JPH09220423A - Diesel exhaust gas purification filter and manufacturing method thereof - Google Patents

Abstract

(57) An object of the present invention is to provide a diesel exhaust gas purification filter having low pressure loss and having a high surface material such as activated alumina carried on the surface of the honeycomb cell side wall and inside the cell side wall. SOLUTION: By alternately plugging both ends of a cell opening portion of a ceramic honeycomb structure, exhaust gas is caused to flow to adjacent cells through pores of the cell side wall of the honeycomb, and particulates contained in the exhaust gas are separated from the cell side wall. In a diesel exhaust gas purification filter having a porous ceramic filter adapted to collect on the surface and inside, the honeycomb structure has a cell wall having a porosity of 40 to 60% and an average pore diameter of 5 to 35 μm. It is coated with a slurry containing activated alumina having a particle size smaller than the average pores.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention removes at least particulates contained in a gas discharged from an internal combustion engine such as a diesel engine to remove hydrocarbons (HC) and carbon monoxide in exhaust gas. The present invention relates to a filter for collecting particulates that is used to purify air.

[0002]

2. Description of the Related Art Particulates discharged from an internal combustion engine such as a diesel engine contain substances harmful to the human body, and it is an environmental issue to remove them. For this reason, conventionally, a method has been used in which particulates are collected by a filter provided in an exhaust system of a diesel engine, and after collecting a certain amount, the particulates are burned and removed by an electric heater or a burner. There is also a method of lowering the combustion temperature of the particulates with a platinum group metal catalyst supported on a filter and burning the collected particulates continuously. In the case of the former method of burning and removing the collected particulates with an electric heater or burner, the maximum filter temperature during combustion rises as the amount of particulates collected increases, and the filter is damaged by the thermal stress applied to the filter. In some cases, it is important to control the collection amount of particulates, and the collection amount has not been completely controlled. In the latter case of combustion with a catalyst, the combustion temperature is relatively low and the thermal stress applied to the filter is small, so that it is excellent in heat resistance.

In the above method, a ceramic honeycomb structure is mainly used for collecting particulates, and cordierite, which exhibits low thermal expansion, is generally used as the material.

The diesel exhaust gas purifying filter to which the present invention is applied has a cell opening on one end of a ceramic monolith having a honeycomb structure, for example, a cell opening on the gas inlet side, which is closed every other cell, and a cell on the gas outlet side. The openings are plugged only in the cells whose openings on the inlet side are not plugged. Therefore, the exhaust gas passes through the pores on the side wall of the cell,
The particulates that flow with the exhaust gas are collected on the surface of the cell wall and inside the pores of the cell wall.

[0005]

In the porous ceramic filter having the honeycomb structure, the both ends of the monolith honeycomb are alternately plugged as described above, so that the gas passes through the pores of several μm to several tens μm of the cell wall. Because of the structure in which they flow into adjacent cells, the collection efficiency of particulates is higher than that of filters of other structures. In order to support the catalyst on this filter, when a high specific surface area material such as activated alumina is coated on the cell side wall surface and inside the pores of the cell side wall as the carrier, the high specific surface area material blocks the pores of the cell side wall. Therefore, there is a problem that the pressure loss becomes higher than that of an uncoated filter. High pressure loss in the filter leads to a reduction in engine output, so the pressure loss should be as low as possible. However, if the porosity and the pore diameter of the filter are too large in order to reduce the pressure loss, there is a possibility that the collection efficiency of particulates may be reduced.

When coating a ceramic carrier having a honeycomb structure with a material having a high specific surface area such as activated alumina, there are some known methods in which the particle diameter of activated alumina is specified. In Japanese Patent Publication No. 55-1818, the particle size of activated alumina is specified to be 0.1 to 100 μm, but it is premised that it is mixed with amorphous alumina such as boehmite. It is said that the adhesive strength will be improved. Further, in Japanese Examined Patent Publication (Kokoku) No. 4-80736, it is effective to ensure a stable dispersibility of the activated alumina slurry by defining the average particle size of the activated alumina to be 20 μm or less. It is said that the adhesive strength will be improved. However, both of them are intended to improve the adhesive strength in order to prevent peeling of the coating layer when a ceramic carrier having a honeycomb structure is coated with activated alumina. And these are flow-through type exhaust gas purifying devices, in which the exhaust gas does not pass through the wall of the honeycomb but passes through the tubular passage formed by the wall, and it cannot be called a filter but the wall Generally, the wall has a low porosity and a small pore diameter in order to increase the strength.

