JPH10280950A - Diesel exhaust emission controlling catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the outer peripheral of straight-flow type catalyst from suffering from SOF poison and prevent sulfate from being generated at high temperature. SOLUTION: By changing carrier amount of noble metal, average particle diameter of the noble metal, or specific surface of a coat layer in relation to central part and outer peripheral part, oxidation force in the outer peripheral part is higher than that in the central part. Because high oxidation force oxidizes and cleans SOF even when much SOF contacts with the outer peripheral part, a catalyst is prevented from suffering from SOF poison. In addition, because highest temperature exhaust gas flows in the central part where oxidation force is lowest, sulfate is prevented from being generated.

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 for purifying HC, CO and SOF (Soluble Organic Fraction) contained in exhaust gas discharged from a diesel engine (hereinafter referred to as DE). The catalyst of the present invention can particularly efficiently purify SOF.

[0002]

2. Description of the Related Art With respect to gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of responding to the strict regulations. However, regarding DE, harmful components are mainly particulates (PM)
Due to the unique circumstances of being discharged as gas, regulation and development of technology are behind in comparison with gasoline engines, and the development of an exhaust gas purifying catalyst that can reliably purify harmful components is desired.

[0003] As a DE exhaust gas purifying apparatus which has been developed up to the present time, a straight flow type SOF can be roughly classified.
A decomposition catalyst and a method using a trap (without catalyst and with catalyst) are known. Among them, the method using a trap captures PM and regulates emission, and is particularly effective for exhaust gas with a high dry suit ratio. For example, a PM filter is used which has a large number of honeycomb cells arranged in parallel in the axial direction and whose inlet-side openings and outlet-side openings are alternately closed in a checkered pattern. However, in this PM filter, PM is trapped in the honeycomb cells and the cell walls and gradually becomes clogged. Therefore, it is necessary to periodically burn and remove the trapped PM.

On the other hand, as a straight-flow type SOF cracking catalyst, for example, as shown in Japanese Patent Application Laid-Open No. 3-38255, a coated layer is formed from activated alumina or the like on a honeycomb-shaped substrate, and a gasoline engine and a gasoline engine are formed on the coated layer. Similarly, a catalyst supporting a noble metal such as a platinum group metal is known. This straight-flow type SOF decomposition catalyst has a CO
It oxidizes and decomposes SOF in PM together with HC and HC to purify it.

[0005] This straight-flow type SOF cracking catalyst has a drawback that the removal rate of dry suit is low.
Since the amount of dry suit can be reduced by improving the DE and fuel itself, there is a great advantage that a reprocessing unit is not required and pressure loss is small, so further improvement of technology is expected in the future. I have.

[0006]

By the way, in the straight-flow type SOF decomposition catalyst, SOF in exhaust gas is first adsorbed on the coat layer, and then oxidized and purified by the action of the noble metal carried on the coat layer, and H ₂ O and CO ₂ are removed. And is discharged. However, the flow velocity of the exhaust gas in the exhaust gas channel has a distribution such that it is largest at the center and smaller near the pipe wall of the channel. Therefore, a similar flow velocity distribution occurs even in a straight flow type catalyst, and the flow velocity is higher at the center of the catalyst and lower at the outer periphery. Therefore, SOF in the exhaust gas is more likely to be deposited on the catalyst at the outer peripheral portion, and is less likely to be deposited at the central portion.

[0007] Then, an amount of SOF exceeding the oxidizing ability of the noble metal may be deposited on the outer peripheral portion, and a problem may occur in which the excess SOF covers the noble metal and loses its activity. Such a phenomenon is called SOF poisoning. Such a defect can be avoided by increasing the oxidizing power by increasing the amount of the noble metal carried.
However, if the oxidizing power is increased, there is a problem that SO2 in the exhaust gas is oxidized at high temperatures to generate sulfate, and the PM emission amount is increased on the contrary.

The present invention has been made in view of such circumstances, and it is an object of the present invention to suppress the SOF poisoning at the outer peripheral portion of a straight flow type catalyst and to suppress the formation of sulfate at a high temperature. .

