JP2004330118A - Filter for clarifying exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a filter for clarifying an exhaust gas that is effective for a long period without catalyst falling under practical service condition and has a high collection efficiency of PM (particulate matter) and a low-pressure drop. <P>SOLUTION: The filter for clarifying an exhaust gas is a wall flow type filter comprising a porous sintered material of silicon carbide, wherein particles of an oxide of ceramics having pores coat the surface of silicon carbide particles constituting the above porous sintered material of silicon carbide, and an oxidation catalyst for burning and oxidizing a particulate matter in the exhaust gas collected by said filter is contained in the pores in the above particles of an oxide of ceramics. An average diameter of the particles of an oxide of ceramics is 0.002 to 0.2 times as large as an average diameter of the silicon carbide particles. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to an exhaust gas purifying filter for collecting and burning and removing fine particulate matter (also simply referred to as particulate matter) contained in exhaust gas of an engine.
[0002]
[Prior art]
In recent years, particulate matter (PM) emitted from a diesel engine or the like has become a major environmental health problem. PM mainly consists of solid carbon fine particles (SOOT) and liquid hydrocarbon fine particles (SOF: soluble organic fraction). Typical examples of the exhaust gas purifying filter include a honeycomb filter, a foam filter, and a fiber filter. In particular, cordierite and silicon carbide wall flow type honeycomb filters have been receiving attention as exhaust gas purifying filters for automobiles in view of heat resistance and trapping performance.
[0003]
As a typical method of burning and removing the collected PM, two honeycomb filters (hereinafter, referred to as filters) called an alternate regeneration system are used, and while one of the filters collects the PM, the PM is removed. The other filter deposited is heated to 600 ° C. or higher with a burner or an electric heater to burn the PM to regenerate the filter, and a method of repeated use or blowing back the PM deposited on the filter called a backwash method with air. After discharge from the filter, a method of burning the PM by heating it to 600 ° C. or higher with a burner, an electric heater or the like has been studied.
[0004]
However, in these methods, when the temperature of the filter rises rapidly due to the burning of PM, the problem that the filter is broken or melted occurs, and the cost is increased, the apparatus becomes large, and the filter can be easily mounted on an automobile or the like. There were problems such as not.
[0005]
Therefore, attention has been paid to a method in which an oxidation catalyst is supported on a honeycomb or a filter and PM is constantly burned at a low temperature. As the catalyst, for example, a technique for oxidizing NO in exhaust gas to NO ₂ and oxidizing and burning PM with NO ₂ (see Patent Document 1), or a combustion catalyst for a mixture of a platinum group metal and an alkaline earth metal oxide (See Patent Document 2) and the like have been proposed.
[0006]
[Patent Document 1] Japanese Patent No. 3012249.
[0007]
[Patent Document 2] Japanese Patent Publication No. 07-106290.
[0008]
[Problems to be solved by the invention]
In order to support a catalyst on a filter, a method is known in which a material having a high specific surface area such as γ-alumina is slurried as a carrier and coated on a cell side wall surface and inside pores of a cell side wall by wash coating. However, there is also a problem that the carrier falls off due to gas circulation, vibration, heat cycle, and the like in running the automobile for a long time, and the catalyst does not work effectively.
[0009]
Further, the formation of the coating layer has a problem that the pressure loss is higher than that of a filter without coating. Since a high pressure loss of the filter leads to a decrease in engine output, the pressure loss should be as low as possible. On the other hand, if the porosity and the pore diameter of the filter are too large to reduce the pressure loss, the PM collection efficiency will be reduced.
[0010]
[Means for Solving the Problems]
The present inventor has conducted intensive studies in view of the above-described conventional art, and has set the particle size ratio of the silicon carbide particles and the carrier constituting the filter to a predetermined range, thereby further improving the pore characteristics after forming the coating layer. By further optimizing, the above-mentioned problem can be further improved and the knowledge that the problem can be solved has been obtained, and the present invention has been achieved.The purpose of the present invention is that even under actual use conditions, An object of the present invention is to provide an exhaust gas purifying filter which is effective for a long time without falling off of a catalyst, has a high PM collection efficiency, and has a low pressure loss.
