Exhaust gas purification system

Abstract

The object of the present invention is to provide an exhaust gas purification system comprising: a first exhaust gas treatment section arranged on the upstream side of an exhaust path of an internal combustion engine and a second exhaust gas treatment section arranged on the upstream side of the exhaust path of the internal combustion engine, the exhaust gas purification system being capable of efficiently exerting the catalytic activity of a rhodium element contained in a catalyst layer of the second exhaust gas treatment section, and providing an exhaust gas purification system (1) for purifying exhaust gas discharged from an internal combustion engine, the exhaust gas purification system (1) comprising: an exhaust path (2) through which exhaust gas flows, a first exhaust gas treatment section (3) arranged on the upstream side in the exhaust path (2), and a second exhaust gas treatment section (4) arranged on the downstream side in the exhaust path (2), the first catalyst layer of the first exhaust gas treatment section (3) containing a cerium element, the mass of the cerium element contained in the first catalyst layer in terms of cerium oxide being 5.0 mass % or more and 13.0 mass % or less relative to the mass of the first catalyst layer, and the second catalyst layer of the second exhaust gas treatment section (4) containing a rhodium element.

Description

Technical Field

[0001] The invention relates to an exhaust gas purification system. Background Art

[0002] The exhaust gas discharged from the internal combustion engines of cars, motorcycles, etc. contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). In order to purify these harmful components and make them harmless, a three-way catalyst is used. As a three-way catalyst, precious metal catalysts such as platinum (Pt), palladium (Pd), and rhodium (Rh) are used. Pt and Pd are mainly involved in the oxidation purification of HC and CO, and Rh is mainly involved in the reduction purification of NOx.

[0003] It is desired that the air / fuel ratio (air-fuel ratio A / F) supplied to the internal combustion engine be controlled to be close to the theoretical air-fuel ratio (stoichiometry). However, the actual air-fuel ratio varies with the stoichiometry as the center, toward the rich side (fuel-rich atmosphere) or the poor side (fuel-lean atmosphere) due to the driving conditions of the vehicle, etc. Therefore, the air-fuel ratio of the exhaust gas also varies toward the rich side or the poor side. Therefore, in order to mitigate the variation of the oxygen concentration in the exhaust gas and improve the exhaust gas purification ability of the catalyst, a material (hereinafter sometimes referred to as "OSC material") having the ability to absorb and store oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low (i.e., oxygen storage capacity (OSC)) is used, such as CeO2-ZrO2 composite oxides.

[0004] Exhaust gas contains harmful components such as HC, CO, NOx and particulate matter (PM), which are known to cause air pollution. For example, the direct injection engine (GDI: Gasoline Direct Injection engine) used in vehicles equipped with gasoline engines has low fuel consumption and high output power, but it is known that the amount of PM emitted in the exhaust gas is larger than that of the conventional port injection engine. In order to comply with environmental regulations related to PM, it is required that a filter (GPF: Gasoline Particulate Filter) with a PM trapping function be installed in vehicles equipped with gasoline engines such as GDI, just like vehicles equipped with diesel engines.

[0005] As the GPF, for example, a substrate having a so-called wall-flow type structure is used. In the wall-flow type substrate, when the exhaust gas flowing in from the cell inlet passes through the porous partition wall portion that partitions the cell and flows out from the cell outlet, PM in the exhaust gas is captured in the pores inside the partition wall portion.

[0006] Generally, the space for carrying exhaust gas purification catalysts is limited, and therefore, studies have been conducted to carry precious metal catalysts such as Pt, Pd, and Rh on the GPF to purify harmful components such as HC, CO, and NOx while capturing PM.

[0007] For example, Patent Document 1 describes a GPF in which a catalyst layer is formed on the surface of a partition wall on the chamber inlet side of a substrate and a catalyst layer is formed on the surface of a partition wall on the chamber outlet side of the substrate.

[0008] Patent Document 2 describes an exhaust gas purification system that arranges a catalytic converter on the upstream side of an exhaust passage of an internal combustion engine and arranges a GPF on the downstream side of the exhaust passage of the internal combustion engine, thereby purifying harmful components in the exhaust gas and capturing PM in the exhaust gas.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2013-099748

[0012] Patent Document 2: Japanese Patent Application Publication No. 2018-189092 Summary of the invention

[0013] Problem that the invention aims to solve

[0014] When a catalytic converter is arranged on the upstream side of the exhaust path of an internal combustion engine and a GPF having a catalyst layer containing rhodium is arranged on the downstream side of the exhaust path of the internal combustion engine, it is necessary to find a way to efficiently exert the catalytic activity (e.g. NOx purification performance) of the rhodium contained in the catalyst layer of the GPF.

[0015] For example, if the rhodium element is over-oxidized, the catalytic activity of the rhodium element (e.g., NOx purification performance) is reduced. On the other hand, the oxygen concentration in the exhaust gas supplied to the GPF through the catalytic converter is affected by the OSC material contained in the catalytic converter. Therefore, in order to prevent the catalytic activity of the rhodium element contained in the catalyst layer of the GPF (especially, the catalyst layer located immediately behind the catalytic converter and first contacted with the exhaust gas supplied to the GPF through the catalytic converter, that is, the catalyst layer formed on the chamber inlet side of the substrate in the GPF) from being affected by the oxygen concentration in the exhaust gas supplied to the GPF through the catalytic converter and reduced, it is desirable to adjust the amount of OSC material contained in the catalytic converter. In addition, in order to efficiently exert the catalytic activity of the rhodium element contained in the catalyst layer of the GPF, it is desirable to extend the time for the rhodium element contained in the catalyst layer of the GPF to contact the exhaust gas supplied to the GPF through the catalytic converter.

[0016] Therefore, an object of the present invention is to provide an exhaust gas purification system comprising a first exhaust gas treatment section (equivalent to a catalytic converter) arranged on the upstream side of an exhaust path of an internal combustion engine and a second exhaust gas treatment section (equivalent to a GPF) arranged on the downstream side of the exhaust path of the internal combustion engine, wherein the exhaust gas purification system can efficiently exert the catalytic activity (e.g., NOx purification performance) of the rhodium element contained in the catalyst layer of the second exhaust gas treatment section (in particular, the catalyst layer located immediately after the first exhaust gas treatment section and first contacting the exhaust gas supplied to the second exhaust gas treatment section through the first exhaust gas treatment section, that is, the catalyst layer formed on the chamber inlet side of the substrate in the second exhaust gas treatment section).

[0017] Solutions for solving problems

[0018] In order to solve the above-mentioned problems, the present invention provides an exhaust gas purification system for purifying exhaust gas discharged from an internal combustion engine.

[0019] The aforementioned exhaust gas purification system has:

[0020] an exhaust passage through which exhaust gas flows;

[0021] A first exhaust gas treatment unit is provided on the upstream side of the exhaust passage; and

[0022] The second exhaust gas treatment unit is arranged on the downstream side of the exhaust passage.

[0023] The first exhaust gas treatment unit comprises a flow-through substrate and a first catalyst layer.

[0024] The aforementioned flow-through type substrate comprises:

[0025] a plurality of chambers extending in the exhaust gas flow direction, wherein both ends on the exhaust gas inlet side and the exhaust gas outlet side in the exhaust gas flow direction are open; and

[0026] a partition wall that separates the aforementioned multiple chambers,

[0027] The first catalyst layer has a portion formed on the surface of the partition wall.

[0028] The second exhaust gas treatment unit comprises a wall flow type substrate, a second catalyst layer and a third catalyst layer.

[0029] The wall flow type substrate comprises:

[0030] an inflow side chamber extending in the exhaust gas flow direction, with an end portion on the exhaust gas inflow side in the exhaust gas flow direction being open and an end portion on the exhaust gas outflow side in the exhaust gas flow direction being closed;

[0031] an outflow side chamber extending in the exhaust gas flow direction, with an end portion on the exhaust gas inflow side in the exhaust gas flow direction being closed and an end portion on the exhaust gas outflow side in the exhaust gas flow direction being open; and

[0032] a porous partition wall that separates the inflow side chamber from the outflow side chamber,

[0033] The second catalyst layer has a portion formed along the exhaust gas flow direction from an end portion of the partition wall on the exhaust gas flow direction on the exhaust gas flow direction, on the surface of the partition wall on the inlet side chamber side.

[0034] The third catalyst layer has a portion formed on the surface of the outflow side chamber of the partition wall and extending from an end of the partition wall on the exhaust gas outflow side in the exhaust gas flow direction in a direction opposite to the exhaust gas flow direction.

[0035] The first catalyst layer contains cerium.

[0036] a percentage P1 of the mass of the cerium element contained in the first catalyst layer in terms of cerium oxide relative to the mass of the first catalyst layer of 5.0 mass % or more and 13.0 mass % or less,

[0037] The second catalyst layer contains rhodium element.

[0038] Effects of the Invention

[0039] According to the present invention, there is provided an exhaust gas purification system comprising a first exhaust gas treatment section (equivalent to a catalytic converter) arranged on the upstream side of an exhaust path of an internal combustion engine and a second exhaust gas treatment section (equivalent to a GPF) arranged on the downstream side of the exhaust path of the internal combustion engine, wherein the exhaust gas purification system is capable of efficiently exerting the catalytic activity (e.g., NOx purification performance) of the rhodium element contained in the catalyst layer of the second exhaust gas treatment section (in particular, the catalyst layer located immediately after the first exhaust gas treatment section and which first contacts the exhaust gas supplied to the second exhaust gas treatment section through the first exhaust gas treatment section, i.e., the catalyst layer formed on the chamber inlet side of the substrate in the second exhaust gas treatment section). BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a plan view of an exhaust gas purification system according to an embodiment of the present invention.

[0041] Figure 2 for Figure 1 AA line section view.

[0042] Figure 3 For Figure 2 An enlarged view of the area indicated by symbol R1 in FIG.

[0043] Figure 4 It is a cross-sectional view showing one embodiment of the stacked structure of the catalyst layer.

[0044] Figure 5 for Figure 1 BB line cross-sectional view.

[0045] Figure 6 For Figure 5 An enlarged view of the area indicated by symbol R2 in FIG.

[0046] Figure 7 For Figure 5 An enlarged view of the area indicated by symbol R3 in FIG.

[0047] Figure 8 This is a SEM observation image obtained by cutting the exhaust gas purifying catalyst along a plane parallel to the axial direction of the substrate and observing the cut surface using a scanning electron microscope (SEM). DETAILED DESCRIPTION

[0048] Hereinafter, embodiments of the exhaust gas purification system according to the present invention will be described with reference to the accompanying drawings. Figure 1 is a plan view of an exhaust gas purification system 1 according to an embodiment of the present invention, Figure 2 for Figure 1 AA line cross-sectional view (cross-sectional view of the first exhaust gas treatment unit 3 provided in the exhaust gas purification system 1), Figure 3 For Figure 2 An enlarged view of the area indicated by symbol R1 in FIG. Figure 4 is a cross-sectional view showing one embodiment of a stacked structure of a catalyst layer. Figure 5 for Figure 1 BB line cross-sectional view (cross-sectional view of the second exhaust gas treatment unit 4 provided in the exhaust gas purification system 1), Figure 6 For Figure 5 An enlarged view of the area indicated by symbol R2 in FIG. Figure 7 For Figure 5 An enlarged view of the area indicated by symbol R3 in FIG.

[0049] An exhaust gas purification system 1 according to an embodiment of the present invention is arranged in an exhaust path of an internal combustion engine, purifies harmful components such as HC, CO, and NOx in the exhaust gas discharged from the internal combustion engine, and captures particulate matter in the exhaust gas. The internal combustion engine is, for example, a gasoline engine (such as a GDI engine).

[0050] like Figure 1As shown, the exhaust gas purification system 1 includes an exhaust pipe 2, a first exhaust gas treatment unit 3 provided upstream in the exhaust pipe 2, and a second exhaust gas treatment unit 4 provided downstream in the exhaust pipe 2. The first exhaust gas treatment unit 3 corresponds to a catalytic converter, and the second exhaust gas treatment unit 4 corresponds to a GPF.

[0051] One end 21 of the exhaust pipe 2 is connected to the internal combustion engine, and the exhaust gas discharged from the internal combustion engine flows in the exhaust pipe 2 from the one end 21 to the other end 22 of the exhaust pipe 2. That is, the exhaust pipe 2 forms an exhaust passage for the exhaust gas to flow. In each of the drawings, the exhaust gas flow direction is represented by the symbol X. The exhaust gas flowing in the exhaust pipe 2 is supplied to the first exhaust gas treatment section 3 provided on the upstream side of the exhaust pipe 2, and is treated in the first exhaust gas treatment section 3. The exhaust gas passing through the first exhaust gas treatment section 3 is supplied to the second exhaust gas treatment section 4 provided on the downstream side of the exhaust pipe 2, and is treated in the second exhaust gas treatment section 4.

[0052] It should be noted that, in this specification, the upstream side of the exhaust gas flow direction X is sometimes referred to as the "exhaust gas inflow side", and the downstream side of the exhaust gas flow direction X is sometimes referred to as the "exhaust gas outflow side". In addition, in this specification, "length" refers to the dimension in the direction parallel to the exhaust gas flow direction X, unless otherwise specified, and "thickness" refers to the dimension in the direction perpendicular to the exhaust gas flow direction X, unless otherwise specified.

[0053] 《First Waste Gas Treatment Department》

[0054] Hereinafter, the first exhaust gas treatment unit 3 will be described.

[0055] like Figure 2 As shown, the first exhaust gas treatment unit 3 includes a flow-through type substrate 31 and a first catalyst layer 32 .

[0056] <Flow-through type substrate>

[0057] Hereinafter, the flow-through type substrate 31 will be described.

[0058] The material constituting the flow-through type substrate 31 can be appropriately selected from materials commonly used as the material of the substrate of the exhaust gas purification catalyst. The material constituting the flow-through type substrate 31 is preferably a material that can maintain a stable shape even when the flow-through type substrate 31 is exposed to high-temperature (e.g., 400° C. or higher) exhaust gas. Examples of the material of the flow-through type substrate 31 include ceramics such as cordierite, silicon carbide (SiC), and aluminum titanate, and alloys such as stainless steel.

[0059] The flow-through type substrate 31 is, for example, a honeycomb structure.

[0060] like Figure 2As shown, the flow-through type substrate 31 has a plurality of chambers 311 and a partition wall portion 312 that separates the plurality of chambers 311. In the flow-through type substrate 31, there is a partition wall portion 312 between two adjacent chambers 311, and the two adjacent chambers 311 are separated by the partition wall portion 312. It should be noted that the flow-through type substrate 31 has a cylindrical portion (not shown) that limits the outer shape of the flow-through type substrate 31, and the partition wall portion 312 is formed in the cylindrical portion. The shape of the cylindrical portion is, for example, cylindrical, and may also be other shapes. As other shapes, for example, elliptical cylindrical, polygonal cylindrical, etc. can be listed. The axial direction of the flow-through type substrate 31 is consistent with the axial direction of the cylindrical portion. The flow-through type substrate 31 is configured in such a way that the axial direction of the cylindrical portion is substantially consistent with the exhaust gas flow direction X.

[0061] like Figure 2 As shown, the flow-through type substrate 31 is formed with a plurality of holes that are open both on the exhaust gas inlet side and the exhaust gas outlet side, and the chamber 311 is formed using the space within these holes.

[0062] like Figure 2 As shown, the plurality of chambers 311 extend in the exhaust gas flow direction X and have an end on the exhaust gas inflow side in the exhaust gas flow direction X and an end on the exhaust gas outflow side in the exhaust gas flow direction X. Figure 2 As shown, both the end on the exhaust gas inlet side of the exhaust gas flow direction X and the end on the exhaust gas outlet side of the exhaust gas flow direction X are open. Hereinafter, the end on the exhaust gas inlet side of the chamber 311 is sometimes referred to as the "opening portion on the exhaust gas inlet side of the chamber 311", and the end on the exhaust gas outlet side of the chamber 311 is sometimes referred to as the "opening portion on the exhaust gas outlet side of the chamber 311".

