Particulate filter

Abstract

Provided is a particulate filter capable of achieving both PM collection performance and pressure loss suppression performance at a high level. The particulate filter (1) collects particulate matter (PM) in exhaust gas. A particulate filter (1) is provided with: a base material (10) having an inlet-side cell (12) that opens only at the end (12a) of the exhaust gas inflow side, an outlet-side cell (14) that opens only at the end (14b) of the exhaust gas outflow side, and a porous partition wall (16) that partitions the inlet-side cell (12) and the outlet-side cell (14); and an outer coating layer (20) formed on the entry surface (16a) of the partition wall (16). The D90 pore diameter of the pores (22) of the outer coating layer (20) is 1 [mu] m or more and 10 [mu] m or less. In addition, the thickness (t1) of the pores (22) of the outer coating layer (20) is 20 [mu] m or more and 80 [mu] m or less, and the porosity (Pcoat) is 30% or more and 70% or less. By means of the outer coating layer (20) of this configuration, particulate matter (PM) can be suitably collected while suitably suppressing an increase in pressure loss.

Description

Technical Field

[0001] The technology disclosed herein relates to a particulate filter. In addition, this international application claims priority based on Japanese Patent Application No. 2022-141708 filed on September 6, 2022, the entire contents of which are incorporated herein by reference. Background Art

[0002] Exhaust gas from internal combustion engines such as automobile engines contains hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO x ) and other harmful gas components, it also contains particulate matter (PM) with carbon as the main component. There is a concern that the particulate matter will have the same impact on the human body and the environment as the harmful gas components. Therefore, a particulate filter is configured in the exhaust system of the internal combustion engine to remove the particulate matter.

[0003] For example, a wall-flow type substrate is used in a particulate filter. The wall-flow type substrate has an inlet compartment (channel / cell / unit) with only an end opening on the exhaust gas inlet side, an outlet compartment with only an end opening on the exhaust gas outlet side, and a porous partition wall separating the two compartments. The exhaust gas supplied to the wall-flow type substrate flows into the inlet compartment and passes through the partition wall before being discharged from the outlet compartment. At this time, particulate matter in the exhaust gas is captured by the partition wall.

[0004] In addition, in such a particulate filter, a coating is sometimes formed on the partition wall of the substrate. This can improve the PM capture performance. For example, in the filter described in Patent Document 1, a carrier coating (washcoat layer / elution coating / catalyst supporting layer) (external coating) is formed to cover the surface of the partition wall. A plurality of pores through which exhaust gas can pass are formed in the external coating. When the exhaust gas passes through the pores, the particulate matter in the exhaust gas is captured by the external coating. For example, in the external coating of Patent Document 1, pores with an average inter-particle pore size (average pore size) of 5.0 nm to 5.0 μm are formed.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Publication No. 2012-518753 Summary of the invention

[0008] Technical problem to be solved by the invention

[0009] However, in recent years, as concerns about the environment have further increased, particulate filters with better PM collection performance have been required. On the other hand, if the average pore size of the outer coating layer is reduced in order to improve the PM collection performance, there is a possibility that the pressure loss will increase due to the reduction in gas flowability. That is, in the existing particulate filters, it is difficult to achieve both high levels of PM collection performance and pressure loss suppression performance.

[0010] The technology disclosed herein has been accomplished in view of the above circumstances, and a main object of the technology is to provide a particulate filter capable of achieving both PM trapping performance and pressure loss suppression performance at a high level.

[0011] Technical solutions to solve problems

[0012] In order to achieve the above object, a particulate filter having the following structure is provided.

[0013] The particulate filter (1) disclosed herein captures particulate matter in the exhaust gas discharged from an internal combustion engine. The particulate filter comprises at least: a wall-flow type substrate having an inlet compartment with only an end opening on the exhaust gas inlet side, an outlet compartment with only an end opening on the exhaust gas outlet side, and a porous partition wall separating the inlet compartment from the outlet compartment; and an outer coating layer, which is a porous layer formed by a plurality of granular bodies and is formed on the surface of the partition wall that is in contact with the inlet compartment. Furthermore, in the particulate filter disclosed herein, the D of the pores of the outer coating layer in the pore size distribution based on the volume basis of the mercury intrusion method is 90 The pore diameter is 1 μm or more and 10 μm or less. In cross-sectional SEM observation, the thickness of the outer coating layer is 20 μm or more and 80 μm or less, and the porosity (air void ratio) is 30% or more and 70% or less.

[0014] First, as shown in Patent Document 1, the size of the pores in the outer coating layer is generally determined by the average pore diameter, D 50 However, among the multiple pores formed in the outer coating, the exhaust gas permeation resistance of the pores with large pore diameters (large pores) is low, so the gas flow rate is larger than that of other pores. Therefore, in the large pores, the penetration of particulate matter through the outer coating without contacting the pore wall surface is extremely increased. Therefore, by setting the pore diameter (D 90 The pore diameter of the outer coating layer is limited to a certain value or less, which can particularly appropriately prevent the penetration of PM in the outer coating layer. From this point of view, in the particulate filter with the above structure, the pore diameter of the outer coating layer is limited to a certain value or less. 90 The pore size is limited to less than 10 μm. On the other hand, unlike the average pore size, D 50 The pore size is not as significant, but when the external coating D 90As the pore diameter decreases, there is a tendency that the pressure loss when PM is easily accumulated tends to increase. Therefore, in the particulate filter with the above structure, the pore size D of the outer coating layer is 90 The pore diameter is set to be 1 μm or more.

[0015] Next, if the flow distance of the particulate matter in the outer coating is extended, the chance of the particulate matter contacting the wall surface of the pores increases. As a result, it is easy to capture the particulate matter inside the outer coating. From this point of view, in the particulate filter of the above structure, the thickness of the outer coating is specified to be 20 μm or more. On the other hand, if the outer coating is too thick, the air permeability of the outer coating may be reduced. Therefore, in the particulate filter of the above structure, the thickness of the outer coating is specified to be 80 μm or less.

[0016] Furthermore, when the porosity of the external coating is too low, the pores of the external coating are easily clogged due to the accumulation of particulate matter. In addition, the effective area of ​​the wall of the pores of the external coating with low porosity (small number of pores formed) becomes narrower, so the PM capture performance is reduced. Furthermore, when the porosity of the external coating becomes lower, the linear velocity of the exhaust gas passing through the pores increases, so the possibility of particulate matter penetrating the external coating becomes higher. From these viewpoints, in the particulate filter of the above structure, the porosity of the external coating is set to 30% or more. On the other hand, when the porosity becomes too high, the mechanical strength of the external coating is reduced, so there is a possibility of damage and peeling caused by the inflow of exhaust gas. Therefore, in the particulate filter of the above structure, the porosity of the external coating is set to 70% or less.

[0017] As described above, according to the particulate filter disclosed herein, it is possible to achieve both PM trapping performance and pressure loss suppression performance at a high level.

[0018] In the particulate filter (2) disclosed herein, in the particulate filter (1), the flow path length of the pores in the 100 μm measurement area of ​​the outer coating along the extension direction is 200 μm or more. This can further improve the PM trapping performance inside the outer coating.

[0019] The particulate filter (3) disclosed herein is a particulate filter (1) or (2) in which the outer coating layer has a volume-based pore size distribution of D based on mercury intrusion porosimetry. 50 The pore diameter is 0.8 μm or more and 3 μm or less. This makes it possible to achieve both PM collection performance and pressure loss suppression performance at a higher level.

[0020] The particulate filter (4) disclosed herein is a particulate filter (1) to (3) wherein D in the particle size distribution based on the number of particles in the outer coating layer determined by a laser diffraction light scattering method is 50The particle size is X, and D in the pore distribution based on the volume of the voids in the partition wall by mercury intrusion porosimetry is 50 When the pore diameter is Y, the following formula (1) is satisfied: Thus, an outer coating layer can be easily formed on the entrance side surface of the partition wall.

[0021] 0.1≤X / Y≤3(1)

[0022] The particulate filter (5) disclosed herein is any one of the particulate filters (1) to (4), wherein the granular body contains at least one selected from the group consisting of aluminum oxide, cerium oxide, zirconium oxide, silicon dioxide, magnesium oxide, and calcium oxide. This can improve the heat resistance of the outer coating layer.

[0023] The particulate filter (6) disclosed herein is any one of the particulate filters (1) to (5) above, wherein the particulate body contains a precious metal catalyst for purifying harmful gas components in the exhaust gas. Thus, a particulate filter having a purification performance for harmful gas components such as NOx can be realized.

[0024] The particulate filter (7) disclosed herein is any one of the particulate filters (1) to (6) described above, further comprising an inner coating layer formed on the wall surface of the gap in a predetermined area from the surface of the partition wall in contact with the outlet compartment toward the inlet compartment, and containing a precious metal catalyst for purifying harmful gas components in the exhaust gas. Thus, a particulate filter having excellent gas purification performance can be realized.

