JPH0832291B2 - Filter for particulate collection - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a filter for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a filter for collecting particulates discharged from a diesel engine. Is.

(Prior Art) Conventionally, as a filter for collecting particulates discharged from a diesel engine, a hanical filter made of heat-resistant ceramics such as cordierite,
A filter such as a form filter is used. In this filter, it has been proposed to support a catalyst on the partition wall surface in order to increase the combustion efficiency of particulates during filter regeneration. Further, in order to disperse the catalyst to a high degree and to improve the adsorbability of particulates, it is considered preferable to support activated alumina on the partition wall surface.

However, since the partition walls of the ceramic filter substrate used for the filter have many pores, the pores are filled with the activated alumina when the activated alumina is coated, and as a result, the thermal shock resistance of the filter is increased. There was a problem of deterioration of sex. This is because the filter base material is usually made of a material with a low coefficient of thermal expansion such as cordierite, whereas activated alumina has a high coefficient of thermal expansion, so that the activated alumina that has entered the pores expands. It is considered that cracks are generated in the filter substrate.

Even in the case of a monolith catalyst carrier, since the porous ceramic substrate is coated with activated alumina, the crack generation temperature becomes low as in the case of the particulate trapping filter. To solve this problem, JP-A-58-89950 proposes a method of coating a water-soluble polymer before coating the activated alumina, thereby preventing the activated alumina from entering the pores. ing.

(Problems to be Solved by the Invention) The method of coating the water-soluble polymer described above is carried out by forming a polymer film on the inner walls of the pores to close the openings of the pores, thus It is a method to prevent invasion, and has a relatively small pore diameter and pore volume (pore amount).
The effect is recognized in the monolith carrier having a small value. However, in a filter having a large pore size such as a particulate collection filter, particularly a diesel particulate collection filter (particularly a honeycomb filter), only a polymer film is formed on the inner wall of the pores, Since there is still a gap in the opening of the hole, the activated alumina will enter into the pore from this and be filled. Further, a filter having a large pore volume has a large amount of activated alumina to be filled therein, and has a high crack occurrence rate. Therefore, in such a filter having a large pore diameter and a large pore volume, it is not possible to prevent the activated alumina from being filled in the pores by the above method, and it is not possible to sufficiently improve the thermal shock resistance. Therefore, in the present invention, in a filter having a large pore size and a large pore volume such as a diesel particulate trapping filter, even if the partition wall surface is coated with a porous ceramic such as activated alumina, the crack generation temperature is reduced. It is an object of the present invention to maintain a high thermal shock resistance without doing so.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the diesel particulate trapping filter of the present invention has a partition wall surface of a cell of a ceramic filter substrate, and a thermal expansion coefficient of the filter substrate. A first coating layer made of ceramic particles having the same or lower coefficient of thermal expansion is formed to close the openings of the pores of the partition walls of the substrate, and the catalyst supporting ability or fine particle adsorption on the first coating layer. It is characterized in that a second coat layer composed of porous ceramic particles having the ability is formed.

As the ceramic filter substrate, those conventionally used for collecting particulates can be used, and examples thereof include honeycomb filters or foam filters made of porous ceramics having a low thermal expansion coefficient, for example, cordierite.

As the material of the first coat layer, any ceramic particles having a thermal expansion coefficient equal to or lower than the thermal expansion coefficient of the filter substrate may be used, for example, cordierite, titanium. Aluminum acid,
Lithia-silica-aluminum ternary compounds and the like can be used.
Further, the particles are of a size capable of blocking the openings of the pores to prevent invasion of particles for the second coat layer such as activated alumina particles, and preferably have a particle size of 5 μm to 20 μm. . The coating amount of the first coat layer is sufficient to block the openings of the pores of the substrate and prevent the particles for the second coat layer from being filled.

The porous ceramic particles forming the second coat layer have a catalyst carrying ability or a fine particle adsorbing ability, and in addition to activated alumina, for example, calcium oxide, magnesium oxide, titanium oxide, zirconium oxide, silica-alumina, etc. Although it can be used, activated alumina is particularly preferable because it has excellent catalyst supporting properties and fine particle adsorption properties.