The present invention relates to a porous honeycomb filter having a structure in which a gas flows through pores of a cell wall to adjacent cells by alternately plugging both ends of a porous monolith honeycomb having a honeycomb structure. It is an object of the present invention to uniformly coat not only the surface of the cell side wall but also the inside of the pores of the cell side wall with a high surface material such as, and to avoid the problem of high pressure loss.

[0008]

According to the invention described in claim 1, in a diesel exhaust gas purification filter having a porous ceramic honeycomb structure, the honeycomb structure has a cell side wall with a porosity of 40 to 65%, Average pore size is 5
˜35 μm, and 90 wt% or more of the high specific surface area material supported thereon is smaller than the average pore diameter of the side wall of the honeycomb structure cell, whereby the diesel exhaust gas purification having the coating of the high specific surface material of the present invention is performed. The filter can have low pressure loss.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The diesel exhaust gas purifying filter of the present invention comprises a ceramic monolith honeycomb structure having a cell side wall surface and pores inside the cell side wall coated with a high specific surface area material such as activated alumina, on which a catalytic metal is formed. And then alternately plugging the ends of the cell openings of the resulting honeycomb.

Ceramic monolith hanika used in the present invention
The structure is conventionally known as low coefficient of thermal expansion ceramics.
Cordierite (Chemical composition 2MgO.2Al
_TwoO _Three・ 5SiO_Two) Is preferably made on the side
The porosity of the wall is preferably about 40% to 65%,
More preferably, it is about 45% to 60%, and the average pore size is about
It is preferably 5 μm to 35 μm, more preferably
It is about 10 μm to 30 μm.

On the other hand, the particle size of the high specific surface area material such as activated alumina coated on the honeycomb structure is 9
It is desirable that 0 wt% or more, preferably 95 wt% or more, has a particle size smaller than the average pore size of the honeycomb structure. When the particle size of the high specific surface area material is larger than 10 wt% of the average pore diameter of the honeycomb structure, the high specific surface material does not enter the pores inside the cell sidewall of the honeycomb structure, and the cell sidewall surface The material having a high specific surface area for covering is relatively increased, the thickness of the coating layer is increased, and the pressure loss is increased, which is not preferable. In addition, 9 of high specific surface area material
When the particle size of 0 wt% or more is smaller than the average pore diameter of the honeycomb structure, the amount of the high specific surface area material that enters the pores inside the cell sidewall increases. At this time, air blow or
An increase in pressure loss can be suppressed by sufficiently sucking with a cleaner to remove an excess slurry of the high surface material and by uniformly dispersing and coating the pores without blocking the pores. Pressure loss is the difference between the air on the inlet side and the air on the outlet side when the air flows into the filter from the inlet side of the filter and out of the filter from the outlet side of the filter. Is 2000
Measured under the condition of L / min (linear velocity 1.8 cm / sec), 45
It is desirable to be less than mmAq (column of water).

The diesel exhaust gas purification filter of the present invention has a structure for collecting particulates contained in the exhaust gas of a diesel engine, because the gas flowing from the cell on the inlet side is plugged on the cell on the outlet side. ,
It passes through the side wall of the cell and is discharged from the outlet of the adjacent cell. When passing through the cell wall, only particulates in the exhaust gas are collected. At this time, if the porosity and the average pore diameter of the honeycomb before coating the activated alumina forming the filter are smaller than the above range, the collection efficiency of particulates is improved, but the pressure loss of the filter is increased and the engine output is reduced. Is not preferred. Further, if it is larger than this range, the collection efficiency of particulates is lowered, which is not preferable.