[0025] The particulate filter (8) disclosed herein is a particulate filter (7) in which an uncoated region substantially free of coating is provided between the outer coating and the inner coating in the thickness direction of the partition wall, thereby achieving both high-level gas purification performance and pressure loss suppression performance.

[0026] In the particulate filter (9) disclosed herein, in the particulate filter (8), the size of the uncoated region in the thickness direction of the partition wall is 10 μm or more, thereby achieving both gas purification performance and pressure loss suppression performance at a higher level.

[0027] Regarding the particulate filter (10) disclosed herein, in any of the above-mentioned particulate filters (1) to (9), the internal combustion engine is a gasoline engine. The exhaust gas generated in the gasoline engine is higher in temperature than the exhaust gas discharged from other engines (diesel engines, etc.), so the particulate matter captured by the outer coating and the partition wall can be easily burned. From this point of view, the particulate filter disclosed herein can be particularly suitable for use as a particulate filter (GPF: Gasoline Particulate Filter) for gasoline engines. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 FIG. 1 is a diagram schematically showing an exhaust system in which a particulate filter is arranged.

[0029] Figure 2 It is a perspective view schematically showing the particulate filter according to the first embodiment.

[0030] Figure 3 It is a diagram schematically showing a cross section along the axial direction of the particulate filter according to the first embodiment.

[0031] Figure 4 This is an enlarged cross-sectional view schematically showing a cross section of a partition wall of the particulate filter according to the first embodiment.

[0032] Figure 5 This is a graph showing the pore size distribution of the coating peeling sample A and the coating remaining sample B by mercury intrusion porosimetry.

[0033] Figure 6 It is a diagram schematically showing a cross section along the axial direction of the particulate filter according to the second embodiment.

[0034] Figure 7 It is an enlarged cross-sectional view schematically showing a cross section of a partition wall of the particulate filter according to the second embodiment. DETAILED DESCRIPTION

[0035] Hereinafter, preferred embodiments of the technology disclosed herein will be described based on the accompanying drawings. In addition, contents other than matters specifically mentioned in this specification and contents required for the implementation of the technology disclosed herein (for example, the detailed structure of the exhaust path, etc.) can be grasped as design matters for those skilled in the art based on the prior art in this field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in this field. In addition, in this specification, the expression "A to B" indicating a numerical range means "above A and below B", including "greater than A and less than B".

[0036] [Exhaust system of internal combustion engine]

[0037] First, the use of the particulate filter disclosed herein will be described. Figure 1 FIG. 1 is a diagram schematically showing an exhaust system in which a particulate filter is arranged.

[0038] Figure 1 The reference numeral 2 in the figure is an internal combustion engine. The internal combustion engine 2 generates kinetic energy by burning a mixed gas containing oxygen and fuel gas. In addition, the exhaust gas generated by the combustion of the mixed gas contains harmful gas components (HC, CO, NO xThe exhaust gas is discharged to the exhaust system composed of the exhaust manifold 3 and the exhaust pipe 4. In addition, in this specification, the direction in which the exhaust gas discharged from the internal combustion engine 2 flows is referred to as the "exhaust gas flow direction D". In addition, the side close to the internal combustion engine 2 in the exhaust gas flow direction D is referred to as the "upstream", and the side away from the internal combustion engine 2 is referred to as the "downstream".

[0039] Next, a sensor 8 is installed in the exhaust pipe 4. The sensor 8 detects information related to the composition and temperature of the exhaust gas. In addition, the sensor 8 is connected to the engine control unit (ECU: Engine Control Unit) 7. The information detected by the sensor 8 is sent to the ECU 7. When controlling the operation of the internal combustion engine 2, the ECU 7 appropriately refers to the detection result of the sensor 8.

[0040] Furthermore, in the exhaust system of the internal combustion engine 2 (in Figure 1 The exhaust pipe 4 is provided with a particulate filter 1, an exhaust gas purification catalyst 5 and an underfloor catalyst 9. The exhaust gas purification catalyst 5 and the underfloor catalyst 9 purify harmful gas components in the exhaust gas. In addition, the detailed structure of the exhaust gas purification catalyst 5 and the underfloor catalyst 9 does not give characteristics to the technology disclosed herein, so the detailed description is omitted.

[0041] On the other hand, the particulate filter 1 captures particulate matter in the exhaust gas discharged from the internal combustion engine 2. The detailed structure of the particulate filter 1 will be described later. In addition, the particulate filter disclosed herein can be used in various internal combustion engines. That is, the particulate filter can be a GPF (Gasoline Particulate Filter) that takes the exhaust gas discharged from a gasoline engine as a target, or it can be a DPF (Diesel Particulate Filter) that takes the exhaust gas discharged from a diesel engine as a target. In addition, the exhaust gas discharged from a gasoline engine is at a higher temperature than the exhaust gas discharged from other types of internal combustion engines. Therefore, the particulate matter captured inside the particulate filter can be easily burned. The particulate filter disclosed herein has excellent PM capture performance, and therefore can be particularly suitable for use in a GPF that can easily remove the captured particulate matter.

[0042] in addition, Figure 1 The particulate filter 1 shown is arranged upstream of the exhaust gas purification catalyst 5 and the bottom catalyst 9. However, the arrangement position of the particulate filter 1 in the exhaust system is not limited to the technology disclosed herein. For example, the particulate filter may be arranged between the exhaust gas purification catalyst and the bottom catalyst.

[0043] [First embodiment]

[0044] Next, a first embodiment of the particulate filter disclosed herein will be described. Figure 2 It is a perspective view schematically showing the particulate filter according to the first embodiment. Figure 3 : is a diagram schematically showing a cross section of the particulate filter according to the first embodiment along the axial direction. Figure 4 It is an enlarged cross-sectional view schematically showing a cross section of a partition wall of the particulate filter according to the first embodiment. In each figure, the symbol X indicates the “extending direction of the partition wall”, and the symbol Y indicates the “thickness direction of the partition wall”.

[0045] A. Structure of the particulate filter

[0046] like Figure 2 to Figure 4 As shown, the particulate filter 1 of the present embodiment includes a substrate 10 and an outer coating layer 20. Hereinafter, each structure will be described.

[0047] 1. Substrate

[0048] The substrate 10 is a member that serves as the skeleton of the particulate filter 1 . Figure 2 The substrate 10 shown is a cylindrical component extending along the exhaust gas flow direction D. In addition, the outer shape of the substrate is not limited to a cylindrical shape, and may also be an elliptical cylindrical shape, a polygonal cylindrical shape, etc. In addition, the total length and capacity of the substrate 10 are not particularly limited, and can be appropriately changed according to the performance of the internal combustion engine 2, the size of the exhaust pipe 4, etc. In addition, the substrate 10 can use previously known materials without particular restrictions. For example, the substrate 10 can use highly heat-resistant materials such as ceramics or alloys. As highly heat-resistant ceramics, cordierite (cordierite), silicon carbide (SiC), aluminum titanate, etc. can be cited. In addition, as highly heat-resistant alloys, stainless steel, etc. can be cited. Among the above materials, cordierite has excellent durability against thermal shock, and therefore can be suitable for use in the substrate 10 for the particulate filter GPF that is easily supplied with high-temperature exhaust gas.

[0049] In addition, the substrate 10 in this embodiment is a wall flow type substrate. Figure 3 As shown, the wall-flow type substrate 10 has an inlet compartment (cell / unit) 12 with only an end 12a on the exhaust gas inlet side open, an outlet compartment (cell / unit) 14 with only an end 14b on the exhaust gas outlet side open, and a porous partition wall 16 that separates the inlet compartment 12 from the outlet compartment 14. In addition, the end of the exhaust gas outlet side of the inlet compartment 12 is blocked by a sealing portion 12b. On the other hand, the end of the exhaust gas inlet side of the outlet compartment 14 is closed by a sealing portion 14a. Furthermore, the inlet compartment 12 and the outlet compartment 14 are alternately formed in a manner adjacent to each other with the partition wall 16 sandwiched therebetween. In addition, as shown in FIG. Figure 2As shown, the inlet compartment 12 and the outlet compartment 14 in this embodiment are roughly square in shape when viewed from the front (viewing along the extension direction X of the partition wall 16). However, the shape of the inlet compartment and the outlet compartment when viewed from the front can be a rectangular shape such as a parallelogram, a rectangle, a trapezoid, a triangle, other polygons (such as a hexagon, an octagon), a circle, and other geometric shapes without particular limitation.

[0050] A plurality of voids 18 are formed in the partition wall 16 as a porous member (see Figure 4 ). The inlet compartment 12 and the outlet compartment 14 are connected via the gap 18. In addition, the D of the gap 18 of the partition wall 16 is 50 The pore diameter is preferably 5 μm or more, and more preferably 10 μm or more. This ensures the air permeability of the partition wall 16 and suppresses the increase in pressure loss. 50 The upper limit of the pore diameter is preferably 100 μm or less, more preferably 50 μm or less, further preferably 40 μm or less, and particularly preferably 30 μm or less. This can improve the PM collection performance inside the partition wall 16. 50 As the pore diameter decreases, there is a tendency that the formation of the outer coating 20 described later becomes easier. In addition, the porosity of the partition wall 16 is preferably 30% or more, more preferably 40% or more, and particularly preferably 50% or more. This can suppress the increase in the pressure loss of the particulate filter 1. On the other hand, the upper limit of the porosity of the partition wall 16 is preferably 90% or less, more preferably 80% or less, and further preferably 70% or less. This can fully ensure the mechanical strength of the substrate 10. In addition, the D of the voids 18 is preferably 20%. 50 The pore diameter and the porosity of the partition wall 16 can be determined based on the pore diameter distribution of the coating peeling sample described later (see Figure 5 A) in the measurement.

