Patent Publication Number: US-2022226763-A1

Title: Exhaust emission control filter

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application Nos. 2021-006575 and 2022-003375, respectively filed on 19 Jan. 2021 and 12 Jan. 2022, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an exhaust emission control filter including an exhaust conversion catalyst. 
     Related Art 
     Conventionally, among gasoline engines mounted on automobiles, adoption of gasoline direct injection engines has been advanced in view of combustion efficiency improvement and the like. Unfortunately, the gasoline direct injection engines emit more particulates, such as PM, than port injection (PI) engines. Accordingly, accompanied by tightening of emission regulations in recent years (PM emission regulation, PN (the number of exhaust particulates) regulation), discussion on a technique of providing an exhaust path of a gasoline engine with an exhaust emission control filter (a gasoline particulate filter; hereinafter called “GPF”) for capturing particulates has been advanced. 
     The exhaust path of a gasoline engine is provided with a three-way catalyst (hereinafter called “TWC”), which removes CO, HC and NOx contained in exhaust, in a state of being held by a honeycomb holder. In particular, in recent years, multiple TWCs are arranged in series on the exhaust path in order to satisfy the performance required for catalytic conversion. Accordingly, it is not preferable to newly provide a GPF on the exhaust path in addition to these TWCs, in view of pressure loss and cost. 
     Accordingly, a technique that causes the GPF to hold the TWCs and provides not only the particulate matter capturing performance but also a three-way conversion function has been proposed (for example, see Patent Document 1).
     Patent Document 1: Japanese Unexamined Patent   Application, Publication No. 2017-082745   