[0011]
That is, the present invention is a wall-flow type filter made of a porous silicon carbide sintered body, wherein ceramic oxide particles having pores on the surface of silicon carbide particles constituting the porous silicon carbide sintered body are provided. An exhaust gas purifying filter, further comprising an oxidation catalyst for oxidizing and burning particulate matter in exhaust gas collected by the filter in pores of the ceramic oxide particles, An exhaust gas purifying filter, wherein the average particle diameter of the particles is 0.002 to 0.2 times the average particle diameter of the silicon carbide particles.
[0012]
Further, in the present invention, when a pore size distribution curve is determined by a mercury intrusion method, the average pore size calculated from peaks other than a peak having a pore size of 0.5 μm or less is 5 to 20 μm, and the porosity is The exhaust gas purifying filter described above, which is 40 to 60%.
[0013]
Further, the present invention is the exhaust gas purifying filter, wherein the specific surface area (m ² / g) of the ceramic oxide particles is 30 or more with respect to the average particle size (μm).
[0014]
Further, the present invention is characterized in that the oxidation catalyst contains one or more elements selected from the group consisting of gold, silver, copper, iron, zinc, manganese, cerium and platinum group elements. It is an exhaust gas purification filter.
[0015]
Further, the present invention is the above exhaust gas purifying filter, wherein the ceramic oxide particles are made of an oxide of at least one element selected from the group consisting of Al, Zr, Ti, and Si.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The exhaust gas purifying filter of the present invention is provided with ceramic oxide particles having pores on the surface of silicon carbide particles constituting the porous silicon carbide sintered body constituting the filter, and the pores of the ceramic oxide particles are provided. It has a structure that contains an oxidation catalyst for oxidizing and burning particulate matter in exhaust gas collected by the filter.
[0017]
The material of the exhaust gas purifying filter of the present invention is made of a porous sintered body made of silicon carbide because of its excellent heat resistance, thermal shock resistance, and corrosion resistance. Of a good wall flow type. As for the filter shape, it is preferable that the filter is a honeycomb filter, because the trapping performance of the particulate matter is more excellent and the production is easy.
[0018]
In the present invention, the average particle size of the ceramic oxide particles serving as the catalyst carrier is 0.002 to 0.2 times, preferably 0.01 to 0.2 times, the average particle size of the silicon carbide particles constituting the filter. It is 15 times. If it is larger than 0.2 times, the ratio of the contact area with the silicon carbide particles on the surface of the ceramic oxide particles is small, and sufficient adhesion cannot be obtained, and it becomes difficult to coat the inside of the pores of the silicon carbide sintered body. However, segregation occurs only on the surface of the honeycomb wall, and even when the carrier is carried in a small amount, the pressure loss may be greatly increased in actual use. On the other hand, powders smaller than 0.002 times are difficult to produce, agglomeration in the slurry drying step becomes extremely difficult, and uniform coating becomes difficult, contact between exhaust gas and the catalyst becomes insufficient, and desired catalyst performance is obtained. It becomes difficult.
[0019]
Since the exhaust gas purifying filter of the present invention has the above configuration, when a pore size distribution curve is measured by a mercury porosimeter, the filter is formed from silicon carbide particles and ceramic oxide particles as illustrated in FIG. A distribution curve having a peak at a pore diameter of 1 μm or more and a distribution curve having a plurality of peaks at a pore diameter of 0.1 μm or less due to pores in the ceramic oxide particles are obtained.
[0020]
According to the present inventors' experimental studies, the former preferably has an average pore diameter of 5 to 20 µm and a porosity of 40 to 60%. If it is smaller than the lower limit of the preferred range, the pressure loss of the filter becomes high and the engine output may decrease in practical use, which is not desirable. It is not preferable because the collection efficiency may be reduced.
[0021]
Further, in the present invention, the specific surface area (m ² / g) of the ceramic oxide particles, which are the catalyst carrier, with respect to the average particle size (μm) in order to enhance the dispersibility of the catalyst component and the gas diffusion property in the initial state. Preferably, the ratio is 30 or more. If the above ratio is less than 30, the amount of catalyst carried per ceramic oxide particle becomes small, and it is necessary to coat a large amount of ceramic oxide particles in order to make the catalyst work effectively. Is difficult to obtain.
