Patent Application
20190091628
The present invention relates to an exhaust gas filter for purifying exhaust gas of an internal combustion engine.
An exhaust pipe of an internal combustion engine is provided with an exhaust gas purification device for trapping particulate matter (i.e., PM) contained in exhaust gas. The exhaust gas purification device is provided with an exhaust gas filter including, for example, cordierite, for trapping PM contained in the exhaust gas (see PTL 1). In order to purify toxic substances contained in the exhaust gas, the exhaust gas filter is coated with a noble metal catalyst and a promoter having oxygen storage capacity (i.e., OSC). The toxic substances include hydrocarbons, carbon monoxide, nitrogen oxides, and the like. The promoter is composed of a ceria-zirconia solid solution, etc.
[PTL 1] JP 2013-530332 A
However, when the exhaust gas filter is coated with a promoter, pores in cell walls may be closed by the promoter. This may lead to an increase in the pressure loss of the exhaust gas filter. For this reason, there is a limitation on the amount of the promoter that can be coated on the cell walls, and the oxygen storage capacity cannot be sufficiently increased. Moreover, when a promoter is coated, the weight of the exhaust gas filter increases, and the heat capacity thus increases. Consequently, the temperature increase performance decreases, thereby making the early activation of the exhaust gas filter difficult.
The present invention has been achieved in view of the above problems, and provides an exhaust gas filter having good oxygen storage capacity and temperature increase performance.
One embodiment of the present invention is an exhaust gas filter (1) includes: a plurality of cell walls (2), a plurality of cell holes (3) surrounded by the cell walls, and plug parts (4) each sealing one of both ends of at least a part of the cell holes, in which the cell walls each have pores (20) that allows adjacent cell holes to communicate with each other, and the cell walls contain at least one promoter (21) selected from the group consisting of ceria, zirconia, and a ceria-zirconia solid solution, as a constituent of the cell walls.
The numerals in parentheses are assigned for reference, and are not intended to limit the invention.
In the aforementioned exhaust gas filter, the cell walls have pores, and the cell walls themselves are composed of a promoter as a constituent, as described above. Accordingly, it is not necessary to separately coat the exhaust gas filter with a promoter. Therefore, an increase in the weight of the exhaust gas filter can be prevented, and an increase in the heat capacity can also be prevented. Consequently, the exhaust gas filter exhibits good temperature increase performance, making the early activation of the exhaust gas filter possible. Moreover, since it is not necessary to coat the exhaust gas filter with a promoter, there is no need to limit the amount of the promoter in order to prevent an increase in pressure loss. Accordingly, the promoter can sufficiently exhibit oxygen storage capacity, while preventing an increase in pressure loss. Therefore, the exhaust gas filter can exhibit good purification performance for exhaust gas.
Furthermore, the cell walls have pores, and the exhaust gas can pass through the pores in the cell walls. Accordingly, particulate matter (hereinafter referred to as “PM”) contained in the exhaust gas can be trapped in the cell walls. In addition, toxic components, such as hydrocarbons, carbon monoxide, and nitrogen oxides, contained in the exhaust gas can be sufficiently purified by the promoter contained in the cell walls. Further, the cell walls themselves have catalytic performance. Accordingly, even if not all the exhaust gas passes through the cell walls, a flow passing through the cell walls is formed as long as part of the exhaust gas passes through the cell walls; thus, good exhaust gas purification performance can be exhibited. Therefore, the exhaust gas filter can reduce PM emission and purify the exhaust gas, since the exhaust gas can pass through the cell walls, and the cell walls themselves can exhibit catalytic performance, as described above.
As described above, the aforementioned embodiment can provide an exhaust gas filter having good oxygen storage capacity and temperature increase performance.
