JPH10113559A - Exhaust gas purifying catalyst layer, exhaust gas purifying catalyst structure and exhaust gas purifying process using them - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst layer and a catalyst coating structural body for removing NOx efficiently in diluted combustion exhaust gas and also provide an exhaust gas purifying process for purifying NOx in diluted combustion exhaust gas with high efficiency and high reliability by using the above-referred layer and the structural body. SOLUTION: An exhaust gas purifying catalyst layer is constituted of a catalyst A composed of alumina and a catalyst B composed of an alumina carrier containing silver having a pore structure in which Y is 70% or more of X, and Z is 20% or less of X when the total value of pore volume of pores of radius of 300 angstrom or smaller is set as X, and the total value of pore volume of pores of radius larger than 25 angstrom and smaller than 100 angstrom is set as Y, and the total value of pore volume of pores of diameter larger than 100 angstrom to smaller than 300 angstrom is set as Z in the relationship of the pore radius and the pore volume measured by the nitrogen gas adsorption process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to exhaust gas used for purifying nitrogen oxides contained in combustion exhaust gas of mobile and stationary internal combustion engines of automobiles, boilers, gas engines, gas turbines, ships and the like. The present invention relates to a purification catalyst layer and an exhaust gas purification catalyst-coated structure, and more specifically, it can purify nitrogen oxides in exhaust gas discharged from an internal combustion engine operated in a lean burn region at a high space velocity and with high efficiency. The present invention relates to an exhaust gas purifying catalyst layer and an exhaust gas purifying method.

[0002]

2. Description of the Related Art In a variety of combustion exhaust gas discharged from an internal combustion engine such as an automobile, nitrogen oxides (NOx) such as nitric oxide and nitrogen dioxide are included together with water and carbon dioxide as combustion products. include. NOx not only adversely affects the human body, particularly the respiratory system, but also is one of the causes of acid rain, which is regarded as a problem from the viewpoint of global environmental protection. Therefore, development of a denitration technology for efficiently removing nitrogen oxides from these various exhaust gases is desired.

On the other hand, in view of the prevention of global warming, lean-burn internal combustion engines have recently attracted attention. A conventional gasoline engine for an automobile has an air-fuel ratio (A / F) = 1.
Combustion at a controlled stoichiometric ratio near 4.7. For exhaust gas treatment, carbon monoxide, hydrocarbons and NOx in the exhaust gas are converted to alumina mainly containing platinum, rhodium, palladium and ceria. A three-way catalyst system has been employed in which three harmful components are simultaneously removed by contact with a catalyst.

However, this three-way catalyst system cannot be applied to exhaust gas purification of a lean burn gasoline engine operated at a lean air-fuel ratio because it is an absolute condition that the engine is operated at a stoichiometric ratio. . In addition, diesel engines are originally lean burn engines, but strict regulations are being imposed on both the suspended particulate matter and NOx in the exhaust gas.

Conventionally, as a method of reducing removing NOx in an oxygen-rich atmosphere, a technique of using NH ₃ also adsorb selectively catalyst small amount as the reducing gas has already been established. This technology has been industrialized as a method for denitration of exhaust gas from boilers and diesel engines, which are so-called fixed sources, but this method requires a special device for the recovery treatment of the reducing agent of the end reaction, and also has an odor. Since it uses highly harmful ammonia, it cannot be applied as a technique for denitration of exhaust gas from mobile sources such as automobiles because of its danger.

In recent years, it has been reported that a NOx reduction reaction can be promoted by using unburned hydrocarbons remaining in a lean combustion exhaust gas in an oxygen-excess atmosphere as a reducing agent. Various catalysts have been developed and reported. For example, there are many reports that alumina or a catalyst in which a transition metal is supported on alumina is effective for a NOx reduction reaction using a hydrocarbon as a reducing agent. Also, JP-A-4-284848 discloses that
0.1 to 4% by weight of Cu, Fe, Cr, Zn, Ni, V
NOx containing alumina or silica-alumina containing
An example of use as a reduction catalyst has been reported.

