 100 g to obtain a finished catalyst (e). This catalyst has 20 magnesium fluoride
It was supported by 0 g / (catalyst liter).

Example 6 In Example 1, 100 g of lithium fluoride was replaced with calcium fluoride [CaF ₂ ] 1
A completed catalyst (f) was obtained in the same manner as in Example 1 except that 100 g was used. This catalyst contains 200g of calcium fluoride
/ (Catalyst liter) was supported.

(Example 7) An aqueous solution of dinitdiammine platinum containing 0.25 g of platinum was added to 100 g of the cerium fluoride used in Example 4, mixed, dried at 120 ° C for 2 hours, and calcined at 500 ° C for 2 hours. did. The obtained powder was wet pulverized with a ball mill to prepare an aqueous slurry. Thereafter, the same procedure as in Example 1 was carried out to obtain a finished catalyst (g). In this catalyst, 0.5 g / (catalyst liter) of platinum and 200 g / (catalyst liter) of cerium fluoride were supported.

(Embodiment 8) In Embodiment 7, platinum 0.
A completed catalyst (h) was obtained in the same manner as in Example 7 except that an aqueous solution of palladium nitrate containing 0.25 g of palladium was used in place of the aqueous solution of dinitrodiammine platinum containing 25 g. This catalyst was loaded with 0.5 g of palladium / (catalyst liter) and cerium fluoride of 200 g / (catalyst liter).

Example 9 100 g of the magnesium fluoride used in Example 5 was added to copper acetate [Cu (CH ₃ COO) ₂ .H
₂ O] An aqueous solution containing 3.2 g was added and mixed, and the temperature was 120 ° C.
And dried at 500 ° C. for 2 hours. The obtained powder was wet pulverized with a ball mill to prepare an aqueous slurry. Thereafter, the same procedure as in Example 1 was carried out to obtain a finished catalyst (i). This catalyst was loaded with copper 2 g / (catalyst liter) and magnesium fluoride 200 g / (catalyst liter). (Example 10) In Example 9, instead of 3.2 g of copper acetate, cobalt acetate [Co (CH ₃ COO) ₂ · 4H ₂ O]
The same procedure as in Example 9 was carried out except that 4.2 g was used to obtain a finished catalyst (j). This catalyst was loaded with cobalt 2 g / (catalyst liter) and magnesium fluoride 200 g / (catalyst liter).

Example 11 In Example 9, nickel acetate [Ni (CH ₃ COO) ₂ was used instead of 3.2 g of copper acetate.
4H ₂ O] The same procedure as in Example 9 was carried out except that 8.5 g was used to obtain a finished catalyst (k). This catalyst is nickel 4g
/ (Catalyst liter), magnesium fluoride 200g /
(Catalyst liter) was supported.

Example 12 In Example 9, iron oxalate [Fe (COO) ₂ .2H ₂ was used instead of 3.2 g of copper acetate.
O] was performed in the same manner as in Example 9 except that 3.4 g was used to obtain a finished catalyst (l). This catalyst was loaded with iron 2 g / (catalyst liter) and magnesium fluoride 200 g / (catalyst liter).

(Comparative Example 1) A method for preparing a ZSM-5 type zeolite is described in the literature (Rapid Crystallizeat).
ion Methods, Proceedings 8t
h International Congress
on Catalysis, Berlin, 1984,
Vol. 3. P569). The obtained zeolite was confirmed to be ZSM-5 type by X-ray diffraction. To 100 g of this ZSM-5 type zeolite, 400 g of pure water was added, and the mixture was stirred at 98 ° C. for 2 hours and then at 80 ° C.
A 2 mol / liter copper ammine complex aqueous solution was slowly added dropwise. After the dropping was completed, the mixture was heated and stirred at 80 ° C. for 12 hours for ion exchange. Further, the ion-exchanged zeolite was filtered, washed thoroughly with water, and then dried at 120 ° C. for 24 hours. The obtained powder was wet pulverized with a ball mill to obtain an aqueous slurry. Thereafter, the same procedure as in Example 1 was carried out to obtain a finished catalyst (I). This catalyst had 5.7% by weight of copper supported on the ZSM-5 type zeolite.