[0063] The top-down shapes of the openings on the exhaust gas inlet side and the exhaust gas outlet side of the chamber 311 (the shape when looking down at the flow-through type substrate 31 from the exhaust gas inlet side and the exhaust gas outlet side in the exhaust gas flow direction X) include quadrilaterals such as squares, parallelograms, rectangles, trapezoids, triangles such as equilateral triangles, hexagons such as regular hexagons, octagons such as regular octagons, and polygons such as circles, ellipses, and other various geometric shapes.

[0064] The area of ​​the opening portion of the chamber 311 on the exhaust gas inlet side in plan view may be the same as or different from the area of ​​the opening portion of the chamber 311 on the exhaust gas outlet side in plan view.

[0065] The density of the chamber 311 can be appropriately adjusted in consideration of the pressure loss, the capture rate of particulate matter, etc. The smaller the density of the chamber 311, the larger the opening area of ​​the opening on the exhaust gas inlet side of the chamber 311 becomes, which can reduce the pressure loss, but the capture rate of particulate matter decreases. Therefore, the density of the chamber 311 per 1 square inch of the flow-through type substrate 31 is preferably 400 chambers or more and 1200 chambers or less, and more preferably 600 chambers or more and 900 chambers or less. The density of the chamber 311 per 1 square inch of the flow-through type substrate 31 is the total number of chambers 311 per 1 square inch when the flow-through type substrate 31 is observed from the exhaust gas inlet side or the exhaust gas outlet side in the exhaust gas flow direction X.

[0066] The volume of the flow-through substrate 31 is preferably greater than 0.7 L and less than 1.6 L, and more preferably greater than 0.8 L and less than 1.2 L. By adjusting the volume of the flow-through substrate 31 to the above range, the inorganic oxide containing the cerium element in the first catalyst layer 32 can more appropriately exert its oxygen storage capacity. The volume of the flow-through substrate 31 is the apparent volume of the flow-through substrate 31 (i.e., the volume of the entire flow-through substrate 31 including the chamber 311 and the partition wall portion 312). When the flow-through substrate 31 is cylindrical, the outer diameter of the cylindrical portion that defines the outer shape of the flow-through substrate 31 is denoted as 2r, and the length of the flow-through substrate 31 is denoted as L. 31 When , the volume of the flow-through type substrate 31 can be calculated according to the following formula.

[0067] Volume of the flow-through substrate 31 = π × r 2 ×L 31

[0068] The thickness of the partition wall portion 312 is, for example, not less than 54 μm and not more than 73 μm. The thickness of the partition wall portion 312 is obtained by the same calculation method as the calculation method of the thickness of the catalyst layer described below.

[0069] <First Catalyst Layer>

[0070] Hereinafter, the first catalyst layer 32 will be described.

[0071] like Figure 2 and Figure 3 As shown in FIG. 1 , the first catalyst layer 32 has a portion 321 formed on the surface of the partition wall portion 312. Specifically, the first catalyst layer 32 has a portion 321 formed on the surface of the partition wall portion 312 along the exhaust gas flow direction X from the end of the exhaust gas inlet side of the partition wall portion 312 to the end of the exhaust gas outflow side of the partition wall portion 312. The "surface of the partition wall portion 312" refers to the outer surface that defines the outer shape of the partition wall portion 312. The "portion formed on the surface of the partition wall portion 312" refers to the portion that bulges from the outer surface of the partition wall portion 312 toward the chamber 311 side.

[0072] like Figure 3 As shown, the first catalyst layer 32 may have a portion 321 and a portion 322 existing inside the partition wall portion 312. For example, when the partition wall portion 312 is porous, when forming the first catalyst layer 32, a portion 322 is usually formed while forming the portion 321. The region where the portion 321 exists does not overlap with the region where the partition wall portion 312 exists, but the region where the portion 322 exists overlaps with the region where the partition wall portion 312 exists. Therefore, by cutting the first catalyst layer 32 and analyzing the cross section by cross-sectional observation using a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA), etc., the portion 321 and the portion 322 can be determined based on the composition, morphology, etc. When observing the cut surface, element mapping of the cut surface can be performed. Element mapping can be performed by combining, for example, observation of the cut surface based on SEM and composition analysis of the cut surface. Element mapping can be performed using, for example, a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX), an electron beam microanalyzer (EPMA), a transmission X-ray inspection device, etc. By element mapping of the cut surface, the portion 321 and the portion 322 can be identified based on the difference in morphology and composition between the first catalyst layer 32 and the partition wall portion 312 .

[0073] The amount of the first catalyst layer 32 can be appropriately adjusted in consideration of the thermal durability of the first catalyst layer 32 and the pressure loss of the first exhaust gas treatment unit 3. From the viewpoint of improving the thermal durability of the first catalyst layer 32, the amount of the first catalyst layer 32 is preferably larger, but as the amount increases, the area of ​​the opening on the exhaust gas inlet side of the chamber 311 decreases and the pressure loss increases. Therefore, the mass (mass after drying and sintering) of the first catalyst layer 32 per unit volume of the flow-through substrate 31 is preferably greater than 135 g / L and less than 320 g / L, further preferably greater than 150 g / L and less than 270 g / L, and further preferably greater than 190 g / L and less than 240 g / L. It should be noted that the volume of the flow-through substrate 31 is the apparent volume of the flow-through substrate 31.

[0074] The method for calculating the mass of the first catalyst layer 32 per unit volume of the flow-through substrate 31 is as follows, for example.

[0075] A portion extending in the axial direction of the flow-through type substrate 31 and having a length L equal to that of the flow-through type substrate 31 is cut out from the first exhaust gas treatment portion 3 . 31For samples of the same length, the sample is cut with a plane perpendicular to the axial direction of the flow-through substrate 31 to prepare a cut piece S1 containing the first catalyst layer 32. The cut piece S1 is, for example, a cylindrical shape with a diameter of 25.4 mm and a length of 10 mm. The values ​​of the diameter and length of the cut piece S1 can be changed as needed. The length of the first catalyst layer 32 contained in the cut piece S1 is equal to the length of the cut piece S1.

[0076] A cut piece of the flow-through type substrate 31 having the same size as the cut piece S1 is prepared. The cut piece of the flow-through type substrate 31 does not include the first catalyst layer 32 .

[0077] The mass of the cut piece S1 and the mass of the cut piece of the flow-through substrate 31 were measured, and the mass of the first catalyst layer 32 per unit volume of the cut piece S1 was calculated according to the following formula.

[0078] The mass of the first catalyst layer 32 per unit volume of the cut piece S1 = ((the mass of the cut piece S1) - (the mass of the cut piece of the flow-through substrate 31)) / (the volume of the cut piece S1)

[0079] The volume of the cut piece S1 is the apparent volume of the cut piece S1. For example, if the cut piece S1 is a cylindrical shape with a diameter of 25.4 mm and a length of 10 mm, the volume of the cut piece S1 is π×(12.7 mm) 2 The volumes of the other cut pieces (cut pieces S2 and S3 described later) are also the same.

[0080] The mass of the first catalyst layer 32 per unit volume of the three cut pieces S1 produced at any position of the first exhaust gas treatment unit 3 was calculated, and their average value was taken as the mass of the first catalyst layer 32 per unit volume of the permeation type substrate 31 .

[0081] The mass of the first catalyst layer 32 per unit volume of the cut piece S1 can also be calculated without using a cut piece of the flow-through substrate 31. An example of such a calculation method is as follows. The mass and volume of the cut piece S1 are measured. The composition of the flow-through substrate 31 contained in the cut piece S1 is determined by element mapping of the cut surface of the cut piece S1, etc. The composition of the cut piece S1 is determined by analyzing using an inductively coupled plasma emission spectrometer, etc. Based on the determined composition of the flow-through substrate 31 and the cut piece S1, the proportion of the mass of the first catalyst layer 32 in the mass of the cut piece S1 is calculated. The mass of the first catalyst layer 32 per unit volume of the cut piece S1 is calculated according to the following formula.

[0082] The mass of the first catalyst layer 32 per unit volume of the cut piece S1 = (the mass of the cut piece S1) x (the ratio of the mass of the first catalyst layer 32 to the mass of the cut piece S1) / (the volume of the cut piece S1)

[0083] The first catalyst layer 32 contains one or more catalytically active components. The catalytically active component can be selected from precious metal elements such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). From the perspective of improving exhaust gas purification performance, it is preferably selected from platinum (Pt), palladium (Pd), and rhodium (Rh). The precious metal element is contained in the first catalyst layer 32 in a form that can function as a catalytically active component, such as a precious metal, an alloy containing a precious metal element, a compound containing a precious metal element (such as an oxide of a precious metal element), etc. From the perspective of improving exhaust gas purification performance, the catalytically active component is preferably in a granular form.

[0084] The first catalyst layer 32 contains cerium (Ce) element. The cerium element contained in the first catalyst layer 32 is derived from an inorganic oxide containing the cerium element. That is, the first catalyst layer 32 contains an inorganic oxide containing the cerium element.

[0085] The inorganic oxide containing the cerium element is an inorganic oxide whose valence of the constituent elements changes due to the working conditions of the exhaust gas purification catalyst, and has oxygen storage capacity (OSC: Oxygen Storage Capacity). The inorganic oxide containing the cerium element is used in the form of, for example, a powder. From the viewpoint of easy loading of catalytically active components, the inorganic oxide containing the cerium element is preferably a porous body. The BET specific surface area of ​​the porous body is, for example, 50 to 150 m 2 / g. It should be noted that the BET specific surface area is measured according to "(3.5) Single-point method" in "6.2 Flow method" of JIS R1626 "Specific surface area measurement method of fine ceramic powder based on gas adsorption BET method". As the gas, a nitrogen-helium mixed gas containing 30 volume % of nitrogen as adsorbed gas and 70 volume % of helium as carrier gas is used. As a measuring device, "BELSORP-MR6" manufactured by MICROTRAC-BELL can be used.

[0086] As inorganic oxides containing cerium elements, for example, cerium oxide (CeO2), composite oxides containing cerium (Ce) elements and zirconium (Zr) elements (hereinafter referred to as "CeO2-ZrO2 composite oxides"), etc. can be listed. Among these, CeO2-ZrO2 composite oxides are preferred. In CeO2-ZrO2 composite oxides, Ce2O and ZrO2 preferably form a solid solution phase. In CeO2-ZrO2 composite oxides, Ce2O and ZrO2 can form a separate phase (Ce2O separate phase or ZrO2 separate phase) on the basis of forming a solid solution phase. CeO2 and ZrO2 forming a solid solution phase can be confirmed by using an X-ray diffraction device (XRD) to confirm that a single phase derived from CeO2-ZrO2 has been formed.

[0087] The percentage P1 of the mass of the cerium element contained in the first catalyst layer 32 in terms of cerium oxide (CeO 2 ) relative to the mass of the first catalyst layer 32 is 5.0 mass % or more and 13.0 mass % or less.

[0088] In order to fully exert the catalytic activity of the rhodium element (such as NOx purification performance), it is necessary to control the oxygen concentration in the exhaust gas within a certain range. For example, if the oxygen concentration in the exhaust gas is too high, the rhodium element is over-oxidized, and the catalytic activity of the rhodium element (such as NOx purification performance) is reduced. On the other hand, in order to fully exert the catalytic activity of the rhodium element, it is preferred that the rhodium element is moderately oxidized. Therefore, if the oxygen concentration in the exhaust gas is too low, the oxidation of the rhodium element is excessively suppressed, and the catalytic activity of the rhodium element becomes insufficient. Therefore, in order to well maintain the catalytic activity of the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment unit 4 (such as NOx purification performance), it is required to control the range of the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 within a certain range. On the other hand, the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 is affected by the inorganic oxide containing the cerium element contained in the first catalyst layer 32. If the percentage P1 is greater than or equal to 5.0 mass % and less than or equal to 13.0 mass %, the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 can be controlled within a range in which the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment unit 4 exhibits sufficient catalytic activity (e.g., NOx purification performance). The detailed reason is not yet clear, but the following reasons can be considered. The more the percentage P1 increases, the more the oxygen storage capacity of the first catalyst layer 32 increases, and the fluctuation range of the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 becomes smaller. It can be considered that: if the percentage P1 is less than 5.0 mass %, the oxygen storage capacity of the first catalyst layer 32 is insufficient, and the fluctuation range of the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment part 4 through the first exhaust gas treatment part 3 is too large. Therefore, the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment part 4 is sometimes affected by the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment part 4 through the first exhaust gas treatment part 3 and is over-oxidized, and the catalytic activity of the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment part 4 (for example, NOx purification performance) will be reduced. On the other hand, it is considered that if the percentage P1 exceeds 13.0% by mass, the oxygen storage capacity of the first catalyst layer 32 becomes excessive, and the fluctuation range of the oxygen concentration in the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 is too small (that is, the oxygen exchanged inside the first catalyst layer 32 increases, and the oxygen released from the first catalyst layer 32 and supplied to the second catalyst layer 42 of the second exhaust gas treatment unit 4 is excessively reduced), so that the oxidation of the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment unit 4 becomes insufficient, and the catalytic activity (for example, NOx purification performance) of the rhodium element contained in the second catalyst layer 42 of the second exhaust gas treatment unit 4 becomes insufficient. The exchange of oxygen inside the first catalyst layer 32 can occur by, for example, storing oxygen released by a certain inorganic oxide particle by another inorganic oxide particle.

[0089] In addition, when the first catalyst layer 32 contains rhodium elements, the catalytic activity (e.g., NOx purification performance) of the rhodium elements contained in the first catalyst layer 32 may be affected by the oxygen concentration in the exhaust gas passing through the first exhaust gas treatment unit 3 and reduced. On the other hand, the oxygen concentration in the exhaust gas passing through the first exhaust gas treatment unit 3 is affected by the inorganic oxide containing the cerium element contained in the first catalyst layer 32. If the percentage P1 is greater than 5.0 mass % and less than 13.0 mass %, it is possible to prevent the catalytic activity (e.g., NOx purification performance) of the rhodium elements contained in the first catalyst layer 32 from being affected by the oxygen concentration in the exhaust gas passing through the first exhaust gas treatment unit 3 and reduced. The detailed reason is not yet certain, but the following reasons can be considered. It can be considered that the more the percentage P1 increases, the more the oxygen storage capacity of the first catalyst layer 32 increases. If the percentage P1 is less than 5.0 mass %, the oxygen storage capacity of the first catalyst layer 32 is insufficient, and the catalytic activity (e.g., NOx purification performance) of the rhodium element contained in the first catalyst layer 32 cannot be sufficiently prevented from being reduced by the influence of the oxygen concentration in the exhaust gas passing through the first exhaust gas treatment unit 3. On the other hand, it is considered that if the percentage P1 exceeds 13.0 mass %, the oxygen exchanged inside the first catalyst layer 32 increases due to the increase in the oxygen storage capacity of the first catalyst layer 32, and the oxygen exchanged between the first catalyst layer 32 and the outside of the first catalyst layer 32 (exhaust gas passing through the first exhaust gas treatment unit 3) is excessively reduced, and the catalytic activity (e.g., NOx purification performance) of the rhodium element contained in the first catalyst layer 32 becomes insufficient. The exchange of oxygen inside the first catalyst layer 32 can occur by, for example, storing oxygen released by a certain inorganic oxide particle by another inorganic oxide particle.