An embodiment of the exhaust gas filter will be described with reference to
As shown in
As shown in
As shown in
As shown in
It is preferable that the cell walls 2 are composed of a material containing a ceria-zirconia solid solution as a main component, and further containing θ alumina and an inorganic binder. In this case, the cell walls 2 of the exhaust gas filter 1 can exhibit more good catalytic performance. In the cell walls 2, the inorganic binder 23 forms a matrix. The promoter 21 composed of ceria-zirconia, and the aggregate 22 composed of θ alumina are dispersed in the matrix. This can be confirmed, for example, by a scanning electron microscope (i.e., SEM). Further, pores 20 are formed, for example, between the promoters 21, between the aggregates 22, between the promoter 21 and the aggregate 22, between the promoter 21 and the inorganic binder 23, and between the aggregate 22 and the inorganic binder 23. These pores 20 allow the cell holes 3 adjacent to each other through the cell walls 2 to communicate with each other, and the cell walls 2 are made of porous materials. In the cell walls 2, the content of the promoter 21 based on 100 parts by mass of the total amount of the promoter 21 and the aggregate 22 can be set to, for example, an amount greater than 50 parts by mass.
Moreover, although the illustration is omitted, the cell walls 2 of the exhaust gas filter 1 may carry a noble metal catalyst. For the noble metal catalyst, at least one noble metal selected from Pt, Pd, Rh, etc., can be used. The noble metal catalyst functions as “three-way catalyst”, and purifies exhaust gas by oxidation or reduction of hydrocarbons, carbon monoxide, nitrogen oxides, etc.
As shown in
Next, the method for producing the exhaust gas filter 1 according to the present embodiment will be described. First, a promoter composed of a ceria-zirconia solid solution, an aggregate made of alumina, an inorganic binder raw material, and a pore-forming material are mixed. Examples of the inorganic binder raw material include sols of various inorganic binders, such as alumina sol and silica sol. Examples of the pore-forming material include organic materials, carbon, and the like that disappears during firing, which will be described later. The amount of the promoter mixed can be adjusted, for example, to an amount greater than 50 parts by mass based on 100 parts by mass of the total amount of the promoter and the aggregate.
Then, an organic binder, a molding assistant, water, etc., are added to the mixture and kneaded to obtain a green body. The green body is then molded into a honeycomb structure to obtain a molded body. Thereafter, the molded body is dried and fired, thereby obtaining an exhaust gas filter with a honeycomb structure. The exhaust gas filter with a honeycomb structure has many cells, and both ends of each cell open. The firing temperature is, for example, 700 to 1200° C., and the firing time is, for example, 2 to 50 hours.
Subsequently, plug parts 4 are formed in the exhaust gas filter in which both ends of the cells open. Specifically, a ceria-zirconia solid solution, water, an organic binder, etc., are first mixed to produce a clay-like plug part-forming material. Then, one of both ends of each cell hole is closed by the plug part-forming material. Subsequently, the plug part-forming material is fired in an electric furnace to form plug parts each closing one of both ends of the cell holes. The formation of the plug parts can be performed before the firing of the honeycomb structure, or the firing of the honeycomb structure and the firing of the plug parts may be performed at the same time. Moreover, the pattern of formation of the plug part-forming material can be suitably changed, and the plug parts can be formed in a desired pattern.
Thereafter, the exhaust gas filter obtained in the above manner can be allowed to carry a noble metal catalyst by a conventional method, for example. Specifically, for example, the exhaust gas filter is first immersed in an aqueous solution containing a noble metal salt. After the aqueous solution containing a noble metal salt is impregnated in the exhaust gas filter, the exhaust gas filter is dried. A repetition of the impregnation and drying process allows the exhaust gas filter to carry a desired amount of the noble metal salt. The exhaust gas filter is then heated, thereby obtaining an exhaust gas filter carrying a noble metal catalyst.
