Particulate filter

Abstract

According to the technology disclosed herein, provided is a particulate filter that is capable of achieving superior PM collection performance by appropriately preventing PM from passing therethrough to an exit-side cell. A particulate filter (1) disclosed herein includes an external coat layer (20) that is formed on an entry-side surface (16a) of a partition (16) of a base material (10). This external coat layer (20) includes at least a bottom layer (22) formed on the entry-side surface (16a) of the partition (16) and a top layer (24) formed so as to cover the bottom layer (22). Also, the average particle diameter of granules contained in the top layer (24) is greater than the average particle diameter of granules contained in the bottom layer (22), the average particle diameter of the granules in the bottom layer (22) is 0.4-2.0 μm, and the average particle diameter of the granules in the top layer (24) is 2.0-7.0 μm. According to said configuration, because it is possible to collect PM in an entry-side cell (12), it is possible to achieve superior PM collection performance by appropriately preventing PM from passing through the particulate filter (1) to an exit-side cell (14).

Description

[Technical Field]

[0001] The present invention relates to a particulate filter, and more particularly to a particulate filter that collects particulate matter (PM) contained in exhaust gas emitted from an internal combustion engine. This international application claims priority based on Japanese Patent Application No. 2021-104292, filed on June 23, 2021, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines such as automobile engines contain hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO x In addition to harmful gas components such as carbon dioxide, PM also contains particulate matter (PM), which is primarily composed of carbon. As with harmful gas components, there are concerns about the effects of PM on the human body. For this reason, particulate filters are installed in the exhaust systems of internal combustion engines to capture PM in the exhaust gas.

[0003] Such a particulate filter includes, for example, a wall-flow type substrate. The wall-flow type substrate includes inlet cells that are open only at the end on the exhaust gas inlet side, outlet cells that are open only at the end on the exhaust gas outlet side, and porous partition walls that separate the two cells. Exhaust gas supplied to such a wall-flow type substrate flows into the inlet cells, passes through the partition walls, and is then discharged from the outlet cells. At this time, PM in the exhaust gas is captured inside the porous partition walls.

[0004] Furthermore, in this type of particulate filter, a coating layer may be formed on the partition walls of the substrate in order to improve the PM trapping performance. For example, in the particulate filter described in Patent Document 1, the pore size of the partition walls is controlled by applying a wash coat layer to the inside of the partition walls (the wall surfaces of the pores in the partition walls). This makes it possible to obtain a particulate filter that can stably trap PM regardless of the driving state. Furthermore, as described in Patent Document 1, in recent years, a technology has also been proposed in which a noble metal catalyst (typically a three-way catalyst) is contained in the coating layer to impart a purification function to harmful gas components to the particulate filter. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2020-81912 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, increasing concern for the environment has led to a demand for the development of particulate filters that can exhibit better PM trapping performance than conventional filters. In conducting research to meet this demand, the present inventors focused on the phenomenon of PM slipping through the pores in the partition walls. Specifically, when large pores are formed in the partition walls of a wall-flow type substrate or when PM with a small particle size is contained in exhaust gas, "PM slippage" occurs, whereby PM is not trapped inside the partition walls but flows out to the outlet cells, resulting in a decrease in PM trapping performance.

[0007] The present invention has been made in view of the above points, and an object of the present invention is to provide a particulate filter that can appropriately prevent PM from passing through to the outlet cells and exhibit excellent PM trapping performance. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides a particulate filter having the following configuration.

[0009] The particulate filter disclosed herein is disposed in the exhaust system of an internal combustion engine and is used to capture particulate matter in exhaust gas emitted from the internal combustion engine. The particulate filter at least comprises a wall-flow type substrate having inlet cells that are open only at their ends on the exhaust gas inlet side, outlet cells that are open only at their ends on the exhaust gas outlet side, and porous partition walls that separate the inlet cells from the outlet cells, and an outer coating layer that is a porous layer formed of a plurality of granules and is formed on the surfaces of the partition walls that contact the inlet cells. The outer coating layer of the particulate filter disclosed herein comprises at least a lower layer that is formed on the surfaces of the partition walls that contact the inlet cells, and an upper layer that is formed to cover the lower layer. The average particle diameter of the granules in the upper layer is larger than that of the granules in the lower layer, the average particle diameter of the granules in the lower layer being 0.4 μm or more and 2.0 μm or less, and the average particle diameter of the granules in the upper layer being 2.0 μm or more and 7.0 μm or less.

[0010] As a result of various studies, the present inventors have come to the conclusion that PM may slip through if it is trapped inside the partition walls as in the conventional art, and have therefore come up with the idea of ​​trapping PM outside the partition walls (typically in the inlet cells). The particulate filter disclosed herein has been made based on this finding, and a porous outer coating layer is formed on the surface of the partition walls that contacts the inlet cells (the inlet side). By covering the entrances of the pores of the partition walls with the outer coating layer in this way, PM can be trapped outside the partition walls, and therefore PM can be suitably prevented from slipping through. Furthermore, a particulate filter configured in this way can also suppress PM from accumulating inside the partition walls. That is, the particulate filter disclosed herein can also contribute to suppressing a sudden increase in pressure drop due to pore blockage.

[0011] Incidentally, as will be described in detail later, in the actual manufacture of a particulate filter, it is difficult to form an outer coating layer that can adequately cover the inlet side of the partition walls (particularly the entrances of the pores). In contrast, in the technology disclosed herein, an outer coating layer with a multilayer structure including an upper layer and a lower layer is formed, and the particle diameter of the granular material in each layer is controlled so as to satisfy the above-mentioned predetermined conditions. This makes it possible to realize an outer coating layer that adequately covers the entrances of the pores, and to suitably prevent PM from passing through.

[0012] In a preferred embodiment of the particulate filter disclosed herein, the granules contain at least one selected from the group consisting of alumina, ceria, zirconia, silica, magnesia, and calcia, thereby forming an outer coating layer with excellent heat resistance.

[0013] In a preferred embodiment of the particulate filter disclosed herein, the granules contain a precious metal catalyst that purifies harmful gas components in exhaust gas. This makes it possible to obtain a particulate filter that is endowed with the function of purifying harmful gas components. Note that, as will be described in detail later, the technology disclosed herein also has the effect of improving the function of this precious metal catalyst in purifying harmful gas components.

[0014] In a preferred embodiment of the particulate filter disclosed herein, the coating amount of the lower layer is greater than the coating amount of the upper layer, thereby making it possible to obtain a particulate filter that is excellent in pressure loss suppression performance.

[0015] In a preferred embodiment of the particulate filter disclosed herein, the coating amount of the lower layer per 1 L of the volume of the substrate is 30 g / L or more and 50 g / L or less. This makes it possible to obtain a particulate filter with more excellent pressure loss suppression performance.

[0016] In a preferred embodiment of the particulate filter disclosed herein, the coating amount of the upper layer per 1 L of the volume of the substrate is 10 g / L or more and 30 g / L or less. This makes it possible to obtain a particulate filter that achieves both high levels of PM trapping performance and high levels of pressure loss suppression performance.

[0017] In a preferred embodiment of the particulate filter disclosed herein, the porosity of the lower layer is greater than the porosity of the upper layer, thereby making it possible to obtain a particulate filter with even more excellent PM trapping performance.

[0018] In a preferred embodiment of the particulate filter disclosed herein, the porosity of the lower layer is 24% or more and 57% or less, thereby making it possible to obtain a particulate filter that achieves both high levels of PM trapping performance and high levels of pressure loss suppression performance.

[0019] In a preferred embodiment of the particulate filter disclosed herein, the porosity of the upper layer is 21% or more and 38% or less, thereby making it possible to obtain a particulate filter that achieves both high levels of PM trapping performance and high levels of pressure loss suppression performance.

[0020] In a preferred embodiment of the particulate filter disclosed herein, the average pore size of the partition walls is 5 μm or more and 50 μm or less. According to the technology disclosed herein, even in a substrate having partition walls with such large pores formed therein, an outer coating layer that appropriately covers the surfaces of the partition walls can be formed.

[0021] In a preferred embodiment of the particulate filter disclosed herein, the porosity of the partition walls is 30% to 90%. According to the technology disclosed herein, even in a substrate having partition walls with such a large number of pores formed therein, an outer coating layer that appropriately covers the surfaces of the partition walls can be formed.

[0022] In a preferred embodiment of the particulate filter disclosed herein, the particulate filter further comprises an inner coating layer formed on the wall surfaces of the pores in a predetermined region from the surface of the partition wall in contact with the outlet cells toward the inlet cells, the inner coating layer containing a precious metal catalyst for purifying harmful gas components in the exhaust gas. According to this configuration, the exhaust gas from which PM has been removed by the outer coating layer can be supplied to the inner coating layer containing the precious metal catalyst. This prevents a decrease in activity of the precious metal catalyst due to adhesion (coating) of PM, and makes it possible to obtain a particulate filter that exhibits excellent purification performance for harmful gas components.

