JPH09103679A - Exhaust gas-purifying catalyst for diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress generation of SO3 and sulfate while oxidizing and decomposing performances of SOF, HC, etc., within a low temperature range are kept at least equivalent to conventional ones. SOLUTION: A catalytic noble metal is supported containing fine particles 3 of at most 5nm mean particle size on an upper stream side of an exhaust gas flow. On a down stream side the catalytic noble metal is supported as a coarse particle 4 of at least 10nm mean particle size, and SO2 is further difficult to be oxidized than HC. Therefore, HC is selectively oxidized on the upper stream side, and oxidation of SO2 is delayed on the down stream side, and so, generation of SO3 and sulfate are suppressed.

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 a diesel engine, and more specifically, carbon monoxide (CO), hydrocarbon (HC) and soluble organic compounds which are harmful components contained in the exhaust gas from a diesel engine. Ingredient (S
The present invention relates to an exhaust gas purification catalyst that purifies OF and reduces the emission amount of sulfate (sulfate).

[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 coping with the regulations. However, for diesel engines, the harmful components are PM
(Particulate materials such as carbon-based particles, sulfur-based particles such as sulfate, and high-molecular-weight hydrocarbon particles)
Due to the peculiar circumstances of being discharged as gas, regulation and technological progress are behind the gasoline engine, and the development of an exhaust gas purifying device that can surely purify exhaust gas is desired.

[0003] As exhaust gas purifying devices for diesel engines that have been developed to date, trap type exhaust gas purifying devices using trap type exhaust gas purifying catalysts and open type exhaust gas purifying devices using open type exhaust gas purifying catalysts have been roughly classified. A purifying device is known. As a trap-type exhaust gas purifying catalyst, a ceramic plugged honeycomb body (diesel particulate filter (DP
F)) and the like are known. In an exhaust gas purifying apparatus using the exhaust gas purifying catalyst, the exhaust gas is filtered by a DPF or the like and the PM is filtered.
Is collected, and if the pressure loss increases, the PM accumulated by a burner or the like is burned to regenerate the DPF or the like. In addition to collecting PM, CO, HC and SOF
In order to oxidize and decompose the exhaust gas, an exhaust gas purifying catalyst in which a catalyst support layer is formed of alumina or the like on a carrier base material such as DPF and platinum (Pt) or the like is supported on the catalyst support layer is also under study.

On the other hand, as an open type exhaust gas purifying catalyst, a carrier base material made of a ceramic straight flow type honeycomb body or the like, a catalyst support layer formed of alumina or the like on the carrier base material, and the catalyst support It is known that the layer is composed of Pt and the like supported in the same manner as a gasoline engine. According to this open type exhaust gas purifying apparatus, CO and the like can be oxidized and decomposed by the catalytic action of Pt and the like.

However, in the above-mentioned trap type or open type exhaust gas purifying device having platinum or the like, the catalyst supporting layer adsorbs SO ₂ in the exhaust gas, and at high temperature SO ₂ is oxidized by the catalytic action of Pt or the like to be converted into SO _3. It will be discharged. Particularly, in a diesel engine, oxygen gas is sufficiently present in the exhaust gas, and SO ₂ is easily oxidized by this oxygen gas and is easily discharged as SO ₃ . And SO ₂ is P
Not measured as M, but SO ₃ is measured as PM. Further, SO ₃ easily reacts with a large amount of water vapor present in the exhaust gas to form a sulfuric acid mist and is discharged as a sulfate. Therefore, these exhaust gas purifying devices have a problem that the amount of PM increases due to the discharge of sulfate at a high temperature.

[0006] Under such circumstances, Japanese Patent Application Laid-Open No. 62-56783.
In Japanese Patent Laid-Open Publication No. 700-1000, after supporting Pt on alumina,
A method for producing a catalyst for removing fine particles in diesel exhaust, in which Pt is sintered by heat treatment at ℃, thereby reducing the oxidizing action of SO ₂ is disclosed.

