Exhaust gas purification catalyst

Abstract

In order to provide an exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided to the substrate, and a second catalyst layer provided to the upper side of the first catalyst layer, the first catalyst layer containing Pd, and the second catalyst layer containing Rh, wherein it is possible to prevent any reduction in exhaust gas purification performance after exposure to high-temperature environments and to prevent any reduction in exhaust gas purification performance after exposure to phosphorus, the present invention provides an exhaust gas purification catalyst (1) comprising a substrate (10), a first catalyst layer (20) provided to the substrate (10), and a second catalyst layer (30) provided to the upper side of the first catalyst layer (20), wherein: the first catalyst layer (20) contains Pd; the second catalyst layer (30) contains Rh and a mixed oxide that includes Ce, Zr, and Al; and the exhaust gas purification catalyst (1) satisfies at least one condition from among (A) the first catalyst layer (20) containing Sr, and the Sr content of the first catalyst layer (20) in terms of metal being 0.1 mass% or greater with reference to the mass of the first catalyst layer (20), and (B) the second catalyst layer (30) containing Sr, and the Sr content of the second catalyst layer (30) in terms of metal being 0.1 mass% or greater with reference to the mass of the second catalyst layer (30).

Description

[Technical Field] 【0001】 This invention relates to a catalyst for exhaust gas purification. [Background technology] 【0002】 Exhaust gases emitted from internal combustion engines of automobiles, motorcycles, and other vehicles contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Catalysts, such as those containing precious metal elements like Pt, Pd, and Rh, are used to purify and neutralize these harmful components. Pt and Pd are primarily involved in the oxidation and purification of HC and CO, while Rh is primarily involved in the reduction and purification of NOx. 【0003】 Pd is susceptible to poisoning by phosphorus (for example, phosphorus in engine oil). As an exhaust gas purification catalyst that can suppress phosphorus poisoning of Pd, an exhaust gas purification catalyst is known that comprises a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided above the first catalyst layer, wherein the first catalyst layer contains Pd and the second catalyst layer contains Rh (for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents] 【0004】 [Patent Document 1] Japanese Patent Publication No. 2011-136319 [Patent Document 2] Japanese Patent Publication No. 2016-185531 [Overview of the project] [Problems that the invention aims to solve] 【0005】 Exhaust gas purification catalysts are required to prevent a decrease in exhaust gas purification performance after exposure to high-temperature environments (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) and to prevent a decrease in exhaust gas purification performance after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine). In this specification, "low temperature" preferably means a temperature of 500°C or lower, more preferably 400°C or lower, and even more preferably 300°C or lower, and "high temperature" preferably means a temperature of 700°C or higher, more preferably 800°C or higher, and even more preferably 900°C or higher. 【0006】 The present invention aims to provide an exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided above the first catalyst layer, wherein the first catalyst layer contains Pd and the second catalyst layer contains Rh, and which can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) and a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine). [Means for solving the problem] 【0007】 This invention provides the following: [1] An exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided above the first catalyst layer, The first catalyst layer contains Pd, The second catalyst layer comprises Rh and a composite oxide containing Ce, Zr, and Al. The exhaust gas purification catalyst is subject to the following conditions A and B: (A) The first catalyst layer contains Sr, and the metal equivalent content of Sr in the first catalyst layer is 0.1% by mass or more, based on the mass of the first catalyst layer; (B) The second catalyst layer contains Sr, and the metal equivalent content of Sr in the second catalyst layer is 0.1% by mass or more, based on the mass of the second catalyst layer. The exhaust gas purification catalyst that satisfies at least one of the following conditions. [2] Under condition A, the metal equivalent content of Sr in the first catalyst layer is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the first catalyst layer. The exhaust gas purification catalyst according to [1], wherein, under condition B, the metal equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the second catalyst layer. [3] Under condition A, the metal equivalent content of Sr in the first catalyst layer is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the first catalyst layer. The exhaust gas purification catalyst according to [2], wherein, under condition B, the metal equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the second catalyst layer. [4] The exhaust gas purification catalyst according to any one of [1] to [3], wherein the composite oxide contains La, and the La2O3 equivalent content of La in the composite oxide is 1.0% by mass or more, based on the mass of the composite oxide. [5] If the exhaust gas purification catalyst satisfies condition A, then the exhaust gas purification catalyst satisfies the following condition C: (C) The ratio of the metal-equivalent content of Sr in the first catalyst layer, based on the mass of the first catalyst layer, to the content of the composite oxide in the second catalyst layer, based on the mass of the second catalyst layer, is 0.0010 or more and 0.0800 or less. Satisfying the conditions, If the exhaust gas purification catalyst satisfies condition B, then the exhaust gas purification catalyst satisfies the following condition D: (D) The ratio of the metal-equivalent content of Sr in the second catalyst layer, based on the mass of the second catalyst layer, to the content of the composite oxide in the second catalyst layer, based on the mass of the second catalyst layer, is 0.0010 or more and 0.0800 or less. An exhaust gas purification catalyst described in any of [1] to [4] that satisfies the following conditions. [6] The exhaust gas purification catalyst according to any one of [1] to [5], wherein the content of the composite oxide in the second catalyst layer is 80.0% by mass or more based on the mass of the second catalyst layer. [7] The exhaust gas purification catalyst according to any one of [1] to [6], wherein the content of Al in terms of Al2O3 in the composite oxide is 30.0% by mass or more and 60.0% by mass or less based on the mass of the composite oxide. [8] The exhaust gas purification catalyst according to any one of [1] to [7], which satisfies at least condition A among conditions A and B. 【Advantages of the Invention】 【0008】 According to the present invention, there is provided an exhaust gas purification catalyst including a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided above the first catalyst layer, wherein the first catalyst layer contains Pd and the second catalyst layer contains Rh, which can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, among the exhaust gas purification performances after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine), and can prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, among the exhaust gas purification performances after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine). 【Brief Description of the Drawings】 【0009】 [Figure 1] FIG. 1 is a partial end view showing a state in which an exhaust gas purification catalyst according to an embodiment of the present invention is disposed in an exhaust passage of an internal combustion engine. [Figure 2] FIG. 2 is an end view taken along line A-A of FIG. 1. [Figure 3] FIG. 3 is an enlarged view of a region indicated by reference numeral R in FIG. 2. [Figure 4] FIG. 4 is an end view taken along line B-B of FIG. 1. 【Embodiments for Carrying Out the Invention】 【0010】 ≪Exhaust Gas Purification Catalyst≫ The exhaust gas purification catalyst of the present invention will be described below. 【0011】 The following describes an exhaust gas purification catalyst 1 (hereinafter sometimes referred to as "catalyst 1") according to one embodiment of the present invention, based on Figures 1 to 4. 【0012】 As shown in Figure 1, catalyst 1 is located in the exhaust passage within the exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas discharged from the internal combustion engine flows through the exhaust passage within the exhaust pipe P from one end to the other and is purified by catalyst 1 installed in the exhaust pipe P. In the drawings, the direction of exhaust gas flow is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas inlet side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outlet side" or "downstream side." 【0013】 In the exhaust passage within the exhaust pipe P, other exhaust gas purification catalysts may be placed upstream and / or downstream of catalyst 1. 【0014】 As shown in Figures 2-4, the catalyst 1 comprises a substrate 10, a first catalyst layer 20 provided on the substrate 10, and a second catalyst layer 30 provided above the first catalyst layer 20. 【0015】 <Base material> The base material 10 will be described below. 【0016】 The material constituting the base material 10 can be appropriately selected from known materials. Examples of materials constituting the base material 10 include ceramic materials and metallic materials, but ceramic materials are preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as alumina, zirconia, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metallic materials include alloys such as stainless steel. 【0017】 As shown in Figures 2-4, the base material 10 has a cylindrical portion 11, a partition wall portion 12 provided inside the cylindrical portion 11, and cells 13 separated by the partition wall portion 12. The base material 10 is preferably a honeycomb structure. 