Next, the working effects of the exhaust gas filter 1 of the present embodiment are described. The exhaust gas filter 1 is used in such a manner that it is placed in an exhaust gas flow passage in order to purify exhaust gas generated in an internal combustion engine. Examples of internal combustion engines include diesel engines, gasoline engines, and the like. As shown in
In the exhaust gas filter 1 of the present embodiment, one of both ends 31 and 32 of the cell holes 3 is sealed with the plug part 4. The plug parts 4 alternately seal the upstream ends 31 or the downstream ends 32 of the adjacent cell holes 3. Therefore, a flow of the exhaust gas is easily formed; more specifically, the exhaust gas introduced into the upstream open cell holes 341 passes through the cell walls 2 and is discharged from the downstream open cell holes 342. That is, the exhaust gas can easily pass through the cell walls 2. Therefore, PM contained in the exhaust gas is easily trapped in the cell walls 2, and the catalyst contained in the cell walls 2 frequently contacts the exhaust gas. Accordingly, the exhaust gas filter 1 exhibits good exhaust gas purification performance, and can sufficiently purify the exhaust gas. The arrows in
In the exhaust gas filter 1, the cell walls 2 themselves include the promoter 21 as a constituent, as shown in
Thus, the exhaust gas filter 1 allows the exhaust gas to pass through the inside of the cell walls 2, and the cell walls 2 themselves can exhibit catalytic performance. Accordingly, the exhaust gas filter 1 can reduce PM emission and purify the exhaust gas.
It is not necessary to separately coat the exhaust gas filter 1 with the promoter 21, as described above. Thus, there is no need to limit the amount of the promoter 21 in order to prevent an increase in pressure loss. Accordingly, in the exhaust gas filter 1, the oxygen storage capacity of the promoter 21 in the cell walls 2 can be sufficiently exhibited, while preventing an increase in pressure loss. Therefore, the exhaust gas filter 1 can show good oxygen storage capacity and exhibit good purification performance for the exhaust gas.
In the exhaust gas filter 1, it is preferable that the plug parts 4 contain the promoter 21 as a constituent thereof. In this case, the promoter 21 contained not only in the cell walls 2, but also in the plug parts 4, can be used to purify the exhaust gas. Moreover, because the coefficient of thermal expansion of the cell walls 2 can be brought close to that of the plug parts 4, the occurrence of cracks, etc., can be prevented.
As described above, the present embodiment can provide the exhaust gas filter 1 that has excellent oxygen storage capacity and temperature increase performance.
Next, an embodiment of an exhaust gas filter that has open cell holes penetrating the exhaust gas filter in the axial direction will be described. As shown in
In the present embodiment, as shown in
Part of the exhaust gas introduced into the open cell holes 33 passes through the pores of the cell walls 2 and is discharged from the plugged cell holes 34. In this case, PM contained in the exhaust gas can be trapped in the cell walls 2. Moreover, the promoter contained in the cell walls 2 can sufficiently exhibit good oxygen storage capacity to purify the exhaust gas. Since the cell walls 2 themselves show catalytic performance, it is not necessary for all the exhaust gas to pass through the cell walls. Due to the formation of a flow of the exhaust gas passing through the cell walls, exhaust gas purification performance can be exhibited. Furthermore, due to the presence of the open cell holes 33, an increase in the pressure loss of the exhaust gas filter 1 can be sufficiently prevented.
Moreover, the cell holes 3 have the open cell holes 33, and the plug parts 4 are respectively disposed in the upstream ends 31 of the plugged cell holes 34. Accordingly, ash including calcium compounds, etc., contained in the exhaust gas together with PM can be discharged from the exhaust gas filter 1. Ash cannot be removed by combustion. Therefore, for example, in an exhaust gas filter provided with plug parts disposed in respective downstream ends 32 of plugged cell holes, ash remains and accumulates in the inside of the filter. In contrast, in the exhaust gas filter 1 of the present embodiment, the exhaust gas is separated by the cell walls 2 when passing through the cell walls 2, and ash remains in the open cell holes 33. Since the open cell holes 33 penetrate the exhaust gas filter 1 in the axial direction X, the ash can be easily discharged from the open cell holes 33, and the ash can be prevented from remaining in the exhaust gas filter 1. This can reduce a reduction in the purification performance of the exhaust gas filter 1.
