JP2006159069A - Exhaust gas cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning system suitable for a diesel engine operated in lean atmosphere, and high in space velocity of the exhaust gas, and a ternary oxidation catalyst excellent in thermal storage effect, used for the same. <P>SOLUTION: The exhaust gas cleaning system is arranged for use in an exhaust gas passage of the diesel engine, and has the ternary oxidation catalyst upstream against the exhaust gas flow direction, and a catalyst for the exhaust gas cleaning, having NOx trapping function and HC trapping function downstream. In the system, a catalyst coat layer containing the ternary oxidation catalyst is formed on a cell wall of a monolithic carrier, and a total thickness of the cell wall thickness and the thinnest portion of the catalyst coat layer thickness formed on the cell wall is 50-650 μm. The ternary oxidation catalyst used for it is provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification system, and more particularly, an exhaust gas purification system suitable for use in a diesel engine that is operated in a lean atmosphere and has a large exhaust gas space velocity, and has an excellent heat storage effect. It relates to a three-way oxidation catalyst.

Conventionally, in exhaust gas purifying catalysts used for gasoline engines, the cell wall thickness of the honeycomb carrier and the coating layer thickness of the catalyst component are reduced from the viewpoint of improving the temperature rise performance in order to activate the catalyst component early. It has been proposed to do.
When such an exhaust gas purification catalyst is applied to a diesel engine that is mainly operated in a lean atmosphere and has a large exhaust gas space velocity, the temperature rise performance is excellent, but the exhaust gas space velocity is large, For example, during deceleration, the catalyst temperature is significantly lower than the activation temperature of the catalyst component, and the exhaust gas conversion rate cannot be sufficiently improved as a whole.

Therefore, it has been proposed to improve the decrease in the catalyst temperature by engine control such as retarding at start-up and control of excess air ratio (see, for example, Patent Document 1).
As a conventional exhaust gas purification system for a diesel engine, a system has been proposed in which an oxidation catalyst is disposed on the upstream side of the exhaust gas passage and a NOx purification catalyst is disposed on the downstream side.
JP 2002-235589 A

However, in the prior art described in the above-mentioned Patent Document 1, although a slight improvement effect of the conversion rate of nitrogen oxide (NOx) was observed due to the increase in the catalyst temperature, carbonization in a trade-off relationship. As the amount of hydrogen (HC) discharged increases, the conversion rate decreases, so there is a limit to improving the conversion rate by engine control.
Further, the prior art described in Patent Document 2 also has a problem that the exhaust gas conversion rate cannot be sufficiently improved.

  The present invention has been made in view of such problems of the prior art, and the object of the present invention is to operate exhaust gas in a lean atmosphere and to use exhaust gas suitable for a diesel engine having a high exhaust gas space velocity. An object of the present invention is to provide a purification system and a three-way oxidation catalyst having an excellent heat storage effect.

  As a result of intensive studies to achieve the above object, the present inventors have arranged a predetermined three-way oxidation catalyst on the upstream side of the exhaust gas flow path, and an exhaust having a NOx trap function and an HC trap function on the downstream side. It has been found that the above object can be achieved by constructing an exhaust gas purification system in which a gas purification catalyst is arranged, and the present invention has been completed.

  That is, the exhaust gas purification system of the present invention is arranged and used in the exhaust gas flow path of a diesel engine, has a three-way oxidation catalyst on the upstream side with respect to the exhaust gas flow direction, and has a NOx trap function on the downstream side. An exhaust gas purification system having an exhaust gas purification catalyst having an HC trap function, wherein the ternary oxidation catalyst is formed by forming a catalyst coat layer containing a ternary oxidation catalyst on a cell wall of a monolith support, The total thickness at the thinnest part of the cell wall thickness and the thickness of the catalyst coat layer formed on the cell wall is 50 to 650 μm.

  According to the present invention, since a predetermined three-way oxidation catalyst is disposed upstream of the exhaust gas flow path, and an exhaust gas purification catalyst having a NOx trap function and an HC trap function is disposed downstream, the lean An exhaust gas purification system suitable for use in a diesel engine that is operated in an atmosphere and has a high exhaust gas space velocity, and a three-way oxidation catalyst excellent in the heat storage effect used therefor can be provided.

Hereinafter, the exhaust gas purification system of the present invention will be described.
As described above, the exhaust gas purification system of the present invention is arranged and used in the exhaust gas flow path of a diesel engine, has a three-way oxidation catalyst on the upstream side with respect to the exhaust gas flow direction, and has a NOx trap on the downstream side. An exhaust gas purification system having an exhaust gas purification catalyst having an HC trap function and an HC trap function, wherein the three-way oxidation catalyst is formed by forming a catalyst coat layer containing the three-way oxidation catalyst on the cell wall of the monolith support. The total thickness at the thinnest part of the cell wall thickness and the thickness of the catalyst coat layer formed on the cell wall is 50 to 650 μm.
Here, the “three-way oxidation catalyst” refers to an oxidation catalyst or a three-way catalyst having an oxidation function.

