Exhaust gas purifying apparatus

Abstract

To provide an exhaust emission control device capable of reducing pressure loss and improving exhaust emission control performance.SOLUTION: An exhaust gas purification device of the present invention includes a honeycomb substrate and an outflow side catalyst, wherein the honeycomb substrate has a porous partition wall defining a plurality of cells extending from an inflow side end face to an outflow side end face, and the plurality of cells include an inflow cell and an outflow cell adjacent to each other with the partition wall interposed therebetween, the outflow side catalyst is disposed in an inner region on the outflow cell side of the partition wall in an outflow side catalyst disposition range extending from the outflow side end of the partition wall to a position separated toward the inflow side along the extending direction, in the outflow-side catalyst installation range of the partition wall, the ratio of the space filled with the outflow-side catalyst to the space in the outflow-cell-side surface layer region from the outflow-cell-side surface of the partition wall to a depth one fourth the thickness of the partition wall is not less than 61% but not more than 78%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification device in which a catalyst is provided in a filter having a wall-flow structure. [Background technology]

[0002] Exhaust gas from internal combustion engines, such as automobiles, contains particulate matter (hereinafter sometimes abbreviated as "PM"), which causes air pollution. Gasoline particulate filters (hereinafter sometimes abbreviated as "GPF") are known as wall-flow filters that remove PM from exhaust gas. Meanwhile, exhaust gas also contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). These harmful components can be removed from exhaust gas by a catalyst containing catalytic metals that is provided on the filter's partition walls. In recent years, exhaust gas purification devices that have catalysts provided on the filter's partition walls have been used to remove both PM and harmful components from exhaust gas. As such a device, for example, an inlet cell side catalyst layer is provided on the surface of the inlet cell side in an inlet cell side catalyst region extending from the inlet side end of the partition wall along the extension direction to a position away from the outlet side, and at a reference position of the inlet cell side catalyst region of the partition wall, the proportion of the filled portion of the inlet cell side catalyst layer in voids in an internal region up to a depth of 50% of the thickness of the partition wall is known to be 40% or less (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-170972 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional exhaust gas purification devices, the conditions for the structure in which the catalyst is provided in the area on the outlet side extending from the outlet end of the partition wall along the extension direction to a position away from the inlet side are not specified. As a result, the pressure loss and exhaust gas purification performance characteristics are insufficient. The present invention has been made in consideration of these points, and its object is to provide an exhaust gas purification device that can reduce pressure loss and improve exhaust gas purification performance. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, an exhaust gas purification apparatus of the present invention is an exhaust gas purification apparatus comprising a honeycomb substrate and an outlet-side catalyst, wherein the honeycomb substrate has porous partition walls that define a plurality of cells extending from an inlet-side end face to an outlet-side end face, the plurality of cells including inlet cells and outlet cells adjacent to each other with the partition wall in between, the inlet cells have an open inlet-side end and a sealed outlet-side end, and the outlet cells have a sealed inlet-side end and an open outlet-side end, the outlet-side catalyst is provided in an internal region on the outlet cell side of the partition wall in an outlet-side catalyst installation range that extends from the outlet-side end of the partition wall along an extension direction to a position away from the inlet side, and in the outlet-side catalyst installation range of the partition wall, a proportion of the filled portion of the outlet-side catalyst in voids in an outlet cell-side surface layer region from the surface of the partition wall on the outlet cell side to a depth of 1 / 4 of the thickness of the partition wall is 61% or more and 78% or less. [Effects of the Invention]

[0006] According to the present invention, it is possible to reduce pressure loss and improve exhaust gas purification performance. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a perspective view schematically illustrating an exhaust gas purification device according to one embodiment. [Figure 2] 1 is a cross-sectional view schematically illustrating a main part of an exhaust gas purification device according to one embodiment, the cross-section being parallel to the extension direction. [Figure 3]10 is an adjusted cross-sectional image of a region between adjacent corners of an inlet cell in a cross section perpendicular to the extension direction of the reference position of the outlet-side catalyst installation range of the partition wall in the exhaust gas purification apparatus of Example 1, taken by X-ray CT. [Figure 4] 10 is a flowchart showing a procedure for determining the proportion of the outflow side catalyst filled portion among voids in the outflow cell side surface layer region in the reference region of the outflow side catalyst installation range of the partition wall. [Figure 5] 1 is a graph showing the relationship between the initial pressure loss and the NOx conversion rate at an inlet gas temperature of 500°C relative to the proportion of the filled portion of the outflow side catalyst among the voids in the outflow cell side surface layer region in the reference region of the outflow side catalyst installation range of the partition walls for the exhaust gas purification devices of Examples 1 to 3 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of the exhaust gas purification apparatus of the present invention will be described. In the exhaust gas purification apparatus according to the embodiment, the "inlet side" refers to the side into which exhaust gas flows, and the "outlet side" refers to the side from which exhaust gas flows out. The "extension direction of the partition walls" refers to the direction in which the partition walls extend. The axial direction of the honeycomb substrate and the extension direction of the cells (direction in which the cells extend) are usually approximately the same as the extension direction of the partition walls. The "thickness direction of the partition walls" refers to the direction perpendicular to the surface of the partition walls on the cell (inlet cell and outlet cell) side. The "width direction of the partition walls" refers to the direction perpendicular to both the extension direction and thickness direction of the partition walls. Hereinafter, the "extension direction" refers to the extension direction of the partition walls, which is approximately the same as the axial direction of the honeycomb substrate and the extension direction of the cells. The "thickness direction" refers to the thickness direction of the partition walls. The "width direction" refers to the width direction of the partition walls.

