Exhaust gas purification catalyst

Abstract

The purpose of the present invention is to provide an exhaust gas purification catalyst capable of improving the exhaust gas purification performance of a Rh-containing layer and improving the exhaust gas purification performance of a portion below the Rh-containing layer. In order to achieve this purpose, provided is an exhaust gas purification catalyst (1) comprising: a substrate (10); a first catalyst layer (20) disposed on the substrate (10); a second catalyst layer (30) disposed on the first catalyst layer (20); and a third catalyst layer (40) disposed on the second catalyst layer (30), wherein the first catalyst layer (20) contains Pd, the second catalyst layer (30) contains Rh and Ce, the third catalyst layer (40) contains Rh, and Al and / or Zr, the content of Rh in terms of metal in the third catalyst layer (40) is greater than the content of Rh in terms of metal in the second catalyst layer (30), the content of Ce in terms of CeO2 in the second catalyst layer (30) is at least 7 mass%, the content of Ce in terms of CeO2 in the third catalyst layer (40) is less than 7 mass%, and the average thickness of the third catalyst layer (40) is 10 μm or less.

Description

Exhaust gas purification catalyst

[0001] The present invention relates to a catalyst for purifying exhaust gases.

[0002] Exhaust gases emitted from internal combustion engines of automobiles, motorcycles, etc. contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. In order to purify and detoxify these harmful components, precious metal elements such as Pd and Rh are used as catalytically active components in exhaust gas purification catalysts.

[0003] Since Pd and Rh are expensive, it is required to maximize the exhaust gas purification performance while using limited amounts of Pd and Rh.

[0004] As an example of an exhaust gas purification catalyst that satisfies such requirements, Patent Document 1 describes an exhaust gas purification catalyst that includes a substrate, a lower layer containing Pd provided on the substrate, and an upper layer provided on the lower layer, in which a Rh-containing layer is formed on the surface of the upper layer. In the exhaust gas purification catalyst described in Patent Document 1, the Rh-containing layer is formed on the surface of the upper layer that is likely to come into contact with exhaust gas, thereby improving the contact between Rh and exhaust gas.

[0005] When catalytically active components such as Rh are exposed to a high-temperature environment, they aggregate and the active sites decrease. This phenomenon is more likely to occur as the concentration of the catalytically active components increases. In this specification, the resistance of the catalytically active components contained in the catalytic layer to aggregation when the catalytic layer is exposed to a high-temperature environment is referred to as the "heat resistance of the catalytic layer." In this specification, "high temperature" refers to a temperature of 850°C or higher, particularly 900°C or higher.

[0006] The Rh-containing layer in Patent Document 1 contains Al and Zr, which are components that improve the heat resistance of the Rh-containing layer (hereinafter, may be referred to as "heat-resistant components").

[0007] Japanese Patent Application Laid-Open No. 2022-030627

[0008] The Rh-containing layer in Patent Document 1 is advantageous in terms of heat resistance because it contains a heat-resistant component; however, since it does not contain Ce, which is a component having oxygen storage capacity (OSC), it has a low OSC and therefore has a problem of insufficient exhaust gas purification performance.

[0009] When Ce is incorporated into the Rh-containing layer to solve the above-mentioned problems, the following problem arises. Increasing the amount of Ce in the Rh-containing layer and correspondingly decreasing the amount of the heat-resistant component in the Rh-containing layer improves the OSC of the Rh-containing layer, but reduces the heat resistance of the Rh-containing layer, resulting in a decrease in the exhaust gas purification performance of the Rh-containing layer (particularly, the exhaust gas purification performance of the Rh-containing layer after exposure to a high-temperature environment). Conversely, increasing the amount of the heat-resistant component in the Rh-containing layer and correspondingly decreasing the amount of Ce in the Rh-containing layer improves the heat resistance of the Rh-containing layer, but reduces the OSC of the Rh-containing layer and the exhaust gas purification performance of the Rh-containing layer. On the other hand, increasing the amounts of Ce and the heat-resistant component in the Rh-containing layer can improve the heat resistance and OSC of the Rh-containing layer, but increases the thickness of the Rh-containing layer, which deteriorates the contact between the catalytically active component contained in the portion below the Rh-containing layer and the exhaust gas, thereby reducing the exhaust gas purification performance of the portion below the Rh-containing layer.

[0010] Therefore, with the conventional technology, it is difficult to improve the exhaust gas purification performance of the Rh-containing layer and the portion below the Rh-containing layer.

[0011] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst that can improve the exhaust gas purification performance of the Rh-containing layer and the exhaust gas purification performance of the portion below the Rh-containing layer.

[0012] The present inventors have discovered a catalyst for purifying exhaust gases, which comprises a substrate, a Pd-containing layer (first catalytic layer) provided on the substrate, and a Rh-containing layer provided on the Pd-containing layer, in which the Rh-containing layer is divided into a lower layer (second catalytic layer) and an upper layer (third catalytic layer), the Rh concentration in the third catalytic layer is higher than the Rh concentration in the second catalytic layer, Ce is contained in the second catalytic layer, Al and / or Zr is contained in the third catalytic layer, and Ce in the second catalytic layer is converted to CeO ２The percentage of the converted mass of Ce in the third catalyst layer relative to the mass of the second catalyst layer is adjusted to 7 mass% or more, and the CeO ２ The present inventors found that by adjusting the percentage of the converted mass of the catalyst to less than 7 mass% relative to the mass of the third catalytic layer and adjusting the average thickness of the third catalytic layer to 10 μm or less, it is possible to improve the contact between the Rh contained in the third catalytic layer and the exhaust gas, improve the heat resistance of the third catalytic layer, improve the OSC of the second catalytic layer, and improve the contact between the catalytically active components contained in the portion below the third catalytic layer and the exhaust gas, thereby improving the exhaust gas purification performance of the second catalytic layer, the third catalytic layer, and the portion below the third catalytic layer, and thus completing the present invention.

[0013] That is, the present invention provides the following exhaust gas purifying catalyst: [1] A catalyst for exhaust gas purifying comprising a substrate, a first catalytic layer provided on the substrate, a second catalytic layer provided on the first catalytic layer, and a third catalytic layer provided on the second catalytic layer, wherein the first catalytic layer contains Pd, the second catalytic layer contains Rh and Ce, and the third catalytic layer contains Rh and Al and / or Zr, and the second catalytic layer and the third catalytic layer satisfy the following formula: a>b (wherein a represents the percentage of the mass of Rh in the third catalytic layer in terms of metal relative to the mass of the third catalytic layer, and b represents the percentage of the mass of Rh in the second catalytic layer in terms of metal relative to the mass of the second catalytic layer), and the Ce in the second catalytic layer is CeO ２ the percentage of the mass of the second catalytic layer converted into CeO is 7 mass% or more, and the percentage of Ce in the third catalytic layer is CeO ２ The catalyst for purifying exhaust gas, wherein the percentage of the mass of the third catalyst layer converted into Al is less than 7 mass % relative to the mass of the third catalyst layer, and the average thickness of the third catalyst layer is 10 μm or less. ２ O ３ The converted mass of Zr in the third catalyst layer and ZrO ２[1] The catalyst for purifying exhaust gas according to [1], wherein the total of the mass of the third catalyst layer and the mass of the third catalyst layer converted from carbon black is 80 mass% or more. [3] The catalyst for purifying exhaust gas according to [1] or [2], wherein the ratio a / b of a to b is 2 or more and 10 or less. [4] The catalyst for purifying exhaust gas according to [3], wherein b is 0.01 mass% or more and 5 mass% or less. [5] The catalyst for purifying exhaust gas according to any one of [1] to [4], wherein the average thickness of the third catalyst layer is 0.5 μm or more and 5 μm or less.