[0007]

SUMMARY OF THE INVENTION However, Japanese Unexamined Patent Publication No.
The catalyst disclosed in JP-A-2-56783 has a large effect of suppressing the formation of sulfate, but has a problem that the effect of oxidizing and purifying SOF and HC is low in a low temperature range because the oxidation activity of Pt decreases. The present invention has been made in view of the above circumstances, and S in a low temperature range is
The object is to maintain the oxidation / decomposition performance of OF, HC and the like at a level equal to or higher than that of the conventional one, and to suppress the production of SO ₃ and sulfate. Since SOF is a relatively high molecular weight hydrocarbon, hereinafter, low molecular weight HC and SOF are collectively referred to as HC.

[0008]

The features of the exhaust gas purifying catalyst for a diesel engine of the present invention for solving the above-mentioned problems are characterized by comprising a porous carrier base material and a catalytic noble metal supported on the carrier base material. The noble metal is carried on the upstream side of the exhaust gas flow while containing fine particles having an average particle size of 5 nm or less, and on the downstream side as coarse particles having an average particle size of 10 nm or more.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION A porous carrier substrate is alumina,
It can be formed of a refractory inorganic oxide such as silica, titania, zeolite, silica-alumina and titania-alumina. The refractory inorganic oxide has an average particle size of 20.
It is preferably μm or less and a specific surface area of 10 m ² / g or more. If the refractory inorganic oxide has an average particle size of more than 20 μm and a specific surface area of less than 10 m ² / g, sufficient HC purification performance may not be obtained. A honeycomb-shaped or pellet-shaped carrier substrate may be formed from these refractory inorganic oxides, or a carrier substrate formed by coating the above refractory inorganic oxide on a substrate formed of cordierite or metal. It can also be made of wood.

Typical catalyst noble metals include Pt, palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os) and iridium (Ir).
At least one of the above can be adopted. For example, P
The carried amount of t is 0.0 per unit volume of the exhaust gas purifying catalyst.
It is preferably from 1 to 10.0 g / L. If the amount of Pt supported is less than 0.01 g / L, sufficient oxidation / decomposition performance may not be obtained. On the contrary, even if Pt is supported in an amount of more than 10.0 g / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purifying catalyst becomes expensive. Particularly, when the supported amount of Pt is 0.1 to 3.0 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

The amount of Pd supported is preferably 0.01 to 20.0 g / L per unit volume of the exhaust gas purifying catalyst. If the supported amount of Pd is less than 0.01 g / L, there is a possibility that sufficient oxidation / decomposition performance may not be obtained. Conversely, 20.0
Even if Pd is supported in excess of g / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purification catalyst becomes expensive. Especially,
When the supported amount of Pd is 0.5 to 3.0 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

The supported amount of Rh is preferably 0.01 to 1.0 g / L per unit volume of the exhaust gas purifying catalyst. If the supported amount of Rh is less than 0.01 g / L, there is a possibility that sufficient oxidation / decomposition performance may not be obtained. Conversely, 1.0g
Even if Rh is supported in excess of / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purification catalyst becomes expensive. In particular, R
When the supported amount of h is 0.05 to 0.5 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

By the way, comparing the oxidizability of SO ₂ and HC, SO ₂ has a characteristic that it is harder to oxidize than HC. That is, when the catalytic noble metal is brought into contact with the exhaust gas in which SO ₂ and HC coexist, HC is preferentially oxidized.
Therefore, when HC is not present, SO ₂ is oxidized and sulfate is easily generated. This phenomenon becomes remarkable especially when the exhaust gas is heated to a high temperature.

In the catalytic noble metal, the smaller the particle size, the greater the oxidizing activity of both HC and SO ₂ . On the other hand, as the particle size increases, the oxidizing activity of HC slightly decreases at low temperature, but the activity hardly decreases at high temperature, whereas the oxidizing activity of SO ₂ greatly decreases at high temperature. Therefore, the exhaust gas purifying catalyst of the present invention has a structure in which a noble metal having a particle size of 5 nm or less is carried on the upstream side of the exhaust gas flow and a noble metal having a particle size of 10 nm or more is carried on the downstream side, so that at low temperature It is possible to preferentially oxidize and decompose HC and prevent the formation of sulfate at high temperature.

