Exhaust gas purification catalyst and exhaust gas purification system

Abstract

The purpose of the present invention is to provide an exhaust gas purifying catalyst which has improved exhaust gas purification performance. In order to achieve this purpose, the present invention provides an exhaust gas purifying catalyst (1A) which is provided with a base material (10) and a catalyst layer (20A) that is provided on the base material (10), wherein: the content of aluminum element in terms of oxide and the content of barium element in a first layer (21) of the catalyst layer (20A) are respectively 15% by mass or more and 3% by mass or more based on the mass of the first layer (21); and the total content of zirconium element and rare earth elements in terms of oxides, the content of aluminum element in terms of oxide and the content of barium element in a second layer (22) of the catalyst layer (20A) are respectively 80% by mass or more, less than 15% by mass and less than 3% by mass based on the mass of the second layer (22).

Description

[Technical Field] 【0001】 This invention relates to an exhaust gas purification catalyst and an exhaust gas purification system. [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). As an exhaust gas purification catalyst that cleanses and neutralizes these harmful components, a three-way catalyst is used that has catalytic activity to oxidize HC to water and carbon dioxide, oxidize CO to carbon dioxide, and reduce NOx to nitrogen. 【0003】 In exhaust gas purification catalysts such as three-way catalysts, composite oxides containing alumina, zirconium (Zr), and rare earth elements (Ln) (hereinafter referred to as "Zr-Ln composite oxides") are used as carriers to support catalytic active components containing platinum group elements such as platinum (Pt), palladium (Pd), and rhodium (Rh), and barium (Ba) is used as a co-catalyst. For example, composite oxides containing zirconium (Zr) and cerium (Ce) (hereinafter referred to as "Zr-Ce composite oxides") are used as Zr-Ln composite oxides. Zr-Ce composite oxides have oxygen storage capacity (OSC), which is advantageous in that they mitigate fluctuations in oxygen concentration in exhaust gas and expand the operating window of the catalyst. 【0004】 Patent Document 1 describes an exhaust gas purification catalyst comprising a substrate and a catalyst layer provided on the substrate. The catalyst layer in Patent Document 1 comprises a lower layer containing Pt or Pd, a Zr-Ce composite oxide, alumina, and barium sulfate, and an upper layer containing Rh or Pt, a Zr-Ce composite oxide, and alumina. The content of the Zr-Ce composite oxide, alumina, and Ba in the lower layer is approximately 58% by mass, approximately 29% by mass, and approximately 7% by mass, respectively, based on the mass of the lower layer, and the content of the Zr-Ce composite oxide and alumina in the upper layer is approximately 49% by mass and approximately 49% by mass, respectively, based on the mass of the upper layer. [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Publication No. 2008-12410 [Overview of the Initiative] [Problems that the invention aims to solve] 【0006】 Further improvements in the exhaust gas purification performance of exhaust gas purification catalysts are required. 【0007】 Therefore, the present invention aims to provide an exhaust gas purification catalyst with improved exhaust gas purification performance and an exhaust gas purification system equipped with the exhaust gas purification catalyst. [Means for solving the problem] 【0008】 The present inventors have found that in a catalyst layer comprising a first layer containing a first platinum group element and a second layer containing a second platinum group element, the exhaust gas purification performance of the exhaust gas purification catalyst is improved by setting the content of aluminum element (in oxide terms) and barium element in the first layer to be above a predetermined value, the content of aluminum element (in oxide terms) and barium element in the second layer to be below a predetermined value, and the total content of zirconium element and rare earth element (in oxide terms) in the second layer to be above a predetermined value. 【0009】 This invention is a completed invention based on the above findings and includes the following inventions. 【0010】 [1] An exhaust gas purification catalyst comprising a substrate and a catalyst layer provided on the substrate, The catalyst layer comprises a first layer containing a first platinum group element, an oxide containing an aluminum element, and an element of barium, and a second layer containing a second platinum group element, a composite oxide containing an element of zirconium, and an element of rare earth. The content of aluminum element in terms of oxide and the content of barium element in the first layer are 15% by mass or more and 3% by mass or more, respectively, based on the mass of the first layer. An exhaust gas purification catalyst wherein the total content of zirconium and rare earth elements in terms of oxides, the content of aluminum in terms of oxides, and the content of barium in the second layer are 80% by mass or more, less than 15% by mass, and less than 3% by mass, respectively, based on the mass of the second layer. [2] An exhaust gas purification system for purifying exhaust gases emitted from an internal combustion engine, The exhaust gas purification system comprises an exhaust passage through which exhaust gas flows, a first exhaust gas purification catalyst provided on the upstream side of the exhaust passage, and a second exhaust gas purification catalyst provided on the downstream side of the exhaust passage. The exhaust gas purification system wherein the second exhaust gas purification catalyst is the exhaust gas purification catalyst described in [1] above. [Effects of the Invention] 【0011】 According to the present invention, an exhaust gas purification catalyst and an exhaust gas purification system equipped with the exhaust gas purification catalyst are provided, which have improved exhaust gas purification performance, particularly after exposure to a high-temperature environment. [Brief explanation of the drawing] 【0012】 [Figure 1] Figure 1 is a partial end view showing an exhaust gas purification catalyst according to the first embodiment of the present invention, arranged in the exhaust passage of an internal combustion engine. [Figure 2] Figure 2 is an end view taken along the line A-A of Figure 1. [Figure 3] Figure 3 is an enlarged view of the region indicated by the symbol R in Figure 2. [Figure 4] Figure 4 is an end view taken along the line B-B of Figure 1. [Figure 5] Figure 5 is an end view (the end view corresponding to Figure 4) of the exhaust gas purification catalyst according to the second embodiment of the present invention. [Figure 6] Figure 6 is an end view (the end view corresponding to Figure 4) of the exhaust gas purification catalyst according to the third embodiment of the present invention. [Figure 7] Figure 7 is an end view (the end view corresponding to Figure 4) of the exhaust gas purification catalyst according to the fourth embodiment of the present invention. [Figure 8] Figure 8 is a plan view of the exhaust gas purification system according to an embodiment of the present invention. 【Mode for Carrying Out the Invention】 【0013】 ≪Explanation of Terms≫ Hereinafter, the terms used in this specification will be explained. 【0014】 <BET Specific Surface Area> The BET specific surface area is measured according to the "one-point method" in "(3.5)" of "6.2 Flow method" in JIS R1626 "Method for Measuring Specific Surface Area of Fine Ceramic Powders by Gas Adsorption BET Method". As the gas, a nitrogen-helium mixed gas containing 30% by volume of nitrogen as the adsorption gas and 70% by volume of helium as the carrier gas is used. As the measuring device, "BELSORP-MR6" manufactured by Microtrac·BEL is used. 【0015】 <Platinum Group Element Group> The platinum group element group means a group of elements consisting of platinum element (Pt), palladium element (Pd), rhodium element (Rh), ruthenium element (Ru), osmium element (Os), and iridium element (Ir). 【0016】 <Rare Earth Elements> Examples of rare earth elements (Ln) include cerium (Ce), yttrium (Y), praseodymium (Pr), scandium (Sc), lanthanum (La), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). 【0017】 <Oxides> The oxides of aluminum (Al) are Al2O3, silicon (Si) are SiO2, zirconium (Zr) are ZrO2, chromium (Cr) are Cr2O3, boron (B) are B2O3, magnesium (Mg) are MgO, calcium (Ca) are CaO, strontium (Sr) are SrO, and barium (Ba) are BaO. The oxides of rare earth elements (Ln) are sesquioxides (Ln2O3), with the exception of the oxides of Ce, Pr, and Tb; the oxide of Ce is CeO2, and the oxide of Pr is Pr6O 11 The oxide of Tb is Tb4O7. 【0018】 <Oxides containing aluminum (Al-based oxides)> Al-based oxides are used as carriers for catalytically active components. Al-based oxides are distinguished from alumina (hereinafter referred to as "alumina binder"), which is used as a binder. Al-based oxides are, for example, particulate. From the viewpoint of improving the support capacity for catalytically active components, it is preferable that Al-based oxides are porous. The BET specific surface area of ​​Al-based oxides is preferably 50 m². 2 / g or more 250m 2 / g or less, more preferably 80m 2 / g or more 200m 2 It is less than / g. 【0019】 Al-based oxides may or may not contain elements other than Al and O. 【0020】 The Al-based oxide according to one embodiment (hereinafter referred to as "the first Al-based oxide") does not contain any elements other than Al and O. That is, the first Al-based oxide is alumina. 【0021】 An Al-based oxide according to another embodiment (hereinafter referred to as the "second Al-based oxide") contains one or more elements other than Al and O. Examples of the second Al-based oxide include oxides obtained by modifying the surface of alumina with elements other than Al and O, and oxides obtained by solid-solving elements other than Al and O in alumina. 【0022】 Elements other than Al and O may be nonmetallic or metallic elements. Examples of nonmetallic elements include B and Si, while examples of metallic elements include Zr, Cr, Ln (e.g., Ce, La, Nd), Mg, Ca, Sr, and Ba. From the viewpoint of improving the heat resistance of the second Al-based oxide, it is preferable to select elements other than Al and O from La, Ce, Sr, and Ba. 