[0014] According to the present invention, there is provided an exhaust gas purification catalyst that can realize an improvement in the exhaust gas purification performance of the Rh-containing layer and an improvement in the exhaust gas purification performance of the portion below the Rh-containing layer.

[0015] Fig. 1 is a partial end view showing a state in which an exhaust gas purification catalyst according to one embodiment of the present invention is disposed in the exhaust path of an internal combustion engine. Fig. 2 is an end view taken along line A-A in Fig. 1. Fig. 3 is an enlarged view of the area indicated by symbol R in Fig. 2. Fig. 4 is an end view taken along line B-B in Fig. 1. Fig. 5 is a diagram showing a BED observation image of the exhaust gas purification catalyst of Comparative Example 2. Fig. 6 is a diagram showing an element mapping image of the exhaust gas purification catalyst of Comparative Example 2.

[0016] <Explanation of Terms> The terms used in this specification will be explained below. The following explanations apply to the entire specification unless otherwise specified.

[0017] <Abbreviations> "SEM" means scanning electron microscope, "EDX" means energy dispersive X-ray spectroscopy, "SEM-EDX" means scanning electron microscope-energy dispersive X-ray analysis, "EPMA" means electron probe microanalyzer, "XRF" means X-ray fluorescence analysis, "WDX" means wavelength dispersive X-ray analysis, and "ICP-AES" means inductively coupled plasma atomic emission spectroscopy.

[0018] <Metallic Elements> The term "metallic elements" also includes metalloid elements such as Si and B.

[0019] <Rare Earth Elements> "Rare earth elements" include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0020] <Noble Metal Elements> "Noble metal elements" include Pt, Pd, Rh, Ru, Os, Ir, Au, and Ag.

[0021] <Oxide> The meaning of "oxide" is as follows: Oxides of rare earth elements other than Ce, Pr, and Tb are called sesquioxides (M ２ O ３ , M represents a rare earth element other than Ce, Pr, and Tb), and the oxide of Ce is CeO ２ Pr oxide is Pr ６ O １１ The oxide of Tb is Tb ４ O ７ The oxide of Al is Al ２ O ３ Zr oxide is ZrO ２ The oxide of Si is SiO ２ The oxide of B is B ２ O ３ Cr oxide is Cr ２ O ３ , Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe ３ O ４ Mn oxide is Mn ３ O ４ Ni oxide is NiO, Ti oxide is TiO ２ Zn oxide is ZnO, Sn oxide is SnO ２ means.

[0022] <Mass of Metal Element in Metal Equivalent> The "mass of a metal element in metal equivalent" refers to the mass of a metal that is determined on the assumption that the metal element exists as a metal composed of the metal element.

[0023] <Mass of Metal Element in Equivalent to Oxide> The "mass of metal element in equivalent to oxide" refers to the mass of the oxide of a metal element that is determined on the assumption that the metal element exists as an oxide of the metal element.

[0024] <Mass of catalytic layer> The "mass of catalytic layer" refers to the sum of the mass of the precious metal elements contained in the catalytic layer, which is determined by classifying all the metal elements contained in the catalytic layer into precious metal elements and metal elements other than precious metal elements, calculating the mass of the precious metal elements in metal equivalent, and the mass of the metal elements other than precious metal elements in oxide equivalent. In other words, the "mass of catalytic layer" refers to the calculated mass determined by summing the mass of the precious metal elements contained in the catalytic layer in metal equivalent and the mass of the metal elements other than precious metal elements contained in the catalytic layer in oxide equivalent.

[0025] When information (e.g., composition, amount, etc.) of the raw materials used in manufacturing the catalyst layer is known, the mass of the catalyst layer can be determined from the information of the raw materials used in manufacturing the catalyst layer.

[0026] <Content of Metal Element in Catalyst Layer in Metal Equivalent or Oxide Equivalent> The "content of metal element in catalyst layer in metal equivalent" is defined by the formula: content of metal element in catalyst layer in metal equivalent (mass %)=(mass of metal element in catalyst layer in metal equivalent) / (mass of catalyst layer)×100.

[0027] The "content of the metal element in the catalytic layer in terms of oxide" is defined by the formula: content of the metal element in the catalytic layer in terms of oxide (mass %)=(mass of the metal element in the catalytic layer in terms of oxide) / (mass of the catalytic layer)×100.

[0028] When information (e.g., composition, amount, etc.) of the raw materials used to form the catalyst layer is known, the content (mass %) of the metal element in the catalyst layer in terms of metal or oxide can be determined from the information on the raw materials.

[0029] When information about the raw materials used to form the catalyst layer is unknown, the content (mass %) of the metal element in the catalyst layer in terms of metal or oxide can be determined by a conventional method such as SEM-EDX. Specifically, this is as follows.

[0030] The catalyst layer is subjected to elemental analysis using a conventional method such as SEM-EDX to identify the types of constituent elements of the catalyst layer and determine the mole percentage of each identified metal element. The mole percentage of each metal element is determined for each of 10 SEM fields, and the average mole percentage of each metal element in the 10 fields is taken as the mole percentage of each metal element in the catalyst layer.

[0031] The V value of each precious metal element in the catalyst layer is calculated using the following formula: V value of each precious metal element = (mol % of each precious metal element in the catalyst layer) x (molar mass of each precious metal element)

[0032] The W value of each metal element other than the noble metal element in the catalyst layer is calculated using the following formula: W value of each metal element = (mol % of each metal element in the catalyst layer) x (molar mass of the oxide of each metal element)

[0033] The metal-equivalent content (mass%) of each precious metal element in the catalyst layer is calculated using the following formula: Metal-equivalent content (mass%) of each precious metal element in the catalyst layer = (V value of each precious metal element) / {(total V values ​​of all precious metal elements) + (total W values ​​of all metal elements other than precious metal elements)} × 100

[0034] The content (mass %) of each metal element other than precious metal elements in the catalyst layer in terms of oxide is calculated using the following formula: Content (mass %) of each metal element other than precious metal elements in the catalyst layer in terms of oxide = (W value of each metal element other than precious metal elements) / {(total V values ​​of all precious metal elements) + (total W values ​​of all metal elements other than precious metal elements)} × 100

[0035] <Metal Oxide> The term "metal oxide" refers to an oxide containing one or more metal elements. Examples of metal oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, and Ce-Zr-based composite oxides.