[0023] In an embodiment in which an inner coating layer is formed, it is preferable that an uncoated region in which no coating layer is substantially present is provided between the outer coating layer and the inner coating layer in the thickness direction of the partition wall, thereby preventing minute PM that has passed through the outer coating layer from accumulating in the region in which the pores are narrowed by the inner coating layer and clogging the pores.

[0024] In the embodiment in which the uncoated region is formed, the dimension of the uncoated region in the thickness direction of the partition wall is preferably 10 μm or more and 100 μm or less, which makes it possible to adequately secure a region for forming the inner coating layer and to suitably prevent adhesion of PM to the inner coating layer.

[0025] In a preferred embodiment of the particulate filter disclosed herein, the internal combustion engine is a gasoline engine. Exhaust gas generated in a gasoline engine has a higher temperature than exhaust gas emitted from other engines (such as diesel engines), and PM trapped on the outside of the partition walls (inlet cells) can be easily combusted. From this perspective, the particulate filter disclosed herein can be particularly suitably used as a particulate filter (GPF: Gasoline Particulate Filter) for a gasoline engine. [Brief explanation of the drawings]

[0026] [Figure 1]FIG. 1 is a diagram that schematically shows an exhaust system in which a particulate filter is arranged. [Figure 2] FIG. 2 is a perspective view that schematically shows the particulate filter according to the first embodiment. [Figure 3] FIG. 3 is a diagram that schematically shows a cross section along the cylindrical axis direction of the particulate filter according to the first embodiment. [Figure 4] FIG. 4 is an enlarged cross-sectional view that schematically shows a cross section of a partition wall of the particulate filter according to the first embodiment. [Figure 5] FIG. 5 is a view that schematically shows a cross section along the cylindrical axis direction of the particulate filter according to the second embodiment. [Figure 6] FIG. 6 is an enlarged cross-sectional view that schematically shows a cross section of a partition wall of a particulate filter according to a second embodiment. [Figure 7] FIG. 7 is a cross-sectional SEM photograph of a cross section of a partition wall of the particulate filter of Sample 1 taken at a magnification of 200 times. [Figure 8] FIG. 8 is an image obtained by binarizing the cross-sectional SEM photograph of FIG. [Figure 9] FIG. 9 is an enlarged cross-sectional view that schematically shows a cross section of a partition wall of a conventional particulate filter. [Figure 10] FIG. 10 is an enlarged cross-sectional view that schematically shows a cross section of a partition wall of a particulate filter that was manufactured in the process leading to the technique disclosed herein. [Figure 11] FIG. 11 is an enlarged cross-sectional view that schematically shows a cross section of a partition wall of a particulate filter that was manufactured in the process leading to the technique disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0027] Preferred embodiments of the present invention will be described below with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification but necessary for implementing the present invention (e.g., general matters relating to the placement of a particulate filter in an exhaust path) can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, the expression "A to B" indicating a numerical range in this specification means "greater than or equal to A and less than or equal to B."

[0028] [Exhaust system of internal combustion engine] First, the use of the particulate filter disclosed herein will be described. Figure 1 is a diagram showing a schematic view of an exhaust system in which a particulate filter is arranged.

[0029] Reference numeral 2 in FIG. 1 denotes an internal combustion engine. An air-fuel mixture containing oxygen and fuel gas is supplied to this internal combustion engine 2. The internal combustion engine 2 burns this mixture to generate kinetic energy. Exhaust gas generated by the combustion of this mixture is then discharged into an exhaust system made up of an exhaust manifold 3 and an exhaust pipe 4, as shown by the arrows in FIG. 1. For ease of explanation, in this specification, the side closer to the internal combustion engine 2 in the flow direction of the exhaust gas is referred to as the upstream side, and the side farther from the internal combustion engine 2 is referred to as the downstream side.

[0030] In addition, a sensor 8 that detects information related to the components and temperature of exhaust gas is attached to the exhaust pipe 4. This sensor 8 is connected to an engine control unit (ECU) 7. The information detected by the sensor 8 is transmitted to the ECU 7 and used as one piece of information for correcting the operation control of the internal combustion engine 2.

[0031] An exhaust system (exhaust pipe 4) of the internal combustion engine 2 is provided with a particulate filter 1, an exhaust gas purification catalyst 5, and an underfloor catalyst 9. The particulate filter 1 collects particulate matter (PM) in the exhaust gas. The specific configuration of the particulate filter 1 will be described later. The exhaust gas from which PM has been removed by the particulate filter 1 passes through the exhaust gas purification catalyst 5 and the underfloor catalyst 9 and is then discharged to the outside of the exhaust system. The exhaust gas purification catalyst 5 and the underfloor catalyst 9 are components including a catalytic body that purifies harmful gas components (NOx, HC, CO) in the exhaust gas. Note that the specific configurations of the exhaust gas purification catalyst 5 and the underfloor catalyst 9 do not characterize the technology disclosed herein, and therefore detailed description thereof will be omitted. Note that in the exhaust system shown in FIG. 1, the exhaust gas purification catalyst 5 is disposed downstream of the particulate filter 1, but their positions are not particularly limited. For example, the exhaust gas purification catalyst may be disposed upstream of the particulate filter. The particulate filter disclosed herein may also be placed at the underfloor catalyst location.

[0032] [First embodiment] Hereinafter, a first embodiment of the particulate filter disclosed herein will be described. Fig. 2 is a perspective view that schematically shows the particulate filter according to the first embodiment. Fig. 3 is a view that schematically shows a cross section along the cylindrical axis direction of the particulate filter according to the first embodiment. Also, Fig. 4 is an enlarged cross-sectional view that schematically shows a cross section of the partition walls of the particulate filter according to the first embodiment. Note that in each drawing referred to in this specification, the symbol A indicates the "flow direction of exhaust gas". Also, the symbol X indicates the "extension direction of the partition walls", and the symbol Y indicates the "thickness direction of the partition walls of the substrate".

[0033] 2 to 4, the particulate filter 1 according to this embodiment includes a substrate 10 and an outer coat layer 20. Each of these will be described below.

[0034] 1. Base material The substrate 10 forms the framework of the particulate filter 1. As shown in FIG. 2, in this embodiment, a cylindrical substrate 10 extending along the flow direction A of exhaust gas is used. The outer shape of the substrate is not limited to a cylindrical shape and may be an elliptical cylinder, a polygonal cylinder, or the like. The overall length and volume of the substrate 10 are also not particularly limited and can be appropriately changed depending on the performance of the internal combustion engine 2 (see FIG. 1) and the dimensions of the exhaust pipe 4. The substrate 10 in this embodiment can be made of any conventionally known material that can be used for the substrate of a particulate filter, without any particular restrictions. Examples of materials for the substrate 10 include ceramics such as cordierite, silicon carbide (SiC), and aluminum titanate, and highly heat-resistant materials such as alloys such as stainless steel. For example, cordierite has excellent resistance to thermal shock and is therefore particularly suitable for use as a material for the substrate of a particulate filter (GPF) for a gasoline engine, which is likely to be supplied with high-temperature exhaust gas.

[0035] The substrate 10 is a wall-flow type substrate. Specifically, as shown in FIGS. 2 and 3 , the substrate 10 includes inlet cells 12 that are open only at their ends on the exhaust gas inlet side, outlet cells 14 that are open only at their ends on the exhaust gas outlet side, and porous partition walls 16 that separate the inlet cells 12 and the outlet cells 14. Specifically, the inlet cells 12 are gas flow paths that are open at their ends on the exhaust gas inlet side and blocked with plugs 12 a at their ends on the exhaust gas outlet side. On the other hand, the outlet cells 14 are gas flow paths that are blocked with plugs 14 a at their ends on the exhaust gas inlet side and open at their ends on the exhaust gas outlet side. In the substrate 10, the inlet cells 12 and the outlet cells 14 are alternately arranged adjacent to each other. The partition walls 16 that separate the inlet cells 12 and the outlet cells 14 have a plurality of pores 18 (see FIG. 4 ) that connect the inlet cells 12 and the outlet cells 14 to each other. In Fig. 2, the shape of the inlet cells 12 and the outlet cells 14 in a cross section perpendicular to the stretching direction X of the partition walls 16 (typically a cross section along the radial direction) is approximately square. However, the cross-sectional shape of each cell is not particularly limited, and various geometric shapes can be used, such as a parallelogram, a rectangle, a trapezoid, or other rectangular shape, a triangle, other polygonal shapes (e.g., a hexagon, an octagon), or a circle. The thickness of the partition walls 16 is preferably about 0.05 mm to 2 mm, and more preferably about 0.1 mm to 1 mm.