Further, when a catalyst in which Pt is supported on alumina is used, the NOx reduction reaction proceeds in a low temperature range of about 200 to 300 ° C., as disclosed in JP-A-4-267946 and JP-A-5-68855. JP-A-5-1039
No. 49, for example. However, when these supported noble metal catalysts are used, the combustion reaction of hydrocarbons as a reducing agent is excessively promoted, and N ₂ O which is one of the substances causing global warming is by-produced, be advanced reduction reaction to harmless N ₂ selectively had disadvantage becomes difficult.

The inventor of the present applicant has previously found that the use of a catalyst containing silver as a reducing agent with a hydrocarbon in an oxygen-excess atmosphere allows the NOx reduction reaction to proceed selectively. No. 4-281844.

Even after this disclosure is made, a NOx reduction and removal technology similar to the technology described in this publication is disclosed in Japanese Patent Laid-Open No.
JP-A-354536, JP-A-5-92124, JP-A-5-92125 and JP-A-6-27745
No. 4 and the like.

[0010]

However, the alumina-supported silver catalysts described in these conventional publications are not suitable for SOx.
And the denitration performance in the presence of steam was still insufficient for practical use.

The present invention has been made to solve the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide an exhaust gas purifying catalyst layer and a catalyst capable of efficiently removing NOx in a lean combustion exhaust gas. An object of the present invention is to provide a coating structure and an exhaust gas purifying method for purifying NOx in lean combustion exhaust gas with high efficiency and high reliability using the same.

[0012]

Means for Solving the Problems The present inventors have developed an exhaust gas purifying catalyst layer and an exhaust gas purifying catalyst coating structure having high denitration performance in a lean burn region where steam and SOx coexist, and using these. As a result of intensive studies on the exhaust gas purification method, the catalyst A composed of alumina was placed in the first stage in the flow direction of exhaust gas, and the catalyst B containing silver in alumina having a specific pore structure was placed in the second stage. It has been found that the above problems can be solved by arranging them, and the present invention has been completed.

That is, a first embodiment of the present invention for solving the above-mentioned problem is that the relationship between the catalyst A made of alumina and the pore radius and the pore volume measured by the nitrogen gas adsorption method is as follows. The total value of the pore volume occupied by pores having a radius of 300 Å or less is defined as 、, the total value of pore volume occupied by pores having a pore radius of 25 Å or more and less than 100 Å is defined as Y, and a pore radius of 100 Å or more is defined. When the total value of the pore volume occupied by pores of 300 Å or less is defined as Ζ, Y is 70
% Or more and Ζ is 20% or less of Χ. The exhaust gas-purifying catalyst layer is constituted by a catalyst B comprising silver contained in an alumina carrier having a pore structure. The catalyst layer is preferably disposed in a flow space of the exhaust gas in a powdered or molded state.

In a second embodiment of the present invention, a support substrate made of a refractory material having a large number of through-holes, and the above-mentioned catalyst is coated on at least the inner surface of the through-holes of the support substrate. The present invention is characterized by the catalyst coated structure described above.

Still further, a third embodiment of the present invention is directed to a method of removing NOx in exhaust gas by bringing combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio into contact with a catalyst-containing layer. The catalyst contained is the catalyst layer in the first embodiment or the catalyst-coated structure in the second embodiment, and is divided such that the catalyst A is at the front stage and the catalyst B is at the rear stage in the flow direction of the exhaust gas. It is characterized by an exhaust gas purification method arranged in a manner as described above.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention and its operation will be described more specifically below. (Catalyst Structure and Method for Producing the Same) Alumina, which is one of the main components of the catalyst A and the catalyst B in the exhaust gas purifying catalyst layer of the present invention, is, for example, boehmite, pseudoboehmite, vialite or norstrandite in terms of mineralogy. The aluminum hydroxide powder or gel classified in the above is heated and dehydrated in air or vacuum at 300 to 800 ° C., preferably 400 to 900 ° C. to crystallographically γ-type, η-type, δ
Those obtained by phase transition to alumina classified into-type, χ-type or a mixed type thereof are preferable from the viewpoint of denitration performance. Alumina having another crystal structure, for example, α-type alumina, is unsuitable as the catalyst component of the present invention because of its extremely small specific surface area and poor solid acidity.

