JP2000225323A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To significantly enhance the NOx cleaning performance by providing a catalyst for oxidizing NO into NO2 and a catalyst for reducing NO2 into N2 and in addition, a catalyst for reducing NO into N2 and further, bringing the catalyst for reducing NO into N2 into contact with an exhaust gas ahead of the other catalysts. SOLUTION: In the catalyst for cleaning an exhaust gas to be discharged from an engine operated at a lean air fuel ratio, a first catalyst 1 for reducing NO contained in the exhaust gas into N2 in the presence of HC, a second catalyst 2 for oxidizing NO into NO2 and a third catalyst 3 for reducing NO2 produced by oxidation using the second catalyst 2, into N2 in the presence of HC, are arranged sequentially from an upstream side to a downstream side in the exhaust gas flow direction A. In this case, the NO → N2 reduction reaction by the first catalyst 1 can be promoted while HC required for the reduction reaction by the third catalyst 3 is secured by adopting a catalyst consisting of zeolite and Pt borne by the former through ion exchange especially as the first catalyst 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art JP-A-63-049255 discloses that
The Ag catalyst layer and the zeolite catalyst layer are arranged on the upstream and downstream sides in the exhaust gas flow direction, NO in the exhaust gas is oxidized to NO ₂ by the Ag catalyst layer, and this NO ₂ is converted into N ₂ by the zeolite catalyst layer. Is described.

[0003]

However, NO is converted to NO ₂
A catalyst for oxidation, only provided a catalyst for reducing NO ₂ to N _2, even change the structure of the both catalysts in a variety, in improving the NOx purification rate is limited. Therefore, the invention of this application is intended to further increase the NOx purification rate.

[0004]

The present inventor has made the above-mentioned NO.
In addition to a catalyst for oxidizing NO to NO ₂ and a catalyst for reducing NO ₂ to N ₂ , a catalyst for reducing NO to N ₂ is provided, and this catalyst is provided with exhaust gas before other catalysts. It has been found that the NOx purification performance is improved if the contact is made.

[0005] That is, the invention of this application relates to NO and HC.
A first catalyst for promoting a reaction of reducing NO to N ₂ in the presence of HC, and a first catalyst for promoting a reaction of oxidizing NO to NO _2. And a third catalyst that promotes the reaction of reducing NO ₂ to N ₂ in the presence of HC, wherein the first to third catalysts are such that the second catalyst is located after the first catalyst. The third catalyst is disposed in the exhaust passage so as to contact the exhaust gas and the third catalyst contacts the exhaust gas after the second catalyst.

In the present invention, first, NO in exhaust gas is reduced to N ₂ by reacting with HC on the first catalyst, or N is reacted by reacting with HC after NO is oxidized to NO _2. Reduced to ₂ . And the first
NO that has not been purified on the catalyst is oxidized to NO ₂ on the second catalyst and reacts with HC on the third catalyst to form N 2.
Reduced to ₂ .

As means for making one of the two catalysts come into contact with the exhaust gas first and the other coming into contact with the exhaust gas later, the two catalysts are arranged upstream and downstream of the exhaust passage. Alternatively, both catalysts may be provided in layers on a carrier. In the latter case, the catalyst is to be provided in a layered manner so that the catalyst to be brought into contact with the exhaust gas first is on the outside and the catalyst to be contacted later is inside.

As the first catalyst, a catalyst having a catalyst component in which a noble metal is supported on zeolite can be used. In this case, the noble metal is preferably supported on the zeolite by ion exchange. That is, when the noble metal acts as a reduction catalyst, the reducing agent for that purpose is oxidized at the same time. If the function of the noble metal as an oxidation catalyst is too high, HC in the exhaust gas becomes NO or NO _2. Is oxidized by the oxygen in the exhaust gas, and the amount of HC required for the reduction of NO ₂ → N ₂ in the third catalyst is reduced.

[0009] In this respect, the one in which the noble metal is supported on the zeolite by ion exchange is not so high in the function as an oxidation catalyst, so that it is effective only as a first catalyst in promoting the reduction reaction of NO → N _2. Therefore, HC is not excessively oxidized, which is advantageous for securing HC necessary for the reduction reaction on the third catalyst.