[0036] The pore size of the pores 18 and the porosity of the partition walls 16 can affect the PM trapping performance and pressure loss suppression performance of the particulate filter 1. Specifically, the average pore size of the pores 18 is preferably 5 μm or more, more preferably 10 μm or more, and particularly preferably 15 μm or more. The porosity of the partition walls 16 is preferably 30% or more, more preferably 40% or more, and particularly preferably 50% or more. The partition walls 16 having many relatively large pores 18 in this manner have excellent breathability and can appropriately suppress an increase in pressure loss. However, in a substrate 10 in which many large pores 18 are formed, it is difficult to form the outer coating layer 20 on the inlet side 16 a of the partition walls 16. However, in the technology disclosed herein, the particle size of the granular material used to form the outer coating layer 20 is appropriately controlled, and therefore, it is possible to form the outer coating layer 20 that appropriately covers the inlet side 16 a of the partition walls 16 even in a substrate 10 in which many large pores 18 are formed. In this specification, the "average pore size of the pores in the partition walls" and the "porosity of the partition walls" are values ​​measured by mercury intrusion porosimetry.

[0037] On the other hand, the upper limit of the average pore size of the pores 18 is preferably 50 μm or less, more preferably 40 μm or less, and particularly preferably 30 μm or less. Furthermore, the upper limit of the porosity of the partition walls 16 is preferably 90% or less, more preferably 80% or less, even more preferably 70% or less, and particularly preferably 65% ​​or less. This allows a substrate 10 with excellent strength to be obtained. Furthermore, as the average pore size or porosity of the partition walls 16 decreases, the outer coating layer 20 tends to be more easily formed on the inlet side surface 16a of the partition walls 16.

[0038] 2.Outer coating layer (1) Overall structure of the outer coating layer As shown in FIGS. 3 and 4, in the particulate filter 1 according to this embodiment, an outer coating layer 20 is formed on the surface (inlet side 16a) of the partition wall 16 of the substrate 10 that contacts the inlet cell 12. This outer coating layer 20 is a porous layer in which particulate matter is aggregated. The particulate matter of this outer coating layer 20 preferably contains a heat-resistant material defined in JIS R2001 as a main component. Examples of such heat-resistant materials include alumina (Al2O3), ceria (CeO2), zirconia (ZrO2), silica (SiO2), magnesia (MgO), and calcia (CaO). Among these heat-resistant materials, alumina, ceria, and zirconia are more preferable. Note that the phrase "containing as a main component" means that the heat-resistant material accounts for 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) of the total weight of the outer coating layer 20. This makes it possible to prevent a decrease in breathability due to sintering of the outer coat layer 20.

[0039] The outer coating layer 20 includes a lower layer 22 formed on the inlet side 16a of the partition wall 16 and an upper layer 24 formed to cover the lower layer 22. In this embodiment, the average particle size of the granular material in each of the lower layer 22 and the upper layer 24 is controlled to satisfy a predetermined condition. This allows the inlets 18a of the pores 18 to be covered with the outer coating layer 20, so that PM is trapped outside the partition wall 16 (inlet cells 12) and the PM can be prevented from entering the pores 18. Therefore, the particulate filter 1 according to this embodiment can appropriately prevent PM from passing through to the outlet cells 14 and exhibit excellent PM trapping performance. The control of the average particle size of the granular material in each layer will be described in detail later. The granular material contained in the lower layer 22 and the granular material contained in the upper layer 24 may be made of the same material or different materials.

[0040] The granules of the outer coating layer 20 may contain a precious metal catalyst, which can reduce harmful gas components (HC, CO, NO) in the exhaust gas. x), a particulate filter 1 can be obtained that is endowed with a purification function for PM. Furthermore, the precious metal catalyst promotes the combustion of PM trapped in the outer coating layer 20, and therefore can also contribute to improving pressure loss suppression performance. The type of such precious metal catalyst is not particularly limited, and conventionally known materials can be used without particular limitations. Specific examples of precious metal catalysts include gold (Au), silver (Ag), palladium (Pd), platinum (Pt), rhodium (Rh), and ruthenium (Ru). These precious metal catalysts oxidize CO and HC in exhaust gas and oxidize NO x The noble metal catalyst may be added to both the lower layer 22 and the upper layer 24, or to only one of them.

[0041] In addition, although details will be described later, in the particulate filter 1 according to this embodiment, in order to form an appropriate outer coating layer 20 on the inlet side 16a of the partition wall 16, the lower limit of the average particle diameter of the granular material in the lower layer 22 is controlled to 0.4 μm or more. Meanwhile, according to experiments conducted by the present inventors, it has been confirmed that when a precious metal catalyst is added to the lower layer 22, in addition to the effect of forming the appropriate outer coating layer 20 described above, the purification performance for harmful gas components is also significantly improved. That is, the technology disclosed herein can exhibit particularly favorable effects in a particulate filter (i.e., a filter catalyst) in which a precious metal catalyst is catalyzed in the coating layer. The reason for this effect is presumed to be that the decrease in catalytic activity due to sintering of the precious metal catalyst can be suppressed.

[0042] Furthermore, when a precious metal catalyst is added to the outer coating layer 20, it is preferable to also add an additive that enhances the catalytic activity of the precious metal catalyst. One example of such an additive is an OSC material. This OSC material refers to a material that has oxygen storage capacity (OSC) and absorbs and releases oxygen. Adding this OSC material makes it easier to maintain the exhaust gas atmosphere in contact with the outer coating layer 20 near the stoichiometric air-fuel ratio (theoretical air-fuel ratio), thereby stabilizing the catalytic action of the precious metal catalyst. One example of such an OSC material is a ceria-zirconia composite oxide. Furthermore, additives other than the OSC material, such as a NOx adsorbent or stabilizer having NOx absorption capacity, may be added. Furthermore, the outer coating layer 20 may contain trace components derived from the raw materials or manufacturing process. For example, the outer coating layer 20 may contain one or more compounds (oxides, sulfates, carbonates, nitrates, chlorides) of alkaline earth metals (Be, Mg, Ca, Ba, etc.), rare earth metals (Y, La, Ce, etc.), alkali metals (Li, Na, K, etc.), transition metals (Mn, Fe, Co, Ni, etc.), etc.

[0043] (2) Structure of the upper and lower layers As described above, in the particulate filter 1 according to this embodiment, the outer coat layer 20 is formed to have a multilayer structure including the lower layer 22 and the upper layer 24, and the average particle size of each layer is controlled within a predetermined range. Specifically, in this embodiment, the average particle size of the granular material in the upper layer 24 is made larger than the average particle size of the granular material in the lower layer 22, and the average particle size of the granular material in the lower layer 22 is controlled to be 0.4 μm or more and 2.0 μm or less, and the average particle size of the granular material in the upper layer 24 is controlled to be 2.0 μm or more and 7.0 μm or less. This allows the outer coat layer 20 to be formed on the inlet side surface 16 a of the partition wall 16 so as to appropriately cover the inlets 18 a of the pores 18, thereby preventing PM from passing through to the outlet cells 14. Below, the structures of the lower layer 22 and the upper layer 24 of the outer coat layer 20 will be described in comparison with the coat layers of conventional particulate filters.

[0044] First, a conventional particulate filter and a particulate filter manufactured in the process leading to the technology disclosed herein will be described. Fig. 9 is an enlarged cross-sectional view schematically showing a cross section of a partition wall of a substrate of a conventional particulate filter. Also, Figs. 10 and 11 are enlarged cross-sectional views schematically showing a cross section of a partition wall of a substrate of a particulate filter manufactured in the process leading to the technology disclosed herein.

[0045] As shown in Fig. 9, in the conventional particulate filter 100, a coating layer 120 is formed inside the partition walls 116 of the substrate (specifically, on the wall surfaces 118b of the pores 118). This narrows the pores 118, thereby reducing the amount of PM passing through to the outlet cells 114. However, the particulate filter 100 having such a structure cannot sufficiently prevent the PM from passing through, and when the pore diameter of the pores 118 is large or the particle diameter of the PM is small, there is a possibility that the PM will pass through to the outlet cells 114 through the pores 118. After various studies, the present inventors have concluded that in order to appropriately prevent this type of PM from passing through, it is sufficient to cover the inlets 118a of the pores 118 with a porous coating layer and collect the PM in the inlet cells 112, thereby preventing the PM from entering the partition walls 116 at all.

[0046] However, in actual production of particulate filters, it has been difficult to form an outer coating layer that can adequately cover the entrances of the pores in the partition walls. Specifically, when forming a coating layer on the partition walls of a substrate, a precursor (raw material slurry) of the coating layer is injected into the inlet cells, and then the outlet cells are suctioned to supply the raw material slurry into the interior of the substrate. By adjusting the suction force at this time, as in the particulate filter 200 shown in FIG. 10 , an outer coating layer 220 can be formed on the inlet side of the partition walls 216, and some PM can be collected by the inlet cells 212. However, if the particulate matter contained in the raw material slurry is small, some of the raw material slurry may pass through the pores 218 of the partition walls 216, and openings 220a may be formed in the outer coating layer 220. Such an outer coating layer 220 cannot adequately cover the entrances 218a of the pores 218, and PM may enter the interior of the partition walls 216 through the openings 220a, making it difficult to adequately prevent PM from passing through to the outlet cells 214.