【0018】 As shown in Figure 2, the cylindrical portion 11 defines the outer shape of the base material 10, and the axial direction of the cylindrical portion 11 coincides with the axial direction of the base material 10. As shown in Figure 2, the shape of the cylindrical portion 11 is cylindrical, but it may also be an elliptical cylinder, a polygonal cylinder, or other shapes. 【0019】 As shown in Figures 2-4, a partition wall 12 exists between adjacent cells 13, and adjacent cells 13 are separated by the partition wall 12. The partition wall 12 may have a porous structure through which exhaust gas can pass. The thickness of the partition wall 12 is, for example, 20 μm to 1500 μm. 【0020】 As shown in Figure 4, cell 13 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. 【0021】 As shown in Figure 4, both the exhaust gas inlet and exhaust gas outlet ends of cell 13 are open. Therefore, exhaust gas entering from the exhaust gas inlet end (opening) of cell 13 flows out from the exhaust gas outlet end (opening) of cell 13. This type of configuration is called a flow-through type. 【0022】 As shown in Figures 2 and 3, the plan view shape of the exhaust gas inlet end (opening) of cell 13 is a rectangle, but it may be a hexagon, octagon, or other shape. The same applies to the plan view shape of the exhaust gas outlet end (opening) of cell 13. 【0023】 The cell density per square inch of the substrate 10 is, for example, between 100 and 1000 cells. The cell density per square inch of the substrate 10 refers to the total number of cells 13 per square inch in a cross-section obtained by cutting the substrate 10 with a plane perpendicular to the exhaust gas flow direction X. 【0024】 The volume of the base material 10 is, for example, 0.1 L or more and 20 L or less. The volume of the base material 10 refers to the apparent volume of the base material 10. For example, if the base material 10 is cylindrical, the outer diameter of the base material 10 is 2r and the length of the base material 10 is L. 10 Therefore, the volume of base material 10 is given by the formula: Volume of base material 10 = π × r 2 ×L 10 It is represented as follows. 【0025】 <First catalyst layer> The first catalyst layer 20 will be described below. 【0026】 As shown in Figures 3 and 4, the first catalyst layer 20 is provided on the cell 13 side surface of the partition wall 12. "Cell 13 side surface of the partition wall 12" means the outer surface of the partition wall 12 that extends in the exhaust gas flow direction X. The first catalyst layer 20 may be provided directly on the cell 13 side surface of the partition wall 12 or via other layers, but it is usually provided directly on the cell 13 side surface of the partition wall 12. 【0027】 The first catalyst layer 20 may consist of a portion that rises from the cell 13 side surface of the partition wall portion 12 toward the cell 13 (hereinafter referred to as the "raised portion"), or it may consist of a portion that exists inside the partition wall portion 12 (hereinafter referred to as the "internal portion"), or it may have both a raised portion and an internal portion. The "first catalyst layer 20 provided on the substrate 10" includes any embodiment in which the first catalyst layer 20 is composed of a raised portion, an embodiment in which the first catalyst layer 20 is composed of an internal portion, and an embodiment in which the first catalyst layer 20 has both a raised portion and an internal portion. 【0028】 As shown in Figure 4, the first catalyst layer 20 extends along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 to the exhaust gas outlet end of the partition wall 12. The first catalyst layer 20 may extend along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 so as not to reach the exhaust gas outlet end of the partition wall 12, or it may extend along the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12 so as not to reach the exhaust gas inlet end of the partition wall 12. 【0029】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalyst layer 20 per unit volume of the portion of the substrate on which the first catalyst layer 20 is formed (mass after firing) is preferably 50 g / L or more and 230 g / L or less, more preferably 70 g / L or more and 180 g / L or less, and even more preferably 80 g / L or more and 150 g / L or less. The mass of the first catalyst layer 20 per unit volume of the portion of the substrate 10 on which the first catalyst layer 20 is formed is given by the formula: (mass of the first catalyst layer 20) / ((volume of the substrate 10) × (average length L of the first catalyst layer 20) 20 / Length L of base material 10 10 It is calculated from ). In this specification, unless otherwise specified, "length" means the axial dimension of the base material 10. 【0030】 In this specification, "mass of the first catalyst layer 20" means the sum of the masses of all metal elements contained in the first catalyst layer 20, calculated as metal equivalent for noble metal elements and as oxide equivalent for metal elements other than noble metal elements. In other words, "mass of the first catalyst layer 20" means the calculated mass obtained by adding the mass of the noble metal elements contained in the first catalyst layer 20 as metal equivalent and the mass of the metal elements other than noble metal elements contained in the first catalyst layer 20 as oxide equivalent. Note that "metal elements" also include metalloid elements such as Si and B. 【0031】 In this specification, "precious metal elements" include Pt (platinum), Pd (palladium), Rh (rhodium), Ru (ruthenium), Os (osmium), Ir (iridium), Au (gold), and Ag (silver). 【0032】 In this specification, oxides of rare earth elements other than Ce, Pr, and Tb are referred to as sesquioxides (M2O3, where M represents a rare earth element other than Ce, Pr, and Tb), oxides of Ce are referred to as CeO2, and oxides of Pr are referred to as Pr6O 11 This means that Tb oxide is Tb4O7, Al oxide is Al2O3, Zr oxide is ZrO2, Si oxide is SiO2, B oxide is B2O3, Cr oxide is Cr2O3, Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe3O4, Mn oxide is Mn3O4, Ni oxide is NiO, Ti oxide is TiO2, Zn oxide is ZnO, and Sn oxide is SnO2. 【0033】 Average length L of the first catalyst layer 20 20 An example of the measurement method is as follows: 【0034】 A catalyst extends from the catalyst 1 in the axial direction of the substrate 10, along the length L of the substrate 10. 10Cut out a sample having the same length as the first catalyst layer. The sample is, for example, cylindrical with a diameter of 25.4 mm. Note that the diameter of the sample can be changed as needed. If the first catalyst layer 20 extends from the exhaust gas inlet end of the partition wall 12 along the exhaust gas flow direction X, cut the sample at 5 mm intervals using a plane perpendicular to the axial direction of the base material 10, and obtain the first cut piece, second cut piece, ..., nth cut piece in order from the exhaust gas inlet end of the sample. If the first catalyst layer 20 extends from the exhaust gas outlet end of the partition wall 12 along the direction opposite to the exhaust gas flow direction X, cut the sample at 5 mm intervals using a plane perpendicular to the axial direction of the base material 10, and obtain the first cut piece, second cut piece, ..., nth cut piece in order from the exhaust gas outlet end of the sample. In either case, the length of the cut piece is 5 mm. The composition of the cut piece is analyzed using an X-ray fluorescence analyzer (XRF) (e.g., energy-dispersive X-ray analyzer (EDX), wavelength-dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma emission spectrometer (ICP-AES), a scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDX), etc.), and based on the composition of the cut piece, it is confirmed whether or not the cut piece contains a portion of the first catalyst layer 20. 【0035】 For sections that clearly contain a portion of the first catalyst layer 20, compositional analysis is not necessarily required. For example, the cross-section can be observed using a scanning electron microscope (SEM), electron beam microanalyzer (EPMA), etc., to confirm whether or not the section contains a portion of the first catalyst layer 20. Elemental mapping of the cross-section may also be performed when observing the cross-section. 【0036】 After confirming whether the cut piece contains a portion of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula. Length of the first catalyst layer 20 in the sample = 5 mm × (number of cut pieces containing a portion of the first catalyst layer 20) 【0037】 For example, when the first to kth cut pieces include a part of the first catalyst layer 20 and the (k + 1)th to nth cut pieces do not include a part of the first catalyst layer 20, the length of the first catalyst layer 20 included in the sample is (5 × k) mm. 【0038】 An example of a more detailed method for measuring the length of the first catalyst layer 20 included in the sample is as follows. When the kth cut piece (when the first catalyst layer 20 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition wall portion 12, it is the cut piece obtained from the most exhaust gas outflow side of the sample among the cut pieces including a part of the first catalyst layer 20; when the first catalyst layer 20 extends along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition wall portion 12, it is the cut piece obtained from the most exhaust gas inflow side of the sample among the cut pieces including a part of the first catalyst layer 20)) is cut in the axial direction of the base material 10, and a part of the first catalyst layer 20 present on the cut surface is observed using SEM, EPMA, etc., to measure the length of a part of the first catalyst layer 20 in the kth cut piece. Then, based on the following formula, the length of the first catalyst layer 20 included in the sample is calculated. Length of the first catalyst layer 20 included in the sample = (5 mm × (k - 1)) + (Length of a part of the first catalyst layer 20 included in the kth cut piece) 【0039】 Regarding 8 to 16 samples arbitrarily cut out from the catalyst 1, measure the length of the first catalyst layer 20 included in each sample, and take their average value as the average length L of the first catalyst layer 20 20 and. 