[0009]

According to a first aspect of the present invention, there is provided a catalyst for purifying diesel exhaust gas which comprises a carrier and a noble metal carried on the carrier, and is mounted on an exhaust system of a diesel engine. It is a flow type exhaust gas purifying catalyst, in which the oxidizing power of the outer peripheral portion is higher than that of the central portion.

A feature of the catalyst for purifying diesel exhaust gas according to claim 2 is that, in the catalyst for purifying diesel exhaust gas according to claim 1, more noble metal is carried on an outer peripheral portion than on a central portion. A feature of the catalyst for purifying diesel exhaust gas according to claim 3 is that, in the catalyst for purifying diesel exhaust gas according to claim 1, the average particle size of the noble metal is smaller at the outer periphery than at the center.

[0011] Further, a feature of the catalyst for purifying diesel exhaust gas according to claim 4 is that, in the catalyst for purifying diesel exhaust gas according to claim 1, the carrier is coated on a straight flow type substrate to form a coating layer, The specific surface area of the coat layer is larger at the outer periphery than at the center.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The catalyst according to the first aspect of the present invention is configured so that the oxidizing power of the outer peripheral portion is higher than that of the central portion. Therefore, even if a large amount of SOF comes into contact with the outer peripheral portion, the SOF is oxidized and purified by a high oxidizing power, so that SOF poisoning is suppressed. Further, the exhaust gas has a temperature distribution in the exhaust flow path, and the temperature at the portion in contact with the flow path wall is lowest and the temperature at the center is highest. In a straight-flow type catalyst, the temperature gradient of the exhaust gas in the catalyst is further increased because the outer peripheral portion is cooled from the outside.

That is, in the catalyst according to the first aspect, since the hottest exhaust gas flows through the central portion having the lowest oxidizing power, the production of sulfate is suppressed. In order to increase the oxidizing power at the outer peripheral portion than at the central portion in this way, various configurations are conceivable. However, in practice, for example, as in the catalyst according to claim 2, the amount of the noble metal carried is set higher than the central portion. More to the department. As a result, the oxidizing power of the outer peripheral portion becomes higher than that of the central portion, and even if more SOF is deposited on the outer peripheral portion, the SOF poisoning is suppressed because the SOF is oxidized and purified by the high oxidizing power.

The effect can be obtained if the amount of the noble metal carried on the outer peripheral portion is slightly larger than that of the central portion.
It is desirable to make it about 400 times. If it is less than twice, almost no effect can be obtained, and if it exceeds 400 times, the effect is saturated and the cost rises. The amount of the noble metal carried may be different between the central portion and the outer peripheral portion in two stages, or may be different in multiple stages. Further, the carrying amount can be continuously increased from the central portion toward the outer peripheral portion.

Further, even when the outer diameter of the noble metal is smaller than that of the center, the surface area of the noble metal in the center is increased. The oxidizing power of the outer periphery increases, and SO
Even if a large amount of F is deposited, SOF is oxidized and purified by a high oxidizing power, so that SOF poisoning is suppressed. The effect can be obtained if the average particle size of the noble metal in the outer peripheral portion is slightly smaller than that in the central portion. However, it is preferable that the average particle size is approximately 1/10 to 1/2 of the central portion. If the average particle size of the noble metal in the outer peripheral portion is 1/2 or more of the average particle size of the noble metal in the central portion, almost no effect can be obtained, and if it is 1/10 or less, it tends to aggregate during use.

The average particle size of the supported noble metal may be different between the central portion and the outer peripheral portion in two stages or in multiple stages. Further, the average particle diameter can be continuously reduced from the center to the outer periphery.
Furthermore, as in the catalyst according to the fourth aspect, the specific surface area of the coat layer may be larger at the outer peripheral portion than at the central portion. With this configuration, the catalytic reaction in the outer peripheral portion becomes active, and the oxidizing power of the outer peripheral portion is higher than that of the central portion. Even if a large amount of SOF is deposited on the outer peripheral portion, the SOF is oxidized and purified by the high oxidizing power. Therefore, SOF poisoning is suppressed.