A catalytic metal is preferably supported on the porous ceramic of the second coat layer.

(Operation) In the filter for collecting diesel particulates of the present invention, the openings of the pores of the base material are closed by the ceramic particles having a low coefficient of thermal expansion, and most of the porous ceramics having a high coefficient of thermal expansion are present in the pores. Since it does not exist, the crack generation temperature is almost the same as the temperature of the base material.

(Example) Example 1 A commercially available cylindrical honeycomb filter having a diameter of 100 mm and a length of 100 mm (made of cordierite, average pore diameter 34 µm, pore volume 0.
40 cm ³ / g) is immersed in distilled water, treated with water and blown off, and then aluminum titanate with an average particle size of 10 μm,
Immerse in a slurry consisting of organic binder, distilled water, surfactant, etc., pull up and blow off excess slurry with compressed air, then dry and calcine at 600 ° C for 2 hours, One coat layer was formed. Next, after subjecting this filter to water absorption treatment, it is dipped in a slurry of activated alumina (γ-alumina) powder having an average particle size of 10 μm, an organic binder, a surfactant, distilled water, etc., in the same manner as described above. The excess slurry was blown off, dried and fired to form a second coat layer made of activated alumina. FIG. 1 shows the filter 1 for collecting diesel particulates obtained by this example. A in the figure
2 is a sectional view taken along the line -A 'and FIG. 2 is a part B of the partition wall 3 in FIG.
An enlarged view of is shown in FIG. As can be seen from the figure, the openings of the pores 4 of the partition walls 3 of the cells 2 of the diesel particulate filter 1 of the present invention are closed by the ceramic particles 7 forming the first coat layer 5, The γ-alumina forming the coat layer 6 does not exist in the pores 4. Although no catalyst is supported in this example, an ordinary diesel particulate collection filter carries an oxidation catalyst for increasing combustion efficiency.

Example 2 8 parts by weight of a commercially available lithium oxide powder (Li ₂ O), 27.5 parts by weight of alumina (Al ₂ O ₃ ) and 64.5 parts by weight of silica (SiO ₂ ) were sufficiently mixed and fired at 1000 ° C. for 5 hours. Next, this fired product was pulverized to obtain β-spodumene (Li ₂ O-) having an average particle size of 15 μm.
Al ₂ O ₃ -4SiO ₂ ternary compound) powder was obtained. A slurry was prepared by thoroughly stirring this powder with distilled water, an organic binder, a surfactant and the like. Next, a commercially available cordierite foam filter (20 cells / inch, diameter 100 m
m, a length of 100 mm) is subjected to a water absorption treatment, immersed in a slurry containing the lithia-alumina-silica powder as a main component, and the excess slurry is blown off by a centrifugation method, followed by drying and firing, and lithia A first coat layer of alumina-silica was formed. Next, this filter is used for the same γ − as in the first embodiment.
The second coating layer made of γ-alumina was formed by immersing the slurry in a slurry containing alumina powder as a main component, removing excess slurry by a centrifugal separation method, and drying and firing.

Example 3 A commercially available cordierite having an average particle size of 15 μm (2MgO · 2Al ₂
The O ₃ .5Al ₂ O ₃ ) powder was sufficiently stirred with distilled water, an organic binder, a surfactant, etc. to prepare a slurry. Next, the same honeycomb filter as in Example 1 was subjected to water absorption treatment, immersed in the slurry, pulled up, blown off excess slurry with compressed air, and then dried and fired to form a first coat layer. . After the obtained filter was subjected to water absorption treatment, a second coat layer made of γ-alumina was formed by the same method as in Example 1.

Comparative Example 1 A γ-alumina coat layer was formed on the same cordierite honeycomb filter substrate as that used in Example 1 by the same method as in Example 1.

Comparative Example 2 A γ-alumina coat layer was formed on the same cordierite layer foam filter substrate as used in Example 2 by the same method as in Example 2.

