JP2002370032A - Exhaust cleaning catalyst, catalyst molding and catalyst-coated structure each using the catalyst, and exhaust cleaning method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust cleaning catalyst having a high ability to denitrify exhaust containing SOx and water vapor and produced during lean combustion and being excellent in retentivity of that ability, to provide a catalyst molding and a catalyst-coated structure each using the catalyst, and to provide an exhaust cleaning method. SOLUTION: There are provided a catalyst used for cleaning exhaust leaving an internal combustion engine operated in a lean air-fuel ratio and comprising a mixed oxide comprising alumina having a specific surface area of at least 120 m<2> , a bulk density of at least 0.6 g/cm<3> and a true density of at most 2.8 g/cm<3> and silver and tin supported by the alumina, wherein the amount of the supported tin is 0.03-5 wt.% based on the entire catalyst, a catalyst molding and an exhaust-cleaning-catalyst-coated structure each using the catalyst, and an exhaust cleaning method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst which is effective for removing nitrogen oxides contained in combustion exhaust gas of internal combustion engines such as automobiles, boilers, gas engines, gas turbines and ships, and uses the same. The present invention relates to a catalyst molded body, a catalyst coated structure and an exhaust gas purifying method.

[0002]

2. Description of the Related Art Various combustion exhaust gases emitted from internal combustion engines such as automobiles contain nitrogen oxides (NOx) such as nitric oxide and nitrogen dioxide together with water and carbon dioxide as combustion products. Have been. NOx not only has an adverse effect on the human body, especially on 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 technology for efficiently removing nitrogen oxides from these various exhaust gases is desired.

Conventionally, as a method for reducing and removing NOx in an oxygen-excess atmosphere, a technique has been already established in which NH ₃ is used as a reducing gas, which is selectively adsorbed to a catalyst even in a small amount. This technology has been industrialized as a method for denitration of exhaust gas from boilers and diesel engines, which are so-called stationary sources. However, this method requires a special device to recover the unreacted reducing agent, and uses harmful ammonia that has a strong odor, so there is a danger as a technology for denitration of exhaust gas from mobile sources such as automobiles. Yes, not applicable.

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

[0005] Further, when a supported noble metal catalyst such as a catalyst in which Pt is supported on alumina is used, the NOx reduction reaction can be 200 to 3 times.
Proceeding in a low temperature region of about 00 ° C.
946, JP-A-5-68855, JP-A-5-6885
This is reported in, for example, JP-A-10-103949. 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 produced as a by-product. , it is made to proceed the reduction reaction of the harmless N ₂ selectively had disadvantage is difficult.

One of the present applicants has previously found that the use of a catalyst containing silver with a hydrocarbon as a reducing agent in an oxygen-excess atmosphere causes the NOx reduction reaction to proceed selectively. -281844. After this disclosure, similar NOx reduction and removal techniques using a catalyst containing silver are disclosed in JP-A-4-354536 and JP-A-5-9.
No. 2,124, JP-A-5-92125, JP-A-6-277454, and the like.

[0007]

However, in the exhaust gas purifying method using an alumina-supported silver catalyst described in these publications, the denitration performance in the coexistence of steam and SOx is still insufficient for practical use, and moreover, The service life of the denitration performance was short and the durability was poor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art.
Provided is an exhaust gas purifying catalyst having high denitration performance and excellent durability for lean combustion exhaust gas in which water and steam coexist, and a molded catalyst, a catalyst coated structure using the catalyst, and the catalyst An object of the present invention is to provide an exhaust gas purifying method that can remove NOx in lean combustion exhaust gas in which SOx and water vapor coexist with high efficiency and high reliability.

[0009]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by using a catalyst comprising silver and tin supported on alumina having specific physical properties. Thus, the present invention has been completed.

That is, according to the present invention, first, the specific surface area is
120 m ² / g or more, a bulk density of 0.6 g / cm ³ or more, and a true density of 2.8 g / cm ³ or less, comprising silver and tin supported on the alumina; Provided is a catalyst for purifying exhaust gas from an internal combustion engine operated at a lean air-fuel ratio, characterized in that the carried amount with respect to the entire catalyst is 0.03% by weight or more and 5% by weight or less in terms of a metal element. In addition, the present invention secondly provides a catalyst molded body obtained by molding the above-mentioned catalyst into a fixed shape. Thirdly, the present invention has a support substrate having an integral structure made of a refractory material having a large number of through-holes, and the above-described catalyst coated on at least the inner surface of the through-hole of the support substrate in a layered manner. An object of the present invention is to provide an exhaust gas purifying catalyst-coated structure.

