JP2002370031A - Exhaust cleaning catalyst, catalyst body, exhaust- cleaning-catalyst-coated structure each using the catalyst, and exhaust cleaning method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust cleaning catalyst which can efficiently remove NOx from exhaust produced during lean combustion, to provide a catalyst body and a catalyst-coated structure each using the catalyst and being excellent in durability, and to provide an exhaust cleaning method which removes NOx from exhaust during lean combustion highly efficiently and highly reliably by using the catalyst. 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 and 3-40 pts.wt., per 100 pts.wt. alumina, cerium dioxide and silver supported by the mixed oxide, a catalyst body 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 catalytic body, an exhaust gas purifying 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 purification method using an alumina-supported silver catalyst described in these publications, the denitration performance at a relatively low temperature of about 300 ° C. to 400 ° C. in the presence of steam and SOx. Is still insufficient for practical use.
In addition, when the catalyst is actually used, the catalyst is coated in a layer on a substrate having an integral structure to form a catalyst-coated structure, molded into a fixed shape, or filled in a container for use. And there was a problem with the durability of the molded body.

SUMMARY OF THE INVENTION 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 capable of efficiently removing NOx in lean combustion exhaust gas, and a catalyst for the same. It is an object of the present invention to provide a catalyst body and a catalyst-coated structure having excellent durability and an exhaust gas purifying method for removing NOx in lean combustion exhaust gas with high efficiency and high reliability using the catalyst.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, have found that a catalyst comprising silver supported on an oxide composed of alumina and cerium dioxide (CeO ₂ ) has been developed. It has been found that the above problems can be solved by using the same, and the present invention has been completed.

That is, the present invention firstly comprises a mixed oxide comprising alumina and 3 to 40 parts by weight of cerium dioxide per 100 parts by weight of alumina, and silver supported on the mixed oxide. The present invention provides a catalyst for purifying exhaust gas from an internal combustion engine operated at a lean air-fuel ratio.

The present invention also provides, secondly, a catalyst body obtained by molding or filling the above-mentioned catalyst into a predetermined 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.

Fourth, the present invention provides a method for purifying exhaust gas, which comprises contacting exhaust gas from an internal combustion engine operated at a lean air-fuel ratio with the exhaust gas purifying catalyst in the presence of a hydrocarbon. is there.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the details of the present invention and its operation will be described more specifically.

(Catalyst and Method for Producing the Same) The exhaust gas purifying catalyst of the present invention is a catalyst in which silver is supported on a mixed oxide of alumina and cerium dioxide. As alumina, γ-alumina or the like can be used. In the alumina-cerium dioxide mixture, which is a carrier, the ratio of both components is such that cerium dioxide is 3 to 40 parts per 100 parts by weight of alumina.
Parts by weight. If the amount is less than 3 parts by weight or exceeds 40 parts by weight, the durability and / or the denitration performance in the coexistence of SOx and water vapor are reduced.

The state of silver as an active metal supported on such a carrier 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 may be exposed to both a reducing atmosphere and an oxidizing atmosphere. The state of silver can change depending on the atmosphere.

The amount of silver carried on the oxide consisting of alumina and cerium dioxide in terms of metal is not particularly limited, but the silver content in the entire catalyst is O.O. It is preferably from 1 to 10% by weight, particularly preferably from 1.5 to 8% by weight.

The method for producing the catalyst of the present invention is not particularly limited, and a conventional method such as an adsorption method, a pore filling method, an incipient wetness method, an evaporation to dryness method, or an impregnation method such as a spray method. Any known method, such as a kneading method, a physical mixing method, or a combination thereof, may be used.

For example, after alumina or an alumina precursor substance and cerium dioxide powder are mixed in advance, drying and firing are carried on a silver source. Examples of alumina precursor materials that can be used include, for example, aluminum hydroxide such as boehmite, pseudoboehmite, vialite, norstrandite, and the like. Further, a method of drying and firing a precipitate obtained by a coprecipitation method in which an alkali is added to a mixed aqueous solution of an aluminum source, a cerium salt and a silver salt to cause precipitation is also applicable.

The drying temperature of the precipitate obtained by the above-mentioned coprecipitation is not particularly limited, but is usually about 80 to 120 ° C. In addition, the temperature of the subsequent firing is usually 300 to 1000 ° C,
Preferably it is about 400 to 900 ° C. If the calcination temperature exceeds 1000 ° C., the specific surface area of the catalyst often becomes extremely small, which is not preferable. The atmosphere at this time is not particularly limited, but in the air, in an inert gas, in oxygen,
Each atmosphere such as in steam may be appropriately selected, and each atmosphere may be alternately changed at regular intervals.