The effect can be obtained if the specific surface area of the outer peripheral coat layer is slightly larger than that of the central part. However, it is desirable that the specific surface area is about 2 to 20 times the specific surface area of the central coat layer.
If it is less than 2 times, almost no effect can be obtained, and if it exceeds 20 times, the noble metal becomes highly dispersed and easily aggregates during use. 5. The catalyst according to claim 1, wherein the noble metal is P.
t, Rh, Pd, Ir and the like are used. Among them, Pt is particularly preferred because of its high oxidizing activity. The amount of the noble metal carried varies depending on the type of the noble metal, but is about 0.05 to 10 g for Pt, about 0.05 to 1 g for Rh, and about Pd for 1 liter of catalyst volume per liter of catalyst. 0.5-10
About g is standard.

As the carrier, those having high heat resistance and high specific surface area are desirable, and alumina, silica, zirconia, titania, silica-alumina, zeolite and the like are used. A straight flow type substrate may be formed from this carrier, or a coat layer composed of the carrier powder may be formed on the surface of a substrate formed of cordierite, metal, or the like.

In order to produce the catalyst according to claim 2 in which the amount of the noble metal carried is larger in the outer periphery than in the center, the carrier powder is made to carry the noble metal in a different amount and the outer periphery is coated. There is a method in which the layer is formed of a powder having a large amount of support and the coat layer at the center is formed of a powder having a small amount of support. Alternatively, a solution having a high concentration of the noble metal salt may be brought into contact with the outer peripheral portion, and a solution having a low concentration of the noble metal salt may be brought into contact with the central portion to carry the solution.

In order for the average particle diameter of the noble metal to be smaller at the outer peripheral part than at the central part, the noble metal is supported on a carrier powder and the powder coated on the central part is fired at a high temperature. For example, a method of sintering the noble metal carried thereon to increase the average particle size, a method of making the carrying method different between the outer peripheral portion and the central portion, or the like can be used.

Further, in order to form the coat layer so that the specific surface area is larger at the outer peripheral portion than at the central portion, the outer peripheral coat layer is formed of a carrier powder having a large specific surface area, A method of forming a central coat layer with a carrier powder having a small specific surface area is exemplified. In order to reduce the specific surface area of the carrier powder, the carrier powder may be fired at a high temperature.

[0022]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) FIG. 1 shows a catalyst of this example. This catalyst comprises a honeycomb-shaped carrier substrate 1 having a diameter of 100 mm, coat layers 20 and 21 formed on the surface of the carrier substrate 1, and Pt 3 supported on the coat layers 20 and 21. P supported on the coat layer 20 of the central part 10 having a diameter of 80 mm from the center
From t3, the coat layer 2 on the outer peripheral portion 11 outside the central portion 10
The amount of Pt3 supported on No. 1 is larger than that on Pt3. Hereinafter, a method for producing the catalyst will be described, and the detailed description of the configuration will be substituted.

200 g of silica powder (average particle size: 15 μm) was immersed in 2000 ml of a tetraammine platinum hydroxide solution containing 0.4 g of Pt and stirred for 1 hour. After stirring, 120
The mixture was evaporated to dryness in a drying oven at a temperature of 250 ° C., and further subjected to a heat treatment at 250 ° C. for 1 hour to prepare a carrier powder for the center. Then, the whole amount of the carrier powder for the center, 200 g of alumina sol having an alumina concentration of 10% by weight, and 200 g of water were mixed for 6 hours by a ball mill to prepare a slurry for the center.

On the other hand, 40 g of silica powder (average particle size: 15 μm) was immersed in 400 ml of a tetraammine platinum hydroxide solution containing 0.6 g of Pt, and stirred for 1 hour. After stirring, 12
The residue was evaporated to dryness in a drying oven at 0 ° C., and further heat-treated at 250 ° C. for 1 hour to prepare a carrier powder for the outer peripheral portion. Then, the entire amount of the outer peripheral portion carrier powder, 40 g of alumina sol having an alumina concentration of 10% by weight, and 40 g of water were mixed by a ball mill for 6 hours to prepare a slurry for the outer peripheral portion.

