JP2006007100A - Exhaust gas purification filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress cracks due to erosion or thermal stress and reduce the combustion residue of PMs at an outer peripheral part. <P>SOLUTION: The cells in the outer peripheral part are formed of a material with higher thermal conductivity than that of the cells in the inner peripheral part. The heat accumulated in the inner peripheral part being transmitted to the outer peripheral part causes the temperature of the outer peripheral part to rise thereby to reduce the combustion residue of the PMs in the outer peripheral part. Further, the heat generated when the PMs are burnt can be effectively released outside and the generation of breakage or cracks due to erosion or thermal stress can be suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification filter that is attached to an exhaust passage of a diesel engine or the like and collects particulate matter (PM) in exhaust gas to suppress emission.

  As for gasoline engines, toxic components in exhaust gas are steadily decreasing due to strict regulations on exhaust gas and technological advances that can cope with it. However, with respect to diesel engines, regulations and technological advances are delayed compared to gasoline engines due to the unique situation that harmful components are discharged as PM.

  2. Description of the Related Art As an exhaust gas purification device for a diesel engine that has been developed so far, a trap type exhaust gas purification device and an open type exhaust gas purification device are known. Among these, as a trap type exhaust gas purifying apparatus, a ceramic plugged DPF such as cordierite is known. In this DPF, a plurality of cells of the ceramic honeycomb structure are composed of an inflow side cell in which the exhaust gas downstream end is clogged and an outflow side cell adjacent to the inflow side cell and clogged in the exhaust gas upstream end. It has a cell, and is a so-called wall flow type that suppresses discharge by filtering exhaust gas through the pores of the cell partition and collecting PM in the cell partition.

  However, in the DPF, the pressure loss increases due to the accumulation of PM. Therefore, it is necessary to periodically remove and regenerate PM accumulated by some means. Thus, conventionally, when the pressure loss increases, the DPF is regenerated by burning the accumulated PM by heating with a burner, an electric heater or the like, or by supplying high-temperature exhaust gas. However, in this case, as the amount of accumulated PM increases, the temperature at the time of combustion rises, which may cause the DPF to melt or break or crack due to thermal stress.

  A continuous regeneration type DPF has been developed in which a coating layer is formed of alumina or the like on the cell partition of the DPF, and a platinum group noble metal is supported on the coating layer. According to this continuous regeneration type DPF, the PM collected in the pores of the cell partition wall is oxidized and burned by the catalytic activity of the noble metal, so that the DPF is regenerated by burning simultaneously with the collection or continuously in the collection. can do. Since the catalytic activity occurs at a relatively low temperature and can be burned while the amount collected is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.

  However, in the continuous regeneration type DPF, since the activity of the noble metal is not expressed at a low temperature, there is a phenomenon that PM accumulates on the cell partition wall in a low temperature region and burns at a time when the temperature becomes high. In such a case, the DPF formed from cordierite or the like may crack microscopically due to thermal stress. This cause is considered as follows.

  Combustion of the deposited PM does not occur uniformly throughout the DPF, but first, combustion occurs from the portion where the temperature is highest, and the combustion propagates. Microscopically, a temperature difference is generated between the combustion part and the non-combustion part. However, since cordierite has low thermal conductivity, a portion with a large temperature difference is generated, and cracks are generated due to thermal stress.

  Therefore, it is conceivable to use ceramics such as SiC. For example, SiC has a melting temperature about 800 ° C higher than that of cordierite, and its thermal conductivity is about 24 times that of cordierite, so heat from PM combustion can be quickly released to the outside. Therefore, it is possible to prevent the above-described problems occurring in the DPF formed from cordierite.

  However, because SiC has a high bulk density, the DPF made of SiC is heavier than the one made of cordierite when compared at the same volume, and it goes against the trend of reducing fuel consumption by reducing weight. Resulting in. Further, when the accumulated PM is burned and removed, the combustion of PM does not occur uniformly from the outer periphery to the inner portion, and the PM residue is more likely to occur in the outer peripheral portion than in the central portion. Since DPF formed from SiC has high thermal conductivity, heat easily escapes compared to that formed from cordierite, and more PM remains on the outer periphery.