[0022]
In the present invention, the oxidation catalyst preferably contains one or more selected from the group consisting of gold, silver, copper, iron, zinc, manganese, cerium, and a white metal element. Further, the content is preferably 0.3 to 7 g / l (per unit volume of honeycomb). When the amount is less than 0.3 g / l, the function is not sufficiently exhibited in the low-temperature activity and the purification performance, and when the amount exceeds 7 g / l, the catalytic activity is saturated, which is not economically advantageous.
[0023]
As the ceramic oxide particles in the present invention, an oxide composed of at least one element selected from the group consisting of Al, Zr, Ti, and Si is preferable, and γ-alumina is particularly suitably selected. This is because γ-alumina is easily available as fine powder having a very large specific surface area, and it is easy to obtain the high ratio (specific surface area / average particle size).
[0024]
In the present invention, as a method of supporting various oxidation catalysts on a catalyst carrier composed of ceramic oxide particles such as γ-alumina, for example, an appropriate method is selected from known methods such as an impregnation method and a kneading method. be able to. As the raw material compound for the oxidation catalyst, any compound can be used as long as it is a water-soluble compound such as dinitrodiamminate, chloride and nitrate. The removal of water can be appropriately selected from known methods such as a filtration method and an evaporation to dryness method.
[0025]
The loading of the catalyst component on the filter can be obtained by immersing the filter in a slurry containing the catalyst component, or by spraying the slurry containing the catalyst component from the end face. For coating, a slurry immersion method is preferred.
[0026]
The total supported amount of the oxidation catalyst (the amount of the catalyst component and the carrier such as alumina) is preferably 20 to 200 g / l (per unit volume of the honeycomb). The larger the coating amount, the more preferable in terms of catalytic activity and catalyst life. However, if the coating layer is too thick, it becomes difficult to obtain the above-mentioned pore characteristics, causing an increase in pressure loss, which is not preferable.
[0027]
【Example】
(Examples 1, 2, Comparative Examples 1, 2, 3)
[0028]
<< Production of exhaust gas purification filter >>
(Example 1)
<Filter material>
A wall made of a silicon carbide sintered body having a cell end portion sealed in a checkered pattern at an outer diameter of 143.8 mm, an axial length of 152.4 mm, a wall thickness of 0.34 mm, and a cell density of 200 cells / square inch. A flow type honeycomb filter was used. The average particle size of the silicon carbide particles of the sintered body determined by the intercept method was 18 μm. The average pore diameter determined by the mercury intrusion method was 18 μm, and the porosity was 55%.
[0029]
<Preparation of catalyst-containing slurry>
An aqueous solution of dinitrodiammine platinum is sprayed onto γ-alumina powder having a specific surface area of 120 m ² / g and an average particle size of 50 μm while stirring, dried at 150 ° C. for 24 hours, and then calcined at 400 ° C. for 1 hour and then at 600 ° C. for 1 hour. Thus, Pt-supported alumina powder was obtained. The Pt concentration of this powder was 1.0% by mass. Next, 40% by mass of the powder, 2% by mass of acetic acid alumina sol (manufactured by Nissan Chemical Co., Alumina Sol 200) and 58% by mass of distilled water were charged into a magnetic ball mill, and mixed and pulverized for 100 hours to obtain a slurry. . The average particle size of the powder in the slurry measured by a laser diffraction / scattering method according to JIS R1629 was 1.0 μm, the ratio of the average particle size of the silicon carbide particles to 0.06, and the specific surface area of γ-alumina The ratio of (m ² / g) / average particle diameter was 120.