Test Example 1 Catalysts (a) prepared in Examples 1 to 6 and Comparative Example 1
A catalytic activity test was conducted on (f) and (I) under the following conditions.

[Initial Performance Test] Diameter 34.5 mmφ,
After filling a 300 mm long stainless steel reaction tube with a catalyst, a reaction gas having the following composition was introduced under the condition of a space velocity of 20000 Hr- ¹ . The catalyst performance was evaluated by measuring the NOx purification rate at the catalyst layer inlet temperature of 400 ° C. (initial performance). The results are shown in Table 1.

[Reaction Gas Composition] Nitric oxide (NO) 75
0 ppm, propylene (C ₃ H ₆ ) 1000 ppm (methane conversion), carbon monoxide (CO) 0.2% by volume, oxygen 2.
0% by volume, 10% by volume of water, 13.5% by volume of carbon dioxide,
The rest is nitrogen.

[Aging Performance Test] Each catalyst was packed in a multi-converter, and the packed catalyst bed was mixed with the exhaust gas at the time of cruising of a commercially available gasoline electronically controlled engine with air to obtain an air-fuel ratio (A / F) of 20. / (Catalyst liter), space velocity (SV) 160,000 / H
The catalyst was passed under conditions of r and catalyst bed temperature of 700 ° C. for 20 hours. Then, the same operation as in the initial performance test was performed to measure the NOx purification rate to evaluate the catalyst performance (time-dependent performance). The results are shown in Table 1.

[0048]

[Table 1]

Test Example 2 The oxygen concentration in the reaction gas was 2.
The same procedure as in Test Example 1 was performed except that the content was changed from 0% by volume to 10% by volume, and the initial performance and aging performance of each catalyst were evaluated. The results are shown in Table 2.

[0050]

[Table 2]

Test Example 3 A catalyst activity test was conducted on the catalysts (g) to (l) and (I) prepared in Examples 7 to 12 and Comparative Example 1, and the catalyst layer inlet temperature in Test Example 1 was 350 ° C.
I went there. The results of initial performance and aging performance are shown in Table 3.

[0052]

[Table 3]

Test Example 4 The catalyst used in Test Example 3 was tested in the same manner as in Test Example 2, and the results of initial performance and aging performance are shown in Table 4.

[0054]

[Table 4]

Example 13 BET surface area 100 m ² /
An aqueous solution of dinitrodiammine platinum containing 5 g of platinum was added to 100 g of activated alumina having 100 g and mixed, and the mixture was heated at 120 ° C.
And dried at 500 ° C. for 2 hours. The obtained powder and 10 g of magnesium fluoride [MgF ₂ ] were wet pulverized with a ball mill to obtain an aqueous slurry, and a commercially available cordierite honeycomb carrier (manufactured by Nippon Insulator, 400 cross sections per 1 inch square) Has a gas distribution cell of
After dipping 33 mmφ in diameter, 76 mm in length, and 65 ml in volume), the excess slurry was blown off by compressed air. Then, it was dried at 120 ° C for 2 hours and calcined at 500 ° C for 2 hours to obtain a finished catalyst (A). This catalyst contains 5% by weight of platinum and 10% of magnesium fluoride based on activated alumina.
% By weight.

(Example 14) In Example 13, 5 g of platinum was replaced by 2 g of platinum instead of the dinitrodiammine platinum aqueous solution.
A completed catalyst (B) was obtained in the same manner as in Example 13 except that a dinitrodiammine platinum aqueous solution containing was used. This catalyst contained 2% by weight of platinum and 10% by weight of magnesium fluoride based on activated alumina.

(Example 15) A completed catalyst (C) was prepared in the same manner as in Example 13 except that 5g of magnesium fluoride was used instead of 10g of magnesium fluoride.
Got This catalyst contained 5% by weight of platinum and 5% by weight of magnesium fluoride with respect to activated alumina.

(Example 16) In Example 13, 20 g of magnesium fluoride was used instead of 10 g of magnesium fluoride.
The same procedure as in Example 13 was carried out except that g was used to obtain a finished catalyst (D). This catalyst contained 5% by weight of platinum and 20% by weight of magnesium fluoride with respect to activated alumina.