Furthermore, the present invention fourthly provides an exhaust gas purifying method characterized by contacting exhaust gas from an internal combustion engine operated at a lean air-fuel ratio with the above-mentioned exhaust gas purifying catalyst in the presence of a hydrocarbon. To provide.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

(Catalyst and Method for Producing the Catalyst) The exhaust gas purifying catalyst of the present invention is a catalyst in which silver and tin are supported on a specific alumina as a carrier. The alumina used is, for example, a powder or gel of aluminum hydroxide classified as boehmite, pseudoboehmite, vialite or norstrandite in mineralogy, at 300 to 800 ° C. in air or vacuum,
Preferably, by dehydrating by heating at 400 to 900 ° C., a phase transition to alumina which is crystallographically classified into γ-type, η-type, δ-type, χ-type or a mixed type thereof has a denitration performance. Above.

The alumina has a specific surface area of 120 m ² / g or more, a bulk density of 0.6 g / cm ³ or more, and a true density of 2.8
g / cm ³ or less. If alumina that does not satisfy this condition is used as a carrier, the exhaust gas purifying catalyst constituted by the catalyst has insufficient denitration performance of exhaust gas in the presence of steam. The method for measuring the specific surface area, bulk density and true density of the alumina is not particularly limited. Examples of the method for measuring the specific surface area include a nitrogen gas adsorption method, and the bulk density and the true density are measured by a mercury intrusion method. Is possible.

Here, the mercury intrusion method is as follows. When a porous body is put into mercury and pressure is applied to the mercury surface, mercury intrudes sequentially from the pores of larger diameter, and the pore diameter is reduced from the pressure at which mercury is injected. The volume density of the pores is determined from the amount of intrusion, and the bulk density measured by the mercury intrusion method is determined from the sample weight and the sample volume at the time of initial injection of mercury at atmospheric pressure. In other words, before mercury injection, mercury is injected at atmospheric pressure to fill the gap in the sample container in which the porous body is placed, and the sample volume is determined from the amount of mercury injected at this time, the mercury density, and the volume of the sample container. The bulk density is calculated by weight / sample volume.
The true density measured by the mercury intrusion method is obtained by performing the same calculation as the bulk density based on the pore volume obtained by injecting mercury at the maximum pressure at the time of measurement by the mercury intrusion method. Things.

The state of silver and tin supported on alumina having the above-mentioned specific properties is not particularly limited, and examples thereof include a metal state, an oxide state, and a mixed state thereof. In particular, since the composition of the combustion exhaust gas of an internal combustion engine such as an automobile changes depending on the operating conditions, the catalyst is exposed to a reducing atmosphere and an oxidizing atmosphere. Therefore, it is considered that the state of the active metal constituting the catalyst changes in the atmosphere.

The method of supporting silver and tin on alumina is not particularly limited, and conventional methods such as an adsorption method, a pore filling method, an incipient wetness method, an evaporation to dryness method, a spray method, and the like can be used. Any commonly used known method such as an impregnation method, a kneading method, a physical mixing method, and a combination method thereof can be arbitrarily adopted. If the catalyst components are substantially the same, one having the same effect can be obtained. For example, alumina or alumina precursor substances, for example, boehmite, silver and tin salts are simultaneously supported on aluminum hydroxide such as pseudo-boehmite, and then dried and calcined. , And then dried and fired. Further, as described above, a method for producing a catalyst containing an active metal species at the time of producing alumina having specific physical properties as described above, for example, an alcohol solution of aluminum alkoxide is mixed with an alcohol solution of silver salt and tin salt, and then heated and hydrolyzed. The precipitate obtained by the alkoxide method or the coprecipitation method in which an alkali is added to a mixed aqueous solution of an aluminum salt, a silver salt and a tin salt to cause precipitation is dried.
A firing method is also applicable.

The silver content in terms of metal in the entire catalyst is not particularly limited, but is preferably in the range of 0.1 to 10% by weight, and particularly preferably in the range of 1.5 to 8% by weight in terms of denitration performance. Further, the content of tin in terms of metal with respect to the entire catalyst is 0.03
It is necessary to be not less than 5% by weight and not more than 5% by weight. If the tin content exceeds 5% by weight, the performance of silver is not exhibited and the denitration performance is reduced. On the other hand, when the content of tin is less than 0.03% by weight, the synergistic effect due to the addition of tin is not sufficiently exhibited, so that the content needs to be in the above range.