With such a configuration, when used in a diesel engine that is operated in a lean atmosphere and has a high exhaust gas space velocity, the conversion rate of the exhaust gas can be improved.
That is, the ternary oxidation catalyst used in the exhaust gas purification system of the present invention is formed by forming the catalyst coat layer containing the ternary oxidation catalyst on the cell wall (rib) of the monolith support as described above. The total thickness at the thinnest part of the wall thickness and the thickness of the catalyst coat layer formed on the cell wall is 50 to 650 μm, and the total thickness is more preferably 150 to 600 μm, and preferably 300 to 600 μm. Particularly preferred.

By making the total thickness at the thinnest part of the cell wall thickness and the thickness of the catalyst coat layer formed on the cell wall as described above within the above range, the heat storage effect of the catalyst can be enhanced and it is arranged downstream. It is possible to suppress a decrease in the catalyst temperature of the exhaust gas purifying catalyst having the NOx trap function and the HC trap function. Furthermore, the exhaust gas in the three-way catalyst itself, in particular, the HC conversion rate can be improved, and since it is an exothermic reaction, the catalyst temperature of the exhaust gas purifying catalyst arranged on the downstream side can also be improved.
In other words, it is possible to suppress a decrease in the catalyst temperature of the exhaust gas purification catalyst disposed on the downstream side, and further improve the catalyst temperature, thereby increasing the total exhaust gas conversion rate of the exhaust gas purification system. It becomes possible to improve.
If the total thickness is less than 50 μm, a sufficient heat storage effect cannot be exhibited. If the total thickness exceeds 650 μm, the exhaust gas passage is clogged, and the improvement of the exhaust gas conversion rate is hindered. There is a high possibility.

Moreover, it is desirable to use a three-way catalyst as the three-way oxidation catalyst from the viewpoint of improving the exhaust gas conversion rate. The ternary oxidation catalyst used in the present invention may contain γ-alumina and δ-alumina having a function as a heat storage material, and may further contain zirconia and titania that improve their heat resistance, and β-zeolite, ZSM, ZSM5 And other zeolites.
Furthermore, by using a monolithic carrier made of ceramic and having a polygonal cell shape, the heat storage effect is higher than in the case of a metal monolithic carrier, and the adhesion between the monolithic carrier and the catalyst coat layer can be improved.
The thinnest part of the monolithic carrier made of ceramics and having a polygonal cell shape is usually the central part between adjacent vertices in the polygonal cell, and in this specification, in particular, the “flat part” That's it.
Furthermore, the conversion rate of the cold HC can be improved by performing engine control such as retard when starting the engine.

In the present invention, the thickness of the catalyst coat layer in the flat portion relative to the total thickness in the thinnest portion of the cell wall thickness of the three-way catalyst used and the thickness of the catalyst coat layer formed on the cell wall is preferably 50% or more. 60% or more is more preferable, and 70% or more is particularly preferable.
If it is less than 50%, the heat obtained by the above-described exothermic reaction may not be sufficiently stored, which is not preferable.

The exhaust gas purifying catalyst used in the present invention is not particularly limited as long as it has a NOx trap function and an HC trap function, and a conventionally known catalyst can be used.
For example, a catalyst in which a coat layer having an HC trap function is arranged in the first layer and a coat layer having a NOx trap function is arranged in the second layer, or a rear catalyst of the catalyst with a three-way function according to the present invention, It is possible to obtain the same effect with a tandem catalyst in which a catalyst having an NOx trap function and a catalyst having an HC trap function are arranged in the rear stage, or a catalyst having an HC trap function in the former stage and a catalyst having the NOx trap function in the rear stage.

Further, the exhaust gas purifying catalyst used in the present invention is not particularly limited with respect to the monolith carrier that coats the exhaust gas purifying catalyst, but it is, for example, a monolith made of ceramic and having a polygonal cell shape. In the case where the support has a laminated structure as described above, for example, the total thickness in the flat portion of the rib thickness and the thickness of the catalyst coat layer is 50 to 50 from the viewpoint of effectively performing both the NOx trap function and the HC trap function. It is preferably 300 μm, more preferably 50 to 200 μm, and particularly preferably 50 to 150 μm.
That is, in the exhaust gas purification system of the present invention, in particular, the total thickness of the three-way catalyst arranged on the upstream side is thicker than the exhaust gas purification catalyst arranged on the downstream side, in other words, It is desirable that the heat storage effect of the catalyst is high.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example and a comparative example, this invention is not limited to these Examples.