Example 17 In Example 13, 10 g of magnesium fluoride was replaced by calcium fluoride [Ca
F ₂ ] 10 g, but in the same manner as in Example 13,
A finished catalyst (E) was obtained. This catalyst contained 5% by weight of platinum and 10% by weight of calcium fluoride with respect to activated alumina.

Example 18 A completed catalyst (F) was obtained in the same manner as in Example 13 except that 10 g of strontium fluoride [SrF ₂ ] was used instead of 10 g of magnesium fluoride. This catalyst contained 5% by weight of platinum and 10% of strontium fluoride based on activated alumina.
% By weight.

Example 19 In Example 13, barium fluoride [BaF] was used instead of 10 g of magnesium fluoride.
₂ ] The same procedure as in Example 13 was carried out except that 10 g was used to obtain a finished catalyst (G). This catalyst contained 5% by weight of platinum and 10% by weight of barium fluoride with respect to activated alumina.

Comparative Example 2 An aqueous solution of dinitrodiammineplatinum containing 5 g of platinum was added to 100 g of the activated alumina used in Example 13, mixed and dried at 120 ° C. for 2 hours.
Calcination was carried out for 2 hours. The obtained powder was wet pulverized with a ball mill to obtain an aqueous slurry.

Thereafter, the same procedure as in Example 13 was carried out to obtain a finished catalyst (II). This catalyst contained 5% by weight of platinum based on activated alumina.

Comparative Example 3 Comparative Example 1 ZSM-5 obtained
Type zeolite was ion-exchanged with copper in the same manner as in Comparative Example 1, and ZSM-5 containing 5.6% by weight of copper.
A type zeolite was obtained. Thereafter, the completed catalyst (III) was obtained in the same manner as in Example 13.

(Example 20) Catalysts (A) to (G) and (II) to prepared in Examples 13 to 19 and Comparative Examples 2 to 3
For (III), a catalyst activity test was conducted under the following conditions.

In a stainless steel reaction tube having a diameter of 34.5 mmφ and a length of 300 mm, propylene was 1000 ppm (in terms of methane), CO was 0.2% by volume, O ₂ was 2.2% by volume, water vapor was 10% by volume, and dioxide was added. 13.5% carbon
And the balance is nitrogen gas with a space velocity of 20,0
It was introduced under the condition of 00Hr- ¹ . Catalyst inlet temperature is 200 ℃
The catalyst was evaluated in the range of up to 400 ° C, and the results are shown in Table 5.

Further, the reaction gas of the above catalyst evaluation is oxygen 2.
Change 0% by volume to 10% by volume, and similarly evaluate the catalyst,
The results are shown in Table 6.

[0068]

[Table 5]

[0069]

[Table 6]

Claims (6)

[Claims] 1. Nitrogen containing a catalytically active substance containing at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. Oxide removal catalyst. 2. At least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. A catalyst for removing nitrogen oxides, which comprises a catalytically active substance comprising at least one kind of fluoride. 3. At least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. A catalyst for removing nitrogen oxides, which comprises a catalytically active substance composed of at least one kind of fluoride and a refractory inorganic oxide. 4. At least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron is selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. The catalyst for removing nitrogen oxides according to claim 2, comprising a catalytically active substance obtained by being supported on at least one kind of fluoride. 5. A powder obtained by supporting a platinum on a refractory inorganic oxide is mixed with at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. A catalyst for removing nitrogen oxides, comprising the obtained catalytically active substance. 6. A catalytically active substance containing (a) at least one fluoride selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals,
(B) At least one metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and at least one selected from the group consisting of manganese, lead, lithium, rare earth elements and alkaline earth metals. A catalytically active substance containing one kind of fluoride, or (c) at least one kind of metal selected from the group consisting of platinum, palladium, copper, cobalt, nickel and iron, and manganese, lead, lithium and a rare earth element. And at least one fluoride selected from the group consisting of alkaline earth metals and a catalytically active substance containing a refractory inorganic oxide in the presence of a hydrocarbon in a catalyst formed by coating the monolithic structure. The method for removing nitrogen oxides is characterized in that the exhaust gas in an oxidizing atmosphere is passed through.