The noble metal of the first catalyst is Pt.
It is particularly preferable that the Pt is supported on zeolite by ion exchange.

On the other hand, it is preferable that the second catalyst for oxidizing NO to NO ₂ contains Ag from the viewpoints of improvement in purification rate, heat resistance, and the like. The Ag is supported on, for example, alumina. What should I do? NO ₂ above
As the third catalyst for reducing the N _2, is improved purification rate zeolite transition metal is one having a catalyst component comprising supported, from the viewpoint of heat resistance and the like. In this case, as the transition metal, a noble metal, for example, Pt, or a transition metal other than the noble metal, for example, Cu can be used.
As the catalyst component of the third catalyst, one in which Pt is supported on zeolite by ion exchange is particularly preferable.

[0012]

As described above, according to the invention of this application, the first catalyst for reducing NO to N ₂ in the presence of HC and the second catalyst for oxidizing NO to NO ₂ , NO
_A third catalyst for reducing ₂ to N ₂ in the presence of HC, wherein the second catalyst contacts the exhaust gas after the first catalyst,
Since the third catalyst is disposed in the exhaust passage so as to come into contact with the exhaust gas later than the second catalyst, the NOx purification rate can be improved. In particular, the first catalyst in which Pt is carried on zeolite by ion exchange is used. According to the one adopted,
While securing HC necessary for the reduction reaction on the third catalyst,
The NO → N ₂ reduction reaction on the first catalyst can be promoted, and the NOx purification performance can be dramatically improved.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

<Structure of Exhaust Gas Purifying Catalyst> FIG. 1 shows an air-fuel ratio lean (for example, A / F> 16 or A / F> 2 such as a diesel engine or a so-called lean burn engine).
0, NOx of a high concentration of oxygen discharged from the engine was operated at O ₂ concentration of 5% or more) in the exhaust gas exhaust gas
1 shows a configuration of an exhaust gas purifying catalyst suitable for purifying exhaust gas. In the figure, reference numeral 1 denotes NO in exhaust gas
The first catalyst for reducing to N ₂ in the presence of
Reference numeral 3 denotes a second catalyst for oxidizing O to NO _2. Reference numeral 3 denotes NO ₂ oxidized and generated by the _second
This is a third catalyst for reducing to ₂ , and these catalysts 1 to 3 are arranged in order from the upstream side to the downstream side in the exhaust gas flow direction A.

The first catalyst 1 is a catalyst in which a transition metal, for example, Pt is supported on a zeolite as a base material (support material) by ion exchange or the like, and is supported on a honeycomb carrier (made of cordierite) by a binder. . The second catalyst 2 is obtained by supporting Ag and a transition metal having a higher ionization tendency, for example, Ni, on γ-alumina as a base material, and is also supported on a honeycomb carrier. The third catalyst 3 is a catalyst in which a transition metal, for example, Pt is supported on zeolite as a base material by ion exchange or the like, and is also supported on a honeycomb carrier.

[0016]

EXAMPLES Exhaust gas purifying catalysts of the following Examples 1 to 8 were prepared and their purifying performance was evaluated.

<Preparation of Example of Exhaust Gas Purifying Catalyst> -Example 1-Preparation of First Catalyst An aqueous solution of hexaammineplatinum tetrachloride was added to H-type MFI powder having a cubic ratio of 30 and ion exchange treatment was performed at 80 ° C. for about 3 hours. Was performed. After cooling to room temperature, filtration was performed, and the obtained solid was washed with water, dried, and dried.
By calcining at 2 ° C. for 2 hours, Pt ion-exchange supported MFI (Pt ⁺ -MFI) was obtained. This was mixed with hydrated alumina, water was added to form a slurry, and the slurry was wash-coated on a cordierite honeycomb carrier (capacity: 12.5 mL, cell density: 400 cells / square inch, cell partition thickness: 6 mil), and dried. And firing. The Pt amount of this catalyst was adjusted to 0.5 g / L (grams per liter of the support). The amount of impurities in the obtained catalyst is 1% or less. This is the same for the other catalysts described below.