The details of the present invention will be described below. In the exhaust gas-purifying catalyst of the present invention, the noble metal is supported on the upstream side of the exhaust gas flow, including fine particles having a particle size of 5 nm or less.
The smaller the particle size of the catalytic noble metal, the larger the surface area and the higher the oxidation activity. Therefore, by setting the particle size of the catalytic noble metal to 5 nm or less on the upstream side where SO ₂ and HC coexist, HC is preferentially oxidized, High HC purification performance can be obtained. When the fine particles have a particle size of more than 5 nm, the oxidizing activity in the low temperature range is lowered, and the purification performance of HC is lowered.

It is sufficient that fine particles of catalytic noble metal having a particle diameter of 5 nm or less are carried on the upstream side.
It does not prevent the loading of catalytic noble metal particles having a size of not less than nm.
Even if particles having a size of more than 5 nm are carried, no particular problem occurs except for cost. On the other hand, SO _{2 that} has not been oxidized on the upstream side flows to the downstream side as it is. In the exhaust gas purifying catalyst of the present invention, the catalytic noble metal is carried on the downstream side of the exhaust gas stream as coarse particles having a particle size of 10 nm or more. Therefore, on the downstream side, since the oxidizing activity is small even at high temperature, SO ₂ is not oxidized and is discharged as it is.
_It is prevented from becoming ₃ or sulfate. As a result, the emission amount of PM is reduced. Even if there is HC that has not been oxidized on the upstream side, HC is relatively easily oxidized even with a catalytic precious metal that is coarser than SO _2. Therefore, the existing HC is oxidized and purified and unpurified HC is discharged. Is prevented.

If the particle size of the coarse particles of the catalytic noble metal supported on the downstream side is smaller than 10 nm, the oxidation activity is high and S
This is not preferable because O ₂ will be oxidized. The upper limit of the particle size of the coarse particles carried on the downstream side is not particularly limited, but is preferably 60 nm or less. If it exceeds 60 nm, the HC purification performance on the downstream side is also deteriorated, and the HC purification performance is deteriorated as a whole.

In the carrier substrate, the volume ratio of the upstream side, on which fine particles having a particle size of 5 nm or less are carried, to the downstream side, on which coarse particles having a particle size of 10 nm or more are carried, is upstream / downstream = 1 /
It is preferably 4 to 1/1. If the volume on the upstream side is smaller than this, the purification performance of HC is deteriorated, and if the volume on the upstream side is larger than this, SO ₂ is easily oxidized, which is not preferable. The upstream side and the downstream side may be continuous in one carrier base material or may be separated. Further, a tandem catalyst device in which a carrier substrate is separated is used, and a carrier substrate carrying fine particles of 5 nm or less is arranged on the upstream side, and a carrier substrate carrying coarse particles of 10 nm or more is arranged on the downstream side. It can also be used as a purification catalyst. Also in this case, it is desirable to set the volume ratio of the upstream carrier substrate to the downstream carrier substrate within the above range.

[0019]

[Test Example 1]

(Embodiment 1) FIG. 1 and FIG. 2 are schematic configuration diagrams of an exhaust gas purifying catalyst of this embodiment. This exhaust gas purifying catalyst comprises a cordierite honeycomb carrier (1), a coat layer (2) coated on the surface of the honeycomb carrier (1), and an average particle size uniformly supported on the entire honeycomb carrier (1). Coarse Pt (3) having a size of 10 nm or more and fine Pt (4) having an average particle size of 5 nm or less carried by the coat layer (2) in a volume range of 40% from the upstream end surface of the honeycomb carrier (1). Has been done.