[0051] In addition, the thickness t2 of the partition wall 16 is preferably 0.01 mm or more, more preferably 0.05 mm or more, and particularly preferably 0.1 mm or more. Thus, the mechanical strength of the substrate 10 can be fully ensured. On the other hand, the thickness t2 of the partition wall 16 is preferably 2 mm or less, more preferably 1 mm or less. Thus, the air permeability of the partition wall 16 can be ensured, and the increase in pressure loss can be suppressed.

[0052] 2. External coating

[0053] The outer coating layer 20 is a porous layer formed of a plurality of granular bodies. Figure 3 As shown in FIG. 1 , the outer coating 20 is formed on the surface of the partition wall 16 (the inlet side surface 16a) that contacts the inlet compartment 12. Figure 4As shown, the external coating 20 covers the entrance of the void 18 of the partition wall 16 (the end face in contact with the entry compartment 12). In addition, the term "formed on the entry side surface 16a" in this specification means that a certain amount of external coating 20 is attached to the entry side surface 16a of the partition wall 16, and does not intend to prohibit a portion of the external coating 20 from entering the interior of the partition wall 16 (in the void 18). Typically, in an analysis based on a cross-sectional SEM image described later, if more than 20% (typically more than 30%, for example more than 40%) of the external coating 20 is attached to the entry side surface 16a of the partition wall 16, it can be said that "the external coating 20 is formed on the entry side surface 16a of the partition wall 16". In addition, if a large amount of the external coating 20 enters the interior of the partition wall 16, it is easy to cause an increase in pressure loss caused by clogging of the void 18. Therefore, the external coating 20 attached to the entry side surface 16a of the partition wall 16 is preferably more than 80% relative to the entire external coating 20, more preferably more than 82%, further preferably more than 86%, and particularly preferably more than 90%.

[0054] (1) External coating material

[0055] Next, the granular body forming the external coating 20 is described. The granular body of the external coating 20 preferably contains a heat-resistant material specified in JISR2001 as a main component. As an example of the heat-resistant material, aluminum oxide (Al2O3), cerium oxide (CeO2), zirconium oxide (ZrO2), silicon dioxide (SiO2), magnesium oxide (MgO), calcium oxide (CaO) and the like can be cited. Among these heat-resistant materials, aluminum oxide, cerium oxide and zirconium oxide are more preferred. In addition, the above-mentioned "as a main component contains" means that when the total weight of the external coating 20 is set to 100% by mass, the heat-resistant material contains 50% by mass or more (preferably 60% by mass or more, more preferably 70% by mass or more, and particularly preferably 80% by mass or more). Thereby, the sintering (Sintering / sintering / sintering) of the external coating 20 caused by high-temperature exhaust gas can be prevented. As a result, the air permeability of the external coating 20 can be maintained at a high state.

[0056] In addition, the granular body of the outer coating layer 20 may also contain a precious metal catalyst. Thus, the particulate filter 1 can be provided with resistance to harmful gas components (HC, CO, NO x) purification function. In addition, the precious metal catalyst also has the function of promoting the combustion of particulate matter PM, and therefore can also contribute to the suppression of the increase in pressure loss. In addition, the type of precious metal catalyst can use previously known materials without particular limitation. Specific examples of precious metal catalysts include gold (Au), silver (Ag), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), etc. In addition, the total amount of the precious metal catalyst in the particulate filter 1 is preferably 0.1 g / L or more relative to the volume of the substrate 10, more preferably 0.5 g / L or more, and particularly preferably 1.0 g / L or more. Thereby, the purification function of harmful gas components can be appropriately exerted. On the other hand, from the viewpoint of material cost, the total amount of the precious metal catalyst is preferably 10 g / L or less, more preferably 8 g / L or less, and particularly preferably 7 g / L or less.

[0057] In addition, when using a granular body containing a precious metal catalyst, the outer coating 20 preferably contains an additive that improves the catalytic activity of the precious metal catalyst. As an example of such an additive, an OSC material can be cited. The OSC material has an oxygen storage capacity (OSC: Oxygen Storage Capacity) for absorbing and releasing oxygen. As a result, the exhaust gas can be maintained near the stoichiometric (theoretical air-fuel ratio) to stabilize the catalytic action of the precious metal catalyst. In addition, as an example of an OSC material, a zirconium oxide-cerium oxide composite oxide (ZC composite oxide) and the like can be cited.

[0058] In addition, in addition to the OSC material, the outer coating 20 may also include an existing well-known additive that can be added to the coating of the particulate filter. Examples of such additives include NH3 adsorbents (zeolites, etc.), NOx adsorbents, stabilizers, etc. In addition, the outer coating 20 may also contain trace components from raw materials and manufacturing processes. For example, the outer coating 20 may include one or more compounds (oxides, sulfates, carbonates, nitrates, chlorides) containing alkaline earth metal elements (Be, Mg, Ca, Ba, etc.), rare earth accumulator elements (Y, La, Ce, etc.), alkali metal elements (Li, Na, K, etc.), transition metal elements (Mn, Fe, Co, Ni, etc.), etc.

[0059] In addition, the D of the granular body forming the outer coating layer 20 is 50 The particle size is preferably 2 μm or more, more preferably 3 μm or more, further preferably 4 μm or more, and particularly preferably 5 μm or more. This can prevent the granules from entering the gaps 18 of the partition wall 16, thereby ensuring the air permeability of the partition wall 16. 50If the particle size is too large, the number of contact points between the outer coating 20 (granular body) and the partition wall 16 decreases, so the outer coating 20 is easily peeled off. 50 The particle size is preferably 30 μm or less, more preferably 25 μm or less, further preferably 20 μm or less, and particularly preferably 15 μm or less.

[0060] In addition, the term "D of the granular body" in this specification 50 The particle size is measured based on the laser diffraction and scattering method. Specifically, the powder of the outer coating 20 is separated from the surface of the partition wall by scraping with a brush or the like. The powder of the outer coating 20 is put into a laser diffraction and scattering particle size distribution measuring device (LA-960 manufactured by HORIBA). Then, the powder is dispersed in pure water in the measuring device, and the particle size distribution based on the number of particles constituting the outer coating is obtained based on the scattered light of the suspension separated by ultrasonic wave. And, in the particle size distribution, the particle size corresponding to the cumulative 50% from the smaller particle size is regarded as the "D of the granular body". 50 Particle size".

[0061] (2) Structure of external coating

[0062] like Figure 4 As shown, a plurality of pores 22 are formed in the outer coating 20 as a porous layer. And the exhaust gas supplied to the inlet compartment 12 is discharged to the outlet compartment 14 side through the pores 22 of the outer coating 20 and the gaps 18 of the partition wall 16. At this time, the particulate matter PM in the exhaust gas is captured inside the outer coating 20. Specifically, the particulate matter PM discharged from a conventional internal combustion engine is very small (particle size is less than 300nm), so it does not flow uniformly along the streamline of the exhaust gas, but vibrates irregularly by Brownian diffusion. When the particulate matter PM flowing while vibrating contacts the wall surface of the pore 22, it is captured by the wall surface of the pore 22.

[0063] Here, in consideration of the above-mentioned mechanism for collecting particulate matter PM, the outer coating layer 20 in the present embodiment is configured to be able to appropriately collect the particulate matter PM. Hereinafter, the specific structure of the outer coating layer 20 will be described.

[0064] (a) D of the pores in the outer coating 90 Pore ​​diameter

[0065] First, the D of the pores 22 of the outer coating layer 20 in this embodiment is 90The pore diameter is 10 μm or less. As described above, the particulate matter PM that has penetrated into the outer coating 20 flows while vibrating by Brownian diffusion. At this time, when the particulate matter PM contacts the wall of the pore 22, it is captured by the wall of the pore 22. In contrast, if the pore diameter of the pore 22 is reduced, the effective area of ​​the wall of the pore 22 becomes larger. In addition, the particulate matter PM that has penetrated into the small pore 22 can contact the wall of the pore 22 even when the amount of movement (vibration amplitude) generated by Brownian diffusion is small. From these viewpoints, if the pore diameter of the pore 22 is reduced, there is a tendency for the PM capture performance to improve. In addition, among the multiple pores 22 formed in the outer coating 20, the pores 22 (large pores) with large pore diameters have low exhaust gas permeation resistance, so the gas flow rate is larger than that of other pores. In this case, even if vibrations generated by Brownian diffusion are generated, the possibility that the particulate matter PM does not contact the wall of the pore 22 and passes through the outer coating 20 becomes higher. From these viewpoints, in the present embodiment, the average pore diameter or D is not used as a general parameter for defining the pore diameter. 50 The pore diameter is not the same as the pore diameter, but D, which represents the pore diameter of the large pores. 90 The pore diameter is set to 10 μm or less. As a result, it is confirmed that the penetration of particulate matter PM in the outer coating layer 20 is greatly suppressed, so the PM collection performance is significantly improved. In addition, from the viewpoint of further improving the PM collection performance in the outer coating layer 20, the D of the pores 22 is set to 10 μm or less. 90 The pore diameter is preferably 5 μm or less, more preferably 4.5 μm or less, further preferably 4 μm or less, and particularly preferably 3.5 μm or less.