【0040】 The first catalyst layer 20 contains Pd as a catalytic active component. Pd is included in the first catalyst layer 20 in the form of a catalytic active component containing Pd, such as metallic Pd, an alloy containing Pd, a compound containing Pd (for example, an oxide of Pd), etc. From the perspective of improving the exhaust gas purification performance, the catalytic active component containing Pd is preferably in a particulate form. 【0041】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the metal equivalent content of Pd in ​​the first catalyst layer 20 is preferably 0.010% by mass or more and 20% by mass or less, more preferably 0.050% by mass or more and 15% by mass or less, and even more preferably 0.10% by mass or more and 10% by mass or less, based on the mass of the first catalyst layer 20. 【0042】 The first catalyst layer 20 may contain noble metal elements other than Pd as catalytically active components. Examples of noble metal elements other than Pd include Pt, Rh, Ir, Ru, Os, Au, Ag, etc. The noble metal elements other than Pd are included in the first catalyst layer 20 in a form that can function as a catalytically active component, such as a metal, an alloy containing the noble metal element, or a compound containing the noble metal element (e.g., an oxide of the noble metal element). From the viewpoint of improving exhaust gas purification performance, it is preferable that the catalytically active component containing the noble metal element other than Pd be in particulate form. 【0043】 If the first catalyst layer 20 contains Pd and other precious metal elements, the Pd and other precious metal elements may form an alloy, potentially reducing the number of active sites of Pd that are involved in exhaust gas purification performance. Therefore, it is preferable that the first catalyst layer 20 substantially does not contain any precious metal elements other than Pd. 【0044】 "The first catalyst layer 20 substantially does not contain any precious metal elements other than Pd" means that the metal equivalent content of precious metal elements other than Pd in ​​the first catalyst layer 20 is preferably 0.050% by mass or less, more preferably 0.010% by mass or less, based on the mass of the first catalyst layer 20. The lower limit is zero. "Metal equivalent content of precious metal elements other than Pd in ​​the first catalyst layer 20" means the metal equivalent content of one precious metal element other than Pd if the first catalyst layer 20 contains one such precious metal element, and the sum of the metal equivalent content of two or more precious metal elements if the first catalyst layer 20 contains two or more precious metal elements other than Pd. 【0045】 The metal equivalent content of each metal element in the first catalyst layer 20 can be calculated using the formula: (metal equivalent mass of each metal element in the first catalyst layer 20) / (mass of the first catalyst layer 20) × 100. For example, the metal equivalent content of Pd in ​​the first catalyst layer 20 can be calculated using the formula: (metal equivalent mass of Pd in ​​the first catalyst layer 20) / (mass of the first catalyst layer 20) × 100. 【0046】 If the composition of the raw materials used to form the first catalyst layer 20 is known, the metal equivalent content of each metal element contained in the first catalyst layer 20 can be determined from the composition of the raw materials used to form the first catalyst layer 20. 【0047】 If the composition of the raw materials used to form the first catalyst layer 20 is unknown, the metal equivalent content of each metal element contained in the first catalyst layer 20 can be determined using conventional methods such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). Specifically, it is as follows: 【0048】 Elemental analysis is performed on the sample obtained from the first catalyst layer 20 using conventional methods such as SEM-EDX to identify the types of constituent elements in the entire sample and to determine the content (mass%) of each identified metal element. The content (mass%) of each metal element is determined for each of the 10 fields of view of the SEM, and the average value of the content (mass%) of each metal element in the 10 fields of view is taken as the content (mass%) of each metal element in the first catalyst layer 20. 【0049】 The first catalyst layer 20 preferably comprises one or more types of carriers, and at least a portion of the catalytically active component is supported on one or more types of carriers. 【0050】 "At least a portion of the catalytically active component is supported on the support" means that at least a portion of the catalytically active component is physically or chemically adsorbed or retained on the outer surface and / or the inner surface of the pores of the support. The fact that at least a portion of the catalytically active component is supported on the support can be confirmed, for example, using SEM-EDX. Specifically, if at least a portion of the catalytically active component and the support are present in the same region in the elemental mapping obtained by analyzing the cross-section of the first catalyst layer 20 with SEM-EDX, it can be determined that at least a portion of the catalytically active component is supported on the support. 【0051】 The support material can be selected from, for example, inorganic oxides. Inorganic oxides are, for example, particulate. From the viewpoint of improving the support of catalytically active components, it is preferable that the inorganic oxide is porous. Inorganic oxides may or may not have oxygen storage capacity (OSC). In this specification, inorganic oxides having OSC may be referred to as "OSC materials". Inorganic oxides used as support materials are distinguished from inorganic oxides used as binders (for example, inorganic oxide binders such as alumina binders, zirconia binders, titania binders, and silica binders). 【0052】 Examples of inorganic oxides include Al-based oxides, Ce-based oxides, Ce-Zr composite oxides, oxides of rare earth elements other than Ce, zirconia (ZrO2), silica (SiO2), titania (TiO2), zeolites (aluminosilicates), and oxides based on MgO, ZnO, SnO2, etc. 【0053】 Al-based oxides are oxides that contain Al, where Al is the element with the highest mass content among the elements other than O that make up the oxide. However, Ce-Zr composite oxides are not considered Al-based oxides. 【0054】 Ce-based oxides are oxides containing Ce, where Ce is the element with the highest mass content among the elements other than O that make up the oxide. However, Ce-Zr composite oxides are not considered Ce-based oxides. 【0055】 Ce-Zr composite oxides refer to composite oxides containing Ce and Zr, wherein the Ce content in the composite oxide, when converted to CeO2, is 5.0% by mass or more and 95.0% by mass or less, based on the mass of the composite oxide; the Zr content in the composite oxide, when converted to ZrO2, is 5.0% by mass or more and 95.0% by mass or less, based on the mass of the composite oxide; and the Al content in the composite oxide, when converted to Al2O3, is less than 5.0% by mass, based on the mass of the composite oxide. 【0056】 The oxide content of each element in Al-based oxides, Ce-based oxides, or Ce-Zr-based composite oxides can be determined in the same manner as the oxide content of each element in Ce-Zr-Al-based composite oxides, as described later. 【0057】 The first catalyst layer 20 may contain other components such as a binder and stabilizer. Examples of binders include inorganic oxide binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol. 【0058】 <Second catalyst layer> The second catalyst layer 30 will be described below. 【0059】 As shown in Figures 3 and 4, the second catalyst layer 30 is provided above the first catalyst layer 20. 【0060】 "The second catalyst layer 30 is provided on the upper side of the first catalyst layer 20" means that part or all of the second catalyst layer 30 is located on the main surface of the first catalyst layer 20 that is opposite to the main surface on the partition wall 12 side. "Main surface of the first catalyst layer 20" means the outer surface of the first catalyst layer 20 that extends in the exhaust gas flow direction X. The second catalyst layer 30 may be provided directly on the main surface of the first catalyst layer 20 or via another layer, but is usually provided directly on the main surface of the first catalyst layer 20. The second catalyst layer 30 may be provided so as to cover a part of the main surface of the first catalyst layer 20, or so as to cover the entire main surface of the first catalyst layer 20. The phrase "second catalyst layer 30 provided above the first catalyst layer 20" includes both embodiments in which the second catalyst layer 30 is directly provided on the main surface of the first catalyst layer 20, and embodiments in which the second catalyst layer 30 is provided on the main surface of the first catalyst layer 20 via another layer. 【0061】 As shown in Figure 4, the second catalyst layer 30 extends along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 to the exhaust gas outlet end of the partition wall 12. The second catalyst layer 30 may extend along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 so as not to reach the exhaust gas outlet end of the partition wall 12, or it may extend along the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12 so as not to reach the exhaust gas inlet end of the partition wall 12. 【0062】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the second catalyst layer 30 per unit volume of the portion of the base material 10 on which the second catalyst layer 30 is formed (mass after firing) is preferably 20 g / L or more and 150 g / L or less, more preferably 50 g / L or more and 120 g / L or less, and even more preferably 70 g / L or more and 100 g / L or less. The mass of the second catalyst layer 30 per unit volume of the portion of the base material 10 on which the second catalyst layer 30 is formed is given by the formula: (mass of the second catalyst layer 30) / ((volume of the base material 10) × (average length L of the second catalyst layer 30) 30 / Length L of base material 10 10 It is calculated from ). 