[0090] The percentage P1 is not particularly limited as long as it is 5.0 mass % to 13.0 mass %, but is preferably 6.0 mass % to 12.0 mass %, more preferably 7.0 mass % to 11.0 mass %, and even more preferably 8.0 mass % to 10.0 mass %.

[0091] The percentage P1 is calculated as follows.

[0092] The same operation as above is performed to prepare the cut piece S1, and the mass of the cerium element contained in the cut piece S1 in terms of cerium oxide is measured by conventional methods such as inductively coupled plasma emission spectrometry (ICP-AES), and the mass of the cerium element contained in the cut piece S1 in terms of cerium oxide per unit volume is calculated. For three cut pieces S1 prepared from any part of the first exhaust gas treatment unit 3, the mass of the cerium element contained in the cut piece S1 in terms of cerium oxide per unit volume is calculated, and the average value thereof is used as the mass of the cerium element contained in the cut piece S1 in terms of cerium oxide per unit volume of the flow-through substrate 31. The percentage P1 is calculated according to the following formula.

[0093] Percentage P1=(mass of cerium element per unit volume of the flow-through substrate 31 in terms of cerium oxide) / (mass of the first catalyst layer 32 per unit volume of the flow-through substrate 31)×100

[0094] The amount of cerium element contained in CeO2-ZrO2 system composite oxide can be appropriately adjusted in consideration of the balance between oxygen storage capacity and cost. From the viewpoint of the balance between oxygen storage capacity and cost, the amount of cerium oxide converted in CeO2-ZrO2 system composite oxide is preferably 5% by mass or more and 40% by mass or less, and more preferably 5% by mass or more and 30% by mass or less, based on the mass of CeO2-ZrO2 system composite oxide. The amount of cerium oxide converted in CeO2-ZrO2 system composite oxide can be measured using conventional methods such as inductively coupled plasma emission spectrometry (ICP-AES). The amount of oxides converted in other metal elements contained in CeO2-ZrO2 system composite oxide is also the same.

[0095] The amount of zirconium element contained in the CeO2-ZrO2 composite oxide can be appropriately adjusted in consideration of the balance between heat resistance and cost, etc. From the viewpoint of the balance between heat resistance and cost, etc., the amount of zirconium element contained in the CeO2-ZrO2 composite oxide in terms of zirconium oxide is preferably 50% by mass or more and 90% by mass or less, and more preferably 55% by mass or more and 80% by mass or less, based on the mass of the CeO2-ZrO2 composite oxide.

[0096] From the viewpoint of the balance between oxygen storage capacity and heat resistance, the ratio of the amount of zirconium element contained in the CeO2-ZrO2 composite oxide converted into zirconium oxide to the amount of cerium element contained in the CeO2-ZrO2 composite oxide converted into cerium oxide (the mass of zirconium element converted into zirconium oxide / the mass of cerium element converted into cerium oxide) is preferably greater than 1.0 and less than 9.0, and more preferably greater than 1.2 and less than 4.0.

[0097] CeO2-ZrO2 composite oxides may contain metal elements other than cerium and zirconium. One or more metal elements other than cerium and zirconium or their oxides may form a solid solution phase with Ce2O and / or ZrO2, or may form a separate phase. One or more metal elements other than cerium and zirconium or their oxides may form a solid solution phase with Ce2O and / or ZrO2, which may be confirmed by using an X-ray diffraction device (XRD) in the same manner as described above.

[0098] The one or two or more metal elements other than cerium and zirconium can be selected from, for example, rare earth elements other than cerium, alkaline earth metal elements, and transition metal elements, and are preferably selected from rare earth elements other than cerium.

[0099] The total amount of oxides of one or more metal elements other than cerium and zirconium contained in the CeO2-ZrO2 composite oxide is preferably 7 mass % or more and 20 mass % or less, and further preferably 10 mass % or more and 15 mass % or less, based on the mass of the CeO2-ZrO2 composite oxide.

[0100] As rare earth elements other than cerium, for example, yttrium (Y) element, praseodymium (Pr) element, scandium (Sc) element, lanthanum (La) element, neodymium (Nd) element, samarium (Sm) element, europium (Eu) element, gadolinium (Gd) element, terbium (Tb) element, dysprosium (Dy) element, holmium (Ho) element, erbium (Er) element, thulium (Tm) element, ytterbium (Yb) element, lutetium (Lu) element, etc., among which lanthanum element, neodymium element, praseodymium element, yttrium element, scandium element and ytterbium element are preferred, and lanthanum element, neodymium element and yttrium element are further preferred. Rare earth element oxides are all sesquioxides (Ln2O3, Ln represents rare earth elements) except praseodymium element and terbium element. Praseodymium oxide is usually Pr6O 11 , terbium oxide is usually Tb4O7.

[0101] Examples of the alkaline earth metal element include calcium (Ca) element, strontium (Sr) element, barium (Ba) element, and radium (Ra) element. Among these, calcium element, strontium element, and barium element are preferred.

[0102] Examples of the transition metal element include manganese (Mn) element, iron (Fe) element, cobalt (Co) element, nickel (Ni) element, and copper (Cu) element.

[0103] The first catalyst layer 32 preferably contains an inorganic oxide other than the inorganic oxide containing the cerium element (hereinafter referred to as "other inorganic oxide"). The other inorganic oxide is used in the form of, for example, a powder. From the viewpoint of facilitating the loading of the catalytically active component, the other inorganic oxide is preferably a porous body. The BET specific surface area of ​​the porous body is, for example, 50 to 150 m 2 / g. Examples of other inorganic oxides include alumina, silica, silica-alumina, aluminosilicates, alumina-zirconia, alumina-chromia, and alumina-lanthanum oxide. Among these, alumina is preferred from the viewpoint of heat resistance.

[0104] The amount of other inorganic oxides contained in the first catalyst layer 32 can be appropriately adjusted in consideration of the thermal durability of the first catalyst layer 32 and the pressure loss of the first exhaust gas treatment unit 3. From the viewpoint of improving the thermal durability of the first catalyst layer 32, the amount of other inorganic oxides contained in the first catalyst layer 32 is preferably large, but as the amount increases, the area of ​​the opening on the exhaust gas inflow side of the chamber 311 decreases and the pressure loss increases. Therefore, the mass of other inorganic oxides contained in the first catalyst layer 32 is preferably 86% by mass or more and 95% by mass or less, more preferably 88% by mass or more and 94% by mass or less, and even more preferably 90% by mass or more and 92% by mass or less, based on the mass of the first catalyst layer 32. The method for calculating the percentage of the mass of other inorganic oxides contained in the first catalyst layer 32 relative to the mass of the first catalyst layer 32 is the same as the method for calculating the percentage P1.

[0105] Inorganic oxides other than inorganic oxides containing cerium elements may be modified with inorganic oxides containing cerium elements, and inorganic oxides containing cerium elements may be supported. For example, the inner surface or outer surface of the pore portion of aluminum oxide or the like may be modified with an inorganic oxide containing cerium elements. In addition, inorganic oxides containing cerium elements may be supported in a dispersed state on the inner surface or outer surface of the pore portion of aluminum oxide or the like.

[0106] From the viewpoint of efficiently exerting the exhaust gas purification performance brought about by the catalytically active component, the catalytically active component is preferably supported on an inorganic oxide containing a cerium element and / or other inorganic oxides. "The catalytically active component is supported on an inorganic oxide" means that the catalytically active component is physically or chemically adsorbed or retained on the outer surface of the inorganic oxide particles or the inner surface of the pores. For example, in the elemental mapping obtained by analyzing the cross section of the catalyst layer using an energy dispersive X-ray analyzer (EDX), when the inorganic oxide and the catalytically active component exist in the same area, it can be determined that the catalytically active component is supported on the inorganic oxide.

[0107] The first catalyst layer 32 may have a single-layer structure or a laminated structure. When the first catalyst layer 32 has a laminated structure, the composition of a certain layer may be the same as or different from the composition of other layers. For example, the catalytically active component contained in a certain layer may be the same as or different from the catalytically active component contained in other layers.

[0108] One embodiment of the stacked structure of the first catalyst layer 32 is shown in FIG. Figure 4 .like Figure 4 As shown, the first catalyst layer 32 may have a two-layer structure including a lower layer 32a and an upper layer 32b. It should be noted that the lower layer 32a is a layer closer to the partition wall portion 312 than the upper layer 32b.

[0109] When the first catalyst layer 32 has a laminated structure, the portion 321 of the first catalyst layer 32 may be formed by the entirety or a portion of one layer, or by the entirety of more than one layer and the entirety or a portion of another layer. For example, when the first catalyst layer 32 has a two-layer structure, the portion 321 of the first catalyst layer 32 may be formed by the entirety or a portion of the upper layer, or by the entirety of the upper layer and a portion of the lower layer. Figure 4 In the illustrated embodiment, the portion 321 of the first catalyst layer 32 is formed by the entirety of the upper layer 32b and a portion of the lower layer 32a.

[0110] Figure 4 In the embodiment shown, the amount of the lower layer 32a and the upper layer 32b provided in the flow-through type substrate 31 can be appropriately adjusted in consideration of the balance between the exhaust gas purification performance and the cost. From the viewpoint of the balance between the exhaust gas purification performance and the cost, the mass of the lower layer 32a per unit volume of the flow-through type substrate 31 (the mass after drying and firing) is preferably 90 g / L or more and 200 g / L or less, more preferably 110 g / L or more and 185 g / L or less, and further preferably 140 g / L or more and 175 g / L or less, and the mass of the upper layer 32b per unit volume of the flow-through type substrate 31 (the mass after drying and firing) is preferably 45 g / L or more and 120 g / L or less, more preferably 40 g / L or more and 90 g / L or less, and further preferably 50 g / L or more and 66 g / L or less. It should be noted that the volume of the flow-through type substrate 31 is the apparent volume of the entire flow-through type substrate 31.

[0111] Figure 4In the illustrated embodiment, the lower layer 32a and the upper layer 32b each contain one or more catalytically active components. The catalytically active component contained in the lower layer 32a is preferably different from the catalytically active component contained in the upper layer 32b. For example, the catalytically active component contained in the lower layer 32a is palladium, and the catalytically active component contained in the upper layer 32b is a precious metal element other than palladium (e.g., rhodium). By covering the lower layer 32a containing the palladium element with the upper layer 32b, phosphorus poisoning of the palladium element contained in the lower layer 32a (reduction in the catalytic activity of the palladium element caused by phosphorus contained in the exhaust gas) can be reduced.

[0112] Figure 4 In the embodiment shown, at least one of the lower layer 32a and the upper layer 32b contains an inorganic oxide containing a cerium element. In one embodiment, the lower layer 32a contains an inorganic oxide containing a cerium element, and the upper layer 32b does not contain an inorganic oxide containing a cerium element. In other embodiments, the lower layer 32a does not contain an inorganic oxide containing a cerium element, and the upper layer 32b contains an inorganic oxide containing a cerium element. Furthermore, in other embodiments, both the lower layer 32a and the upper layer 32b contain an inorganic oxide containing a cerium element. In any embodiment, the percentage P1 of the mass of the cerium element contained in the first catalyst layer 32 (upper layer 32a and lower layer 32b) in terms of cerium oxide relative to the mass of the first catalyst layer 32 (upper layer 32a and lower layer 32b) is 5.0 mass% or more and 13.0 mass% or less.

[0113] The amount of the inorganic oxide containing the cerium element contained in the lower layer 32a can be appropriately adjusted in consideration of the oxygen storage capacity and the pressure loss of the first exhaust gas treatment unit 3. From the viewpoint of improving the oxygen storage capacity, the amount of the inorganic oxide containing the cerium element contained in the lower layer 32a is preferably large, but as the amount increases, the area of ​​the opening portion on the exhaust gas inflow side of the chamber 311 decreases, and the pressure loss increases. Therefore, the percentage of the mass of the cerium element contained in the lower layer 32a in terms of cerium oxide relative to the mass of the lower layer 32a is preferably 7% by mass or more and 12% by mass or less, and more preferably 10% by mass or more and 12% by mass or less.

[0114] From the same viewpoint as the lower layer 32a, the amount of the inorganic oxide containing the cerium element contained in the upper layer 32b can be appropriately adjusted in consideration of the oxygen storage capacity and the pressure loss of the first exhaust gas treatment unit 3. The percentage of the mass of the cerium element contained in the upper layer 32b in terms of cerium oxide relative to the mass of the upper layer 32b is preferably 1% by mass or more and 7% by mass or less, and more preferably 3% by mass or more and 7% by mass or less.

[0115] like Figure 4 As shown, when the first catalyst layer 32 has a two-layer structure including a lower layer 32a and an upper layer 32b, the lower layer 32a and the upper layer 32b each preferably contain other inorganic oxides.

[0116] The amount of other inorganic oxides contained in the lower layer 32a can be appropriately adjusted in consideration of the thermal durability of the lower layer 32a and the pressure loss of the first exhaust gas treatment unit 3. From the viewpoint of improving the thermal durability of the lower layer 32a, the amount of other inorganic oxides contained in the lower layer 32a is preferably large, but as the amount increases, the area of ​​the opening on the exhaust gas inflow side of the chamber 311 decreases, and the pressure loss increases. Therefore, the mass of other inorganic oxides contained in the lower layer 32a is preferably 60 mass% or more and 76 mass% or less, and more preferably 64 mass% or more and 72 mass% or less, based on the mass of the lower layer 32a.

[0117] From the same viewpoint as the lower layer 32a, the amount of other inorganic oxides contained in the upper layer 32b can be appropriately adjusted in consideration of the thermal durability of the upper layer 32b and the pressure loss of the first exhaust gas treatment unit 3. The mass of other inorganic oxides contained in the upper layer 32b is preferably 33 mass % or more and 100 mass % or less, and more preferably 33 mass % or more and 56 mass % or less, based on the mass of the upper layer 32b.

[0118] Hereinafter, a method for forming the first catalyst layer 32 will be described.

[0119] The flow-through type substrate 31 and a slurry for forming the first catalyst layer 32 are prepared. When the first catalyst layer 32 has a laminated structure, two or more slurries are prepared as the slurry for forming the first catalyst layer 32.

[0120] The composition of the slurry used to form the first catalyst layer 32 is adjusted according to the composition of the first catalyst layer 32. The slurry includes, for example, a source of precious metal elements, inorganic oxide particles, a binder, a pore former, a solvent, and the like. As a source of precious metal elements, for example, salts of precious metal elements can be listed, and as salts of precious metal elements, for example, nitrates, ammine complex salts, acetates, chlorides, and the like can be listed. As inorganic oxides constituting inorganic oxide particles, for example, inorganic oxides containing cerium elements, other inorganic oxides, and the like can be listed. The descriptions related to inorganic oxides containing cerium elements and other inorganic oxides are the same as above. As a binder, for example, alumina sol, zirconia sol, titania sol, silica sol, and the like can be listed. As a pore former, for example, cross-linked poly(methyl)acrylate particles, cross-linked poly(butyl)acrylate particles, cross-linked polystyrene particles, cross-linked polyacrylate particles, melamine resins, and the like can be listed. As a solvent, for example, water, an organic solvent, and the like can be listed. Examples of the organic solvent include alcohols, acetone, dimethyl sulfoxide, dimethylformamide, etc. The solvent may be a single solvent or a mixture of two or more solvents.

[0121] The slurry for forming the first catalyst layer 32 is applied to the flow-through substrate 31, dried, and fired as needed. When the first catalyst layer 32 has a stacked structure, this operation is repeated. Thus, a precursor layer of the first catalyst layer 32 is formed. By adjusting the solid content concentration, viscosity, etc. of the slurry, the length of the precursor layer of the first catalyst layer 32 (and thus the length of the first catalyst layer 32) can be adjusted. In addition, by adjusting the coating amount of the slurry, the type of material constituting the slurry, the particle size of the pore-forming agent contained in the slurry, etc., the thickness of the precursor layer of the first catalyst layer 32 (and thus the thickness of the first catalyst layer 32) and the mass of the precursor layer of the first catalyst layer 32 (and thus the mass of the first catalyst layer 32) can be adjusted. The drying temperature is generally above 70°C and below 200°C, preferably above 90°C and below 150°C, and the drying time is generally above 1 hour and below 3 hours, preferably above 1.5 hours and below 2 hours.