[0036] <Mass of Metal Oxide> The "mass of metal oxide" refers to the total mass of oxides of metal elements that can be determined on the assumption that each metal element in the metal oxide exists as an oxide.

[0037] <Content of Metal Element in Metal Oxide in Terms of Oxide> The "content of metal element in metal oxide in terms of oxide" is defined by the formula: content of metal element in metal oxide in terms of oxide (mass %) = (mass of metal element in metal oxide in terms of oxide) / (mass of metal oxide) × 100.

[0038] When the composition of the metal oxide is known, the content (mass %) of the metal element in the metal oxide calculated as oxide can be determined from the composition of the metal oxide.

[0039] When the composition of the metal oxide is unknown, the content (mass %) of the metal element in the metal oxide calculated as the oxide can be determined by a conventional method such as SEM-EDX. Specifically, this is as follows.

[0040] The metal oxide is subjected to elemental analysis using a conventional method such as SEM-EDX to identify the types of constituent elements of the metal oxide, and to determine the content (mass %) of each identified metal element in terms of oxide.

[0041] <Average particle diameter of metal oxide> The "average particle diameter of metal oxide" refers to the average value of the unidirectional diameters (Feret diameters) of 100 metal oxide particles, which is determined by observing a sample containing a metal oxide with a scanning electron microscope and measuring the unidirectional diameters (Feret diameters) of 100 metal oxide particles randomly selected from within the field of view.

[0042] <Al-based oxide> "Al-based oxide" refers to an oxide containing Al, in which Al is the metal element with the highest content by mass of the metal elements constituting the oxide. However, Ce-Zr-based composite oxides are not considered to be Al-based oxides. Ce-Zr-based composite oxides will be described later. Al-based oxides are distinguished from alumina used as a binder. In this specification, alumina used as a binder may be referred to as "alumina binder."

[0043] The Al-based oxide is, for example, in the form of particles. The Al-based oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Al-based oxide is preferably porous.

[0044] Al-based oxides have high heat resistance, so when the catalyst layer contains an Al-based oxide, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0045] The Al-based oxide may contain one or more metal elements (hereinafter referred to as "additional element M1") other than Al and O. The additional element M1 can be selected from, for example, rare earth elements (e.g., Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), B, Si, Zr, Cr, etc.

[0046] In the Al-based oxide, the additional element M1 is a solid solution phase (e.g., Al ２ O ３ The additional element M1 may form a solid solution phase (a solid solution phase of the additional element M1 and an oxide of the additional element M1), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M1), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M1 forms a solid solution phase.

[0047] Examples of Al-based oxides include alumina (Al ２ O ３ Examples of the Al-based oxide containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-ceria, and alumina-lanthana.

[0048] From the viewpoint of improving the heat resistance of the Al-based oxide, the Al content of the Al in the Al-based oxide is ２ O ３ The converted content is preferably 50% by mass or more, more preferably 75% by mass or more, and even more preferably 95% by mass or more, based on the mass of the Al-based oxide, with the upper limit being 100% by mass.

[0049] <Ce-based oxide> "Ce-based oxide" refers to an oxide containing Ce, in which Ce is the element with the largest content by mass among the metal elements constituting the oxide. However, an oxide that falls under the category of Ce-Zr-based composite oxide is not considered to fall under the category of Ce-based oxide. Ce-Zr-based composite oxides will be described later. Ce-based oxides are distinguished from ceria used as a binder. In this specification, ceria used as a binder may be referred to as a "ceria binder."

[0050] The Ce-based oxide is, for example, in the form of particles. The Ce-based oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Ce-based oxide is preferably porous.

[0051] Ce-based oxides have the OSC (the ability to store oxygen when the oxygen concentration in exhaust gas is high and release oxygen when the oxygen concentration in exhaust gas is low), which alleviates fluctuations in the oxygen concentration in exhaust gas and widens the operating window of the catalytically active components. Therefore, by including Ce-based oxides in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0052] The Ce-based oxide may contain one or more metal elements (hereinafter referred to as "additional element M2") other than Ce and O. The additional element M2 can be selected from, for example, rare earth elements other than Ce (e.g., Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0053] In the Ce-based oxide, the additional element M2 forms a solid solution phase (e.g., CeO ２ The additional element M2 may form a solid solution phase (a solid solution phase of the additional element M2 and an oxide of the additional element M2), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M2), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M2 forms a solid solution phase.

[0054] Examples of Ce-based oxides include ceria (CeO ２), an oxide obtained by modifying the surface of ceria with the additional element M2, an oxide obtained by dissolving the additional element M2 in ceria, and the like.

[0055] From the viewpoint of improving the OSC of the Ce-based oxide, the CeO ２ The content in terms of the mass of the Ce-based oxide is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more, with the upper limit being 100 mass %.

[0056] <Zr-based oxide> "Zr-based oxide" refers to an oxide containing Zr, in which Zr is the element with the largest content by mass among the metal elements constituting the oxide. However, Ce-Zr-based composite oxides are not considered to be Zr-based oxides. Ce-Zr-based composite oxides will be described later. Zr-based oxides are distinguished from zirconia used as a binder. In this specification, zirconia used as a binder may be referred to as a "zirconia binder."

[0057] The Zr-based oxide is, for example, in a particulate form. The Zr-based oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Zr-based oxide is preferably porous.

[0058] Zr-based oxides have high heat resistance, and therefore, by including Zr-based oxides in the catalyst layer, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0059] The Zr-based oxide may contain one or more metal elements (hereinafter referred to as "additional element M3") other than Zr and O. The additional element M3 can be selected from, for example, rare earth elements (e.g., Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), B, Si, Al, Cr, etc.

[0060] In the Zr-based oxide, the additional element M3 forms a solid solution phase (e.g., ZrO ２The additional element M3 may form a solid solution phase (a solid solution phase of the additional element M3 and an oxide of the additional element M3), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M3), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M3 forms a solid solution phase.

[0061] Examples of Zr-based oxides include zirconia (ZrO ２ ), an oxide obtained by modifying the surface of zirconia with the additional element M3, an oxide obtained by dissolving the additional element M3 in zirconia, and the like.

[0062] From the viewpoint of improving the heat resistance of the Zr-based oxide, Zr in the Zr-based oxide is ２ The converted content is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, based on the mass of the Zr-based oxide, with the upper limit being 100% by mass.

[0063] <Ce—Zr-based composite oxide> The Ce—Zr-based composite oxide is a composite oxide containing Ce and Zr, and the Ce in the composite oxide is represented by CeO ２ The content of Zr in the composite oxide in terms of ZrO is 5 mass % or more and 95 mass % or less based on the mass of the composite oxide. ２ The term "oxide" refers to an oxide whose content, calculated based on the mass of the composite oxide, is 5 mass % or more and 95 mass % or less.