Furthermore, as shown in
Next, an embodiment of an exhaust gas filter that has cell holes with an octagonal inner peripheral shape and cell holes with a square inner peripheral shape will be described. As shown in
The hydraulic diameter of each octagonal cell hole 3a is larger than the hydraulic diameter of each square cell hole 3b. In the exhaust gas filter 1, it is preferable that the octagonal cell holes 3a and the square cell holes 3b are alternately arranged. In this case, the difference between each hydraulic diameter of the octagonal cell hole 3a and each hydraulic diameter of the square cell hole 3b can be increased. Thereby, for example, when the octagonal cell holes 3a and the square cell holes 3b are suitably allocated as plugged cell holes 34 and open cell holes 33, respectively, each plugged cell holes 34 and each open cell holes 33 can be made adjacent. This arrangement can effectively increase the pressure difference between each plugged cell hole 34 and each open cell hole 33.
By taking advantage of this pressure difference, the exhaust gas flowing into the open cell holes 33 can be efficiently circulated to the plugged cell holes 34 through the pores. Moreover, the pressure difference between each open cell hole 33 and each plugged cell hole 34 is more reduced from upstream of the exhaust gas filter 1 toward downstream. However, the circulation of the exhaust gas into the pores is continued within the range in which a pressure difference occurs between each open cell hole 33 and each plugged cell hole 34. Accordingly, the exhaust gas can pass through the cell walls 2 in a broader range of the exhaust gas filter 1 by increasing the pressure difference between each open cell hole 33 and each plugged cell hole 34, as described above. PM contained in the exhaust gas can thereby be effectively trapped.
On the other hand, when the plugged cell holes 34 are adjacent to each other, or when the open cell holes 33 are adjacent to each other, it is difficult for a pressure difference to occur between the plugged cell holes 34 or between the open cell holes 33. Accordingly, there are few useful functions in terms of trapping performance. Moreover, the cell shape is preferably a shape with a large hydraulic diameter, in terms of the pressure loss of the exhaust gas filter 1. Therefore, cell holes 3 formed in a triangular shape, etc., are likely to cause an increase in the pressure loss of the exhaust gas filter 1. From the above viewpoint, the purification performance can be efficiently improved by forming the octagonal cell holes 3a and the square cell holes 3b in an alternate arrangement. In addition, the present embodiment has the same working effects as those of Embodiment 1.
In the exhaust gas filter 1 of the present embodiment, the square cell holes 3b were used as the open cell holes 33, and the octagonal cell holes 3a were used as the plugged cell holes 34. The open cell holes 33 and the plugged cell holes 34 are formed in an alternate arrangement; however, any shapes other than this shape may be employed. For example, as shown in
Note that the present invention is not limited to the embodiments described above, and can be applied to various embodiments within a range that does not depart from the gist of the invention. For example, a single cylindrical exhaust gas filter is used in each of the above-mentioned embodiments; however, a joined exhaust gas filter configured of a plurality of exhaust gas filters that are joined together can also be used. Specifically, for example, a plurality of exhaust gas filters in a square columnar shape, such as a rectangular parallelepiped shape, may be produced, and the produced exhaust gas filters may be integrated by joining them on their side surfaces.
Next, the oxygen storage capacity and temperature increase performance are compared between the Example and Comparative Examples of the exhaust gas filters. In the present experimental example, 3 types of exhaust gas filters of Example 1, Comparative Example 1, and Comparative Example 2 are evaluated. The exhaust gas filters all have a cylindrical shape, a diameter Φ of 103 mm, and a length L in the axial direction of 105 mm.
The exhaust gas filter of Example 1 has the same structure as that of Embodiment 1 described above. The cell walls themselves are made up of, as a constituent, a promoter made of a ceria-zirconia solid solution, and plug parts are respectively formed at the ends of the cells. The exhaust gas filter of Example 1 has a cell wall thickness of 8 mil and a cell number of 300 meshes. The term “mil” represents the thickness of the cell wall, and its unit is 1/1000 inch. Further, the term “mesh” represents the number of cells per square inch. Moreover, the cell walls carry a noble metal catalyst (specifically Pd). The total amount of the promoter and the noble metal catalyst in the exhaust gas filter of Example 1 is 300 g/L, as shown in Table 1, provided later.