[0047] On the other hand, if the granules contained in the raw slurry are made larger, the raw slurry can be prevented from passing through the pores of the partition walls, and therefore, it is possible to form a coating layer 320 that does not have openings, as in the particulate filter 300 shown in Fig. 11. However, if such large granules are used, a large amount of the coating layer 320 will enter the pores 318 from the inlet side surface 316a of the partition walls 316, and the pores 318 may be blocked by the large amount of the coating layer 320. In this case, it is possible to prevent the PM from passing through, but there is a risk that the pressure loss will increase to a level that makes it difficult to use.

[0048] In contrast, the outer coating layer 20 in this embodiment has a multilayer structure including a lower layer 22 formed of granules with a small particle diameter and an upper layer 24 formed of granules with a large particle diameter (see FIG. 4). In forming the outer coating layer 20 with such a multilayer structure, first, the lower layer 22 is formed using a lower layer slurry containing granules with a small particle diameter. In forming this lower layer 22, as in the particulate filter 200 shown in FIG. 10, part of the lower layer slurry passes through the pores 18, so that openings 22a may be generated in the formed lower layer 22. However, in this embodiment, after forming this lower layer 22, the upper layer 24 is formed using an upper layer slurry containing granules with a large particle diameter. As a result, the upper layer 24 is formed so as to close the openings 22a of the lower layer 22, so that the outer coating layer 20 that appropriately covers the entrances 18a of the pores 18 can be formed. Furthermore, in this embodiment, since the lower layer 22 is formed in advance, unlike the particulate filter 300 shown in FIG. 11, it is possible to prevent the pores 18 from being blocked by the upper layer slurry.

[0049] As described above, the particulate filter 1 according to this embodiment has the inlets 18a of the pores 18 covered by the outer coat layer 20 having a multilayer structure including the lower layer 22 and the upper layer 24, and therefore can prevent PM from entering the interior of the partition walls 16. This prevents PM from passing through to the outlet cells 14, and high PM trapping performance can be exhibited. Note that in the particulate filter 1 according to this embodiment, PM is trapped in the inlet cells 12. These inlet cells 12 have a larger volume than the pores 18 of the partition walls 16, and are therefore less likely to be clogged even if a large amount of PM accumulates. Therefore, the particulate filter 1 according to this embodiment can also contribute to suppressing an increase in pressure loss due to PM accumulation.

[0050] According to the inventors' investigations, simply specifying the relative particle sizes of the granules in the lower layer 22 and the upper layer 24 may not result in the formation of an outer coating layer 20 that adequately covers the entrances 18a of the pores 18. For example, if the granules in the lower layer 22 are too small, most of the lower layer slurry supplied to the inlet cells 12 may penetrate into the pores 18. In this case, the lower layer 22 may not be formed to partially cover the entrances 18a of the pores 18, and a coating layer (see coating layer 120 in FIG. 9 ) may be formed that adheres to the wall surfaces 18b of the pores 18. From this perspective, the average particle size of the granules in the lower layer 22 needs to be controlled to 0.4 μm or more. As the granules in the lower layer 22 become larger, the entrances 18a of the pores 18 tend to be more easily covered by the lower layer 22, and the openings 22a of the lower layer 22 tend to become smaller. From this perspective, the average particle size of the granules in the lower layer 22 is preferably 0.45 μm or more, and particularly preferably 0.5 μm or more. On the other hand, if the granules in the lower layer 22 become too large, there is a risk of forming a coating layer that blocks the pores 18 (see coating layer 320 in FIG. 11). For this reason, the average particle size of the granules for the lower layer 22 needs to be controlled to 2.0 μm or less. Note that, from the perspective of more effectively preventing the blocking of the pores 18 by the coating layer, the upper limit of the average particle size of the granules in the lower layer 22 is preferably 1.75 μm or less, and particularly preferably 1.5 μm or less.

[0051] Furthermore, if the granules for the upper layer 24 are too small, the upper layer slurry may pass through the openings 22a of the lower layer 22. In this case, a coating layer made of large-diameter granules may be formed inside the partition walls 16, potentially blocking the pores 18. For this reason, the average particle size of the granules for the upper layer 24 must be controlled to 2.0 μm or more. From the viewpoint of more appropriately suppressing the inflow of the upper layer slurry into the pores 18, the average particle size of the granules for the upper layer 24 is preferably 2.5 μm or more, more preferably 3.0 μm or more, and particularly preferably 3.5 μm or more. On the other hand, if the granules for the upper layer 24 are too large, the contact area at the interface between the lower layer 22 and the upper layer 24 may decrease, potentially causing peeling of the upper layer 24. For this reason, the average particle size of the granules for the upper layer 24 must be controlled to 7.0 μm or less. From the viewpoint of more appropriately preventing peeling of the upper layer 24, the upper limit of the average particle size of the granules for the upper layer 24 is preferably 6.5 μm or less, more preferably 6 μm or less, even more preferably 5.5 μm or less, and particularly preferably 5 μm or less.

[0052] In this specification, the term "average particle size" refers to the cumulative 50% particle size (D 50 ) Specifically, in measuring this "average particle size," first, SEM images of the outer coating layers (upper and lower layers) are subjected to image analysis, and the circle-equivalent diameters of 10 particles constituting each layer are measured to create a particle size distribution based on the number. Then, in this particle size distribution, the particle diameter corresponding to the smallest cumulative 50% of the particle size is defined as the "average particle size D 50 ". The average particle size in this specification is calculated by including the particle sizes of secondary particles formed by aggregation, sintering, etc. That is, when calculating the average particle size, the circle-equivalent diameters of particles confirmed by electron microscope observation are measured without distinguishing between primary particles and secondary particles, and the average particle size is calculated based on the circle-equivalent diameters of the particles thus measured.

[0053] Furthermore, in this specification, the formation position of the lower layer 22 is described as "formed on the surface (inlet side surface 16a) of the partition wall 16 that is in contact with the inlet cell 12." Here, "formed on the inlet side surface 16a" means that most of the lower layer 22 is attached to the inlet side surface 16a of the partition wall 16, and does not prohibit part of the lower layer 22 from penetrating into the partition wall 16. Typically, it can be said that "the lower layer 22 is formed on the inlet side surface 16a of the partition wall 16" when 20% or more (preferably 30% or more, more preferably 40% or more) of the lower layer 22 is attached to the inlet side surface 16a of the partition wall 16 in an analysis based on an SEM image described later.

[0054] Although not limiting the technology disclosed herein, the thickness t1 of the outer coating layer 20 is preferably 3 μm or more, more preferably 5 μm or more, and particularly preferably 10 μm or more. This ensures sufficient PM trapping performance in the outer coating layer 20 and effectively prevents PM from penetrating into the pores 18. On the other hand, if the thickness t1 of the outer coating layer 20 is too large, the inlet cells 12 become narrower, which may cause an increase in pressure loss. From this perspective, the thickness t1 of the outer coating layer 20 is preferably 50 μm or less, more preferably 40 μm or less, and particularly preferably 30 μm or less.

[0055] Furthermore, the porosity of the lower layer 22 is preferably greater than the porosity of the upper layer 24. This allows for a particulate filter with even better PM trapping performance. While not intending to limit the technology disclosed herein, it is presumed that this effect is achieved because covering the lower layer 22 with the upper layer 24, which has a relatively low porosity, reduces PM passing through the outer coating layer 20. The specific porosity of the lower layer 22 is preferably 20% or more, more preferably 24% or more, even more preferably 30% or more, and particularly preferably 35% or more. This ensures a certain level of breathability in the outer coating layer 20 and suppresses a sudden increase in pressure drop. The upper limit of the porosity of the lower layer 22 is preferably 65% ​​or less, more preferably 60% or less, and particularly preferably 57% or less. This allows for more suitable PM trapping performance. On the other hand, the specific porosity of the upper layer 24 is preferably 15% or more, more preferably 20% or more, and particularly preferably 24% or more. This ensures a certain level of breathability in the outer coating layer 20 and prevents a sudden increase in pressure loss. The upper limit of the porosity of the upper layer 24 is preferably 60% or less, more preferably 50% or less, and particularly preferably 40% or less. This allows for more suitable PM trapping performance. The "porosity of the outer coating layer (upper layer, lower layer)" can be measured by performing a predetermined image analysis on a cross-sectional SEM image, as will be described in detail later.