     SUMMARY OF THE INVENTION 
     However, achievement of a desired particulate matter capturing performance requires use of a filter base material that is included in the GPF and has a small gas pore diameter. 
     In this case, there is a problem in that the pressure loss increases and causes reduction in output. This problem is further significant because the longer the traveling distance is, the more particulates, such as ash derived from oil, are captured by the exhaust emission control filter. 
     There is a method of improving the particulate matter capturing performance by coating the catalyst. However, conventionally, holding of the catalyst on a filter base material having a typical gas pore diameter has a restriction on the amount of catalyst held by GPF in view of pressure loss described above. Accordingly, the exhaust conversion performance has not been expected in comparison with the conventional TWCs. That is, the pressure loss, the exhaust conversion performance and the particulate matter capturing performance have a tradeoff relationship. 
     The present invention has been made in view of the above description, and has an object to provide an exhaust emission control filter that can reduce the pressure loss and has a high exhaust conversion performance and particulate matter capturing performance. 
     (1) To achieve the object described above, the present invention provides an exhaust emission control filter (e.g., a GPF  32  described later) provided on an exhaust path (e.g., exhaust pipe  3  described later) of an internal combustion engine (e.g., an engine  1  described later) and capturing and removing particulates in exhaust of the internal combustion engine. The exhaust emission control filter includes: a filter base material (e.g., a filter base material  320  described later) which includes a plurality of cells extending from an inlet end face of exhaust to an outlet end face and is partitioned by porous partitions (e.g., partitions  323  described later) and in which inlet cells with an opening at the inlet end face being sealed, and outlet cells with an opening at the outlet end face being sealed are arranged alternately; and an exhaust conversion catalyst (e.g., TWC  33  described later) carried on the partitions. The filter base material has a median gas pore diameter (D50) of 17 μm or more after the exhaust conversion catalyst is carried on the filter base material, a half width of a gas pore distribution of the filter base material ranges from 7 to 15 μm, the exhaust conversion catalyst is ununiformly carried in a high-density layer (e.g., a high-density layer  331  described later) having a relatively high density of the exhaust conversion catalyst and a low-density layer (e.g., a low-density layer  332  described later) having a relatively low density of the exhaust conversion catalyst, and the high-density layer has a maximum gas pore diameter of 11.7 μm or less. 
     According to the aspect (1), in the exhaust emission control filter that includes what is called a wall flow type filter base material, and the exhaust conversion catalyst carried on the material, the median gas pore diameter (D50) of the filter base material after the exhaust conversion catalyst is carried is 17 μm or more, which is relatively large. The exhaust conversion catalyst is carried on the filter base material ununiformly in the layer with a relatively high density and in the layer with a relatively low density. 
     According to the aspect (1), some of partitions in the thickness direction where relatively large gas pore diameters are secured after the exhaust conversion catalyst is carried include the high-density layer where exhaust conversion catalyst is arranged in a layered manner at high density. Accordingly, the exhaust flow path is sufficiently secured, and the uniformity of the exhaust flow is secured. As a result, increase in pressure loss can be suppressed in an acceptable range. Here, the present applicant has found out that there is a correlation between the initial increase in pressure loss due to particulates and the increase in pressure loss after particulate matter deposition. That is, if the increase in pressure loss due to particulates can be suppressed, the increase in pressure loss after particulate matter deposition can be reduced. In view of this point, the effect of suppressing increase in pressure loss by the aspect (1) described above is exerted from the initial stage. Accordingly, the aspect (1) can reduce the increase in pressure loss after particulate matter deposition. 
     According to the aspect (1), the high-density layer where exhaust conversion catalyst is arranged in a layered manner at high density at some part of partitions  323  in the thickness direction is included, and the maximum gas pore diameter of the high-density layer is 11.7 μm or less, which is relatively small. Consequently, exhaust surely passes through the flow paths narrowed by the exhaust conversion catalyst arranged at the high density. Accordingly, a high particulate matter capturing performance and a high exhaust conversion performance can be achieved. According to the aspect (1), the increase in pressure loss due to particulates can be suppressed, and the pressure loss after particulate matter deposition can be reduced. In turn, the pressure loss can be reduced without limiting the amount of carrying the exhaust conversion catalyst. Accordingly, the pressure loss can be reduced, and the exhaust emission control filter that has a high exhaust conversion performance and particulate matter capturing performance can be provided. 
     Furthermore, according to the aspect (1), the half width of the peak of the gas pore distribution of the filter base material ranges from 7 to 15 μm. That is, in the exhaust emission control filter according to the aspect (1), the gas pore diameter is large, and the half width of the gas pore distribution is narrow. Accordingly, when the exhaust conversion catalyst is carried on the filter base material, blockage of the gas pores by the slurry that contains the exhaust conversion catalyst flowing preferentially into gas pores having small gas pore diameters owing to the capillary action can be prevented. Accordingly, reduction of the exhaust flow paths in the partitions can be suppressed, and the exhaust emission control filter that can further suppress the increase in pressure loss even after the exhaust conversion catalyst is carried can be provided. Since the number of flow paths is large, the probability of contact between the exhaust containing particulates and the exhaust conversion catalyst is increased. Accordingly, the exhaust emission control filter that has a higher exhaust conversion performance and particulate matter capturing performance can be provided. 