In particular, the alumina of the catalyst B is defined as the sum of the pore volume occupied by pores having a pore radius of 300 angstroms or less as measured by a nitrogen gas adsorption method, and 100% when the pore radius is 25 angstroms or more. When the total value of the pore volume occupied by pores smaller than Å is defined as Y, and the total value of the pore volume occupied by pores having a pore radius of 100 Å or more and 300 Å or less is defined as Ζ, Y is 70 of Χ. % Or more and Ζ is 20% or less of Χ. In the case where alumina having a pore structure not satisfying the above-mentioned conditions is used as a carrier in the catalyst B of the present invention, the exhaust gas purifying catalyst constituted thereby has insufficient denitration performance of exhaust gas in the presence of steam. there were. Therefore, it can be said that alumina having the above-mentioned crystal structure and pore characteristics is suitable as the alumina effective as a component of the catalyst B of the present invention.

The exhaust gas purifying catalyst layer of the present invention is the following catalyst. The catalyst layer according to the present invention is composed of a catalyst A made of alumina and a catalyst B made of alumina having the above-mentioned crystal structure and pore characteristics containing silver. The state of silver contained in the alumina of the catalyst B is not particularly limited, and examples thereof include a metal state, an oxide state, and a mixed state thereof. In particular, the composition of the combustion exhaust gas of an internal combustion engine such as an automobile changes each time depending on the operation state, and thus the catalyst is exposed to a reducing atmosphere and an oxidizing atmosphere. Therefore, it is assumed that the state of the active metal constituting the catalyst changes with the change of the atmosphere. The starting material of silver in the catalyst B is not particularly limited.

The method of incorporating silver into alumina in the catalyst B according to the present invention is not particularly limited, and any of the conventional methods such as an adsorption method, a pore filling method,
Any known method such as an impregnating method such as an incipient wetness method, an evaporation to dryness method, and a spray method, a kneading method, a physical mixing method, and a combination thereof can be arbitrarily adopted. In this case, the alumina is dried and fired after the silver salt is supported on the alumina precursor material. Further, a method for producing a catalyst containing an active metal at the time of producing alumina or an alumina carrier having a specific pore structure as described above, for example, mixing an alcohol solution of aluminum alkoxide and an alcohol solution of silver salt, followed by heating. An alkoxide method for hydrolysis and a coprecipitation method in which an alkali is added to a mixed aqueous solution of an aluminum salt and a silver salt for precipitation are also applicable. The content of silver in terms of metal relative to alumina in catalyst B is not particularly limited, but is 0.1 to 10% by weight.
Is preferable. If the amount of silver carried is less than 0.1% by weight, the effect is not exhibited, and if it exceeds 10% by weight, the combustion reaction of hydrocarbons as a reducing agent proceeds preferentially, and NOx
The removal characteristics deteriorate.

The drying temperature of the catalyst B is not particularly limited, and it is usually dried at about 80 to 120 ° C. The firing temperature is 300 to 1000 ° C., preferably 400 to 900.
It is about ° C. If the firing temperature exceeds 1000 ° C, α-
It is not preferable because phase transformation to alumina occurs. The atmosphere at this time is not particularly limited, but each atmosphere such as in air, in an inert gas, or in oxygen may be appropriately selected according to the catalyst composition. In addition, each atmosphere may be alternately changed at regular intervals.

In the first embodiment of the present invention, when a catalyst layer for purifying exhaust gas is formed, the catalyst layer is formed into a predetermined shape or a predetermined shape through which a target exhaust gas flows in a powder state. Fill in the space. When forming the catalyst layer into a molded body, the shape is not particularly limited, for example, powder, spherical, cylindrical, honeycomb, spiral, granular,
Various shapes such as a pellet shape and a ring shape can be adopted. These shapes, sizes, and the like may be arbitrarily selected according to use conditions.