Japanese Patent Laid-Open No. 2003-321280 discloses a diesel particulate filter in which a heat conductive layer selected from SiC or SiN is formed on the pore surface of a cell partition wall. According to this filter, heat at the time of PM combustion is quickly diffused through the heat conductive layer, so that generation of cracks due to thermal stress can be suppressed. In addition, since most of the cordierite is formed, the bulk density is low as a whole, and the weight can be reduced. However, even with this filter, it has been difficult to solve the problem of a large amount of unburned PM in the outer periphery.
JP2003-321280

  This invention is made | formed in view of an above-described situation, and makes it the problem which should be solved while suppressing the crack by melting damage and a thermal stress, and reducing PM unburned residue of an outer peripheral part.

The features of the exhaust gas purification filter of the present invention that solves the above-described problems include an inflow side cell that is clogged on the exhaust gas downstream side, an outflow side cell that is adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell A honeycomb-shaped exhaust gas purification filter having a porous cell partition wall partitioning an outflow side cell and having a large number of pores,
The outer peripheral cell is made of a material having higher thermal conductivity than the material of the inner peripheral cell.

  Preferably, the inner peripheral cell is made of cordierite, and the outer peripheral cell is made of at least one selected from silicon carbide and silicon nitride.

  Further, it is desirable that the outer peripheral cells occupy a range of 30 to 90% of the total number of cells.

  Furthermore, a catalyst layer containing a porous oxide and a noble metal may be formed on the surface of the pores of the cell partition wall.

  According to the exhaust gas purification filter of the present invention, the outer peripheral cells are made of a material having a higher thermal conductivity than the inner peripheral cell material, so that heat during PM combustion can be effectively released to the outside. It is possible to suppress the occurrence of breakage and cracks due to melting damage and thermal stress.

  In addition, since the inner peripheral cell is formed of a material having a lower thermal conductivity than the outer peripheral cell, the temperature of the outer peripheral portion rises by transferring the heat stored in the inner peripheral portion to the outer peripheral portion, PM unburned residue on the outer periphery decreases. Therefore, PM collection efficiency after regeneration is improved.

  If the inner peripheral cell is made of cordierite or the like, the bulk density as a whole can be lowered and the weight can be reduced. Further, since the exhaust gas has a higher flow rate toward the center, most of the PM is collected in the inner periphery. Therefore, if the inner peripheral cell is formed of cordierite or the like, the pore diameter of the cell partition wall can be easily controlled, and the characteristics as a DPF can be easily satisfied.

  In the exhaust gas purification filter of the present invention, the outer peripheral cell is formed of a material having a higher thermal conductivity than the material of the inner peripheral cell. As the material of the inner peripheral cell, the same material as the conventional DPF can be used, and examples thereof include alumina, zirconia, ceria, titania, silica, silica-alumina, cordierite and the like. Of these, cordierite, which has a proven record and has a low bulk density, is particularly preferred.

  The material of the outer peripheral cell is only required to have a relatively higher thermal conductivity than the material of the inner peripheral cell. SiC, SiN, metal, alumina, zirconia, ceria, titania, silica, silica-alumina, It can be used by selecting from cordierite. For example, when the inner peripheral cell is made of cordierite, it is desirable to use at least one selected from SiC and SiN as the material of the outer peripheral cell. Moreover, it is also possible to comprise so that heat conductivity may become high gradually from an inner peripheral part toward an outer peripheral part. This can be done, for example, by gradually changing the ratio of both from the inner periphery to the outer periphery when forming a cell from a mixed powder of SiC and cordierite.

  The composition ratio of the outer peripheral portion with high thermal conductivity is not particularly limited, but in the filter arranged in the exhaust passage of a general diesel engine, the outer peripheral cell occupies a range of 30 to 90% of the total number of cells. Is desirable. If the ratio of the number of cells in the outer peripheral portion is outside this range, the effect of the present invention may not be obtained.