Comparative Examples 1 and 2 are exhaust gas filters composed of cordierite. Comparative Example 1 is a straight flow-type exhaust gas filter in which no plug parts are formed at both ends of the cells, and both ends of each cell open. Comparative Example 2 is an exhaust gas filter in which plug parts composed of cordierite are formed at both ends of the cells, and the pattern of formation of the plug parts is the same as that of Example 1. Moreover, the cell walls of the exhaust gas filter of Comparative Example 2 have many pores, as in Example 1, and the exhaust gas can pass through the cell walls. The cell walls of the exhaust gas filters of Comparative Examples 1 and 2 carry a promoter and a noble metal catalyst, and these catalysts are carried after the production of the exhaust gas filters. The exhaust gas filters of Comparative Examples 1 and 2 are produced, for example, by a known method. The total amount of the promoter and the noble metal catalyst is 240 g/L in Comparative Example 1 and 100 g/L in Comparative Example 2, as shown in Table 1, provided later.
The exhaust gas filters of Example 1, Comparative Example 1, and Comparative Example 2 were each mounted in a gasoline engine exhaust system with a displacement of 2.5 liter. The temperature of the gas entering each exhaust gas filter was adjusted to about 600° C., and the air-fuel ratio A/F of the exhaust gas was adjusted to the theoretical air-fuel ratio, i.e., 14.6. In each exhaust gas filter, the side facing the exhaust gas flow is regarded as the upstream of the exhaust gas filter. The side opposite to the upstream side of the exhaust gas filter is regarded as the downstream side of the exhaust gas filter. Then, while monitoring the output of an O2 sensor, the air-fuel ratio was switched from the theoretical air-fuel ratio to the rich condition, i.e., 14.1, and to the lean condition, i.e., 1.51. The O2 sensor is disposed in the downstream of the exhaust gas filter in the flow direction of the exhaust gas. The oxygen storage amount of the exhaust gas filter was determined by measuring the output delay of the 02 sensor at the time of switching. Table 1 shows the results.
The exhaust gas filters of Example 1, Comparative Example 1, and Comparative Example 2 were each mounted in a gasoline engine exhaust system with a displacement of 2.5 liter. Each exhaust gas filter was disposed in a position apart from an engine exhaust manifold through a water-cooling pipe. The engine was driven at the theoretical air-fuel ratio, and the inlet temperature of each exhaust gas filter was adjusted to 100° C. by means of cooling water flowing through the inside of the water-cooling pipe. The term “inlet temperature” refers to the temperature of the upstream end of the exhaust gas filter in the flow direction of the exhaust gas, the upstream end facing the exhaust gas flow. Then, the flow rate of cooling water was controlled to thereby increase the inlet temperature of each exhaust gas filter, as shown in
As is known from Table 1, because the filter of Example 1 itself contained a promoter as a constituent, the amount of catalyst could be increased, and a higher oxygen storage amount was shown, compared with Comparative Examples 1 and 2. Comparatively, in Comparative Examples 1 and 2, in which the produced filter was used as a substrate, and a promoter and a noble metal catalyst were carried on the substrate, there is a limitation on the amount of the promoter in order to avoid the situation in which the pores in the cell walls, which serve as the flow passage of the exhaust gas, are buried and closed by the promoter, etc. In particular, in Comparative Example 2, in which plug parts are formed at the ends of the cells, there is a tendency that pressure loss significantly increases because the catalysts are carried; thus, the limit value of the amount of the promoter carried decreases, as shown in Table 1.
Moreover, the temperature increase performance of Comparative Example 2 is low, as is known from
In the present experimental example, an exhaust gas filter with the same plug part formation pattern as that of Embodiment 1 shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-254097 | Dec 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/083879 | 11/16/2016 | WO | 00 |