The method for producing the exhaust gas purifying catalyst will be described in detail below, and will replace the detailed description of the structure of the exhaust gas purifying catalyst of this embodiment. Cordierite straight flow type honeycomb carrier (1) (400 cells / in ² , diameter 117 mm, length 120 mm, volume 1.
3 liters) is prepared. Next, a slurry containing silica and alumina in a weight ratio of SiO ₂ : Al ₂ O ₃ = 9: 1 was prepared, and the honeycomb carrier (1) was immersed in this slurry and pulled up to blow off excess slurry. After 1
After drying at 00 ° C for 1 hour and firing at 500 ° C for 1 hour, 100 g of the coating layer (2) per liter of the honeycomb carrier (1) was formed.

Next, the honeycomb carrier (1) having the coat layer (2) formed thereon is dipped in a tetraammine hydroxide platinum solution having a predetermined concentration for 1 hour, pulled up, and then excess water droplets are blown off, followed by 100 ° C. for 1 hour. After drying, heat treatment was performed at 300 ° C. for 1 hour to remove ammine and the like. The amount of Pt carried is 1.2 g per liter of the honeycomb carrier (1). After that, heat treatment is performed at 600 ° C. for 10 hours in the atmosphere, and P
The t was sintered to obtain coarse Pt (3). As a result of microscopic observation, the average particle diameter of coarse Pt (3) was 10 nm.

Next, the honeycomb carrier (1) carrying the coarse Pt (3) has a total volume of 40 from the end face on the upstream side of the exhaust gas.
% Of tetraammine hydroxide platinum solution was supported on the volume range. The supporting method was such that the tip of the carrier was slightly dipped in a tetraammine hydroxide platinum solution and dried with a drier to carry all the solution containing 0.39 g of Pt. The amount of fine Pt (4) loaded is 4 on the upstream side.
The volume of 0% is 0.39 g, and the total amount of coarse Pt (3) and fine Pt (4) combined is 1.5 g per liter of the honeycomb carrier (1). The fine Pt particle size at that time was measured and found to be 3 nm. (Example 2) An exhaust gas purifying catalyst of Example 2 was prepared in the same manner as in Example 1 except that the heat treatment temperature for making coarse Pt (3) was 700 ° C. The average particle size of coarse Pt (3) is 22 nm. (Example 3) An exhaust gas purifying catalyst of Example 3 was prepared in the same manner as in Example 1 except that the heat treatment temperature for making coarse Pt (3) was 800 ° C. The average particle size of the coarse Pt (3) is 36 nm. (Example 4) An exhaust gas purifying catalyst of Example 4 was prepared in the same manner as in Example 1 except that the heat treatment temperature for making coarse Pt (3) was 900 ° C. The average particle size of the coarse Pt (3) is 60 nm. (Comparative Example 1) An exhaust gas purifying catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that the heat treatment temperature for making coarse Pt (3) was 500 ° C. The average particle size of the coarse Pt (3) is 3 nm. (Comparative Example 2) An exhaust gas purifying catalyst of Comparative Example 2 was prepared in the same manner as in Example 1 except that the heat treatment temperature for forming coarse Pt (3) was 1000 ° C. The average particle size of the coarse Pt (3) is 94 nm. (Performance test) 2.6 each of the above exhaust gas purifying catalysts was used.
Installed in the exhaust system of L diesel engine, the rotation speed is 200
After performing an aging process of operating at 0 rpm and an inlet gas temperature of 500 ° C. for 1 hour, the inlet gas temperature is lowered by 50 ° C. to analyze PM before and after the catalyst, and H before and after the catalyst is analyzed.
The amount of C was measured. The result shows that PM zero% reduction temperature and HC5
It is converted into 0% purification temperature and shown in FIG.