Next, a cordierite honeycomb-shaped carrier substrate 1 having a volume of 1.3 liters is prepared, and as shown in FIG. 2, an outer peripheral portion of the carrier substrate 1 is formed using an outer peripheral sealing material 4 made of Teflon. 11 were sealed in a ring shape at both end surfaces. As a result, only the central portion 10 of the honeycomb passage of the carrier substrate 1 is exposed. Thereafter, the carrier substrate 1 sealed with the outer peripheral sealing material 4 is immersed in the slurry for the center part, pulled up and blow off excess slurry, and then dried at 120 ° C. for 6 hours and dried.
Baking for 1 hour at 0 ° C.
Was formed. The coat layer 20 is formed in an amount of 100 g per liter of the volume of the carrier substrate 1, and Pt3 is supported in an amount of 0.2 g per liter of the volume of the carrier substrate 1.

Next, the outer peripheral sealing material 4 was peeled off, and both end surfaces of the central portion 10 were sealed in a perfect circle with a central sealing material 5 made of Teflon as shown in FIG. Thereby, the coat layer 20
The honeycomb passage of the central portion 10 where the is formed is not exposed, and only the honeycomb passage of the outer peripheral portion 11 is exposed. Thereafter, the carrier base material 1 sealed with the center sealing material 5 is immersed in the slurry for the outer peripheral portion, pulled up and blow off excess slurry, dried at 120 ° C. for 6 hours, and baked at 500 ° C. for 1 hour,
The coat layer 21 was formed only on the outer peripheral portion 11. Coat layer 2
1 is formed at 100 g per liter of the volume of the carrier substrate 1, and Pt 3 is formed at 1.5 g per liter of the volume of the carrier substrate 1.
g is carried.

(Comparative Example 1) 200 g of dinitrodiammine platinum nitrate aqueous solution containing 1.7 g of Pt
Was immersed and stirred for 1 hour. After stirring, the mixture was evaporated to dryness in a drying oven at 120 ° C., and further dried for 2 hours.
Heat treatment was performed at 50 ° C. for 1 hour to prepare a carrier powder.
The total amount of the carrier powder, 200 g of alumina sol having an alumina concentration of 10% by weight, and 200 g of water were mixed by a ball mill for 6 hours to prepare a slurry.

Next, the outer peripheral sealing material 4 and the central sealing material 5
Without using, the same carrier substrate 1 as in Example 1 is immersed in the slurry, pulled up and blow off excess slurry, dried at 120 ° C. for 6 hours, and baked at 500 ° C. for 1 hour to form a coat layer. Formed. The coat layer is formed in an amount of 100 g per 1 liter of the volume of the carrier substrate 1, and 0.85 g of Pt is uniformly carried on the entire carrier substrate 1 per 1 liter of the volume.

(Comparative Example 2) The entire carrier substrate 1 similar to that of Example 1 was immersed in the same slurry for the central portion as in Example 1 without using the outer peripheral sealing material 4 and the central sealing material 5, and was pulled up to obtain an excess. The slurry was blown off, dried at 120 ° C. for 6 hours, and fired at 500 ° C. for 1 hour to form a coat layer. The coat layer is formed in a volume of 100 g per liter of the carrier substrate 1, and Pt is uniformly supported on the entire carrier substrate 1 in an amount of 0.2 g per liter in volume.

(Comparative Example 3) The same carrier substrate 1 as in Example 1 was immersed in the same slurry for the outer peripheral portion as in Example 1 without using the outer peripheral sealing material 4 and the center sealing material 5, and was lifted up to remove excess. The slurry was blown off, dried at 120 ° C. for 6 hours, and fired at 500 ° C. for 1 hour to form a coat layer. The coat layer is formed in an amount of 100 g per 1 liter of the volume of the carrier substrate 1, and 1.5 g of Pt is uniformly supported on the entire carrier substrate 1 per 1 liter of the volume.