[0122] The particle size of the pore former can be adjusted appropriately. From the viewpoints of suppressing peeling, suppressing the increase in pressure loss, and PM collection performance, the median particle size D of the pore former is preferably 50 It is usually 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The larger the particle size of the pore former, the greater the thickness of the first catalyst layer 32. 50 It is the particle size at which the cumulative volume in the volume-based particle size distribution measured by laser diffraction scattering particle size distribution measurement method reaches 50%. 50 The measurement of D is performed by, for example, using an automatic sample feeder for a laser diffraction scattering particle size distribution measuring device ("Microtorac SDC" manufactured by MICROTRAC-BELL), placing a pore-forming agent into an aqueous dispersion medium, irradiating 40W ultrasonic waves for 360 seconds at a flow rate of 26 mL / sec, and then using a laser diffraction scattering particle size distribution measuring device ("MICROTRAC MT3300EXII" manufactured by MICROTRAC-BELL). The particle refractive index is set to 1.5, the particle shape is set to "true sphere", the solvent refractive index is set to 1.3, the "zero setting" is set to 30 seconds, and the measurement time is set to 30 seconds. The average value of the obtained measured values ​​is taken as D 50 As the aqueous dispersion medium, pure water was used.

[0123] The coating of the slurry for forming the first catalyst layer 32 can be performed by, for example, immersing the entire flow-through type substrate 31 in the slurry for forming the first catalyst layer 32, or by immersing the end of the exhaust gas inlet side or the exhaust gas outlet side of the flow-through type substrate 31 in the slurry for forming the first catalyst layer 32 and sucking the slurry from the opposite side.

[0124] After forming the precursor layer of the first catalyst layer 32, calcination is performed. Thus, the first catalyst layer 32 is formed. The calcination temperature is usually 450° C. to 700° C., preferably 450° C. to 600° C., and the calcination time is usually 1 hour to 4 hours, preferably 1.5 hours to 2 hours. Calcination can be performed, for example, in an air atmosphere.

[0125] 《The second waste gas treatment unit》

[0126] Next, the second exhaust gas treatment unit 4 will be described.

[0127] like Figure 5 As shown, the second exhaust gas treatment unit 4 includes a wall flow type substrate 41 , a second catalyst layer 42 , and a third catalyst layer 43 .

[0128] <Wall flow type substrate>

[0129] Next, the wall flow type substrate 41 will be described.

[0130] The material constituting the wall flow type substrate 41 can be appropriately selected from materials commonly used as substrates of exhaust gas purifying catalysts. For example, the same material as that constituting the above-mentioned percolation type substrate 31 can be used.

[0131] like Figure 5 As shown, the wall-flow type substrate 41 has an inlet side chamber 411, an outlet side chamber 412, and a porous partition wall 413 that separates the inlet side chamber 411 from the outlet side chamber 412. In the wall-flow type substrate 41, a plurality of (for example, four) outlet side chambers 412 are arranged adjacent to the periphery of one inlet side chamber 411, and the inlet side chamber 411 and the outlet side chamber 412 adjacent to the inlet side chamber 411 are separated by a porous partition wall portion 413. It should be noted that the wall-flow type substrate 41 has a cylindrical portion (not shown) that limits the outer shape of the wall-flow type substrate 41, and the partition wall 413 is formed in the cylindrical portion. The shape of the cylindrical portion is, for example, cylindrical, and may also be other shapes. As other shapes, for example, an elliptical cylindrical shape, a polygonal cylindrical shape, etc. can be listed. The axial direction of the wall-flow type substrate 41 is consistent with the axial direction of the cylindrical portion. The wall flow type substrate 41 is arranged so that the axial direction of the cylindrical portion substantially coincides with the exhaust gas flow direction X.

[0132] like Figure 5 As shown, the wall flow type substrate 41 is formed with a recessed portion opening on the exhaust gas inlet side and a recessed portion opening on the exhaust gas outlet side. An inlet side chamber 411 is formed by utilizing the space within the recessed portion opening on the exhaust gas inlet side, and an outlet side chamber 412 is formed by utilizing the space within the recessed portion opening on the exhaust gas outlet side.

[0133] like Figure 5As shown, the inlet side chamber 411 extends in the exhaust gas flow direction X and has an end on the exhaust gas inlet side and an end on the exhaust gas outlet side. Figure 5 As shown, the end of the exhaust gas inlet side of the inlet side chamber 411 is open, and the end of the exhaust gas outlet side of the inlet side chamber 411 is closed. Hereinafter, the end of the exhaust gas inlet side of the inlet side chamber 411 is sometimes referred to as the "opening of the inlet side chamber 411".

[0134] like Figure 5 As shown, the wall flow type substrate 41 is provided with a first sealing portion 44 for sealing the end of the inflow side chamber 411 on the exhaust gas outflow side. The end of the inflow side chamber 411 on the exhaust gas outflow side is closed by the first sealing portion 44 .

[0135] like Figure 5 As shown, the outflow side chamber 412 extends in the exhaust gas flow direction X and has an end on the exhaust gas inflow side and an end on the exhaust gas outflow side. Figure 5 As shown, the end of the exhaust gas inlet side of the outlet chamber 412 is closed, and the end of the exhaust gas outlet side of the outlet chamber 412 is open. Hereinafter, the end of the exhaust gas outlet side of the outlet chamber 412 is sometimes referred to as the "opening of the outlet chamber 412".

[0136] like Figure 5 As shown, the wall flow type substrate 41 is provided with a second sealing portion 45 that seals the end of the outflow side chamber 412 on the exhaust gas inflow side, and the end of the outflow side chamber 412 on the exhaust gas inflow side is closed by the second sealing portion 45 .

[0137] The top-view shapes of the openings of the inlet side chamber 411 and the outlet side chamber 412 (the shape of the wall flow type substrate 41 when looking down from the exhaust gas inlet side and the exhaust gas outlet side in the exhaust gas flow direction X) include, for example, squares, parallelograms, rectangles, trapezoids and other quadrilaterals, equilateral triangles and other triangles, regular hexagons and other hexagons, regular octagons and other octagons; polygons such as circles, ellipses and other various geometric shapes.

[0138] The area of ​​the opening of the inlet-side chamber 411 in a plan view and the area of ​​the opening of the outlet-side chamber 412 in a plan view may be the same as or different from each other.

[0139] The density of the inflow side chamber 411 and the outflow side chamber 412 can be appropriately adjusted in consideration of pressure loss, the capture rate of particulate matter, etc. The smaller the density of the inflow side chamber 411 and the outflow side chamber 412, the larger the opening area of ​​the opening of the inflow side chamber 411 becomes, which can reduce the pressure loss, but the capture rate of particulate matter is also reduced. Therefore, the density of the inflow side chamber 411 and the outflow side chamber 412 per 1 square inch of the wall-flow type substrate 41 is preferably more than 200 chambers and less than 400 chambers, and more preferably more than 200 chambers and less than 300 chambers. It should be noted that the density of the inflow side chamber 411 and the outflow side chamber 412 per 1 square inch of the wall-flow type substrate 41 is the total number of the inflow side chamber 411 and the outflow side chamber 412 per 1 square inch in the cross section obtained by cutting the wall-flow type substrate 41 with a plane perpendicular to the exhaust gas flow direction X.

[0140] The volume of the wall-flow type substrate 41 is preferably greater than 1.0L and less than 2.2L, and more preferably greater than 1.3L and less than 1.7L. By adjusting the volume of the wall-flow type substrate 41 to the above range, when the inorganic oxide containing the cerium element is contained in the second catalyst layer 42 and / or the third catalyst layer 43, the inorganic oxide containing the cerium element can more appropriately exert the oxygen storage capacity. It should be noted that the volume of the wall-flow type substrate 41 is the apparent volume of the wall-flow type substrate 41 (that is, the volume of the wall-flow type substrate 41 as a whole including the inlet side chamber 411, the outlet side chamber 412, the partition wall 413, the first sealing portion 44 and the second sealing portion 45). When the wall-flow type substrate 41 is cylindrical, the outer diameter of the cylindrical portion that limits the outer shape of the wall-flow type substrate 41 is denoted as 2r, and the length of the wall-flow type substrate 41 is denoted as L. 41 When , the volume of the wall flow type substrate 41 can be calculated according to the following formula.

[0141] Volume of wall flow type substrate 41 = π × r 2 ×L 41

[0142] The partition wall 413 has a porous structure through which exhaust gas can pass. The thickness of the partition wall 413 is, for example, 200 μm or more and 280 μm or less. The thickness of the partition wall 413 is calculated by the same calculation method as the calculation method of the thickness of the catalyst layer described below.

[0143] <Second Catalyst Layer>

[0144] Next, the second catalyst layer 42 will be described.

[0145] like Figure 5 and Figure 6As shown, the second catalyst layer 42 has a portion 421 formed on the surface of the partition wall portion 413 on the side of the inflow side chamber 411. Specifically, the second catalyst layer 42 has a portion 421 formed on the surface of the partition wall portion 413 on the side of the inflow side chamber 411 along the exhaust gas flow direction X from the end of the exhaust gas inflow side of the partition wall portion 413. The "surface of the partition wall portion 413 on the side of the inflow side chamber 411" refers to the outer surface of the inflow side chamber 411 that defines the outer shape of the partition wall portion 413. The "portion formed on the surface of the partition wall portion 413 on the side of the inflow side chamber 411" refers to: a portion that bulges from the outer surface of the partition wall portion 413 on the side of the inflow side chamber 411 toward the inflow side chamber 411.

[0146] Since the second catalyst layer 42 has a portion 421 formed on the surface of the partition wall portion 413 on the side of the inflow side chamber 411, the exhaust gas that passes through the first exhaust gas treatment portion 3 and is supplied to the second exhaust gas treatment portion 4 is easily in contact with the rhodium element contained in the second catalyst layer 42. As a result, the catalytic activity of the rhodium element contained in the second catalyst layer 42 can be efficiently exerted.

[0147] like Figure 6 As shown, the second catalyst layer 42 can have a portion 421 and a portion 422 that exists inside the partition wall portion 413. The partition wall portion 413 is porous, so when forming the second catalyst layer 42, a portion 422 is usually formed while forming the portion 421. The region where the portion 421 exists does not overlap with the region where the partition wall portion 413 exists, but the region where the portion 422 exists overlaps with the region where the partition wall portion 413 exists. Therefore, by cutting the second catalyst layer 42 and analyzing the cross-section by cross-sectional observation using a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA), etc., it is possible to determine the portion 421 and the portion 422 based on the composition, morphology, etc. When observing the cut surface, element mapping of the cut surface can be performed. Element mapping can be performed by the same operation as described above. By element mapping of the cut surface, it is possible to determine the portion 421 and the portion 422 based on the difference in morphology and composition between the second catalyst layer 42 and the partition wall portion 413.

[0148] The second catalyst layer 42 is formed on the inflow side chamber 411 side of the wall flow type substrate 41. The second catalyst layer 42 is formed on the inflow side chamber 411 side of the wall flow type substrate 41 because the portion 421 of the second catalyst layer 42 has a predetermined thickness.

[0149] From the viewpoint of improving the contact efficiency between the exhaust gas supplied to the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 and the rhodium element contained in the second catalyst layer 42, the portion 421 preferably has an appropriate thickness. 421The thickness is preferably 15 μm or more and 55 μm or less, more preferably 25 μm or more and 45 μm or less, and still more preferably 30 μm or more and 40 μm or less.

[0150] The thickness T of the portion 421 of the second catalyst layer 42 is 421 The calculation method of is as follows.

[0151] The second exhaust gas treatment section 4 (for example, a portion 10 mm away from the end of the exhaust gas inflow side of the wall-flow type substrate 41 in the exhaust gas flow direction X) is cut with a plane perpendicular to the axial direction of the wall-flow type substrate 41, and a scanning electron microscope (SEM) is used to observe the second catalyst layer 42 existing in one of the inflow side chambers 411 selected from the cut surface, and the existence area of ​​the partition wall portion 413 of the wall-flow type substrate 41 and the existence area of ​​the second catalyst layer 42 are determined. In the observation of the cut surface based on SEM, the field of view magnification is, for example, 300 times, and the full width of the field of view (the length in the direction perpendicular to the axial direction of the wall-flow type substrate 41) is, for example, 500 to 600 μm. The area observed by SEM is set in a manner that does not include the corner of the inflow side chamber 411. The existence area of ​​the partition wall portion 413 of the wall-flow type substrate 41 and the existence area of ​​the second catalyst layer 42 can be determined based on the morphological difference between the partition wall portion 413 of the wall-flow type substrate 41 and the second catalyst layer 42. At this time, element mapping of the cut surface can be performed. Element mapping can be performed by the same operation as described above. By element mapping of the cut surface, the area where the partition wall portion 413 of the wall flow type substrate 41 exists and the area where the second catalyst layer 42 exists can be determined based on the difference in morphology and composition between the partition wall portion 413 of the wall flow type substrate 41 and the second catalyst layer 42.

[0152] In the SEM observation image, the first to Nth grid lines parallel to the thickness direction of the partition wall portion 413 of the wall flow type substrate 41 are drawn in sequence at intervals of 15 μm from the left end or the right end, and the contour line of the existence area of ​​the partition wall portion 413 of the wall flow type substrate 41 is connected to the intersection points of each grid line with a straight line to determine the position of the surface of the partition wall portion 413 of the wall flow type substrate 41. N is, for example, an integer of 30 to 50. Similarly, the contour line of the existence area of ​​the second catalyst layer 42 is connected to the intersection points of each grid line with a straight line to determine the position of the surface of the second catalyst layer 42. When the amount of change in the thickness direction from a certain intersection P1 toward an intersection P2 adjacent to the intersection P1 exceeds the interval (15 μm) of the grid lines, it is preferred not to use the intersection P2 for determining the surface position (i.e., remove the intersection P2 from the intersections connected by the straight line). The amount of change in the thickness direction from a certain intersection P1 toward an intersection P2 adjacent to the intersection P1 refers to the distance between a straight line passing through the intersection P1 and perpendicular to the thickness direction of the partition wall portion 413 of the wall flow type substrate 41 and a straight line passing through the intersection P2 and perpendicular to the thickness direction of the partition wall portion 413 of the wall flow type substrate 41. When the amount of change in the thickness direction from the intersection P1 toward the intersection P2 adjacent to the intersection P1 exceeds the interval (15 μm) of the grid lines, and the amount of change in the thickness direction from the intersection P1 toward the intersection P3 adjacent to the intersection P2 also exceeds the interval (15 μm) of the grid lines, it is preferred not to use the intersection P2 and the intersection P3 to determine the position of the surface (i.e., to remove the intersection P2 and the intersection P3 from the intersections connected by the straight line). In this way, when five intersections are continuously removed from the intersections connected by the straight line, it is preferred not to measure the thickness of the SEM image.

[0153] After determining the surface position of the partition wall portion 413 of the wall flow type substrate 41 and the surface position of the second catalyst layer 42, use image analysis software to find the area of ​​the region surrounded by the second grid line, the (N-1)th grid line, the surface of the partition wall portion 413 of the wall flow type substrate 41 and the surface of the second catalyst layer 42. As image analysis software, for example, AreaQ (manufactured by STEC), ImageJ (public domain), Photoshop (Adobe Systems), etc. can be used. It should be noted that since the two ends of the image tend to become unclear, it is difficult to determine the surface position of the partition wall portion 413 of the wall flow type substrate 41 and the surface position of the second catalyst layer 42, and therefore, the first grid line and the Nth grid line are not used.

