JP2003340284A - Exhaust gas cleaning catalyst and catalyst coated structure thereof and exhaust gas cleaning method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an exhaust gas cleaning catalyst having an excellent NOx removal capacity and durability in combustion exhaust gas containing sulfur oxide and not lowering in denitration capacity even if the temperature of the exhaust gas has a relatively low temperature of about 300-400°C, and to provide an exhaust gas cleaning method. <P>SOLUTION: The exhaust gas cleaning catalyst comprising a mixture of a first catalyst containing titania and iridium and a second catalyst containing sulfuric acid treated zirconia and palladium or a catalyst coated structure obtained by applying this catalyst to a support substrate having an integrated structure comprising a refractory material having a large number of through- holes is used to bring the oxygen excessive combustion exhaust gas containing sulfur oxide into contact with combustion exhaust gas in the presence of methanol to reduce and remove nitrogen oxides. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to exhaust gas purification for removing nitrogen oxides contained in combustion exhaust gas having a high oxygen concentration, such as exhaust gas from a boiler, a diesel engine generator, or a diesel engine automobile. catalyst,
And a method for purifying exhaust gas using the catalyst.

[0002]

2. Description of the Related Art Nitrogen oxides (NOx) such as nitric oxide and nitrogen dioxide are contained in various combustion exhaust gases emitted from factories, power generation facilities, automobiles, etc., as well as water and carbon dioxide which are combustion products. include. NOx not only adversely affects the human body, especially the respiratory system, but 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 technique for efficiently removing nitrogen oxides from these various combustion exhaust gases is desired.

Conventionally, as a method for reducing and removing NOx in an atmosphere of excess oxygen, a V ₂ O ₅ —TiO ₂ catalyst has been used,
A method of reducing and removing ammonia (NH ₃ ) using a reducing agent is well known. However, this method uses ammonia, which has a strong odor and is harmful, so that it is not easy to handle, and a special device is required to prevent the discharge of unreacted ammonia, and the equipment becomes large. It is not suitable for application to an exhaust gas source or a mobile source, and it is not economically preferable.

In recent years, it has been reported that the reduction reaction of NOx can be promoted by using unburned hydrocarbons remaining in the lean-burn exhaust gas with excess oxygen as a reducing agent. Since then, various catalysts for accelerating this reaction have been developed. For example, alumina and a catalyst in which a transition metal is supported on alumina are effective for the reduction and removal reaction of NOx using hydrocarbon as a reducing agent. Many reports have been made.

[0005]

As a catalyst for reducing and removing nitrogen oxides in exhaust gas with excess oxygen by using such a hydrocarbon as a reducing agent, in addition to a catalyst in which a transition metal is supported on alumina or alumina, In Kaihei 4-284848, 0.1 to 4% by weight of Cu, Fe, Cr, Zn, N is disclosed.
Reduction catalysts made of alumina or silica-alumina containing i or V have been reported.

When a catalyst in which Pt is supported on alumina is used, the reduction reaction of NOx proceeds in a low temperature range of about 200 to 300 ° C., as disclosed in JP-A-4-267946 and JP-A-5-68855. Japanese Patent Laid-Open No. 5-1039
It is reported in Japanese Patent Publication No. 49 and the like. However, in these noble metal-supported catalysts, the combustion reaction of hydrocarbon as a reducing agent is excessively promoted, and a large amount of N ₂ O, which is one of the causative agents of global warming, is produced as a by-product, and harmless N _It has a drawback that it is difficult to selectively proceed the reduction reaction to ₂ .

Further, JP-A-4-281844 discloses that a catalyst in which silver is supported on alumina or the like selectively causes a reduction reaction of NOx by using hydrocarbon as a reducing agent in an oxygen excess atmosphere. Has been done. Then, a similar NOx reduction and removal method using a silver-containing catalyst was performed.
JP-A-4-354536, JP-A-5-92124
JP-A-5-92125 and JP-A-6-92125.
It is disclosed in Japanese Patent Publication No. 277454.

However, the above-mentioned conventional exhaust gas purification method using a denitration catalyst has a problem that the NOx removal performance is easily deteriorated in the combustion exhaust gas containing sulfur oxides and the practical durability is insufficient. there were. Also, 300 ° C to 40
At a relatively low temperature of about 0 ° C., there is also a problem that the NOx removal performance tends to deteriorate.

The present invention has been made in view of such conventional circumstances.
It has excellent NOx removal performance and durability even in combustion exhaust gas containing sulfur oxides, and the combustion exhaust gas temperature is 300 ° C to
An object of the present invention is to provide an exhaust gas purifying catalyst that does not deteriorate in denitration performance even at a relatively low temperature of about 400 ° C., and an exhaust gas purifying method using the same.