【0063】 The above explanation regarding the mass of the first catalyst layer 20 also applies to the second catalyst layer 30. When applying this explanation, "first catalyst layer 20" should be read as "second catalyst layer 30". 【0064】 Average length L of the first catalyst layer 20 20 The above explanation regarding the measurement method refers to the average length L of the second catalyst layer 30. 30 This also applies to the measurement method. When applied, the "first catalyst layer 20" is "second catalyst layer 30" and the "average length L 20 " is "Average length L 30 This can be reinterpreted as ". 【0065】 The second catalyst layer 30 contains Rh as a catalytically active component. Rh is included in the second catalyst layer 30 in a form that can function as a catalytically active component, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Rh is preferably in particulate form. 【0066】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the metal equivalent content of Rh in the second catalyst layer 30 is preferably 0.010% by mass or more and 5% by mass or less, more preferably 0.050% by mass or more and 3% by mass or less, and even more preferably 0.10% by mass or more and 2% by mass or less, based on the mass of the second catalyst layer 30. 【0067】 The second catalyst layer 30 may contain noble metal elements other than Rh as catalytically active components. Examples of noble metal elements other than Rh include Pt, Pd, Ir, Ru, Os, Au, Ag, etc. The noble metal elements other than Rh are included in the second catalyst layer 30 in a form that can function as a catalytically active component, such as a metal, an alloy containing the noble metal element, or a compound containing the noble metal element (e.g., an oxide of the noble metal element). From the viewpoint of improving exhaust gas purification performance, it is preferable that the catalytically active component containing the noble metal element other than Rh be in particulate form. 【0068】 If the second catalyst layer 30 contains Rh and other precious metal elements, the Rh and other precious metal elements may form an alloy, potentially reducing the number of Rh active sites involved in exhaust gas purification performance. Therefore, it is preferable that the second catalyst layer 30 substantially does not contain any precious metal elements other than Rh. 【0069】 "The second catalyst layer 30 substantially does not contain any noble metal elements other than Rh" means that the metal equivalent content of noble metal elements other than Rh in the second catalyst layer 30 is preferably 0.050% by mass or less, more preferably 0.010% by mass or less, based on the mass of the second catalyst layer 30. The lower limit is zero. "Metal equivalent content of noble metal elements other than Rh in the second catalyst layer 30" means the metal equivalent content of one noble metal element other than Rh if the second catalyst layer 30 contains one noble metal element other than Rh, and the sum of the metal equivalent content of two or more noble metal elements other than Rh if the second catalyst layer 30 contains two or more noble metal elements other than Rh. 【0070】 The metal equivalent content of each metal element in the second catalyst layer 30 can be calculated using the formula: (metal equivalent mass of each metal element in the second catalyst layer 30) / (mass of the second catalyst layer 30) × 100. For example, the metal equivalent content of Rh in the second catalyst layer 30 can be calculated using the formula: (metal equivalent mass of Rh in the second catalyst layer 30) / (mass of the second catalyst layer 30) × 100. 【0071】 The metal equivalent content of each metal element in the second catalyst layer 30 can be determined in the same manner as the metal equivalent content of each metal element in the first catalyst layer 20. 【0072】 The second catalyst layer 30 contains a composite oxide containing Ce, Zr, and Al (hereinafter sometimes referred to as "Ce-Zr-Al composite oxide"). 【0073】 Whether or not the second catalyst layer 30 contains a Ce-Zr-Al composite oxide can be determined by conventional methods such as SEM-EDX. Specifically, if the second catalyst layer 30 is observed with SEM-EDX and Ce, Zr, and Al are present at the same location within the second catalyst layer 30, it can be determined that the second catalyst layer 30 contains a Ce-Zr-Al composite oxide. 【0074】 It is desirable that the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine be controlled to be near the stoichiometric air-fuel ratio. However, the actual air-fuel ratio fluctuates around the stoichiometric ratio, either rich (excess fuel atmosphere) or lean (lean fuel atmosphere), depending on the vehicle's driving conditions, and similarly, the air-fuel ratio of the exhaust gas also fluctuates to the rich or lean side. Ce-Zr-Al composite oxides have oxygen storage capacity, so by using Ce-Zr-Al composite oxides, fluctuations in the oxygen concentration in the exhaust gas can be mitigated, thereby expanding the operating window of the catalyst. Furthermore, Ce-Zr-Al composite oxides have heat resistance, so in high-temperature environments, aggregation of Ce-Zr-Al composite oxides, loss of pores in Ce-Zr-Al composite oxides (i.e., decrease in specific surface area), Rh burial, and Rh sintering can be prevented. Therefore, by including a Ce-Zr-Al composite oxide in the second catalyst layer 30, it is possible to prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (in particular, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine, among the exhaust gas purification performance after exposure to a high-temperature environment). 【0075】 Ce-Zr-Al composite oxides refer to composite oxides containing Ce, Zr, and Al, wherein the Ce content in the composite oxide, based on the mass of the composite oxide, is 1.0% by mass or more and 94.0% by mass or less, the Zr content in the composite oxide, based on the mass of the composite oxide, is 1.0% by mass or more and 94.0% by mass or less, and the Al content in the composite oxide, based on the mass of the composite oxide, is 5.0% by mass or more and 98.0% by mass or less, based on the mass of the composite oxide. 【0076】 Ce-Zr-Al composite oxides are, for example, particulate. Ce-Zr-Al composite oxides are used as carriers for catalytically active components. From the viewpoint of improving the support capacity of catalytically active components, it is preferable that Ce-Zr-Al composite oxides are porous. 【0077】 In the second catalyst layer 30, it is preferable that at least a portion of the catalytically active component is supported on a Ce-Zr-Al composite oxide. The explanation of the significance of the support and the method of confirmation is the same as in the explanation for the first catalyst layer 20. 【0078】 From the viewpoint of improving oxygen storage capacity and heat resistance, the Ce content of Ce in the Ce-Zr-Al composite oxide, in terms of CeO2, is preferably 2.0% by mass or more and 40.0% by mass or less, more preferably 3.0% by mass or more and 20.0% by mass or less, and even more preferably 5.0% by mass or more and 15.0% by mass or less, based on the mass of the Ce-Zr-Al composite oxide. 【0079】 From the viewpoint of improving oxygen storage capacity and heat resistance, the Zr content of Zr in the Ce-Zr-Al composite oxide, in terms of ZrO2, is preferably 10.0% by mass or more and 70.0% by mass or less, more preferably 15.0% by mass or more and 60.0% by mass or less, and even more preferably 20.0% by mass or more and 50.0% by mass or less, based on the mass of the Ce-Zr-Al composite oxide. 【0080】 From the viewpoint of improving oxygen storage capacity and heat resistance, the Al content of Al in the Ce-Zr-Al composite oxide, in terms of Al2O3, is preferably 30.0% by mass or more and 60.0% by mass or less, more preferably 35.0% by mass or more and 55.0% by mass or less, and even more preferably 40.0% by mass or more and 50.0% by mass or less, based on the mass of the Ce-Zr-Al composite oxide. 【0081】 From the viewpoint of improving oxygen storage capacity and heat resistance, the sum of the Ce content of Ce (calculated as CeO2), Zr content of Zr (calculated as ZrO2), and Al content of Al (calculated as Al2O3) in the Ce-Zr-Al composite oxide is preferably 70.0% by mass or more, more preferably 80.0% by mass or more, and even more preferably 90.0% by mass or more, based on the mass of the Ce-Zr-Al composite oxide. The upper limit is 100% by mass. 【0082】 From the viewpoint of improving oxygen storage capacity and heat resistance, the ratio of the sum of the Ce content (calculated as CeO2) and Zr content (calculated as ZrO2) in the Ce-Zr-Al composite oxide to the Al content (calculated as Al2O3) in the Ce-Zr-Al composite oxide is preferably 0.5 to 1.2, more preferably 0.6 to 1.1, and even more preferably 0.7 to 1.0. 【0083】 Ce-Zr-Al composite oxides may contain one or more metallic elements other than Ce, Zr, and Al. Examples of metallic elements other than Ce include rare earth elements other than Ce. Examples of rare earth elements other than Ce include Y, Pr, Sc, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. 【0084】 The oxide content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al composite oxide can be appropriately adjusted considering the CeO2 content of Ce, the ZrO2 content of Zr, and the Al2O3 content of Al in the Ce-Zr-Al composite oxide. From the viewpoint of improving oxygen storage capacity and heat resistance, the oxide content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al composite oxide is preferably 30.0% by mass or less, more preferably 20.0% by mass or less, and even more preferably 15.0% by mass or less. The lower limit is zero. "Oxide-equivalent content of metal elements other than Ce, Zr, and Al in Ce-Zr-Al composite oxides" means the oxide-equivalent content of one metal element other than Ce, Zr, and Al if the Ce-Zr-Al composite oxide contains one such metal element, and the sum of the oxide-equivalent content of two or more metal elements other than Ce, Zr, and Al if the Ce-Zr-Al composite oxide contains two or more such metal elements. 