Further, the reason why the particle size of the high specific surface area material such as activated alumina is in the above range is that the high specific surface area material needs to penetrate into the pores of the cell side wall of the honeycomb structure. Is. Conventionally, it was only the surface of the cell side wall that coats the high specific surface area material on the honeycomb type monolithic carrier, but in the case of a honeycomb type filter having a structure in which the exhaust gas passes through the inside of the pores of the cell side wall, it is included in the exhaust gas. Because the particulates that remain remain on the surface of the filter cell sidewalls and inside the pores of the cell sidewalls, it is necessary for the particulates to contact this high surface area material inside the pores in order to be catalyzed. Is. Therefore, the high specific surface area material requires the above particle size. Further, the coating amount (carrying amount) of the high specific surface area material
Is preferably 20 to 75 g / L. Coat amount is 20g / L
If it is less than the above range, the exhaust gas purification capability is low, which is not preferable. Further, when it is larger than 75 g / L, the pressure loss of the filter becomes high and the engine output decreases, which is not preferable.

As the material having a high specific surface area in the present invention, in addition to activated alumina, silica, zirconia, titania, or a material containing two or more of them can be used.

The diesel exhaust gas purification filter of the present invention collects at least particulates contained in the exhaust gas of a diesel engine, and burns and removes them. The coating of the filter with a high specific surface area material such as activated alumina is preferably to provide a support for coating the platinum group catalytic metal. Generally, platinum group catalytic metals are used as catalysts for lowering the combustion temperature of particulates, and are also used as oxidation catalysts for carbon monoxide and hydrocarbons. The filter of the present invention is preferably a filter carrying a metal catalyst composed of at least one platinum group element.

Next, the diesel exhaust gas purification filter of the present invention will be specifically described with reference to FIGS. FIG.
As described above, the porous ceramic filter having the honeycomb structure is alternately plugged with the plugging materials 1 at both ends of the monolith honeycomb to form the coating layer 4 made of the activated alumina particles 3 on the cell side wall 2 of the honeycomb filter. doing. When activated alumina having a particle size smaller than the average pore size of the filter is used as shown in FIG. 2 which is an enlarged view of the portion A of FIG. 1, coating is performed without blocking the inside of the pores 5 on the side wall of the cell. Little rise. However, when activated alumina having a particle size larger than the average pore size of the filter is used as shown in FIG. 3 in which the part A of FIG. 1 is enlarged,
Since the pores of the cell wall are closed, the pressure loss of the filter is significantly increased. In addition, by supporting the platinum group catalytic metal on the coating portion of activated alumina, particulates and other exhaust gas components (HC, C
(O etc.) purification efficiency is improved. Incidentally, the description of the catalyst metal layer is omitted in these drawings.

Diesel exhaust gas containing particulates enters the cell from the cell inlet side 6, passes through the cell wall 2 and exits from the cell outlet side 7. At this time, the particulates are collected on the cell wall surface and inside pores. The platinum group catalytic metal is usually coated again after coating the activated alumina, but it is also possible to coat the platinum group catalytic metal with a solution mixed with the activated alumina.

[0018]

EXAMPLE A filter coated with the above materials can be suitably used as a low pressure loss diesel particulate filter. Below, the Example and the comparative example are shown.

[Example 1] Silica, aluminum hydroxide, and talc were used as main raw materials, and cordierite (2MgO) was used.
.2Al ₂ O ₃ .5SiO ₂ ) composition, and then 20% by weight of carbon for making it porous is added to these main raw materials to produce a ceramic honeycomb structure by a known extrusion method. 1350 ℃ ~ 1450
Calcination at a maximum temperature of 5 ° C., a temperature rising rate of 5 ° C. to 200 ° C., and a holding time of 2 to 20 hours provides pore characteristics with a porosity of 55% and an average pore diameter of 28 μm, and a cell sidewall thickness of 0. 45mm,
140m diameter with 150 cells per square inch
A porous cordierite honeycomb structure having a length of m and a length of 130 mm was obtained.

On the other hand, as a material having a high specific surface area, the central particle size is 5
670 g of activated alumina (manufactured by Sumitomo Chemical Co., Ltd.) having a particle size of 5 μm and having a particle size of more than 28 μm and not more than 28 μm and 330 g of alumina sol (manufactured by Nissan Kagaku) were mixed with 4 liters of water and stirred to prepare an activated alumina slurry.