[0010]

To achieve the above object, the present invention is a catalyst for reducing and removing nitrogen oxides in oxygen-rich combustion exhaust gas, which comprises a first catalyst containing titania and iridium, An exhaust gas purifying catalyst comprising a mixture of a sulfuric acid-treated zirconia and a second catalyst containing palladium.

In the above exhaust gas purifying catalyst of the present invention, the mixing ratio of the second catalyst to the first catalyst is 3 to 5.
It is preferably a weight times.

Further, the present invention comprises the above exhaust gas purifying catalyst and a supporting substrate having an integral structure made of a refractory material having a large number of through holes, and at least the inner surface of the through holes of the supporting substrate is coated with the catalyst. A catalyst coating structure for exhaust gas purification is provided.

Further, the present invention comprises contacting the exhaust gas-purifying catalyst or the exhaust-gas-purifying catalyst coating structure with oxygen-rich combustion exhaust gas containing sulfur oxides in the presence of methanol,
An exhaust gas purification method characterized by reducing and removing nitrogen oxides in combustion exhaust gas.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The exhaust gas purifying catalyst according to the present invention is a catalyst obtained by mixing a first catalyst and a second catalyst,
The first catalyst is titania (TiO ₂ ) and iridium (Ir)
The second catalyst contains sulfuric acid-treated zirconia (ZrO ₂ ).
And palladium (Pd). First catalyst and second
From the standpoint of denitration performance, it is preferable that the mixing ratio of the catalyst be larger than that of the first catalyst in terms of denitration performance. Specifically, the mixing ratio of the second catalyst to the first catalyst is 3 to 5 times by weight. It is a range. If the mixing ratio of the second catalyst is less than 3 times the weight ratio of the first catalyst, the denitrification performance will be reduced, and if it exceeds 5 times, the content of the first catalyst will be small. The NOx oxidation performance, which is important as a step, becomes insufficient, and as a result, the denitration performance decreases.

The structure of titania in the first catalyst is not particularly limited, and examples thereof include anatase type, rutile type, or a mixed state thereof. Further, the state of iridium contained in the first catalyst is not particularly limited, and may be a metal state, an oxide state,
Alternatively, a mixed state thereof may be mentioned, and the starting material is not particularly limited. The content of iridium in the first catalyst is preferably 0.1 to 10% by weight, and more preferably 0.1 to 5% by weight in terms of metal. If the content of iridium is less than 0.1% by weight, the NOx oxidation performance, which is important as one step in the progress of the denitration reaction, is insufficient, while if it exceeds 10% by weight, the denitration performance deteriorates.

Next, the sulfuric acid-treated zirconia in the second catalyst is, for example, 500 to 600 in air after impregnating commercially available zirconia hydroxide with an aqueous solution of ammonium sulfate.
Obtained by firing at ° C. The state of palladium contained in the second catalyst is not particularly limited, and may be a metal state, an oxide state, or a mixed state thereof,
The starting material is also not particularly limited. From the viewpoint of denitration performance, the palladium content in the second catalyst is preferably 0.05 to 5% by weight in terms of metal, as in the case of iridium, and is preferably 0.05% by weight.
1-2% by weight is more preferable.

The method for producing the first catalyst and the second catalyst is not particularly limited, and a conventional method can be adopted. For example,
Adsorption method, pore filling method, incipient wetness method, evaporation dryness method, impregnation method such as spray method, kneading method,
Well-known methods that are usually adopted can be used, such as a physical mixing method and a method combining these methods.

In the above-mentioned catalyst production, the drying temperature is usually preferably about 80 to 120 ° C. The firing temperature is preferably 300 to 900 ° C., and 400 to 900 ° C.
About 700 ° C. is more preferable. Regarding the atmosphere at this time, an atmosphere such as air, an inert gas, oxygen, or steam may be appropriately selected according to the catalyst composition, and each atmosphere may be alternately changed at regular intervals.

The exhaust gas-purifying catalyst obtained by mixing the first catalyst and the second catalyst is, as usual, filled in a container in a fixed shape in a powder state, or a molded body molded into a predetermined shape as it is or It can be used by filling it in a container having a fixed shape.

In order to make the catalyst into a molded product having a desired shape, the powdery catalyst is mixed with a suitable binder such as methyl cellulose, or is extruded without a binder using a molding machine having a die having a desired shape. The shape of the catalyst is not particularly limited, and may be various shapes such as spherical shape, honeycomb shape, pellet shape, and ring shape. Also,
The size of the catalyst having these shapes may be arbitrarily selected according to the usage conditions.

Further, the exhaust gas-purifying catalyst obtained by mixing the first catalyst and the second catalyst can be used as a catalyst coating structure in which a supporting substrate is coated with the catalyst. A preferred catalyst-coated structure is a monolithic support substrate composed of a refractory material having a large number of through holes, at least the inner surface of the through holes being coated with a catalyst by a known method such as a wash coating method. .