Next, an exhaust gas purifying catalyst-coated structure according to a second embodiment of the present invention will be described. The catalyst-coated structure referred to here has a structure in which at least the inner surface of the through-hole is coated with the above-mentioned catalyst in a support substrate having an integral structure made of a refractory material having a large number of through-holes. It is.

The support substrate is provided with a large number of through holes along the flow direction of the exhaust gas. In a section perpendicular to the flow direction, the porosity is usually 60 to 90%, preferably 70 to 90%. And the number is one square inch (5.0
30 to 700, preferably 200 to 6, 6 cm ² )
00. The catalyst is coated on at least the inner surface of the through hole, but may be coated separately on the end face or side face of the supporting substrate.

As the material of the refractory support substrate, ceramics such as α-alumina, mullite, cordierite, and silicon carbide, and metals such as austenitic and ferritic stainless steels are used. Conventional shapes such as honeycombs and foams can be used. Preferred is a honeycomb-shaped support substrate made of cordierite or stainless steel.

The method of coating the support substrate with a catalyst includes:
A so-called ordinary wash coat method or a sol-gel method, in which the catalyst of the present invention sized to a predetermined particle size is coated separately with or without a binder on the inner surface of the support substrate, can be applied. Alternatively, the support substrate may be coated with alumina in advance, and then the catalyst active layer of the present invention may be subjected to a treatment to form a catalyst coating layer. The coating amount of the catalyst layer on the supporting substrate is not limited,
It is preferably about 50 to 250 g / l, more preferably about 100 to 200 g / l, per unit volume of the supporting substrate.

Next, an exhaust gas purifying method according to a third embodiment of the present invention will be described. A third embodiment of the present invention provides:
By using the catalyst layer of the first embodiment and the catalyst-coated structure of the second embodiment, CO, ΗC and Η in the exhaust gas are used.
By _two such reducing component contacting a flue gas containing excess oxygen from the stoichiometry near required to complete oxidation with oxidizing components such as NOx and O _2,
NOx is reduced and decomposed to N ₂ and { ₂ O}
Reducing agent such as C are also oxidized to Eta ₂ O and CΟ _2.

In the present invention, the reason why the catalyst A is disposed at the front stage and the catalyst B is disposed at the rear stage is that SOx is adsorbed and removed by the catalyst A at the front stage, so that S in the total catalyst system is removed.
This is for improving the Ox durability. The ratio between the catalyst A and the catalyst B may be arbitrarily selected according to the SOx durability performance and the NOx removal performance. When the ΔC / NOx ratio of the exhaust gas itself is low, such as the exhaust gas of a diesel engine,
A sufficiently high NOx removal rate can be achieved by employing a system in which the fuel ΗC is added to the exhaust gas at a concentration of several hundreds to several thousands ppm in terms of methane and then brought into contact with the catalyst of the present invention. Here, ΔC is a paraffinic hydrocarbon,
It means those containing olefinic hydrocarbons and aromatic hydrocarbons, oxygen-containing organic compounds such as alcohols, aldehydes, ketones, and ethers, gasoline, kerosene, light oil, and heavy oil A.

The gas space velocity (SV) when purifying the combustion exhaust gas of the internal combustion engine operated in the region of the lean air-fuel ratio using the catalyst layer according to the present invention is not particularly limited. 200,000h over 000h ^-1
It is preferably set to ^-1 or less.

The denitration rate when the gas composition is constant depends on the type of catalyst and the type of ΔC. When the catalyst layer of the present invention is used, for example, C _{2 to} C ₆ paraffin,
4 for olefins and C ₆ -C ₉ aromatic ΔC
50-600 ° C., 350-550 ° C. for C ₆ -C ₉ paraffins and olefins, 250-500 ° C. for C ₁₀ -C ₂₅ paraffins and olefins.
In order to exhibit a high denitrification rate, it is necessary to set the catalyst layer inlet temperature at 100 ° C. or higher to 700 ° C. or lower, preferably at 200 ° C. or higher to 600 ° C. or lower.