[0030]
<Loading of catalyst component on filter>
After diluting the slurry to a predetermined concentration with distilled water, the filter was immersed, a catalyst component was attached thereto, and excess slurry in the cell was removed with an air stream and dried. Thereafter, the filter was fired at 600 ° C. for 3 hours to obtain an exhaust gas purifying filter having a coating amount of 50 g / l per unit volume of the filter. FIG. 1 shows pore size distribution curves measured by a mercury porosimeter before and after carrying the catalyst component. Due to the coating of the catalyst component, the peak before coating shifted to the low pore size side, and a peak based on pores in γ-alumina particles was observed at around 0.05 μm. The average pore diameter calculated from the peaks other than those having a pore diameter of 0.5 μm or less was 14 μm, and the porosity was 52%.
[0031]
(Example 2)
<Filter material>
A filter made of a silicon carbide sintered body having an outer diameter of 143.8 mm at the end face, an axial length of 152.4 mm, a wall thickness of 0.20 mm, and a cell density of 300 cells / square inch was used. The average particle diameter of silicon carbide was 18 μm, the average pore diameter measured by a mercury porosimeter was 30 μm, and the porosity was 60%.
[0032]
<Preparation of catalyst-containing slurry>
A catalyst-containing slurry was prepared in the same manner as in Example 1.
[0033]
<Loading of catalyst component on filter>
After the same catalyst component attachment treatment as in Example 1 was repeated four times, it was fired at 600 ° C. for 3 hours to obtain an exhaust gas purifying filter. The coating amount was 180 g / l, the average pore diameter calculated from the peaks other than the peak having a pore diameter of 0.5 μm or less measured by a mercury porosimeter was 19 μm, and the porosity was 52%.
[0034]
(Comparative Example 1)
<Filter material>
The same one as in Example 1 was used.
[0035]
<Preparation of catalyst-containing slurry>
A catalyst-containing slurry was prepared in the same manner as in Example 1. However, the pulverization mixing time by the ball mill was 5 hours. The average particle size of the powder in the slurry was 10 μm, the ratio to the average particle size of the silicon carbide particles was 0.56, and the ratio of the specific surface area (m ² / g) / average particle size of γ-alumina was 12. Was.
[0036]
<Loading of catalyst component on filter>
The same catalyst component adhesion treatment as in Example 1 was performed. The coating amount was 50 g / l, the average pore size calculated from the peaks other than the peak having a pore size of 0.5 μm or less measured by a mercury porosimeter was 13 μm, and the porosity was 52%.
[0037]
(Comparative Example 2)
<Filter material>
A wall made of a silicon carbide sintered body having a cell end portion sealed in a checkered pattern at an outer diameter of 143.8 mm, an axial length of 152.4 mm, a wall thickness of 0.34 mm, and a cell density of 200 cells / square inch. A flow type honeycomb filter was used. The average particle size of the silicon carbide particles of the sintered body determined by the intercept method was 23 μm. The average pore diameter determined by the mercury intrusion method was 9 μm, and the porosity was 39%.
[0038]
<Preparation of catalyst-containing slurry>
The same slurry as in Example 1 was used. The average particle size of the γ-alumina particles was 0.04 times the average particle size of the silicon carbide particles, and the ratio of specific surface area (m ² / g) / average particle size of γ-alumina was 120.
[0039]
<Loading of catalyst component on filter>
The same catalyst component adhesion treatment as in Example 1 was performed. The coating amount was 50 g / l, the average pore diameter calculated from the peaks other than the peak having a pore diameter of 0.5 μm or less measured by a mercury porosimeter was 4 μm, and the porosity was 35%.
[0040]
(Comparative Example 3)
<Filter material>
The same one as in Example 2 was used.
[0041]
<Preparation of catalyst-containing slurry>
A catalyst-containing slurry was prepared in the same manner as in Example 1.
[0042]
<Loading of catalyst component on filter>
The same catalyst component adhesion treatment as in Example 1 was performed. The coating amount was 50 g / l, the average pore diameter calculated from the peaks other than the peak having a pore diameter of 0.5 μm or less measured by a mercury porosimeter was 26 μm, and the porosity was 57%.
