JPH10118489A - Catalyst layer for purifying exhaust gas, catalyst structure for purifying exhaust gas, and method for purifying exhaust gas thereby - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst layer which can remove NOx in dilute combustion exhaust gas efficiently, a catalyst-coated structure, and a method for purifying exhaust gas using them. SOLUTION: The catalyst A is composed of at least one kind of alkaline earth metal oxide selected from CaO, MgO, and BaO. A catalyst layer for purifying exhaust gas consists of the catalyst A and a catalyst B in which silver is contained in an alumina carrier having such pore structure that when in the relation between pore radii and pore volume measured by a nitrogen gas adsorption method, the total of pore volume occupied by pores whose radii are not exceeding 300 angstrom if made X, the total of pore volume occupied by pores whose radii are 25-100Å is made Y, and the total of pore volume occupied by pores whose radii are 100-300 angstrom is made Z, Y is 70% or more of X, and Z is 20% or less of X.

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, especially the respiratory system, but also becomes 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 a catalyst coating structure having high denitration performance in a lean burn region where SOx and water vapor coexist, and an exhaust gas purifying structure using them. As a result of intensive studies on the method, the above-mentioned problems have been solved by using a catalyst A comprising a specific alkaline earth metal oxide and a catalyst B comprising silver in alumina having a specific pore structure. The inventors have found that the present invention can be solved, and have completed the present invention.

[0013] That is, a first embodiment of the present invention for solving the above-mentioned problems includes CaO, MgO, SrO, and Ba.
The catalyst A comprising at least one kind of alkaline earth metal oxide selected from O, and the relationship between the pore radius and the pore volume measured by the nitrogen gas adsorption method are as follows. The total value of pore volumes occupied by pores having a pore radius of 25 Å or more and less than 100 Å is defined as Y, and the total value of pore volumes occupied by pores having a pore radius of 25 Å or more and less than 100 Å is defined as Y. The total value of the occupied pore volume is Ζ
Is more than 70% of Χ, and Ζ is 20 of Χ
%, Characterized in that it has a catalyst layer for purifying exhaust gas composed of a catalyst B comprising silver contained in an alumina carrier having a pore structure of not more than 10%. 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 layer at least on the inner surface of the through-hole in the support substrate are divided. It features a coated catalyst-coated structure.

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 of the first embodiment or the catalyst-coated structure of the second embodiment, and the catalyst A contained in the denitration catalyst layer in the exhaust gas flow direction And catalyst B
Are characterized by an exhaust gas purifying method which is divided and arranged at a later stage.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention and its operation will be described more specifically below. (Structure of Catalyst and Production Method Thereof) Alumina, which is one of the main components of catalyst B in the catalyst layer for purifying exhaust gas of the present invention,
For example, an aluminum hydroxide powder or gel classified as boehmite, pseudo-boehmite, vialite, or norstrandite in mineralogy is placed in air or in vacuum.
By heating and dehydrating at 0 to 800 ° C, preferably 400 to 900 ° C, crystallographically, γ-type, η-type, δ-
It is preferable from the viewpoint of denitration performance that a phase transition is made to alumina classified into a type, a χ-type or a mixed type thereof. 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.

The alumina has a pore diameter measured by a nitrogen gas adsorption method having a pore radius of 300 .ANG.
When the total value of the pore volume occupied by pores of not less than 100 Å and not more than 100 Å is Y, the pore radius is 1
When the total value of the pore volume occupied by the pores of not less than 00 Å and not more than 300 Å is represented by Ζ, Y
Is alumina having a pore structure such that ア ル ミ ナ is 70% or more of Χ 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 appropriate as the alumina effective as the catalyst component of the present invention.