(Comparative Example 4) Carrier substrate 1 with outer peripheral sealing material 4
After sealing both end surfaces of the outer peripheral portion 11, it is immersed in the slurry for the outer peripheral portion to form a coat layer 20 on the central portion 10, and then, after sealing both end surfaces of the central portion 10 with the central sealing material 5, The coating layer 21 is immersed in the slurry and
Comparative Example 4 in the same manner as in Example 1 except that
Was prepared.

The supported amount of Pt in the catalyst of Comparative Example 4 was 1.5 g per liter of the volume of the carrier substrate 1 at the center 10 and 0.2 g per liter of the volume of the carrier substrate 1 at the outer periphery 11. Thus, the configuration is exactly opposite to that of the catalyst of the first embodiment. (Test / Evaluation) Each of the above catalysts was used for 2.
It was attached to the exhaust system of a 6 L diesel engine and bench evaluated. The conditions were an engine rotation speed of 2000 rpm, and the PM purification rates were measured at two levels of an incoming gas temperature of 200 ° C. and 400 ° C. The catalysts of Example 1 and Comparative Example 3 were further subjected to a durability test in which the catalyst was operated at 650 ° C. for 100 hours on the engine bench, and then the PM purification rate was measured in the same manner as above. Table 1 shows the results.

[0033]

[Table 1] In the catalyst of Comparative Example 3, since Pt was supported at a high loading amount on the whole, the amount of sulfate generated at a high temperature was increased, and the PM purification rate was significantly negative. In addition, the degree of reduction in the purification rate after durability is large. This is Pt
This is due to the fact that the Pt particles aggregated during the durability test because the average particle size of the Pt particles was as fine as 50 °.

In the catalyst of Comparative Example 2, the amount of sulfate produced at high temperatures is small, but the PM purification rate at low temperatures is low, because Pt is carried at a low amount throughout. The catalyst of Example 1 and the catalysts of Comparative Example 1 and Comparative Example 4 have the same Pt carrying amount. However, the catalyst of Example 1 has the best PM purification rate at low temperature, and the production of sulfate is suppressed. I have. This is the P
The amount of t carried is larger than that of the central portion 10 and the P
It is clear that this is the effect of reducing the amount of t carried.

Further, the catalyst of Example 1 has a smaller degree of reduction in the PM purification rate after durability than the catalyst of Comparative Example 3, and is excellent in durability. (Embodiment 2) FIG. 4 shows a catalyst of this embodiment. This catalyst comprises a honeycomb-shaped carrier substrate 1 having a diameter of 100 mm, coat layers 20 and 21 formed on the surface of the carrier substrate 1, and Pt 30 and 31 carried on the coat layers 20 and 21. And the coat layer 20 at the center 10 having a diameter of 80 mm from the center.
The average particle diameter of Pt31 carried on the coat layer 21 of the outer peripheral portion 11 outside the central portion 10 is smaller than that of Pt30 carried on the central portion 10. Hereinafter, a method for producing the catalyst will be described, and the detailed description of the configuration will be substituted.

75 g of silica powder (average particle size: 15 μm) was immersed in 750 ml of tetraammineplatinum nitrate aqueous solution containing 1.1 g of Pt, and stirred for 1 hour. After stirring, 120 ° C
And then heat-treated at 250 ° C. for 1 hour to prepare an outer peripheral carrier powder. Carried P
The average particle size of t is 50 °. The total amount of the carrier powder for the outer peripheral portion and 75 g of alumina sol having an alumina concentration of 10% by weight
And 75 g of water in a ball mill for 6 hours to prepare an outer peripheral slurry.

On the other hand, a carrier powder for the outer peripheral portion is prepared in the same manner as described above, and baked in air at 800 ° C. for 3 hours to sinter the supported Pt to have an average particle size of 200 μm.
And This carrier powder was used as the center carrier powder, and slurried in the same manner as above to prepare a center slurry. Using the obtained slurry for the outer peripheral portion and the slurry for the central portion, coat layers 20 and 21 were formed in the same manner as in Example 1 to prepare a catalyst of Example 2. Pt is uniformly supported on the entire coating layer, and the amount of Pt is 1.5 g per liter of the volume of the carrier substrate. The average particle size of Pt in the central portion 10 is 200 °, and the average particle size of Pt in the outer peripheral portion 11 is 50 °.