[0066] On the other hand, D of the pore 22 90 The pore diameter is the pore diameter of the large pores formed in a small number. Therefore, even if the D of the pore 22 is reduced 90 The pore size does not produce an average pore size or D 50 The smaller the pore diameter, the more significant the increase in pressure loss. 90 If the pore diameter is too small, the pressure loss may increase slightly when PM accumulates. Therefore, in the present embodiment, the diameter D of the pore 22 is set to 90 The pore diameter is set to be 1 μm or more. In order to more appropriately suppress the increase in pressure loss due to clogging of the pores, the D 90 The pore diameter is preferably 1.2 μm or more, more preferably 1.4 μm or more, further preferably 1.6 μm or more, and particularly preferably 1.9 μm or more.

[0067] In addition, the D of the pores of the above-mentioned "external coating 90The pore size is measured in the following steps. First, two cubic test pieces (one side: 10 mm) are cut out from the particulate filter 1. Then, one of the test pieces is subjected to a pore size measurement from the outlet compartment 14 toward the inlet compartment 12 (i.e., the same as the outlet compartment 14). Figure 4 Then, the test piece after the blowing is ultrasonically cleaned in water (frequency: 45kHz, cleaning time: 10 minutes). Then, the test piece is dried and the blowing in the direction opposite to the exhaust gas flow direction D is performed again. Then, the above-mentioned stripping treatment is repeated until the complete stripping of the external coating 20 is confirmed in the SEM observation. In this way, a sample of the partition wall 16 with the external coating 20 stripped off (coating stripping sample) can be obtained.

[0068] Next, mercury intrusion porosimetry was performed to obtain the pore size distribution of each of the coating peeled sample and the untreated test piece (coating remaining sample). Figure 5 An example of the obtained pore size distribution is shown in FIG. Figure 5 "A" in the figure is the pore size distribution of the coating peeling sample, and "B" is the pore size distribution of the coating remaining sample. Figure 5 As shown, in the coating peeling sample A, a pore diameter peak A1 is confirmed at a position above 10 μm (for example, near 30 μm). On the other hand, in the coating residual sample B, a pore diameter peak B3 is also confirmed at the same position as the pore diameter peak A1 of the coating peeling sample A. These pore diameter peaks A1 and B3 are understood to come from the gap 18 of the partition wall 16 which is a common structure of both the coating peeling sample A and the coating residual sample B. Based on this situation, by removing the pore diameter peak B3 from the pore diameter distribution of the coating residual sample B, the pore diameter distribution from the external coating 20 can be obtained.

[0069] Next, in the pore size distribution of the coating residue sample B, a pore size peak B1 of 0.01 μm or more and less than 0.1 μm and a pore size peak B2 of 0.1 μm or more and less than 1 μm were confirmed. Among them, the pore size peak B1 on the small diameter side is very fine, so it can be understood as a concave portion (micropore) from the surface of the granular body. Based on this situation, by removing the pore size peak B1, the pore size distribution (pore size peak B2) of the pore 22 from the external coating 20 can be separated. In addition, when the total volume in the pore size distribution from the pore 22 is set to 100%, by calculating the pore size at which the cumulative volume becomes 90%, the "D of the pore" can be measured. 90 "Pore diameter".

[0070] (b) Thickness of the outer coating t1

[0071] Second, the thickness t1 of the outer coating 20 in the present embodiment is 20 μm or more. As described above, the particulate matter PM that has penetrated into the pores 22 is captured by the wall of the pores 22 when it contacts the wall of the pores 22. In contrast, by extending the flow distance of the particulate matter PM in the outer coating 20, the chance of the particulate matter PM contacting the wall of the pores 22 can be increased. From this point of view, in the present embodiment, the thickness t1 of the outer coating 20 is set to be 20 μm or more. In addition, the thickness t1 of the outer coating 20 may be 25 μm or more, 28 μm or more, or 30 μm or more. In addition, from the perspective of further improving the PM capture performance inside the outer coating 20, the thickness t1 of the outer coating 20 is preferably 33 μm or more, more preferably 35 μm or more, further preferably 38 μm or more, and particularly preferably 43 μm or more.

[0072] On the other hand, if the thickness t1 of the outer coating 20 is too thick, the air permeability of the outer coating 20 may be reduced. Therefore, in the present embodiment, the thickness t1 of the outer coating is set to 80 μm or less. Thus, the increase in pressure loss caused by the outer coating 20 can be suppressed. In addition, from the viewpoint of more appropriately suppressing the increase in pressure loss, the thickness t1 of the outer coating is preferably 70 μm or less, more preferably 65 μm or less, further preferably 60 μm or less, and particularly preferably 56 μm or less.

[0073] In addition, the "thickness t1 of the external coating" is measured according to the following steps. First, the particulate filter 1 to be inspected is disassembled to prepare a sample piece in which the substrate 10 is embedded with a resin. Next, the sample piece is cut to expose the cross section of the partition wall 16. Then, the cross section of the exposed partition wall 16 is observed using a scanning electron microscope (SEM) to obtain a cross-sectional SEM photograph (reflected electron image, observation magnification 200 times). Next, the cross-sectional SEM photograph of the partition wall 16 is automatically binarized using two-dimensional image analysis software (product name: ImageJ (registered trademark)). In this automatic binarization process, the coating (external coating 20 and internal coating 30 described later) is blackened, and other structures (pores 22, partition wall 16, voids 18) are whitened. The images before and after the binarization process are compared, and the coating (black portion) present on the entrance side 16a of the partition wall 16 is regarded as the "external coating 20". Then, the thickness of the outer coating 20 is measured at a plurality of locations (eg, three locations) at intervals of 50 μm in the lateral direction of the processed image (eg, the extending direction X of the partition 16 ). The average of the plurality of measurement results is taken as “the outer coating thickness t1 ”.

[0074] (c) Porosity P of the outer coating coat

[0075] Third, the porosity P of the outer coating layer 20 in this embodiment is coat When the porosity P of the outer coating 20 is coat When the porosity P is lower, the flow rate (linear velocity) of the exhaust gas passing through the fine pores 22 increases, so the possibility of particulate matter PM penetrating the outer coating layer 20 increases. coat The number of pores 22 formed in the low-profile outer coating 20 is small, and the effective area of ​​the wall of the pores 22 for capturing particulate matter PM is narrowed. From this point of view, the PM capture rate may be reduced. coat If the porosity P of the outer coating layer 20 is lowered, there is also a disadvantage that the pores 22 are easily clogged due to the accumulation of particulate matter PM. coat The porosity P of the outer coating 20 is set to 30% or more. This can improve the PM collection performance inside the outer coating 20 and prevent a sharp increase in pressure loss. In addition, from the perspective of more appropriately improving both the PM collection performance and the pressure loss suppression performance, the porosity P of the outer coating 20 is set to 30% or more. coat It is preferably 35% or more, more preferably 40% or more, and particularly preferably 43% or more.

[0076] On the other hand, when the porosity P of the outer coating 20 is coat If it becomes too high, the mechanical strength of the outer coating 20 decreases. Therefore, when the flow rate of the exhaust gas increases sharply, the outer coating 20 may be damaged or peeled off. Therefore, the porosity P of the outer coating 20 in this embodiment is coat In addition, from the viewpoint of forming the outer coating layer 20 having a better mechanical strength, the porosity P of the outer coating layer 20 is preferably 70%. coat It is preferably 65% ​​or less, more preferably 60% or less, and particularly preferably 55% or less.

[0077] In addition, the porosity of the external coating coat ” is measured based on cross-sectional SEM observation. Specifically, similarly to the measurement method of the “thickness t1 of the external coating” described above, the cross-sectional SEM photograph of the partition wall 16 is automatically binarized. And, a specified measurement area is arbitrarily set in the external coating 20 (black portion) of the binarized image. The measurement area is a quadrilateral area having one side with a length of 100 μm along the horizontal direction of the binary image and another side with a length equal to the thickness t1 of the external coating. Then, the “total area AR of the measurement area” is obtained by counting the total number of pixels in the set measurement area. all In addition, the “area AR of the outer coating” is obtained by counting the number of pixels of the outer coating 20 (black portion) in the measurement area. coatAnd, by finding the total area AR of the measurement area all The area AR of the external coating coat The difference between the two is used to obtain the "pore area AR pore "And," the porosity of the external coating P coat As shown in the following formula (2), it is possible to calculate the total area AR of the measurement area based on all and the area of ​​the pore AR pore to calculate.