[0056] Furthermore, the coating amount of the lower layer 22 is preferably greater than the coating amount of the upper layer 24. This makes it possible to obtain a particulate filter with even better pressure drop suppression performance. While not intending to limit the technology disclosed herein, it is presumed that the reason for such an effect is that forming a sufficient coating amount of the lower layer 22 can prevent the upper layer slurry from penetrating into the pores 18 of the partition walls 16. The specific coating amount of the lower layer 22 is preferably 30 g / L or more, more preferably 35 g / L or more, and particularly preferably 40 g / L or more. This can more suitably prevent the upper layer slurry from penetrating into the pores 18 of the partition walls 16. Furthermore, the upper limit of the coating amount of the lower layer 22 is not particularly limited, as long as it is not an excessive coating amount that clogs the inlet cells 12. For example, the upper limit of the coating amount of the lower layer 22 may be 60 g / L or less, 55 g / L or less, or 50 g / L or less. On the other hand, the specific coating amount of the upper layer 24 is preferably 1 g / L or more, more preferably 5 g / L or more, and particularly preferably 10 g / L or more. This allows the openings 22a of the lower layer 22 to be more appropriately covered, thereby achieving even more favorable PM capture performance. The upper limit of the coating amount of the upper layer 24 is preferably 50 g / L or less, more preferably 40 g / L or less, and particularly preferably 30 g / L or less. This allows for more favorable suppression of the upper layer slurry from penetrating into the pores 18 of the partition walls 16. In this specification, the term "coating amount (g / L)" refers to the weight (g) of the coating layer relative to the volume (L) of the substrate.

[0057] [Second embodiment] The particulate filter disclosed herein is not limited to the above-described first embodiment, and various forms can be adopted. Hereinafter, a particulate filter according to a second embodiment will be described. Fig. 5 is a view schematically showing a cross section along the cylindrical axis direction of the particulate filter according to the second embodiment. Also, Fig. 6 is an enlarged cross section schematically showing a cross section of a partition wall of the particulate filter according to the second embodiment.

[0058] 1. Inner coating layer As described above, in the particulate filter 1 according to the first embodiment, no coating layer is formed inside the partition walls 16 (see FIGS. 3 and 4). However, the technology disclosed herein does not limit the formation of a coating layer inside the partition walls. For example, as shown in FIGS. 5 and 6, the particulate filter 1A according to the second embodiment includes an inner coating layer 30 formed inside the partition walls 16, in addition to an outer coating layer 20 formed on the inlet side surface 16a of the partition walls 16. This inner coating layer 30 is formed on the wall surfaces 18b of the pores 18 in a predetermined region extending from the surface (outlet side surface 16b) of the partition walls 16 that contacts the outlet cells 14 toward the inlet cells 12.

[0059] This inner coat layer 30 contains a precious metal catalyst that purifies harmful gas components in exhaust gas. In the particulate filter 1A according to this embodiment, as shown in FIG. 6, the inlets 18a of the pores 18 are covered by the outer coat layer 20 (the lower layer 22 and the upper layer 24), so that most of the PM in the exhaust gas is trapped in the inlet cells 12. For this reason, in this embodiment, the improvement in PM trapping performance achieved by narrowing the pores 18 as in the prior art is hardly achieved. However, when a precious metal catalyst is added to this inner coat layer 30, an unprecedented effect occurs in which exhaust gas in which PM has been sufficiently trapped can be brought into contact with the precious metal catalyst. As a result, it is possible to suppress a decrease in catalytic activity due to PM adhering to the precious metal catalyst and to demonstrate excellent purification performance for harmful gas components. That is, the particulate filter 1A according to the second embodiment can appropriately trap PM in the outer coat layer 20 and then appropriately purify harmful gas components in the inner coat layer 30, so that it can efficiently purify exhaust gas.

[0060] As with the outer coating layer 20, various heat-resistant materials can be used for the granules that make up the inner coating layer 30. The precious metal catalyst added to the inner coating layer 30 is not particularly limited, and conventionally known materials can be used without any particular restrictions. These heat-resistant materials and precious metal catalysts have already been described, so a duplicate description will be omitted. Similarly to the outer coating layer 20, various additives (such as OSC materials, NOx adsorbents, and stabilizers) may be added to the inner coating layer 30 in order to improve the catalytic activity of the precious metal catalyst.

[0061] Furthermore, the average particle size of the granules in the inner coating layer 30 is preferably smaller than that of the granules in the upper layer 24 of the outer coating layer 20. If the inner coating layer 30 is formed using granules with a large particle size (average particle size: 2.0 μm or more and 7.0 μm or less) like those in the upper layer 24, the pores 18 may be blocked by the inner coating layer 30, causing a sudden increase in pressure drop. It is more preferable that the average particle size of the granules in the inner coating layer 30 is smaller than that of the lower layer 22 of the outer coating layer 20. This allows the inner coating layer 30 to be formed so as to be properly adhered to the wall surfaces 18b of the pores 18. Specifically, the average particle size of the granules in the inner coating layer 30 is preferably 2.0 μm or less, more preferably 1.5 μm or less, and particularly preferably 1.0 μm or less.

[0062] 2. Uncoated area Moreover, in the particulate filter 1A according to the second embodiment, an uncoated region 40 where substantially no coating layer is present is provided between the outer coating layer 20 and the inner coating layer 30 in the thickness direction Y of the partition walls 16. In other words, this uncoated region 40 is a region provided from the inlet side surface 16a of the partition walls 16 toward the outlet cells 14, and is a region where most of the wall surfaces 18b of the pores 18 are exposed. As shown in Fig. 6, in the particulate filter 1A according to the present embodiment, most of the PM in the exhaust gas is captured by the outer coating layer 20 and deposited in the inlet cells 12. However, when very small PM is contained in the exhaust gas, there is a possibility that PM will pass through the outer coating layer 20 and enter the pores 18. In response to this, by providing an uncoated region 40 between the outer coating layer 20 and the inner coating layer 30 and separating the inner coating layer 30 from the entrances 18a of the pores 18, it is possible to prevent minute PM that has passed through the outer coating layer 20 from accumulating in the region where the pores 18 are narrowed by the inner coating layer 30 and clogging the pores 18. This makes it possible to more effectively suppress a sudden increase in pressure drop.

[0063] In this specification, "substantially no coating layer or catalyst layer is formed" refers to the fact that no coating layer is intentionally formed on the wall surface of the pores. Therefore, if a trace amount of coating layer exists in the region between the outer coating layer 20 and the inner coating layer 30 due to manufacturing errors or the like, this is included in the concept of "substantially no coating layer or catalyst layer is formed" in this specification. Whether or not an uncoated region is formed (i.e., whether or not a coating layer is actively formed in the region between the outer coating layer and the inner coating layer) can be determined according to the procedure described below in "Determination of Each Region."

[0064] Furthermore, the dimension t2 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 more effectively prevents extremely small PM that has passed through the outer coating layer 20 from adhering to the inner coating layer 30. On the other hand, the upper limit of the dimension t2 of the uncoated region 40 is preferably 100 μm or less, more preferably 90 μm or less, and particularly preferably 80 μm or less. This makes it possible to sufficiently ensure the dimension t3 of the region where the inner coating layer 30 is formed, thereby achieving better harmful gas purification performance.

[0065] The particulate filter 1A having the uncoated region 40 can be manufactured according to the following procedure. First, organic solids are filled into the pores 18 of the partition walls 16 from the inlet cell 12 side. Specifically, a pore-filling slurry is prepared by dispersing the organic solids in a predetermined dispersion medium, and the pore-filling slurry is introduced into the interior of the substrate 10 through the inlet cell 12. Then, by suctioning through the outlet cell 14 of the substrate 10, the pore-filling slurry is permeated into a predetermined region extending from the inlet side surface 16a of the partition walls 16 toward the interior of the partition walls 16. Then, a drying process is performed to remove the dispersion medium, thereby filling the region where the uncoated region 40 is to be formed with the organic solids. Note that any conventionally known material can be used as the organic solids without any particular limitations, as long as it is burned away in the firing process described below. Examples of the organic solids include resin beads mainly composed of polyethylene, polypropylene, melamine resin, polymethyl methacrylate (PMMA) resin, etc. As another example, resin fibers such as cellulose microfibers can also be used. The particle size of the organic solid content is preferably adjusted so that it can be suitably filled into the pores 18 of the partition walls 16. For example, when resin beads are used as the organic solid content, it is preferable that the average particle size be approximately 100 μm or less (more preferably 50 μm or less, and even more preferably 30 μm or less). The concentration of the organic solid content in the pore-filling slurry is preferably adjusted appropriately taking into account the porosity of the partition walls 16 of the substrate 10, etc. The dispersion medium can be any liquid material that does not dissolve the organic solid content described above, without any particular limitations. Examples of such dispersion mediums include water and alcohol. The pore-filling slurry may also contain a resin component (cellulose-based resin) that is soluble in this type of dispersion medium.