【0023】 Examples of the second type of Al oxide include alumina-silica, alumina-silicate, alumina-zirconia, alumina-chromia, alumina-celia, and alumina-lantana. 【0024】 In the second Al-based oxide, elements other than Al and O may form a solid solution phase with Al and O, or they may form a single phase that is either crystalline or amorphous (for example, an oxide phase of elements other than Al and O), or they may form both a solid solution phase and a single phase. 【0025】 From the viewpoint of improving heat resistance, the content of Al in the second Al-based oxide, in terms of oxide, is preferably 80% by mass or more and 99.9% by mass or less, more preferably 90% by mass or more and 99.8% by mass or less, and even more preferably 95% by mass or more and 99.5% by mass or less, based on the mass of the second Al-based oxide. 【0026】 From the perspective of improving heat resistance, the content of elements other than Al and O in terms of oxide conversion in the second Al-based oxide is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.2% by mass or more and 10% by mass or less, still more preferably 0.5% by mass or more and 5% by mass or less, based on the mass of the second Al-based oxide. The "content of elements other than Al and O in terms of oxide conversion" means the total content in terms of oxide conversion of two or more elements other than Al and O when the second Al-based oxide contains two or more elements other than Al and O. 【0027】 <Composite oxide containing zirconium element and rare earth element (Zr-Ln based composite oxide)> The Zr-Ln based composite oxide is used as a carrier for the catalytic active component. The Zr-Ln based composite oxide is, for example, particulate. From the perspective of improving the supportability of the catalytic active component, the Zr-Ln based composite oxide is preferably porous. The BET specific surface area of the Zr-Ln based oxide is preferably 20 m 2 / g or more and 120 m 2 / g or less, more preferably 30 m 2 / g or more and 80 m 2 / g or less. 【0028】 The Zr-Ln based composite oxide according to one embodiment (hereinafter referred to as "the first Zr-Ln based composite oxide") contains Zr and Ce. Zr mainly contributes to improving the heat resistance of the Zr-Ln based composite oxide, and Ce mainly contributes to improving the oxygen storage capacity of the Zr-Ln based composite oxide. From the perspective of improving heat resistance and oxygen storage capacity, the total content of Zr and Ce in terms of oxide conversion in the first Zr-Ln based composite oxide is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, based on the mass of the first Zr-Ln based composite oxide. The upper limit is 100% by mass. 【0029】 In the first Zr-Ln composite oxide, Zr may form a solid solution phase with Ce and O, or it may form a single phase that is either crystalline or amorphous (for example, a ZrO2 single phase), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Zr forms a solid solution phase. 【0030】 From the viewpoint of improving heat resistance, the content of Zr in the first Zr-Ln composite oxide, in terms of oxide, is preferably 20% by mass or more and 95% by mass or less, more preferably 30% by mass or more and 85% by mass or less, and even more preferably 40% by mass or more and 75% by mass or less, based on the mass of the first Zr-Ln composite oxide. 【0031】 In the first Zr-Ln composite oxide, Ce may form a solid solution phase with Zr and O, or it may form a single phase that is either crystalline or amorphous (for example, a CeO2 single phase), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Ce forms a solid solution phase. 【0032】 From the viewpoint of improving oxygen storage capacity and heat resistance, the content of Ce in the first Zr-Ln composite oxide, in terms of oxide, is preferably 5% by mass or more and 80% by mass or less, more preferably 10% by mass or more and 60% by mass or less, and even more preferably 15% by mass or more and 50% by mass or less, based on the mass of the first Zr-Ln composite oxide. 【0033】 From the viewpoint of balancing oxygen storage capacity and heat resistance, the ratio of the amount of Ce (in oxide form) to the total amount of Zr and Ce (in oxide form) in the first Zr-Ln composite oxide is preferably 0.001 to 0.9, more preferably 0.05 to 0.7, and even more preferably 0.1 to 0.6, in terms of mass ratio. 【0034】 The first Zr-Ln composite oxide may contain one or more Ln elements other than Ce, or it may not contain any Ln elements other than Ce. 【0035】 From the viewpoint of improving heat resistance, the Ln other than Ce in the first Zr-Ln composite oxide is preferably selected from La, Nd, Pr, Y, Gd, and Sm, and more preferably selected from La, Nd, Pr, and Y. 【0036】 When the first Zr-Ln composite oxide contains Ln other than Ce, the Ln other than Ce may form a solid solution phase with Zr and / or Ce and O, or form a standalone phase that is either crystalline or amorphous (for example, a standalone oxide phase of Ln other than Ce), or form both a solid solution phase and a standalone phase, but it is preferable that at least a portion of the Ln other than Ce forms a solid solution phase. 【0037】 When the first Zr-Ln composite oxide contains Ln other than Ce, from the viewpoint of improving heat resistance, the content of Ln other than Ce in the first Zr-Ln composite oxide in terms of oxides is preferably 1% by mass or more and 40% by mass or less, more preferably 3% by mass or more and 30% by mass or less, and even more preferably 5% by mass or more and 20% by mass or less, based on the mass of the first Zr-Ln composite oxide. "Content of Ln other than Ce in terms of oxides" means the total content of the two or more Ln other than Ce in terms of oxides when the first Zr-Ln composite oxide contains two or more Ln other than Ce. 【0038】 Another embodiment of the Zr-Ln composite oxide (hereinafter referred to as the "second Zr-Ln composite oxide") contains Zr and one or more Ln other than Ce, but does not contain Ce. 【0039】 In the second Zr-Ln composite oxide, Zr may form a solid solution phase with Ln and O other than Ce, or it may form a single phase that is either crystalline or amorphous (for example, a ZrO2 single phase), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Zr forms a solid solution phase. 【0040】 From the viewpoint of improving heat resistance, the content of Zr in the second Zr-Ln composite oxide, in terms of oxide, is preferably 60% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 99% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less, based on the mass of the second Zr-Ln composite oxide. 【0041】 From the viewpoint of improving heat resistance, the Ln other than Ce in the second Zr-Ln composite oxide is preferably selected from Y, La, Nd, and Pr, and more preferably selected from Y, La, and Nd. 【0042】 In the second Zr-Ln composite oxide, Ln other than Ce may form a solid solution phase with Zr and O, or it may form a single phase that is either crystalline or amorphous (for example, a single oxide phase of Ln other than Ce), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Ln other than Ce forms a solid solution phase. 【0043】 From the viewpoint of improving heat resistance, the content of Ln other than Ce in the second Zr-Ln composite oxide, in terms of oxides, is preferably 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 25% by mass or less, and even more preferably 5% by mass or more and 20% by mass or less, based on the mass of the second Zr-Ln composite oxide. "Content of Ln other than Ce, in terms of oxides" means the total content of the two or more Ln other than Ce, in terms of oxides, if the second Zr-Ln composite oxide contains two or more Ln other than Ce. 【0044】 The first or second Zr-Ln composite oxide may contain one or more alkali metal elements. From the viewpoint of improving the cocatalytic effect on platinum group elements, the alkaline earth metal element is preferably selected from Ca, Sr, and Ba. 【0045】 From the viewpoint of improving the cocatalytic effect on platinum group elements, the oxide content of alkaline earth metal elements in the first or second Zr-Ln composite oxide is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.2% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 3% by mass or less, based on the mass of the first or second Zr-Ln composite oxide. If the first or second Zr-Ln composite oxide contains two or more alkaline earth metal elements, "oxide content of alkaline earth metal elements" means the total oxide content of those two or more alkaline earth metal elements. 【0046】 <Binder> Examples of binders include inorganic binders such as alumina, zirconia, titania, silica, and ceria. 【0047】 <Aluminum (Al) oxide content> When calculating the content of Al in terms of oxides in a particular layer, the Al source is not particularly limited as long as it is an Al-containing oxide. Examples of Al sources include Al-based oxides and alumina binders. 【0048】 The content of Al in oxide terms in a given layer means the content of Al in oxide terms derived from a single Al source if the layer contains one Al source, and the total content of Al in oxide terms derived from two or more Al sources if the layer contains two or more Al sources. 【0049】 The content of Al in terms of oxides in a given layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0050】 <Barium (Ba) content> When calculating the Ba content in a particular layer, the Ba source is not particularly limited as long as it contains Ba. Examples of Ba sources include barium carbonate, barium oxide, barium aluminate, barium zirconate, Al-based oxides containing Ba, and Zr-Ln-based composite oxides containing Ba. 【0051】 The Ba content in a given layer refers to the Ba content derived from a single Ba source if the layer contains one Ba source, or the total Ba content derived from two or more Ba sources if the layer contains two or more Ba sources. The Ba content refers to the Ba content in metallic terms. 【0052】 The Ba content in a particular layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0053】 <Zirconium (Zr) content in terms of oxide> When calculating the oxide content of Zr in a given layer, the Zr source is not particularly limited as long as it is an oxide containing Zr. Examples of Zr sources include Zr-Ln composite oxides, Zr-containing Al oxides, and zirconia binders. 【0054】 The content of Zr in oxide terms in a given layer means the content of Zr in oxide terms derived from a single Zr source if the layer contains one Zr source, and the total content of Zr in oxide terms derived from two or more Zr sources if the layer contains two or more Zr sources. 