[0009] First, an exhaust gas purification apparatus according to one embodiment will be described. Fig. 1 is a perspective view that schematically shows the exhaust gas purification apparatus according to one embodiment. Fig. 2 is a cross-sectional view that schematically shows a main part of the exhaust gas purification apparatus according to one embodiment in a cross section parallel to the extension direction. The dashed-line frame in Fig. 2 shows an adjusted cross-sectional image of the region between adjacent corners of the inlet cell 12A (outlet cell 12B) in a cross section perpendicular to the extension direction of the reference position 14c of the outlet-side catalyst installation range 14Y of the partition wall 14, which was captured by X-ray CT.

[0010] As shown in FIGS. 1 and 2 , an exhaust gas purification device 1 according to one embodiment includes a honeycomb substrate 10, plugs 16, an inlet-side catalyst 20, and an outlet-side catalyst 30. The honeycomb substrate 10 is a substrate integrally formed with a cylindrical frame 11 and partition walls 14 that divide the space inside the frame 11 into a honeycomb shape. The partition walls 14 are porous and define a plurality of cells 12 extending from an inlet-side end face 10Sa to an outlet-side end face 10Sb of the honeycomb substrate 10. The partition walls 14 include a plurality of wall portions 14A that are spaced apart and arranged parallel to each other, and a plurality of wall portions 14B that are perpendicular to the plurality of wall portions 14A and spaced apart and arranged parallel to each other, so that the cross section of the plurality of cells 12 perpendicular to the extension direction is rectangular. The cross section perpendicular to the extension direction has a lattice pattern. The plurality of cells 12 include inlet cells 12A and outlet cells 12B that are adjacent to each other with the partition wall 14 sandwiched therebetween. The inlet cell 12A has an open inlet end 12Aa and an outlet end 12Ab sealed with a sealing portion 16, and the outlet cell 12B has an open inlet end 12Ba and an open outlet end 12Bb sealed with a sealing portion 16. The cross-sectional shape perpendicular to the extension direction of the inlet cell 12A and the outlet cell 12B is rectangular and has four corners.

[0011] The inlet-side catalyst 20 is provided on a surface 14SA of the partition wall 14 on the inlet cell 12A side in an inlet-side catalyst installation range 14X that extends from the inlet-side end 14a of the partition wall 14 along the extension direction to a position 14b on the outlet side that is 50% of the length of the partition wall 14 in the extension direction. The inlet-side catalyst 20 includes a powdered carrier, catalytic metal particles containing at least one of platinum (Pt) and palladium (Pd) supported on the carrier, and an OSC material. The outlet-side catalyst 30 is provided in a gap in an internal region 14NB on the outlet cell 12B side of the partition wall 14 in an outlet-side catalyst installation range 14Y that extends from the outlet-side end 14d of the partition wall 14 along the extension direction to a position 14e on the inlet side that is 70% of the length of the partition wall 14 in the extension direction. The outlet-side catalyst 30 includes a powdered carrier, catalytic metal particles containing rhodium (Rh) supported on the carrier, and an OSC material.

[0012] At a reference position 14c in the extension direction of the outlet-side catalyst installation range 14Y of the partition wall 14, the proportion of the filling portion of the outlet-side catalyst 30 in the voids of the outflow cell-side surface layer region 14SR from the surface 14SB on the outflow cell 12B side of the partition wall 14 to a depth of ¼ of the thickness of the partition wall 14 is 61% or more and 78% or less. The reference position 14c of the outlet-side catalyst installation range 14Y of the partition wall 14 is a position that is a predetermined distance away from the outlet-side end 14d of the partition wall 14 on the inflow side along the extension direction (for example, a position as close to the outlet side as possible within a range that does not overlap with the arrangement region of the plugs 16 in the extension direction). Specifically, the proportion of the filled portion of the outlet cell side surface region 14SR of the voids in the reference position 14c of the outlet side catalyst installation range 14Y of the partition wall 14 that is filled with the outlet side catalyst 30 is obtained as the proportion of the filled portion of the outlet cell side surface region 14SR of the voids in the reference region of the outlet side catalyst installation range 14Y of the partition wall 14, when the reference region is the region of the partition wall 14 between adjacent corners of the inlet cell 12A (outlet cell 12B) within a range of ±1 mm in the extension direction from the reference position 14c of the outlet side catalyst installation range 14Y of the partition wall 14.