【0085】 From the viewpoint of improving heat resistance, it is preferable that the Ce-Zr-Al composite oxide contains La. From the viewpoint of improving heat resistance, the La2O3 content of La in the Ce-Zr-Al composite oxide is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 4.0% by mass or more, based on the mass of the Ce-Zr-Al composite oxide. The upper limit of the La2O3 content of La in the Ce-Zr-Al composite oxide can be appropriately adjusted by considering the CeO2 content of Ce, the ZrO2 content of Zr, and the Al2O3 content of Al in the Ce-Zr-Al composite oxide. The La2O3 content of La in the Ce-Zr-Al composite oxide is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, and even more preferably 8.0% by mass or less. Each of these upper limits may be combined with any of the lower limits described above. 【0086】 From the viewpoint of improving heat resistance, the Ce-Zr-Al composite oxide may contain rare earth elements other than Ce and La. From the viewpoint of improving heat resistance, the oxide content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 4.0% by mass or more, based on the mass of the Ce-Zr-Al composite oxide. The upper limit of the oxide content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide can be appropriately adjusted by considering the CeO2 content of Ce, the ZrO2 content of Zr, and the Al2O3 content of Al in the Ce-Zr-Al composite oxide. The oxide content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, and even more preferably 8.0% by mass or less. Each of these upper limits may be combined with any of the lower limits mentioned above. "Oxide-equivalent content of rare earth elements other than Ce and La in Ce-Zr-Al composite oxides" means the oxide-equivalent content of one rare earth element other than Ce and La if the Ce-Zr-Al composite oxide contains one such rare earth element, and the sum of the oxide-equivalent content of two or more rare earth elements other than Ce and La if the Ce-Zr-Al composite oxide contains two or more such rare earth elements. 【0087】 If the composition of a Ce-Zr-Al composite oxide is known, the oxide content of each element in the Ce-Zr-Al composite oxide can be determined from the composition of the Ce-Zr-Al composite oxide. 【0088】 If the composition of the Ce-Zr-Al composite oxide is unknown, the oxide content of each element in the Ce-Zr-Al composite oxide can be determined by analyzing a sample obtained from the second catalyst layer 30 using energy-dispersive X-ray spectroscopy (EDX), and then measuring the resulting elemental mapping and the EDX elemental analysis of specified particles. Specifically, by qualitatively identifying (color-coding) Ce-Zr-Al composite oxide particles and other particles using elemental mapping, and then performing compositional analysis (elemental analysis) on the specified particles, the oxide content of each element in the specified particles can be measured. 【0089】 In Ce-Zr-Al composite oxides, Ce may form a solid solution phase (for example, a solid solution phase of CeO2 and ZrO2, a solid solution phase of CeO2 and Al2O3, a solid solution phase of CeO2, ZrO2, and Al2O3, etc.), a single phase which is either a crystalline or amorphous phase (for example, a single CeO2 phase), or both a solid solution phase and a single phase. However, it is preferable that at least a portion of the Ce forms a solid solution phase. 【0090】 In Ce-Zr-Al composite oxides, Zr may form a solid solution phase (for example, a solid solution phase of CeO2 and ZrO2, a solid solution phase of ZrO2 and Al2O3, a solid solution phase of CeO2, ZrO2 and Al2O3, etc.), a single phase which is either a crystalline or amorphous phase (for example, a single ZrO2 phase), or both a solid solution phase and a single phase may be formed, but it is preferable that at least a portion of Zr forms a solid solution phase. 【0091】 In Ce-Zr-Al composite oxides, Al may form a solid solution phase (for example, a solid solution phase of CeO2 and Al2O3, a solid solution phase of ZrO2 and Al2O3, a solid solution phase of CeO2, ZrO2 and Al2O3, etc.), or it may form a single phase (Al2O3 single phase) which is either a crystalline or amorphous phase. 【0092】 When a Ce-Zr-Al composite oxide contains one or more metal elements other than Ce, Zr, and Al, the metal elements other than Ce, Zr, and Al may form solid solution phases (for example, a solid solution phase of CeO2 and an oxide of a metal element other than Ce, Zr, and Al; a solid solution phase of ZrO2 and an oxide of a metal element other than Ce, Zr, and Al; a solid solution phase of CeO2, ZrO2, and an oxide of a metal element other than Ce, Zr, and Al, etc.), or they may form single phases that are crystalline or amorphous (single oxide phases of metal elements other than Ce, Zr, and Al). However, it is preferable that at least a portion of the metal elements other than Ce, Zr, and Al form solid solution phases. 【0093】 From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine), the content of Ce-Zr-Al composite oxide in the second catalyst layer 30 is preferably 80.0% by mass or more, more preferably 85.0% by mass or more, and even more preferably 88.0% by mass or more, based on the mass of the second catalyst layer 30. The upper limit of the content of Ce-Zr-Al composite oxide in the second catalyst layer 30 can be appropriately adjusted considering the content of other components (e.g., Rh) in the second catalyst layer 30. The content of Ce-Zr-Al composite oxide in the second catalyst layer 30 is preferably 98.0% by mass or less, more preferably 97.0% by mass or less, and even more preferably 95.0% by mass or less, based on the mass of the second catalyst layer 30. Each of these upper limits may be combined with any of the lower limits described above. 【0094】 If the composition of the raw materials used to form the second catalyst layer 30 is known, the content of Ce-Zr-Al composite oxide in the second catalyst layer 30 can be determined from the composition of the raw materials used to form the second catalyst layer 30. 【0095】 If the composition of the raw materials used to form the second catalyst layer 30 is unknown, the content of Ce-Zr-Al composite oxide in the second catalyst layer 30 can be determined by conventional methods such as SEM-EDX. Specifically, it is as follows. 【0096】 (1) The sample obtained from the second catalyst layer 30 is subjected to elemental analysis using a standard method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the content (mass%) of each identified element. (2) Elemental mapping is performed on the sample obtained from the second catalyst layer 30 using a conventional method such as SEM-EDX to identify the types of particles contained in the sample (for example, Ce-Zr-Al composite oxide particles and, if applicable, other particles). (3) For each type of particle, several (e.g., 50) arbitrarily selected particles are subjected to elemental analysis using SEM-EDX to identify the constituent elements of the particles and determine the content (mass %) of each identified element. For each type of particle, the average value of the content (mass %) of each element is calculated. (4) The content of each type of particle in the sample is calculated by creating and solving an equation that expresses the relationship between the content (mass%) of each element in the sample, the content (mass%) of each element in each type of particle, and the content (mass%) of each type of particle in the sample, and this is taken as the content (mass%) of each type of particle in the second catalyst layer 30. 【0097】 The second catalyst layer 30 may contain one or more supports other than Ce-Zr-Al composite oxides (hereinafter sometimes referred to as "other supports"). When the second catalyst layer 30 contains other supports, it is preferable that at least a portion of the catalytically active components are supported on the other supports. The explanation regarding the supports (including the explanation regarding inorganic oxides), as well as the significance of support and the method of confirmation, are the same as those explained in the first catalyst layer 20. 【0098】 <Conditions A and B> Catalyst 1 is subject to the following conditions A and B: (A) The first catalyst layer 20 contains Sr, and the metal equivalent content of Sr in the first catalyst layer 20 is 0.1% by mass or more, based on the mass of the first catalyst layer 20; (B) The second catalyst layer 30 contains Sr, and the metal equivalent content of Sr in the second catalyst layer 30 is 0.1% by mass or more, based on the mass of the second catalyst layer 30. It satisfies at least one of the following conditions. 【0099】 Since Pd in ​​the first catalyst layer 20 is susceptible to poisoning by phosphorus (for example, phosphorus in engine oil), the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 (especially the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine) tends to decrease after exposure to phosphorus. In contrast, if catalyst 1 satisfies at least one of conditions A and B, Sr in the first catalyst layer 20 and / or the second catalyst layer 30 captures phosphorus, thereby preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (especially the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine). The inventors investigated various alkaline earth metal elements to suppress phosphorus poisoning and found that Mg and Ca combine more readily with Al2O3 present in the first catalyst layer 20 or the second catalyst layer 30 compared to Sr, thereby reducing heat resistance; Ba combines more readily with Al2O3 present in the first catalyst layer 20 or the second catalyst layer 30 compared to Sr, thereby reducing heat resistance and causing a decrease in Rh exhaust gas purification performance; and Sr combines less readily with Al2O3 present in the first catalyst layer 20 or the second catalyst layer 30 compared to Mg, Ca, and Ba, thereby reducing heat resistance and causing a decrease in Rh and Pd exhaust gas purification performance. Based on these findings, Sr was selected. 