The drying temperature of the catalyst is not particularly limited, and it is usually dried at about 80 to 120 ° C. Further, the firing temperature is about 300 to 1000 ° C, preferably about 400 to 900 ° C. If the firing temperature exceeds 1000 ° C., the specific surface area of the catalyst becomes extremely small, which is not preferable. The atmosphere at this time is not particularly limited, but each atmosphere such as air, inert gas, oxygen, or steam may be appropriately selected according to the catalyst composition, and the atmospheres are alternately changed at regular intervals. You may. The catalyst of the present invention can be used by filling it into a certain space in a powder state, or can be used as described below.

(Catalyst molded article) The catalyst molded article of the present invention is obtained by molding the above-mentioned catalyst into a predetermined shape. The shaped catalyst body is configured such that the exhaust gas to be purified flows through it. When the catalyst body is a molded body, the shape of the molded body is not particularly limited, and may be, for example, a layered (sheet or coated), spherical, cylindrical, honeycomb, spiral,
Various shapes such as a granular shape, a pellet shape, and a ring shape are exemplified. These shapes, sizes, and the like may be arbitrarily selected according to use conditions.

The method of forming the powdered catalyst into a desired shaped catalyst molded body is not particularly limited, and a molding machine having a desired shaped die with the powdered catalyst mixed with an appropriate binder or without a binder. Extrusion molding. As the binder, for example, methyl cellulose,
Examples include polyvinyl alcohol and polyethylene glycol. The molded catalyst is used by filling it in a container having a desired shape provided with, for example, a stainless steel mesh at both ends.

(Catalyst-Coated Structure) The catalyst-coated structure of the present invention is to coat the above-mentioned catalyst on at least the inner surfaces of the through-holes of a support substrate having an integral structure composed of a refractory material having a large number of through-holes. It is made. A large number of through-holes are provided in the support substrate used for the catalyst-coated structure, and the through-holes are arranged so as to be along the flow direction of the exhaust gas during use. In a cross section perpendicular to its flow direction, usually the hole area ratio 60% to 90%, preferably a 70% to 90%, the number is 5.06cm ² (1 square inch) per usually 30 to 700 pieces, preferably 200 ~ 600 pieces. The catalyst is coated on at least the inner surface of the through hole, but may be coated on the end surface or side surface of the supporting substrate.

As the material of the refractory support substrate, ceramics such as α-type alumina, mullite, cordierite, and silicon carbide, and metals such as austenitic and ferritic stainless steel are used. Conventional shapes such as a honeycomb shape and a continuous foam shape can be used. Preferred supporting substrates are cordierite or stainless steel honeycomb.

The method of coating the support substrate with a catalyst includes:
A so-called ordinary wash coat method or sol-gel method, in which the catalyst of the present invention sized to a certain particle size is coated on the inner surface of the support substrate with or without a binder, can be applied. Examples of the binder include alumina sol, silica sol, titania sol and the like.
Further, a predetermined surface of the above-mentioned support substrate may be coated with alumina in advance, and a catalyst coating layer may be formed by carrying out a treatment for supporting a silver component which is a catalytically active substance of the present invention. The coating amount of the catalyst layer on the support substrate is not limited, but is preferably about 50 to 250 g / L, more preferably about 100 to 200 g / L per unit volume (apparent volume) of the support substrate.

(Exhaust Gas Purification Method) The exhaust gas purification method of the present invention is characterized in that exhaust gas from an internal combustion engine operated at a lean air-fuel ratio is brought into contact with the above-mentioned exhaust gas purification catalyst of the present invention. It removes NOx and purifies exhaust gas. The form of the purification catalyst that is brought into contact with the exhaust gas is not limited at all. The catalyst may be in the form of the catalyst body described above, or may be in the form of a coated catalyst layer provided on the catalyst coated structure.

The exhaust gas from an internal combustion engine to be burned in a lean air-fuel ratio, generally, CO, HC and reducing component such as (hydrocarbons) and H _2, although containing an oxidizing component such as NOx and O _2, both It contains oxygen in excess of the stoichiometric amount required for mutual complete redox reactions. According to the exhaust gas purifying method of the present invention, the exhaust gas is brought into contact with the catalyst of the present invention under such an oxygen-excess condition.
Ox is reduced and decomposed to N ₂ and H ₂ O,
Is also completely oxidized to CO ₂ and H ₂ O.