The support substrate used in the catalyst coated structure is
A large number of through holes are provided, and the through holes are arranged along the exhaust gas flow direction during use. The porosity of the supporting substrate is 60 to 90% in a cross section perpendicular to the exhaust gas flow direction.
Is preferred. As the material of the supporting substrate, ceramics such as cordierite and metals such as stainless steel can be used. Also, regarding the shape, a conventionally used one such as a honeycomb shape or a continuous foam shape can be used.

In the exhaust gas purification method of the present invention, the combustion exhaust gas may be brought into contact with the above-mentioned exhaust gas purification catalyst or exhaust gas purification catalyst coating structure in the presence of methanol. The amount of methanol added to the exhaust gas as a reducing agent may be appropriately selected according to the denitration rate and running cost required for operation, but normally, the molar ratio to nitrogen oxide is preferably about 0.5 to 5.

Combustion exhaust gas from an internal combustion engine operated at a lean air-fuel ratio is generally CO, HC (hydrocarbon) and H ₂
And a oxidative component such as NOx and O _2, but contains an excess of oxygen in excess of the stoichiometric amount required for a complete redox reaction between them.
By contacting the combustion exhaust gas with the catalyst of the present invention under such an excess oxygen condition, NOx is reductively decomposed to N ₂ and H ₂ O, and at the same time, a reducing agent such as HC is also reduced to CO ₂ and H ₂ O. It is completely oxidized to O.

The gas space velocity (SV) in the exhaust gas purification method using the catalyst of the present invention is not particularly limited, but it is preferably 1,000 / h or more and 100,000 / h or less.

[0026]

Example (1) Preparation of catalyst After immersing 10 g of commercially available titania in 120 ml of an aqueous solution containing 0.19 g of iridium tetrachloride hydrate, 80 ° C.
It was evaporated to dryness. This was further dried at 110 ° C. and then calcined in air at 500 ° C. for 3 hours to obtain a catalyst 1 composed of Ir / TiO ₂ . The Ir content of the catalyst 1 in terms of metal is 1% by weight based on the total weight of the catalyst 1.
Is. Further, a catalyst 2 composed of Ir / TiO ₂ was prepared in the same manner as above except that the Ir content in terms of metal was 3% by weight.

On the other hand, 12 containing 10 g of ammonium sulfate
80 ml of zirconium hydroxide in 0 ml of aqueous solution
Was dipped and then evaporated to dryness at 80 ° C. This is 11 more
After drying at 0 ° C., it was calcined in air at 550 ° C. for 3 hours to obtain sulfuric acid-treated ZrO ₂ . 20 g of this sulfuric acid-treated zirconia was added to a palladium nitrate aqueous solution (Pd: 10%) of 0.
Immerse 4g in a solution diluted with 50ml pure water,
After evaporating to dryness and drying at 110 ° C in air, 50
It was calcined at 0 ° C. to obtain a catalyst 3 composed of Pd / sulfuric acid treated ZrO ₂ . The Pd content in terms of metal in the catalyst 3 is 0.2% by weight based on the weight of the entire catalyst 3.

The catalyst 1 and the catalyst 3 obtained as described above are
Catalyst A was prepared by physically mixing the catalyst 1: catalyst 3 in a weight ratio of 1: 4. Further, the catalyst 2 and the catalyst 3 were physically mixed so that the weight ratio of the catalyst 2 to the catalyst 3 was 1: 4, to prepare the catalyst B.

Next, 60 g of the above catalyst A was charged into a ball mill pot together with 4 g of silica sol (SiO ₂ solid content 20% by weight) and 120 ml of water, and wet pulverized to obtain a slurry. Into this slurry, a cylindrical core with a diameter of 1 inch and a length of 2.5 inches hollowed out from a commercially available 400 cpsi (cell / inch ² ) cordierite honeycomb substrate was immersed, and after pulling up, the excess slurry was air blown. Removed and dried. Then, it was fired at 500 ° C. for 30 minutes to prepare a catalyst-coated structure coated with 120 g of solid content per liter of honeycomb in terms of dry matter, and this honeycomb-shaped catalyst-coated structure was designated as catalyst C.

(2) Evaluation of denitration performance Using the catalysts A to B and the catalyst 1 or 3 produced as described above, the denitration performance was evaluated. First, each of these catalysts is pressure-molded and then pulverized to have a particle size of 350 to 500 μm.
The particles were sized. Each of the obtained catalyst powders (1.2 g) was filled in a Pyrex (registered trademark) glass reaction tube having an inner diameter of 7 mm, and the glass reaction tube was mounted in an atmospheric fixed bed flow reactor.