【0055】 The oxide content of Zr in a given layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0056】 <Oxide content of rare earth elements (Ln)> When calculating the oxide content of linn in a given layer, the linn source is not particularly limited as long as it is an oxide containing linn. Examples of linn sources include Zr-linn composite oxides, Al oxides containing linn, and binders containing linn (e.g., ceria binders). The linn source does not need to be a different oxide from the Zr source; it may be the same oxide as the Zr source. For example, a Zr-linn composite oxide is both a linn source and a Zr source. Similarly, an Al oxide containing both Zr and linn is both a linn source and a Zr source. 【0057】 The oxide content of Ln in a given layer means the oxide content of Ln derived from a single Ln source if the layer contains one Ln source, and the total oxide content of Ln derived from two or more Ln sources if the layer contains two or more Ln sources. 【0058】 The oxide content of Ln in a given layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0059】 <Content of cerium element (Ce) in terms of oxide> When calculating the content of Ce in terms of oxides in a certain layer, the Ce source is not particularly limited as long as it is an oxide containing Ce. Examples of Ce sources include Ce-containing Zr-Ln composite oxides, Ce-containing Al oxides, and ceria binders. The Ce source does not need to be a different oxide from the Zr source; it may be the same oxide as the Zr source. For example, a Ce-containing Zr-Ln composite oxide is both a Ce source and a Zr source. Similarly, an Al oxide containing both Zr and Ce is both a Ce source and a Zr source. 【0060】 The content of Ce in terms of oxides in a given layer means the content of Ce derived from one Ce source if the layer contains one Ce source, and the total content of Ce derived from two or more Ce sources if the layer contains two or more Ce sources. 【0061】 The content of Ce in terms of oxides in a particular layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0062】 <Oxide content of rare earth elements (Ln) other than cerium (Ce)> When calculating the oxide content of luminescence other than Ce in a given layer, the source of luminescence other than Ce is not particularly limited as long as it is an oxide containing luminescence other than Ce. Examples of sources of luminescence other than Ce include Zr-Ln composite oxides containing luminescence other than Ce, and Al-based oxides containing luminescence other than Ce. Furthermore, the source of luminescence other than Ce does not need to be a different oxide from the Zr source; it may be the same oxide as the Zr source. For example, a Zr-Ln composite oxide containing luminescence other than Ce is both a source of luminescence other than Ce and a Zr source. Similarly, an Al-based oxide containing Zr and luminescence other than Ce is both a source of luminescence other than Ce and a Zr source. 【0063】 The oxide content of non-Ce Ln in a given layer means, if the layer contains one non-Ce Ln source, the oxide content of non-Ce Ln derived from that one non-Ce Ln source; if the layer contains two or more non-Ce Ln sources, it means the total oxide content of non-Ce Ln derived from those two or more non-Ce Ln sources. 【0064】 The content of Ln other than Ce in a given layer can be measured, for example, using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX). 【0065】 ≪Exhaust gas purification catalyst≫ The exhaust gas purification catalyst of the present invention will be described below. 【0066】 <First Embodiment> The exhaust gas purification catalyst 1A according to the first embodiment of the present invention will be described below with reference to Figures 1 to 4. 【0067】 As shown in Figure 1, the exhaust gas purification catalyst 1A is located in the exhaust passage within the exhaust pipe P of the internal combustion engine. The internal combustion engine is, for example, a gasoline engine. The internal combustion engine may also be a lean-burn engine. The 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 the exhaust gas purification catalyst 1A installed in the exhaust pipe P. In the drawing, the direction of exhaust gas flow is indicated by the symbol X. The upstream side of the exhaust gas flow direction X is sometimes called the "exhaust gas inlet side," and the downstream side of the exhaust gas flow direction X is sometimes called the "exhaust gas outlet side." 【0068】 As shown in Figures 2-4, the exhaust gas purification catalyst 1A comprises a base material 10 and a catalyst layer 20A provided on the base material 10. 【0069】 The material constituting the base material 10 can be appropriately selected from materials commonly used as base materials for exhaust gas purification catalysts. Preferably, the material constituting the base material 10 is one that allows the base material 10 to maintain a stable shape even when the base material 10 is exposed to exhaust gas at, for example, 400°C or higher. Examples of materials for the base material 10 include ceramics such as cordierite, silicon carbide, and aluminum titanate, and alloys such as stainless steel. 【0070】 The base material 10 is, for example, a honeycomb structure. 【0071】 As shown in Figures 2-4, the base material 10 has a cylindrical portion 11 that defines the outer shape of the base material 10, a partition wall portion 12 provided inside the cylindrical portion 11, and cells 13 separated by the partition wall portion 12. 【0072】 As shown in Figure 2, the shape of the cylindrical portion 11 is, for example, cylindrical, but it may also be other shapes such as elliptical or polygonal. 【0073】 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 is preferably porous. The thickness of the partition wall 12 is, for example, 20 μm or more and 1500 μm or less. 【0074】 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. 【0075】 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. 【0076】 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. 【0077】 The cell density per square inch of the substrate 10 is, for example, between 200 and 1000 cells. The cell density per square inch of the substrate 10 is 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. 【0078】 The volume of the base material 10 is, for example, between 0.1 L and 20 L. The volume of the base material 10 refers to the apparent volume of the base material 10. If the base material 10 is cylindrical, and its outer diameter is 2r and its length is L, then the volume of the base material 10 is given by the formula: Volume of base material 10 = π × r 2 It is represented by ×L. 【0079】 As shown in Figure 4, the catalyst layer 20A is provided on the partition wall portion 12 of the substrate 10. The catalyst layer 20A may be provided directly on the partition wall portion 12, or it may be provided on the partition wall portion 12 via another layer. 【0080】 As shown in Figure 4, the catalyst layer 20A 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 catalyst layer 20A 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. 【0081】 From the viewpoint of balancing the catalyst's heating ability and purification performance, the mass of the catalyst layer 20A per unit volume of the substrate 10 (mass after drying and calcination) is preferably 50 g / L or more and 500 g / L or less, more preferably 70 g / L or more and 400 g / L or less, and even more preferably 90 g / L or more and 300 g / L or less. 【0082】 As shown in Figure 4, the catalyst layer 20A comprises a first layer 21 and a second layer 22. 【0083】 <First layer> The first layer, 21, will be explained below. 【0084】 The first layer 21 contains a first platinum group element, an oxide containing aluminum (Al-based oxide), and barium (Ba). 【0085】 The first platinum group element contained in the first layer 21 consists of one or more elements selected from the platinum group elements. 【0086】 The first platinum group element is included in the first layer 21 in a form that can function as a catalytically active component, such as a metal, alloy, or compound (e.g., oxide). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the first platinum group element is preferably in particulate form. 【0087】 The Ba contained in the first layer 21 acts as a co-catalyst for Pd, improving exhaust gas purification performance. Therefore, it is preferable that the first platinum group element contains Pd. When the first layer 21 contains Pd, exhaust gas purification performance at low temperatures is particularly improved. This effect is especially pronounced when the first layer 21 contains Pd and the second layer 22 contains Rh. In this specification, "low temperature" usually means 100°C to 500°C, preferably 150°C to 400°C. 【0088】 When the first platinum group element includes Pd, it is preferable that the total molar content of platinum group elements other than Pd in ​​the first layer 21 is less than the molar content of Pd, from the viewpoint of preventing deactivation of Pd due to alloying with other platinum group elements. In the first layer 21, it is preferable that the ratio of the total molar amount of platinum group elements other than Pd to the molar amount of Pd is 0.2 or less. The lower limit is zero. 【0089】 The catalytically active component containing the first platinum group element is preferably supported on an Al-based oxide. "Supported" means that the catalytically active component containing the first platinum group element is physically or chemically adsorbed or retained on the outer surface or inner surface of the pores of the Al-based oxide. The fact that the catalytically active component containing the first platinum group element is supported on an Al-based oxide can be confirmed, for example, using SEM-EDX. 【0090】 From the viewpoint of balancing exhaust gas purification performance and cost, the content of the first platinum group element in the first layer 21 is preferably 0.01% by mass or more and 15% by mass or less, more preferably 0.05% by mass or more and 10% by mass or less, and even more preferably 0.1% by mass or more and 7.5% by mass or less, based on the mass of the first layer 21. If the first platinum group element is composed of two or more elements, the content of the first platinum group element refers to the total content of those two or more elements. The mass of the first platinum group element is the mass in terms of metal equivalent. 