[0078] P coat (%) = (AR pore / AR all )×100(2)

[0079] (d) Summary

[0080] As described above, the D of the pores 22 of the outer coating layer 20 in this embodiment is 90 The pore diameter is 1 μm or more and 10 μm or less, the thickness t1 is 20 μm or more and 80 μm or less, and the porosity P coat is 30% or more and 70% or less. Thus, the particulate matter PM can be properly captured by the inside of the outer coating 20 (the wall surface of the pores 22). Furthermore, the above-mentioned structures are set in a manner that does not cause a sharp increase in pressure loss. Therefore, according to the particulate filter 1 of this embodiment, it is possible to achieve both PM capture performance and pressure loss suppression performance at a high level.

[0081] (e) Other structures

[0082] In addition, the D of the pore 22 50 The pore diameter is preferably 1.8 μm or less, more preferably 1.75 μm or less, and particularly preferably 1.7 μm or less. 90 In addition to the pore diameter, by making D 50 When the upper limit of the pore diameter is less than a certain value, the PM collection performance inside the outer coating layer 20 can be further improved. 50 The pore diameter is preferably 0.5 μm or more, more preferably 0.7 μm or more, further preferably 1 μm or more, and particularly preferably 1.3 μm or more. This can more appropriately prevent the increase in pressure loss due to clogging of the pores 22. 50 The pore size is the pore size at which the cumulative volume in the pore size distribution of the pores 22 is 50%. This "pore size distribution of the pores 22" is the same as the above D 90 Measurement of pore diameter Similarly, the pore diameter distribution of the coating residue sample B can be separated from the pore diameter distribution of the pore 22 ( Figure 5 It is obtained by measuring the pore diameter peak B2).

[0083] In addition, the flow path length of the pores 22 in the outer coating 20 is preferably 200 μm or more, more preferably 300 μm or more, further preferably 400 μm or more, and particularly preferably 430 μm or more. The "flow path length of the pores 22" in this specification refers to the total length of the pores 22 in the measurement area of ​​100 μm along the extension direction X of the outer coating 20. Figure 4 As shown, the pores 22 are branched or meandered inside the outer coating 20. Therefore, by making the flow path length of the pores 22 above a certain value, the PM collection performance inside the outer coating 20 can be further improved. On the other hand, when the flow path length of the pores 22 is too long, the air permeability of the particulate filter 1 is greatly reduced. From this point of view, the flow path length of the pores 22 is more preferably less than 2000 μm, more preferably less than 1600 μm, and particularly preferably less than 1400 μm. In addition, the "100 μm measurement area along the extension direction of the outer coating" can be calculated based on the following formula (3).

[0084] Flow path length = pore area AR pore / ((D of pores 50 Pore ​​diameter / 2) 2 ×π)(3)

[0085] In addition, the length of the outer coating 20 in the extension direction X of the partition wall 16 is preferably 85% or more, preferably 90% or more, preferably 95% or more, and preferably 97.5% or more relative to the total length (100%) of the partition wall 16. Thus, it is possible to prevent the particulate matter PM from penetrating from the portion where the outer coating 20 is not formed. On the other hand, the upper limit of the coating length of the outer coating 20 is not particularly limited, and may be 100% or less, 99.5% or less, or 99% or less.

[0086] In addition, the coating amount of the external coating 20 relative to the capacity of the substrate 10 is preferably 10 g / L or more, more preferably 20 g / L or more, and particularly preferably 30 g / L or more. Thus, the external coating 20 is fully formed, so that a more suitable PM capture performance can be exerted. On the other hand, when the coating amount of the external coating 20 is too much, it is possible to produce an increase in pressure loss caused by clogging of the inlet compartment 12. From this point of view, the coating amount of the external coating 20 is preferably 100 g / L or less, more preferably 70 g / L or less, further preferably 50 g / L or less, and particularly preferably 40 g / L or less.

[0087] B. Method of Manufacturing Particulate Filter

[0088] Next, a method for producing the particulate filter 1 of the present embodiment will be described. This method includes a substrate preparation step, a slurry application step, and a firing step.

[0089] 1. Substrate preparation process

[0090] In the substrate preparation step, a wall flow type substrate 10 is prepared. The detailed structure of the substrate 10 has been described above, and therefore, repeated description will be omitted.

[0091] 2. Slurry application process

[0092] In the slurry imparting process, the slurry for the external coating is imparted to the inlet side surface 16a of the partition wall 16 of the substrate 10. The slurry for the external coating contains at least the above-mentioned granular body, the organic pore-forming material and the dispersion medium. In addition, the organic pore-forming material can be used without particular restrictions as a powder made of resin that is burned out in the firing process. As an example of the organic pore-forming material, cellulose, polyethylene, polypropylene, melamine resin, polymethyl methacrylate resin (PMMA) and the like can be cited. When the slurry for the external coating is fired, the granular body is sintered to form the external coating 20, and the organic pore-forming material is burned out to form pores 22. In addition, when imparting the slurry for the external coating, a coating method known in the past can be used without particular restrictions. For example, after the slurry for the external coating is supplied to the inlet compartment 12 of the substrate 10, it is sucked from the outlet compartment 14. By adjusting the suction speed at this time, the external coating slurry flows along the inlet-side cell 12 , and the external coating slurry is applied to the inlet side surface 16 a of the partition wall 16 .

[0093] Furthermore, the D of the pores 22 in the outer coating layer 20 after firing is 90 The pore size is easily affected by the particle size and aspect ratio of the organic pore-forming material. 50 The particle size can be set to be 3 μm or more (preferably 4 μm or more, more preferably 5 μm or more). 50 The upper limit of the particle size can be set to 20 μm or less (preferably 15 μm or less, more preferably 10 μm or less). In addition, the aspect ratio of the organic pore-forming material can be set to 1 or more (preferably 1.5 or more, more preferably 2 or more). On the other hand, the upper limit of the aspect ratio of the organic pore-forming material can be set to 5 or less (preferably 4 or less, more preferably 3 or less). In addition, the D of the pore 22 90 The pore size may also be affected by factors other than the particle size and aspect ratio of the organic pore-forming material (such as the firing temperature, the amount of gas generated during firing, and the dispersibility of the organic pore-forming material). Therefore, the particle size and aspect ratio of the organic pore-forming material do not limit the technology disclosed herein. 90 The conditions for the pore diameter are preferably obtained based on preliminary experiments in which various conditions including the particle diameter and aspect ratio of the organic pore-forming material are adjusted.

[0094] In addition, the porosity P of the outer coating layer 20 after firing is coatIt is easily affected by the content of the organic pore-forming material, the particle size of the granules, etc. For example, the content of the organic pore-forming material can be set to 10 wt% or more (preferably 20 wt% or more, more preferably 30 wt% or more) relative to the total weight of the solid components (granules, etc.) in the slurry. On the other hand, the upper limit of the content of the organic pore-forming material can be set to 60 wt% or less (preferably 55 wt% or less, more preferably 50 wt% or less). On the other hand, since it has been described that the D of the granules 50 The particle size is 0.04 mm, so repeated description is omitted. In addition, the porosity P of the outer coating 20 is coat It can also be changed by factors other than the organic pore-forming material and the granules (such as the firing temperature and firing time). 50 The particle size is not limited to the technology disclosed herein. That is, in order to obtain the desired porosity P coat The conditions are preferably obtained based on preliminary experiments in which various conditions including the content of the organic pore-forming material and the particle size of the granular body are adjusted.

[0095] Next, the thickness t1 of the fired external coating 20 is easily affected by the viscosity of the slurry, the supply amount, etc. For example, the viscosity of the external coating slurry can be set to 40mPa·s or more (preferably 60mPa·s or more, more preferably 80mPa·s or more) in an E-type viscometer set at 100rpm. On the other hand, the upper limit of the slurry viscosity can be set to 300mPa·s or less (preferably 200mPa·s or less, more preferably 150mPa·s or less). In addition, the external coating slurry may contain a thickener for viscosity adjustment. As a thickener, cellulose polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), and hydroxyethyl methyl cellulose (HEMC) can be cited. In addition, the slurry supply amount is appropriately set according to the solid content ratio in the dispersion medium. For example, when the solid content ratio of the slurry is about 16%, the slurry supply amount relative to the capacity of the substrate 10 can be set to 200g / L or more (preferably 250g / L or more, more preferably 300g / L or more). In addition, the upper limit of the slurry supply amount can be set to 500 g / L or less (preferably 450 g / L or less, more preferably 400 g / L or less). In addition, the thickness t1 of the outer coating 20 can also vary according to factors other than the slurry viscosity and the supply amount (suction speed, suction time, etc.). Therefore, the slurry viscosity and the supply amount are not limited to the technology disclosed herein. That is, the conditions for obtaining the desired thickness t1 of the outer coating 20 are preferably obtained based on preliminary tests that adjust various conditions including the slurry viscosity and the supply amount.