The PM zero% reduction temperature is a temperature at which the amount of sulfate that increases as the temperature rises becomes the same as the amount of reduction of other PM components such as SOF in the catalyst.
When the reduction amount of SOF or the like is the same, it means that the generation of sulfate is suppressed as the PM zero% reduction temperature becomes higher. The 50% HC purification temperature is the temperature at which 50% of the HC in the exhaust gas before the catalyst is purified,
The lower the 50% purification temperature of HC, the higher the oxidizing power of HC. (Evaluation) From FIG. 3, the average particle size of coarse Pt (3) is 3 nm.
In Comparative Example 1, the PM zero% reduction temperature is low and SO ₂ oxidation cannot be prevented. Further, in Comparative Example 2 in which the coarse Pt (3) has an average particle size of 94 nm, the HC purification activity is reduced. However, in the catalysts of the respective examples, the PM zero% reduction temperature is as high as 350 ° C. or higher, the SO ₂ oxidation is prevented, and the H 2
Efficient HC at low temperature with C50% purification temperature of less than 250 ° C
Is oxidatively purified, and it is clear that this is a function and effect obtained by setting the average particle diameter of the coarse Pt (3) to 10 nm to 60 nm. [Test Example 2] (Comparative Example 3) Pt was loaded on the honeycomb carrier (1) having the coat layer (2) used in Example 1 in the same manner as in Example 1. The loading amount of Pt was set to be 1.5 g per liter of the honeycomb carrier (1).

Then, heat treatment was carried out in the same manner as in Examples 1 to 4 and Comparative Examples 1 and 2 to prepare respective catalysts without supporting fine Pt (4). The coarse Pt (3) particle size of each catalyst was similar to the corresponding catalyst of Test Example 1. For each catalyst, the PM zero% reduction temperature and the HC 50% purification temperature were measured in the same manner as in Test Example 1, and the results are shown in FIG. 4 together with the results of Test Example 1.

From FIG. 4, the respective catalysts of Comparative Example 3 were
Although the PM zero% reduction temperature is higher than the corresponding catalysts of Examples 1 to 4 and Comparative Examples 1 and 2, the HC 50% reduction temperature is considerably high and the HC purification performance is poor. It is clear that this is due to the fact that fine Pt (4) was not supported. (Comparative Example 4) The heat treatment of the carried Pt was not carried out, the carried amount of Pt was set to be 1.5 g per liter of the honeycomb carrier (1), and the fine Pt (4)
A catalyst of Comparative Example 4 was prepared in the same manner as in Example 1 except that the catalyst was not supported. The supported Pt particle size is 0.5
nm is extremely small.

Then, the PM zero% reduction temperature and the HC 50% purification temperature were measured in the same manner as in Test Example 1, and the results are shown in FIG. As shown in FIG. 4, although the catalyst of Comparative Example 4 is extremely excellent in the purification performance of HC, the PM zero% reduction temperature is 300 ° C.
As a result, SO ₂ cannot be prevented from being oxidized. It is clear that this is because fine Pt is supported on the entire surface and coarse Pt (3) is not supported. [Test Example 3] (Example 5) In the same manner as in Example 1, the honeycomb carrier (1) on which the coat layer (2) was formed had Pt in an amount of 1.2 g per liter of the honeycomb carrier (1). So that it was carried. After that, heat treatment was performed in the air at 800 ° C. for 5 hours to sinter Pt to obtain coarse Pt (3). As a result of microscopic observation, the average particle size of coarse Pt (3) was 3
6 nm.