When the HC / NOx ratio of the exhaust gas itself is low, such as the exhaust gas of a diesel engine, after adding about several hundreds to several thousand ppm of fuel HC or the like in terms of methane concentration in the exhaust gas, the present invention is applied. If a method of contacting with a catalyst is adopted, a sufficiently high NOx removal rate can be achieved. Here, HC refers to oxygen-containing organic compounds such as paraffinic hydrocarbons, olefinic hydrocarbons and aromatic hydrocarbons, alcohols, aldehydes, ketones, and ethers.
It includes gasoline, kerosene, light oil, heavy oil A, etc.

The gas space velocity (SV) when purifying the combustion exhaust gas of the internal combustion engine operated in the lean air-fuel ratio range using the catalyst according to the present invention is not particularly limited,
SV5, oooh 200 ^-1 or more, it is preferable to O00h ^-1 or less.

[0029]

The present invention will be described more specifically with reference to the following examples and comparative examples. However, the present invention is not limited to the following examples. (1) Selection of alumina carrier In order to select the alumina to be used, specific surface area (nitrogen gas adsorption method) and bulk density (mercury intrusion method) were measured to obtain various γ-type aluminas as shown in Table 1. . a to c are aluminas falling within the scope of the present invention, and d to f are aluminas outside the scope of the present invention.

[0030]

[Table 1]

(2) Preparation of catalyst Example 1 300 g of γ-type alumina a shown in Table 1 was replaced with silver nitrate 1
After being immersed in a 500 mL aqueous solution containing 7.2 g, the mixture was heated with stirring to evaporate water. After drying this with air at 110 ° C,
To obtain a Ag / Al ₂ O ₃ and then calcined 3 hours at 600 ° C. in air. Next, this Ag / Al ₂ O ₃ was immersed in a 500 mL aqueous solution containing 1.84 g of stannic chloride, and then a catalyst 1 was obtained in the same manner as in the above and Ag / Al ₂ O ₃ . Incidentally, Ag and metal in terms of metal in the catalyst 1
The Sn content was 3.5% by weight based on the entire catalyst,
0.2% by weight.

Examples 2 to 7 and Comparative Examples 1 to 6 The same procedure as in Example 1 was carried out except that the contents of silver and tin and the γ-type alumina carrier used were changed.
(Example 2), Catalyst 3 (Example 3), Catalyst 4 (Comparative Example 1), Catalyst 5 (Comparative Example 2), Catalyst 6 (Comparative Example 3), Catalyst 7
(Example 4), Catalyst 8 (Example 5), Catalyst 9 (Comparative Example 4), Catalyst 10 (Comparative Example 5), Catalyst 11 (Comparative Example 6), Catalyst
12 (Example 6) and catalyst 13 (Example 7) were obtained. Table 2 shows the supported amounts (% by weight) of silver and tin in the whole catalyst and the alumina carrier of each catalyst.

[0033]

[Table 2]

Next, the above Examples 1 to 7 and Comparative Examples 1 to
The denitration performance of the catalysts 1 to 13 of Table 2 obtained in Table 6 was evaluated under the following conditions.

[Performance Evaluation Example 1] Each of the catalysts of Examples 1 to 7 and Comparative Examples 1 to 6 was molded under pressure and then pulverized to a particle size of 35.
The mixture was sized to 0 to 500 μm, filled in a stainless steel reaction tube having an inner diameter of 15 mm to form a catalyst bed, and this was attached to a normal pressure fixed bed flow reactor. The temperature of the exhaust gas in the reaction tube is
Maintained at 400 ° C., NO as a model exhaust gas: 750ppm, O _2: 10
%, Light oil _{(C1): 4500ppm, H 2} O: 10%, SOx: 5ppm, the balance of the mixed gas of N ₂ was passed at a space velocity of 75,000h ^-1 10 hours to evaluate the denitration performance. In the analysis of the reaction tube exit gas composition, chemiluminescence NOx is the concentration of NO and N ₂
The concentration of N ₂ O was measured using a gas chromatograph / thermal conductivity detector equipped with a PorapackQ column.
The denitration rate is calculated by the following formula:

[0036]

(Equation 1)

Was determined according to the following formula: Incidentally, N ₂ O and NO ₂ were hardly produced by any of the catalysts of the present invention. Table 3 shows the initial denitration performance and the denitration performance 10 hours after the start of model exhaust gas distribution.