[0096] In addition, from the viewpoint of appropriately forming an outer coating on the surface of the partition wall 16, the D of the granular body in the slurry is50 The particle size is preferably considered to be D of the gap 18 between the particle and the partition wall 16. 50 Specifically, if the D of the particles in the slurry is 50 Particle size relative to gap 18 D 50 If the pore diameter is too small, the granules will enter the interior of the partition wall 16, making it difficult to form the outer coating 20 on the inlet side surface 16a of the partition wall 16. 50 If the particle size is too large, the number of contact points between the partition wall 16 and the granular body (external coating 20) will decrease, and the external coating 20 will be easily peeled off. 50 The particle size is "X", and the D of the gap 18 is 50 When the pore diameter is defined as "Y", it is preferable that the following formula (1) is satisfied.

[0097] 0.1≤X / Y≤3(1)

[0098] In addition, from the viewpoint of more reliably preventing the intrusion of granular bodies into the partition wall 16, the above-mentioned X / Y is more preferably greater than 0.5, and particularly preferably greater than 1. In addition, from the viewpoint of more reliably preventing the peeling of the external coating 20, the upper limit of the above-mentioned X / Y is preferably less than 2.5, and more preferably less than 2. In addition, the adjustment of the above-mentioned X / Y is an example of a method for forming an appropriate external coating 20, and is not limited to the technology disclosed herein. For example, if an organic solid component is filled in the gap 18 of the partition wall 16 before the slurry is supplied, the intrusion of granular bodies into the interior of the partition wall 16 can be prevented even when the above-mentioned X / Y is significantly lower than 0.1. In addition, the organic solid component filled in the gap 18 is burned out in the firing process described later. By using this method, even when very fine granular bodies are used, the external coating 20 can be appropriately formed on the entry side 16a of the partition wall 16.

[0099] 3. Firing process

[0100] In the firing step, the substrate 10 to which the outer coating slurry is applied is fired. As a result, the granular body is sintered to form the outer coating 20, and the organic pore-forming material is burned away to form the pores 22. In this way, the particulate filter 1 of the present embodiment can be manufactured.

[0101] In addition, the firing temperature in this step is preferably 400° C. or higher, more preferably 500° C. or higher, and further preferably 600° C. or higher. As a result, the organic pore-forming material can be appropriately burned away, so that the pores 22 of the desired structure can be reliably formed. In addition, from the viewpoint of suppressing excessive sintering of the granular body, the firing temperature is preferably 1000° C. or lower, more preferably 900° C. or lower, and further preferably 800° C. or lower. In addition, the firing time is preferably about 2 to 4 hours.

[0102] [Second embodiment]

[0103] The particulate filter disclosed herein is not limited to the first embodiment described above, and various forms can be adopted. Hereinafter, a particulate filter according to a second embodiment will be described. Figure 6 : is a diagram schematically showing a cross section of a particulate filter according to the second embodiment along the cylinder axis direction. Figure 7 It is an enlarged cross-sectional view schematically showing a cross section of a partition wall of the particulate filter according to the second embodiment.

[0104] A. Structure of the particulate filter

[0105] like Figure 6 As shown, the particulate filter 1A of the second embodiment is different from the particulate filter 1 of the first embodiment (see Figure 3 ) are different. The following describes each structure.

[0106] 1. Internal coating

[0107] like Figure 6 and Figure 7 As shown, the inner coating 30 is formed on the wall surface 18b of the gap 18 in a predetermined area from the surface of the partition wall 16 that is in contact with the outlet compartment 14 (outlet side surface 16b) toward the inlet compartment 12. In addition, the inner coating 30 contains a precious metal catalyst for purifying harmful gas components in the exhaust gas. Therefore, the inner coating 30 is used to purify harmful gas components (HC, CO, NO X Here, in the particulate filter 1A, since the inlet side surface 16a of the partition wall 16 is covered by the outer coating layer 20, most of the particulate matter PM in the exhaust gas is captured before reaching the inner coating layer 30. As a result, it is possible to suppress the reduction in catalyst activity caused by the particulate matter PM adhering to the precious metal catalyst of the inner coating layer 30. Therefore, the particulate filter 1A of the second embodiment can exert high exhaust gas purification performance.

[0108] The granular body forming the inner coating 30 can use the same heat-resistant material as the outer coating 20. In addition, the precious metal catalyst added to the inner coating 30 is not particularly limited, and the existing well-known materials can be used without particular restrictions. In addition, various additives (OSC materials, NOx adsorbents, stabilizers, etc.) can be added to the inner coating 30 in the same manner as the outer coating 20. The materials of these coatings have been described, so repeated descriptions are omitted.

[0109] Furthermore, when the pore size distribution of the particulate filter 1A of the present embodiment is measured by mercury penetration porosimetry, not only the pore size peak ( Figure 5 B1, B2 in the figure), the pore diameter peak value ( Figure 5 B3) in the figure also includes the pore size peak from the inner coating 30. When measuring the D of the pores 22 based on such a pore size distribution 90 In the case of a pore diameter, it is preferable to perform the following steps.

[0110] First, a plurality of test pieces are cut out from the particulate filter 1A, and each test piece is subjected to a stripping treatment of the external coating 20. In addition, since the steps of the stripping treatment of the external coating 20 have been described, repeated descriptions are omitted. Furthermore, a cross-sectional SEM observation is performed on the plurality of test pieces after the stripping treatment, and a test piece in which the external coating 20 is stripped and the internal coating 30 remains is selected. Hereinafter, the selected test piece is referred to as an "external coating stripping sample". Furthermore, the pore size distribution of each of the external coating stripping sample and the test piece (coating residual sample) that has not been subjected to the stripping treatment is obtained by mercury penetration. Here, in the pore size distribution of the coating residual sample, the pore size peak from the external coating 20 is confirmed. On the other hand, in the pore size distribution of the external coating stripping sample, the pore size peak from the external coating 20 disappears. Therefore, by performing a comparative study of the two obtained pore size distributions, the pore size peak from the external coating 20 can be determined. Furthermore, by separating the pore size peak from the external coating 20 from the pore size distribution of the coating residual sample, the D of the pores 22 of the external coating 20 can be measured. 90 Small pore size.

[0111] In addition, the granules of the inner coating layer 30 are preferably smaller than the granules of the outer coating layer 20. As a result, the granules can be easily introduced into the gap 18, so that the inner coating layer 30 can be easily formed. Specifically, the D of the granules of the inner coating layer 30 is 50 The particle size is preferably 2.0 μm or less, more preferably 1.5 μm or less, and particularly preferably 1.0 μm or less.

[0112] In addition, the total amount of the precious metal catalyst in the particulate filter is as described in the first embodiment above. In addition, as shown in the present embodiment, when an internal coating 30 containing a precious metal catalyst is formed, it is preferred that the content ratio of the precious metal catalyst in the internal coating 30 relative to the total amount of the precious metal catalyst be set to 20% or more (more preferably 40% or more, and further preferably 60% or more). As a result, a more suitable exhaust gas purification performance can be exerted. On the other hand, the upper limit of the content ratio of the precious metal catalyst in the internal coating 30 is not particularly limited, and can be 100% (the precious metal catalyst exists only in the internal coating 30). In addition, from the perspective of imparting a certain exhaust gas purification performance to the external coating 20, the upper limit of the content ratio of the precious metal catalyst in the internal coating 30 is preferably 90% or less, more preferably 80% or less, and particularly preferably 70% or less.

[0113] In addition, the coating amount of the inner coating 30 relative to the capacity of the substrate 10 is preferably 5 g / L or more, more preferably 10 g / L or more, and particularly preferably 20 g / L or more. Thus, the purification performance of harmful gas components can be further improved. On the other hand, when the coating amount of the inner coating 30 is too much, it is easy to produce a pressure loss increase caused by the clogging of the gap 18 of the partition wall 16. From this point of view, the coating amount of the inner coating 30 is preferably 100 g / L or less, more preferably 70 g / L or less, and particularly preferably 40 g / L or less.

[0114] In addition, the dimension t4 of the inner coating 30 in the thickness direction Y of the partition wall 16 is preferably 20 μm or more, more preferably 60 μm or more, and particularly preferably 100 μm or more. Thus, the purification performance of harmful gas components can be further improved. On the other hand, the upper limit of the dimension t4 of the inner coating 30 is preferably 190 μm or less, more preferably 170 μm or less, and particularly preferably 150 μm or less. Thus, the increase in pressure loss caused by the clogging of the gap 18 of the partition wall 16 can be appropriately prevented.

[0115] 2. Uncoated area

[0116] In the particulate filter 1A of the second embodiment, an uncoated region 40 where substantially no coating layer exists is provided between the outer coating layer 20 and the inner coating layer 30 in the thickness direction Y of the partition wall 16. In other words, the uncoated region 40 is provided in a region in contact with the inlet side surface 16a inside the partition wall 16.