[0030]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples. However, the present invention is not limited to the following examples. (1) Selection of alumina for catalyst B: In order to select an alumina carrier to be used for catalyst B, a to c of various γ-aluminas having a specific surface area and a pore distribution as shown in Table 1 correspond to the present invention. Alumina falling within the range, and d to g are aluminas outside the scope of the present invention. In addition, the pore distribution of alumina of a to g was measured by a soapmatic manufactured by Carlo Elba.

[0031]

[Table 1] ア ル ミ ナ Alumina specific surface area pore distribution (m ² / g) Y / Χ (%) Ζ / Χ (%) ａa 241 83.2 2. 4b 219 87.0 3.9 c 174 88.4 4.4 d 199 47.0 0.7 e 177 68.5 4.9 f 241 51.0 45.9 g 266 71.1 22.7} ──────────────────────────────

(2) Preparation of catalyst layer: Preparation examples of each catalyst for constituting the catalyst layer of the present invention are shown below as reference examples. REFERENCE EXAMPLE 1 300 g of alumina hydrate, which is a precursor of γ-alumina a in Table 1, was mixed with 16.1 g of silver nitrate.
After being immersed in a 00 ml aqueous solution, the mixture was heated with stirring to evaporate water. This was air-dried at 110 ° C., and then calcined in air at 600 ° C. for 3 hours to obtain Catalyst 1. The content of Ag in the catalyst 1 in terms of metal was 4.5% by weight with respect to alumina.

Reference Examples 2 to 11 Similarly to Reference Example 1, except that alumina hydrate, which is a precursor substance from which γ-alumina b to g shown in Table 1, was obtained, was used. Catalyst 2 (Reference Example 2), Catalyst 3 (Reference Example 3), Catalyst 4 (Reference Example 4), Catalyst 5 (Reference Example 5), Catalyst 6 (Reference Example 6), and Catalyst 7 (Reference Example 7) were obtained. Was. Further, in preparing Catalyst 1 of Reference Example 1, the content of silver was 0% by weight, 1% by weight,
Catalyst 8 (Reference Example 8) was replaced with Catalyst 9 (Reference Example 9) and Catalyst 10 in the same manner as in Reference Example 1 except that 3% by weight and 8% by weight were used.
(Reference Example 10) and Catalyst 11 (Reference Example 11) were obtained.

Reference Examples 12 and 13 Production of honeycomb catalyst: 60 g of the above powdered catalyst 1 was charged into a ball mill pot together with 8 g of alumina sol (Al ₂ O ₃ solid content of 10% by weight) and 120 ml of water, and wet-pulverized. A slurry was obtained. In this slurry, commercially available 4
A cylindrical core having a diameter of 1 inch and a length of 2.5 inches penetrated from a 00 cpsi (cell / inch ² ) cordierite honeycomb substrate was immersed, pulled up, and then the excess slurry was removed by air blow and dried. After that, 500 ° C
For 30 minutes at a dry conversion rate of 15L / L honeycomb
By covering the solid content of 0 g, a honeycomb catalyst 12 (Reference Example 12) having a composition of 4.5% Αg / A1 ₂ O ₃ was obtained. Table 1
A1 ₂ O was prepared in the same manner as in the above-mentioned honeycomb catalyst preparation method except that the catalyst 8 comprising only alumina a (Reference Example 8) was used.
A honeycomb catalyst 13 having _three compositions (Reference Example 13) was obtained.

Hereinafter, the results of evaluating the denitration performance of the exhaust gas catalyst layer formed using the catalysts of Reference Examples 1 to 13 under various conditions will be described.