【0091】 The Al-based oxide contained in the first layer 21 can be selected from the first and second Al-based oxides. The first layer 21 may contain one or both of the first and second Al-based oxides. 【0092】 From the viewpoint of ensuring the specific surface area of ​​the catalyst, the content of Al in terms of oxides in the first layer 21 is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, based on the mass of the first layer 21. Furthermore, from the viewpoint of ensuring oxygen storage capacity, the content of Al in terms of oxides in the first layer 21 is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less, based on the mass of the first layer 21. These upper limits may be combined with any of the lower limits above. The proportion of the mass of Al in terms of oxides in the first layer 21 that is derived from Al-based oxides is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. The upper limit of this proportion is 100% by mass. 【0093】 Examples of Ba sources included in the first layer 21 include barium carbonate, barium oxide, barium aluminate, and barium zirconate. The Ba source may also be an Al-based oxide containing Ba, a Zr-Ln-based composite oxide containing Ba, and the like. 【0094】 From the viewpoint of enhancing the co-catalyst effect on Pd, the Ba content in the first layer 21 is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, based on the mass of the first layer 21. Furthermore, from the viewpoint of suppressing the decrease in heat resistance due to the reaction of Ba with Al-based oxides or Zr-Ln-based oxides, the Ba content in the first layer 21 is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, based on the mass of the first layer 21. Each of these upper limits may be combined with any of the lower limits above. The mass of Ba is the mass in terms of metal (same throughout this specification). 【0095】 When the first platinum group element contains Pd and the second platinum group element contains Rh, the Ba content in the first layer 21 is preferably 8% by mass or more, more preferably 10% by mass or more, and even more preferably 12% by mass or more, based on the mass of the first layer 21. The upper limit is the same as above. Each of the above upper limits may be combined with any of the above lower limits. Ba absorbs NOx, while Rh has NOx purification properties. Therefore, NOx absorbed by Ba is purified by Rh. This effect is particularly pronounced when the Ba content in the first layer 21 is within the above range. 【0096】 The first layer 21 may contain one or more Zr-Ln composite oxides, or it may not contain any Zr-Ln composite oxides. If the first layer 21 contains a Zr-Ln composite oxide, the catalytically active component containing the first platinum group element may be supported on the Zr-Ln composite oxide. The significance of support and the method for confirming that support is present are the same as described above. 【0097】 The Zr-Ln composite oxide contained in the first layer 21 can be selected from the first and second Zr-Ln composite oxides. The first layer 21 may contain one of the first and second Zr-Ln composite oxides, or it may contain both. 【0098】 When the first layer 21 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the total content of Zr and Ln in the first layer 21 in terms of oxides is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 75% by mass or less, and even more preferably 30% by mass or more and 65% by mass or less, based on the mass of the first layer 21. The proportion of the total mass of Zr and Ln in terms of oxides in the first layer 21 that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0099】 When the first layer 21 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the content of Zr in the first layer 21 in terms of oxide is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 15% by mass or more and 30% by mass or less, based on the mass of the first layer 21. The proportion of the mass of Zr in the first layer 21 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0100】 When the first layer 21 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the content of Ln in the first layer 21 in terms of oxide is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% 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 first layer 21. The proportion of the mass of Ln in the first layer 21 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0101】 When the first layer 21 contains a Zr-Ln composite oxide containing Ce, from the viewpoint of improving oxygen storage capacity, the content of Ce in the first layer 21 in terms of oxide is preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 15% by mass or more and 30% by mass or less, based on the mass of the first layer 21. The proportion of the mass of Ce in the first layer 21 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0102】 When the first layer 21 contains a Zr-Ln composite oxide containing Ln other than Ce, from the viewpoint of improving heat resistance, the content of Ln other than Ce in the first layer 21 in terms of oxide is preferably 1% by mass or more and 15% by mass or less, more preferably 1.5% by mass or more and 10% by mass or less, and even more preferably 2.0% by mass or more and 7.5% by mass or less, based on the mass of the first layer 21. The proportion of the mass of Ln other than Ce in the first layer 21 in terms of oxide is that which is derived from the Zr-Ln composite oxide, preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0103】 As shown in Figure 4, the first layer 21 is located between the base material 10 and the second layer 22. That is, the first layer 21 is provided below the second layer 22. The expression "the first layer 21 is provided below the second layer 22" means that part or all of the second layer 22 is located on the main surface of the first layer 21 that is opposite to the main surface of the base material 10 that faces the partition wall portion 12. "Main surface of the first layer 21" means the outer surface of the first layer 21 that extends in the exhaust gas flow direction X. The second layer 22 may be provided directly on the main surface of the first layer 21 or via other layers. The first layer 21 may be provided directly on the partition wall portion 12 or via other layers. 【0104】 The first layer 21 may be provided on the upper side of the second layer 22. The expression "the first layer 21 is provided on the upper side of the second layer 22" means that part or all of the first layer 21 is located on the main surface of the second layer 22 that is opposite to the main surface on the partition wall portion 12 side of the base material 10. "Main surface of the second layer 22" means the outer surface of the second layer 22 that extends in the exhaust gas flow direction X. The first layer 21 may be provided directly on the main surface of the second layer 22, or it may be provided via another layer. 【0105】 However, if the first layer 21 is located above the second layer 22, the Ba contained in the first layer 21 may dissolve and mix into the second layer 22, potentially promoting the sintering of the Zr-Ln composite oxide contained in the second layer 22. Therefore, it is preferable that the first layer 21 be located below the second layer 22. This prevents the Ba contained in the first layer 21 from dissolving and mixing into the second layer 22, thereby promoting the sintering of the Zr-Ln composite oxide contained in the second layer 22. As a result, the oxygen absorption and release performance of the Zr-Ln composite oxide is improved, the dispersibility of the second platinum group element in the second layer 22 is improved, and consequently, the exhaust gas purification performance is improved. In particular, the exhaust gas purification performance after exposure to a high-temperature environment is improved. In this specification, "high temperature" means a temperature exceeding low temperature. 【0106】 <Second Layer> The second layer, 22, will be explained below. 【0107】 The second layer 22 contains a second platinum group element and a composite oxide (Zr-Ln composite oxide) containing zirconium and rare earth elements. 【0108】 The second platinum group element contained in the second layer 22 consists of one or more elements selected from the platinum group element group. 【0109】 The second platinum group element is included in the second layer 22 in a form that can function as a catalytically active component, such as a metal, alloy, or compound (e.g., oxide). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the second platinum group element is preferably in particulate form. 【0110】 Since the content of Al in oxide terms in the second layer 22 is less than 15% by mass, Rh is easily reduced. Also, although the content of Ba in the second layer 22 is less than 3% by mass, Rh exhibits exhaust gas purification performance without the presence of Ba. Therefore, it is preferable that the second platinum group element contains Rh. When the second layer 22 contains Rh, the exhaust gas purification performance is particularly improved at low temperatures. This effect is especially pronounced when the first layer 21 contains Pd and the second layer 22 contains Rh. 【0111】 When the second platinum group element contains Rh, it is preferable that the total molar content of platinum group elements other than Rh in the second layer 22 is less than the molar content of Rh, from the viewpoint of preventing the deactivation of Rh due to alloying with platinum group elements other than Rh. In the second layer 22, it is preferable that the ratio of the total molar amount of platinum group elements other than Rh to the molar amount of Rh is 0.2 or less. The lower limit is zero. 【0112】 The catalytically active component containing the second platinum group element is preferably supported on a Zr-Ln composite oxide. The significance of support and the method for confirming that it is supported are the same as described above. 【0113】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the content of the second platinum group element in the second layer 22 is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 7.5% by mass or less, and even more preferably 0.1% by mass or more and 5.0% by mass or less, based on the mass of the second layer 22. If the second platinum group element is composed of two or more elements, the content of the second platinum group element refers to the total content of those two or more elements. The mass of the second platinum group element is the mass in terms of metal equivalent. 