【0100】 Ce-Zr-Al composite oxides have a high bulk density, meaning they have a small apparent volume per unit mass. Therefore, when the second catalyst layer 30 is exposed to phosphorus, the Ce-Zr-Al composite oxides in the second catalyst layer 30 have low physical barrier properties against phosphorus intrusion. Consequently, the higher the content of Ce-Zr-Al composite oxides in the second catalyst layer 30, the easier it becomes for phosphorus to pass through the second catalyst layer 30 and reach the first catalyst layer 20, and the more susceptible the Pd in ​​the first catalyst layer 20 is to phosphorus poisoning. Therefore, when the content of Ce-Zr-Al composite oxides in the second catalyst layer 30 is 80.0% by mass or higher, the effect of catalyst 1 satisfying at least one of conditions A and B is significant. 【0101】 From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (particularly the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine), under condition A, the metal equivalent content of Sr in the first catalyst layer 20 is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, based on the mass of the first catalyst layer 20. 【0102】 From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (particularly the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine), under condition B, the metal equivalent content of Sr in the second catalyst layer 30 is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, based on the mass of the second catalyst layer 30. 【0103】 It is desirable that the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine be controlled to be close to the stoichiometric air-fuel ratio. However, the actual air-fuel ratio fluctuates around the stoichiometric ratio, either rich (excess fuel atmosphere) or lean (lean fuel atmosphere), depending on the vehicle's driving conditions, and similarly, the air-fuel ratio of the exhaust gas also fluctuates to either the rich or lean side. In addition to its property of collecting phosphorus, syrup (Sr) has the property of adsorbing NOx when the exhaust gas is lean and releasing NOx when the exhaust gas is rich. Therefore, if the amount of Sr is excessive, when the exhaust gas switches from a lean state to a rich state, a large amount of NOx may be released from the Sr all at once, and there is a risk that the released NOx cannot be completely purified by catalyst 1. The NOx released from Sr when the exhaust gas switches from a lean state to a rich state may be referred to as "transient NOx" below. Therefore, it is preferable to moderately limit the upper limit of the amount of Sr to prevent an excessive increase in transient NOx and to keep the transient NOx within a range that catalyst 1 can purify. 【0104】 From the viewpoint of more effectively preventing an excessive increase in transient NOx levels, under condition A, the metal-equivalent content of Sr in the first catalyst layer 20 is preferably 7.0% by mass or less, more preferably 4.0% by mass or less, and even more preferably 3.0% by mass or less, based on the mass of the first catalyst layer 20. Each of these upper limits may be combined with any of the lower limits mentioned above. 【0105】 From the viewpoint of more effectively preventing an excessive increase in transient NOx levels, under condition B, the metal-equivalent content of Sr in the second catalyst layer 30 is preferably 7.0% by mass or less, more preferably 4.0% by mass or less, and even more preferably 3.0% by mass or less, based on the mass of the second catalyst layer 30. These upper limits may be combined with any of the lower limits mentioned above. 【0106】 In a preferred embodiment, the metal-equivalent content of Sr in the first catalyst layer 20 is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the first catalyst layer 20, and / or the metal-equivalent content of Sr in the second catalyst layer 30 is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the second catalyst layer 30. This makes it possible to more effectively prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (in particular, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine) and to more effectively prevent an excessive increase in transient NOx. 【0107】 In a more preferred embodiment, the metal-equivalent content of Sr in the first catalyst layer 20 is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the first catalyst layer 20, and / or the metal-equivalent content of Sr in the second catalyst layer 30 is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the second catalyst layer 30. This makes it possible to more effectively prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (in particular, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine) and to more effectively prevent an excessive increase in transient NOx. 【0108】 If catalyst 1 satisfies condition A, catalyst 1 may or may not satisfy condition B. "Catalyst 1 does not satisfy condition B" includes embodiments in which the second catalyst layer 30 does not contain Sr, and embodiments in which the second catalyst layer 30 contains Sr, and the metal equivalent content of Sr in the second catalyst layer 30 is less than 0.1 mass% based on the mass of the second catalyst layer 30. In the latter embodiment, the metal equivalent content of Sr in the second catalyst layer 30 is preferably 0.05 mass% or less, more preferably 0.03 mass% or less, and even more preferably 0.01 mass% or less, based on the mass of the second catalyst layer 30. 【0109】 If catalyst 1 satisfies condition B, catalyst 1 may or may not satisfy condition A. "Catalyst 1 does not satisfy condition A" includes embodiments in which the first catalyst layer 20 does not contain Sr, and embodiments in which the first catalyst layer 20 contains Sr, and the metal equivalent content of Sr in the first catalyst layer 20 is less than 0.1 mass% based on the mass of the first catalyst layer 20. In the latter embodiment, the metal equivalent content of Sr in the first catalyst layer 20 is preferably 0.05 mass% or less, more preferably 0.03 mass% or less, and even more preferably 0.01 mass% or less, based on the mass of the first catalyst layer 20. 【0110】 The statement "Catalyst 1 satisfies at least one of conditions A and B" includes (a) embodiments in which Catalyst 1 satisfies conditions A and B, (b) embodiments in which Catalyst 1 satisfies condition A but does not satisfy condition B, and (c) embodiments in which Catalyst 1 satisfies condition B but does not satisfy condition A. 【0111】 When catalyst 1 satisfies condition B, phosphorus contained in the exhaust gas can be captured by Sr in the second catalyst layer 30 before it reaches the first catalyst layer 20. This has the advantage of more effectively preventing phosphorus poisoning of Pd in ​​the first catalyst layer 20 compared to when the first catalyst layer 20 satisfies condition A. On the other hand, when catalyst 1 satisfies condition B, Sr in the second catalyst layer 30 has the disadvantage of reducing the exhaust gas purification performance of Rh in the second catalyst layer 30. In contrast, when catalyst 1 satisfies condition A, although the effect of preventing phosphorus poisoning of Pd in ​​the first catalyst layer 20 is inferior compared to when catalyst 1 satisfies condition B, there is no disadvantage of Sr in the first catalyst layer 20 reducing the exhaust gas purification performance of Pd in ​​the first catalyst layer 20. Therefore, from the viewpoint of achieving a good balance between preventing phosphorus poisoning of Pd by Sr and preventing a decrease in the exhaust gas purification performance of precious metal elements by Sr, embodiments a and b (i.e., catalyst 1 satisfies at least condition A of conditions A and B) are preferred, and embodiment b (i.e., catalyst 1 satisfies condition A but does not satisfy condition B) is more preferred. 【0112】 The metal content of Sr in the first catalyst layer 20 and the metal content of Sr in the second catalyst layer 30 can be determined in the same manner as the metal equivalent content of each metal element in the first catalyst layer 20. 【0113】 If catalyst 1 satisfies condition A, the first catalyst layer 20 contains one or more Sr sources, and if catalyst 1 satisfies condition B, the second catalyst layer 30 contains one or more Sr sources. 【0114】 The Sr source is not particularly limited as long as it is a compound containing Sr. The form in which Sr exists is not particularly limited. Sr may exist in one or more forms selected from oxides, carbonates, and sulfates, for example. From the viewpoint of more effectively exerting the effects of Sr, it is preferable that Sr exists in one or two forms selected from SrO and SrCO3. 【0115】 If catalyst 1 satisfies condition A, then catalyst 1 satisfies the following condition C: (C) The ratio of the metal-equivalent content (mass%) of Sr in the first catalyst layer 20, based on the mass of the first catalyst layer 20, to the content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30, based on the mass of the second catalyst layer 30 (metal-equivalent content (mass%) of Sr in the first catalyst layer 20 / content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30) is 0.0010 or more and 0.0800 or less. It is preferable that the following conditions are met. The phenomenon in which Pd in ​​the first catalyst layer 20 is more susceptible to phosphorus poisoning due to the high bulk density of the Ce-Zr-Al composite oxide is particularly pronounced when a large amount of Ce-Zr-Al composite oxide is contained in the second catalyst layer 30. Therefore, by including an amount of Sr in the first catalyst layer 20 that is commensurate with the amount of Ce-Zr-Al composite oxide in the second catalyst layer 30, that is, by ensuring that catalyst 1 satisfies condition C, phosphorus poisoning of Pd in ​​the first catalyst layer 20 can be prevented more effectively. 