Example 1 A catalyst 8 consisting solely of alumina a shown in Table 1 (Reference Example 8) and a catalyst 1 of Reference Example 1 were each subjected to pressure molding, and then pulverized and sized to a particle size of 350 to 500 μm. The catalyst 8 is located upstream of the exhaust gas flow direction,
Was filled in a stainless steel reaction tube having an inner diameter of 21 mm so as to form a catalyst layer, and this was attached to a normal-pressure fixed-bed flow reactor. The weight ratio between the catalyst 8 and the catalyst 1 was 1:
It is one.

[Performance Evaluation Example 1] In this catalyst layer, as a model exhaust gas, NΟ: 750 ppm, kerosene (C ₁ ): 450
0 ppm, O ₂ : 10%, Δ ₂ O: 10%, balance: N ₂
Was passed at a space velocity of 78,000 h ^-1 . In the analysis of the gas composition at the outlet of the reaction tube, NO and N
The concentration of O ₂ was measured with a chemiluminescent NOx meter,
_The 2O concentration was measured using a gas chromatograph / thermal conductivity detector equipped with a Ρorapack Q column. The catalyst layer inlet temperature was set to a predetermined temperature in the range of 100 to 700 ° C., and the value at the time when the gas composition at the outlet of the reaction tube became stable at each predetermined temperature was used to define the denitration rate by the following equation. Further, N ₂ O and NO ₂ were hardly produced by any of the catalysts of the present invention.

[0038]

(Equation 1)

[Examples 2 to 7 and Comparative Examples 1 to 6] The catalyst 8 was used as the first stage, and the catalysts 2, 3, and 9 of Reference Examples 2, 3, and 9 to 11
Evaluation tests using model gases were performed in the same manner as above, except that the catalysts 4 to 7 and 8 of Reference Examples 9 to 11 and Reference Examples 4 to 7 and 8 were used instead of the catalyst 1 of Example 1. Catalyst layers using catalysts 2, 3, and 9 to 11 at the subsequent stage were referred to as Examples 2 to 6, respectively, and catalyst layers using catalysts 4 to 7, 8 at the subsequent stage were referred to as Comparative Examples 1 to 5, respectively. Further, a catalyst layer similar to the above was formed by using the catalyst 14 consisting only of alumina f and the catalyst 14 consisting of commercially available silica in the preceding stage instead of the catalyst 8 of Example 1, respectively. An evaluation test using gas was performed. The catalyst layers using the catalysts 14 and 15 were Example 7 and Comparative Example 6, respectively. Table 2 shows the denitration ratio C ₄₂₅ (%) at the catalyst temperature of 425 ° C. for the catalyst layers of the above Examples and Comparative Examples.
The catalyst layer of the example of the present invention was 70 times larger than the catalyst layer of the comparative example.
% Or higher denitration performance.

[Performance Evaluation Example 2]
The honeycomb catalyst 13 of Reference Example 13 and the honeycomb catalyst 12 of Reference Example 12 were each processed into a cylindrical shape having a diameter of 1.5 cm and a length of 3.2 cm, and the honeycomb catalyst 13 was positioned upstream in the exhaust gas flow direction. Was filled in a stainless steel reaction tube having an inner diameter of 15 mm so as to be a subsequent stage (Example 8). Performance evaluation example 1 except that the space velocity of the gas to be fed to the latter catalyst layer was 13,000 h ^-1.
An evaluation test was performed using the same model gas as in Example 1 and the results are shown in Table 2 together with the results of Performance Evaluation Example 1. As shown in Table 2, it can be seen that the honeycomb catalyst layer also shows high denitration performance of 70% or more.