     (2) In the exhaust emission control filter according to (1), wherein the median gas pore diameter (D50) after the exhaust conversion catalyst is carried on the filter base material may be 20 μm or more. 
     In the aspect (2), the median gas pore diameter (DSO) of the filter base material after the exhaust conversion catalyst is carried is 20 μm or more. Accordingly, the increase in pressure loss can be further suppressed, and the effect of the aspect (1) can be more improved. 
     (3) In the exhaust emission control filter according to (1) or (2), the maximum gas pore diameter of the high-density layer may be 7.7 μm or less. 
     According to the aspect (3), the maximum gas pore diameter of the high-density layer may be 7.7 μm or less. Accordingly, a higher particulate matter capturing performance and a higher exhaust conversion performance can be achieved, which can further improve the effect of the invention (1). 
     (4) In the exhaust emission control filter according to any of (1) to (3), the half width of the gas pore distribution of the filter base material may range from 7 to 9 μm. 
     Furthermore, according to the aspect (4), the half width of the peak of the gas pore distribution of the filter base material before the exhaust conversion catalyst is carried ranges from 7 to 9 μm. Accordingly, the reduction in the exhaust flow paths in the partitions can be suppressed even after the exhaust conversion catalyst is carried, thereby further improving the advantageous effect of the aspect (1). 
     (5) In the exhaust emission control filter according to any of (1) to (4), the gas pore rate of the filter base material may range from 55% to 70%. 
     According to the aspect (5), the gas pore rate of the filter base material before the exhaust conversion catalyst is carried ranges from 55% to 70%. Accordingly, the exhaust flow paths can be more sufficiently secured, thereby further improving the advantageous effect of the aspect (1). 
     According to the present invention, an exhaust emission control filter can be provided that can reduce the pressure loss and has a high exhaust conversion performance and particulate matter capturing performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the configuration of an exhaust emission control device of an internal combustion engine according to one embodiment of the present invention; 
         FIG. 2  is a sectional view of a GPF according to the embodiment; 
         FIG. 3  is a sectional view of partitions of the GPF according to the embodiment; 
         FIG. 4  is a schematic sectional view showing an example of the structure of the partition of the GPF according to the embodiment; 
         FIG. 5  is a schematic sectional view showing another example of the structure of the partition of the GPF according to the embodiment; 
         FIG. 6  is a schematic sectional view showing still another example of the structure of the partition of the GPF according to the embodiment; 
         FIG. 7  shows measurement points of a perm porometer and a mercury porosimeter. 
         FIG. 8  shows the relationship between the median gas pore diameter and the initial pressure loss; 
         FIG. 9  shows the relationship between the maximum gas pore diameter of a high-density layer and the PN reduction rate; 
         FIG. 10  shows the relationship between the maximum gas pore diameter of a high-density layer and the CPI; 
         FIG. 11  shows the relationship between the PN collecting efficiency and the pressure loss after ash deposition in each of Examples and Comparative Examples; and 
         FIG. 12  shows the relationship between the CPI and the pressure loss after ash deposition in each of Examples and Comparative Examples. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
       FIG. 1  shows the configuration of an exhaust emission control device  2  of an internal combustion engine (hereinafter called “engine”)  1  according to this embodiment. The engine  1  is a direct injection type gasoline engine. As shown in  FIG. 1 , the exhaust emission control device  2  includes a TWC  31  and a GPF  32  as an exhaust emission control filter, which are provided in an order from the upstream of the exhaust pipe  3  through which exhaust flows. 
     The TWC  31  oxidizes or reduces HC to HO and CO 2 , CO to CO 2 , and NO x  to N 2  in exhaust, thereby achieving emission control. The TWC  31  may be what includes, for example, a carrier made of an oxide, such as of alumina, silica, zirconia, titania, ceria or zeolite, and a noble metal, such as Pd or Rh, carried as a catalytic metal by the carrier. Typically, the TWC  31  is carried on a honeycomb holder. 
     The TWC  31  includes an OSC material having an OSC capability. The OSC material may be not only CeO 2  but also a complex oxide of CeO 2  and ZrO 2  (hereinafter “CeZr complex oxide”) and the like is used. Among them, the CeZr complex oxide is preferably used because it has a high durability. Note that the catalytic metal may be carried on the OSC material. 
     The method of preparing the TWC  31  is not specifically limited. The conventionally publicly known slurry process or the like may be used for preparation. For example, after the slurry including the oxide, noble metal and OSC material described above is prepared, a honeycomb holder made of cordierite is coated with the prepared slurry, and is calcined, thereby achieving the preparation. 
     The GPF  32  captures and removes particulates in exhaust. Specifically, when exhaust passes through fine pores in partitions described later, the particulates are deposited on the surfaces of the partitions, thereby capturing the particulates. 
     Here, the particulates in this specification encompasses soot (carbon soot), soluble organic fractions (SOF), ash that is oil cinders, and particulates, such as PM. In recent years, emission regulations for these particulates have been tightened. Not only the regulation (PM regulation) for the gross emission weight of particulates (g/km, g/kW), but also, for example, the number of exhaust particulates having particle diameters 2.5 μm or less, such as PM 2.5 (PN regulation) are becoming effective. Meanwhile, the GPF  32  according to this embodiment can conform to these PM regulations and PN regulations. 