[Production of three-way catalyst]
Example 1
Alumina powder containing 3 mol% of Ce (Al 97 mol%) was impregnated with an aqueous solution of palladium nitrate or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, calcined at 400 ° C. for 1 hour, and then calcined at 600 ° C. for 1 hour. Pd-supported alumina powder (powder a) was obtained. The Pd concentration of this powder a was 4.0%.
A cerium oxide powder containing 1 mol% La and 32 mol% Zr (Ce 67 mol%) is impregnated with an aqueous palladium nitrate solution or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, then 400 ° C. for 1 hour, then 600 ° C. for 1 hour. Firing was performed to obtain Pd-supported cerium oxide powder (powder b). The Pd concentration of this powder b was 2.0%.

The Pd-carried alumina powder (Powder a) 314 g, Pd supported cerium oxide powder (Powder b) 314 g, sol obtained by adding 10% nitric acid to nitric acid alumina sol 320 g (10% boehmite alumina _Al 2 O ₃ g of 32 g) and 51.5 g of barium carbonate (40 g as BaO) were added to 2000 g of pure water in a magnetic ball mill, mixed and ground to obtain a slurry liquid. This slurry liquid was attached to a monolith support (cell number: 400 cells / 4 mil, rib thickness: 104 μm), excess slurry in the cells was removed by air flow, dried, and fired at 400 ° C. for 1 hour. A coat layer weight of 150 g / L was applied to obtain a three-way catalyst A for use in the present invention having a catalyst coat layer thickness of 70 μm in the flat part.
FIG. 1A is a conceptual diagram showing a state in which a catalyst coat layer 20 is formed on one cell wall (rib) 10 of the obtained three-way catalyst A. FIG.

(Example 2)
A three-way catalyst A was prepared except that a slurry liquid was prepared in the same manner as the three-way catalyst A, and a coating layer weight of 150 g / L was applied to a monolith support (number of cells: 400 cells / 10 mil, rib thickness: 260 μm). The three-way catalyst B used in the present invention having a catalyst coat layer thickness of 320 μm in the flat part was obtained.
FIG. 1B is a conceptual diagram showing a state in which a catalyst coat layer 20 is formed on one cell wall (rib) 10 of the obtained three-way catalyst B.

(Example 3)
Further, a slurry liquid was prepared in the same manner as the method for preparing the three-way catalyst A except that the powder having a metal concentration of 1/3 used in Example 1 was used, and a monolith carrier (cell number: 400 cells / 4 mil) was prepared. Rib thickness: 104 μm), a coating layer weight of 450 g / L was applied, and the coating amount was tripled, and the same was prepared as in the three-way catalyst A, and the catalyst coating layer thickness in the flat part was 500 μm. A three-way catalyst C used in the present invention was obtained.
FIG. 1A is a conceptual diagram showing a state in which a catalyst coat layer 20 is formed on one cell wall (rib) of the obtained three-way catalyst X.

[Production of oxidation catalyst]
Example 4
Alumina powder containing 3 mol% of Ce (Al 97 mol%) was impregnated with a platinum nitrate aqueous solution or sprayed at high speed stirring, dried at 150 ° C. for 24 hours, calcined at 400 ° C. for 1 hour, and then calcined at 600 ° C. for 1 hour. Pt-supported alumina powder (powder c) was obtained. The Pt concentration of this powder c was 4.0%.
A cerium oxide powder containing 1 mol% La and 32 mol% Zr (Ce 67 mol%) is impregnated with a platinum nitrate aqueous solution or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, then at 400 ° C. for 1 hour, and then at 600 ° C. for 1 hour. Firing was performed to obtain Pt-supported cerium oxide powder (powder d). The Pt concentration of this powder d was 2.0%.

The Pt-carried alumina powder (Powder c) 314 g, Pt supported cerium oxide powder (Powder d) 314 g, sol obtained by adding 10% nitric acid to nitric acid alumina sol 320 g (10% boehmite alumina _Al 2 O ₃ g of 32 g) and 51.5 g of barium carbonate (40 g as BaO) were added to 2000 g of pure water in a magnetic ball mill, mixed and ground to obtain a slurry liquid. This slurry liquid was attached to a monolith support (cell number: 400 cells / 4 mil, rib thickness: 104 μm), excess slurry in the cells was removed by air flow, dried, and fired at 400 ° C. for 1 hour. A coating layer weight of 150 g / L was applied to obtain an oxidation catalyst D used in the present invention having a catalyst coating layer thickness of 70 μm in the flat part.
FIG. 1A is a conceptual diagram showing a state in which a catalyst coat layer 20 is formed on one cell wall (rib) 10 of the obtained oxidation catalyst.
FIG. 1C is a conceptual diagram showing a state in which a catalyst coat layer 20 is formed on one cell wall (rib) 10 of a conventional catalyst.