[0013] In the exhaust gas purification device 1 according to one embodiment, the proportion of the filled portions of the outlet-side catalyst 30 in the voids in the outflow cell-side surface layer region 14SR at the reference position 14c of the outflow-side catalyst installation range 14Y of the partition wall 14 is 61% or more, thereby ensuring a sufficient amount of the outflow-side catalyst 30 present in the outflow cell-side surface layer region 14SR of the outflow-side catalyst installation range 14Y of the partition wall 14. This ensures that the exhaust gas flowing into the outflow cells 12B is sufficiently purified by the outflow-side catalyst 30 present in the outflow cell-side surface layer region 14SR, thereby improving the exhaust gas purification performance, particularly in the gas diffusion-controlled region. Meanwhile, the proportion of the filled portions of the outlet-side catalyst 30 in the voids in the outflow cell-side surface layer region 14SR at the reference position 14c of the outflow-side catalyst installation range 14Y of the partition wall 14 is 78% or less, thereby ensuring a sufficient number of communication holes from the inflow cell 12A side to the outflow cell 12B side in the outflow-side catalyst installation range 14Y of the partition wall 14. Therefore, it becomes sufficiently easy for the exhaust gas to pass through the outlet-side catalyst installation range 14Y of the partition wall 14 from the inlet cell 12A side to the outlet cell 12B side. This reduces pressure loss. Furthermore, since the exhaust gas is sufficiently purified by the outlet-side catalyst 30 while passing through the partition wall 14, it is possible to improve the exhaust gas purification performance, particularly in the gas diffusion rate-controlled region. Next, details of each component of the exhaust gas purification device according to the embodiment will be described.

[0014] 1.Honeycomb substrate The honeycomb substrate is a substrate in which a frame and partition walls that divide the space inside the frame into a honeycomb shape are integrally formed. The axial length of the honeycomb substrate is preferably, for example, 10 mm or more and 500 mm or less. The capacity (total cell volume) of the honeycomb substrate can be a general capacity. The material of the honeycomb substrate is not particularly limited, and examples include ceramics such as cordierite. The length of the partition walls in the extension direction is usually approximately the same as the axial length of the honeycomb substrate. The thickness of the partition walls is preferably, for example, 50 μm or more and 2000 μm or less. The partition walls are porous structures containing voids made of pores that allow exhaust gas to pass through. The porosity of the partition walls is preferably, for example, 40% or more and 70% or less. This is because pressure loss can be effectively suppressed and sufficient mechanical strength can be ensured. The average pore diameter of the voids in the partition walls (the average pore diameter of the voids in the partition walls alone) is preferably, for example, 1 μm or more and 60 μm or less. This is because sufficient PM collection performance can be obtained and pressure loss can be sufficiently suppressed. The "average pore size of the pores in the partition walls" refers to a value measured by, for example, mercury intrusion porosimetry.

[0015] The inflow cells and outflow cells are formed by dividing the space inside the frame with a partition wall, and are adjacent to each other with the partition wall in between. The inflow cells and outflow cells are usually surrounded by the partition wall in a direction perpendicular to the stretching direction. The inflow side end of the inflow cells is usually sealed with a sealing part. The outflow side end of the outflow cells is usually sealed with a sealing part. The length of the sealing part in the stretching direction is preferably, for example, 2 mm or more and 20 mm or less. The cross-sectional shape of the inflow cells and outflow cells perpendicular to the stretching direction can be, for example, a rectangle such as a square. The cross-sectional area of ​​the inflow cells and outflow cells perpendicular to the stretching direction can be, for example, 1 mm 2 More than 7mm 2 The arrangement of the inflow cells and outflow cells may be, for example, as in one embodiment, a checkerboard pattern in which the inflow cells and outflow cells are alternately arranged.

[0016] 2. The proportion of the voids in the outlet catalyst and the surface layer area of ​​the outlet cell that are filled with the outlet catalyst The outlet-side catalyst is provided in an internal region of the partition wall on the outlet cell side within the outlet-side catalyst installation range of the partition wall. The outlet-side catalyst installation range of the partition wall is not particularly limited as long as it is a range extending from the outlet-side end of the partition wall along the extension direction to a position away from the inlet side by a predetermined distance, but for example, a range extending from the outlet-side end of the partition wall along the extension direction to a position away from 50% to 100% of the length of the partition wall in the extension direction on the inlet side is preferred, and particularly a range extending to a position away from 55% to 90% of the length of the partition wall in the extension direction, and particularly a range extending to a position away from 60% to 80% of the length of the partition wall in the extension direction is preferred. This is because pressure loss can be effectively reduced and exhaust gas purification performance can be effectively improved.