【0116】 From the viewpoint of more effectively preventing phosphorus poisoning of Pd in ​​the first catalyst layer 20, the ratio of the metal equivalent content (mass%) of Sr in the first catalyst layer 20, based on the mass of the first catalyst layer 20, to the content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30, based on the mass of the second catalyst layer 30, is preferably 0.0010 to 0.0800, more preferably 0.0020 to 0.0500, and even more preferably 0.0040 to 0.0130. 【0117】 If catalyst 1 satisfies condition B, then catalyst 1 satisfies the following condition D: (D) The ratio of the metal-equivalent content (mass%) of Sr in the second catalyst layer 30, based on the mass of the second catalyst layer 30, to the content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30, based on the mass of the second catalyst layer 30 (metal-equivalent content (mass%) of Sr in the second catalyst layer 30 / content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30) is 0.0010 or more and 0.0800 or less. It is preferable that the following conditions are met. The phenomenon in which Pd in ​​the first catalyst layer 20 is more susceptible to phosphorus poisoning due to the high bulk density of the Ce-Zr-Al composite oxide is particularly pronounced when a large amount of Ce-Zr-Al composite oxide is present in the second catalyst layer 30. Therefore, by including an amount of Sr in the second catalyst layer 30 that is commensurate with the amount of Ce-Zr-Al composite oxide in the second catalyst layer 30, that is, by ensuring that catalyst 1 satisfies condition D, phosphorus poisoning of Pd in ​​the first catalyst layer 20 can be prevented more effectively. 【0118】 From the viewpoint of more effectively preventing phosphorus poisoning of Pd in ​​the first catalyst layer 20, the ratio of the metal equivalent content (mass%) of Sr in the second catalyst layer 30, based on the mass of the second catalyst layer 30, to the content (mass%) of Ce-Zr-Al composite oxide in the second catalyst layer 30, based on the mass of the second catalyst layer 30, is preferably 0.0010 to 0.0800, more preferably 0.0020 to 0.0500, and even more preferably 0.0040 to 0.0130. 【0119】 <Catalyst action> In catalyst 1, the second catalyst layer 30 is provided above the first catalyst layer 20. Therefore, exhaust gas flowing in from the exhaust gas inlet end (opening) of cell 13 comes into contact with the second catalyst layer 30 and then with the first catalyst layer 20. Exhaust gas that comes into contact with the second catalyst layer 30 is purified by the catalytically active component (e.g., Rh) in the second catalyst layer 30, and exhaust gas that comes into contact with the first catalyst layer 20 is purified by the catalytically active component (e.g., Pd) in the first catalyst layer 20. By including a Ce-Zr-Al composite oxide in the second catalyst layer 30, it is possible to prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (in particular, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine, among the exhaust gas purification performance after exposure to a high-temperature environment). Furthermore, by ensuring that catalyst 1 satisfies at least one of conditions A and B, it is possible to prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (in particular, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine). Therefore, catalyst 1 can prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (in particular, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) and prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (in particular, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in the low-temperature environment immediately after starting the internal combustion engine). 【0120】 <Catalyst Manufacturing> Catalyst 1 can be manufactured by forming a first catalyst layer 20 on a substrate 10, and then forming a second catalyst layer 30 on top of the first catalyst layer 20. 【0121】 The first catalyst layer 20 can be formed by preparing a first slurry by mixing a Pd source (e.g., a Pd salt), optionally a Sr source (e.g., a Sr salt), and optionally other components (e.g., inorganic oxides, binders, solvents, etc.), applying the first slurry onto the substrate 10, drying it, and firing it. 【0122】 The second catalyst layer 30 can be formed by preparing a second slurry by mixing a source of Rh (e.g., a Rh salt), a Ce-Zr-Al composite oxide, optionally a source of Sr (e.g., a Sr salt), and optionally other components (e.g., inorganic oxides other than Ce-Zr-Al composite oxides, binders, solvents, etc.), applying the second slurry onto the first catalyst layer 20, drying it, and firing it. 【0123】 Examples of Pd sources include Pd salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of Rh sources include Rh salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of Sr sources include Sr salts, such as nitrates, acetates, carbonates, and chlorides. Examples of binders include alumina sol, zirconia sol, titania sol, silica sol, and ceria sol. Examples of solvents include water and organic solvents. 【0124】 The drying temperature is, for example, between 70°C and 150°C, and the drying time is, for example, between 5 minutes and 1 hour. The firing temperature is, for example, between 200°C and 700°C, and the firing time is, for example, between 0.5 hours and 5 hours. Firing can be carried out, for example, in an air atmosphere. [Examples] 【0125】 [Example 1] (1) Preparation of slurry for forming the lower layer We prepared an OSC material (Ce-Zr composite oxide, sometimes referred to as "CZ") with the following composition. Ce content (calculated as CeO2): 40.0% by mass, Zr content (calculated as ZrO2): 50.0% by mass, Oxide content of other rare earth elements: 10.0% by mass 【0126】 In a mixing container, an aqueous solution of palladium nitrate, a Ce-Zr composite oxide, La2O3-modified alumina (La2O3 modification amount: 1% by mass), strontium nitrate, alumina sol, and water were added, mixed, and stirred to prepare a slurry for forming the lower layer. The amounts of each component in the slurry for forming the lower layer were adjusted based on the mass of the lower layer after calcination (100% by mass) so that Pd was 4.0% by mass in terms of metal equivalent, Ce-Zr composite oxide was 40.0% by mass, La2O3-modified alumina was 45.4% by mass, Sr was 0.5% by mass in terms of metal equivalent (0.6% by mass in terms of SrO), and the solid content of alumina sol was 10.0% by mass. Note that strontium nitrate is converted to strontium oxide by calcination. 【0127】 (2) Formation of the lower layer As a flow-through substrate, axially extending cells, partitioned by partitions with a thickness of 50-70 μm, are arranged at a rate of 600 cells / inch in a plane perpendicular to the axial direction. 2 A flow-through type substrate with a density of 1.0 L and a volume of 1.0 L was prepared. 【0128】 A flow-through substrate was immersed in a slurry for forming the lower layer. The flow-through substrate coated with the slurry was dried at 150°C for 0.5 hours, and then baked at 500°C for 1 hour to form the lower layer. The mass of the lower layer per unit volume of the portion of the flow-through substrate where the lower layer was formed was 100 g / L. 【0129】 (3) Preparation of slurry for forming the upper layer We prepared an OSC material (Ce-Zr-Al composite oxide, sometimes referred to as "CZA") with the following composition. Ce content (calculated as CeO2): 10.0 mass, Zr content (calculated as ZrO2): 35.0 mass, Al content (calculated as Al2O3): 45.0 mass, La content (calculated as La2O3): 5.0 mass, Oxide content of rare earth elements other than Ce and La: 5.0 mass 【0130】 In a mixing container, rhodium nitrate aqueous solution, Ce-Zr-Al composite oxide, alumina sol, and water were added, mixed, and stirred to prepare a slurry for forming the upper layer. The amounts of each component in the slurry for forming the upper layer were adjusted so that, based on the mass of the upper layer after calcination (100% by mass), Rh was 1.0% by mass in terms of metal equivalent, Ce-Zr-Al composite oxide was 89.0% by mass, and the solid content of alumina sol was 10.0% by mass. 【0131】 (4) Formation of the upper layer A flow-through substrate with a lower layer formed on top was immersed in a slurry for forming the upper layer. The flow-through substrate coated with the slurry for forming the upper layer was dried at 150°C for 0.5 hours, and then baked at 500°C for 1 hour to form the upper layer on top of the lower layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate where the upper layer was formed was 80 g / L. 【0132】 As described above, an exhaust gas purification catalyst comprising a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer was manufactured. 【0133】 [Examples 2-4] In the lower layer forming slurry, the amount of strontium nitrate added was adjusted so that, based on the mass of the lower layer after calcination (100% by mass), Sr was 1.0% by mass (Example 2), 2.5% by mass (Example 3), or 5.0% by mass (Example 4) in terms of metal equivalent. The exhaust gas purification catalysts of Examples 2 to 4 were manufactured in the same manner as in Example 1, except that the amount of strontium nitrate added was increased and the amount of La2O3-modified alumina added was decreased. 【0134】 [Comparative Example 1] The preparation of the slurry for forming the lower layer and the formation of the lower layer were carried out in the same manner as in Example 2. 【0135】 We prepared an OSC material (Ce-Zr composite oxide, sometimes referred to as "CZ") with the following composition. Ce content (calculated as CeO2): 15.0% by mass, Zr content (calculated as ZrO2): 75.0% by mass, Oxide content of other rare earth elements: 10.0% by mass 【0136】 In a mixing container, aqueous rhodium nitrate solution, Ce-Zr composite oxide, La2O3-modified alumina (La2O3 modification amount: 1% by mass), alumina sol, and water were added, mixed, and stirred to prepare a slurry for forming the upper layer. The amounts of each component in the slurry for forming the upper layer were adjusted so that, based on the mass of the upper layer after calcination (100% by mass), Rh was 1.0% by mass in terms of metal equivalent, Ce-Zr composite oxide was 50.0% by mass, La2O3-modified alumina was 39.0% by mass, and the solid content of the alumina sol was 10.0% by mass. The upper layer was formed in the same manner as in Example 2, except that this slurry for forming the upper layer was used. 