[0041]

[Table 2] ──────────────────────────── Catalyst layer First stage Second stage Denitration rate (%) ──────── ──────────────────── Example 1 Catalyst 8 Catalyst 1 78.7 Example 2 Catalyst 8 Catalyst 2 74.9 Example 3 Catalyst 8 Catalyst 3 74.2 Comparative Example 1 Catalyst 8 Catalyst 4 17.6 Comparative Example 2 Catalyst 8 Catalyst 5 2.5 Comparative Example 3 Catalyst 8 Catalyst 6 27.1 Comparative Example 4 Catalyst 8 Catalyst 7 23.1 Comparative Example 5 Catalyst 8 Catalyst 8 33.7 Example 4 Catalyst 8 Catalyst 9 83.1 Example 5 Catalyst 8 Catalyst 10 87.3 Example 6 Catalyst 8 Catalyst 11 75.9 Example 7 Catalyst 14 Catalyst 1 74.2 Comparative Example 6 Catalyst 15 Catalyst 1 86.8 Example 8 Catalyst 13 Catalyst 12 70.1

[Performance Evaluation Example 3] The catalyst layers of Examples 1, 7 and Comparative Example 6 were reacted for 1 hour in the gas composition of Performance Evaluation Example 1 in the presence of 50 ppm of SO ₂ . Table 3 shows the denitration ratio C ₄₂₅ (%) at a catalyst layer temperature of 425 ° C. after one hour. The catalysts of the examples of the present invention maintained an activity of 55% or more as compared with the catalyst of the comparative example.

[0043]

[Table 3] 触媒 Catalyst layer performance evaluation example 3 ─────────────────── Example 1 61.6 Example 7 57.9 Comparative Example 6 43.6

[0044]

As described above, according to the exhaust gas purifying catalyst layer and the exhaust gas purifying catalyst coating structure of the present invention and the exhaust gas purifying method using the same, water vapor is contained in the lean combustion exhaust gas coexisting. Since it can reduce and purify nitrogen oxides with a high denitration rate and has SOx durability, it is useful for purifying nitrogen oxides in combustion exhaust gas of an internal combustion engine.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/28 301 B01D 53/36 102H 102B (72) Inventor Atsushi Katakei 3-18-5 Chugoku-ku, Ichikawa-shi, Chiba Inside the Central Research Laboratory, Metal Mining Co., Ltd. (72) Inventor Ken Ito 2-4-1 Hamamatsucho, Minato-ku, Tokyo NE Chemcat Corporation (72) Inventor Yukio Ozaki 678 Ipponmatsu Numazu City, Shizuoka Prefecture Within the Company (72) Inventor Makoto Nagata 3-19-3, Chugoku, Ichikawa, Chiba Pref.

Claims (5)

[Claims] 1. The relationship between the catalyst A made of alumina and the pore radius and pore volume measured by a nitrogen gas adsorption method is expressed by the following formula: X is the total value of the pore volume occupied by pores having a pore radius of 300 Å or less. And the total value of the pore volume occupied by pores having a pore radius of 25 Å or more and less than 100 Å is Y, and the total value of the pore volume occupied by pores having a pore radius of 100 Å or more and 300 Å or less is Z. Wherein Y is 70% or more of X and Z is 20% or less of X. The catalyst B comprises silver in an alumina carrier having a pore structure having a pore structure. Exhaust gas purifying catalyst layer. 2. A catalyst-coated structure for purifying an exhaust gas, wherein at least the inner surface of the through-hole is coated with the catalyst according to claim 1 in a support substrate having an integral structure made of a refractory material having a large number of through-holes. . 3. A method for purifying exhaust gas using a hydrocarbon as a reducing agent, comprising contacting combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio with a catalyst-containing layer, wherein the catalyst contained in the exhaust gas catalyst-containing layer is Item 7. An exhaust gas purifying method comprising the exhaust gas purifying catalyst layer according to Item 1. 4. A method for purifying exhaust gas using a hydrocarbon as a reducing agent, comprising contacting combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio with a catalyst-containing layer, wherein the catalyst contained in the exhaust gas-purifying catalyst layer is An exhaust gas purifying method comprising the exhaust gas purifying catalyst-coated structure according to claim 2. 5. The exhaust gas purifying catalyst layer according to claim 3, wherein the catalyst A contained in the exhaust gas purifying catalyst layer is disposed at a front stage and the catalyst B is disposed at a rear stage in a flow direction of the exhaust gas. Exhaust gas purification method.