[0154] After the area of ​​the above region is obtained, the thickness of the above region is calculated according to the following formula.

[0155] The thickness of the above region = the area of ​​the above region / (grid line spacing × number of grid line spacings)

[0156] It should be noted that the interval between the grid lines is 15 μm, and the number of intervals between the grid lines is (N-3).

[0157] The thickness of the above region is calculated for 20 selected inlet side chambers 411 from the cut surface, and the average value thereof is used as the thickness T of the portion 421 of the second catalyst layer 42. 421 .

[0158] The length L of the second catalyst layer 42 42 The length L of the wall flow type substrate 41 41 The percentage (L 42 / L 41 × 100) is preferably 10% to 80%, more preferably 30% to 60%, and still more preferably 40% to 50%.

[0159] The length L of the second catalyst layer 42 42 The calculation method of is as follows.

[0160] A portion extending in the axial direction of the wall-flow type substrate 41 and having a length L equal to that of the wall-flow type substrate 41 is cut out from the second exhaust gas treatment portion 4 . 41 are samples of the same length. The sample is, for example, cylindrical with a diameter of 25.4 mm. It should be noted that the diameter value of the sample can be changed as needed. Using a plane perpendicular to the axial direction of the wall flow type substrate 41, the sample is cut at intervals of 5 mm, and the first cut piece, the second cut piece, ..., the nth cut piece are obtained in sequence from the end side of the exhaust gas inflow side of the sample. The length of the cut piece is 5 mm. Use a fluorescent X-ray analyzer (XRF) (such as an energy dispersive X-ray analyzer (EDX), a wavelength dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma emission spectrometer (ICP-AES), etc. to analyze the composition of the cut piece, and confirm whether the cut piece contains the second catalyst layer 42 based on the composition of the cut piece.

[0161] It is not necessary to perform composition analysis on the cut piece that clearly contains the second catalyst layer 42. For example, a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA), etc. can be used to observe the cut surface to confirm whether the cut surface contains the second catalyst layer 42. When observing the cut surface, element mapping of the cut surface can be performed. Element mapping can be performed in the same manner as described above.

[0162] After confirming whether the cut piece includes the second catalyst layer 42 , the length of the second catalyst layer 42 included in the sample is calculated according to the following formula.

[0163] The length of the second catalyst layer 42 included in the sample = 5 mm × (the number of cut pieces including the second catalyst layer 42)

[0164] For example, when the first to kth cut pieces include the second catalyst layer 42 but the (k+1)th to nth cut pieces do not include the second catalyst layer 42, the length of the second catalyst layer 42 included in the sample is (5×k) mm.

[0165] When measuring the length of the second catalyst layer 42 in more detail, calculation is performed as follows.

[0166] The k-th cut piece (i.e., the cut piece obtained from the exhaust gas outflow side of the sample among the cut pieces containing the second catalyst layer 42) is cut along the axial direction of the wall-flow type substrate 41, and the second catalyst layer 42 existing on the cut surface is observed using a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA), etc., thereby measuring the length of the second catalyst layer 42 in the k-th cut piece. And, the length of the second catalyst layer 42 contained in the sample is calculated according to the following formula.

[0167] The length of the second catalyst layer 42 included in the sample = (5 mm × (k-1)) + (the length of the second catalyst layer 42 in the k-th cut piece)

[0168] The length of the second catalyst layer 42 included in each of the 8 to 16 samples randomly cut out from the second exhaust gas treatment unit 4 is calculated, and the average value thereof is taken as the length L of the second catalyst layer 42 42 .

[0169] The amount of the second catalyst layer 42 provided on the wall-flow type substrate 41 can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost. From the viewpoint of the balance between exhaust gas purification performance and cost, the mass of the second catalyst layer 42 per unit volume of the wall-flow type substrate 41 (the mass after drying and calcination) is preferably 15 g / L or more and 30 g / L or less, and more preferably 17 g / L or more and 24 g / L or less. It should be noted that the volume of the wall-flow type substrate 41 is the apparent volume of the wall-flow type substrate 41.

[0170] The method for calculating the mass of the second catalyst layer 42 per unit volume of the wall-flow type substrate 41 is as follows, for example.

[0171] A portion extending in the axial direction of the wall-flow type substrate 41 and having a length L equal to that of the wall-flow type substrate 41 is cut out from the second exhaust gas treatment portion 4 . 41For samples of the same length, the samples are cut with a plane perpendicular to the axial direction of the wall flow type substrate 41, and a cutting piece S2 containing the second catalyst layer 42 but not containing the third catalyst layer 43 is prepared. The cutting piece S2 is, for example, a cylindrical shape with a diameter of 25.4 mm and a length of 10 mm. The values ​​of the diameter and length of the cutting piece S2 can be changed as needed. When the third catalyst layer 43 does not extend to the end of the exhaust gas inlet side of the partition wall 413, the second catalyst layer 42 exists near the end of the exhaust gas inlet side of the partition wall 413, but the third catalyst layer 43 does not exist. Therefore, the cutting piece S2 can be obtained near the end of the exhaust gas inlet side of the partition wall 413. The length of the second catalyst layer 42 contained in the cutting piece S2 is equal to the length of the cutting piece S2.

[0172] A cut piece having a wall flow type substrate 41 having the same size as the cut piece S2 is prepared. The cut piece of the wall flow type substrate 41 does not contain both the second catalyst layer 42 and the third catalyst layer 43 .

[0173] The mass of the cut piece S2 and the mass of the cut piece of the wall flow type substrate 41 were measured, and the mass of the second catalyst layer 42 per unit volume of the cut piece S2 was calculated according to the following formula.

[0174] The mass of the second catalyst layer 42 per unit volume of the cut piece S2 = ((the mass of the cut piece S2) - (the mass of the cut piece of the wall-flow type substrate 41)) / (the volume of the cut piece S2)

[0175] The mass of the second catalyst layer 42 per unit volume of the three cut pieces S2 produced at any position of the second exhaust gas treatment unit 4 was calculated and their average value was calculated. The mass of the second catalyst layer 42 per unit volume of the wall flow type substrate 41 was calculated according to the following formula.

[0176] The mass of the second catalyst layer 42 per unit volume of the wall-flow substrate 41 = (the average mass of the second catalyst layer 42 per unit volume of the cut piece S2) × (the length L of the second catalyst layer 42 42 / Length L of the wall flow type substrate 41 41 )

[0177] The mass of the second catalyst layer 42 per unit volume of the cut piece S2 can be calculated without using the cut piece of the wall flow type substrate 41. An example of such a calculation method is the same as an example of a method for calculating the mass of the first catalyst layer 32 per unit volume of the cut piece S1 without using the cut piece of the through-flow type substrate 31.

[0178] The second catalyst layer 42 contains rhodium (Rh) as a catalytically active component. The rhodium element is contained in the second catalyst layer 42 in a form that can function as a catalytically active component, such as rhodium metal, an alloy containing the rhodium element, a compound containing the rhodium element (such as an oxide of the rhodium element), etc. From the viewpoint of improving exhaust gas purification performance, the catalytically active component is preferably in a granular form.

[0179] The amount of rhodium contained in the second catalyst layer 42 can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost. From the perspective of the balance between exhaust gas purification performance and cost, the mass of the rhodium contained in the second catalyst layer 42 is preferably 1.3% by mass or more and 2.7% by mass or less, and more preferably 1.7% by mass or more and 2.4% by mass or less, based on the mass of the second catalyst layer 42. The method for calculating the percentage of the mass of the rhodium contained in the second catalyst layer 42 relative to the mass of the second catalyst layer 42 is the same as the method for calculating the percentage P1. It should be noted that the mass of the rhodium element is the mass converted to metal.

[0180] The second catalyst layer 42 may contain one or more catalytically active components other than the rhodium element. The catalytically active components other than the rhodium element may be selected from precious metal elements such as platinum (Pt), palladium (Pd), ruthenium (Ru), iridium (Ir), and osmium (Os), but from the viewpoint of improving the exhaust gas purification performance, it is preferably selected from platinum (Pt) and palladium (Pd). The precious metal element is contained in the second catalyst layer 42 in a form that can function as a catalytically active component, such as a precious metal, an alloy containing the precious metal element, a compound containing the precious metal element (such as an oxide of the precious metal element), etc. From the viewpoint of improving the exhaust gas purification performance, the catalytically active component is preferably in a granular form.

[0181] In one embodiment, the second catalyst layer 42 contains rhodium and no catalytically active components other than rhodium. In other embodiments, the second catalyst layer 42 contains rhodium and palladium and no catalytically active components other than rhodium and palladium.

[0182] When the second catalyst layer 42 contains catalytically active components other than rhodium, the amount of catalytically active components other than rhodium contained in the second catalyst layer 42 can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost. From the viewpoint of the balance between exhaust gas purification performance and cost, the mass of the catalytically active components other than rhodium contained in the second catalyst layer 42 is preferably 1.3% by mass or more and 2.7% by mass or less, and more preferably 1.7% by mass or more and 2.4% by mass or less, based on the mass of the second catalyst layer 42. The method for calculating the percentage of the mass of the catalytically active components other than rhodium contained in the second catalyst layer 42 relative to the mass of the second catalyst layer 42 is the same as the method for calculating the percentage P1.

[0183] The second catalyst layer 42 preferably contains an inorganic oxide containing cerium. The relevant description of the inorganic oxide containing cerium is the same as above. The metal oxide containing cerium is preferably a CeO2-ZrO2 composite oxide. The relevant description of the CeO2-ZrO2 composite oxide is the same as above.

[0184] The amount of the inorganic oxide containing the cerium element contained in the second catalyst layer 42 can be appropriately adjusted in consideration of the amount of the inorganic oxide containing the cerium element contained in the third catalyst layer 43 .

[0185] The percentage P of the sum of the mass of the cerium element contained in the second catalyst layer 42 in terms of cerium oxide and the mass of the cerium element contained in the third catalyst layer 43 in terms of cerium oxide relative to the sum of the mass of the second catalyst layer 42 and the mass of the third catalyst layer 43 23 It is preferably greater than the percentage P1. Since the oxygen concentration in the exhaust gas flowing into the second exhaust gas treatment unit 4 through the first exhaust gas treatment unit 3 may vary, the catalytic activity of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains rhodium elements, the rhodium elements contained in the second catalyst layer 42 and the rhodium elements contained in the third catalyst layer 43) may be affected by the oxygen concentration in the exhaust gas flowing into the second exhaust gas treatment unit 4 and may not be properly exerted. In this regard, it can be considered that the percentage P 23When it is greater than the percentage P1, the influence of the oxygen concentration in the exhaust gas flowing into the second exhaust gas treatment unit 4 can be suppressed, and the catalytic activity of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, it is the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) can be appropriately expressed. That is, it can be considered that: by increasing the oxygen storage capacity from the upstream side to the downstream side of the exhaust gas flow direction X, the catalytic activity (for example, NOx purification performance) of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, it is the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) can be improved.

[0186] Percentage P 23 It is preferably 1.1 times or more and 5.3 times or less of the percentage P1, more preferably 1.3 times or more and 4.2 times or less, and still more preferably 1.7 times or more and 2.9 times or less. 23 If the percentage P is too small, sufficient oxygen for the catalytic activity to be adequately exerted cannot be supplied. 23 If the percentage P is too large, excessive oxygen will be supplied, which will reduce the catalytic activity. 23 When the amount is within the above range, sufficient oxygen for exhibiting appropriate catalytic activity can be supplied.

[0187] The percentage P2 of the mass of the cerium element contained in the second catalyst layer 42 in terms of cerium oxide relative to the mass of the second catalyst layer 42 is preferably greater than the percentage P1. Since the oxygen concentration in the exhaust gas that passes through the first exhaust gas treatment section 3 and flows into the second exhaust gas treatment section 4 may vary, the catalytic activity of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) may be affected by the oxygen concentration in the exhaust gas that flows into the second exhaust gas treatment section 4 and may not be properly exerted. In this regard, it is considered that when the percentage P2 is greater than the percentage P1, the influence of the oxygen concentration in the exhaust gas that flows into the second exhaust gas treatment section 4 can be suppressed, and the catalytic activity of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) can be properly exerted. That is, it can be considered that by increasing the oxygen storage capacity from the upstream side to the downstream side of the exhaust gas flow direction X, the catalytic activity (for example, NOx purification performance) of the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, it is the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) can be improved.

[0188] The percentage P2 is preferably 1.1 times or more and 5.3 times or less of the percentage P1, more preferably 1.2 times or more and 2.9 times or less, and even more preferably 1.2 times or more and 1.4 times or less. It can be considered that if the percentage P2 is too small, sufficient oxygen for exerting appropriate catalytic activity cannot be supplied. On the other hand, if the percentage P2 is too large, excessive oxygen is supplied, which reduces the catalytic activity. In this regard, it can be considered that if the percentage P2 is within the above range, sufficient oxygen for exerting appropriate catalytic activity can be supplied.

[0189] The calculation method of percentage P2 is the same as the calculation method of percentage P1.

[0190] The second catalyst layer 42 preferably includes other inorganic oxides. The description of other inorganic oxides is the same as above.

[0191] The amount of other inorganic oxides contained in the second catalyst layer 42 can be appropriately adjusted in consideration of the thermal durability of the second catalyst layer 42 and the pressure loss of the second exhaust gas treatment unit 4. From the viewpoint of improving the thermal durability of the second catalyst layer 42, the amount of other inorganic oxides contained in the second catalyst layer 42 is preferably large, but as the amount increases, the area of ​​the opening portion on the exhaust gas inflow side of the inflow side chamber 411 decreases, and the pressure loss increases. Therefore, the mass of other inorganic oxides contained in the second catalyst layer 42 is preferably 20% by mass or more and 72% by mass or less, more preferably 20% by mass or more and 57% by mass or less, and even more preferably 20% by mass or more and 40% by mass or less, based on the mass of the second catalyst layer 42.

[0192] The method for calculating the percentage of the mass of other inorganic oxides contained in the second catalyst layer 42 relative to the mass of the second catalyst layer 42 is the same as the method for calculating the percentage P1.

[0193] From the viewpoint of efficiently exerting the exhaust gas purification performance brought about by the catalytically active component, the catalytically active component is preferably supported on an inorganic oxide containing a cerium element and / or other inorganic oxides. The meaning of “the catalytically active component is supported on an inorganic oxide” is the same as described above.

[0194] The second catalyst layer 42 may have a single layer structure or a laminated structure. When the second catalyst layer 42 has a laminated structure, the composition of a certain layer may be the same as or different from the composition of other layers. For example, the catalytically active component contained in a certain layer may be the same as or different from the catalytically active component contained in other layers.

[0195] <Third Catalyst Layer>

[0196] Next, the third catalyst layer 43 will be described.

[0197] like Figure 5 and Figure 7 As shown, the third catalyst layer 43 has a portion 431 formed on the surface of the outflow side chamber 412 side of the partition wall portion 413. Specifically, the third catalyst layer 43 has a portion 431 formed on the surface of the outflow side chamber 412 side of the partition wall portion 413 from the end of the exhaust gas outflow side of the partition wall portion 413 along the direction opposite to the exhaust gas flow direction X. The "surface of the outflow side chamber 412 side of the partition wall portion 413" refers to the outer surface of the outflow side chamber 412 side that defines the outer shape of the partition wall portion 413. The "portion formed on the surface of the outflow side chamber 412 side of the partition wall portion 413" refers to: a portion that bulges from the outer surface of the outflow side chamber 412 side of the partition wall portion 413 toward the outflow side chamber 412 side.