Comparative Example 5 Coating layers 20 and 21 were formed on the entire carrier substrate in the same manner as in Comparative Example 1 except that only the slurry for the center prepared in Example 2 was used. Pt is uniformly supported on the entire coating layer, and the supported amount is the volume 1 of the carrier substrate.
1.5 g per liter, average particle size 200 °.

Comparative Example 6 The catalyst of Comparative Example 3 was used as Comparative Example 6 as it was. In the catalyst of Comparative Example 6, Pt was uniformly loaded on the whole at a loading amount of 1.5 g / l, and the average particle size was 50 °. Comparative Example 7 Comparative Example 7 was performed in the same manner as in Example 2 except that the coat layer 20 was formed on the central portion 10 with the slurry for the outer peripheral portion, and the coat layer 21 was formed on the outer peripheral portion 11 with the slurry for the central portion. Was prepared.

Pt is uniformly supported on the entire coating layer, and the amount of Pt is 1.5 g per liter of the volume of the carrier substrate. The average particle size of Pt in the central portion 10 was 50 ° and the average particle size of Pt in the outer peripheral portion 11 was 200 °.
It has the exact opposite configuration. (Test / Evaluation) The PM purification rate of each of the catalysts was measured in the same manner as in Example 1, and the results are shown in Table 2.

[0041]

[Table 2] In the catalyst of Comparative Example 6, the activity was strong because the particle size of Pt was small as a whole, the amount of sulfate produced at high temperatures was large, and the PM purification rate was significantly negative.

In the catalyst of Comparative Example 5, the activity was low because the particle size of Pt was large as a whole, and although the amount of sulfate produced at high temperatures was small, the purification rate at low temperatures was also low. From the comparison between Example 2 and Comparative Example 7, the catalyst of Example 2 has a higher purification rate at low temperatures, which is due to the effect of making the average particle diameter of Pt in the outer peripheral portion 11 smaller than that in the central portion 10. It is clear that there is.

Example 3 FIG. 5 shows a catalyst of this example. This catalyst comprises a honeycomb-shaped carrier substrate 1 having a diameter of 100 mm, and coat layers 20 and 2 formed on the surface of the carrier substrate 1.
1 and Pt carried on the coat layers 20 and 21. The specific surface area of the coat layer 20 in the central portion 10 having a diameter of 80 mm from the center is smaller than the specific surface area of the coat layer 21 in the outer peripheral portion 11 outside the central portion 10. Less than,
The method for producing this catalyst will be described, and the detailed description of the structure will be used instead.

75 g of silica powder (average particle size: 15 μm, specific surface area: 200 m ² / g) was immersed in 750 ml of an aqueous solution of dinitrodiammine platinum nitrate containing 1.1 g of Pt, and stirred for 1 hour. After stirring, evaporate to dryness in a drying oven at 120 ° C,
Further, heat treatment was performed at 250 ° C. for 1 hour to prepare an outer peripheral carrier powder. The average particle size of the supported Pt is 50 °.

Then, the whole amount of the carrier powder for the outer periphery, 75 g of alumina sol having an alumina concentration of 10% by weight, and 75 g of water were mixed in a ball mill for 6 hours to prepare a slurry for the outer periphery. On the other hand, the above silica powder was calcined at 950 ° C. for 3 hours in the atmosphere. The specific surface area after firing is as small as 20 m ² / g. Using the calcined silica powder, Pt was carried in the same manner as above to prepare a carrier powder for the center. Then, the whole amount of the carrier powder for the center, 130 g of alumina sol having an alumina concentration of 10% by weight, and 130 g of water were mixed by a ball mill for 6 hours to prepare a slurry for the center.