【0114】 The second layer 22 may contain one type of Zr-Ln composite oxide, or it may contain two or more types of Zr-Ln composite oxides. The second layer 22 may contain one of the first Zr-Ln composite oxide and the second Zr-Ln composite oxide, or it may contain both. 【0115】 From the viewpoint of improving heat resistance, the total content of Zr and Ln in oxide form in the second layer 22 is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the mass of the second layer 22. Furthermore, from the viewpoint of ensuring the amount of other components such as platinum group elements and co-catalyst components, the total content of Zr and Ln in oxide form in the second layer is preferably 99.9% by mass or less, more preferably 99.8% by mass or less, and even more preferably 99.7% by mass or less, based on the mass of the second layer. These upper limits may be combined with any of the lower limits above. The proportion of the total mass of Zr and Ln in oxide form in the second layer 22 that is derived from Zr-Ln composite oxides is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0116】 From the viewpoint of improving heat resistance, the content of Zr in the second layer 22 in terms of oxide is preferably 30% by mass or more and 95% by mass or less, more preferably 40% by mass or more and 90% by mass or less, and even more preferably 50% by mass or more and 85% by mass or less, based on the mass of the second layer 22. The proportion of the mass of Zr in terms of oxide in the second layer 22 that is derived from Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0117】 When the second layer 22 contains a Zr-Ln composite oxide containing Ce, from the viewpoint of improving oxygen storage capacity, the content of Ce in the second layer 22 in terms of oxide is preferably 1% by mass or more and 50% by mass or less, more preferably 3% by mass or more and 30% by mass or less, and even more preferably 6% by mass or more and 20% by mass or less, based on the mass of the second layer 22. The proportion of the mass of Ce in the second layer 22 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0118】 When the second layer 22 contains a Zr-Ln composite oxide containing Ln other than Ce, from the viewpoint of improving heat resistance, the content of Ln other than Ce in the second layer 22 in terms of oxide is preferably 1% by mass or more and 30% by mass or less, more preferably 2% by mass or more and 20% by mass or less, and even more preferably 3% by mass or more and 15% by mass or less, based on the mass of the second layer 22. The proportion of the mass of Ln other than Ce in the second layer 22 in terms of oxide is that which is derived from the Zr-Ln composite oxide, preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0119】 The second layer 22 may contain an Al-based oxide and / or Ba. If the second layer 22 contains an Al-based oxide, the catalytically active component containing the second platinum group element may be supported on the Al-based oxide. The significance of support and the method for confirming that support exists are the same as described above. 【0120】 The Al-based oxide contained in the second layer 22 can be selected from the first and second Al-based oxides. The second layer 22 may contain one or both of the first and second Al-based oxides. 【0121】 To prevent the acceleration of sintering of Zr-Ln composite oxides by Al, improve the oxygen absorption and release performance of Zr-Ln composite oxides, improve the dispersibility of the second platinum group element in the second layer 22, and consequently improve exhaust gas purification performance (especially exhaust gas purification performance after exposure to a high-temperature environment), the content of Al in terms of oxides in the second layer 22 is preferably less than 15% by mass, more preferably 5% by mass or less, and even more preferably 1% by mass or less, based on the mass of the second layer 22. The lower limit of the content of Al in terms of oxides in the second layer 22 is zero. The content of Al in terms of oxides in the second layer 22 may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 0.5% by mass or more, based on the mass of the second layer 22. Each of these lower limits may be combined with any of the upper limits above. 【0122】 Examples of Ba sources include barium carbonate, barium oxide, barium nitrate, barium aluminate, and barium zirconate. The Ba source may also be an Al-based oxide containing Ba, a Zr-Ln-based composite oxide containing Ba, etc. 【0123】 To prevent the acceleration of sintering of Zr-Ln composite oxides by Ba, improve the oxygen absorption and release performance of Zr-Ln composite oxides, improve the dispersibility of the second platinum group element in the second layer 22, and consequently improve exhaust gas purification performance (especially exhaust gas purification performance after exposure to a high-temperature environment), the Ba content in the second layer 22 is preferably less than 3% by mass, more preferably 1.5% by mass or less, and even more preferably 0.5% by mass or less, based on the mass of the second layer 22. The lower limit of the Ba content in the second layer 22 is zero. The Ba content in the second layer 22 may be, for example, 0.01% by mass or more, 0.05% by mass or more, or 0.1% by mass or more, based on the mass of the second layer 22. Each of these lower limits may be combined with any of the upper limits mentioned above. 【0124】 As shown in Figure 4, the second layer 22 is the outermost layer of the catalyst layer 20A. The "outermost layer" refers to the layer that forms the main surface of the catalyst layer 20A opposite to the main surface on the side of the partition wall 12 of the substrate 10. 【0125】 Another layer may be provided above the second layer 22 (in which case the other layer would be the outermost layer), but from the viewpoint of improving the contact efficiency between the platinum group elements contained in the second layer 22 and the exhaust gas, and improving the exhaust gas purification performance, it is preferable that the second layer 22 be the outermost layer. 【0126】 <Method for forming a catalyst layer> The method for forming the catalyst layer 20A will be described below. Prepare the base material 10, a slurry for forming the first layer 21, and a slurry for forming the second layer 22. 【0127】 The composition of the slurry for forming the first layer 21 and the second layer 22 is adjusted according to the composition of the first layer 21 and the second layer 22, respectively. The slurry includes, for example, a source of platinum group elements, a source of Ba, Al-based oxides, Zr-Ln-based complex oxides, a binder, a solvent, etc. Examples of sources of noble metal elements include salts of noble metal elements, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of sources of Ba include barium carbonate, barium nitrate, and barium acetate. Examples of binders include alumina sol, zirconia sol, titania sol, silica sol, and ceria sol. Examples of solvents include water and organic solvents. 【0128】 A catalyst layer 20A can be formed by applying a slurry for forming the first layer 21 to the substrate 10, drying it, firing it, and then applying a slurry for forming the second layer 22 to the substrate 10, drying it, and firing it. The slurry can be applied, for example, by immersing the entire substrate 10 in the slurry, or by immersing the exhaust gas inlet or exhaust gas outlet end of the substrate 10 in the slurry and sucking the slurry from the opposite side. The drying temperature is, for example, 50°C to 200°C, the drying time is, for example, 0.1 hours to 12 hours, the firing temperature is, for example, 400°C to 700°C, and the firing time is, for example, 0.5 hours to 8 hours. Firing can be carried out, for example, in an atmospheric environment. 【0129】 <Second Embodiment> Hereinafter, an exhaust gas purification catalyst 1B according to a second embodiment of the present invention will be described with reference to Figure 5. In the exhaust gas purification catalyst 1B, the same components and parts as those of the exhaust gas purification catalyst 1A are indicated by the same reference numerals as those of the exhaust gas purification catalyst 1A. Unless otherwise specified, the above description relating to the exhaust gas purification catalyst 1A also applies to the exhaust gas purification catalyst 1B. 【0130】 As shown in Figure 5, the exhaust gas purification catalyst 1B differs from the exhaust gas purification catalyst 1A in that it includes a catalyst layer 20B. 【0131】 As shown in Figure 5, catalyst layer 20B differs from catalyst layer 20A in that it includes a third layer 23. Unless otherwise specified, the above description relating to catalyst layer 20A also applies to catalyst layer 20B. 【0132】 From the viewpoint of fully exhibiting oxygen absorption and release performance, it is preferable that the third layer 23 be located between the first layer 21 and the second layer 22, as shown in Figure 5. 【0133】 The third layer 23 contains a third platinum group element. 【0134】 The third platinum group element consists of one or more elements selected from the platinum group elements. 【0135】 The third platinum group element is included in the third layer 23 in a form that can function as a catalytically active component, such as a metal, alloy, or compound (e.g., oxide). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the third platinum group element is preferably in particulate form. 【0136】 From the viewpoint of fully exhibiting oxygen absorption and release performance, it is preferable that the third platinum group element includes Pd. 【0137】 When the third platinum group element includes Pd, it is preferable that the total molar content of platinum group elements other than Pd in ​​the third layer 23 is less than the molar content of Pd, from the viewpoint of preventing deactivation of Pd due to alloying with other platinum group elements. In the third layer 23, it is preferable that the ratio of the total molar amount of platinum group elements other than Pd to the molar amount of Pd is 0.2 or less. The lower limit is zero. 【0138】 The third platinum group element is preferably supported on an Al-based oxide and / or a Zr-Ln-based composite oxide. The significance of support and the method for confirming support are the same as described above. 