[0064] The Ce—Zr-based composite oxide is, for example, in the form of particles. The Ce—Zr-based composite oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Ce—Zr-based composite oxide is preferably porous.

[0065] The Ce—Zr-based composite oxide has an OSC, which reduces fluctuations in the oxygen concentration in exhaust gas and widens the operating window of the catalytically active component. Therefore, by including the Ce—Zr-based composite oxide in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0066] The Ce—Zr-based composite oxide may contain one or more metal elements (hereinafter referred to as “additional element M4”) other than Ce, Zr, and O. The additional element M4 can be selected from, for example, rare earth elements other than Ce (e.g., Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0067] In the Ce-Zr based composite oxide, Ce exists in a solid solution phase (e.g., CeO ２ and ZrO ２ or a single phase that is a crystalline phase or an amorphous phase (e.g., a solid solution phase with CeO ２ Although Ce may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Ce forms a solid solution phase.

[0068] In the Ce-Zr composite oxide, Zr exists in a solid solution phase (e.g., CeO ２ and ZrO ２ or a single phase that is a crystalline phase or an amorphous phase (e.g., ZrO ２ Although Zr may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Zr forms a solid solution phase.

[0069] When the Ce—Zr-based composite oxide contains the additional element M4, the additional element M4 forms a solid solution phase (for example, CeO ２ and a solid solution phase of the oxide of the additional element M4, ZrO ２ and a solid solution phase of the oxide of the additional element M4, CeO ２ and ZrO ２ The additional element M4 may form a solid solution phase (e.g., a solid solution phase of the additional element M4 and an oxide of the additional element M4), or may form a single phase that is a crystalline phase or an amorphous phase (e.g., a single phase of an oxide of the additional element M4), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M4 forms a solid solution phase.

[0070] From the viewpoint of improving the OSC of the Ce-Zr based composite oxide, the Ce in the Ce-Zr based composite oxide is converted to CeO. ２The converted content is preferably 7% by mass or more, more preferably 10% by mass or more, and even more preferably 13% by mass or more, based on the mass of the Ce-Zr-based composite oxide. The upper limit can be adjusted appropriately taking into consideration heat resistance, structural stability, the contents of other components, and the like. The upper limit is preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0071] From the viewpoint of improving the heat resistance of the Ce-Zr based composite oxide, the Zr in the Ce-Zr based composite oxide is converted to ZrO. ２ The converted content is preferably 25% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be adjusted appropriately taking into consideration oxygen storage capacity, structural stability, the contents of other components, etc. The upper limit is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0072] From the viewpoint of improving the heat resistance and OSC of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２ Conversion rate and ZrO ２ The total content in terms of these elements is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, based on the mass of the Ce—Zr-based composite oxide, with the upper limit being 100% by mass.

[0073] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the Ce—Zr-based composite oxide preferably contains one or more rare earth elements selected from, for example, Y, Pr, La, Nd, Sm, Eu, Gd, etc.

[0074] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide, is preferably 5% by mass or more and 30% by mass or less, more preferably 7% by mass or more and 25% by mass or less, and even more preferably 9% by mass or more and 20% by mass or less, based on the mass of the Ce—Zr-based composite oxide. Each of the above lower limits may be combined with any of the above upper limits. When the Ce—Zr-based composite oxide contains one rare earth element, the "content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide" refers to the content of that one rare earth element, calculated as an oxide, and when the Ce—Zr-based composite oxide contains two or more rare earth elements, the "content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide" refers to the total content of the two or more rare earth elements, calculated as an oxide.

[0075] <<Catalyst for Purifying Exhaust Gas>> The catalyst for purifying exhaust gas of the present invention will be described below.

[0076] An exhaust gas purifying catalyst 1 (hereinafter referred to as "catalyst 1") according to one embodiment of the present invention will be described below with reference to FIGS.

[0077] As shown in Figure 1, a catalyst 1 is disposed in an exhaust passage in an exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas emitted from the internal combustion engine flows through the exhaust passage in the exhaust pipe P from one end to the other end of the exhaust pipe P and is purified by the catalyst 1 provided in the exhaust pipe P. In the drawing, the exhaust gas flow direction 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 inflow side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outflow side" or "downstream side."

[0078] In the exhaust passage in the exhaust pipe P, other exhaust gas purifying catalysts may be arranged upstream and / or downstream of the catalyst 1 .

[0079] As shown in FIGS. 2 to 4, the catalyst 1 includes a substrate 10, a first catalytic layer 20 provided on the substrate 10, a second catalytic layer 30 provided on the first catalytic layer 20, and a third catalytic layer 40 provided on the second catalytic layer 30.

[0080] In the catalyst 1, the first catalytic layer 20 contains Pd, the second catalytic layer 30 contains Rh and Ce, and the third catalytic layer 40 contains Rh and Al and / or Zr, and the second catalytic layer 30 and the third catalytic layer 40 satisfy the following formula: a>b (wherein a represents the percentage of the mass of Rh in the third catalytic layer 40 converted into metal relative to the mass of the third catalytic layer 40, and b represents the percentage of the mass of Rh in the second catalytic layer 30 converted into metal relative to the mass of the second catalytic layer 30), and the Ce in the second catalytic layer 30 is CeO ２ The percentage of the converted mass of Ce in the third catalytic layer 40 relative to the mass of the second catalytic layer 30 is 7 mass % or more, and the CeO ２ The percentage of the converted mass relative to the mass of the third catalyst layer 40 is less than 7 mass %, and the average thickness of the third catalyst layer 40 is 10 μm or less.

[0081] The effects of the catalyst 1 will be described below.

[0082] The second catalytic layer 30 and the third catalytic layer 40 provided on the second catalytic layer 30 satisfy the formula: a > b. Therefore, the catalyst 1 can improve the contact between the Rh contained in the third catalytic layer 40 and the exhaust gas, thereby improving the exhaust gas purification performance of the third catalytic layer 40.

[0083] From the viewpoint of more effectively improving the contact between the Rh contained in the third catalytic layer 40 and the exhaust gas, the ratio of a to b, a / b, is preferably 2 or more and 10 or less, more preferably 3 or more and 6 or less, and even more preferably 4 or more and 6 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0084] The third catalytic layer 40 contains Al and / or Zr, and the Ce in the third catalytic layer 40 is CeO ２ The percentage of the converted mass to the mass of the third catalytic layer 40 is less than 7 mass %. Therefore, the catalyst 1 can improve the heat resistance of the third catalytic layer 40, and thereby improve the exhaust gas purification performance of the third catalytic layer 40 (in particular, the exhaust gas purification performance of the third catalytic layer 40 after exposure to a high-temperature environment).