The obtained slurry for the outer peripheral portion and the slurry for the central portion were obtained.
And coat layers 20 and 21 in the same manner as in Example 1.
Was formed to prepare the catalyst of Example 3. Pt is the coat layer
It is uniformly supported on the whole, and the supported amount is the volume 1 of the carrier base material.
1.5 g per liter. The average particle size of Pt is
100 °. However, the specific surface area of the coat layer is different,
The specific surface area of the coat layer 20 in the central part 10 is 20 m^Two/ G,
The specific surface area of the coat layer 21 on the outer peripheral portion 11 is 200 m^Two/ G
(Comparative Example 8) The center slurry prepared in Example 3 was used.
Using, in the same manner as in Comparative Example 1, a coat layer was formed on the entire carrier substrate.
Formed. The specific surface area of this coat layer is uniform and 2
0m ^Two/ G.

Comparative Example 9 The catalyst of Comparative Example 3 was used as Comparative Example 9 as it was. In the catalyst of Comparative Example 9, the specific surface area of the coat layer was uniform and 200 m ² / g. (Comparative Example 10) Comparative Example 10 was performed in the same manner as in Example 3 except that the coat layer 20 was formed on the central portion 10 with the slurry for the outer peripheral portion, and the coat layer 21 was formed on the outer peripheral portion 11 with the slurry for the central portion. Was prepared.

In the catalyst of Comparative Example 10, the specific surface area of the coat layer was 200 m ² / g at the central portion 20 and ² m ² / g at the outer peripheral portion 21.
0 m ² / g, which is the exact opposite configuration of the third embodiment. (Test / Evaluation) The PM purification rate of each of the above catalysts was measured in the same manner as in Example 1, and the results are shown in Table 3.

[0049]

[Table 3] In the catalyst of Comparative Example 8, the activity was low because the specific surface area of the coat layer was small as a whole, and the amount of sulfate generated at high temperatures was small, but the PM purification rate at low temperatures was also low.

On the other hand, in the catalyst of Comparative Example 9, the activity was strong because the specific surface area of the coat layer was large as a whole, the amount of sulfate generated at high temperatures was large, and the PM purification rate was significantly negative. From the comparison between Example 3 and Comparative Example 10, the catalyst of Example 3 has a higher purification rate at low temperature, which is the effect of increasing the specific surface area of the coat layer at the outer periphery from the center. Is evident.

[0051]

According to the catalyst for purifying diesel exhaust gas of the present invention, SOF poisoning is suppressed, so that the SOF purifying performance at low temperatures is excellent, and a high SOF purifying rate can be secured.
In addition, since the production of sulfate at a high temperature is also suppressed, the emission of PM can be suppressed in a wide temperature range.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of a catalyst according to one embodiment of the present invention.

FIG. 2 is an explanatory view showing a state in which an outer peripheral portion is masked in the method for producing a catalyst according to one embodiment of the present invention.

FIG. 3 is an explanatory view showing a state in which a central portion is masked in the method for producing a catalyst according to one embodiment of the present invention.

FIG. 4 is an explanatory diagram of a configuration of a catalyst according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of a configuration of a catalyst according to a third embodiment of the present invention.

[Explanation of symbols]

1: support base material 3: Pt 10:
Central part 11: Outer part 20, 21: Coat layer

Claims (4)

[Claims] 1. A straight-flow type exhaust gas purifying catalyst comprising a carrier and a noble metal carried on the carrier and mounted on an exhaust system of a diesel engine, wherein an oxidizing power of an outer peripheral portion is higher than that of a central portion. A catalyst for purifying diesel exhaust gas, characterized in that: 2. The catalyst for purifying diesel exhaust gas according to claim 1, wherein the noble metal is carried more in the outer peripheral part than in the central part. 3. The catalyst for purifying diesel exhaust gas according to claim 1, wherein the average particle size of the noble metal is smaller at the outer peripheral portion than at the central portion. 4. The carrier according to claim 1, wherein the carrier is coated on a straight-flow type substrate to form a coat layer, and the specific surface area of the coat layer is larger at the outer periphery than at the center. The catalyst for purifying diesel exhaust gas as described in the above.