The exhaust gas purifying catalyst of the present invention is as follows. The catalyst layer according to the present invention comprises CaO, Mg
A catalyst A comprising at least one kind of alkaline earth metal oxide selected from O, SrO and BaO, and a catalyst B comprising silver in alumina having the above-mentioned crystal structure and pore characteristics, The state of silver contained in the alumina 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. In the present invention, the method for incorporating silver into alumina in the catalyst B is not particularly limited, and any of the conventional methods such as an adsorption method, a pore filling method, an incipient wet method, an evaporation to dryness method, a spray method, and the like. Any known method such as impregnation method, kneading method, physical mixing method and combination thereof can be arbitrarily adopted.

The silver content in terms of metal with respect to the catalyst B is not particularly limited, but is not limited to 0.1. It is preferably 1 to 10% by weight. 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 the hydrocarbon as the reducing agent proceeds preferentially, and the NOx 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. Here, the catalyst-coated structure has a structure in which at least the inner surface of the through-hole is coated with the above-mentioned catalyst in a unitary support substrate 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 for coating the catalyst layer on the support substrate is as follows: the catalyst of the present invention, sized to a fixed particle size, is coated separately on the inner surface of the support substrate with or without a binder. That is, a so-called ordinary wash coat method or sol-gel method can be applied. In the catalyst B, the above-mentioned support substrate may be coated with alumina in advance, and then the catalyst-active substance of the present invention may be subjected to a supporting treatment to form a catalyst coating layer. The coating amount of the catalyst layer on the support substrate is not particularly limited, but may be 50 to 50 per unit volume of the support substrate.
It is preferably about 250 g / l, more preferably about 100 to 200 g / l.

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 intended to oxidize the Eta _{2 O} and CΟ _2.

In the present invention, the catalyst A is placed before the catalyst B,
Is arranged in the latter stage because SOx is adsorbed and removed by the former catalyst A, so that SO in the total catalyst system is removed.
x It is for improving 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 In order to select an alumina carrier to be used, in various γ-aluminas having a specific surface area and a pore distribution as shown in Table 1, a to c are aluminas falling within the scope of the present invention; d ~
g is alumina 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] ─────────────────────────────── Alumina specific surface area pore distribution (m ² / g) Y / Χ (%) Ζ / Χ (%) ───────────────────────────────a 241 83.2 2.4 b 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 Hereinafter, preparation examples of preparation of each catalyst for constituting the catalyst layer of the present invention will be shown as reference examples. (A) Production of Catalyst B: Reference Example 1; 300 g containing 100 g of alumina hydrate, which is a precursor of γ-alumina a in Table 1, containing 4.0 g of silver nitrate
After immersion in 10 ml of an aqueous solution for 10 hours, the mixture was evaporated to dryness at 80 ° C. This was air-dried at 110 ° C., and then calcined in air at 550 ° 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 based on the entire catalyst.
It is.

Reference Examples 2 to 12 Similarly, the same procedures as in Reference Example 1 were carried out except that alumina hydrate, which is a precursor substance from which γ-aluminas b to g shown in Table 1, were 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. In preparing Catalyst 1 of Reference Example 1, the content of silver was 0% by weight, 2% by weight,
Catalyst 8 (Reference Example 8) was replaced by Catalyst 9 (Reference Example 9), Catalyst 10 (Reference Example 10), and Catalyst 11 (Except for 3%, 8% and 12% by weight). Reference Example 1
1) and Catalyst 12 (Reference Example 12) were obtained.

(B) Production of Catalyst A: Reference Examples 13 to 17: Commercially available calcium oxide, magnesium oxide, strontium oxide, barium oxide and silica were respectively converted into Catalyst 13 (Reference Example 13) and Catalyst 1
4 (Reference Example 14), Catalyst 15 (Reference Example 15), Catalyst 16
It was used as (Reference Example 16) and Catalyst 17 (Reference Example 17).