[0198] Since the third catalyst layer 43 has the portion 431 formed on the surface of the outflow side chamber 412 side of the partition wall portion 413, the exhaust gas supplied to the second exhaust gas treatment portion 4 through the first exhaust gas treatment portion 3 is easy to contact with the rhodium element contained in the second catalyst layer 42. As a result, the catalytic activity of the rhodium element contained in the second catalyst layer 42 can be efficiently exerted. In addition, when the third catalyst layer 43 contains the rhodium element, the exhaust gas supplied to the second exhaust gas treatment portion 4 through the first exhaust gas treatment portion 3 is easy to contact with the rhodium contained in the third catalyst layer 43, thereby The catalytic activity of the rhodium element contained in the third catalyst layer 43 can be efficiently exerted. When the third catalyst layer 43 has the portion 431, the flow rate of the exhaust gas contacting the second catalyst layer 42 and the third catalyst layer 43 is reduced compared to the case where the third catalyst layer 43 does not have the portion 431, so it is presumed that it contributes to increasing the opportunity for the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43) to contact with the exhaust gas. It should be noted that it can be inferred that: since there is no porous partition wall portion 413 in portion 431 of the third catalyst layer 43 (in contrast, there is a porous partition wall portion 413 in portion 432 of the third catalyst layer 43 described later), when the third catalyst layer 43 has portion 431, the apparent density of the third catalyst layer 43 increases compared to the case where the third catalyst layer 43 does not have portion 431, thereby reducing the flow rate of the exhaust gas in contact with the second catalyst layer 42 and the third catalyst layer 43.

[0199] like Figure 7As shown, the third catalyst layer 43 has a portion 431 and a portion 432 that exists inside the partition wall portion 413. The partition wall portion 413 is porous, and therefore, when forming the third catalyst layer 43, the portion 432 is usually formed while forming the portion 431. The region where the portion 431 exists does not overlap with the region where the partition wall portion 413 exists, but the region where the portion 432 exists overlaps with the region where the partition wall portion 413 exists. Therefore, by cutting the third catalyst layer 43 and analyzing the cross section by cross-sectional observation using a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA), or the like, the portion 431 and the portion 432 can be determined based on the composition, morphology, and the like.

[0200] The third catalyst layer 43 is formed on the outflow chamber 412 side of the wall flow type substrate 41. The formation of the third catalyst layer 43 on the outflow chamber 412 side of the wall flow type substrate 41 can be judged by the fact that the portion 431 of the third catalyst layer 43 has a predetermined thickness.

[0201] From the viewpoint of improving the contact efficiency between the exhaust gas that passes through the first exhaust gas treatment unit 3 and is supplied to the second exhaust gas treatment unit 4 and the rhodium element contained in the second catalyst layer 42 (when the third catalyst layer 43 contains the rhodium element, the rhodium element contained in the second catalyst layer 42 and the rhodium element contained in the third catalyst layer 43), the portion 431 preferably has an appropriate thickness. The thickness T of the portion 431 of the third catalyst layer 43 is 431 The thickness is preferably 20 μm or more and 100 μm or less, more preferably 40 μm or more and 80 μm or less, and still more preferably 45 μm or more and 65 μm or less.

[0202] The thickness T of the portion 431 of the third catalyst layer 43 is 431 The calculation method of the thickness T of the portion 421 of the second catalyst layer 42 is 421 It should be noted that the thickness T of the portion 431 of the third catalyst layer 43 is 431 In the calculation method, the second exhaust gas treatment section 4 (for example, a portion 10 mm away from the end of the exhaust gas outflow side of the wall-flow type substrate 41 in the direction opposite to the exhaust gas flow direction X) is cut with a plane perpendicular to the axial direction of the wall-flow type substrate 41, and a scanning electron microscope (SEM) is used to observe the third catalyst layer 43 existing in the outflow side chamber 412 selected from the cut surface.

[0203] The length L of the third catalyst layer 43 43 The length L of the wall flow type substrate 41 41 The percentage (L 43 / L 41× 100) is preferably 30% to 90%, more preferably 50% to 80%, and still more preferably 65% ​​to 75%.

[0204] The length L of the third catalyst layer 43 43 The calculation method of the length L of the second catalyst layer 42 42 It should be noted that the length L of the third catalyst layer 43 is 43 In the calculation method, the sample is cut at intervals of 5 mm using a plane perpendicular to the axial direction of the wall flow type substrate 41, and the first cut piece, the second cut piece,..., and the nth cut piece are obtained in sequence from the end side of the exhaust gas outflow side of the sample.

[0205] The amount of the third catalyst layer 43 provided on the wall-flow type substrate 41 can be appropriately adjusted in consideration of the balance between the exhaust gas purification performance and the cost. From the viewpoint of the balance between the exhaust gas purification performance and the cost, the mass of the third catalyst layer 43 per unit volume of the wall-flow type substrate 41 (the mass after drying and calcining) is preferably 27 g / L or more and 53 g / L or less, and more preferably 27 g / L or more and 37 g / L or less. It should be noted that the volume of the wall-flow type substrate 41 is the apparent volume of the wall-flow type substrate 41.

[0206] The method for calculating the mass of the third catalyst layer 43 per unit volume of the wall-flow type substrate 41 is the same as the method for calculating the mass of the second catalyst layer 42 .

[0207] For example, first, a portion extending in the axial direction of the wall-flow type substrate 41 and having a length L of the wall-flow type substrate 41 is cut out from the second exhaust gas treatment portion 4. 41 The sample is cut with a plane perpendicular to the axial direction of the wall-flow type substrate 41 to prepare a cut piece S3 containing the third catalyst layer 43 but not containing the second catalyst layer 42. Next, a cut piece of the wall-flow type substrate 41 having the same size as the cut piece S3 is prepared. The cut piece of the wall-flow type substrate 41 does not contain both the second catalyst layer 42 and the third catalyst layer 43.

[0208] The mass of the cut piece S3 and the mass of the cut piece of the wall flow type substrate 41 were measured, and the mass of the third catalyst layer 43 per unit volume of the cut piece S3 was calculated according to the following formula.

[0209] The mass of the third catalyst layer 43 per unit volume of the cut piece S3 = ((the mass of the cut piece S3) - (the mass of the cut piece of the wall-flow type substrate 41)) / (the volume of the cut piece S3)

[0210] The mass of the third catalyst layer 43 per unit volume of the three cut pieces S3 produced at any position of the second exhaust gas treatment unit 4 was calculated and their average value was calculated. The mass of the third catalyst layer 43 per unit volume of the wall flow type substrate 41 was calculated according to the following formula.

[0211] The mass of the third catalyst layer 43 per unit volume of the wall-flow substrate 41 = (the average mass of the third catalyst layer 43 per unit volume of the cut piece S3) × (the length L of the third catalyst layer 43 43 / Length L of the wall flow type substrate 41 41 )

[0212] The mass of the third catalyst layer 43 per unit volume of the cut piece S3 can be calculated without using the cut piece of the wall flow type substrate 41. An example of such a calculation method is the same as an example of a method for calculating the mass of the first catalyst layer 32 per unit volume of the cut piece S1 without using the cut piece of the through-flow type substrate 31.

[0213] The third catalyst layer 43 contains one or more catalytically active components. The catalytically active component can be selected from precious metal elements such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os), but from the viewpoint of improving exhaust gas purification performance, it is preferably selected from platinum (Pt), palladium (Pd) and rhodium (Rh). The precious metal element is contained in the third catalyst layer 43 in a form that can function as a catalytically active component, such as a precious metal, an alloy containing a precious metal element, a compound containing a precious metal element (such as an oxide of a precious metal element), etc. From the viewpoint of improving exhaust gas purification performance, the catalytically active component is preferably in a granular form.

[0214] In one embodiment, the third catalyst layer 43 contains rhodium and no catalytically active components other than rhodium. In other embodiments, the third catalyst layer 43 contains rhodium and palladium and no catalytically active components other than rhodium and palladium.

[0215] The third catalyst layer 43 may contain a catalytically active component different from the catalytically active component contained in the second catalyst layer 42. Thus, the exhaust gas purification performance of the second exhaust gas treatment unit 4 can be improved. In one embodiment, the third catalyst layer 43 contains palladium. In this embodiment, the third catalyst layer 43 may contain a catalytically active component other than palladium, or may not contain it.

[0216] The amount of catalytically active components contained in the third catalyst layer 43 can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost. From the viewpoint of the balance between exhaust gas purification performance and cost, the mass of the rhodium element or palladium element contained in the third catalyst layer 43 is preferably 0.8 mass % or more and 1.5 mass % or less, and further preferably 1.1 mass % or more and 1.5 mass % or less, based on the mass of the third catalyst layer 43. When the third catalyst layer 43 contains rhodium and palladium elements, the total amount of rhodium and palladium elements is preferably within the above range. The method for calculating the percentage of the mass of the rhodium or palladium element contained in the third catalyst layer 43 relative to the mass of the third catalyst layer 43 is the same as the method for calculating the percentage P1. It should be noted that the mass of the rhodium element and the mass of the palladium element are metal-converted masses.

[0217] The third catalyst layer 43 preferably contains an inorganic oxide containing cerium. The relevant description of the inorganic oxide containing cerium is the same as above. The metal oxide containing cerium is preferably a CeO2-ZrO2 composite oxide. The relevant description of the CeO2-ZrO2 composite oxide is the same as above.

[0218] The amount of the inorganic oxide containing the cerium element contained in the third catalyst layer 43 can be appropriately adjusted in consideration of the amount of the inorganic oxide containing the cerium element contained in the second catalyst layer 42 .

[0219] As mentioned above, the percentage P 23 It is preferably larger than the percentage of the mass of the cerium element contained in the first catalyst layer 32 in terms of cerium oxide relative to the mass of the first catalyst layer 32 .

[0220] As mentioned above, the percentage P 23 It is preferably 1.1 times or more and 5.3 times or less of the percentage P1, more preferably 1.3 times or more and 4.2 times or less, and still more preferably 1.7 times or more and 2.9 times or less.

[0221] The percentage P3 of the mass of the cerium element contained in the third catalyst layer 43 in terms of cerium oxide relative to the mass of the third catalyst layer 43 is preferably larger than the percentage P1. The reason why the percentage P3 is preferably larger than the percentage P1 is the same as the reason why the percentage P2 is preferably larger than the percentage P1.

[0222] The percentage P3 is preferably 1.1 times or more and 5.3 times or less of the percentage P1, more preferably 2.4 times or more and 4.4 times or less, and even more preferably 2.4 times or more and 3.0 times or less. It can be considered that if the percentage P3 is too small, sufficient oxygen for exerting appropriate catalytic activity cannot be supplied, while on the other hand, if the percentage P3 is too large, excessive oxygen is supplied, which reduces the catalytic activity. In this regard, it can be considered that if the percentage P3 is within the above range, sufficient oxygen for exerting appropriate catalytic activity can be supplied.

[0223] The calculation method of percentage P3 is the same as the calculation method of percentage P1.

[0224] The third catalyst layer 43 preferably includes other inorganic oxides. The description of other inorganic oxides is the same as above.

[0225] The amount of other inorganic oxides contained in the third catalyst layer 43 can be appropriately adjusted in consideration of the thermal durability of the third catalyst layer 43 and the pressure loss of the second exhaust gas treatment unit 4. From the viewpoint of improving the thermal durability of the third catalyst layer 43, the amount of other inorganic oxides contained in the third catalyst layer 43 is preferably large, but as the amount increases, the area of ​​the opening portion on the exhaust gas inflow side of the inflow side chamber 411 decreases, and the pressure loss increases. Therefore, the mass of other inorganic oxides contained in the third catalyst layer 43 is preferably 19 mass% or more and 63 mass% or less, and more preferably 19 mass% or more and 43 mass% or less, based on the mass of the third catalyst layer 43.

[0226] The method for calculating the percentage of the mass of other inorganic oxides contained in the third catalyst layer 43 relative to the mass of the third catalyst layer 43 is the same as the method for calculating the percentage P1.

[0227] From the viewpoint of efficiently exerting the exhaust gas purification performance brought about by the catalytically active component, the catalytically active component is preferably supported on an inorganic oxide containing a cerium element and / or other inorganic oxides. The meaning of “the catalytically active component is supported on an inorganic oxide” is the same as described above.

[0228] The third catalyst layer 43 may have a single layer structure or a laminated structure. When the third catalyst layer 43 has a laminated structure, the composition of a certain layer may be the same as or different from the composition of other layers. For example, the catalytically active component contained in a certain layer may be the same as or different from the catalytically active component contained in other layers.

[0229] Hereinafter, a method for forming the second catalyst layer 42 and the third catalyst layer 43 will be described.

[0230] A wall flow type substrate 41, a slurry for forming the second catalyst layer 42, and a slurry for forming the third catalyst layer 43 are prepared. When the second catalyst layer 42 has a laminated structure, two or more slurries are prepared as slurries for forming the second catalyst layer 42. When the third catalyst layer 43 has a laminated structure, two or more slurries are prepared as slurries for forming the third catalyst layer 43.

[0231] The composition of the slurry for forming the second catalyst layer 42 is adjusted according to the composition of the second catalyst layer 42. The composition of the slurry for forming the third catalyst layer 43 is adjusted according to the composition of the third catalyst layer 43. The slurry contains, for example, a supply source of a noble metal element, inorganic oxide particles, a binder, a pore-forming agent, a solvent, etc. As the supply source of the noble metal element, the inorganic oxide particles, the binder, the pore-forming agent, the solvent, etc., the same slurry as the slurry for forming the first catalyst layer 32 can be used.

[0232] The slurry for forming the second catalyst layer 42 is applied to the wall flow type substrate 41, dried, and fired as needed. When the second catalyst layer 42 has a stacked structure, this operation is repeated. Thus, a precursor layer of the second catalyst layer 42 is formed. By adjusting the solid content concentration, viscosity, etc. of the slurry, the length of the precursor layer of the second catalyst layer 42 can be adjusted (and thus the length of the second catalyst layer 42 can be adjusted). In addition, by adjusting the coating amount of the slurry, the type of material constituting the slurry, the particle size of the pore-forming agent contained in the slurry, etc., the thickness of the precursor layer of the second catalyst layer 42 (and thus the thickness of the second catalyst layer 42) and the mass of the precursor layer of the second catalyst layer 42 (and thus the mass of the second catalyst layer 42) can be adjusted.

[0233] The slurry for forming the third catalyst layer 43 is applied to the wall flow type substrate 41, dried, and fired as needed. When the third catalyst layer 43 has a stacked structure, the operation is repeated. Thus, a precursor layer of the third catalyst layer 43 is formed. By adjusting the solid content concentration, viscosity, etc. of the slurry, the length of the precursor layer of the third catalyst layer 43 can be adjusted (and thus the length of the third catalyst layer 43 can be adjusted). In addition, by adjusting the coating amount of the slurry, the type of material constituting the slurry, the particle size of the pore-forming agent contained in the slurry, etc., the thickness of the precursor layer of the third catalyst layer 43 (and thus the thickness of the third catalyst layer 43) and the mass of the precursor layer of the third catalyst layer 43 (and thus the mass of the third catalyst layer 43) can be adjusted.

[0234] The drying temperature is usually 70° C. to 200° C., preferably 90° C. to 150° C., and the drying time is usually 1 hour to 3 hours, preferably 1.5 hours to 2 hours.

[0235] The application of the slurry for forming the second catalyst layer 42 can be performed by, for example, dipping the end of the wall flow type substrate 41 on the exhaust gas inflow side into the slurry for forming the second catalyst layer 42 and sucking the slurry from the opposite side.