The above porous cordierite honeycomb structure was completely immersed in the activated alumina slurry (wash coat). Then, the excess slurry was removed as completely as possible with an air cleaner and compressed air. The coating was repeated to prepare five types of honeycombs having different coating amounts. After that, at 120 ℃ 2
It was dried for an hour and calcined at 800 ° C. The coat amount per unit volume was calculated from the difference in honeycomb weight before and after wash coating <coat amount [g / L] = (weight before coating−weight after coating) / honeycomb volume). After this, 0.1 mol / L
Was immersed in the chloroplatinic acid aqueous solution for 30 minutes, dried at 120 ° C. for 2 hours, and then baked at 800 ° C. to support platinum.
The amount of platinum supported was about 2 g / L.

Every other cell opening portion on the gas inlet side of the honeycomb structure supporting platinum was plugged, and on the gas outlet side, only cells that were not plugged on the inlet side were plugged. The plugging material is not particularly limited as long as it is a ceramic material having a heat resistance of 1000 ° C. or higher, such as cordierite, alumina, or zirconia, and a ceramic adhesive may be used. Cordierite was used in this example.
In this way, filters with catalyst carriers having different active alumina coat amounts were prepared (carrier A-1 to carrier A-
4).

[Example 2] A filter similar to the porous cordierite honeycomb filter used in Example 1 was produced by the same method to obtain a high specific surface area material having a central particle size of 2 µm.
m, 670 g of activated alumina (Sumitomo Chemical Co., Ltd.) in which particles larger than 28 μm were 5% or less were mixed with 330 g of alumina sol (Nissan Chemical Co., Inc.) and 4 liters of water, and the stirred activated alumina slurry was washcoated with the filter. The coating was repeated to prepare five types of filters having different coating amounts. Then, the excess slurry was removed as completely as possible with an air cleaner and compressed air. After that, it was dried at 120 ° C. for 2 hours and calcined at 800 ° C. After checking the coat amount,
It was immersed in a chloroplatinic acid aqueous solution for 30 minutes, dried at 120 ° C. for 2 hours, and then baked at 800 ° C. to support platinum. The amount of platinum supported was about 2 g / L.

Thereafter, using cordierite, every other cell opening on the gas inlet side of the honeycomb structure supporting platinum was plugged, and on the gas outlet side, cells not plugged on the inlet side were plugged. Only the catalyst was sealed to produce a filter with a catalyst carrier (carrier B-1 to carrier B-5).

[Comparative Example 1] A filter similar to the porous cordierite honeycomb filter used in Example 1 was produced by the same method, and a high specific surface area material having a center particle size of 50 was prepared.
670 g of activated alumina (manufactured by Sumitomo Chemical Co., Ltd.) having a diameter of 330 μm was mixed with 330 g of alumina sol (manufactured by Nissan Kagaku) and 4 liters of water, and the stirred activated alumina slurry was wash-coated with the filter. Then, the excess slurry was removed as completely as possible with an air cleaner and compressed air. The coating was repeated to prepare three types of filters having different coating amounts. And then
It was dried at 120 ° C. for 2 hours and calcined at 800 ° C. After calculating the coating amount, it was immersed in a 0.1 mol / L chloroplatinic acid aqueous solution for 30 minutes and dried at 120 ° C. for 2 hours.
It was calcined at ° C to support platinum. Platinum loading is 2g /
L.

Then, using cordierite, every other cell opening on the gas inlet side of the honeycomb structure supporting platinum was plugged, and on the gas outlet side, cells not plugged on the inlet side were plugged. Only the catalyst was sealed to produce a filter with a catalyst carrier (carrier C-1 to carrier C-3).