Preparation of Second Catalyst A high specific surface area γ-alumina powder and a hydrated alumina powder (binder) were mixed at a ratio of 10: 1, and water was added to the mixed powder to prepare a slurry. By repeating the operation of dipping a cordierite honeycomb carrier (capacity: 12.5 mL, cell density: 400 cells / square inch, cell partition thickness: 6 mil) into the slurry, pulling it up, and drying it in an oven, the above γ is obtained. -150 g / L of alumina was loaded on the honeycomb carrier.

An aqueous solution of nickel nitrate was impregnated into the alumina coat layer of the honeycomb carrier so that the amount of Ni was 26 g / L, followed by drying at 200 ° C. for 2 hours and firing at 500 ° C. for 2 hours. The obtained product was heated at 600 ° C. in a vacuum heating furnace at a vacuum degree of 10 ⁻⁴ to 10 ⁻⁵ Torr (O ₂ concentration: 2.0 × 10 ^{−3 to} 2.0 × 10 ⁻⁴ %). Heat treatment was performed for a time.

Next, an aqueous solution of silver acetate was impregnated into the above-mentioned coat layer carrying Ni so that the amount of Ag was 30 g / L, followed by drying at 200 ° C. for 2 hours and firing at 500 ° C. for 2 hours. . The obtained product was placed in a vacuum heating furnace for 10 hours.
Heat treatment was performed at 600 ° C. for 6 hours at a degree of vacuum of ⁻⁴ to 10 ⁻⁵ Torr.

Preparation of Third Catalyst A catalyst having the same constitution as the first catalyst was prepared.

Example 2 In this example, Pt dry solid supported MFI (Pt / MFI) was employed as the first catalyst, and the capacity of the honeycomb support was 12.
5 mL, Pt amount is 1.0 g / L. Pt-supported MFI
Was prepared by adding an aqueous solution of dinitrodiamineplatinum chloride to H-type MFI powder having a Caban ratio of 30 and performing spray drying and firing. Other configurations are the same as in Example 1.

Example 3 In this example, a Pd ion-exchange supported MFI (Pd
⁺ -MFI) and the honeycomb carrier capacity is 1
2.5 mL, Pd amount is 0.5 g / L. The Pd ion exchange supported MFI is the Pt ion exchange supported MFI of Example 1.
(Pt ⁺ -MFI). Other configurations are the same as in Example 1.

Example 4 In this example, Rh ion exchange supported MFI (Rh
⁺ -MFI) and the honeycomb carrier capacity is 1
2.5 mL, Rh amount is 0.5 g / L. The Rh ion exchange supported MFI is the Pt ion exchange supported MFI of Example 1.
(Pt ⁺ -MFI). Other configurations are the same as in Example 1.

Example 5 In this example, an Ir ion-exchange supported MFI (Ir
⁺ -MFI) and the honeycomb carrier capacity is 1
2.5 mL, Rh amount is 0.5 g / L. The Ir ion-exchange supported MFI is the Pt ion-exchange supported MFI of Example 1.
(Pt ⁺ -MFI). Other configurations are the same as in Example 1.

Example 6 In this example, the first catalyst was eliminated and the third catalyst was replaced with the catalyst of Example 1
(Honeycomb carrier capacity 25.
0 mL). Other configurations are the same as in Example 1.

<Evaluation test of catalyst>
The first to third catalysts (however, the second catalyst and the third catalyst in Example 6) were incorporated in the exhaust gas circulation device so as to be arranged in order from the upstream side to the downstream side. Then, the next simulated exhaust gas was flown, the gas temperature at the catalyst inlet was raised to 160 ° C. and held for 5 minutes, and then the NOx purification rate when the temperature was raised at a rate of 25 ° C./min was determined.