[0017] In the outlet-side catalyst installation range of the partition wall, the proportion of the outlet-side catalyst-filled portions in the voids in the outflow cell-side surface layer region from the surface on the outflow cell side of the partition wall to a depth of ¼ of the thickness of the partition wall is 61% or more and 78% or less. The "proportion of the outlet-side catalyst-filled portions in the voids in the outflow cell-side surface layer region" refers to, for example, the proportion of the outlet-side catalyst-filled portions in the voids in the outflow cell-side surface layer region at a reference position in the extension direction of the outflow-side catalyst installation range of the partition wall. The reference position of the outlet-side catalyst installation range of the partition wall is not particularly limited as long as it is any position in the extension direction of the outflow-side catalyst installation range of the partition wall, but is preferably as close to the outlet as possible within a range that does not overlap in the extension direction with the arrangement region of the plugs that plug the outlet-side ends of the inflow cells. This is because such a position has a significant effect on pressure loss and exhaust gas purification performance. As a method for obtaining the proportion of the filling portion of the outflow-side catalyst among the voids in the surface layer region on the outflow cell side at the reference position of the outflow-side catalyst installation range of the partition wall, for example, there may be mentioned a method in which the region of the partition wall between adjacent corners of the inlet cells (outflow cells) in a range of ±1 mm in the extension direction from the reference position of the outflow-side catalyst installation range of the partition wall (however, limited to the range included in the outflow-side catalyst installation range of the partition wall) is used as the reference region, and the proportion of the filling portion of the outflow-side catalyst among the voids in the surface layer region on the outflow cell side in the reference region of the outflow-side catalyst installation range of the partition wall is obtained.

[0018] As a structure in which the proportion of the outflow-side catalyst filled portions in the voids in the outflow cell side surface layer region at the reference position of the outflow-side catalyst installation range of the partition wall is 61% or more and 78% or less, a structure in which the proportion of the outflow-side catalyst filled portions in the voids in the outflow cell side surface layer region over the entire extension direction of the outflow-side catalyst installation range of the partition wall is 61% or more and 78% or less is particularly preferred. This is because this can effectively reduce pressure loss and effectively improve exhaust gas purification performance. Note that, as a method for obtaining the proportion of the outflow-side catalyst filled portions in the voids in the outflow cell side surface layer region over the entire extension direction of the outflow-side catalyst installation range of the partition wall, for example, there may be mentioned a method in which the region of the partition wall between adjacent corners of inlet cells (outflow cells) over the entire extension direction of the outflow-side catalyst installation range of the partition wall is used as a calculation region, and the proportion is obtained as the proportion of the outflow-side catalyst filled portions in the voids in the outflow cell side surface layer region over the entire calculation region over the extension direction of the outflow-side catalyst installation range of the partition wall.

[0019] The outlet-side catalyst typically includes catalytic metal particles and a carrier supporting the catalytic metal particles. The mass ratio of the catalytic metal particles to the total mass of the catalytic metal particles and the carrier is not particularly limited and is preferably, for example, 0.01 mass% to 10 mass%. The material of the catalytic metal particles is not particularly limited and may be, for example, platinum (Pt), palladium (Pd), rhodium (Rh), etc. The average particle size of the catalytic metal particles is not particularly limited and is preferably, for example, 0.1 nm to 20 nm. The average particle size of the catalytic metal particles refers to, for example, the average value determined from particle sizes measured using a transmission electron microscope (TEM). The content of the catalytic metal particles is not particularly limited and is preferably 0.01 g to 2 g per 1 L of the honeycomb substrate. Here, the content of the catalytic metal particles per 1 L of the substrate volume refers to the mass of the catalytic metal particles contained in the outlet-side catalyst divided by the volume of a portion of the honeycomb substrate in the axial direction where the axial length of the extension direction of the partition wall where the outlet-side catalyst is installed is the same as the axial length. The material of the support is not particularly limited, and examples thereof include metal oxides such as alumina, ceria (CeO2), and zirconia (ZrO2), and solid solutions such as ceria-zirconia (CeO2-ZrO2) composite oxide. The shape of the support is not particularly limited, and a powder form is preferred. The average particle size D50 of the powder support is not particularly limited, and a range of 1 μm to 15 μm is preferred. The average particle size D50 can be determined, for example, by laser diffraction / scattering.

[0020] The outlet-side catalyst may further include a promoter such as an OSC (Oxygen Storage Capacity) material. The promoter material is, for example, the same material as the carrier. The promoter shape is, for example, the same shape as the carrier. The average particle size D50 of the powdered promoter is not particularly limited, and may be, for example, the same average particle size as the powdered carrier. The mass ratio of the promoter to the total mass of the catalytic metal particles, the carrier, and the promoter can be a general mass ratio. The density of the outlet-side catalyst is not particularly limited, but is preferably, for example, 5 g / L or more and 100 g / L or less. The "density of the outlet-side catalyst" refers to the mass of the outlet-side catalyst divided by the volume of a portion of the axial direction of a honeycomb substrate having the same axial length as the extension direction length of the outlet-side catalyst installation range of the partition wall.