       FIG. 2  is a sectional view of the GPF  32  according to this embodiment. As shown in  FIG. 2 , the GPF  32  includes a filter base material  320 , and an exhaust conversion catalyst (TWC  33  in this embodiment) carried on the partitions  323  of the filter base material  320 . The filter base material  320  has, for example, a cylindrical shape elongated in the axial direction is made of a porous body, such as of cordierite, mullite, or silicon carbide (SiC). The filter base material  320  is provided with a plurality of cells extending from an inlet end face  32   a  to an outlet end face  32   b . These cells are partitioned by the partitions  323 . 
     The filter base material  320  includes inlet seals  324  that seal openings at the inlet end face  32   a . The cells with the openings at the inlet end face  32   a  being sealed by the inlet seals  324  are blocked at the inlet end while being opened at the outlet end, thus constituting outlet cells  322  that allow exhaust having passed through the partitions  323  to flow downstream. The inlet seals  324  are formed by injecting sealing cement from the inlet end face  32   a  of the filter base material  320  to achieve enclosure. 
     The filter base material  320  includes outlet seals  325  that seal openings at the outlet end face  32   b . The cells with the openings at the outlet end face  32   b  being sealed by the outlet seals  325  are blocked at the outlet end while being opened at the inlet end, thus constituting inlet cells  321  that allow exhaust flow thereinto from an exhaust pipe  3 . The outlet seals  325  are formed by injecting sealing cement from the outlet end face  32   b  of the filter base material  320  to achieve enclosure. 
     Note that the openings of the cells at the inlet end face  32   a  and the openings at the outlet end face  32   b  are alternately sealed, thereby alternately arranging the inlet cells  321  with the openings at the outlet end face  32   b  being sealed and the outlet cells  322  with the openings at the inlet end face  32   a  being sealed. More specifically, the inlet cells  321  and the outlet cells  322  are arranged adjacent to each other to form a lattice shape (in a checkered manner). 
     As indicated by an arrow in  FIG. 2 , exhaust flowing into the inlet cells  321 . flows from air flow layers into the partitions  323 , subsequently passes in the partitions  323  and flows to the outlet cells  322 . A side where the exhaust flows into the partitions  323  is the inlet side. A side where the exhaust flows from the partitions  323  is an outlet side. 
     The gas pore distribution of the filter base material  320  according to this embodiment is measured by a mercury porosimeter. The gas pore distribution is represented with the abscissa axis being the gas pore diameter (μm) and with the ordinate axis being Log differential gas pore volume distribution dV/d(log D) (ml/g). In this embodiment, the median gas pore diameter (D50) according to the volume standard of the filter base material  320  after the exhaust conversion catalyst is carried is 17 μm or more. The more preferable median gas pore diameter (D50) according to the volume standard of the filter base material  320  after the exhaust conversion catalyst is carried is 20 μm or more. 
     That is, the filter base material  320  of this embodiment has a median gas pore diameter of 17 μm or more, which is relatively large, even after the exhaust conversion catalyst is carried. Accordingly, the exhaust flow paths for flows into the partitions  323  can be sufficiently secured. In particular, as described later, according to this embodiment, the position of carrying the TWC  33  as the exhaust conversion catalyst is devised, which suppresses reduction (blockage) in the gas pore diameters of gas pores in the filter base material  320  due to the TWC  33 . Consequently, the exhaust flow paths are sufficiently secured. As a result, the pressure loss can be reduced. 
     Here, the half width of the gas pore distribution is an indicator that indicates the degree of sharpness of the peak of the gas pore distribution. In this embodiment, the half width of the gas pore distribution of the filter base material  320  before the exhaust conversion catalyst is carried ranges from 7 to 15 μm, which are narrow. The more preferable half width ranges from 7 to 9 μm. 
     That is, in the filter base material  320  in this embodiment, the gas pore diameter is large and the half width of the gas pore distribution is narrow before the exhaust conversion catalyst is carried. The half width thus ranges from 7 to 15 μm. Accordingly, when the TWC  33  is carried on the filter base material  320 , blockage of the gas pores by the slurry that contains the TWC  33  flowing preferentially into gas pores having small gas pore diameters owing to the capillary action can be prevented. Accordingly, reduction of the exhaust flow paths in the partitions  323  can be suppressed, and the GPF  32  that can further suppress the increase in pressure loss even after the exhaust conversion catalyst is carried can be provided. Since the number of flow paths is large, the probability of contact between the exhaust containing particulates and the TWC  33  is increased. Accordingly, a higher exhaust conversion performance and particulate matter capturing performance can be achieved. 
     Preferably, the gas pore rate of the filter base material  320  before the exhaust conversion catalyst is carried ranges from 55% to 70%. When the gas pore rate of the filter base material  320  before the exhaust conversion catalyst is carried ranges from 55% to 70%, the abrupt pressure loss when the TWC  33  is carried can be suppressed. 
     Preferably, the average gas pore diameter of the filter base material before the exhaust conversion catalyst is carried ranges from 20 to 30 μm. When the average gas pore diameter of the filter base material before the exhaust conversion catalyst is carried ranges from 20 to 30 μm, the median gas pore diameter of the filter base material  320  can be configured as 17 μm or more even after the exhaust conversion catalyst is carried. 
     Preferably, the thicknesses of the partitions  323  range from 5 to 15 mil. When the thicknesses of the partitions  323  range from 5 to 15 mil, the pressure loss can be reduced, and a high exhaust conversion performance and particulate matter capturing performance can be achieved. 