(Catalyst Body) The catalyst body of the present invention is obtained by molding the above-mentioned catalyst into a predetermined shape or filling a predetermined space in a powder state. The catalyst is configured so that the exhaust gas to be purified flows through it. When the catalyst body is formed into a molded body, its shape is not particularly limited, and may be, for example, a layer (sheet or film), a sphere, a cylinder,
Various shapes such as a honeycomb shape, a spiral shape, a granular shape, a pellet shape, a ring shape and the like can be given. These shapes, sizes, and the like may be arbitrarily selected according to use conditions.

As a method of forming the powdery catalyst into a catalyst body having a desired shape, a commonly used known method may be used. If necessary, a binder is usually used. As the binder, a commonly used known binder can be used.

(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.

The support substrate used in the catalyst-coated structure is provided with a large number of through-holes, and the through-holes are arranged 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 steels 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. As the binder used here,
For example, alumina sol, silica sol, titania sol and the like can be mentioned. 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 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 (apparent volume) of the supporting 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-described 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.

Exhaust gas from an internal combustion engine burned at a lean air-fuel ratio generally contains reducing components such as CO, HC (hydrocarbon) and H ₂ and oxidizing components such as NOx and O _2. 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 hundred to several thousand ppm of fuel HC or the like in terms of methane concentration into the exhaust gas, the present invention is applied. A sufficiently high NOx removal rate can be achieved by employing a method of contacting with a catalyst. In addition,
As used herein, HC refers to those containing paraffinic hydrocarbons, olefinic hydrocarbons and aromatic hydrocarbons, oxygen-containing organic compounds such as alcohols, aldehydes, ketones, and ethers, gasoline, kerosene, light oil, and heavy oil A. means.

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 and O00h ^-1 or less.

[0030]

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) Preparation of Catalyst Hereinafter, an example illustrating the preparation of the catalyst of the present invention will be described.

Example 1 An oxide obtained by previously mixing 270 g of γ-alumina and 30 g of cerium dioxide was mixed with an oxide containing 24.5 g of silver nitrate.
After immersion in 0 ml of an aqueous solution, the mixture was heated with stirring to evaporate water. This was air-dried at 110 ° C. and calcined in air at 600 ° C. for 3 hours to obtain Catalyst 1. The content of Ag in terms of metal in Catalyst 1 was 4.5% by weight based on the weight of the entire catalyst.
It is.

Examples 2 to 7 and Comparative Examples 1 to 3
When preparing the catalyst 1 in the above, except that the amount of cerium dioxide was 100 parts by weight of alumina, 0 parts by weight, 2 parts by weight, 5 parts by weight, 20 parts by weight, 35 parts by weight, 40 parts by weight and 45 parts by weight, respectively. In the same manner as in Example 1, catalyst C1 (Comparative Example 1),
Catalyst C2 (Comparative Example 2), Catalyst 2 (Example 2), Catalyst 3 (Example 3), Catalyst 4 (Example 4), Catalyst 5 (Example 5), Catalyst C
3 (Comparative Example 3) was obtained. Catalyst 6 (Example 6) and Catalyst 6 were prepared in the same manner as in Example 1 except that the content of silver was 2% by weight and 8% by weight, respectively, based on the weight of the entire catalyst when preparing Catalyst 1 in Example 1. 7 (Example 7) was obtained.

Example 8 Production of Catalyst-Coated Structure: Catalyst 1 in powder form (Example 1) 60
g was charged into a ball mill pot together with 8 g of alumina sol (Al ₂ O ₃ solid content 10% by weight) and 120 ml of water, and wet-milled to obtain a slurry. In this slurry, commercially available 400c
A cylindrical core having a diameter of 1 inch and a length of 2.5 inches penetrated from a psi (cells / inch ² ) cordierite honeycomb substrate was immersed and pulled up, and then excess slurry was removed by air blow and dried. Thereafter, the resultant was baked at 500 ° C. for 30 minutes, and was coated with 150 g of solid content in terms of dry weight per liter of honeycomb to obtain a catalyst-coated structure (hereinafter, referred to as a honeycomb catalyst).

The following Examples 1 to 7 and Comparative Example 1
The denitration performance of the exhaust gas purifying catalysts formed using the catalysts of Nos. 1 to 3 and the honeycomb catalyst of Example 8 was evaluated under various conditions. The results will be described.

Evaluation Example 1 After the catalyst 1 was molded under pressure,
The powder was pulverized to a particle size of 350 to 500 μm, filled in a stainless steel reaction tube having an inner diameter of 15 mm to form a catalyst, and this was mounted on a normal pressure fixed bed flow reactor.