[0085] The second catalytic layer 30 contains Ce, and the Ce in the second catalytic layer 30 is converted into CeO ２The percentage of the converted mass to the mass of the second catalyst layer 30 is 7 mass % or more. Therefore, the catalyst 1 can improve the OSC of the second catalyst layer 30, and thereby improve the exhaust gas purification performance of the second catalyst layer 30.

[0086] The average thickness of the third catalytic layer 40 is 10 μm or less. Therefore, the catalyst 1 can improve the contact between the catalytically active components contained in the portion below the third catalytic layer 40 (e.g., Pd contained in the first catalytic layer 20 and Rh contained in the second catalytic layer 30) and the exhaust gas, thereby improving the exhaust gas purification performance of the first catalytic layer 20 and the second catalytic layer 30.

[0087] As described above, catalyst 1 can improve the contact between the Rh contained in the third catalytic layer 40 and the exhaust gas, improve the heat resistance of the third catalytic layer 40, improve the OSC of the second catalytic layer 30, and improve the contact between the catalytically active components contained in the portion below the third catalytic layer 40 and the exhaust gas. These effects combine to provide excellent exhaust gas purification performance.

[0088] <Substrate> The substrate 10 will be described below.

[0089] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of materials constituting the substrate 10 include ceramic materials and metal materials, with ceramic materials being 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 metal materials include alloys such as stainless steel.

[0090] 2 to 4, the substrate 10 has a cylindrical portion 11, partition walls 12 provided in the cylindrical portion 11, and cells 13 separated by the partition walls 12. The substrate 10 is preferably a honeycomb structure.

[0091] As shown in Fig. 2, the tubular portion 11 defines the outer shape of the substrate 10, and the axial direction of the tubular portion 11 coincides with the axial direction of the substrate 10. As shown in Fig. 2, the shape of the tubular portion 11 is cylindrical, but it may be other shapes such as an elliptical cylinder or a polygonal cylinder.

[0092] 2 to 4 , partition walls 12 are present between adjacent cells 13, and the adjacent cells 13 are separated by the partition walls 12. The partition walls 12 may have a porous structure that allows exhaust gas to pass through. The thickness of the partition walls 12 is, for example, 20 μm or more and 1500 μm or less.

[0093] As shown in FIG. 4, the 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.

[0094] As shown in Fig. 4, both the end on the exhaust gas inlet side and the end on the exhaust gas outlet side of the cell 13 are open. Therefore, the exhaust gas that flows in from the end (opening) on ​​the exhaust gas inlet side of the cell 13 flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13. This type of configuration is called a flow-through type.

[0095] 2 and 3, the end (opening) of the cell 13 on the exhaust gas inlet side has a rectangular shape in plan view, but may have other shapes such as a hexagon, an octagon, etc. The end (opening) of the cell 13 on the exhaust gas outlet side also has a similar shape in plan view.

[0096] The cell density per square inch of the substrate 10 is, for example, 100 cells or more and 1200 cells or less. The cell density per square inch of the substrate 10 means the total number of cells 13 per square inch in a cross section obtained by cutting the substrate 10 along a plane perpendicular to the exhaust gas flow direction X.

[0097] The volume of the substrate 10 is, for example, 0.1 L or more and 20 L or less. The volume of the substrate 10 means the apparent volume of the substrate 10. For example, when the substrate 10 is cylindrical, if the outer diameter of the substrate 10 is 2r and the length of the substrate 10 is L10, the volume of the substrate 10 can be calculated by the formula: Volume of substrate 10 = π × r ２ × L10. In this specification, "length" refers to the dimension of the substrate 10 in the axial direction, unless otherwise specified.

[0098] <First catalytic layer> The first catalytic layer 20 will now be described.

[0099] As shown in Figures 3 and 4, the first catalytic layer 20 is provided on the substrate 10. Specifically, the first catalytic layer 20 is provided on the cell 13-side surface of the partition wall section 12. The "cell 13-side surface of the partition wall section 12" refers to the outer surface of the partition wall section 12 that extends in the exhaust gas flow direction X and is in contact with the cells 13. The first catalytic layer 20 may be provided directly on the cell 13-side surface of the partition wall section 12, or may be provided via another layer. However, the first catalytic layer 20 is typically provided directly on the cell 13-side surface of the partition wall section 12. The "first catalytic layer 20 provided on the substrate 10" encompasses an embodiment in which the first catalytic layer 20 is provided directly on the cell 13-side surface of the partition wall section 12, and an embodiment in which the first catalytic layer 20 is provided on the cell 13-side surface of the partition wall section 12 via another layer.

[0100] The first catalytic layer 20 may be composed of a portion that protrudes from the cell 13-side surface of the partition wall section 12 toward the cell 13 (hereinafter referred to as a "protruding portion"), or may be composed of a portion that exists inside the partition wall section 12 (hereinafter referred to as an "internal portion"), or may have both a protruding portion and an internal portion. The "first catalytic layer 20 provided on the substrate 10" includes an embodiment in which the first catalytic layer 20 is composed of a protruding portion, an embodiment in which the first catalytic layer 20 is composed of an internal portion, and an embodiment in which the first catalytic layer 20 has both a protruding portion and an internal portion.

[0101] 4 , the first catalytic layer 20 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side. The first catalytic layer 20 may extend along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, or may extend in the direction opposite to the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas outlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0102] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalytic layer 20 per unit volume of the portion of the substrate 10 on which the first catalytic layer 20 is formed is preferably 50 g / L or more and 160 g / L or less, more preferably 60 g / L or more and 140 g / L or less, and even more preferably 70 g / L or more and 120 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0103] The mass of the first catalytic layer 20 per unit volume of the portion of the substrate 10 on which the first catalytic layer 20 is formed is calculated from the formula: (mass of the first catalytic layer 20) / ((volume of the substrate 10)×(average length L20 of the first catalytic layer 20 / length L10 of the substrate 10)).

[0104] An example of a method for measuring the average length L20 of the first catalyst layer 20 is as follows.

[0105] A sample extending in the axial direction of the substrate 10 and having the same length as the length L10 of the substrate 10 is cut out from the catalyst 1. The sample is, for example, cylindrical with a diameter of 25.4 mm. The diameter of the sample can be changed as needed. When the first catalyst layer 20 extends from the end of the partition wall 12 on the exhaust gas inlet side along the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas inlet side. When the first catalyst layer 20 extends from the end of the partition wall 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas outlet side. In either case, the length of the cut pieces is 5 mm. The composition of the cut pieces is analyzed using XRF (e.g., EDX, WDX, etc.), ICP-AES, SEM-EDX, etc., and based on the composition of the cut pieces, it is determined whether or not the cut pieces contain a part of the first catalyst layer 20.

[0106] It is not necessary to perform composition analysis on a cut piece that is clearly containing a part of the first catalytic layer 20. For example, the cut surface can be observed using an SEM, an EPMA, or the like to confirm whether the cut piece contains a part of the first catalytic layer 20. When observing the cut surface, elemental mapping of the cut surface may be performed.