【0139】 From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the content of the third platinum group element in the third layer 23 is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 7.5% by mass or less, and even more preferably 0.1% by mass or more and 5.0% by mass or less, based on the mass of the third layer 23. If the third platinum group element is composed of two or more elements, the content of the third platinum group element means the total content of those two or more elements. The mass of the third platinum group element is the mass in terms of metal equivalent. 【0140】 The third layer 23 preferably contains a Zr-Ln composite oxide. The Zr-Ln composite oxide contained in the third layer 23 can be selected from the first and second Zr-Ln composite oxides. The third layer 23 may contain one or both of the first and second Zr-Ln composite oxides. 【0141】 When the third layer 23 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the total content of Zr and Ln in the third layer 23 in terms of oxides is preferably 80% by mass or more and 99.9% by mass or less, more preferably 90% by mass or more and 99.7% by mass or less, and even more preferably 95% by mass or more and 99.5% by mass or less, based on the mass of the third layer 23. The proportion of the total mass of Zr and Ln in terms of oxides in the third layer 23 that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0142】 When the third layer 23 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the content of Zr in the third layer 23 in terms of oxide is preferably 20% by mass or more and 90% by mass or less, more preferably 30% by mass or more and 80% by mass or less, and even more preferably 35% by mass or more and 70% by mass or less, based on the mass of the third layer 23. The proportion of the mass of Zr in terms of oxide in the third layer 23 that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0143】 When the third layer 23 contains a Zr-Ln composite oxide, from the viewpoint of improving heat resistance, the content of Ln in the third layer 23 in terms of oxide is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 70% by mass or less, and even more preferably 30% by mass or more and 60% by mass or less, based on the mass of the third layer 23. The proportion of the mass of Ln in the third layer 23 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0144】 When the third layer 23 contains a Zr-Ln composite oxide containing Ce, from the viewpoint of improving oxygen storage capacity, the content of Ce in the third layer 23 in terms of oxide is preferably 5% by mass or more and 80% by mass or less, more preferably 10% by mass or more and 70% by mass or less, and even more preferably 20% by mass or more and 60% by mass or less, based on the mass of the third layer 23. The proportion of the mass of Ce in the third layer 23 in terms of oxide that is derived from the Zr-Ln composite oxide is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0145】 When the third layer 23 contains a Zr-Ln composite oxide containing Ln other than Ce, from the viewpoint of improving heat resistance, the content of Ln other than Ce in the third layer 23 in terms of oxide is preferably 1.0% by mass or more and 40% by mass or less, more preferably 2.5% by mass or more and 30% by mass or less, and even more preferably 4.0% by mass or more and 20% by mass or less, based on the mass of the third layer 23. The proportion of the mass of Ln other than Ce in the third layer 23 in terms of oxide is that which is derived from the Zr-Ln composite oxide, preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of this proportion is 100% by mass. 【0146】 The third layer 23 may contain an Al-based oxide. 【0147】 The Al-based oxide contained in the third layer 23 can be selected from the first and second Al-based oxides. The third layer 23 may contain one or both of the first and second Al-based oxides. 【0148】 To prevent the acceleration of sintering of Zr-Ln composite oxides by Al, improve the oxygen absorption and release performance of Zr-Ln composite oxides, improve the dispersibility of the third platinum group element in the third layer 23, and consequently improve exhaust gas purification performance (especially exhaust gas purification performance after exposure to a high-temperature environment), the content of Al in terms of oxides in the third layer 23 is preferably less than 15% by mass, more preferably 5% by mass or less, and even more preferably 1% by mass or less, based on the mass of the third layer 23. The lower limit of the content of Al in terms of oxides in the third layer 23 is zero. The content of Al in terms of oxides in the third layer 23 may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 0.5% by mass or more, based on the mass of the third layer 23. Each of these lower limits may be combined with any of the upper limits above. 【0149】 The third layer 23 may contain Ba. Ba can be included in the third layer 23 in the form of, for example, barium carbonate, barium oxide, barium nitrate, barium aluminate, barium zirconate, etc. The Ba source included in the third layer 23 may be an Al-based oxide containing Ba, a Zr-Ln-based composite oxide containing Ba, etc. 【0150】 To prevent the acceleration of sintering of Zr-Ln composite oxides by Ba, improve the oxygen absorption and release performance of Zr-Ln composite oxides, improve the dispersibility of the third platinum group element in the third layer 23, and consequently improve exhaust gas purification performance (especially exhaust gas purification performance after exposure to a high-temperature environment), the Ba content in the third layer 23 is preferably 8% by mass or less, more preferably 6% by mass or less, even more preferably less than 3% by mass, even more preferably 1% by mass or less, and even more preferably 0.5% by mass or less, based on the mass of the third layer 23. The lower limit of the Ba content in the third layer 23 is zero. The Ba content in the third layer 23 may be, for example, 0.01% by mass or more, 0.05% by mass or more, or 0.1% by mass or more, based on the mass of the third layer 23. Each of these lower limits may be combined with any of the upper limits above. 【0151】 The following describes the method for forming the catalyst layer 20B. Unless otherwise specified, the above description of the method for forming the catalyst layer 20A also applies to the method for forming the catalyst layer 20B. 【0152】 Prepare the base material 10, a slurry for forming the first layer 21, a slurry for forming the second layer 22, and a slurry for forming the third layer 23. 【0153】 The composition of the slurry for forming the first layer 21, the second layer 22, and the third layer 23 is adjusted according to the composition of the first layer 21, the second layer 22, and the third layer 23, respectively. 【0154】 A catalyst layer 20B can be formed by applying a slurry to the substrate 10 to form the first layer 21, drying and firing it, then applying a slurry to the substrate 10 to form the third layer 23, drying and firing it, and then applying a slurry to the substrate 10 to form the second layer 22, drying and firing it. 【0155】 <Third Embodiment> Hereinafter, an exhaust gas purification catalyst 1C according to a third embodiment of the present invention will be described with reference to Figure 6. In the exhaust gas purification catalyst 1C, the same components and parts as those of the exhaust gas purification catalyst 1A are indicated by the same reference numerals as those of the exhaust gas purification catalyst 1A. Unless otherwise specified, the above description relating to the exhaust gas purification catalyst 1A also applies to the exhaust gas purification catalyst 1C. 【0156】 As shown in Figure 6, the exhaust gas purification catalyst 1C is The base material 10 is provided with a first sealing portion 14 that seals the exhaust gas outlet end of some of the cells 13, and a second sealing portion 15 that seals the exhaust gas inlet end of the remaining cells 13. As a result, some of the cells 13 are inlet-side cells 13a, where the exhaust gas inlet end is open and the exhaust gas outlet end is closed by the first sealing portion 14, while the remaining cells 13 are outlet-side cells 13b, where the exhaust gas inlet end is closed by the second sealing portion 15 and the exhaust gas outlet end is open. A catalyst layer 30 is provided on the inlet side cell 13a side of the partition wall portion 12 of the base material 10, and a catalyst layer 20A is provided on the outlet side cell 13b side of the partition wall portion 12 of the base material 10. Therefore, it differs from exhaust gas purification catalyst 1A. 【0157】 As shown in Figure 6, multiple (e.g., four) outlet cells 13b are arranged adjacent to one inlet cell 13a, and the inlet cell 13a and the outlet cells 13b adjacent to it are separated by a porous partition 12. 【0158】 As shown in Figure 6, the catalyst layer 30 extends from the exhaust gas inlet side end of the partition wall 12 along the exhaust gas flow direction X, and the catalyst layer 20A extends from the exhaust gas outlet side end of the partition wall 12 along the direction opposite to the exhaust gas flow direction X. That is, the catalyst layer 30 is located upstream of the catalyst layer 20A. 【0159】 In the exhaust gas purification catalyst 1C, exhaust gas flows in from the exhaust gas inlet end (opening) of the inlet cell 13a, passes through the porous partition wall 12, and flows out from the exhaust gas outlet end (opening) of the outlet cell 13b. This type of configuration is called a wall-flow type. 【0160】 In the exhaust gas purification catalyst 1C, when exhaust gas flowing in from the exhaust gas inlet end (opening) of the inlet cell 13a passes through the porous partition wall 12, particulate matter (PM) in the exhaust gas is collected in the pores of the partition wall 12. Therefore, the exhaust gas purification catalyst 1C is useful as a gasoline particulate filter for gasoline engines or a diesel particulate filter for diesel engines. 【0161】 As shown in Figure 6, the catalyst layer 20A comprises a first layer 21 and a second layer 22. The above description of the catalyst layer 20A also applies to the third embodiment. 【0162】 As shown in Figure 6, the catalyst layer 30 has a single-layer structure, but it may also have a multilayer structure. The catalyst layer 30 can be constructed in the same way as known catalyst layers. 【0163】 In the exhaust gas purification catalyst 1C, a catalyst layer 20A may be provided on the inlet side cell 13a side of the partition wall portion 12 of the base material 10, and a catalyst layer 30 may be provided on the outlet side cell 13b side of the partition wall portion 12 of the base material 10. 