       FIG. 3  is a sectional view of the partition  323  of the GPF  32  according to the embodiment. In  FIG. 3 , hatched parts represent the filter base material  320 , white parts represent gas pores, and black parts represent TWC (three-way catalyst)  33  as the exhaust conversion catalyst. The upper side in  FIG. 3  indicates the inlet of the partition  323 , and the lower side indicates the outlet of the partition  323 . That is, the inlet of the partition  323  constitutes an inner wall surface of the inlet cell  321 , and the outlet of the partition  323  constitutes an inner wall surface of the outlet cell  322 . 
     At part of the partition  323  in the thickness direction, i.e., the inlet side of the partition  323  in the example shown in  FIG. 3 , a high-density layer  331  on which the TWC  33  is carried at a high density is arranged. As described above, in the GPF  32  in this embodiment, the TWC  33  is carried biasedly in the high-density layer  331  having a relatively high density and in the low-density layer  332  having a relatively low density. 
     As described above, the GPF  32  of this embodiment includes the high-density layer  331  where the TWC  33  is arranged in a layered manner at high density, at some of partitions  323  in the thickness direction where relatively large gas pore diameters of 20 μm or more. Accordingly, the exhaust flow paths are sufficiently secured, and the uniformity of the exhaust flow is secured. As a result, increase in pressure loss can be suppressed in an acceptable range. Here, the present applicant has found out that there is a correlation between the initial increase in pressure loss due to particulates and the increase in pressure loss after particulate matter deposition. That is, if the initial increase in pressure loss due to particulates can be suppressed, the increase in pressure loss after particulate matter deposition can be reduced. In view of this point, the effect of suppressing increase in pressure loss described above is exerted from the initial stage. Accordingly, this embodiment can reduce the increase in pressure loss after particulate matter deposition. 
     As shown in  FIG. 3 , in the GPF  32  in this embodiment, the gas pore diameters of the gas pores  34  in the high-density layer  331  are narrowed by the TWC  33  carried on the inner wall surfaces of the gas pores in comparison with those in the low-density layer  332 . Specifically, in this embodiment, the maximum gas pore diameter of the high-density layer  331  is 11.7 μm or less. More preferably, the maximum gas pore diameter of the high-density layer  331  is 7.7 μm or less. 
     The high-density layer  331  where the TWC  33  is arranged at part of the partition  323  in the thickness direction in a layered manner at high density is configured, and the maximum gas pore diameter of the high-density layer  331  is 11.7 μm or less, which is relatively small. Consequently, the relatively large gas pore diameters of 20 μm or more are secured as a whole, while partially, exhaust securely passes through the flow paths narrowed by the TWC  33  arranged at high density in the high-density layer  331 . Accordingly, a high particulate matter capturing performance and a high exhaust conversion performance can be achieved. 
     Consequently, according to this embodiment, the increase in pressure loss due to particulates can be suppressed, and the pressure loss after particulate matter deposition can be reduced. In turn, the pressure loss can be reduced without limiting the amount of carrying the TWC  33 . Accordingly, the pressure loss can be reduced, and a high exhaust conversion performance and particulate matter capturing performance can be achieved. 
       FIG. 4  is a schematic sectional view showing an example of the structure of the partition  323  of the GPF  32  according to the embodiment. More specifically, this diagram schematically shows the structure of the partition  323  of the GPF  32  shown in  FIG. 3 . As shown in  FIGS. 3 and 4 , the TWC  33  is carried on the inner wall surfaces of the gas pores  34  over the entire partition  323 . In particular, the TWC  33  is carried at high density at the part of the partition  323  closer to the inlet (high-density layer  331 ). Note that the arrangement of the high-density layer  331  is not limited thereto. The layer may be arranged at any part in the thickness direction of the partition  323 . 
       FIG. 5  is a schematic sectional view showing another example of the structure of the partition  323  of the GPF  32  according to this embodiment. In the example shown in  FIG. 5 , the high-density layer  331  where the TWC  33  is arranged in a layered manner at high density is arranged on the external surface of the partition  323  and adjacent thereto. More specifically, the high-density layer  331  is arranged on the external surface on the inlet side of the partition  323  and adjacent thereto. 
       FIG. 6  is a schematic sectional view showing another example of the structure of the partition  323  of the GPF  32  according to this embodiment. In the example shown in  FIG. 6 , the high-density layer  331  where the TWC  33  is arranged in a layered manner at high density is arranged at the substantially center in the thickness direction of the partition  323 . 
     Preferably, in each high-density layer  331  in each example described above, at least 50 percent by mass of TWC  33  in the entire TWC  33  carried on one partition  323  is allocated. Accordingly, each advantageous effect of this embodiment described above is more securely exerted. The pressure loss can be further reduced, and a higher exhaust conversion performance and particulate matter capturing performance can be achieved. 
     Similar to the TWC  31  described above, the TWC  33  oxidizes or reduces HC to H 2 O and CO 2 , CO to CO 2 , and NOx to N 2 , thereby achieving emission control. The TWC  33  may be what includes, for example, a carrier made of an oxide, such as of alumina, silica, zirconia, titania, ceria or zeolite, and a noble metal, such as Pd or Rh, carried as a catalytic metal by the carrier. 