[Comparative Example 2] A filter similar to the porous cordierite honeycomb filter used in Example 1 was produced by the same method, and as a high specific surface area material, the central particle diameter was about 2
670 g of activated alumina (Sumitomo Chemical Co., Ltd.) containing 45 wt% of particles larger than 28 μm at 5 μm was mixed with 330 g of alumina sol (Nissan Chemical Co., Ltd.) and 4 liters of water, and the stirred activated alumina slurry was wash-coated with the filter. Then, the excess slurry was removed as completely as possible with an air cleaner and compressed air. Repeat coating to change the coating amount 4
Different types of filters were made. After that, it was dried at 120 ° C. for 2 hours and calcined at 800 ° C. After the coating amount was calculated, it was immersed in a 0.1 mol / L chloroplatinic acid aqueous solution for 30 minutes, dried at 120 ° C. for 2 hours, and then baked at 800 ° C. to support platinum. The amount of platinum supported was about 2 g / L.

Thereafter, using cordierite, every other cell opening on the gas inlet side of the honeycomb structure supporting platinum was plugged, and on the gas outlet side, cells not plugged on the inlet side were plugged. Only the catalyst was sealed to prepare a filter with a catalyst carrier (carrier D-1 to carrier D-4).

[Comparative Example 3] A filter similar to the porous cordierite honeycomb filter used in Example 1 was prepared by the same method, and platinum was similarly coated at about 2 g / L without coating with activated alumina. Supported, plugging every other cell opening on the gas inlet side of the honeycomb structure,
On the gas outlet side, only the cells that were not plugged on the inlet side were plugged to prepare a filter with a catalyst carrier (carrier E).

Table 1 shows the coating amounts of the carriers prepared in Examples and Comparative Examples.

[0031]

[Table 1]

(Measurement of Pressure Loss of Filter with Catalyst Carrier) Compressed air was introduced from the inlet side of the filters for all the filters of carrier A to carrier E obtained in Examples 1 and 2 and Comparative Examples 1, 2, and 3. It was made to flow and the differential pressure between the inlet side and the outlet side was measured. The measurement result of the pressure loss is shown in FIG. From these results, it can be seen that the product of the present invention has a lower pressure loss than Comparative Examples 1 and 2 which are coated.

(Measurement of Pore Distribution of Filter) Example 1,
2. The pore distributions of all the filters of carrier A to carrier E obtained in Comparative Examples 1, 2, and 3 were measured. The relationship between the average pore size of the carrier and the coating amount of activated alumina is shown in FIG. As the coating amount increases, the average pore size of the carrier becomes smaller. The average pore size that is advantageous as a low-pressure loss diesel purification filter is 5 μm to 35 μm.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a diesel exhaust gas purification filter according to an example of the present invention or a comparative example.

FIG. 2 is an enlarged view of a portion A in FIG.

FIG. 3 is an enlarged view of a portion A in FIG. 1;

FIG. 4 is a graph showing measurement results of pressure loss of filters of Examples and Comparative Examples.

FIG. 5 is a graph showing the relationship between the average pore size of the carrier and the coated amount of activated alumina in the filters of Examples and Comparative Examples.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI technical display location B01J 23/40 B01D 53/36 ZAB F01N 3/02 301 104B (72) Inventor Kageyama Teraka Aichi 1-chome, Showa-cho, Kariya city, Nihon Denso Co., Ltd. (72) Inventor, Juji Kondo 1-1-chome, Showa-cho, Kariya city, Aichi prefecture, Nihon Denso Co., Ltd.

Claims (2)

[Claims] 1. A ceramic honeycomb structure is configured such that both ends of cell openings are alternately plugged to allow exhaust gas to flow to adjacent cells through pores on the side walls of the cells of the honeycomb, and only particulates contained in the exhaust gas are supplied to the cells. In a diesel exhaust gas purification filter having a porous ceramic filter so as to collect on the surface and inside of the side wall, and carrying a catalyst metal that burns the particulates on the side wall surface and inside, the ceramic honeycomb structure,
The porosity of the side wall of the cell is 40 to 65%, the average pore size is 5 to 35 μm, and the carrier carrying the catalyst metal has a particle size smaller than the average pore size of 90 wt%.
A diesel exhaust gas purification filter, which carries a high specific surface area material occupying the above. 2. The filter according to claim 1, wherein a catalytic metal composed of at least one kind of platinum group element is carried.