[0236] The application of the slurry for forming the third catalyst layer 43 can be performed by, for example, dipping the end of the wall flow type substrate 41 on the exhaust gas outflow side into the slurry for forming the third catalyst layer 43 and sucking the slurry from the opposite side.

[0237] After forming the precursor layer of the second catalyst layer 42 and the precursor layer of the third catalyst layer 43, calcination is performed. Thus, the second catalyst layer 42 and the third catalyst layer 43 are formed. The calcination temperature is usually 450° C. to 700° C., preferably 450° C. to 600° C., and the calcination time is usually 1 hour to 4 hours, preferably 1.5 hours to 2 hours. Calcination can be performed, for example, in an air atmosphere.

[0238] The particle size of the pore former can be adjusted appropriately. From the viewpoint of suppressing peeling, suppressing the increase in pressure loss, and PM collection performance, the median particle size D of the pore former is preferably 50 It is usually 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The larger the particle size of the pore former, the thicker the second catalyst layer 42 and the third catalyst layer 43 become. 50 The determination is the same as above.

[0239] Example

[0240] Hereinafter, embodiments of the present invention will be described in detail with reference to Examples, but the present invention is not limited to these Examples.

[0241] Preparation of the first exhaust gas treatment unit A

[0242] (1) Preparation of slurry for lower layer formation

[0243] An OSC material (inorganic oxide containing cerium element) having the following composition and BET specific surface area was prepared.

[0244] CeO2: 30 mass%, ZrO2: 55 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g

[0245] It should be noted that in OSC materials, CeO2, Y2O3, La2O3, Nd2O3 and ZrO2 form a solid solution.

[0246] OSC material and lanthanum oxide modified alumina (La2O3 modification amount: 1 mass %, BET specific surface area: 100 m2) were added to the ball mill. 2 / g), barium acetate, alumina sol and water, mixed and crushed by a ball mill to obtain a base slurry. The base slurry is mixed with a palladium nitrate aqueous solution to obtain a slurry for forming a lower layer. The amount ratio of each component in the slurry for forming the lower layer is set to the following amount: in the fired lower layer, the OSC material reaches 28.2 mass%, the lanthanum oxide modified alumina reaches 47.0 mass%, the barium carbonate reaches 10.0 mass%, the alumina reaches 10.0 mass%, and the palladium reaches 4.8 mass%.

[0247] (2) Formation of the lower layer

[0248] The flow-through substrate was immersed in the obtained slurry for forming the lower layer, and then the flow-through substrate was dried at 150°C for 2.5 hours and then calcined at 450°C for 2.5 hours to form the lower layer. The mass of the lower layer per unit volume of the flow-through substrate was 130 g / L. The palladium content in the lower layer per unit volume of the flow-through substrate was 6.2 g / L in terms of metal. It should be noted that as the flow-through substrate, a Figure 2 The structure shown in the figure is 600 rooms / inch on the plane perpendicular to the axial direction. 2 A flow-through type substrate having a density of 1.0 L and having chambers, wherein the chambers are partitioned by partition walls having a thickness of 50 to 70 μm and extending in the axial direction.

[0249] (3) Preparation of slurry for upper layer formation

[0250] A ZrO2-based material having the following composition and BET specific surface area was prepared.

[0251] ZrO2: 85 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g

[0252] It should be noted that in ZrO2-based materials, ZrO2, Y2O3, La2O3 and Nd2O3 form a solid solution.

[0253] ZrO2-based materials and lanthanum-modified alumina (La2O3 modification amount: 1 mass %, BET specific surface area: 100 m2) were added to the ball mill. 2 / g), alumina sol and water, mixed and crushed by a ball mill to obtain a base slurry. The base slurry is mixed with an aqueous solution of rhodium nitrate to obtain a slurry for forming an upper layer. The amount ratio of each component in the slurry for forming the upper layer is set to the following amount: in the upper layer after firing, the ZrO2-based material reaches 66.7% by mass, the lanthanum-modified alumina reaches 22.0% by mass, the alumina reaches 10.0% by mass, and the rhodium reaches 1.3% by mass in terms of metal conversion.

[0254] (4) Formation of the upper layer

[0255] The flow-through type substrate formed with the lower layer is impregnated in the obtained slurry for forming the upper layer, and then the flow-through type substrate is dried at 150°C for 2.5 hours and then calcined at 450°C for 2.5 hours. Thus, a catalyst for purifying exhaust gas having a wall-flow type substrate and a first catalyst layer formed on the flow-through type substrate is manufactured, and the manufactured catalyst for purifying exhaust gas is used as a first exhaust gas treatment unit. The first catalyst layer has a two-layer structure including a lower layer and an upper layer formed on the entire surface of the lower layer. The mass of the upper layer per unit volume of the flow-through type substrate after calcination is 60g / L. The rhodium content in the upper layer per unit volume of the flow-through type substrate is 0.8g / L in terms of metal.

[0256] Fabrication of the first exhaust gas treatment unit B

[0257] In forming the lower layer of the first exhaust gas treatment unit, the amount ratio of each component in the slurry for forming the lower layer is changed to an amount in which the OSC material in the fired lower layer reaches 35.9 mass%, the lanthanum oxide modified alumina reaches 39.3 mass%, the barium carbonate reaches 10.0 mass%, the alumina reaches 10.0 mass%, and the palladium reaches 4.8 mass%. In forming the upper layer of the first exhaust gas treatment unit, the ZrO2-based material in the slurry for forming the upper layer is changed to CeO2: 5 mass%, ZrO2: 80 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g of OSC material, and the BET specific surface area were 0.04475 W / g. The first exhaust gas treatment part B was produced in the same manner as the first exhaust gas treatment part A except for the above-mentioned points.

[0258] Fabrication of the first exhaust gas treatment unit C

[0259] In forming the lower layer of the first exhaust gas treatment unit, the amount ratio of each component in the slurry for forming the lower layer is changed to an amount in which the OSC material in the fired lower layer reaches 35.9 mass%, the lanthanum oxide modified alumina reaches 39.3 mass%, the barium carbonate reaches 10.0 mass%, the alumina reaches 10.0 mass%, and the palladium reaches 4.8 mass%. In forming the upper layer of the first exhaust gas treatment unit, the ZrO2-based material in the slurry for forming the upper layer is changed to CeO2: 10 mass%, ZrO2: 75 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g of OSC material, and except for these points, the first exhaust gas treatment part C was produced in the same manner as the first exhaust gas treatment part A.

[0260] Fabrication of the first exhaust gas treatment unit D

[0261] In the lower layer after firing of the first exhaust gas treatment unit, the amount ratio of each component in the slurry for forming the lower layer is changed to an amount in which the OSC material reaches 46.2 mass%, the lanthanum oxide modified alumina reaches 29.0 mass%, the barium carbonate reaches 10.0 mass%, the alumina reaches 10.0 mass%, and the palladium reaches 4.8 mass%. In the formation of the upper layer of the first exhaust gas treatment unit, the ZrO2-based material in the slurry for forming the upper layer is changed to CeO2: 10 mass%, ZrO2: 75 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g of OSC material, and except for these points, the first exhaust gas treatment part D was produced in the same manner as the first exhaust gas treatment part A.

[0262] Fabrication of the first exhaust gas treatment unit E

[0263] In the formation of the lower layer of the first exhaust gas treatment unit, the amount ratio of each component in the slurry for forming the lower layer is changed to an amount in which the OSC material in the fired lower layer reaches 23.1 mass%, lanthanum oxide modified alumina reaches 52.1 mass%, barium carbonate reaches 10.0 mass%, alumina reaches 10.0 mass%, and palladium reaches 4.8 mass%. Except for this point, the first exhaust gas treatment unit E is produced by the same operation as the first exhaust gas treatment unit A.

[0264] Fabrication of the first exhaust gas treatment unit F

[0265] In forming the lower layer of the first exhaust gas treatment unit, the amount ratio of each component in the slurry for forming the lower layer is changed to an amount in which the OSC material in the fired lower layer reaches 51.3 mass%, lanthanum oxide modified alumina reaches 23.9 mass%, barium carbonate reaches 10.0 mass%, alumina reaches 10.0 mass%, and palladium reaches 4.8 mass%. In forming the upper layer of the first exhaust gas treatment unit, the ZrO2-based material in the slurry for forming the upper layer is changed to CeO2: 15 mass%, ZrO2: 70 mass%, Y2O3: 5 mass%, La2O3: 5 mass%, Nd2O3: 5 mass%, BET specific surface area: 80m 2 / g of OSC material, and except for these points, the first exhaust gas treatment part F was produced in the same manner as the first exhaust gas treatment part A.

[0266] Production of the second exhaust gas treatment unit a

[0267] (1) Preparation of slurry for forming the second catalyst layer

[0268] CeO2-ZrO2 solid solution powder (containing 15 mass % of CeO2, 70 mass % of ZrO2, and 15 mass % of oxide of rare earth elements other than Ce in the CeO2-ZrO2 solid solution) and alumina powder were prepared. The CeO2-ZrO2 solid solution powder and alumina powder were mixed and impregnated into a rhodium nitrate aqueous solution.

[0269] Next, a pore former (cross-linked poly(methyl)acrylate particles), alumina sol, zirconia sol, and water as a liquid medium were mixed into the mixed solution to prepare a slurry for forming a second catalyst layer. The mass ratio of each component in the slurry was set to the following amount: in the second catalyst layer after firing, rhodium reached 2.3 mass%, the ratio of components other than rhodium reached 81.4 mass% of CeO2-ZrO2 solid solution powder, 10.6 mass% of alumina, 5.0 mass% of alumina sol, and 3.0 mass% of zirconium oxide, and the pore former was set to an amount of 30.0 mass% relative to the mass of the catalyst layer after firing.

[0270] (2) Preparation of slurry for forming the third catalyst layer

[0271] CeO2-ZrO2 solid solution powder (the CeO2-ZrO2 solid solution contains 15 mass % of CeO2, 70 mass % of ZrO2, and 10 mass % of oxides of rare earth elements other than Ce) and alumina powder are impregnated in a rhodium nitrate aqueous solution.

[0272] Next, a pore former (cross-linked poly(methyl)acrylate particles), alumina sol, the zirconia sol, and water as a liquid medium are mixed into the mixed solution to prepare a slurry for forming a third catalyst layer. The mass ratio of each component in the slurry is set to the following amount: in the third catalyst layer after firing, rhodium reaches 1.5 mass%, the ratio of components other than rhodium reaches 81.5 mass% of CeO2-ZrO2 solid solution powder, 10.5 mass% of alumina, 5.0 mass% of alumina sol, and 3.0 mass% of zirconium oxide, and the pore former is set to an amount of 30 mass% relative to the mass of the catalyst layer after firing.

[0273] (3) Formation of Precursor Layer of Second Catalyst Layer

[0274] As a wall flow type substrate, a Figure 5 The structure shown in the figure is 300 cells / inch in a plane perpendicular to the axial direction. 2 A wall-flow type substrate having a density of 1.4 L and a chamber, wherein the chamber is separated by a partition wall having a thickness of 200 to 250 μm and extends in the axial direction. In the wall-flow type substrate, the area of ​​the opening of one inflow side chamber at the inflow side end surface is substantially the same as the area of ​​the opening of one outflow side chamber at the outflow side end surface.

[0275] The upstream end of the wall-flow type substrate in the exhaust gas flow direction was immersed in the slurry for forming the second catalyst layer, and after suction from the downstream side, it was dried at 70° C. for 10 minutes. Thus, a layer containing the solid content of the slurry for forming the second catalyst layer (precursor layer of the second catalyst layer) was formed on the surface of the partition wall of the wall-flow type substrate facing the inflow side chamber.

[0276] (4) Formation of Precursor Layer of Third Catalyst Layer

[0277] After forming the precursor layer of the second catalyst layer, the downstream end of the wall flow type substrate in the exhaust gas flow direction is immersed in the slurry for forming the third catalyst layer, and after suction from the upstream side, it is dried at 70° C. for 10 minutes. Thus, a layer containing the solid content of the slurry for forming the third catalyst layer (precursor layer of the third catalyst layer) is formed on the surface of the partition wall of the wall flow type substrate facing the outflow side chamber.

[0278] (5) Firing

[0279] After forming the precursor layer of the second catalyst layer and the precursor layer of the third catalyst layer, the wall flow type substrate is calcined at 450° C. for 1 hour. Thus, an exhaust gas purification catalyst having a wall flow type substrate and a second catalyst layer and a third catalyst layer formed on the wall flow type substrate is manufactured, and the manufactured exhaust gas purification catalyst is used as a second exhaust gas treatment unit. It should be noted that the second catalyst layer and the third catalyst layer are composed of a single layer.

[0280] The ratio of the length of the second catalyst layer to the total length of the wall-flow type substrate is 45%, and the mass of the second catalyst layer per unit volume relative to the volume of the entire wall-flow type substrate is 17.6 g / L after firing. The ratio of the length of the third catalyst layer to the total length of the wall-flow type substrate is 70%, and the mass of the third catalyst layer per unit volume relative to the volume of the entire wall-flow type substrate is 27.4 g / L after firing.

[0281] The obtained exhaust gas purifying catalyst was cut with a plane perpendicular to the axial direction of the wall flow type substrate, and the second catalyst layer and the third catalyst layer existing in the cut surface were observed using a scanning electron microscope (SEM) to determine the morphology of the second catalyst layer and the third catalyst layer. In the observation of the first catalyst layer, the exhaust gas purifying catalyst was cut at a position 10 mm away from the end of the exhaust gas inflow side of the substrate in the axial direction of the substrate, and in the observation of the second catalyst layer, the exhaust gas purifying catalyst was cut at a position 10 mm away from the end of the exhaust gas outflow side of the substrate in the axial direction of the substrate.

[0282] In the observation of the cut surface by SEM, the field magnification was set to 300 times, and the total field width (length in the direction perpendicular to the axial direction of the substrate) was set to 500 to 600 μm. The area observed by SEM was set so as not to include the corners of the chamber.

[0283] The SEM observation image of the portion where only the second catalyst layer is formed is shown in FIG. Figure 8 .like Figure 8 As shown in FIG. 1 , the area where the partition wall of the wall flow type substrate exists and the area where the second catalyst layer exists can be determined based on the difference in morphological surface between the partition wall of the wall flow type substrate and the second catalyst layer. Similarly, the area where the partition wall of the wall flow type substrate exists and the area where the third catalyst layer exists can be determined based on the difference in morphological surface between the partition wall of the wall flow type substrate and the third catalyst layer.

[0284] The second catalyst layer has a portion formed on the surface of the inflow side chamber side of the partition wall from the upstream end of the exhaust gas flow direction of the wall flow type substrate toward the downstream side (a portion that bulges from the surface of the inflow side chamber side of the partition wall toward the inflow side chamber side). It should be noted that the surface of the inflow side chamber side of the partition wall is the outer surface of the inflow side chamber side that constitutes the outer shape of the partition wall. The portion formed on the surface of the inflow side chamber side of the partition wall is a portion that bulges from the outer surface of the inflow side chamber side of the partition wall toward the inflow side chamber side, and is sometimes referred to as the "bulging portion of the second catalyst layer" below.

[0285] The third catalyst layer has a portion formed on the surface of the outflow side chamber side of the partition wall from the downstream end of the exhaust gas flow direction of the wall flow type substrate toward the upstream side (a portion that bulges from the surface of the partition wall on the outflow side chamber side toward the outflow side chamber side). It should be noted that the surface of the outflow side chamber side of the partition wall is the outer surface of the outflow side chamber side that constitutes the outer shape of the partition wall. The portion formed on the surface of the outflow side chamber side of the partition wall is a portion that bulges from the outer surface of the outflow side chamber side of the partition wall toward the outflow side chamber side, and is sometimes referred to as the "bulging portion of the third catalyst layer" below.