     The TWC  33  contains an OSC material (oxygen absorption and release capacity material). The OSC material may be not only CeO 2  but also a complex oxide of CeO 2  and ZrO 2  (hereinafter “CeZr complex oxide”) and the like is used. Among them, the CeZr complex oxide is preferably used because it has a high durability. Note that the catalytic metal may be carried on the OSC material. To exert the catalytic action of the TWC described above simultaneously and effectively, it is preferable to keep the ratio of air to fuel (hereinafter called “air-fuel ratio”) around the stoichiometric ratio in a complete combustion reaction (hereinafter called “stoichiometry”). An OSC material having an oxygen storing and releasing capability of storing oxygen under an oxidizing atmosphere and releasing oxygen under a reducing atmosphere is used as a promotor together with the catalytic metal, thereby achieving a higher catalytic conversion performance. 
     The method of preparing the TWC  33  is not specifically limited. The conventionally publicly known slurry process or the like may be used for preparation. For example, after the slurry containing the oxide, the noble metal and the OSC material described above is prepared through milling, the filter base material  320  is coated with the prepared slurry, and is calcined, thereby achieving the preparation. 
     Preferably, the amount of wash coat of the TWC  33  having the configuration described above ranges from 30 to 150 g/L. When the amount of wash coat of the TWC  33  is in this range, a high catalytic conversion performance and particulate matter capturing performance can be achieved while reducing the increase in pressure loss. Note that in this embodiment, the TWC  33  may contain another noble metal, e.g., Pt, as a catalytic metal. 
     The GPF  32  according to this embodiment having the configuration described above is manufactured by a piston pushing up method, for example. The piston pushing up method fabricates the slurry containing a predetermined amounts of component materials of the TWC  33  through milling, and causes the filter base material  320  to carry the TWC  33  at a WC amount of 60 g/L according to the piston pushing up method with the inlet end face of the filter base material  320  serving as a slurry inlet. Subsequently, through drying and calcination, the GPF  32  is achieved. 
     An example of forming (arranging) the high-density layer  331  on the external surface of the filter base material  320  and adjacent thereto may be a method of impregnating the filter base material  320  with a slurry having a high viscosity, and setting the suction pressure low. There is another method of using a slurry where relatively large particles remain due to reduction in milling time period in slurry preparation. An example of forming (arranging) the high-density layer  331  in parts of the partition  323  on the inlet side and the outlet side may be a method of impregnating the filter base material  320  with a slurry having a high viscosity, and setting the suction pressure high. An example of forming (arranging) the high-density layer  331  in part of the filter base material  320  at the middle in the thickness direction may be a method of impregnating the filter base material  320  with a slurry having a low viscosity, and setting the suction time period short. 
     According to the GPF  32  in this embodiment manufactured as described above, the median gas pore diameter of the filter base material  320  after the TWC  33  described above is carried is measured by a mercury porosimeter. More specifically, the median gas pore diameter of the filter base material  320  after the TWC  33  is carried is the median gas pore diameter in an entire part P 1  indicated by chain lines in  FIGS. 3 to 6 . 
     In this embodiment, the maximum gas pore diameter in the high-density layer  331  is measured by the perm porometer. More specifically, the maximum gas pore diameter in the high-density layer  331  is the maximum gas pore diameter in part P 2  indicated by broken lines in  FIGS. 3 to 6 . 
     Here,  FIG. 7  shows measurement points of a perm porometer and a mercury porosimeter. In  FIG. 7 , the inlet of the GPF  32  described above is indicated as TOP, the middle part with a distance from the inlet in the inflow gas flow direction being T and with a distance from the outlet being T is indicated as MID, and the outlet is indicated as BTM. 
     Measurement of the maximum gas pore diameter in the high-density layer  331  using a perm porometer measures three points that are TOP, MID and BTM shown in  FIG. 7 , and adopts the average value thereof. Note that for example, when it is determined that the cell length is uniform due to EPMA or the like, the measured value at BTM may be adopted as a representative value. The perm porometer measures the through hole distribution of the partitions  323  according to the bubble point method. More specifically, the through hole distribution is measured from the pressure lost when the GPF  32  is immersed with alcohol and the gas pressure increased. A gas pore diameter distribution when the gas pores penetrating through the partition  323  is observed from the surface of the partition  323  at the inlet cells  321  and the surface of the partition at the outlet cells  322  is obtained. 
     Measurement of the median gas pore diameter of the filter base material  320  using a mercury porosimeter after the TWC  33  is carried measures the three points that are TOP, MID and BTM shown in  FIG. 7 , and adopts the average value thereof. The GPF  32  is immersed with mercury, and the pressure is changed and mercury infiltrates; based on the pressure at this time, the mercury porosimeter measures the gas pore diameter. More specifically, in the gas pore distribution, all the gas pores (including non-through pores) other than closed pores are considered and the gas pore diameters in the entire region from the surface of the partition at the inlet cells  321  to the surface of the partition at the outlet cells  322  are reflected. 
     Next, a result of simulation about the initial pressure loss, PN reduction rate, and honeycomb property of the GPF  32  according to this embodiment having the aforementioned configuration is described. Note that the simulation was performed by causing exhaust to flow in a model conforming to an actual case, in a manner similar to the actual case. 
       FIG. 8  shows the relationship between the median gas pore diameter and the initial pressure loss. As shown in  FIG. 8 , it is understood that when the median gas pore diameter of the filter base material  320  after the TWC  33  is carried is 17 μm or more, the initial pressure loss can be sufficiently reduced. As described above, there is a correlation between the initial increase in pressure loss due to particulates and the increase in pressure loss after particulate matter deposition. Accordingly, in this embodiment, the effect of suppressing increase in pressure loss is exerted from the initial stage. Consequently, it can be said that the increase in pressure loss after particulate matter deposition can be reduced. 