Simulated exhaust gas HC = 170 ppm, CO = 200 ppm, NO = 17
0 ppm, CO ₂ = 4.5%, H ₂ O = 0%, O ₂ = 1
0%, remaining N ₂ SV = 50000 / h <Test Results> The results are shown in FIG. Example 1 and Example 6
The structure differs depending on whether the Pt ion-exchange supported MFI is divided into a first catalyst and a third catalyst, or is combined as a third catalyst. In Example 1, however, the peak of catalytic activity is higher than in Example 6. Temperature shifts by about 50 ° C to the high temperature side, 270 ° C
And the peak NOx purification rate is 1
It is almost 0% higher. From this, it can be seen that configuring the first catalyst that comes into contact with exhaust gas before the second catalyst by the Pt ion-exchange supported MFI is highly effective in improving NOx purification performance.

On the other hand, Example 2 shows that the first catalyst is not Pt ion-exchange supported MFI but Pt dry supported MFI and Pt
Although different from Example 1 in that the amount is large, the temperature at which the catalyst activity peaks shifts to a lower temperature side, and the peak NOx purification rate decreases. This is because, in the case of Example 2, although the oxidation reaction promoting function of the first catalyst is too strong and exhibits NOx purification activity from a relatively low temperature, the oxidation of HC considerably proceeds at the same time. _It is considered that HC necessary for the reduction of ₂ became insufficient.

In Examples 3 to 5, Pt was used as the noble metal of the third catalyst.
, And the catalyst was supported on the MFI by ion exchange.
However, the peak NOx purification rate is low.
Therefore, it is found that it is advantageous to use Pt as the noble metal of the third catalyst.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an arrangement configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is a graph showing a NOx purification rate of each example in which the first catalyst is different.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st catalyst 2 2nd catalyst 3 3rd catalyst

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) B01D 53/36 102B F-term (Reference) 4D048 AA06 AB01 AB02 BA03X BA10X BA11X BA30X BA31Y BA32Y BA33X BA34X BA38X BA41X BA42X BB02 CA01 CC04 CC32 CC38 4G069 AA01 AA03 BA01A BA01B BA07A BA07B BB02A BB02B BC69A BC69B BC71B BC74B BC75A BC75B CA02 CA03 DA06 EA18 ZA01A ZD01

Claims (8)

[Claims] 1. An exhaust gas purifying catalyst provided in an exhaust passage from which NO and HC are discharged, the first catalyst promoting a reaction of reducing NO to N ₂ in the presence of HC, _Second to promote the reaction to oxidize to NO 2
A catalyst, and a third catalyst that promotes a reaction of reducing NO ₂ to N ₂ in the presence of HC, wherein the first to third catalysts are configured such that the second catalyst generates exhaust gas after the first catalyst. An exhaust gas purifying catalyst disposed in the exhaust passage such that the third catalyst comes into contact with the exhaust gas later than the second catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the first catalyst has a catalytic component in which a noble metal is supported on zeolite. 3. The exhaust gas purifying catalyst according to claim 2, wherein the noble metal is carried on zeolite by ion exchange. 4. The exhaust gas purifying catalyst according to claim 2, wherein the noble metal is Pt. 5. The exhaust gas purifying catalyst according to claim 1, wherein the second catalyst contains Ag. 6. The exhaust gas purifying catalyst according to claim 1, wherein the third catalyst has a catalyst component in which a transition metal is supported on zeolite. 7. The exhaust gas purifying catalyst according to claim 6, wherein the third catalyst has a catalytic component in which Pt is supported on zeolite by ion exchange. 8. An exhaust gas purifying catalyst provided in an exhaust passage from which NO and HC are exhausted, comprising: a first catalyst having a catalyst component comprising a noble metal supported on zeolite; and NO oxidizing NO to NO ₂ . A second catalyst that promotes a reaction of reducing NO ₂ to N ₂ in the presence of HC, and the first to third catalysts include a first catalyst that is a first catalyst. An exhaust gas purifying catalyst disposed in the exhaust passage so that the exhaust gas contacts the exhaust gas after the catalyst and the third catalyst contacts the exhaust gas after the second catalyst.