[0286] like Figure 8 As shown, in the SEM observation image, the first to thirty-seventh grid lines perpendicular to the axial direction of the substrate are drawn in sequence at intervals of 15 μm from the left end side, and the contour line of the existence area of ​​the partition wall portion of the substrate and the intersection points of each grid line are connected to each other with a straight line to determine the surface position of the partition wall portion of the substrate. Similarly, the contour line of the existence area of ​​the first catalyst layer and the intersection points of each grid line are connected to each other with a straight line to determine the surface position of the first catalyst layer. When the amount of change in the thickness direction from a certain intersection P1 toward the intersection P2 adjacent to the intersection P1 exceeds the interval (15 μm) of the grid lines, the intersection P2 is not used for determining the surface position (that is, the intersection P2 is removed from the intersections connected by the straight line). In addition, when the amount of change in the thickness direction from the intersection point P1 toward the intersection point P2 adjacent to the intersection point P1 exceeds the interval (15 μm) of the grid lines, and the amount of change in the thickness direction from the intersection point P1 toward the intersection point P3 adjacent to the intersection point P2 also exceeds the interval (15 μm) of the grid lines, the surface position is determined without using the intersection point P2, and the intersection point P3 is not used (that is, the intersection point P2 and the intersection point P3 are removed from the intersection points connected by the straight line). In this way, when five intersection points are continuously removed from the intersection points connected by the straight line, the SEM image is not used for the measurement of thickness.

[0287] After determining the surface position of the partition wall portion of the substrate and the surface position of the second catalyst layer, the image analysis software is used to obtain the area of ​​the region surrounded by the second grid line, the thirty-sixth grid line, the surface of the partition wall portion of the substrate, and the surface of the second catalyst layer. As the image analysis software, AreaQ (STEC company system) is used. It should be noted that the two ends of the image are prone to become unclear, and it is difficult to determine the surface position of the partition wall portion and the surface position of the first catalyst layer, therefore, the first grid line and the thirty-seventh grid line are not used.

[0288] After the area of ​​the above region is obtained, the thickness of the above region is calculated according to the following formula.

[0289] The thickness of the above region = the area of ​​the above region / (grid line spacing × number of grid line spacings)

[0290] It should be noted that the interval between the grid lines is 15 μm, and the number of intervals between the grid lines is 34.

[0291] The thickness of the above region was calculated for 20 randomly selected second catalyst layers from the cut surface, and the average value of the calculated values ​​was 47 μm. This average value was used as the thickness of the raised portion of the second catalyst layer. The thickness of the raised portion of the third catalyst layer was calculated in the same manner, and the result was 41 μm.

[0292] Production of the second exhaust gas treatment unit b

[0293] In forming the second catalyst layer of the second exhaust gas treatment section, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the second catalyst layer is changed to a CeO2-ZrO2 solid solution powder containing 32 mass% of CeO2, 58 mass% of ZrO2, and 10 mass% of oxides of rare earth elements other than Ce in the CeO2-ZrO2 solid solution, the CeO2-ZrO2 solid solution powder is mixed with an alumina powder, and impregnated in a mixed solution of rhodium nitrate and palladium nitrate adjusted so that Rh in the second catalyst layer reaches 0.6 mass% and Pd in ​​the second catalyst layer reaches 1.7 mass% in terms of metal conversion, and in forming the third catalyst layer of the second exhaust gas treatment section, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the third catalyst layer is changed to a CeO2-ZrO2 solid solution containing 32 mass% of CeO2, 58 mass% of ZrO2, and 10 mass% of oxides of rare earth elements other than Ce in the CeO2-ZrO2 solid solution. The second exhaust gas treatment part b is prepared by mixing a CeO2-ZrO2 solid solution powder containing 58 mass % of Ce and 10 mass % of oxides of rare earth elements other than Ce with an alumina powder, and impregnating the mixture with a mixed solution of rhodium nitrate and palladium nitrate adjusted so that the Rh in the third catalyst layer reaches 0.4 mass % and the Pd in ​​the third catalyst layer reaches 1.1 mass % in terms of metal conversion. Except for these aspects, the second exhaust gas treatment part b is prepared by the same operation as the second exhaust gas treatment part a.

[0294] Production of the second exhaust gas treatment unit c

[0295] In forming the second catalyst layer of the second exhaust gas treatment unit, the mixture of rhodium nitrate and palladium nitrate adjusted so that the Rh in the second catalyst layer reaches 0.6 mass % and the Pd in ​​the second catalyst layer reaches 1.7 mass % in terms of metal conversion is impregnated, and in forming the third catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the third catalyst layer is changed to a CeO2-ZrO2 solid solution containing 20 mass % of CeO2 and 1.7 mass % of ZrO2. 70 mass % CeO2-ZrO2 solid solution powder and 10 mass % oxide of rare earth elements other than Ce, the CeO2-ZrO2 solid solution powder is mixed with alumina powder and impregnated in a mixed solution of rhodium nitrate and palladium nitrate adjusted in such a way that the Rh in the third catalyst layer reaches 0.4 mass % on a metal basis and the Pd reaches 1.1 mass % on a metal basis. Except for these aspects, the second exhaust gas treatment part c is produced by the same operation as the second exhaust gas treatment part a.

[0296] Production of the second exhaust gas treatment unit d

[0297] In forming the second catalyst layer of the second exhaust gas treatment unit, the second catalyst layer is impregnated with a rhodium nitrate solution adjusted so that the Rh in the second catalyst layer reaches 1.7 mass % in terms of metal conversion, and the mass of the second catalyst layer per unit volume relative to the volume of the entire wall flow type substrate after sintering is set to 23.5 g / L in terms of the mass after sintering, and in forming the third catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the third catalyst layer is changed to a CeO2-ZrO2 solid solution containing 20 mass % of CeO2 and 20 mass % of ZrO2 The second exhaust gas treatment part d is prepared by impregnating a CeO2-ZrO2 solid solution powder containing 70 mass % Ce and 10 mass % oxides of rare earth elements other than Ce in a rhodium nitrate solution adjusted so that the Rh in the third catalyst layer reaches 1.1 mass % in metal conversion, and the mass of the second catalyst layer per unit volume relative to the overall volume of the wall-flow type substrate after sintering is set to 36.5 g / L in terms of the mass after sintering. Except for these aspects, the second exhaust gas treatment part d is prepared by the same operation as the second exhaust gas treatment part a.

[0298] Production of the second exhaust gas treatment unit e

[0299] In forming the second catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the second catalyst layer is changed to a CeO2-ZrO2 solid solution powder containing 23 mass % of CeO2, 67 mass % of ZrO2, and 10 mass % of oxides of rare earth elements other than Ce in the CeO2-ZrO2 solid solution, and is impregnated with a rhodium nitrate solution adjusted in such a way that the Rh in the second catalyst layer reaches 1.7 mass % in terms of metal conversion, and the mass of the second catalyst layer per unit volume relative to the entire volume of the wall-flow type substrate after sintering is set to 23.5 g / L in terms of the mass after sintering, and, in forming the third catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used for preparing the slurry for forming the third catalyst layer is changed to a CeO2-ZrO2 solid solution powder containing 23 mass % of CeO2, 67 mass % of ZrO2, and 10 mass % of oxides of rare earth elements other than Ce. 30 mass % CeO2-ZrO2 solid solution powder, 60 mass % ZrO2, and 10 mass % oxide of rare earth elements other than Ce are prepared. The CeO2-ZrO2 solid solution powder is mixed with alumina powder. Except for these aspects, the second exhaust gas treatment part e is prepared by the same operation as the second exhaust gas treatment part a.

[0300] Production of the second exhaust gas treatment unit f

[0301] In the formation of the second catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used to prepare the slurry for forming the second catalyst layer is changed to a CeO2-ZrO2 solid solution powder containing 8 mass% of CeO2, 82 mass% of ZrO2, and 10 mass% of oxides of rare earth elements other than Ce in the CeO2-ZrO2 solid solution, and in the formation of the third catalyst layer of the second exhaust gas treatment unit, the CeO2-ZrO2 solid solution powder used to prepare the slurry for forming the third catalyst layer is changed to a CeO2-ZrO2 solid solution powder containing 10 mass% of CeO2, 80 mass% of ZrO2, and 10 mass% of oxides of rare earth elements other than Ce in the CeO2-ZrO2 solid solution. Except for these aspects, the second exhaust gas treatment unit f is manufactured by the same operation as the second exhaust gas treatment unit a.

[0302] Table 1 shows the compositions of the first catalyst layers in the first exhaust gas treatment units A to F, and Table 2 shows the compositions of the second catalyst layers and the third catalyst layers in the second exhaust gas treatment units a to f.

[0303] [Table 1]

[0304]

[0305] [Table 2]

[0306]

[0307] Examples 1 to 8 and Comparative Examples 1 to 3: Evaluation of Exhaust Gas Purification Performance

[0308] The following durability conditions were applied to the first exhaust gas treatment unit and the second exhaust gas treatment unit as a degradation treatment assuming a driving distance of 160,000 to 240,000 kilometers. Specifically, the first exhaust gas treatment unit was arranged on the upstream side of the exhaust passage through which the exhaust gas flows, and the second exhaust gas treatment unit was arranged on the downstream side of the exhaust passage. The engine was operated under the following conditions, and the catalyst temperature was maintained at the following temperature while in contact with the exhaust gas, and the following treatment was performed for the following time.

[0309] (Durability conditions)

[0310] Durable engine: Passenger NA 2L gasoline engine

[0311] Gasoline used: Commercially available regular gasoline

[0312] Durable temperature / time: 1000℃-80hr in the first exhaust gas treatment layer

[0313] · Air-fuel ratio change before the catalyst: A / F = 14.6 (20 sec) → fuel cutoff control (5 sec) → 12.5 (5 sec) repeatedly

[0314] After the durability test is carried out under the above conditions, the first exhaust gas treatment unit after the durability test and the second exhaust gas treatment unit after the durability test are combined as shown in Table 3, and are arranged on the following vehicle in such a manner that the first exhaust gas treatment unit after the durability test is located on the upstream side of the exhaust gas flow path and the second exhaust gas treatment unit after the durability test is located on the downstream side of the exhaust gas flow path. As a vehicle test, the vehicle is operated according to the operating conditions of the World Harmonized Exhaust Gas Test Model (WLTC). The NMHC (hydrocarbons other than methane) and NOx in the exhaust gas passing through the first exhaust gas treatment unit and the second exhaust gas treatment unit from the start of operation to 1800 seconds are measured, and the emission per unit driving distance is calculated. Regarding the obtained emission values, the values ​​when the values ​​of Example 1, which will serve as the benchmark, are set to 1 are shown in Table 3. In addition, the respective emissions of NMHC and NOx are shown in Table 3 together with the emissions of WLTC (total emissions).

[0315] (Purification rate measurement conditions)

[0316] ·Evaluated vehicle: 1.5L direct injection turbo passenger car

[0317] Use gasoline: fuel for certification test

[0318] [Table 3]

[0319]

[0320] As shown in Table 3, the combination of the first exhaust gas treatment part and the second exhaust gas treatment part in Examples 1 to 8 has lower total NMHC and NOx emissions than the combination of the first exhaust gas treatment part and the second exhaust gas treatment part in Comparative Examples 1 to 3. This confirms that the combination of the first exhaust gas treatment part and the second exhaust gas treatment part in Examples 1 to 8 of the present invention reduces the amount of cerium oxide used and exhibits high exhaust gas purification performance.

[0321] Description of Reference Numerals

[0322] 1. Exhaust gas purification system

[0323] 2 Exhaust pipe

[0324] 3. The first exhaust gas treatment unit

[0325] 31 Flow-through substrate

[0326] Room 311

[0327] 312 partition wall

[0328] 32. First catalyst layer

[0329] 4 Second exhaust gas treatment unit

[0330] 41 Wall flow substrate

[0331] 411 Flowing into the side chamber

[0332] 412 Outflow side chamber

[0333] 413 Partition wall

[0334] 42 Second catalyst layer

[0335] 43 Third catalyst layer

[0336] 44 First sealing part

[0337] 45 Second sealing part

[0338] X Exhaust gas flow direction

Claims

1. An exhaust gas purification system for purifying exhaust gas discharged from an internal combustion engine, The exhaust gas purification system comprises: an exhaust passage through which exhaust gas flows; a first exhaust gas treatment unit provided on an upstream side in the exhaust passage; and a second exhaust gas treatment unit disposed on a downstream side in the exhaust passage, The first exhaust gas treatment unit includes a flow-through substrate and a first catalyst layer. The flow-through substrate comprises: a plurality of chambers extending in the exhaust gas flow direction, wherein both ends on the exhaust gas inflow side in the exhaust gas flow direction and ends on the exhaust gas outflow side in the exhaust gas flow direction are open; and a partition wall that partitions the plurality of chambers, The first catalyst layer has a portion formed on the surface of the partition wall, The second exhaust gas treatment unit includes a wall flow type substrate, a second catalyst layer and a third catalyst layer. The wall flow type substrate comprises: an inflow side chamber extending in the exhaust gas flow direction, with an end portion on the exhaust gas inflow side in the exhaust gas flow direction being open and an end portion on the exhaust gas outflow side in the exhaust gas flow direction being closed; an outflow side chamber extending in the exhaust gas flow direction, with an end portion on the exhaust gas inflow side in the exhaust gas flow direction being closed and an end portion on the exhaust gas outflow side in the exhaust gas flow direction being open; and a porous partition wall that separates the inflow side chamber from the outflow side chamber, The second catalyst layer has a portion formed on the surface of the partition wall on the inlet side chamber side and along the exhaust gas flow direction from an end of the partition wall on the exhaust gas inlet side in the exhaust gas flow direction. The third catalyst layer has a portion formed on the surface of the partition wall on the outflow side chamber side and extending from an end of the partition wall on the exhaust gas outflow side in the exhaust gas flow direction in a direction opposite to the exhaust gas flow direction. The first catalyst layer contains cerium element, A percentage P1 of the mass of the cerium element contained in the first catalyst layer in terms of cerium oxide relative to the mass of the first catalyst layer is 5.0 mass % or more and 13.0 mass % or less, The second catalyst layer contains rhodium element, The second catalyst layer and the third catalyst layer contain cerium element, The percentage P of the sum of the mass of the cerium element contained in the second catalyst layer in terms of cerium oxide and the mass of the cerium element contained in the third catalyst layer in terms of cerium oxide relative to the sum of the mass of the second catalyst layer and the mass of the third catalyst layer 23 It is 1.1 times or more and 5.3 times or less of the percentage P1.

2. The exhaust gas purification system according to claim 1, wherein: A percentage P2 of the mass of the cerium element contained in the second catalyst layer in terms of cerium oxide relative to the mass of the second catalyst layer is greater than the percentage P1.

3. The exhaust gas purification system according to claim 2, wherein: The percentage P2 is greater than or equal to 1.1 times and less than or equal to 5.3 times the percentage P1.

4. The exhaust gas purification system according to claim 1 or 2, wherein: A percentage P3 of the mass of the cerium element contained in the third catalyst layer in terms of cerium oxide relative to the mass of the third catalyst layer is greater than the percentage P1.

5. The exhaust gas purification system according to claim 4, wherein: The percentage P3 is greater than or equal to 1.1 times and less than or equal to 5.3 times the percentage P1.

6. The exhaust gas purification system according to claim 1 or 2, wherein: The third catalyst layer contains palladium element.

7. The exhaust gas purification system according to claim 1 or 2, wherein: The wall flow type substrate has a cell density per square inch of 200 cells or more and 400 cells or less.

8. The exhaust gas purification system according to claim 1 or 2, wherein: The cell density per square inch of the flow-through substrate is greater than or equal to 400 cells and less than or equal to 1200 cells.

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