[0021] The method for forming the outlet-side catalyst is not particularly limited, and examples thereof include a method in which catalytic metal particles and a powdered support are mixed in a solvent to prepare an outlet-side catalyst slurry, and the outlet-side catalyst slurry is then supplied to the internal region (voids present in the internal region) of the partition wall on the outlet cell side in the area where the outlet-side catalyst is installed, followed by drying and calcination. In such a method, the outlet-side catalyst slurry may contain additional components such as a co-catalyst, binder, and additives in addition to the catalytic metal particles, support, and solvent (e.g., ion-exchanged water). Examples of additives include pH adjusters such as acetic acid. The method for preparing the outlet-side catalyst slurry is not particularly limited, and examples thereof include the following method. First, a powdered support is immersed in a solution (e.g., an aqueous solution) containing a catalytic metal salt or a catalytic metal complex, and then the resulting solution is dried and calcined to prepare a catalytic metal-supported powder in which the catalytic metal is supported on the support. Next, a solvent is added to the catalyst metal-supported powder, and additional components are added as necessary. The mixture is thoroughly stirred and wet-pulverized until the average particle size D50 of the solid components reaches the desired value. This prepares an outlet-side catalyst slurry. The average particle size D50 of the solid components of the outlet-side catalyst slurry is not particularly limited and may be a general average particle size. The method for supplying the outlet-side catalyst slurry is not particularly limited, and examples include a method in which a honeycomb substrate is immersed in the outlet-side catalyst slurry from the outlet side and then removed from the outlet-side catalyst slurry after a predetermined time has elapsed. The outlet-side catalyst slurry may be supplied to the partition walls by, for example, blowing the outlet-side catalyst slurry using a blower or the like to prevent the outlet-side catalyst slurry from being supplied to unnecessary areas of the catalyst in the internal regions and on the surfaces of the partition walls. Conventional drying and firing conditions can be used.

[0022] The proportion of the outflow-side catalyst-filled portion in the voids in the surface layer region on the outflow cell side in the outflow-side catalyst installation range of the partition wall can be adjusted by adjusting the concentrations of additives such as a pH adjuster such as acetic acid and other solid components in the slurry, the average particle size D50 of the solid components in the slurry, the dispersion state of the solid components in the slurry, the viscosity and other properties, the amount of slurry supplied, the slurry supply method, drying conditions, firing conditions, etc.

[0023] 3.Other The exhaust gas purification device includes a honeycomb substrate and an outlet-side catalyst. The exhaust gas purification device typically further includes plugs that seal the outlet-side ends of the inlet cells and plugs that seal the inlet-side ends of the outflow cells. As in one embodiment, the exhaust gas purification device may further include an inlet-side catalyst provided in an inlet-side catalyst installation range that extends from the inlet-side end of the partition wall along the extension direction to a position a predetermined distance away from the outlet side, or may not include an inlet-side catalyst. In the exhaust gas purification device, regardless of whether or not an inlet-side catalyst is provided, the effects of reduced pressure loss and improved exhaust gas purification performance can be obtained depending on the configuration of the outlet-side catalyst.

[0024] The inlet-side catalyst is provided on at least one of the surface of the partition wall on the inlet cell side in the inlet-side catalyst installation range of the partition wall and the internal region (voids present in the internal region) on the inlet cell side. The inlet-side catalyst installation range of the partition wall is preferably, for example, a range extending from the inlet-side end of the partition wall along the extension direction to a position on the outlet side at a distance of 80% or less of the length of the partition wall in the extension direction. This is because an increase in pressure loss can be suppressed. Furthermore, within such a range, a range extending along the extension direction to a position overlapping with the outlet-side catalyst installation range is preferable. This is because it is possible to prevent exhaust gas from passing through the catalyst-free region of the partition wall. The inlet-side catalyst usually includes catalytic metal particles and a carrier that supports the catalytic metal particles. The catalytic metal particles and the carrier are the same as the catalytic metal particles and the carrier contained in the outlet-side catalyst, respectively. The inlet-side catalyst may include the same promoters as those in the outlet-side catalyst. The "density of the inlet-side catalyst" refers to the value obtained by dividing the mass of the inlet-side catalyst by the volume of a portion of the axial direction of a honeycomb substrate having the same length in the extension direction of the inlet-side catalyst installation range of the partition wall as the axial length. [Example]

[0025] Hereinafter, the exhaust gas purification device according to the embodiment will be described in more detail with reference to examples and comparative examples.

[0026] [Example 1] An example of an exhaust gas purifying apparatus according to one embodiment was produced. Specifically, first, a GPF was prepared that included the following honeycomb substrate 10 and plugs 16, but was not provided with a catalyst.

[0027] Honeycomb substrate material: Cordierite Honeycomb substrate size: outer diameter x axial length = 117 mm x 122 mm Partition wall thickness: 240 μm Porosity of partition wall (percentage of voids in a single partition wall): 61% Average pore size of partition walls (average pore size of partition walls alone): 7 μm Cell density: 200 cells per square inch Length of the sealing part in the extension direction: 5 mm

[0028] Next, a catalyst-loaded support (a powdered support on which catalytic metal particles are supported) was mixed with a solvent to prepare a slurry for the outflow side catalyst. Specifically, a powdered ceria-zirconia composite oxide (support) was immersed in an aqueous solution containing Rh hydroxide (catalytic metal salt), and then dried and calcined to prepare a Rh-loaded powder in which rhodium (Rh) was loaded on the powdered ceria-zirconia composite oxide. Next, alumina (co-catalyst), acetic acid (pH adjuster), a binder, and ion-exchanged water (solvent) were added to the Rh-loaded powder, and the mixture was thoroughly stirred and wet-pulverized. This resulted in the preparation of a slurry for the outflow side catalyst. Acetic acid was added to the outflow side catalyst slurry so that the concentration of acetic acid (pH adjuster) in the outflow side catalyst slurry was 1 wt %.