(C) Production of honeycomb catalyst: Reference Examples 18 and 19: 60 g of the above catalyst 13 was mixed with 5 g of silica sol (SiO ₂ solid content of 20% by weight) and water 1
The mixture was charged into a ball mill pot together with 20 ml, and wet-milled to obtain a slurry. In this slurry, commercially available 400
A cylindrical core having a diameter of 1 inch and a length of 2.5 inches penetrated from a cpsi (cell / inch ² ) cordierite honeycomb substrate was immersed, pulled up, and then the excess slurry was removed by air blow and dried. Then, at 500 ° C, 3
Bake for 0 minutes, 150g dry conversion per 1L of honeycomb
To obtain a CaO-containing honeycomb catalyst 18 (Reference Example 18).

In addition, 60 g of the above-mentioned powder catalyst 1 was mixed with 8 g of alumina sol (Al ₂ O ₃ solid content: 10% by weight).
And 120 ml of water in a ball mill pot,
The slurry was obtained by wet pulverization. A cylindrical core having a diameter of 1 inch and a length of 2.5 inches penetrated from a commercially available 400 cpsi (cell / inch ² ) cordierite honeycomb substrate is immersed in the slurry, and the excess slurry is air blown. And dried. afterwards,
Fired at 500 ° C. for 30 minutes, and coated with 150 g of solid content in terms of dry weight per 1 L of honeycomb, 4.5% Αg / A1 ₂
A honeycomb catalyst 19 having an O ₃ composition (Reference Example 19) was obtained.

The results of evaluating the denitration performance of the catalyst layers for purifying exhaust gas formed using the catalysts of Reference Examples 1 to 19 under various conditions will be described below. Example 1 Catalyst 13 of Reference Example 13 and Catalyst 1 of Reference Example 1
After pressure molding each, pulverized to a particle size of 350 to
The particles are sized to 500 μm, and the catalyst 13
Was filled in a stainless steel reaction tube having an inner diameter of 15 mm so that the catalyst 1 was placed in the former stage to form a catalyst layer, and this was attached to a normal pressure fixed bed flow reactor. The weight ratio between the catalyst 13 and the catalyst 1 is 1: 1. [Performance Evaluation Example 1] The temperature of the exhaust gas in the reaction tube was maintained at 425 ° C in this catalyst layer.
pm, kerosene (C ₁ ): 4500 ppm, O ₂ : 10%,
A mixed gas composed of 5 ppm of SO ₂ , 10% of Η ₂ O, and the balance of N ₂ was passed at a space velocity of 40,000 h ⁻¹ . In the analysis of the gas composition at the outlet of the reaction tube, NO and NO ₂
Was measured with a chemiluminescent NOx meter, and N ₂ O
The concentration was measured using a gas chromatograph / thermal conductivity detector equipped with a Ρorapack Q column. The denitration rate was defined 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 5 and Comparative Examples 1 to 6 Catalysts 2, 3, 9 to 11 of Reference Examples 2, 3, 9 to 11 and Catalysts 4 to 8 of Reference Examples 4 to 8, 12 A catalyst layer similar to the above was formed using each of the catalysts 12 in place of the catalyst 1 of Example 1, and a model gas evaluation test was performed in the same manner.
The catalyst layers using the catalysts 2, 3, and 9 to 11 were Examples 2 to 6, respectively, and the catalyst layers using the catalysts 4 to 8, 12 were Comparative Examples 1 to 6, respectively. The catalyst 13 of Example 1
Instead of using the catalysts 14 to 17 of Reference Examples 14 to 17, a catalyst layer similar to that of Example 1 was formed, and an evaluation test using a model gas was performed in the same manner. The catalyst layers using catalysts 14 to 16 were Examples 7 to 9, respectively, and the catalyst layer using catalyst 17 was Comparative Example 7. Table 2 shows the initial denitration ratio and the denitration ratio 8 hours after the start of the reaction for the catalyst layers of the catalysts of the above Examples and Comparative Examples.