[0117] By forming the uncoated region 40, the harmful gas component purification performance in the inner coating layer 30 can be maintained at a high state. Specifically, in the particulate filter 1A of the present embodiment, D having appropriately set pores 22 is also formed. 90 Pore ​​diameter, thickness t1, porosity Pcoat Therefore, particulate matter PM can be appropriately captured inside the outer coating 20 (the wall surface of the pores 22) (see Figure 7 However, among the particulate matter PM in the exhaust gas, there are also particles that are not captured by the outer coating 20 and penetrate the gaps 18 of the partition wall 16 (hereinafter referred to as "penetrating particles PM"). t ”) When the penetrating particles PM t When reaching the inner coating layer 30, the particulate matter PM adheres to the precious metal catalyst, which may reduce the catalyst activity. However, in this embodiment, since the uncoated area 40 is provided between the outer coating layer 20 and the inner coating layer 30, the penetrating particulate matter PM can be captured in the uncoated area 40 on the upstream side of the inner coating layer 30. t Thus, it is possible to suppress the reduction of the exhaust gas purification performance in the inner coating layer 30. In addition, since the gaps 18 are narrowed in the region where the inner coating layer 30 is formed, the PM penetrating particles are t In contrast, in the uncoated area 40, the gaps 18 are kept wide, so it is not easy to generate PM due to penetration. t The advantage is that the pressure drop increases sharply due to accumulation.

[0118] In addition, in this specification, "substantially no coating or catalyst layer is formed" means that the coating is not intentionally formed on the wall surface of the pore. Therefore, in the case where a trace amount of coating exists in the region between the outer coating 20 and the inner coating 30 due to errors during manufacturing, the concept of "substantially no coating or catalyst layer is formed" in this specification is included. In addition, whether an uncoated region is formed (i.e., whether an intentional coating is formed in the region between the outer coating and the inner coating) can be judged according to the steps described in "Judgment of Each Region" described later.

[0119] In addition, the size t3 of the uncoated region 40 in the thickness direction Y of the partition wall 16 is preferably 10 μm or more, more preferably 15 μm or more, and particularly preferably 20 μm or more. This can more appropriately prevent the penetration of the particles PM. t The upper limit of the size t3 of the uncoated region 40 is not particularly limited as long as an area for forming the inner coating 30 is ensured, and may be 500 μm or less, 400 μm or less, or 300 μm or less.

[0120] B. Method of Manufacturing Particulate Filter

[0121] Hereinafter, an example of a method for manufacturing the particulate filter 1A of the second embodiment is described. In this manufacturing method, before the above-mentioned slurry imparting step is performed, a gap filling step of filling the gap 18 of the partition wall 16 with an organic solid component is performed. Specifically, in the gap filling step, a gap filling slurry containing a predetermined organic solid component is introduced into the inlet compartment 12 of the substrate 10. And, suction is performed from the outlet compartment 14 of the substrate 10. Thus, the gap filling slurry permeates from the inlet compartment 12 to the inside of the partition wall 16. By adjusting the suction force at this time, the length of the slurry penetration area in the thickness direction Y of the partition wall 16 can be controlled. And, by performing a drying process, the predetermined area forming the uncoated area 40 can be filled with the organic solid component.

[0122] Next, similarly to the first embodiment, a slurry application step is performed to apply the external coating slurry to the side entrance 16a of the partition wall 16. Then, by performing a drying process, a precursor of the external coating 20 is formed on the side entrance 16a of the partition wall 16. In addition, the drying process is preferably performed at a temperature at which the dispersion medium of the external coating slurry evaporates and the organic solid components filled in the partition wall 16 are not burned out.

[0123] Next, an internal slurry penetration step is performed to penetrate the internal coating slurry from the side surface 16b of the partition wall 16 into the partition wall 16. The internal coating slurry contains granules that form the internal coating 30 and a dispersion medium that disperses the granules. In addition, as described above, the granules of the internal coating 20 contain the same heat-resistant material and precious metal catalyst as the external coating 20.

[0124] Furthermore, in the internal slurry penetration step, the internal coating slurry is supplied to the outlet compartment 14 of the substrate 10 and sucked from the inlet compartment 12. Thus, the internal coating slurry penetrates from the outlet compartment 14 side to the inside (gap 18) of the partition wall 16. By performing the firing step in this state, the outer coating 20 is formed on the inlet side 16a of the partition wall 16, and the inner coating 30 is formed inside the partition wall 16. In addition, the organic solid component filled in the interior of the partition wall 16 is burned away in the firing step, so the area filled with the organic solid component becomes the uncoated area 40. Thus, the particulate filter 1A having the outer coating 20, the inner coating 30 and the uncoated area 40 can be manufactured.

[0125] In addition, the organic solid component can be any material that is burned away during the firing process, and materials known in the prior art can be used without particular restrictions. As an example of an organic solid component, resin beads mainly composed of polyethylene, polypropylene, melamine resin, polymethyl methacrylate resin, etc. can be cited. In addition, as another example, resin fibers such as cellulose microfibers can also be used. In addition, the concentration of the organic solid component in the gap filling slurry is preferably appropriately adjusted in consideration of the porosity of the partition wall 16 of the substrate 10. In addition, the dispersion medium can be used without particular restrictions as long as it is a liquid material that does not dissolve the above-mentioned organic solid component. As an example of such a dispersion medium, water, alcohol, etc. can be cited. In addition, the pore filling slurry can contain a resin component (cellulose resin) that is soluble in such a dispersion medium. In addition, the particle size of the organic solid component is preferably adjusted in a manner that can be appropriately filled in the gap 18 of the partition wall 16. For example, when resin beads are used as the organic solid component, it is preferred that the D of the resin beads be . 50 The particle size is smaller than the gap 18 D 50 Small pore size.

[0126] In addition, the method of manufacturing the particulate filter 1A of the second embodiment is not limited to the method of implementing the above-mentioned gap filling process. For example, if the supply amount of the internal coating slurry in the internal slurry impregnation process, the suction force, etc. are adjusted, the uncoated area 40 can be formed even if the organic solid component is not filled in the gap 18 of the partition wall 16. Specifically, if the supply amount of the internal coating slurry is reduced and the suction force of the suction side compartment 12 is weakened, the internal coating slurry will not reach the inlet side surface 16a of the partition wall 16, so the uncoated area 40 can be formed even if the gap filling process is not performed.

[0127] [Judgment of each region]

[0128] The outer coating 20, inner coating 30 and uncoated area 40 can be determined based on the following determination steps (a) to (k). That is, in a given particulate filter, whether an outer coating, an inner coating, an uncoated area, etc. are provided can be determined based on the following determination steps.

[0129] (a) A particulate filter to be inspected is disassembled, and 10 test pieces are prepared in which the partition walls of the substrate are embedded with a resin.

[0130] (b) The sample piece was cut to expose the cross section of the partition wall, and the exposed cross section of the partition wall was observed using a scanning electron microscope (SEM) to obtain a cross-sectional SEM observation image (reflected electron image, observation magnification 200 times).

[0131] (c) The cross-sectional SEM observation image was automatically binarized using two-dimensional image analysis software (product name: ImageJ (registered trademark)) to obtain a binary image showing only the coating layer.

[0132] (d) The images before and after the automatic binarization process are compared, and the coating confirmed on the entrance side surface of the partition wall is regarded as the "external coating".

[0133] (e) Count the "total number of pixels of the coating" and the "number of pixels of the entire binary image" confirmed in the above binary image. And, calculate the value obtained by dividing the "total number of pixels of the coating" by the "number of pixels of the entire binary image" and use it as the "coating presence rate of the entire partition wall".

[0134] (f) Compare the images before and after the automatic binarization processing, set an arbitrary area from the exit side of the partition wall toward the entry side compartment, calculate the value obtained by dividing the "number of pixels of the coating in the set area" by the "total number of pixels in the set area", and use it as the "coating presence rate in the set area".

[0135] (g) Calculate the value obtained by dividing the "coating presence rate in the set area" by the "coating presence rate of the entire partition wall", and determine whether the value is above the first threshold. In addition, the first threshold in this process is set to 1.05 (preferably 1.1, more preferably 1.15, further preferably 1.2, and particularly preferably 1.25), for example.

[0136] (h) The "arbitrary area from the exit side of the partition wall toward the entry side compartment" described in (f) above is gradually expanded toward the entry side compartment, and the "coating presence rate in the set area" is calculated each time, and the value obtained by dividing the "coating presence rate in the set area" by the "coating presence rate of the entire partition wall" is calculated. And the set area is expanded until the value is lower than the first threshold.

[0137] (i) When the result of the process in (h) above is that an area below the first threshold is confirmed, the area previously determined is regarded as the "inner coating layer". Furthermore, the area between the "lower coating layer" and the "inner coating layer" is regarded as the "third area".

[0138] (j) Calculate the value obtained by dividing the "number of pixels of the coating in the third area" by the "total number of pixels in the third area", and use it as the "coating existence rate in the third area". In addition, calculate the value obtained by dividing the "coating existence rate in the third area" by the "coating existence rate in the entire image". And, when this value is below the second threshold, it is judged that the "third area" is an "uncoated area" (that is, an uncoated area is provided between the outer coating and the inner coating). In addition, the second threshold in this process is set to, for example, 0.65 (preferably 0.5, more preferably 0.4, further preferably 0.3, and particularly preferably 0.25).