       FIG. 9  shows the relationship between the maximum gas pore diameter of the high-density layer  331  and the PN reduction rate. As shown in  FIG. 9 , it is understood that when the maximum gas pore diameter of the high-density layer  331  is 11.7 μm or less, a PN reduction rate exceeding 801 can be achieved. 
       FIG. 10  shows the relationship between the maximum gas pore diameter of the high-density layer  331  and the CPI. Here the CPI (Coat Performance Index) is obtained by dividing the NOx conversion efficiency of GPF by the NOx conversion efficiency of TWC carried on a typical honeycomb carrier (without sealing), and is a NOx conversion indicator for TWC. 
     As shown in  FIG. 10 , it is understood that even when the maximum gas pore diameter of the high-density layer  331  is 11.7 μm or less, a sufficient conversion performance is achieved. 
     The present invention is not limited to the embodiment described above. Modification and improvement in a range capable of achieving the object of the present invention can be encompassed in the present invention. In the embodiment described above, the exhaust emission control filter according to the present invention is applied to GPF. However, there is no limitation thereto. The exhaust emission control filter according to the present invention may be applied to DPF. In this case, the exhaust conversion catalyst is not limited to TWC. Another exhaust conversion catalyst may be used. For example, an oxidation catalyst, such as a PM combustion catalyst may be used. 
     EXAMPLES 
     Next, Examples of the present invention are described. However, the present invention is not limited to these Examples. 
     Examples 1 to 4, Comparative Examples 1 to 7 
     First, nitrite Pd and nitrite Rh solutions and Al 2 O 3  carrier (commercially available γ-alumina) were put into an evaporator, and the Al 2 O 3  carrier is impregnated with Pd and Rh at a mass ratio of 6/1. Next, after drying, calcination was performed at 600° C., and Pd—Rh/Al 2 O 3  catalyst was achieved. Likewise, nitrite Pd, nitrite Rh and CeO 2  were prepared, and Pd—Rh/CeO 2  catalyst was achieved. In each case, the noble metal carrying amounts were 1.51 percent by mass of Pd and 0.25 percent by mass of Rh. Note that the filter base material (carrier) used herein had a size of ϕ118.4×91 mm and 1 L. The average gas pore diameter of the filter base material used herein ranged from 20 to 30 μm. The half width of the gas pore distribution ranged from 7 to 15 μm. The gas pore rate ranged from 55 to 70%. The wall thickness ranged from 5 to 15 mil. The catalyst carrying amount ranged from 30 to 150 g/L. 
     Next, the same amounts of Pd—Rh/Al 2 O 3  catalyst and Pd—Rh/CeO 2  catalyst were mixed with each other, water and binder were additionally mixed, and milling was performed by a ball mill, thus preparing the slurry. In each of Examples and Comparative Examples, the slurry viscosity was adjusted, and the slurry suction pressure in the catalyst carrying step was adjusted, thereby arranging the high-density layer of the exhaust conversion catalyst on the inlet side as shown in  FIGS. 3 and 4 . Lastly, drying was performed at 150° C. with an air flow, and calcination was performed at 600° C., thereby obtaining each GPF. Table 1 shows the median gas pore diameter (μm) of the filter base material after the exhaust conversion catalyst is carried, and the maximum gas pore diameter (μm) of the high-density layer. 
     [Actual Vehicle Particulate Matter Collecting Test] 
     For each of the GPFs according to Examples and Comparative Examples, the GPF to be tested was mounted after 1 L three-way catalytic converter below a gasoline direct injection engine with a displacement of 1.5 L. Under a condition of a room temperature of 25° C. and a humidity of 50%, a WLTP mode drive was performed, the numbers of PMs (PN) before and after GPF in this case were measured, and the number of PMs (PN) collecting efficiency was calculated. For the measurement, as a preprocess, one-cycle WLTP drive was performed, the remaining particulates were removed by the GPF, subsequently, soaking was performed for 24 hours at a room temperature of 25° C., and measurement was performed from a cold state and data was obtained. 
     [Pressure Loss Test after Ash Endurance] 
     For each of the GPFs according to Examples and Comparative Examples, a durability test using calcium sulfate as mock ash was performed. Specifically, first calcium sulfate was calcined, and subsequently milling was performed until particle diameters close to those of actual ash were obtained. Next, a self-made aspirator (a large dry pump (design displacement of 1850 L/min.) was connected to a tank for vacuuming) was used, and aspiration of a predetermined amount of mock ash was performed through the filter base material, thereby simulating durability of actual drive. Ash deposition was set to 150 g. 
     [Pressure Loss] 
     The pressure loss of GPF according to each of Examples and Comparative Examples was measured using a catalyst carrier pressure loss testing instrument made by Tsukubarikaseiki. Specifically, the GPF full-size (ϕ118.4×91 mm) was set, air was caused to flow at a flow rate of 2.17 m 3 /min (COLD FLOW), and the pressure loss was measured. 
     [Conversion Performance (CPT)] 
     For the exhaust conversion performance according to each of Examples and Comparative Examples, CPT (Coat Performance Index) was calculated. Here the CPI is obtained by dividing the NOx conversion efficiency of GPF by the NOx conversion efficiency of TWC carried on a typical honeycomb carrier (without sealing), and is a NOx conversion indicator of GPF for TWC. Specifically, aging was performed under an aging condition described later, and subsequently, through simulation measurement under a 400° C. stationary SV performance measurement condition, the NOx conversion efficiency of GPF and the NOx conversion efficiency of TWC carried by a typical honeycomb carrier (without sealing) (hereinafter called NOx conversion efficiency of TWC) were measured, and CPI was calculated by the following Expression (1). 
     (Aging Condition) 
       Rich/Air Aging(Rich:80 sec./Air:20 sec.) 
       H 2 O=10% 
       Rich:C 3 H 6 =1%,O 2 =2.5%,N 2 =balance gas 
       Air:O 2 =21%,N 2 =balance gas 
       980° C.×10 hours
 