[0029] Next, the outlet-side catalyst slurry was poured into the outlet cells 12B from the outlet end 12Bb at a supply rate such that the density of the outlet-side catalyst 30 became 52 g / L. This supplied the slurry to the inner region 14NB of the partition wall 14 on the outlet cell 12B side of the partition wall 14 in the outlet-side catalyst installation range 14Y. The outlet-side catalyst installation range 14Y of the partition wall 14 extended from the outlet end 14d of the partition wall 14 to a position 14e at a distance of 70% of the length of the partition wall 14 in the extension direction toward the inlet side. The honeycomb substrate 10 to which the outlet-side catalyst slurry had been supplied was then dried by heating at 120°C for 2 hours in a dryer to remove moisture, and then fired at 500°C for 2 hours in an electric furnace. This produced the outlet-side catalyst 30.

[0030] Next, a catalyst-loaded support (a powdered support on which catalytic metal particles are supported) was mixed with a solvent to prepare an inlet-side catalyst slurry. Specifically, a powdered ceria-zirconia composite oxide (support) was immersed in an aqueous solution containing platinum nitrate (catalytic metal salt), and then dried and calcined to prepare a Pt-loaded powder in which platinum (Pt) was loaded on the powdered ceria-zirconia composite oxide. Next, acetic acid (a pH adjuster), a binder, and ion-exchanged water (a solvent) were added to the Pt-loaded powder, and the mixture was thoroughly stirred and wet-pulverized. This prepared the inlet-side catalyst slurry. Acetic acid was added to the inlet-side catalyst slurry so that the concentration of acetic acid (a pH adjuster) in the inlet-side catalyst slurry was 1 wt %.

[0031] Next, the inlet-side catalyst slurry was poured into the inlet cells 12A from the inlet end 12Aa at a supply rate sufficient to achieve the desired density of the inlet-side catalyst 20. The slurry was then supplied onto the inlet cell 12A-side surface 14SA of the partition wall 14 in the inlet-side catalyst installation range 14X of the partition wall 14. The inlet-side catalyst installation range 14X of the partition wall 14 extended from the inlet end 14a of the partition wall 14 along the extension direction to 14b, a distance 50% of the length of the partition wall 14 in the extension direction. The honeycomb substrate 10 to which the outlet-side catalyst slurry had been supplied was then dried by heating at 120°C for 2 hours in a dryer to remove moisture, and then fired at 500°C for 2 hours in an electric furnace. This resulted in the formation of the inlet-side catalyst 20. The exhaust gas purification device 1 was fabricated in this manner.

[0032] [Examples 2 and 3 and Comparative Examples 1 and 2] In Examples 2 and 3 and Comparative Example 1, the exhaust gas purification device 1 was produced by the same production method as in Example 1, except that when preparing the slurry for the outflow side catalyst, acetic acid was added to the slurry for the outflow side catalyst so that the concentrations of acetic acid (pH adjuster) in the slurry for the outflow side catalyst were 4 wt % (Example 2), 5 wt % (Example 3), and 6 wt % (Comparative Example 1), respectively. In Comparative Example 2, the exhaust gas purification device 1 was produced by the same production method as in Example 1, except that when the slurry for the outflow side catalyst was supplied to the inner region 14NB on the outflow cell 12B side of the partition wall 14 in the outlet-side catalyst installation range 14Y of the partition wall 14, the supply amount of the slurry for the outflow side catalyst was adjusted so that the density of the outflow side catalyst 30 was 109 g / L.

[0033] [evaluation] For the exhaust gas purification devices 1 of Examples 1 to 3 and Comparative Examples 1 and 2, a cross section perpendicular to the extension direction of the reference position 14c in the extension direction of the outlet-side catalyst installation range 14Y of the partition wall 14 was observed. Then, the initial pressure loss and the exhaust gas purification performance after a durability test were evaluated relative to the proportion of the filled portion of the outlet-side catalyst 30 in the voids of the outflow cell-side surface layer region 14SR in the outlet-side catalyst installation range 14Y of the partition wall 14. The reference position 14c in the outlet-side catalyst installation range 14Y of the partition wall 14 is a position 15 mm away from the outlet-side end 14d of the partition wall 14 on the inflow side along the extension direction. The outflow cell-side surface layer region 14SR in the outlet-side catalyst installation range 14Y of the partition wall 14 is a region from the surface 14SB on the outflow cell 12B side in the outlet-side catalyst installation range 14Y of the partition wall 14 to a depth of ¼ (60 μm) the thickness of the partition wall 14.