  In order to form pores in the cell partition walls, the cells and cell partition walls are formed from a slurry in which combustible powders such as carbon powder, wood powder, starch and resin powder are mixed, and the combustible powders are burned off during firing. Pore can be formed. By adjusting the particle size and addition amount of the combustible powder, it is possible to control the distribution of the surface pores and the diameters of the internal pores and the opening area. In the case of the present invention, the diameter distribution and opening area of the surface vacancies and internal pores are not particularly limited.

  The exhaust gas purification filter of the present invention preferably has a catalyst layer containing a porous oxide and a noble metal on the surface of the pores of the cell partition walls that partition the inflow side cell and the outflow side cell. Thereby, it can be set as a continuous regeneration type DPF. As the porous oxide used in the catalyst layer, oxides such as alumina, zirconia, ceria, and titania, or composite oxides composed of a plurality of these can be used.

  The formation amount of the catalyst layer is preferably in the range of 1 to 20 μm, or in the range of 60 to 200 g per liter of filter volume, although it depends on the pore diameter of the partition walls. When the formation amount of the catalyst layer is less than this range, the noble metal is supported at a high density, so that when exposed to a high temperature, grain growth of the noble metal may occur and the activity may decrease. Moreover, when the formation amount of the catalyst layer is larger than this range, the pore diameter and the opening area are reduced, and the pressure loss is increased.

  In order to form the catalyst layer, the porous oxide powder may be made into a slurry together with a binder component such as alumina sol and water, impregnated into the cell partition wall, fired, and then loaded with at least a noble metal. A normal dipping method can be used to impregnate the cell partition walls with the slurry, but it is desirable to remove excess slurry that has entered the pores by air blowing or suction.

  At least a noble metal is supported on the catalyst layer. The noble metal can be used as long as it promotes the oxidation of PM by a catalytic reaction, but it is particularly preferable to support one or more selected from platinum group noble metals such as Pt, Rh, and Pd. . The amount of noble metal supported is preferably in the range of 0.5 to 10 g per liter of filter deposition. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.

In addition to precious metals, NO _x occlusion materials selected from alkaline earth metals such as Ba, Ca and Sr, alkali metals such as Na, K, Li and Cs, or rare earth elements such as La, Nd, Sc and Y Furthermore, it is preferable to carry. By supporting the NO _x storage material, the NO _x storage and release ability is expressed, and the NO _x purification ability is improved. The amount of the NO _x storage material supported is preferably in the range of 0.01 to 2 mol per liter of filter deposition. If the supported amount is less than this, the NOx occlusion / release ability is not expressed, and if it is supported more than this, the noble metal such as Pt is covered and the oxidizing ability is lowered.

In order to support the noble metal and the NO _x storage material, a solution in which nitrate of the noble metal or the NO _x storage element is dissolved may be supported on the porous oxide layer by an adsorption support method, a water absorption support method, or the like. Alternatively, a noble metal may be supported on the porous oxide powder in advance, and the catalyst layer may be formed using the catalyst powder, and then the NO _x storage material may be supported.

  In order to manufacture the exhaust gas purification filter of the present invention, the slurry for forming the outer peripheral portion and the slurry for forming the inner peripheral portion are used, and the slurry for forming the inner peripheral portion is formed by a two-color extrusion molding method. It can be manufactured by forming the inner peripheral portion, forming the outer peripheral portion from the slurry for forming the outer peripheral portion on the outer periphery, firing the outer peripheral portion, and sealing both ends in the same manner as in the prior art. Further, as shown in the examples, each of the cell units independent from the slurry for forming the outer peripheral portion and the slurry for forming the inner peripheral portion is formed, and the inner peripheral portion and the plurality of cell units are joined together. An outer periphery can also be formed. The clogging portion can be formed in the same manner as in the past.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

First, using a paste in which a predetermined amount of an organic binder, water, and carbon powder are mixed with a powder of a cordierite composition made of alumina, talc, kaolin, and silica, and then extruded into a rectangular tube shape having a plurality of cell passages, a plurality of cells One end and the other end are alternately packed to form an inflow side cell and an outflow side cell, fired at 1300 ° C., and as shown in FIG. 1, a 20 mm square, 100 mm length, wall thickness A 300 μm cordierite cell unit 1 was formed. In this cordierite cell unit 1, 300 cells are formed per 1 cm ² .