     400° C. stationary SV performance measurement condition T/P size: ϕ1 inch×30 mm (BTM part on the outlet side in a case with sealing) 
       Gas flow rate:63→51→38→25 L/min.
 
       ( SV =250,000/ h →200,000/ h →150,000/ h →100,000/hours)
 
       Gas composition: CO   2 =14%,O 2 =0.48%.,C 3 H 6 =400 ppm,CO =5000 ppm ,H 2 =1700 ppm,NO =500 ppm ,H 2O =10%,N 2 =balance gas 
     [Expression 1] 
         CPI=NOx  conversion efficiency by GPF/NOx  conversion efficiency by TWC   Expression (1)
 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 MAXIMUM GAS 
               
               
                   
                 PRESSURE LOSS 
                   
                   
                   
                 PORE DIAMETER 
               
               
                   
                 AFTER ASH 
                   
                   
                 MEDIAN GAS 
                 OF HIGH- 
               
               
                   
                 DEPOSITION 
                 PN COLLECTING 
                   
                 PORE DIAMETER 
                 DENSITY LAYER 
               
               
                   
                 (kPa) 
                 EFFICIENCY 
                 CPI 
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 1.8 
                 1.0 
                 0.9 
                 20.4 
                 11.7 
               
               
                 Example 2 
                 2.0 
                 1.0 
                 1.0 
                 23.7 
                 9.0 
               
               
                 Example 3 
                 2.0 
                 1.0 
                 1.0 
                 17.0 
                 2.0 
               
               
                 Example 4 
                 1.8 
                 0.9 
                 0.9 
                 22.5 
                 7.7 
               
               
                 Comparative 
                 1.5 
                 0.9 
                 0.8 
                 18.0 
                 18.0 
               
               
                 Example 1 
               
               
                 Comparative 
                 4.4 
                 0.9 
                 0.9 
                 13.6 
                 13.6 
               
               
                 Example 2 
               
               
                 Comparative 
                 9.0 
                 0.9 
                 1.0 
                 11.7 
                 11.7 
               
               
                 Example 3 
               
               
                 Comparative 
                 8.7 
                 0.8 
                 0.9 
                 13.4 
                 13.4 
               
               
                 Example 4 
               
               
                 Comparative 
                 2.0 
                 0.7 
                 0.2 
                 26.7 
                 24.0 
               
               
                 Example 5 
               
               
                 Comparative 
                 1.1 
                 0.8 
                 0.1 
                 28.4 
                 28.4 
               
               
                 Example 6 
               
               
                 Comparative 
                 3.1 
                 0.6 
                 0.2 
                 25.8 
                 25.8 
               
               
                 Example 7 
               
               
                   
               
            
           
         
       
     
     Each numerical value in Table 1 is a value rounded to the first decimal place. 
     DISCUSSION 
       FIG. 11  shows the relationship between the PN collecting efficiency and the pressure loss after ash deposition in each of Examples and Comparative Examples. In  FIG. 11 , it has been confirmed that when a range of satisfying compatibility between the PN collecting efficiency that is a property required for GPF in an actual vehicle and the pressure loss after deposition of ash of 150 g is adopted so that the PN collecting efficiency of 90% or more and the pressure loss after ash deposition of 150 g is 2.0 kPa or less, only Examples 1 to 4 can achieve the compatibility. 
       FIG. 12  shows the relationship between the emission control CPI and the pressure loss after ash deposition in each of Examples and Comparative Examples. In  FIG. 12 , it has been confirmed that when a range of satisfying compatibility between the CPT and the pressure loss after deposition of ash of 150 g is adopted so that the CPI of 0.9 or more and the pressure loss after ash deposition of 150 g is 2.0 kPa or less, only Examples 1 to 4 can achieve the compatibility. 
     As the result described above, according to the Example where the median gas pore diameter (D50) after the exhaust conversion catalyst is carried on the filter base material was 17 μm or more, the half width of the gas pore distribution of the filter base material ranges from 7 to 15 μm, and the maximum gas pore diameter of the high-density layer is 11.7 μm or less, the pressure loss can be reduced, and a high exhaust conversion performance and particulate matter capturing performance can be achieved. Consequently, the advantageous effects exerted by the present invention have been proved. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           1  . . . Engine (internal combustion engine) 
           2  . . . Exhaust emission control device 
           3  . . . Exhaust pipe (exhaust path) 
           32  . . . GPF (exhaust emission control filter) 
           32   a  . . . Inlet end face 
           32   b  . . . Outlet end face 
           33  . . . TWC (exhaust conversion catalyst) 
           34  . . . Gas pores 
           320  . . . Filter base material 
           323  . . . Partition 
           321  . . . Inlet cell. 
           322  . . . Outlet cell 
           324  . . . Inlet seal 
           325  . . . Outlet seal 
           331  . . . High-density layer 
           332  . . . Low-density layer