[0034] (Cross-section observation) In the exhaust gas purification apparatuses 1 of Examples 1 to 3 and Comparative Examples 1 and 2, an image was taken by X-ray CT of the region between adjacent corners of the inlet cells 12A (outlet cells 12B) in a cross section perpendicular to the extension direction of the reference position 14c of the outlet-side catalyst installation range 14Y of the partition wall 14. Fig. 3 is an adjusted cross-sectional image of the region between adjacent corners of the inlet cells in a cross section perpendicular to the extension direction of the reference position of the outlet-side catalyst installation range of the partition wall in the exhaust gas purification apparatus of Example 1. This cross-sectional image shows the partition wall 14, the inlet cells 12A, and the outlet cells 12B, as well as the portions of the voids in the partition wall 14 that are not filled with the outlet-side catalyst 30, the substrate portion of the partition wall 14, and the outlet-side catalyst 30.

[0035] (Proportion of the voids in the surface layer area on the outflow cell side filled with the outflow catalyst) First, the ratio of the filled portion of the outlet-side catalyst 30 to the voids in the outflow cell-side surface layer region 14SR in the reference region of the outflow-side catalyst installation range 14Y of the partition wall 14 was determined for the exhaust gas purification device 1 of Examples 1 to 3 and Comparative Examples 1 and 2. The procedure for determining the filled portion ratio for the exhaust gas purification device 1 of each example will be described below. Fig. 4 is a flowchart showing the procedure for determining the ratio of the filled portion of the outlet-side catalyst to the voids in the outflow cell-side surface layer region in the reference region of the outflow-side catalyst installation range of the partition wall.

[0036] In this procedure, as shown in Figure 4, first, in the exhaust gas purification device 1 of each example, the region of the partition 14 between adjacent corners of the inlet cell 12A (outlet cell 12B) within a range of ±1 mm in the extension direction from the reference position 14c of the outlet-side catalyst installation range 14Y of the partition 14 was set as the reference region, and cross sections perpendicular to the extension direction at 1,000 shooting positions set at 2 μm intervals in the extension direction in the reference region of the outlet-side catalyst installation range 14Y of the partition 14 were photographed using X-ray CT, and 1,000 cross-sectional images were obtained (STEP 1). Specifically, a ZEISS X-ray CT measurement device (Xradia 510 Versa) was used, and under predetermined measurement conditions (tube voltage: 60 KV, tube current: 87.5 μA, pixel size: 2 μm), a cross section perpendicular to the extension direction at each imaging position in the reference region of the outlet-side catalyst installation range 14Y of the partition wall 14 was imaged at a magnification of 150, and each cross-sectional image was acquired in an 8-bit format of 1280 × 960 pixels. Note that the cross-sectional image of Fig. 3 described above is an image showing a cross section perpendicular to the extension direction at one imaging position (reference position) in the reference region of the outlet-side catalyst installation range of the partition wall according to Example 1.

[0037] Next, as shown in FIG. 4, by using CAE software GEODICT (registered trademark) manufactured by Math2Market, Inc., all 1,000 cross-sectional images were read using the Import Geo module of GEODICT, and the 1,000 cross-sectional images were integrated to construct a 3D (three-dimensional) model representing the reference area of ​​the outlet-side catalyst installation range 14Y of the partition wall 14 (STEP 2).

[0038] Next, as shown in Fig. 4, using GEODICT, a calculation target region of 800 μm (width direction) × 800 μm (extension direction) × 400 μm (thickness direction) that is located at the center of the width and extension directions of the reference region of the outlet-side catalyst installation range 14Y of the partition wall 14 and covers the entire thickness direction of the reference region was cut out from the 3D model into 400 voxels (width direction) × 400 voxels (extension direction) × 200 voxels (thickness direction) and voxelized to obtain a 3D voxel model representing the calculation target region (STEP 3). In this case, a three-dimensional space was adopted in which the width direction is the X-axis direction, the extension direction is the Y-axis direction, and the thickness direction is the Z-axis direction, and the calculation target region of the 3D model was represented by voxels aligned in the X-axis direction, Y-axis direction, and Z-axis direction to obtain the 3D voxel model.

[0039] Next, as shown in Fig. 4, the Threshold menu of the PoroDict module of GEODICT was used to set a predetermined threshold value for binarizing each voxel of the 3D voxel model using the mode method, and each voxel of the 3D voxel model was then binarized to obtain a binarized 3D voxel model in which each voxel was classified into the portion of the void in the partition wall 14 that was not filled with the outflow-side catalyst 30 and other regions (the base portion of the partition wall 14 and the outflow-side catalyst 30) (STEP 4). Next, as shown in Fig. 4, the Crop module of GEODICT was used to cut out a portion of the binarized 3D voxel model that corresponds to the outflow cell side surface region 14SR in the outflow-side catalyst installation range 14Y of the partition wall 14, thereby obtaining a binarized 3D voxel model of the outflow cell side surface region (STEP 5). Next, as shown in Figure 4, the Granulometry menu of the Porodict module of GEODICT was used to calculate the percentage [%] of unfilled voids in the partition wall in the voxels that make up the binary 3D voxel model of the surface region on the outflow cell side (STEP 6).