The temperature of the exhaust gas in the reaction tube was set to 40
Keep at 0 ℃, NO: 750ppm, O as model exhaust gas₂:Ten
%, Kerosene (Cl): 4500ppm, H₂O: 10%, SOx: 2 ppm, residual
Part: N₂Space gas of 75, OOh^-1At 10:00
After passing through, the temperature is adjusted to a predetermined temperature in the range of 300 to 400 ° C.
It was set and the denitration performance was evaluated. Reaction tube outlet gas composition
In the analysis, NO and N₂About the concentration of
Measured with Ox meter, N ₂O concentration was measured using a Porapack Q column.
Using a gas chromatograph and thermal conductivity detector
Specified. The denitration rate defined by the following equation was determined. The result
It is shown in Table 1. Note that N₂O and NO₂Generates almost no
Was.

[0037]

(Equation 1)

[Evaluation Examples 2 to 7 and Evaluation Comparative Examples 1 to
3] Catalysts 2 to 7 and catalyst C in place of catalyst 1
Except for using 1 to C3, the denitration rate was measured in the same manner as in Evaluation Example 1. Table 1 shows the results. In each of the evaluation examples using the catalysts of the examples, N ₂ O and NO ₂
Was hardly generated.

[Evaluation Example 8] Instead of the catalyst 1, Example 8
The honeycomb catalyst 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 to form a catalyst bed.The space velocity of the gas fed to the catalyst bed was set at 30. Evaluation example 1 except that it was set to OOOh ^- 1
The denitration performance was evaluated in the same manner as described above. Table 1 shows the results.

[0040]

[Table 1]

From the results shown in Table 1, the catalysts 1 to 7 and the honeycomb catalysts of the examples of the present invention have higher denitration rates than the catalysts C1 to C3 of the comparative examples.
It can be seen that the effect is remarkable even at a low temperature of 50 ° C.

[0042]

As described above, according to the exhaust gas purifying catalyst, the catalyst body using the catalyst, the exhaust gas purifying catalyst coating structure and the exhaust gas purifying method of the present invention, the lean combustion in which SOx and steam coexist is provided. Nitrogen oxides contained in the exhaust gas can be reduced and removed at a high conversion rate, and the denitration performance is excellent in durability. Therefore, the exhaust gas purifying catalyst of the present invention, the catalyst body using the catalyst, the exhaust gas purifying catalyst coating structure, and the exhaust gas purifying method are useful for purifying combustion exhaust gas of an internal combustion engine.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) F01N 3/08 F01N 3/28 301P 3/10 B01D 53/36 ZABC 3/28 301 102H 104A (72) Invention Person Kengo Soda 3-18-5 China, Ichikawa City, Chiba Prefecture 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. 72) Inventor Makoto Nagata 678, Ipponmatsu, Numazu City, Shizuoka Prefecture NE Chemcat Corporation Numazu Plant (72) Inventor Yasushi Yasushi 678 Ipponmatsu, Numazu City, Shizuoka Prefecture NV Chemcat Corporation Numazu Plant (72) Inventor Takeshi Nagashima 678 Ipponmatsu, Numazu City, Shizuoka Prefecture NE Chemcat Co., Ltd. ) 3G091 AA02 AA12 AB05 BA07 BA14 CA18 FB10 FC02 GA16 GA18 GB04W GB05W GB10X 4D048 AA06 AA13 AA18 AB02 AB07 AC02 BA03X BA19X BA34X BA42X BB02 4G069 AA01 AA03 AA08 BA01A BA01B13 BB02B03 BC BB02B04 BCB BC EC22Y FA01 FA02 FA03 FB14 FB23 FB30 FC08

Claims (5)

[Claims] 1. An alumina and 3 to 100 parts by weight of the alumina
A catalyst for purifying exhaust gas from an internal combustion engine operated at a lean air-fuel ratio, comprising: a mixed oxide comprising 40 parts by weight of cerium dioxide; and silver supported on the mixed oxide. 2. A catalyst formed by molding or filling the catalyst according to claim 1 into a predetermined shape. 3. 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, which is coated in a layer on at least the inner surface of the through hole of the support substrate. A catalyst coated structure for purifying exhaust gas. 4. An exhaust gas purification method comprising contacting exhaust gas from an internal combustion engine operated at a lean air-fuel ratio with the exhaust gas purification catalyst according to claim 1 in the presence of a hydrocarbon. 5. The exhaust gas purification method according to claim 4, wherein the catalyst is in the form of the catalyst body according to claim 2 or the layered coated catalyst according to claim 3.