The gas temperature in the reaction tube was set to 350 in this apparatus.
As the model exhaust gas, NO: 1000 ppm, O ₂ : 10%,
H ₂ O: 10%, SO ₂ : 50 ppm, methanol: 2
A mixed gas consisting of 000 ppm and the balance: N2 was passed at a space velocity of 30,000 / h, and the denitration performance at that time was evaluated. The denitration rate as the denitration performance was calculated according to the following mathematical formula 1, and the obtained results are shown in Table 1 below.

Regarding the analysis of the gas composition at the outlet of the reaction tube, the concentration of NOx was measured by a chemiluminescence type NOx meter, and N ₂
The O concentration was measured using a gas chromatograph / heat conductivity detector equipped with a Porpack Q column. In the case of any of the catalysts, N ₂ O was hardly found in the outlet gas of the reaction tube.

[0033]

[Equation 1]

For comparison, the above catalyst 2 and catalyst 3 are also included.
Two-stage catalysts arranged in order (without mixing) in the exhaust gas flow direction, that is, catalyst 2/3 and catalyst 3/2 (upstream side / downstream side)
The denitration performance was evaluated in the same manner as above, and the results are also shown in Table 1 below.

Further, a catalyst C composed of a honeycomb-shaped catalyst coating structure coated with the above-mentioned catalyst A was used, which had a diameter of 1
After adjusting to a cylindrical shape of 5 mm × length 32 mm, the inner diameter 15
A catalyst bed was constructed by being housed in a stainless steel reaction tube of mm. Denitration performance was evaluated in the same manner as above except that the model exhaust gas was passed through the catalyst bed at a space velocity of 6,000 / h, and the results are also shown in Table 1 below.

[0036]

[Table 1]

Next, using the same catalyst A, catalyst B, and catalyst C consisting of a honeycomb-shaped catalyst coating structure as described above, while maintaining the gas temperature in the reaction tube constant at 350 ° C., 50 hours after the initial start, The change in denitration rate with time was measured. At that time, the space velocity of the model exhaust gas was 30,000 / h for catalyst A and catalyst B and 6.0 for catalyst C.
00 / h. The results obtained are shown in Table 2 below.

[0038]

[Table 2]

From the results shown in Table 1 above, the catalysts A to C of the present invention in which the catalysts 1 to 3 were combined and mixed were comparative examples in which any one of the catalysts 1 to 3 was arranged alone or in two stages. Compared to NO, even in exhaust gas in which a large amount of sulfur oxides coexist
It can be seen that x removal activity is high. Further, from the results in Table 2 above, it can be seen that the catalysts A to C of the examples of the present invention are excellent in durability with little deterioration in denitration performance over time even in the presence of sulfur oxides.

[0040]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide an exhaust gas purifying catalyst that reduces and removes nitrogen oxides in combustion exhaust gas with excess oxygen at an excellent denitration rate. The exhaust gas-purifying catalyst of the present invention maintains excellent NOx removal performance even in combustion exhaust gas containing sulfur oxides, and has excellent durability. Furthermore, even if the exhaust gas temperature is relatively low, such as about 300 ° C to 400 ° C, the denitration performance does not deteriorate.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4D048 AA06 AB02 AB07 AC09 BA07X                       BA08X BA31X BA33X BA39Y                       BA41X BA46X BB01 BB02                       BB16 BC01 CC50                 4G069 AA03 BA04A BA04B BA05A                       BA05B BA13A BA13B BA17                       BB02A BB02B BB10A BB10B                       BC51A BC51B BC74A BC74B                       CA02 CA03 CA08 CA13 EA18                       EA19 EB12Y EB14Y EC09                       EC28 FC08

Claims (5)

[Claims] 1. A catalyst for reducing and removing nitrogen oxides in combustion exhaust gas containing excess oxygen, which is obtained by mixing a first catalyst containing titania and iridium with a second catalyst containing sulfuric acid-treated zirconia and palladium. An exhaust gas purifying catalyst characterized by: 2. The exhaust gas-purifying catalyst according to claim 1, wherein a mixing ratio of the second catalyst to the first catalyst is 3 to 5 times by weight. 3. The exhaust gas purifying catalyst according to claim 1 or 2, and a supporting substrate having an integral structure made of a refractory material having a large number of through holes, wherein the catalyst is provided on at least the inner surface of the through holes. An exhaust gas-purifying catalyst-coated structure characterized by being coated with. 4. The exhaust gas purifying catalyst according to claim 1 or 2 is contacted with oxygen-rich combustion exhaust gas containing sulfur oxides in the presence of methanol to reduce and remove nitrogen oxides in the combustion exhaust gas. Exhaust gas purification method characterized by the above. 5. The exhaust gas-purifying catalyst-coated structure according to claim 3 is contacted with oxygen-rich combustion exhaust gas containing sulfur oxides in the presence of methanol to reduce and remove nitrogen oxides in the combustion exhaust gas. An exhaust gas purification method characterized by the above.