Next, 20% of the volume of the honeycomb carrier (1) carrying the coarse Pt (3) from the end face on the upstream side of the exhaust gas.
The above range was supported by a tetraammine hydroxide platinum solution having a predetermined concentration. The loading method was a method in which the tip of the carrier was slightly dipped in a tetraammine hydroxide platinum solution and dried with a dryer, and all the solutions containing 0.39 g of Pt were loaded. After drying, heat treatment was performed at 300 ° C. for 1 hour to remove ammine and the like, and fine Pt (4) having an average particle size of 0.6 nm was supported. The amount of fine Pt (4) loaded was 0.39 g in the volume portion of the upstream 20%, and the amount of coarse Pt (3) and fine Pt (3)
The total average carrying amount of (4) is 1.5 g per liter of the honeycomb carrier (1). (Example 6) A portion supporting fine Pt (4) is provided on the upstream side 4.
An exhaust gas purification catalyst of Example 6 was prepared in the same manner as in Example 5 except that the volume portion was 0%. (Embodiment 7) The portion supporting the fine Pt (4) is located on the upstream side 5.
An exhaust gas purification catalyst of Example 7 was prepared in the same manner as in Example 5 except that the volume portion was 0%. (Embodiment 8) The portion carrying the fine Pt (4) is placed on the upstream side 6
An exhaust gas purification catalyst of Example 8 was prepared in the same manner as Example 5 except that the volume portion was 0%. (Performance test and evaluation) For the catalysts of Examples 5 to 8 described above, in the same manner as in Test Example 1, the PM zero% reduction temperature and HC5
The 0% purification temperature was measured, and the results are shown in FIG.

From FIG. 5, it can be seen that the PM zero% reduction temperature tends to decrease as the volume of the fine Pt (4) supporting portion increases, and in Example 8, the PM zero% reduction temperature is 35%.
When the temperature is below 0 ° C., SO ₂ is likely to be oxidized.
Therefore, it is understood that it is desirable to stop the portion carrying the fine Pt (4) from the upstream end surface to the portion having a volume of 50%.

In the above embodiment, the amount of fine Pt carried was fixed at 0.39 g. However, the present invention is not limited to this, and the amount carried can be variously changed according to the purpose and effect. It goes without saying that you can do it. Further, in the above-mentioned embodiment, the coarse Pt is also supported on the upstream side together with the fine Pt, but it is known that the same result is obtained even if the coarse Pt on the upstream side is not supported. Further, although only Pt is used as the catalytic noble metal in the above-mentioned examples, the present invention is not limited to this, and it is obvious that the various catalytic noble metals can be used to obtain some effect.

[0030]

[Effects of the Invention] According to the exhaust gas purifying catalyst for a diesel engine of the present invention, it has excellent oxidative purifying activity for HC in a low temperature range, and can prevent SO ₂ from being oxidized in a high temperature range to prevent sulfate emission. . Therefore, it is possible to reduce the emission amount of PM over a wide temperature range while maintaining a high HC purification activity.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a main part of FIG. 1 for explaining the structure of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 3 shows the coarse Pt particle size and PM zero% in Test Example 1.
It is a graph which shows the relationship between reduction temperature and HC50% purification temperature.

FIG. 4 is a graph showing the relationship between the coarse Pt particle size, PM zero% reduction temperature, and HC 50% purification temperature in Test Examples 1 and 2.

FIG. 5 is a graph showing the relationship between the loading volume of fine Pt, the PM zero% reduction temperature, and the HC50% purification temperature in Test Example 3.

[Explanation of symbols]

1: Honeycomb carrier 2: Coat layer 3: Coarse Pt 4: Fine P
t

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Taguchi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Koji Sakano 1 41, Nagakute, Nagakute Town, Aichi County, Aichi Prefecture Toyota Central Research Institute Co., Ltd. (72) Inventor, Kaei Watanabe, Aichi Prefecture, Nagakute Town, Aichi Prefecture 1 41, Yokosuka, Yokouchi Central Research Co., Ltd. (72) Inventor, Koichi Kasahara Chihama, Daito Town, Ogasa County, Shizuoka Prefecture 7800 At Cataler Industry Co., Ltd. (72) Inventor Norihiko Aono Chihama, Daito Town, Ogasa County, Shizuoka Prefecture 7800 At Cataler Industry Co., Ltd.

Claims (1)

[Claims] 1. A porous carrier base material and a catalytic noble metal supported on the carrier base material, wherein the catalytic noble metal has an average particle diameter of 5 n on the upstream side of the exhaust gas flow.
An exhaust gas purification catalyst for a diesel engine, characterized in that it is carried by containing fine particles of m or less, and is carried on the downstream side as coarse particles having an average particle size of 10 nm or more.