[0040] Next, as shown in FIG. 4, using Microsoft Excel (registered trademark), the proportion [%] of the filled portion of the voids in the outflow cell side surface layer region 14SR in the reference region of the outflow side catalyst installation range 14Y of the partition wall 14 was calculated from the proportion [%] of the unfilled portion of the voids in the partition wall in the voxels constituting the binarized 3D voxel model of the outflow cell side surface layer region and the porosity [%] of the partition wall (the proportion of voids in the partition wall alone) (STEP 7). In this case, the proportion [%] of the filled portion was calculated as (partition wall porosity - proportion of unfilled portion of the voids in the partition wall) / partition wall porosity × 100. Table 1 below shows the proportion of the filled portion of the voids in the outflow cell side surface layer region in the reference region of the outflow side catalyst installation range of the partition wall, obtained for each example.

[0041] (Initial pressure loss) For the exhaust gas purification devices 1 of Examples 1 to 3 and Comparative Examples 1 and 2, the initial pressure loss was measured with the outlet-side catalyst and the inlet-side catalyst still formed on the honeycomb substrate. Specifically, the initial pressure loss was measured by blowing 7 m of air into the exhaust gas purification device 1 of each example. 3 The pressure loss [kPa] was measured when the fluid was passed through the device at 20° C. The following Table 1 shows the initial pressure loss obtained for the device of each example.

[0042] (Exhaust gas purification performance after durability testing) For the exhaust gas purification devices 1 of Examples 1 to 3 and Comparative Examples 1 and 2, the NOx reduction rate at an inlet gas temperature of 500°C was determined as an index of exhaust gas purification performance after a durability test. Specifically, first, the exhaust gas purification device of each example was installed in the exhaust system of a V8 engine, and a durability test was conducted by repeatedly flowing exhaust gas in stoichiometric and lean atmospheres for a fixed period of time (at a ratio of 3:1) at a catalyst bed temperature of 900°C for 50 hours. Next, the exhaust gas purification device of each example after the durability test was installed in the exhaust system of an L4 engine, and exhaust gas with an A / F (air-fuel ratio) of 14.4 was supplied, and the inlet gas temperature was raised from 200°C to 600°C (at 20°C / min) under the condition of Ga = 28 g / s. Then, the NOx concentrations of the inlet gas and outlet gas were measured during the inlet gas temperature rise process to calculate the NOx reduction rate, and the NOx reduction rate [%] at an inlet gas temperature of 500°C was determined. Table 1 below shows the NOx purification rate at an inlet gas temperature of 500°C obtained for the devices of each example.

[0043] [Table 1]

[0044] Fig. 5 is a graph showing the relationship between the initial pressure loss and the NOx purification rate at an inlet gas temperature of 500°C versus the proportion of the filled portion of the outflow catalyst in the voids in the surface layer region on the outflow cell side in the reference region of the outflow catalyst installation range of the partition walls for the exhaust gas purification devices of Examples 1 to 3 and Comparative Examples 1 and 2. As shown in Table 1 and Fig. 5 above, when the proportion of the filled portion of the outflow catalyst is less than 61%, the NOx purification rate decreases, and even if the proportion of the filled portion of the outflow catalyst is further reduced, the increase in initial pressure loss becomes less likely to progress. On the other hand, when the proportion of the filled portion of the outflow catalyst exceeds 78%, the initial pressure loss increases significantly, and the NOx purification rate decreases significantly.

[0045] The present invention is not limited to the embodiments described above, and various design modifications can be made without departing from the spirit of the present invention as set forth in the claims. [Explanation of symbols]

[0046] 1: exhaust gas purification device, 10: honeycomb substrate, 11: frame, 12: cell, 12A: inlet cell, 12B: outlet cell, 14: partition wall, 14NB: internal region, 14Y: outlet side catalyst installation range, 14c: reference position, 14SR: outlet cell side surface region, 30: outlet side catalyst

Claims

1. An exhaust gas purification device comprising a honeycomb substrate and an outlet-side catalyst, the honeycomb substrate has porous partition walls that define a plurality of cells extending from an inlet end face to an outlet end face, the plurality of cells include an inflow cell and an outflow cell adjacent to each other with the partition wall interposed therebetween, The inlet cell has an open inlet end and a sealed outlet end, The outflow cell has a sealed inflow end and an open outflow end, the outlet-side catalyst is provided in an internal region on the outlet cell side of the partition wall in an outlet-side catalyst installation range that extends from an outlet-side end of the partition wall along an extension direction to a position away from the inlet side, an outlet-side catalyst installation area of ​​the partition wall, wherein a proportion of voids in an outlet-side surface layer region from a surface of the partition wall on the outlet cell side to a depth of ¼ of a thickness of the partition wall that is filled with the outlet-side catalyst is 61% or more and 78% or less.

2. 2. The exhaust gas purification device according to claim 1, wherein the outlet-side catalyst installation range of the partition wall extends from the outlet-side end of the partition wall along the extension direction to a position on the inlet side at a distance of 50% to 100% of the length of the partition wall in the extension direction.

Images