[Exhaust gas purification catalyst]
A catalyst comprising a monolith honeycomb carrier having a GSA (geometric surface area) of 20 to 50 cm ² / cm ³ coated with zeolite as a first layer and a noble metal and an alkali metal or alkaline earth metal compound as a second layer Coat the components, 100 to 300 g of the first layer zeolite is converted to 100 to 300 g per 1 L of the catalyst carrier, 150 to 400 g of the second layer catalyst component is converted to metal atoms in terms of alkali metal or alkaline earth metal compound Thus, an exhaust gas purifying catalyst E having 0.1 to 0.6 mol and noble metal of 1 to 10 g was prepared.

[Performance evaluation]
An exhaust gas purification system was constructed using the obtained three-way catalysts A, B, and C, an oxidation catalyst D, and an exhaust gas purification catalyst E. That is, FIG. 2 is a schematic diagram showing the configuration of the engine system used for performance evaluation of the exhaust gas purification system. The exhaust gas purification system was constructed by arranging the three-way catalyst A at the catalyst position 1 in FIG. 2 and the exhaust gas purification catalyst E at the catalyst position 2. This was designated as an exhaust gas purification system A (Example 5).
Further, an exhaust gas purification system is constructed by replacing the three-way catalyst A with the three-way catalyst B, the three-way catalyst C, or the oxidation catalyst D, respectively, and the exhaust gas purification system B (Example 6) and the exhaust gas purification system C, respectively. Example 7 or exhaust gas purification system D (Example 8) was used.

After endurance under the following conditions, the NOx conversion rates of Abag (250-505) and Bbag (506-1347) of exhaust gas purification system A and exhaust gas purification system D were measured. The obtained results are shown in Table 1.
Further, after endurance under the same conditions, the NOx conversion rate was measured for 0 to 160 seconds after the exhaust gas purification system A and the exhaust gas purification system B were started. The obtained results are shown in Table 2.

(Endurance conditions)
-Engine displacement 3000cc
・ Fuel Light oil (Class 1)
・ Catalyst inlet gas temperature 700 ℃
-Endurance time 50 hours (performance evaluation conditions)
・ Catalyst capacity Three-way catalyst: 1L
Oxidation catalyst: 1L
Exhaust gas purification catalyst: 1L
・ Vehicle performance test Nissan Motor Co., Ltd. V type 6 cylinder 3.3L engine

From Table 1, when comparing the exhaust gas purification system A belonging to the scope of the present invention and the exhaust gas purification system D, it can be seen that the NOx conversion rate is better when the three-way catalyst is used as the three-way oxidation catalyst.
Further, from Table 2, by comparing the exhaust gas purification system A and the exhaust gas purification system B, the heat storage effect is further improved by increasing the catalyst coat layer thickness compared with the case of increasing the rib thickness, and NOx conversion It can be seen that the rate is improved.

(A), (b) and (c) is a conceptual diagram which shows the state in which the catalyst coat layer was formed on one cell wall of a ternary oxidation catalyst. It is the schematic which shows the structure of the engine system used for the performance evaluation of the exhaust gas purification system.

Explanation of symbols

10 Rib 20 Catalyst coating layer

Claims (6)

An exhaust gas purification catalyst that is disposed in an exhaust gas flow path of a diesel engine, has a three-way oxidation catalyst on the upstream side in the exhaust gas flow direction, and has a NOx trap function and an HC trap function on the downstream side. An exhaust gas purification system comprising:
The ternary oxidation catalyst is formed by forming a catalyst coat layer containing a ternary oxidation catalyst on a cell wall of a monolith support, and the cell wall thickness and the thinnest part of the catalyst coat layer thickness formed on the cell wall The exhaust gas purification system is characterized in that the total thickness in is 50 to 650 μm.   The exhaust gas purification system according to claim 1, wherein 50% or more of the total thickness is a thickness of the catalyst coat layer.   The exhaust gas purification system according to claim 1 or 2, wherein the three-way oxidation catalyst is a three-way catalyst.   The exhaust gas purification system according to any one of claims 1 to 3, wherein the monolith carrier is made of ceramics and has a polygonal cell shape, and the thinnest part is a flat part. .   The exhaust gas purifying catalyst is formed by forming a catalyst coat layer containing an exhaust gas purifying catalyst on the cell wall of the monolith support, and the cell wall thickness and the thickness of the catalyst coat layer formed on the cell wall are the maximum. The exhaust gas purification system according to any one of claims 1 to 4, wherein a total thickness in the thin portion is thinner than a total thickness in the thinnest portion of the three-way oxidation catalyst.   A three-way oxidation catalyst used in the exhaust gas purification system according to any one of claims 1 to 5, wherein a catalyst coat layer containing a catalyst containing a noble metal is formed on a cell wall of a monolith support. And the total thickness at the thinnest part of the cell wall thickness and the thickness of the catalyst coat layer formed on the cell wall is 50 to 650 μm.

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