[0043]
<< Measurement of pressure loss of exhaust gas purification filter >>
With respect to the filters obtained in Examples 1 and 2 and Comparative Examples 1 to 3, compressed air was flowed from the inlet side of the filter, and the differential pressure between the inlet side and the outlet side was measured. FIG. 2 shows the measurement results of the pressure loss. As the average pore diameter and / or the porosity calculated from peaks other than those having a pore diameter measured by a mercury porosimeter of 0.5 μm or less increase, the pressure loss tends to decrease. In Comparative Example 2, the pressure loss significantly increases. Was. In Comparative Example 1, the pressure loss was higher than the values of the pore diameter and the porosity.
[0044]
<< Evaluation of collection efficiency and durability of exhaust gas purification filter >>
The filters obtained in Examples 1 and 2 and Comparative Examples 1 and 3 were mounted on a vehicle equipped with a 2000 cc diesel engine, and PM collection and regeneration were repeated, and a running test of up to 30,000 km was performed.
[0045]
The black smoke density and the PM reduction rate before and after the filter was attached were measured before the actual vehicle running test. The black smoke density of the filters of Examples 1 and 2 and Comparative Example 1 was 100%, and the PM reduction rate was 100%. Was 90% on average. On the other hand, it was found that the filter of Comparative Example 3 had a black smoke density of 80% reduction rate and a PM reduction rate of 50% on average and low collection efficiency.
[0046]
In each of the exhaust gas purifying filters of Example 1 and Example 2, no increase in back pressure was observed after traveling about 30,000 km, and it was found that PM was collected and regeneration by oxidation was performed efficiently. . Further, the black smoke concentration reduction rate after the running test was 100%, and the PM reduction rate was 90% on average, and the purification performance was maintained.
[0047]
On the other hand, in the exhaust gas purifying filter of Comparative Example 1, when the traveling distance exceeded about 3000 km, the back pressure rapidly increased, and the traveling test was stopped. When the microstructure of the filter after the running test was observed, almost no γ-alumina particles as the catalyst carrier were observed on the surface of the honeycomb wall, and the particles were localized in the rear part of the cell. In this case, the adhesion strength between γ-alumina and the silicon carbide particles constituting the filter is insufficient, and the peeling of the γ-alumina coat layer progresses during the running test, so that the regeneration function due to oxidation of PM deteriorates. It turned out that the back pressure had risen sharply.
[0048]
【The invention's effect】
The filter of the present invention, since the ratio of the particle size of the silicon carbide particles and the carrier constituting the carrier is in a predetermined range, furthermore, since the pore characteristics after forming the coating layer is optimized, under actual use conditions, It is very useful in industry because it has the characteristics of low pressure loss, excellent PM trapping performance, and the ability to remove PM under mild conditions over a long period of time by the action of the supported catalyst. .
[Brief description of the drawings]
FIG. 1 is a view showing a pore size distribution curve of an exhaust gas purifying filter according to Example 1 measured by a mercury porosimeter.
FIG. 2 is a graph showing flow rate-pressure loss characteristics of exhaust gas purifying filters according to examples and comparative examples.

Claims (5)

A wall flow type filter made of a porous silicon carbide sintered body, wherein ceramic oxide particles having pores are provided on the surface of silicon carbide particles constituting the porous silicon carbide sintered body; An exhaust gas purifying filter containing an oxidation catalyst for oxidizing and burning particulate matter in exhaust gas collected by the filter in pores in the particles, wherein the average particle size of the ceramic oxide particles is An exhaust gas purifying filter, wherein the average particle size of the silicon carbide particles is 0.002 to 0.2 times. When a pore size distribution curve is obtained by a mercury intrusion method, the average pore size calculated from peaks other than the peak having a pore size of 0.5 μm or less is 5 to 20 μm, and the porosity is 40 to 60%. The exhaust gas purifying filter according to claim 1, wherein: 3. The exhaust gas purifying filter according to claim 1, wherein the specific surface area (m ² / g) of the ceramic oxide particles is 30 or more with respect to the average particle size (μm). The oxidation catalyst contains one or more elements selected from the group consisting of gold, silver, copper, iron, zinc, manganese, cerium, and platinum group elements. An exhaust gas purifying filter according to claim 3. The ceramic oxide particles are made of an oxide of one or more elements selected from the group consisting of Al, Zr, Ti, and Si. An exhaust gas purifying filter according to claim 1.

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