Also, using a paste in which a predetermined amount of organic binder, water, and carbon powder are mixed in SiC powder, after extrusion into a rectangular tube shape with multiple cell passages, one end and the other end of multiple cells are alternately plugged. Then, an inflow side cell and an outflow side cell were formed and fired at 2000 ° C. to form an SiC cell unit 2 having the same shape as the cordierite cell unit 1 as shown in FIG. In this SiC cell unit 2, 300 cells are formed per 1 cm ² .

  Table 1 shows the characteristic values of cordierite and SiC.

Example 1
As shown in Fig. 2, the cordierite cell units 1 are bonded together using ceramic segment material (made by Ibiden) so that they are 4 in length and 4 in width. Pasted together. The SiC cell unit 2 is represented by a thicker line than the cordierite cell unit 1. Then, after forming the shape shown on the left side of FIG. 3, the outer periphery was cut to form a cylindrical filter, and the filter was housed in the outer cylinder 3 as shown on the left side of FIG.

  In this exhaust gas purification filter, the inner peripheral cell is made of cordierite, and the outer peripheral cell is made of SiC. The number of cells in the outer periphery formed from SiC is 43.4% of the total number of cells.

(Example 2)
The cordierite cell units 1 were bonded together using the same segment material as in Example 1 so that they were two in length and two in width, and the SiC cell unit 2 was double bonded around the periphery. Then, after making the shape shown on the left side of FIG. 4, the outer periphery was cut into a cylindrical filter, which was housed in the outer cylinder 3 to form a catalytic converter.

  In this exhaust gas purification filter, the inner peripheral cell is made of cordierite, and the outer peripheral cell is made of SiC. The number of cells in the outer periphery formed from SiC is 85.9% of the total number of cells.

(Comparative Example 1)
The SiC cell units 2 were bonded using the same segment material as in Example 1 so as to be 4 in length and 4 in width, and the cordierite cell 1 was bonded in a single layer around the same. Then, after making the shape shown on the left side of FIG. 5, the outer periphery was cut into a cylindrical filter, which was housed in the outer cylinder 3 to form a catalytic converter.

  In this exhaust gas purification filter, the inner peripheral cell is made of SiC, and the outer peripheral cell is made of cordierite. The number of outer peripheral cells formed from cordierite is 43.4% of the total number of cells.

(Comparative Example 2)
Except for not using the cordierite cell unit 1 but using all the SiC cell units 2, a cylindrical filter was formed in the same manner as in Example 1 and housed in the outer cylinder 3 to form a catalytic converter.

(Comparative Example 3)
Except for using the cordierite cell unit 1 without using the SiC cell unit 2, a cylindrical filter was formed in the same manner as in Example 1 and housed in the outer cylinder 3 to form a catalytic converter.

<Test and evaluation>
Each catalytic converter was attached to the exhaust system of an engine bench equipped with a 2 L direct injection diesel engine and operated at 1500 rpm × 30 Nm. When the PM deposition amount reached 6 g, regeneration treatment was performed by operating at 3800 rpm × 200 Nm for 10 minutes. Thereafter, the catalytic converter was removed, the weight of the filter was measured, and the PM oxidation rate was calculated from the amount of weight change before and after the regeneration treatment. Further, for each removed filter, the presence or absence of PM unburned residue in the cell at the outer peripheral portion 10 mm deep from the outer peripheral surface was visually observed. The results are shown in Table 2.