Example 8 Production of Honeycomb Catalyst: 60 g of the above-mentioned 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-milled to obtain a slurry. Obtained. A commercially available 400 cpsi (cell / inc.)
h ² ) diameter cut out from cordierite honeycomb substrate
A cylindrical core having a length of 1 inch and a length of 2.5 inches was immersed and pulled up, and then excess slurry was removed by air blow and dried. After that, it was baked at 500 ° C for 30 minutes, and coated with 150 g of solid content in terms of dry weight per liter of honeycomb to form a honeycomb catalyst.
14 (Example 8) was obtained.

[Performance Evaluation Example 2] The honeycomb catalyst 14 was
It was processed into a cylindrical shape having a diameter of 15 mm and a length of 32 mm, and filled into a stainless steel reaction tube having an inner diameter of 15 mm. An evaluation test was performed on the catalyst layer using the same model gas as in Performance Evaluation Example 1 except that the space velocity of the supplied gas was 40,000 h ^-1 . The results are shown in Table 3 together with the results of the performance evaluation example 1.

[0040]

[Table 3]

From the results shown in Table 3, it was found that the catalysts of Examples 1 to 8 had higher initial denitration performance than the catalysts of Comparative Examples 1 to 6 and also had excellent durability of the denitration performance.

[0042]

As is apparent from the above description, according to the exhaust gas purifying catalyst of the present invention, the catalyst molded body and the catalyst coating structure using the same, and the exhaust gas purifying method, the lean combustion in which SOx and steam coexist is provided. Nitrogen oxides contained in the exhaust gas can be reduced and purified at a high conversion rate, and the denitration performance is excellent in durability. Therefore, the exhaust gas purifying catalyst of the present invention,
Further, the molded catalyst, the catalyst coated structure, and the method for purifying exhaust gas using the same are useful for purifying exhaust gas from an internal combustion engine burned at a lean air-fuel ratio.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 B01D 53/36 ZAB 102H (72) Inventor Kengo Soda 3-18 Chugoku, Ichikawa, Chiba −5 Sumitomo Metal Mining Co., Ltd. Central Research Laboratory (72) Inventor Yoshiyuki Uekusa 3-18-5 China, Ichikawa City, Chiba Prefecture Sumitomo Metal Mining Co., Ltd. Central Research Laboratory (72) Inventor Makoto Nagata Ichimotomatsu 678, Numazu City, Shizuoka Prefecture Inside N-Cumcat Corporation Numazu Plant (72) Inventor Yasushi Yasushi 678 Ipponmatsu, Numazu-shi, Shizuoka Prefecture N-Emcat Corporation Inside Numazu Plant (72) Inventor Ken Takeshi Nagashima 678, Ipponmatsu, Numazu-shi, Shizuoka Prefecture F-term (reference) in the Numazu Plant of Mcat Corporation 3G091 AA02 AA12 AA19 AB05 BA01 BA07 BA14 GA17 GA18 GB01W GB05W 4D048 AA06 AA13 AA18 AB07 BA03X BA21X BA34X BA41X BB02 BB17 4G069 AA01 AA03 AA08 BA01A BA01B BC22A BC22B BC32A BC32B CA03 CA13 CA14 CA15 DA06 EA19 EC03X EC03Y EC04 EC03 FA03 EC02

Claims (6)

[Claims] 1. A specific surface area of at least 120 m ² / g and a bulk density of 0.6 g / g
cm ³ or more, and alumina true density of 2.8 g / cm ³ or less, it has a silver and tin comprising supported on the alumina, supporting amount on the entire catalyst of tin in terms of metal element 0.03
A catalyst for purifying exhaust gas from an internal combustion engine that is operated at a lean air-fuel ratio, wherein the catalyst is not less than 5% by weight and not more than 5% by weight. 2. The exhaust gas according to claim 1, wherein the specific surface area is a value measured by a nitrogen gas adsorption method, and the bulk density and the true density are values measured by a mercury intrusion method. Gas purification catalyst. 3. A catalyst molded article obtained by molding the catalyst according to claim 1 or 2 into a fixed shape. 4. A support substrate having an integral structure made of a refractory material having a large number of through holes, and the catalyst according to claim 1 coated on at least the inner surface of the through hole of the support substrate in a layered manner. A catalyst coated structure for purifying exhaust gas. 5. An exhaust gas purifying method comprising contacting exhaust gas from an internal combustion engine operated at a lean air-fuel ratio with the exhaust gas purifying catalyst according to claim 1 in the presence of a hydrocarbon. 6. The exhaust gas according to claim 5, wherein the catalyst is in the form of a layered coated catalyst provided on the catalyst molded article according to claim 3 or the catalyst structure according to claim 3. Purification method.