Performance Evaluation Example 2 (Embodiment 10) In Performance Evaluation Example 1, the honeycomb catalyst 18 of Reference Example 18 and the honeycomb catalyst 19 of Reference Example 19 were each processed into a circular shape, and the honeycomb was formed in the exhaust gas flow direction. A stainless steel reaction tube having an inner diameter of 15 mm was filled so that 18 was at the front stage and the honeycomb 19 was at the rear stage. The space velocity of the gas to be fed to the catalyst layer of the honeycomb catalyst 19 is 13,000 h ^-1.
An evaluation test was performed using the same model gas as in Performance Evaluation Example 1 except that the evaluation was made, and the results are shown in Table 2 together with the results of Performance Evaluation Example 1.

[0041]

[Table 2] Catalyst layer Initial performance 8 hours after 1st stage 2nd stage Denitration rate (% ) Denitration rate (%) 実 施 Example 1 Catalyst 13 + Catalyst 1 90 85 Example 2 Catalyst 13 + Catalyst 2 88 82 Example 3 Catalyst 13 + Catalyst 3 88 83 Comparative Example 1 Catalyst 13 + Catalyst 4 29 ... Comparative Example 2 Catalyst 13 + Catalyst 5 12 ... Comparative Example 3 Catalyst 13 + Catalyst 6 38 ... Comparative Example 4 Catalyst 13+ Catalyst 7 35 ... Comparative Example 5 Catalyst 13 + Catalyst 8 24 ... Example 4 Catalyst 13 + Catalyst 9 88 78 Example 5 Catalyst 13 + Catalyst 10 93 87 Example 6 Catalyst 13 + Catalyst 11 80 76 Comparative Example 6 Catalyst 13 + Catalyst 12 48 Example 7 Catalyst 14 + Catalyst 1 92 84 Example 8 15 + catalyst 1 85 87 Example 9 catalyst 16 + catalyst 1 93 85 Comparative Example 7 catalyst 17 + catalyst 1 91 68 Example 10 catalyst 18 + catalyst 19 84 77よ り From Table 2, Examples 1 to 10 and Comparative Example 7 have an initial denitration performance of 80% or more, which is superior to Comparative Examples 1 to 6. Indicated. In Examples 1 to 10, 5 ppm SO ₂
Even after reacting for 8 hours under the coexistence condition, it had a performance of 75% or more, showing a performance superior to that of Comparative Example 7.

[0042]

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, the lean combustion exhaust gas in which SOx and water vapor coexist is contained in the lean combustion exhaust gas. Since nitrogen oxides contained therein can be reduced and purified at a high denitration rate, they are useful for purification of nitrogen oxides in combustion exhaust gas of an internal combustion engine.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 35/10 301 F01N 3/28 301E F01N 3/28 301 B01D 53/36 102H (72) Inventor Atsushi Katakei Ichikawa, Chiba (18) Inventor Kunihide Chino Kunihide Chino 678 Ipponmatsu, Numazu City, Shizuoka Prefecture N-Chem Cat Co., Ltd. Numazu Plant

Claims (5)

[Claims] 1. A catalyst A comprising at least one kind of alkaline earth metal oxide selected from CaO, MgO, SrO, and BaO, and a relationship between a pore radius and a pore volume measured by a nitrogen gas adsorption method. X is the total value of the pore volume occupied by pores having a pore radius of 300 Å or less, and Y is the total value of pore volume occupied by pores having a pore radius of 25 Å or more and less than 100 Å. When the total value of the pore volume occupied by pores of 100 Å or more and 300 Å or less is Z, the pore structure has a pore structure in which Y is 70% or more of X and Z is 20% or less of X. An exhaust gas purifying catalyst layer comprising: a catalyst B comprising silver contained in an alumina carrier. 2. A catalyst coating structure for purifying an exhaust gas, wherein at least an inner surface of a through-hole in a support substrate having an integral structure made of a refractory material having a large number of through-holes is divided and coated with the catalyst layer according to claim 1. body. 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.