[0107] After confirming whether the cut pieces include a part of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is calculated based on the following formula: Length of the first catalytic layer 20 included in the sample = 5 mm × (number of cut pieces including a part of the first catalytic layer 20)

[0108] For example, if the first to kth cut pieces include a portion of the first catalytic layer 20, but the (k+1)th to nth cut pieces do not include a portion of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is (5×k) mm.

[0109] A more detailed example of a method for measuring the length of the first catalyst layer 20 included in the sample is as follows.

[0110] The kth cut piece is cut in the axial direction of the substrate 10, and the part of the first catalytic layer 20 present on the cut surface is observed using an SEM, an EPMA, or the like, to measure the length of the part of the first catalytic layer 20 in the kth cut piece. Then, the length of the first catalytic layer 20 included in the sample is calculated based on the following formula. When the first catalytic layer 20 extends from the end of the exhaust gas inlet side of the partition wall 12 along the exhaust gas flow direction X, the kth cut piece is the cut piece obtained from the nearest exhaust gas outlet side of the sample, among the cut pieces including a part of the first catalytic layer 20. When the first catalytic layer 20 extends from the end of the exhaust gas outlet side of the partition wall 12 along the direction opposite to the exhaust gas flow direction X, the kth cut piece is the cut piece obtained from the nearest exhaust gas inlet side of the sample, among the cut pieces including a part of the first catalytic layer 20. Length of the first catalytic layer 20 included in the sample = (5 mm × (k - 1)) + (length of the part of the first catalytic layer 20 included in the kth cut piece)

[0111] For 8 to 16 samples arbitrarily cut out from the catalyst 1, the length of the first catalyst layer 20 included in each sample is measured, and the average value thereof is taken as the average length L20 of the first catalyst layer 20.

[0112] The first catalytic layer 20 contains Pd as a catalytically active component. Pd is contained in the first catalytic layer 20 in a form that can function as a catalytically active component, such as metallic Pd, an alloy containing Pd, or a compound containing Pd (e.g., an oxide of Pd). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Pd is preferably in a particulate form.

[0113] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the mass of Pd in ​​the first catalytic layer 20 in terms of metal relative to the mass of the first catalytic layer 20 (referred to herein as the "content of Pd in ​​the first catalytic layer 20 in terms of metal") is preferably 0.1 mass% to 10 mass%, more preferably 0.2 mass% to 7 mass%, and even more preferably 0.3 mass% to 5 mass%. Each of the above lower limits may be combined with any of the above upper limits.

[0114] The first catalytic layer 20 may contain one or more precious metal elements other than Pd as catalytically active components. The precious metal elements other than Pd can be selected from, for example, Pt, Rh, Ru, Os, Ir, Au, Ag, etc. The precious metal elements other than Pd are contained in the first catalytic layer 20 in a form that can function as a catalytically active component, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (for example, an oxide of the precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the precious metal element other than Pd is preferably in a particulate form.

[0115] When the first catalytic layer 20 contains Pd and a precious metal element other than Pd, the Pd and the precious metal element other than Pd may form an alloy, which may reduce the active sites of Pd, which is involved in the exhaust gas purification performance. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Pd in ​​the first catalytic layer 20 in terms of metal relative to the mass of the first catalytic layer 20 (referred to herein as the "content of the precious metal elements other than Pd in ​​the first catalytic layer 20 in terms of metal") is small. Specifically, the content of the precious metal elements other than Pd in ​​the first catalytic layer 20 in terms of metal is preferably 0.05% by mass or less, more preferably 0.03% by mass or less, and even more preferably 0.01% by mass or less. The lower limit is 0% by mass. The "metal-equivalent content of precious metal elements other than Pd in ​​the first catalytic layer 20" means, when the first catalytic layer 20 contains one type of precious metal element other than Pd, the metal-equivalent content of that one type of precious metal element, and when the first catalytic layer 20 contains two or more types of precious metal elements other than Pd, the metal-equivalent content of the two or more types of precious metal elements.

[0116] The first catalyst layer 20 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers.

[0117] The phrase "at least a part of the catalytically active component is supported on a carrier" means that at least a part of the catalytically active component is physically or chemically adsorbed or retained on the outer surface and / or the inner pore surfaces of the carrier. This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40).

[0118] Whether at least a part of the catalytically active component in a certain catalyst layer is supported on the carrier can be confirmed, for example, by using SEM-EDX etc. Specifically, when at least a part of the catalytically active component and the carrier are present in the same region in elemental mapping obtained by analyzing a cross section of the catalyst layer with SEM-EDX, it can be determined that at least a part of the catalytically active component is supported on the carrier.

[0119] The support can be selected from, for example, metal oxides. The metal oxide is, for example, particulate. From the viewpoint of improving the supportability of the catalytically active component, the metal oxide is preferably porous. The metal oxide may or may not have an OSC. The metal oxide used as a support is distinguished from the metal oxide used as a binder (e.g., metal oxide-based binders such as alumina binders, zirconia binders, titania binders, and silica binders).

[0120] Examples of metal oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, Ce-Zr-based composite oxides, oxides of rare earth elements other than Ce, and silica (SiO ２ ), titania (TiO ２ ), zeolite (aluminosilicate), MgO, ZnO, SnO ２ and the like.

[0121] From the viewpoint of improving the heat resistance and / or OSC of the first catalytic layer 20, and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the support is preferably selected from an Al-based oxide, a Ce-based oxide, and a Ce—Zr-based composite oxide, and more preferably selected from an Al-based oxide and a Ce—Zr-based composite oxide. In one embodiment, the first catalytic layer 20 contains an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0122] From the viewpoint of improving the OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains Ce.

[0123] From the viewpoint of improving the OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Ce in the first catalytic layer 20 is converted to CeO ２ The percentage of the converted mass of Ce in the first catalytic layer 20 relative to the mass of the first catalytic layer 20 (referred to herein as "CeO ２The total content (referred to as "content in terms of total mass") is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0124] Regarding a certain catalyst layer, "CeO of Ce in the catalyst layer" ２ When the catalyst layer contains one kind of Ce source, the "concentration of CeO converted from Ce derived from the one kind of Ce source" is ２ When the catalyst layer contains two or more Ce sources, the CeO content of Ce derived from the two or more Ce sources is ２ This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40).

[0125] When the first catalytic layer 20 contains Ce, the first catalytic layer 20 contains one or more Ce sources.

[0126] Examples of Ce sources include oxides containing Ce, such as Al-based oxides containing Ce, Ce-based oxides, Zr-based oxides containing Ce, Ce-Zr-based composite oxides, and ceria binders.

[0127] From the viewpoint of improving the heat resistance and OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains a Ce—Zr-based composite oxide as a Ce source. The first catalytic layer 20 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide.