[0139] (k) When the treatments (b) to (j) are performed on all 10 test pieces, and when uncoated areas between the outer coating and the inner coating are confirmed in more than 50% of the test pieces, the particulate filter to be inspected is judged as having "an uncoated area between the outer coating and the inner coating."

[0140] In addition, the above-mentioned "thickness (dimension in thickness direction Y) t1-t4" can be measured based on the boundaries of each region identified in the above-mentioned determination step. Specifically, the "thickness (dimension in thickness direction Y) t1-t4" in this specification is the average value of the dimensions of each region confirmed in each of the 10 sample pieces.

[0141] [Test example]

[0142] Hereinafter, test examples related to the technology disclosed herein will be described, but the technology disclosed herein is not intended to be limited to the technology shown in the test examples.

[0143] In this test, seven types of particulate filters (Samples 1 to 7) were prepared, and the influence of the structure of the outer coating layer on the PM trapping performance and the pressure loss suppression performance was investigated.

[0144] 1. Preparation of test specimens

[0145] First, as a substrate, a cylindrical honeycomb substrate (made of cordierite, number of cells: 300 cpsi, partition wall thickness: 8 mil, average pore diameter: 20 μm, porosity: 63%) having a substrate volume of 1.4 L and a length of 101.6 mm was prepared.

[0146] Next, a mixed solution is prepared in which 50% of Al2O3 powder and 50% of ZC composite oxide powder are dispersed in a dispersion medium (water). Furthermore, a predetermined amount of an organic pore-forming material (cellulose particles) is added to the mixed solution to prepare a slurry for an external coating. Furthermore, the slurry for an external coating is supplied to the inlet compartment of the substrate at a coating amount of 40 g / L and sucked from the outlet compartment. Then, a drying treatment (100° C., 30 minutes) is performed to form a dry film (precursor of an external coating) on ​​the inlet side of the partition wall.

[0147] In addition, in this test example, the D of the organic pore-forming material is 50 The particle size and the amount added (the amount added when the total amount of solid components excluding the organic pore-forming material is taken as 100 wt %) are different in samples 1 to 7. 50 The particle size and addition amount are shown in Table 1.

[0148] [Table 1]

[0149] Table 1

[0150]

[0151] Next, a slurry for internal coating was prepared by dispersing 50% of Al2O3 powder and 50% of ZC composite oxide powder in a dispersion medium (water). The slurry for internal coating was supplied to the outlet compartment of the substrate at a coating amount of 30 g / L and sucked from the inlet compartment. Then, a drying treatment (100°C, 30 minutes) and a firing treatment (500°C, 60 minutes) were performed to prepare particulate filters of samples 1 to 7. In the particulate filter, an external coating was formed on the inlet side of the partition wall, and an internal coating was formed inside the partition wall adjacent to the outlet side.

[0152] 2. Structural analysis of external coating

[0153] In this test, the structure of the outer coating of samples 1 to 7 was analyzed. Specifically, the D 90 Pore ​​diameter, thickness of external coating, porosity of external coating, D of pores 50 The five items of pore diameter and pore flow path length were measured. In addition, the measurement of each item was carried out according to the above-mentioned procedure. The measurement results of each item are shown in Table 2.

[0154] 3. Evaluation test

[0155] (1) Evaluation of PM collection performance

[0156] The particulate filter of each sample was set in a smoke generating device (DPG, Cambustion), and the light oil was burned, thereby supplying the exhaust gas containing particulate matter to the particulate filter. In this test, the PM content (pieces) in the exhaust gas was measured on the upstream and downstream sides of the particulate filter. And, the ratio of the PM content on the downstream side to the PM content on the upstream side was calculated as the PM capture rate (%). In addition, in this evaluation, the measurement of the PM content was started after 0.02 g / L of particulate matter was supplied to the particulate filter. The measurement results are shown in Table 2.

[0157] (2) Evaluation of pressure loss suppression performance

[0158] In addition, in this test, the PM accumulation pressure loss was measured during the evaluation of the above-mentioned PM capture performance. Specifically, in the measurement of PM content, the pressure on the upstream side and the pressure on the downstream side of the particulate filter were measured. And the pressure loss was calculated based on the pressure difference between the upstream side and the downstream side. In addition, in this test, the pressure loss just after the measurement of the PM capture rate (starting pressure loss) and the pressure loss after 1g / L of PM was supplied (PM accumulation pressure loss) were measured. The measurement results are shown in Table 2.

[0159] [Table 2]

[0160] Table 2

[0161]

[0162] As shown in Table 2, the PM collection rate of samples 1 to 3 is significantly improved compared with sample 4. 90 When the pore size is controlled below a certain level, the PM collection performance of the particulate filter is greatly improved. This is believed to be because the number of large pores with large gas flow and easy penetration of particulate matter is reduced. In addition, when comparing samples 1 to 3 with sample 4, no significant difference in the initial pressure loss and PM accumulation pressure loss was confirmed. It can be seen that when D is reduced, the PM collection performance is greatly improved. 90 When the pore diameter is small, the PM collection performance is greatly improved, while at the same time, there is no excessive increase in pressure loss.

[0163] Next, the PM capture rate of samples 1 to 3 is further improved compared with samples 5 to 7. It can be seen that when an external coating with a thick thickness and high porosity is formed, it is possible to achieve both PM capture performance and pressure loss suppression performance at a higher level. It is believed that this is because the flow distance of particulate matter inside the external coating becomes longer and the gas flow path in the external coating becomes more. In addition, among samples 1 to 3, sample 2 has the best PM capture performance. It is expected that this is because the flow path length of the pores in the external coating of sample 2 is longer.

[0164] From the above test results, it can be seen that by 90 By setting the pore diameter, the thickness of the outer coating layer, and the porosity of the outer coating layer within appropriate ranges, it is possible to achieve both PM trapping performance and pressure loss suppression performance at a high level.

[0165] Although specific examples of the technology disclosed herein have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the scope of the claims includes various modifications and changes to the specific examples described above.

[0166] Industrial availability

[0167] According to the present invention, it is possible to provide a particulate filter capable of achieving both PM trapping performance and pressure loss suppression performance at high levels.

Claims

1. A particulate filter for capturing particulate matter in exhaust gas discharged from an internal combustion engine, comprising at least: A wall-flow type substrate having an inlet compartment whose end portion is open only on the exhaust gas inlet side, an outlet compartment whose end portion is open only on the exhaust gas outlet side, and a porous partition wall separating the inlet compartment from the outlet compartment; and an outer coating layer which is a porous layer formed of a plurality of granular bodies and is formed on a surface of the partition wall in contact with the inlet compartment, D of the pores of the outer coating layer in the volume-based pore size distribution by mercury intrusion porosimetry 90 The pore diameter is 1 μm or more and 10 μm or less. In cross-sectional SEM observation, the outer coating layer has a thickness of 20 μm or more and 80 μm or less, and a porosity of 30% or more and 70% or less.

2. The particulate filter according to claim 1, wherein: The flow path length of the pores in a measurement area of ​​100 μm along the extending direction of the outer coating layer is 200 μm or more.

3. The particulate filter according to claim 1 or 2, wherein: In the outer coating layer, D in the volume-based pore size distribution based on mercury intrusion porosimetry is 50 The pore diameter is 0.8 μm or more and 3 μm or less.

4. The particulate filter according to any one of claims 1 to 3, wherein The D in the particle size distribution based on the number of the granular bodies in the outer coating layer by the laser diffraction light scattering method is 50 The particle size is X, and D in the pore distribution based on the volume of the voids in the partition wall by mercury intrusion porosimetry is 50 When the pore diameter is set to Y, the following formula (1) is satisfied: 0.1≤X / Y≤3 (1).

5. The particulate filter according to any one of claims 1 to 4, wherein The granular body contains at least one selected from the group consisting of aluminum oxide, cerium oxide, zirconium oxide, silicon dioxide, magnesium oxide, and calcium oxide.

6. The particulate filter according to any one of claims 1 to 5, wherein The granular body contains a precious metal catalyst for purifying harmful gas components in the exhaust gas.

7. The particulate filter according to any one of claims 1 to 6, wherein An inner coating layer is further provided, which is formed on a wall surface of a gap in a predetermined region of the inlet-side cell from a surface of the partition wall in contact with the outlet-side cell and contains a precious metal catalyst for purifying harmful gas components in the exhaust gas.

8. The particulate filter according to claim 7, wherein: An uncoated region in which substantially no coating layer exists is provided between the outer coating layer and the inner coating layer in the thickness direction of the partition wall.

9. The particulate filter according to claim 8, wherein: The size of the uncoated region in the thickness direction of the partition wall is 10 μm or more.

10. The particulate filter according to any one of claims 1 to 9, wherein The internal combustion engine is a gasoline engine.

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