  According to the filter of each example, PM can be removed at a higher oxidation rate than that of each comparative example during the regeneration process, and there is no PM remaining on the outer peripheral portion. This is clearly an effect of arranging the SiC cell 2 on the outer peripheral portion. In order to investigate the cause of this, the temperature distribution of each filter during the regeneration process was measured. The measurement positions were positions slightly entering the inside from the exhaust gas inflow side end face of the filter, and were the positions of four points No. 1 to No. 4 shown in FIG. 6, and the temperature at each position was measured. The results are shown in FIG.

  FIG. 7 shows that the filter of Example 1 has substantially the same temperature at the outer peripheral portion and the inner peripheral portion, and the temperature of the outer peripheral portion is higher than that of the filter of the comparative example. On the other hand, in the filter of Comparative Example 2, since all the cells are made of SiC, the thermal conductivity is too high, and both the outer peripheral portion and the inner peripheral portion have a lower temperature than the embodiment. Further, in the filter of Comparative Example 3, since all the cells are made of cordierite, the heat conductivity is high, but the thermal conductivity is low, and the inner peripheral temperature is higher than that of Example 1, but the outer peripheral temperature is higher. Remarkably low. In the filter of Comparative Example 1, the temperature of the inner peripheral part is higher than that of Comparative Example 2 because the thermal conductivity of the outer peripheral part is low, but the temperature of the outer peripheral part is an intermediate value between Comparative Example 2 and Comparative Example 3. Is significantly lower than

  In addition, the PM oxidation rate of the filter of Example 1 is higher than that of the filter of Example 2. This is considered to be due to the composition ratio of the SiC cell and the cordierite cell, and the outer periphery formed from SiC. It can be seen that the number of cells in the part is more preferably 43.4% than 85.9% of the total number of cells.

  That is, the filter of Example 1 shows a high PM oxidation rate, and there is no PM burning residue in the outer peripheral part. The temperature of the outer peripheral part rises due to heat conduction from the inner peripheral part, and both the inner peripheral part and the outer peripheral part have high temperatures. This is due to the fact that This is clearly due to the fact that the inner periphery is made of cordierite and the outer periphery is made of SiC.

  The exhaust gas purification filter of the present invention is used in exhaust gas containing PM, such as exhaust from a diesel engine and exhaust from a boiler. If a catalyst layer containing a porous oxide and a noble metal is formed on the surface of the pores of the cell partition walls, PM can be burned and removed at the same time or immediately after collection, so that no special regeneration treatment is required. And is particularly useful.

It is a perspective view of the cell unit formed in one example of the present invention. In one Example of this invention, it is explanatory drawing which shows the arrangement | positioning state at the time of joining cell units. It is explanatory drawing which shows the end surface of the filter of one Example of this invention in the state before and behind cutting. It is explanatory drawing which shows the end surface of the filter of the 2nd Example of this invention in the state before and behind cutting. It is explanatory drawing which shows the end surface of the filter of the comparative example 1 in the state before and behind cutting. It is explanatory drawing which shows the temperature measurement position of a filter. It is a graph which shows the temperature distribution in each measurement position.

Explanation of symbols

1: Cordierite cell unit 2: SiC cell unit 3: Outer cylinder

Claims (4)

An inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, the inflow side cell and the outflow side cell are partitioned, and a large number of pores are formed. A honeycomb-shaped exhaust gas purification filter having a porous cell partition wall,
An exhaust gas purification filter, wherein the outer peripheral cell is made of a material having a higher thermal conductivity than the inner peripheral cell.   2. The exhaust gas purification filter according to claim 1, wherein the inner peripheral cell is made of cordierite, and the outer peripheral cell is made of at least one selected from silicon carbide and silicon nitride.   The exhaust gas purification filter according to claim 1, wherein the cells in the outer peripheral portion occupy a range of 30 to 90% of the total number of cells.   The exhaust gas purification filter according to claim 1, wherein a catalyst layer containing a porous oxide and a noble metal is provided on the surface of the pores of the cell partition wall.

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