[0066] Next, a precursor of the outer coating layer 20 is formed on the inlet side 16a of the partition wall 16. Specifically, a lower layer slurry is supplied to the inlet cell 12 of the substrate 10, and then suction is performed through the outlet cell 14. At this time, because the pores 18 of the partition wall 16 are filled with organic solids, the lower layer slurry is less likely to penetrate into the partition wall 16 and is more likely to adhere to the inlet side 16a of the partition wall 16. However, even when such organic solids are filled, it is difficult to properly adhere all of the slurry to the inlet side 16a of the partition wall 16, and some of the slurry may penetrate into the pores 18. For this reason, when a drying process is performed after filling the slurry, a large number of precursors of the lower layer 22 (dried films of the lower layer slurry) having openings 22a are formed on the inlet side 16a of the partition wall 16. For this reason, even in this embodiment, it is necessary to form an upper layer 24 to block the openings 22a of the lower layer 22. Specifically, the upper layer slurry is supplied to the inlet cells 12 of the substrate 10, and then suction is performed from the outlet cells 14. As a result, the upper layer slurry adheres to the surface of the precursor of the lower layer 22. Thereafter, a drying treatment is performed, whereby a precursor of the outer coating layer 20 having the lower layer 22 and the upper layer 24 is formed on the inlet side 16a of the partition wall 16. Note that in the drying treatment, it is preferable to perform a heat treatment to an extent that the solvent of the raw material slurry evaporates but the organic solid content filled in the partition wall 16 is not burned away.

[0067] Next, a precursor of the inner coating layer 30 is formed inside the partition walls 16. Here, first, an inner coating slurry is permeated into the partition walls 16 from the outlet side surface 16b of the partition walls 16. Specifically, the inner coating slurry is supplied into the outlet cells 14 of the substrate 10, and suction is performed through the inlet cells 12. As a result, the slurry adheres to the wall surfaces 18b of the pores 18. At this time, because the organic solid content is filled in the region from the inlet side surface 16a of the partition walls 16 toward the inside of the partition walls 16 (the region where the uncoated region 40 is to be formed), the slurry does not permeate to the inlet side surface 16a of the partition walls 16 and remains near the outlet cells 14 inside the partition walls 16. Then, by performing a drying treatment and a firing treatment, the outer coating layer 20 including the lower layer 22 and the upper layer 24 and the inner coating layer 30 inside the partition walls 16 are formed. Furthermore, in this firing treatment, the organic solid content is burned away, and an uncoated region 40 is formed in the region where the organic solid content was filled (i.e., a predetermined region from the inlet side 16a of the partition wall 16 toward the inside of the partition wall 16). From the viewpoint of reliably burning away the organic solid content, the temperature of the firing treatment is preferably 400°C or higher, more preferably 500°C or higher, and even more preferably 600°C or higher. Similarly, in order to reliably burn away the organic solid content, the firing time is preferably about 2 to 4 hours.

[0068] As described above, by the above-described procedure, it is possible to manufacture a particulate filter 1A in which the outer coating layer 20 is provided on the inlet side 16a of the partition wall 16, the inner coating layer 30 is provided in a predetermined region from the outlet side 16b of the partition wall 16 toward the inlet cell 12, and an uncoated region 40 is provided between the outer coating layer 20 and the inner coating layer 30. Note that the penetration of each slurry in the above-described manufacturing procedure is affected by many conditions, such as the amount of slurry supplied, the viscosity of the slurry, the components of the slurry (particulates, additives, etc.), the porosity of the partition wall of the substrate, the pressure during suction, and the suction time. For this reason, it is preferable to carry out a preliminary test in which these conditions are changed as appropriate, and to control the various conditions so that the slurry adheres to the desired region based on knowledge obtained from the test.

[0069] It should be noted that the above-described manufacturing method is one example of means for manufacturing the particulate filter 1A according to the second embodiment, and is not intended to limit such particulate filter 1A. That is, the method for manufacturing the particulate filter 1A including the outer coating layer 20, the inner coating layer 30, and the uncoated region 40 is not limited to the above-described method. For example, in this embodiment, the particle diameters of the granular materials in each of the lower layer 22 and the upper layer 24 are appropriately controlled, so that the outer coating layer 20 that covers the inlet side surface 16a of the partition wall 16 can be appropriately formed even if the interior of the partition wall 16 is not filled with organic solid content. Then, by appropriately adjusting the conditions for permeating the inner coating slurry (such as the pressure during suction and the suction time), it is also possible to form an appropriate uncoated region 40.

[0070] [Judgment of each area] The outer coating layer 20, the inner coating layer 30, and the uncoated region 40 in the above-described embodiment can be identified based on the following determination procedures (a) to (k). That is, whether or not an outer coating layer, an inner coating layer, an uncoated region, etc. are provided in a predetermined particulate filter can be determined based on the following determination procedures.

[0071] (a) The particulate filter to be inspected is disassembled, and 10 sample pieces are prepared by embedding the partition walls of the substrate in resin. (b) The sample piece is scraped to expose the cross section of the partition wall. The exposed cross section of the partition wall is then observed with a scanning electron microscope (SEM) to obtain a cross-sectional SEM observation image (backscattered electron image, observation magnification 200x). (c) Using two-dimensional image analysis software (product name: ImageJ (registered trademark)), the cross-sectional SEM observation image is automatically binarized to obtain a binary image showing only the coating layer. (d) The images before and after the automatic binarization process are compared, and the coating layer confirmed on the inlet side of the partition wall is regarded as the "external coating layer." (e) Count the "total number of pixels in the coating layer" and the "number of pixels in the entire binary image" confirmed in the above binary image. Then, calculate the value by dividing the "total number of pixels in the coating layer" by the "number of pixels in the entire binary image" and use this as the "coating layer presence rate for the entire partition wall." (f) Comparing the images before and after automatic binarization processing, an arbitrary area is set from the outlet side of the partition wall toward the inlet cell, and the "number of pixels of the coating layer in the set area" is divided by the "total number of pixels in the set area" to calculate the "coating layer presence rate in the set area." (g) The "coating layer presence rate in the set region" is divided by the "coating layer presence rate of the entire partition wall" to calculate a value, and it is determined whether this value is equal to or greater than a first threshold value. Note that the first threshold value in this process is set to, for example, 1.05 (preferably 1.1, more preferably 1.15, even more preferably 1.2, and particularly preferably 1.25). (h) The "arbitrary region from the outlet side of the partition wall toward the inlet cell" described in (f) above is gradually expanded toward the inlet cell, and each time the "coat layer presence rate in the set region" is calculated, the "coat layer presence rate in the set region" is divided by the "coat layer presence rate of the entire partition wall" to calculate a value. The set region is then expanded until this value falls below a first threshold. (i) If a region below the first threshold is confirmed as a result of the process (h) above, the region where the previous determination was made is regarded as the "internal coating layer." The region between the "external coating layer" and the "internal coating layer" is regarded as the "third region." (j) The "number of pixels of the coating layer in the third region" is divided by the "total number of pixels in the third region" to calculate the "coating layer presence rate in the third region." Furthermore, the "coating layer presence rate in the third region" is divided by the "coating layer presence rate in the entire image" to calculate the "coating layer presence rate." If this value is equal to or less than a second threshold, the "third region" is determined to be an "uncoated region" (i.e., an uncoated region is provided between the outer coating layer and the inner coating layer). The second threshold in this process is set to, for example, 0.65 (preferably 0.5, more preferably 0.4, even more preferably 0.3, and particularly preferably 0.25). (k) The processes (b) to (j) are performed on all of the 10 sample pieces, and if an uncoated area between the outer coating layer and the inner coating layer is confirmed in 50% or more of the sample pieces, the particulate filter being inspected is determined to have "an uncoated area between the outer coating layer and the inner coating layer."

[0072] Next, whether the outer coating layer has a multilayer structure including an "upper layer" and a "lower layer" can be determined based on the following determination procedures (l) to (n). Note that the following determination procedures use the cross-sectional SEM observation image of the partition wall acquired in the process (b) above.

[0073] (l) First, measure the circle-equivalent diameters of multiple (e.g., 10) granular particles present on the outermost surface of the region considered to be the "external coating layer" in the process of (d) above (in other words, the surface closest to the inlet cell), and calculate the "average particle diameter of the upper layer." (m) Next, the circle-equivalent diameters of a plurality of (e.g., 10) granular particles present at the bottom surface of the region regarded as the "external coating layer" (in other words, the interface with the inlet side of the partition wall) are measured, and the "average particle diameter of the lower layer" is calculated. (n) Next, the "average particle size of the upper layer" is compared with the "average particle size of the lower layer" to determine whether the "average particle size of the upper layer" is larger than the "average particle size of the lower layer." Based on this process, it can be determined whether the outer coating layer is formed of an upper layer made of large particles and a lower layer made of small particles.