[0128] From the viewpoint of improving the heat resistance and OSC of the first catalytic layer 20, and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the percentage of the mass of the Ce—Zr-based composite oxide in the first catalytic layer 20 relative to the mass of the first catalytic layer 20 (referred to herein as the "content of Ce—Zr-based composite oxide in the first catalytic layer 20") is preferably 10 mass% or more, more preferably 15 mass% or more, and even more preferably 20 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 80 mass% or less, more preferably 75 mass% or less, and even more preferably 70 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0129] When information (e.g., composition, amount, etc.) about the raw materials used to form the first catalytic layer 20 is known, the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 can be determined from the information about the raw materials used to form the first catalytic layer 20.

[0130] When information about the raw materials used to form the first catalytic layer 20 is unknown, the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 can be determined by a conventional method such as SEM-EDX. Specifically, this is as follows.

[0131] (1) Elemental analysis is performed on the sample obtained from the first catalytic layer 20 using a standard method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the oxide-equivalent content (mass%) of each identified metal element. (2) Elemental mapping is performed on the sample obtained from the first catalytic layer 20 using a standard method such as SEM-EDX to identify the types of particles contained in the sample (e.g., Al-based oxides, Ce-based oxides, Ce-Zr-based composite oxides, etc.). (3) For each type of particle, elemental analysis is performed on a number of arbitrarily selected particles (e.g., 50 particles) using SEM-EDX to identify the types of constituent elements of the particles and to determine the oxide-equivalent content (mass%) of each identified metal element. For each type of particle, the average oxide-equivalent content (mass%) of each metal element is calculated, and this is defined as the oxide-equivalent content (mass%) of each metal element in each type of particle. (4) An equation is created and solved to represent the relationship between the oxide-equivalent content (mass %) of each metal element in the sample, the oxide-equivalent content (mass %) of each metal element in each type of particle, and the content (mass %) of each type of particle in the sample, thereby calculating the content (mass %) of each type of particle in the sample, and this is set as the content (mass %) of each type of particle in the first catalytic layer 20.

[0132] From the viewpoint of improving the heat resistance and OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Ce in the first catalytic layer 20 is converted to CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0133] When the first catalytic layer 20 contains a Ce—Zr-based composite oxide, the average particle size of the Ce—Zr-based composite oxide contained in the first catalytic layer 20 is preferably 2 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0134] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains Al and / or Zr.

[0135] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Al content of the Al in the first catalytic layer 20 is ２ O ３ The converted mass of Zr in the first catalyst layer 20 and ZrO ２ The percentage of the total of the converted mass of the catalyst layer 20 relative to the mass of the first catalyst layer 20 is preferably 30 mass% or more, more preferably 35 mass% or more, and even more preferably 40 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 80 mass% or less, more preferably 78 mass% or less, and even more preferably 76 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0136] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Al content of the Al in the first catalytic layer 20 is ２ O ３ The percentage of the converted mass of Al in the first catalytic layer 20 relative to the mass of the first catalytic layer 20 (in this specification, "Al of the first catalytic layer 20") ２ O ３ The total content (referred to as "content in terms of total mass") is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0137] Regarding a certain catalyst layer, "Al in the catalyst layer" ２ O ３ When the catalyst layer contains one type of Al source, the "concentration of Al converted from Al derived from the one type of Al source" is ２ O ３When the catalyst layer contains two or more Al sources, the content of Al derived from the two or more Al sources is expressed as the Al content in terms of Al. ２ O ３ This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40).

[0138] When the first catalytic layer 20 contains Al, the first catalytic layer 20 contains one or more Al sources.

[0139] The Al source may be an oxide containing Al, such as an Al-based oxide, a Ce-based oxide containing Al, a Zr-based oxide containing Al, a Ce-Zr-based composite oxide containing Al, or an alumina binder.

[0140] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains an Al-based oxide as the Al source. The first catalytic layer 20 may contain, as the Al source, one or more other Al sources in addition to the Al-based oxide.

[0141] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the percentage of the mass of the Al-based oxide in the first catalytic layer 20 relative to the mass of the first catalytic layer 20 (referred to herein as the "content of Al-based oxide in the first catalytic layer 20") is preferably 10 mass% or more, more preferably 15 mass% or more, and even more preferably 20 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 70 mass% or less, more preferably 65 mass% or less, and even more preferably 60 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0142] The content of Al-based oxide in the first catalytic layer 20 can be determined in the same manner as the content of Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0143] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Al content of the Al in the first catalytic layer 20 is ２ O ３ Of the converted mass, Al derived from Al-based oxides ２ O ３ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0144] When the first catalytic layer 20 contains an Al-based oxide, the average particle size of the Al-based oxide contained in the first catalytic layer 20 is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0145] From the viewpoint of improving the heat resistance of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Zr in the first catalytic layer 20 is converted to ZrO ２ The percentage of the converted mass of Zr in the first catalyst layer 20 to the mass of the first catalyst layer 20 (in this specification, "ZrO ２ The total content (referred to as "content in terms of total mass") is preferably 5% by mass or more, more preferably 7% by mass or more, and even more preferably 10% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0146] Regarding a certain catalyst layer, "ZrO of Zr in the catalyst layer" ２ When the catalyst layer contains one type of Zr source, the "content of ZrO converted from Zr derived from the one type of Zr source" is ２ When the catalyst layer contains two or more Zr sources, the ZrO content of Zr derived from the two or more Zr sources is ２ This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40).

[0147] When the first catalytic layer 20 contains Zr, the first catalytic layer 20 contains one or more Zr sources.

[0148] The Zr source may be an oxide containing Zr, such as an Al-based oxide containing Zr, a Ce-based oxide containing Zr, a Zr-based oxide, a Ce-Zr-based composite oxide, or a zirconia binder.

[0149] From the viewpoint of improving the heat resistance and OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains a Ce—Zr-based composite oxide as a Zr source. The first catalytic layer 20 may contain, as a Zr source, one or more other Zr sources in addition to the Ce—Zr-based composite oxide.

[0150] The content of the Ce—Zr-based composite oxide in the first catalytic layer 20 is the same as that described above.

[0151] From the viewpoint of improving the heat resistance and OSC of the first catalytic layer 20 and thereby improving the exhaust gas purification performance of the first catalytic layer 20, the Zr in the first catalytic layer 20 is converted to ZrO ２ Of the converted mass, ZrO derived from Ce-Zr composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0152] The first catalyst layer 20 may contain other components such as a binder and a stabilizer. Examples of the binder include metal oxide binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol. Examples of the stabilizer include nitrates, carbonates, oxides, and sulfates of alkaline earth metal elements (e.g., Sr, Ba, etc.).

[0153] <Second catalytic layer> The second catalytic layer 30 will now be described.

[0154] As shown in FIGS. 3 and 4, the second catalyst layer 30 is disposed on the first catalyst layer 20 .