【0164】 In the exhaust gas purification catalyst 1C, the catalyst layer 20A may be provided on the inlet side cell 13a side of the partition wall portion 12 of the base material 10. That is, the catalyst layer 20A may be provided on either the inlet side cell 13a side or the outlet side cell 13b side of the partition wall portion 12 of the base material 10. 【0165】 <Fourth Embodiment> Hereinafter, an exhaust gas purification catalyst 1D according to a fourth embodiment of the present invention will be described with reference to Figure 7. In the exhaust gas purification catalyst 1D, the same components and parts as those of the exhaust gas purification catalyst 1C are indicated by the same reference numerals as those of the exhaust gas purification catalyst 1C. Unless otherwise specified, the above description relating to the exhaust gas purification catalyst 1C also applies to the exhaust gas purification catalyst 1D. 【0166】 As shown in Figure 7, the exhaust gas purification catalyst 1D differs from the exhaust gas purification catalyst 1C in that it includes a catalyst layer 20B. The above description of the catalyst layer 20B also applies to the fourth embodiment. 【0167】 Exhaust gas purification system The exhaust gas purification system of the present invention will be described below. 【0168】 The exhaust gas purification system 100 according to one embodiment of the present invention will be described below with reference to Figure 8. 【0169】 As shown in Figure 8, the exhaust gas purification system 100 comprises an exhaust pipe P, a first exhaust gas purification catalyst 101 provided on the upstream side of the exhaust passage within the exhaust pipe P, and a second exhaust gas purification catalyst 102 provided on the downstream side of the exhaust passage within the exhaust pipe P. 【0170】 One end P1 of the exhaust pipe P is connected to an internal combustion engine, and exhaust gas discharged from the internal combustion engine flows through the exhaust pipe P from one end P1 to the other end P2. In other words, the exhaust pipe P forms an exhaust passage through which exhaust gas flows. In the drawing, the direction of exhaust gas flow is indicated by the symbol X. The exhaust gas flowing through the exhaust pipe P is treated by a first exhaust gas purification catalyst 101 provided on the upstream side of the exhaust passage in the exhaust pipe P, and the exhaust gas that has passed through the first exhaust gas purification catalyst 101 is treated by a second exhaust gas purification catalyst 102 provided on the downstream side of the exhaust passage in the exhaust pipe P. 【0171】 As the second exhaust gas purification catalyst 102, the exhaust gas purification catalyst of the present invention (for example, exhaust gas purification catalysts 1A, 1B, 1C, or 1D) is used. 【0172】 As the first exhaust gas purification catalyst 101, the exhaust gas purification catalyst of the present invention (for example, exhaust gas purification catalysts 1A, 1B, 1C, or 1D) may be used. When an exhaust gas purification catalyst other than the exhaust gas purification catalyst of the present invention is used as the first exhaust gas purification catalyst 101, the first exhaust gas purification catalyst 101 can be configured in the same way as known exhaust gas purification catalysts. 【0173】 When the first exhaust gas purification catalyst 101 functions as a catalytic converter and the second exhaust gas purification catalyst 102 functions as a GPF, it is preferable that exhaust gas purification catalyst 1C or 1D is used as the second exhaust gas purification catalyst 102. [Examples] 【0174】 The following Zr-Ln composite oxides (CZ material, ZC material, and NZ material) and lanthanum oxide-modified alumina were prepared. 【0175】 [CZ material] Ce oxide equivalent: 45 mass, Zr oxide equivalent: 45 mass, La oxide equivalent: 5 mass, Nd oxide equivalent: 5 mass, Specific surface area: 70 m² 2 / g 【0176】 [ZC material] Ce oxide equivalent: 20 mass, Zr oxide equivalent: 70 mass, Nd oxide equivalent: 10 mass, specific surface area: 65 m² 2 / g 【0177】 [NZ materials] Zr oxide equivalent: 90 mass, Nd oxide equivalent: 10 mass, specific surface area: 80 m² 2 / g 【0178】 [Lanthan oxide-modified alumina] Al oxide equivalent: 99% by mass, La oxide equivalent: 1% by mass, specific surface area: 120 m² 2 / g 【0179】 Furthermore, in all of the CZ, ZC, and NZ materials, a solid solution was formed between the oxide of Zr (ZrO2) and the oxide of a rare earth element (Ln). 【0180】 <Example 1> (1) Formation of the first layer Lanthanum oxide-modified alumina, barium acetate, CZ material, binder (alumina sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain the first base slurry. The first base slurry and an aqueous solution of palladium nitrate were mixed to obtain the first slurry. The composition of the first slurry was adjusted so that the composition of the first layer formed from the first slurry would be as shown in Table 1. 【0181】 Mixing and grinding by ball milling results in D in the cumulative sieve distribution of the base slurry constituent materials. 50 and D 90 This was continued until the sizes were 12 μm or less and 20 μm to 35 μm, respectively. 50 and D 90 These are the particle sizes at which the cumulative volume accounts for 50% and 90% of the volume-based particle size distribution of the base slurry constituent material obtained by laser diffraction scattering particle size distribution analysis. The laser diffraction scattering particle size distribution analysis was performed using an automated sample feeder for laser diffraction scattering particle size distribution analysis (Microtorac SDC, manufactured by Microtorac-Bell). The base slurry constituent material was introduced into an aqueous dispersion medium (e.g., pure water), and after irradiating with 40W ultrasound for 360 seconds at a flow rate of 26 mL / sec, the analysis was performed using a laser diffraction scattering particle size distribution analyzer (Microtorac MT3300EXII, manufactured by Microtorac-Bell). 50 The measurement was performed twice under the following conditions: particle refractive index of 1.5, particle shape "perfectly spherical", solvent refractive index of 1.3, "set zero" for 30 seconds, and measurement time of 30 seconds. The average of the obtained measurements was D 50 D 90 The same applies to the latter. 【0182】 A ceramic honeycomb substrate (25.4 mm in diameter, 40 mm in length, 600 cells / square inch) was immersed in the first slurry, excess slurry was removed, and the first slurry was coated onto the inner walls of the substrate. The substrate coated with the first slurry was dried at 150°C for 2.5 hours, and then fired at 500°C for 2.5 hours to form the first layer on the inner walls of the substrate. The amount of the first layer per unit volume of substrate (wash coat amount) was 114 g / L. 【0183】 (2) Formation of the second layer NZ material, ZC material, binder (zirconia sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain a second base slurry. The second slurry was mixed with an aqueous rhodium nitrate solution to obtain a second slurry. The composition of the second slurry was adjusted so that the composition of the second layer formed from the second slurry would be as shown in Table 1. The mixing and grinding using a ball mill determined the D content of the cumulative sieve distribution of the base slurry constituent materials. 50 The process was continued until the particle size was 10 μm or less. 50 The significance and measurement method are the same as described above. 【0184】 The substrate with the first layer formed on it was immersed in the second slurry, excess slurry was removed, and the second slurry was applied to the first layer. The substrate coated with the second slurry was dried at 150°C for 2.5 hours, and then baked at 500°C for 2.5 hours to form the second layer on the first layer. The amount of the second layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0185】 <Example 2> The first layer was formed on the inner wall surface of the substrate in the same manner as in Example 1, except that lanthanum oxide-modified alumina, barium acetate, a binder (alumina sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain the first base slurry, and the composition of the first slurry was adjusted so that the composition of the first layer formed from the first slurry was as shown in Table 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 47 g / L. 【0186】 Barium acetate, CZ material, binder (alumina sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain a third base slurry. The third base slurry and an aqueous solution of palladium nitrate were mixed to obtain a third slurry. The composition of the third slurry was adjusted so that the composition of the third layer formed from the third slurry would be as shown in Table 1. The mixing and grinding using a ball mill was performed to determine the D content of the cumulative sieve distribution of the base slurry constituent materials. 50 and D 90 This was continued until the sizes were 12 μm or less and 20 μm to 35 μm, respectively. 50 and D 90 The significance and measurement method are the same as described above. 【0187】 The substrate with the first layer formed on it was immersed in the third slurry, excess slurry was removed, and the third slurry was applied onto the first layer. The substrate coated with the third slurry was dried at 150°C for 2.5 hours, and then baked at 500°C for 2.5 hours to form the third layer on the first layer. The amount of the third layer per unit volume of the substrate (wash coat amount) was 67 g / L. 【0188】 A substrate with a third layer formed on it was immersed in a second slurry obtained in the same manner as in Example 1, excess slurry was removed, and the second slurry was applied to the third layer. The substrate coated with the second slurry was dried at 150°C for 2.5 hours, and then baked at 500°C for 2.5 hours to form the second layer on the third layer. The amount of the second layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0189】 <Example 3> The first layer was formed on the inner wall surface of the substrate in the same manner as in Example 2, except that lanthanum oxide-modified alumina, barium acetate, a binder (alumina sol), and water were added to a ball mill pot, and the mixture and grinding were performed using a ball mill to obtain the first base slurry, and the composition of the first slurry was adjusted so that the composition of the first layer formed from the first slurry was as shown in Table 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 52 g / L. 