[0074] The above-mentioned "dimensions t1 to t3 in the thickness direction Y (see FIG. 4 or FIG. 6)" can all be measured based on the boundaries of each region identified in the above-mentioned evaluation procedure. Specifically, the "dimensions t1 to t3 in the thickness direction Y" in this specification are all average values ​​of the dimensions of each region confirmed in each of the 10 test pieces.

[0075] [Other embodiments] The embodiments of the particulate filter disclosed herein have been described above. Note that the particulate filter disclosed herein is not limited to the first and second embodiments described above. For example, in the particulate filter according to the above-described embodiments, an outer coat layer having a two-layer structure including a lower layer and an upper layer is formed. However, the outer coat layer in the particulate filter disclosed herein only needs to include at least the lower layer and the upper layer described above, and may have a multi-layer structure having three or more layers. For example, by forming a third layer that further covers the upper layer, more suitable PM trapping performance can be exhibited. On the other hand, if the number of layers of the outer coat layer is increased too much, pressure loss tends to increase. Therefore, from the viewpoint of suppressing pressure loss, the number of layers of the outer coat layer is preferably four or less, more preferably three or less, and particularly preferably two.

[0076] Furthermore, the particulate filter disclosed herein can be particularly suitably used as a GPF (Gasoline Particulate Filter) that captures PM emitted from a gasoline engine. Exhaust gas from a gasoline engine has a higher temperature than exhaust gas from other engines (such as diesel engines), so the PM captured in the inlet cells can be easily combusted.

[0077] [Test example] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in these test examples.

[0078] <First test> In this test, 10 types of particulate filters were prepared and the effect of the particle size of the granular material contained in the outer coating layer on PM capture performance and pressure drop suppression performance was investigated.

[0079] 1. Preparation of test samples (1) Sample 1 First, a cylindrical honeycomb substrate (made of cordierite, cell count: 300 cpsi, partition wall thickness: 8 mil, average pore diameter: 15 μm, porosity: 64%) with a substrate volume of 1.3 L and a length of 114.3 mm was prepared.

[0080] Next, a pore-filling slurry was prepared by dispersing resin beads (powdered cellulose) with an average particle size of 25 μm in ion-exchanged water. This pore-filling slurry was then supplied into the inlet cells of the honeycomb substrate, and the outlet cells were suctioned to allow the slurry to penetrate into the pores of the partition walls. The pores of the partition walls were then filled with the resin beads by drying (100°C, 30 minutes).

[0081] Next, a mixture of an aqueous palladium nitrate solution, an aqueous rhodium nitrate solution, powdered Al2O3, powdered CeO2, powdered ZrO2, powdered BaO, and an appropriate amount of ion-exchanged water was milled to obtain a lower layer dispersion containing lower layer particles with an average particle size of 0.5 μm. Next, this lower layer dispersion was mixed with a pore-forming material (powdered cellulose with an average major axis of 25 μm) and a solvent (pure water) in a ratio of 10:1:10 to prepare a lower layer slurry. The lower layer slurry was then supplied at a coating amount of 50 g / L into the inlet cells of the honeycomb substrate and suctioned through the outlet cells to adhere the lower layer slurry to the inlet side of the partition wall. A drying treatment (100 °C, 30 minutes) was then performed to form a lower layer precursor on the inlet side of the partition wall.

[0082] Next, an upper layer dispersion containing upper layer granules with an average particle size of 5 μm was obtained by milling in the same manner as the lower layer granules. An upper layer slurry was prepared by mixing the upper layer dispersion, a pore-forming material (powdered cellulose with an average major axis of 25 μm), and a solvent (pure water) in a ratio of 30:1:30. Next, the upper layer slurry was supplied to the inlet cell at a coating amount of 10 g / L and then sucked through the outlet cell, thereby applying the upper layer slurry so as to cover the lower layer precursor. A drying process (100°C, 30 minutes) was then performed to form an upper layer precursor on the lower layer precursor.

[0083] Next, an inner coating slurry containing inner coating granules with an average particle diameter of 0.5 μm was prepared according to the same procedure as for the lower layer granules. The inner coating slurry was supplied into the outlet cell at a coating amount of 30 g / L and sucked through the inlet cell, allowing the slurry to penetrate from the outlet side of the partition wall toward the inlet cell. A drying treatment (100 ° C, 30 minutes) was then performed to form a precursor of the inner coating layer inside the partition wall. A firing treatment (500 ° C, 60 minutes) was then performed to burn off the resin beads and fire the precursors of each layer. This produced a particulate filter equipped with an outer coating layer including a lower layer and an upper layer, an uncoated region, and an inner coating layer.

[0084] (2) Samples 2 to 7, A Seven types of particulate filters were produced under the same conditions as Sample 1, except that the average particle diameter of the granular material used in the upper layer and the lower layer was varied. The average particle diameter of the granular material used in each sample is shown in Table 1.

[0085] (3) Samples 8 and 9 Except for the fact that a single outer coating layer was formed, two types of particulate filters were produced under the same conditions as Samples 1 to 7. The average particle diameter of the granules forming the outer coating layer differs between Sample 8 and Sample 9. The average particle diameter of the granules used in each sample is shown in Table 1 below.

[0086] 2.Evaluation Test (1) Evaluation of PM collection performance Each sample particulate filter was installed in a soot generator (DPG, manufactured by Cambustion), and gas containing soot was supplied to the particulate filter by burning diesel fuel.The number of PM particles (pieces) emitted downstream of the particulate filter after 0.02 g / L of soot had accumulated was measured, and the ratio of this number to the number of PM particles emitted directly below the soot generator was calculated as the PM capture efficiency (%).The measurement results are shown in Table 1.

[0087] (2) Evaluation of pressure loss suppression performance In this test, PM deposition pressure loss was also measured at the same time as the PM capture efficiency was measured. Specifically, while the PM capture efficiency was being measured, the pressure in the pipes upstream and downstream of the particulate filter was measured, and the pressure loss (kPa) was calculated based on these. The pressure loss after 1 g / L of soot deposition was defined as the "PM deposition pressure loss," and the pressure loss suppression performance was evaluated based on this PM deposition pressure loss. The measurement results are shown in Table 1.

[0088] [Table 1]

[0089] As shown in Table 1, Samples 1, 4, 5, and A demonstrated favorable PM capture efficiencies of 85% or higher, while PM deposition pressure loss was suppressed to 91 mbar or less. In contrast, when a single outer coating layer was formed, as in Samples 8 and 9, the PM capture efficiency fell to 80% or less. This is presumably due to the lack of an outer coating layer that adequately covered the pore entrances, resulting in PM slipping through to the outlet cells. Furthermore, as in Samples 2 and 7, when the lower-layer granular material was larger than the upper-layer granular material, a sharp increase in PM deposition pressure loss was observed. This is presumably due to the lower-layer granular material clogging the pores in the partition walls. Furthermore, as in Samples 3 and 6, even when the lower-layer granular material was smaller than the upper-layer granular material, a decrease in PM capture efficiency and an increase in PM deposition pressure loss were observed. This suggests that simply specifying the relative sizes of the lower-layer and upper-layer granular material may not be sufficient to achieve the desired effect, and that it is necessary to control the particle size of the granular material in each layer to meet the desired particle size.

[0090] <Second test> In this test, four types of particulate filters were prepared, and the effects of the coating amount of each of the upper and lower layers on PM capture performance and pressure drop suppression performance were investigated.

[0091] 1. Preparation of test samples In this test, three types of particulate filters (samples 10 to 12) were produced using the same procedure as sample 1 in the first test, except that the coating amount of each layer was varied. In addition to samples 10 to 12, sample 1 produced in the first test was also prepared in this test. The average particle size and coating amount of the outer coating layer in each sample are shown in Table 2.

[0092] 2.Evaluation Test In this test, PM collection performance and pressure loss suppression performance were evaluated according to the same procedures as in the first test. The evaluation results are shown in Table 2.

[0093] [Table 2]

[0094] As shown in Table 2, a favorable PM capture efficiency of 85% or more was confirmed in all of Sample 1 and Samples 10 to 12. On the other hand, it was confirmed that Samples 11 and 12 had higher PM deposition pressure loss compared to the other samples. From this, it was found that when producing a particulate filter with excellent pressure loss suppression performance, it is better to make the coating amount of the lower layer greater than the coating amount of the upper layer.

[0095] <Third Test> In this test, seven types of particulate filters were prepared, and the effects of the porosity of the upper and lower layers on PM capture performance and pressure drop suppression performance were investigated.

[0096] 1. Preparation of test samples In this test, six types of particulate filters (samples 13 to 18) were produced by varying the amount of pore-forming material added from the above sample 1. In addition to these samples 13 to 18, sample 1 produced in the first test was also prepared in this test. Then, in this test, the porosity of each of the upper layer and the lower layer was measured according to the procedure described below. The measurement results are shown in Table 1.