【0137】 [Comparative Example 2] A catalyst for exhaust gas purification for Comparative Example 2 was manufactured in the same manner as in Example 1, except that strontium nitrate was not added to the lower layer forming slurry, and the amount of La2O3-modified alumina added was increased to compensate for the decrease in the amount of strontium nitrate added. 【0138】 [Example Test] (1) Heat treatment of catalyst A 30 mL cored exhaust gas purification catalyst was heat-treated in a quartz tubular furnace at 1000°C for 20 hours under a 10 volume% H2O atmosphere using the following FC mode. The heat-treated catalyst was used as an evaluation sample. FC mode: Model gas (3 L / min) with the following composition and air (3 L / min) were flowed alternately. The model gas flow rates were set as follows: C3H 66 mL / min, O2 71 mL / min, N22 923 mL / min (total 3 L / min). 10 vol% H2O was vaporized from a water-filled tank and mixed with the model gas or air as water vapor. The saturated water vapor pressure was adjusted according to the temperature to obtain the above volume% water vapor amount. 【0139】 (2) Phosphorus poisoning treatment of catalyst In a tubular furnace made of quartz, phosphorus poisoning treatment was performed at 750°C for 20 hours under a 10 volume% H2O atmosphere, while spraying a solution containing zinc dialkyldithiophosphate from the upstream side of the catalyst after the heat treatment described in (1) above, under the gas flow rate conditions described below. The gas flow rates were set as follows: N2 11.9 L / min, CO2 3.0 L / min, O2 0.1 L / min (total 15 L / min). 10 vol% H2O was vaporized from a water-filled tank and mixed with the model gas or air as water vapor. The saturated water vapor pressure was adjusted according to the temperature to obtain the above volume% water vapor amount. 【0140】 (3) Evaluation of exhaust gas purification performance (T50) A catalyst treated according to (1) or (2) above was placed in the exhaust passage, and exhaust model gas (CO 0.50 vol, H2 0.17 vol, O2 0.50 vol, NO 400 vol ppm, C3H6 1180 vol ppm, CO2 14 vol, H2O 10 vol, remaining N2) was flowed through at a space velocity of 100,000 / h while the temperature was raised to 500°C at a heating rate of 20°C / min. The purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) were continuously measured, and the temperature at which the purification rate of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) reached 50% (light-off temperature T50) (°C) was determined. The light-off temperature T50 was determined during the heating process. The results are shown in Table 1. 【0141】 (4) Assessment of transient NOx emissions The catalyst treated in (2) above was placed in the exhaust passage, and the atmosphere was switched in the following order: lean conditions, N2, and rich conditions, and the mixture was circulated for 5 minutes each at a space velocity of 100,000 / h and 400°C. Lean conditions: NO 1000volppm, O25000volppm, N2 balance Rich condition: CO 5000volppm, N2 balance 【0142】 The total amount of NO and NO2 emitted when switching to rich conditions was evaluated as transient NOx emissions (μmol). 【0143】 [Table 1] 【0144】 The exhaust gas purification catalysts of Examples 1 to 4 comprise a substrate, a lower layer containing Pd, and an upper layer containing Rh and Ce-Zr-Al composite oxide, and meet the following conditions A: (A) The lower layer contains Sr, and the metallic equivalent content of Sr in the lower layer is 0.1% by mass or more, based on the mass of the lower layer. It satisfies the condition. 【0145】 Comparative Example 1's exhaust gas purification catalyst comprises a substrate, a lower layer containing Pd, and an upper layer containing Rh, and is subject to the following conditions A: (A) The lower layer contains Sr, and the metallic equivalent content of Sr in the lower layer is 0.1% by mass or more, based on the mass of the lower layer. This satisfies the condition. However, in the exhaust gas purification catalyst of Comparative Example 1, the upper layer contains Ce-Zr-based composite oxides but does not contain Ce-Zr-Al-based composite oxides. 【0146】 The exhaust gas purification catalyst of Comparative Example 2 comprises a substrate, a lower layer containing Pd, and an upper layer containing Rh and a Ce-Zr-Al composite oxide. However, the exhaust gas purification catalyst of Comparative Example 2 is subject to the following condition A: (A) The lower layer contains Sr, and the metallic equivalent content of Sr in the lower layer is 0.1% by mass or more, based on the mass of the lower layer. It does not satisfy the following condition B: (B) The upper layer contains Sr, and the metallic equivalent content of Sr in the upper layer is 0.1% by mass or more, based on the mass of the upper layer. It doesn't even meet that requirement. 【0147】 As shown in Table 1, the exhaust gas purification catalysts of Examples 1 to 4 can more effectively prevent the decline in exhaust gas purification performance after exposure to a high-temperature environment (in particular, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) compared with the exhaust gas purification catalyst of Comparative Example 1. 【0148】 As shown in Table 1, the exhaust gas purification catalysts of Examples 1 to 4 can more effectively prevent the decline in exhaust gas purification performance after exposure to phosphorus (in particular, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) compared with the exhaust gas purification catalyst of Comparative Example 2. 【0149】 Thus, the exhaust gas purification catalysts of Examples 1 to 4 can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine) and prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, the exhaust gas purification performance in the low-temperature environment immediately after starting the internal combustion engine). [Explanation of Symbols] 【0150】 P... Exhaust pipe of an internal combustion engine 1. Catalyst for exhaust gas purification 10...Base material 11. Cylindrical part 12...Bulkhead part 13...cell 20...1st catalyst layer 30...Second catalyst layer

Claims

[Claim 1] An exhaust gas purification catalyst comprising a base material, a first catalyst layer provided on the base material, and a second catalyst layer provided above the first catalyst layer, The first catalyst layer contains Pd, and the metal equivalent content of Pd in ​​the first catalyst layer is 0.010% by mass or more and 20% by mass or less, based on the mass of the first catalyst layer. The second catalyst layer is a composite oxide particle containing Rh, Ce, Zr, and Al, wherein the Ce in the composite oxide particle is CeO ２ The converted content is 2.0% by mass or more and 40.0% by mass or less, based on the mass of the composite oxide particles, and the Zr in the composite oxide particles is ZrO ２ The converted content is 10.0% by mass or more and 60.0% by mass or less, based on the mass of the composite oxide particles, and the Al content in the composite oxide particles is... ２ O ３ The composite oxide particles have a converted content of 30.0% by mass or more and 60.0% by mass or less, based on the mass of the composite oxide particles, and the metal equivalent content of Rh in the second catalyst layer is 0.010% by mass or more and 5% by mass or less, based on the mass of the second catalyst layer, and the content of the composite oxide particles in the second catalyst layer is 80.0% by mass or more, based on the mass of the second catalyst layer. The second catalyst layer is provided so as to cover the entire main surface of the first catalyst layer. The exhaust gas purification catalyst is provided under the following conditions A and B: (A) The first catalyst layer contains Sr, and the metal equivalent content of Sr in the first catalyst layer is 0.3% by mass or more and 7.0% by mass or less, based on the mass of the first catalyst layer; (B) The second catalyst layer contains Sr, and the metal equivalent content of Sr in the second catalyst layer is 0.3% by mass or more and 7.0% by mass or less, based on the mass of the second catalyst layer. Among these, the exhaust gas purification catalyst that satisfies at least condition A. [Claim 2] Under condition A, the metal equivalent content of Sr in the first catalyst layer is 0.3% by mass or more and 4.0% by mass or less, based on the mass of the first catalyst layer. The exhaust gas purification catalyst according to claim 1, wherein, under condition B, the metal equivalent content of Sr in the second catalyst layer is 0.3% by mass or more and 4.0% by mass or less, based on the mass of the second catalyst layer. [Claim 3] The composite oxide particles contain La, and the La in the composite oxide particles ２ O ３ The exhaust gas purification catalyst according to claim 1 or 2, wherein the converted content is 1.0% by mass or more and 15.0% by mass or less, based on the mass of the composite oxide particles. [Claim 4] If the exhaust gas purification catalyst satisfies condition A, then the exhaust gas purification catalyst satisfies the following condition C: (C) The ratio of the metal equivalent content (mass%) of Sr in the first catalyst layer, based on the mass of the first catalyst layer, to the content (mass%) of the composite oxide particles in the second catalyst layer, based on the mass of the second catalyst layer, is 0.0010 or more and 0.0800 or less. Satisfying the conditions, If the exhaust gas purification catalyst satisfies condition B, then the exhaust gas purification catalyst satisfies the following condition D: (D) The ratio of the metal equivalent content (mass%) of Sr in the second catalyst layer, based on the mass of the second catalyst layer, to the content (mass%) of the composite oxide particles in the second catalyst layer, based on the mass of the second catalyst layer, is 0.0010 or more and 0.0800 or less. A catalyst for purifying exhaust gas according to claim 1 or 2, which satisfies the requirements.

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