【0190】 The third layer was formed on the first layer in the same manner as in Example 2, except that CZ material, binder (alumina sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain a third base slurry, and the composition of the third slurry was adjusted so that the composition of the third layer formed from the third slurry was as shown in Table 1. The amount of the third layer per unit volume of the substrate (wash coat amount) was 62 g / L. 【0191】 A second layer was formed on the third layer in the same manner as in Example 2. The amount of the first layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0192】 <Comparative Example 1> A first layer was formed on the inner wall surface of the substrate in the same manner as in Example 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 114 g / L. 【0193】 The second layer was formed on the first layer in the same manner as in Example 1, except that lanthanum oxide-modified alumina, ZC material, binder (alumina sol), and water were added to a ball mill pot, and the mixture was mixed and ground using a ball mill to obtain a second base slurry, and the composition of the second slurry was adjusted so that the composition of the second layer formed from the second slurry was as shown in Table 1. The amount of the second layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0194】 <Comparative Example 2> Except for adding barium acetate, NZ material, CZ material, binder (zirconia sol), and water to a ball mill pot and mixing and grinding with a ball mill to obtain the first base slurry, and adjusting the composition of the first slurry so that the composition of the first layer formed from the first slurry is as shown in Table 1, the first layer was formed on the inner wall surface of the substrate in the same manner as in Example 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 114 g / L. 【0195】 A second layer was formed on the first layer in the same manner as in Example 1. The amount of the second layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0196】 <Comparative Example 3> The first layer was formed on the inner wall surface of the substrate in the same manner as in Example 1, except that NZ material, CZ material, binder (zirconia sol), and water were added to a ball mill pot, and the mixture and grinding were performed using a ball mill to obtain the first base slurry, and the composition of the first slurry was adjusted so that the composition of the first layer formed from the first slurry was as shown in Table 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 114 g / L. 【0197】 A second layer was formed on the first layer in the same manner as in Example 1. The amount of the second layer per unit volume of the substrate (wash coat amount) was 50.5 g / L. 【0198】 <Comparative Example 4> Except for adding NZ material, CZ material, ZC material, binder (zirconia sol), and water to a ball mill pot and mixing and grinding with a ball mill to obtain a first base slurry, adjusting the composition of the first slurry so that the composition of the first layer formed from the first slurry is as shown in Table 1, and not forming a second layer, the first layer was formed on the inner wall surface of the substrate in the same manner as in Example 1. The amount of the first layer per unit volume of the substrate (wash coat amount) was 164 g / L. 【0199】 As described above, we obtained the exhaust gas purification catalysts of Examples 1-3 and Comparative Examples 1-4. 【0200】 The exhaust gas purification catalysts of Example 1 and Comparative Examples 1 to 3 comprise a substrate, a first layer provided on the substrate, and a second layer provided on the first layer. The exhaust gas purification catalysts of Examples 2 and 3 comprise a substrate, a first layer provided on the substrate, a third layer provided on the first layer, and a second layer provided on the third layer. The exhaust gas purification catalyst of Comparative Example 4 comprises a substrate and a first layer provided on the substrate. 【0201】 Table 2 shows the content of Pd (as metal equivalent, mass%), Rh (as metal equivalent, mass%), Al (as oxide equivalent, mass%), Ba (as metal equivalent, mass%), Zr (as oxide equivalent, mass%), Ce (as oxide equivalent, mass%), Ln (other than Ce) (as oxide equivalent, mass%), and the total content of Zr and Ln (as oxide equivalent, mass%) in each layer of the exhaust gas purification catalyst for Examples 1-3 and Comparative Examples 1-4. 【0202】 [Table 1] 【0203】 [Table 2] 【0204】 <Exhaust gas purification performance test> The exhaust gas purification catalysts of Examples 1-3 and Comparative Examples 1-4 were subjected to durability treatment, and their exhaust gas purification performance was evaluated as follows. The durability treatment was performed by heat treatment at 1000°C for 30 hours in an atmosphere in which 0.50% O2 gas, 10% water vapor, and N2 as a balance gas were circulated. 【0205】 A model gas with the following composition and an A / F ratio of 14.6 was flowed through an exhaust gas purification catalyst (catalyst volume 15 mL) after durability treatment at a rate of 25 L / min, while adjusting the CO and O2 concentrations so that the A / F ratio fluctuated within the range of 14.4 to 14.8. The gas temperature flowing into the exhaust gas purification catalyst was gradually increased from room temperature at a predetermined heating rate, and the amount of HC contained in the exhaust gas that passed through the catalyst was determined using the following apparatus, and the purification rate was calculated based on the following formula. Note that X represents the amount detected without the catalyst, and Y represents the amount detected after the catalyst was installed. Purification rate (%)=(XY) / X×100 【0206】 The catalyst inlet gas temperature when the purification rate reached 50% was determined as the light-off temperature T50. T50 was measured during the heating phase. The purification rate when the catalyst inlet gas temperature was 400°C was also determined as η400. The measurement results for T50 and η400 are shown in Table 3. 【0207】 [Model gas (composition is based on volume)] CO: 0.3%, C3H6: 1000ppmC, NO: 500ppm, O2: 0.28%, CO2: 14%, H2O: 10%, N2: balance [Heating rate] 10℃ / min [Evaluation Equipment] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by Horiba, Ltd. 【0208】 As shown in Table 3, the exhaust gas purification catalysts of Examples 1 to 3 all showed good results for both T50 and η400. On the other hand, the exhaust gas purification catalysts of Comparative Examples 1, 3, and 4 showed inferior T50 results. Furthermore, the exhaust gas purification catalysts of Comparative Examples 1 and 2 showed inferior η400 results. 【0209】 [Table 3] [Explanation of Symbols] 【0210】 1A, 1B, 1C, 1D... Catalysts for exhaust gas purification 10...Base material 20A,20B...Catalyst layer 21...First layer 22...Second layer 23...The third layer 100... Exhaust gas purification system 101...First exhaust gas purification catalyst 102...Second exhaust gas purification catalyst

Claims

[Claim 1] An exhaust gas purification catalyst comprising a base material and a catalyst layer provided on the base material, The catalyst layer, A first layer containing a first platinum group element including palladium, an oxide containing aluminum, and a barium element, A second layer comprising a second platinum group element containing rhodium, and a composite oxide containing zirconium and rare earth elements, A third layer comprising a third platinum group element containing palladium, and a composite oxide containing zirconium and rare earth elements, Equipped with, The content of aluminum element in terms of oxide and the content of barium element in the first layer are 15% by mass or more and 3% by mass or more, respectively, based on the mass of the first layer. The total content of zirconium and rare earth elements in the second layer, the content of aluminum in terms of oxides, and the content of barium are, respectively, 80% by mass or more, less than 15% by mass, and less than 3% by mass, based on the mass of the second layer. The total content of zirconium and rare earth elements in the third layer, the content of aluminum in terms of oxides, and the content of barium are, respectively, 80% by mass or more, less than 15% by mass, and 8% by mass or less, based on the mass of the third layer. The first layer is located between the substrate and the second layer. An exhaust gas purification catalyst wherein the third layer is located between the first layer and the second layer. [Claim 2] The exhaust gas purification catalyst according to claim 1, wherein the second layer is the outermost layer of the catalyst layer. [Claim 3] The exhaust gas purification catalyst according to claim 1 or 2, wherein the content of barium element in the first layer is 8% by mass or more, based on the mass of the first layer. [Claim 4] The exhaust gas purification catalyst according to claim 1 or 2, wherein the content of barium element in the first layer is 12% by mass or more, based on the mass of the first layer. [Claim 5] The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the content of aluminum element in terms of oxide and the content of barium element in the third layer are 5% by mass or less and less than 3% by mass, respectively, based on the mass of the third layer. [Claim 6] The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the content of aluminum element in terms of oxide and the content of barium element in the third layer are 1% by mass or less and 1% by mass or less, respectively, based on the mass of the third layer. [Claim 7] An exhaust gas purification system that purifies exhaust gases emitted from an internal combustion engine, The exhaust gas purification system comprises an exhaust passage through which exhaust gas flows, a first exhaust gas purification catalyst provided on the upstream side of the exhaust passage, and a second exhaust gas purification catalyst provided on the downstream side of the exhaust passage. The exhaust gas purification system wherein the second exhaust gas purification catalyst is the exhaust gas purification catalyst according to any one of claims 1 to 6.

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