[0097] (Porosity measurement procedure) (a) A partition wall of a substrate is cut out from a particulate filter, and the partition wall is embedded in a resin to prepare a sample piece. Then, a cross section of the partition wall is cut out from each sample piece, and an SEM observation image (backscattered electron image, observation magnification: 200 times) of the cross section is obtained. (As an example, a cross-sectional SEM observation image of the partition wall of Sample 1 is shown in FIG. 7.) (b) The cross-sectional SEM observation image is subjected to automatic binarization (discriminant analysis) using image analysis software (ImageJ) to obtain a binarized image in which the partition wall skeleton and catalyst layer are extracted. (As an example, the binarized image of Sample 1 is shown in Figure 8.) (c) The total area of ​​the part corresponding to the upper layer of the outer coating layer (the total area of ​​the black and white parts of the binary image) is measured, and the area of ​​the coating layer in the part corresponding to the upper layer (the area of ​​the black parts of the binary image) is measured. (d) Calculate the porosity of the upper layer based on the following formula (1). Porosity (%) = (total area - area of ​​coating layer) / total area × 100 (1) (e) The total area of ​​the area corresponding to the lower layer of the outer coating layer (the total area of ​​the black and white areas in the binary image) is measured, and the area of ​​the coating layer in the area corresponding to the lower layer (the area of ​​the black areas in the binary image) is measured. (f) Calculate the porosity of the lower layer based on the above formula (1).

[0098] 2.Evaluation Test In this test, PM collection performance and pressure loss suppression performance were evaluated according to the same procedures as in the first test. The evaluation results are shown in Table 3.

[0099] [Table 3]

[0100] As shown in Table 3, Sample 1 and Samples 13 to 18 all exhibited favorable PM capture efficiencies of 85% or higher. Among these samples, Samples 1, 14, 15, and 16 exhibited particularly excellent PM capture efficiencies. This indicates that PM capture performance tends to be further improved by increasing the porosity of the lower layer relative to that of the upper layer. However, Sample 18 did not exhibit a significant improvement in PM capture performance, even though the porosity of the lower layer was greater than that of the upper layer. This indicates that to favorably improve PM capture efficiency, the porosity of the lower layer should be 65% or less and the porosity of the upper layer should be 60% or less. Furthermore, Sample 16 exhibited a slight increase in PM deposition pressure drop. This indicates that to achieve favorable pressure drop suppression performance, the porosity of the lower layer should be 20% or more and the porosity of the upper layer should be 15% or more.

[0101] <Fourth Test> In this test, four types of particulate filters were prepared, and the influence of the presence or absence of a coating layer formed inside the partition walls (internal coating layer) on PM capture performance and pressure loss suppression performance was investigated.

[0102] 1. Preparation of test samples In this test, a particulate filter (sample 19) was prepared that was manufactured in the same procedure as sample 1, except that no inner coating layer was formed. In addition to sample 19, samples 1, 8, and 9 having the inner coating layer manufactured in the first test were also prepared in this test. Table 4 shows the configurations of the outer coating layer and the inner coating layer in each sample.

[0103] 2.Evaluation Test In this test, PM collection performance and pressure drop suppression performance were evaluated using the same procedures as in the first test. The evaluation results are shown in Table 4.

[0104] [Table 4]

[0105] As shown in Table 4, Samples 1 and 19 showed improvements in both PM collection efficiency and PM deposition pressure drop compared to Samples 8 and 9. However, there was no significant difference in PM collection efficiency between Sample 1 and Sample 19. This indicates that when the pore entrances are properly covered by the outer coating layer, PM is captured on the outside of the partition walls, and the presence or absence of an inner coating layer does not affect PM collection performance. Furthermore, Sample 1, which has an inner coating layer, showed a slightly lower PM deposition pressure drop compared to Sample 19. This is presumably because minute PM that penetrates the partition walls is easily combusted by the catalytic action of the precious metal catalyst (especially Pd) in the inner coating layer.

[0106] <Fifth Test> In this test, the influence of the particle size of the particulate material contained in the outer coating layer on the catalytic activity of the precious metal catalyst was investigated.

[0107] 1. Preparation of test samples In this test, a particulate filter (sample 20) was prepared by the same procedure as sample 1, except that the average particle diameter of the granular material for the lower layer was changed to 0.3 μm. In addition to sample 20, sample 1 produced in the first test was also prepared in this test. The configuration of the outer coating layer in each sample is shown in Table 5.

[0108] 2.Evaluation Test Unlike the first through fourth tests, this test evaluated the purification performance of harmful gas components in exhaust gas. Specifically, a durability test was first conducted on each sample, in which the filter was exposed to a 950°C environment for 45 hours. The particulate filters after the durability test were then installed in the exhaust system of an engine bench. Exhaust gas was supplied while the temperature was increased from 50°C at a rate of 10°C / min. The concentrations of harmful gas components (here, hydrocarbons (HC)) were measured in the upstream and downstream pipes of the particulate filter. The exhaust gas temperature at which the HC concentration measured in the downstream pipe was 50 mol% or less of the HC concentration measured in the upstream pipe was evaluated as the T50 catalytic activity (°C). The results are shown in Table 5. The lower the T50 catalytic activity, the lower the temperature at which harmful gas components can be purified, indicating superior purification performance for harmful gas components.

[0109] [Table 5]

[0110] As shown in Table 5, the particulate filter of Sample 1 had a lower T50 catalytic activity than Sample 20, confirming that it had excellent harmful gas purification performance. This indicates that if the average particle size of the lower layer granules is too small, the harmful gas purification performance may be reduced when a precious metal catalyst is added. This is presumably because if the lower layer granules are too small, sintering between the granules is promoted, reducing the catalytic activity of the precious metal catalyst. This indicates that the technology disclosed herein can exert particularly favorable effects in particulate filters (filter catalysts) in which a precious metal catalyst is catalyzed in the coating layer.

[0111] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Industrial Applicability]

[0112] According to the present invention, it is possible to provide a particulate filter that can appropriately prevent PM from passing through to the outlet cells and exhibit excellent PM trapping performance.

Claims

1. A particulate filter disposed in an exhaust system of an internal combustion engine to capture particulate matter in exhaust gas emitted from the internal combustion engine, a wall-flow type substrate having inlet cells that are open only at their ends on the exhaust gas inlet side, outlet cells that are open only at their ends on the exhaust gas outlet side, and a porous partition wall that separates the inlet cells from the outlet cells; an outer coating layer which is a porous layer formed of a plurality of granules and is formed on the surface of the partition wall in contact with the inlet cell; At least The outer coating layer comprises at least a lower layer formed on a surface of the partition wall in contact with the front-filling cell; an upper layer formed to cover the lower layer; It is equipped with the average particle size of the granular material in the upper layer is larger than the average particle size of the granular material in the lower layer; The average particle size of the granular material in the lower layer is 0.4 μm or more and 2.0 μm or less, and The average particle size of the granular material in the upper layer is 2.0 μm or more and 7.0 μm or less, the partition wall further includes an inner coating layer formed on wall surfaces of the pores in a predetermined region from a surface of the partition wall in contact with the outlet cells toward the inlet cells, the inner coating layer containing a precious metal catalyst for purifying harmful gas components in the exhaust gas, A particulate filter, wherein an uncoated region where substantially no coating layer is present is provided between the outer coating layer and the inner coating layer in the thickness direction of the partition wall.

2. 2. The particulate filter according to claim 1, wherein said granules contain at least one selected from the group consisting of alumina, ceria, zirconia, silica, magnesia, and calcia.

3. 3. The particulate filter according to claim 1, wherein the granular material contains a precious metal catalyst that purifies harmful gas components in the exhaust gas.

4. The particulate filter according to any one of claims 1 to 3, wherein a coating amount of the lower layer is greater than a coating amount of the upper layer.

5. The particulate filter according to any one of claims 1 to 4, wherein the coating amount of said lower layer per 1 L of volume of said substrate is 30 g / L or more and 50 g / L or less.

6. The particulate filter according to any one of claims 1 to 5, wherein the coating amount of said upper layer per 1 L of volume of said base material is 10 g / L or more and 30 g / L or less.

7. The particulate filter according to any one of claims 1 to 6, wherein the porosity of the lower layer is greater than the porosity of the upper layer.

8. The particulate filter according to any one of claims 1 to 7, wherein the porosity of the lower layer is 20% or more and 65% or less.

9. The particulate filter according to any one of claims 1 to 8, wherein the upper layer has a porosity of 15% or more and 60% or less.

10. The particulate filter according to any one of claims 1 to 9, wherein the partition walls have an average pore diameter of 5 µm or more and 50 µm or less.

11. The particulate filter according to any one of claims 1 to 10, wherein the porosity of the partition walls is 30% to 90%.

12. 2. The particulate filter according to claim 1, wherein a dimension of said uncoated region in the thickness direction of said partition wall is 10 μm or more and 100 μm or less.

13. The particulate filter according to any one of claims 1 to 12, wherein the internal combustion engine is a gasoline engine.

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