Comparative Example 3 The same cordierite honeycomb filter substrate as that used in Example 1 was immersed in an aqueous solution in which 5% by weight of commercially available polyvinyl alcohol (molecular weight 500, saponification rate 90%) was dissolved, and then pulled up and extra. After blowing off the aqueous solution of the above with compressed air, it was dried at 200 ° C. for 2 hours. Next, after water-absorbing treatment of this filter, it was immersed in the same slurry containing γ-alumina powder as the main component as in Example 1 and pulled up to blow off excess slurry, and dried and baked to obtain γ. -Alumina coat layer was formed.

Reference Example In order to clarify the effect of the present invention, the same cordierite honeycomb filter substrate as that used in Example 1 is used in the following test without coating.

Test Example The filters of Examples 1 to 3, Comparative Examples 1 to 3 and Reference Example are placed in an electric furnace maintained at a predetermined temperature, left for 2 hours, then taken out and allowed to cool to room temperature. The presence / absence of cracks in the filter was evaluated by determining the presence / absence of muddy sound when hit with a metal rod and visually determining. First result
Shown in the table.

Cordierite is originally a low thermal expansion material, which is clear from the fact that the reference example in which the activated alumina coat layer is not formed does not crack at high temperatures. Comparative Examples 1 and 2 are examples in which only the γ-alumina coat layer was formed by the conventional method, and as shown in Table 1,
The cracking temperature drops significantly. Comparative Example 3 is an example according to the crack resistance improving method disclosed in JP-A-58-89950, and the crack generation temperature is slightly higher than Comparative Examples 1 and 2, but the effect is not so remarkable. . This is because in the honeycomb filter for collecting diesel particulates, the average pore diameter is set large in order to keep the pressure loss low, so that the method disclosed in JP-A-58-89950 has an activity in the pores. It is presumed that it is difficult to prevent the invasion of alumina particles. On the other hand, in the examples according to the present invention, particles such as aluminum titanate or β-spodumene (Li ₂ O.Al ₂ O ₃ .4SiO ₂ ) having a smaller thermal expansion coefficient than cordierite which is the filter substrate are coated. As a result, the pore openings are closed, and then the activated alumina is coated, so that the activated alumina particles having a large thermal expansion coefficient do not penetrate into the pores, and as a result, cracks are generated as shown in Table 1. It is possible to significantly suppress the decrease in the generated temperature.

(Effect of the invention) In the particulate trapping filter of the present invention, the porous ceramic particles such as activated alumina are coated without entering into the pores of the cell partition walls,
A decrease in crack generation temperature is suppressed, and high thermal shock resistance is maintained. Therefore, the effect of supporting the catalyst by the porous ceramic particles such as activated alumina can be achieved without impairing the thermal shock resistance. Furthermore, by coating the porous ceramic particles, the particulate adsorbability is increased, and the particulate collection efficiency is improved. Also,
The present invention is a filter made of a ceramic base material having a large pore size and a large pore volume, such as a diesel particulate trap filter, which is remarkably excellent in effect as compared with the conventional one. An excellent effect can be achieved even with a ceramic substrate having a small pore size and a small pore volume, and therefore, it can be applied to a monolith carrier catalyst or the like.

[Brief description of drawings]

FIG. 1 is a perspective view showing a filter for collecting particulates of the present invention, FIG. 2 is a sectional view showing a part of the AA ′ section of the filter, and FIG. 3 is an enlarged view of a portion B of the sectional view. is there. 1 ... Diesel particulate trapping filter 2 ... Cell, 3 ... Partition wall 4 ... Pore, 5 ... First coat layer 6 ... Second coat layer, 7 ... Ceramic particle 8 ... Filter base Material

Claims (1)

[Claims] 1. A first coating layer made of ceramic particles having a thermal expansion coefficient equal to or lower than the thermal expansion coefficient of the filter base material is formed on the partition wall surface of the cell of the ceramic filter base material to form the base material. Particulate collection, characterized in that the openings of the pores of the material partition wall are closed and a second coat layer made of porous ceramic particles having a catalyst carrying ability or a fine particle adsorbing ability is formed on the first coat layer. Filter.