[0155] The phrase "the second catalytic layer 30 is provided on the first catalytic layer 20" means that part or all of the second catalytic layer 30 is present on one of the two main surfaces of the first catalytic layer 20 opposite the main surface on the partition wall section 12 side. The "main surface of the first catalytic layer 20" means the outer surface of the first catalytic layer 20 extending in the exhaust gas flow direction X. The second catalytic layer 30 may be provided directly on the main surface of the first catalytic layer 20 or via another layer, but is usually provided directly on the main surface of the first catalytic layer 20. The second catalytic layer 30 may be provided so as to cover part of the main surface of the first catalytic layer 20 or so as to cover the entire main surface of the first catalytic layer 20. The "second catalytic layer 30 provided on the first catalytic layer 20" includes an embodiment in which the second catalytic layer 30 is provided directly on the main surface of the first catalytic layer 20, and an embodiment in which the second catalytic layer 30 is provided on the main surface of the first catalytic layer 20 via another layer.

[0156] 4 , the second catalytic layer 30 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side. The second catalytic layer 30 may extend along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, or may extend in the direction opposite to the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas outlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0157] From the viewpoints of achieving a good balance between exhaust gas purification performance and cost and improving the contact between the catalytically active component (e.g., Pd contained in the first catalytic layer 20) contained in the portion below the second catalytic layer 30 and the exhaust gas, the mass of the second catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed is preferably 30 g / L or more and 160 g / L or less, more preferably 40 g / L or more and 140 g / L or less, and even more preferably 50 g / L or more and 120 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0158] The mass of the second catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed is calculated from the formula: (mass of the second catalytic layer 30) / ((volume of the substrate 10)×(average length L30 of the second catalytic layer 30 / length L10 of the substrate 10)).

[0159] The above description regarding the method for measuring the average length L20 of the first catalytic layer 20 also applies to the second catalytic layer 30. When applied, the "first catalytic layer 20" is replaced with the "second catalytic layer 30," and the "average length L20" is replaced with the "average length L30."

[0160] The second catalytic layer 30 contains Rh as a catalytically active component. Rh is contained in the second catalytic 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 a particulate form.

[0161] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the mass of Rh in the second catalytic layer 30 in terms of metal relative to the mass of the second catalytic layer 30 (i.e., the above b) is preferably 0.01 mass% or more and 5 mass% or less, more preferably 0.02 mass% or more and 4 mass% or less, and even more preferably 0.03 mass% or more and 3 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0162] The second catalytic layer 30 may contain one or more precious metal elements other than Rh as catalytically active components. The precious metal elements other than Rh can be selected from, for example, Pt, Pd, Ru, Os, Ir, Au, Ag, etc. The precious metal elements other than Rh are contained in the second catalytic layer 30 in a form that can function as a catalytically active component, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (for example, an oxide of the precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the precious metal element other than Rh is preferably in a particulate form.

[0163] When the second catalytic layer 30 contains Rh and a precious metal element other than Rh, the Rh and the precious metal element other than Rh may form an alloy, which may reduce the active sites of Rh, which is involved in the exhaust gas purification performance. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Rh in the second catalytic layer 30 in terms of metal relative to the mass of the second catalytic layer 30 (referred to herein as the "content of the precious metal elements other than Rh in the second catalytic layer 30 in terms of metal") is small. Specifically, the content of the precious metal elements other than Rh in the second catalytic layer 30 in terms of metal is preferably 0.01% by mass or less, more preferably 0.005% by mass or less, and even more preferably 0.003% by mass or less. The lower limit is 0% by mass. The "metal-equivalent content of precious metal elements other than Rh in the second catalytic layer 30" means, when the second catalytic layer 30 contains one type of precious metal element other than Rh, the metal-equivalent content of that one type of precious metal element, and when the second catalytic layer 30 contains two or more types of precious metal elements other than Rh, the metal-equivalent content of that two or more types of precious metal elements.

[0164] The metal-equivalent content of the predetermined noble metal element in the second catalytic layer 30 can be determined in the same manner as the metal-equivalent content of the predetermined noble metal element in the first catalytic layer 20 .

[0165] The second catalytic layer 30 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers. The carrier may be selected from, for example, metal oxides. The description of the metal oxides is the same as above.

[0166] From the viewpoint of improving the heat resistance and / or OSC of the second catalytic layer 30, and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the support is preferably selected from an Al-based oxide, a Ce-based oxide, and a Ce—Zr-based composite oxide, and more preferably selected from an Al-based oxide and a Ce—Zr-based composite oxide. In one embodiment, the second catalytic layer 30 contains an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0167] The second catalytic layer 30 contains Ce. CeO of Ce in the second catalytic layer 30 ２The percentage of the converted mass of Ce in the second catalytic layer 30 to the mass of the second catalytic layer 30 (referred to herein as "CeO ２ The content (concentration in terms of carbon dioxide equivalent) of the second catalyst layer 30 is 7 mass % or more. This can improve the OSC of the second catalyst layer 30 and the exhaust gas purification performance of the second catalyst layer 30.

[0168] From the viewpoint of more effectively improving the OSC of the second catalytic layer 30 and thereby more effectively improving the exhaust gas purification performance of the second catalytic layer 30, the Ce in the second catalytic layer 30 is ２ The converted content is preferably 8% by mass or more, more preferably 10% by mass or more, and even more preferably 11% by mass or more. The upper limit can be appropriately adjusted taking into consideration the balance with cost, the content of other components, etc. The upper limit is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0169] The second catalyst layer 30 contains one or more Ce sources. The Ce sources are the same as those described above.

[0170] From the viewpoint of improving the OSC of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the second catalytic layer 30 preferably contains a Ce—Zr-based composite oxide as a Ce source. The second catalytic layer 30 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide.

[0171] From the viewpoint of improving the heat resistance and OSC of the second catalytic layer 30, and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the percentage of the mass of the Ce—Zr-based composite oxide in the second catalytic layer 30 relative to the mass of the second catalytic layer 30 (referred to herein as the "content of Ce—Zr-based composite oxide in the second catalytic layer 30") is preferably 20 mass% or more, more preferably 25 mass% or more, and even more preferably 30 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 80 mass% or less, more preferably 75 mass% or less, and even more preferably 70 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0172] The content of the Ce—Zr-based composite oxide in the second catalytic layer 30 can be determined in the same manner as the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0173] From the viewpoint of improving the heat resistance and OSC of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Ce in the second catalytic layer 30 is converted to CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0174] When the second catalytic layer 30 contains a Ce—Zr-based composite oxide, the average particle size of the Ce—Zr-based composite oxide contained in the second catalytic layer 30 is preferably 2 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0175] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the second catalytic layer 30 preferably contains Al and / or Zr.

[0176] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Al content of the Al in the second catalytic layer 30 is ２ O３ The converted mass of Zr in the second catalyst layer 30 and ZrO ２ The percentage of the total of the converted mass of the catalyst layer 30 relative to the mass of the second catalyst layer 30 is preferably 50 mass% or more, more preferably 55 mass% or more, and even more preferably 65 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 96 mass% or less, more preferably 93 mass% or less, and even more preferably 90 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0177] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Al content of the Al in the second catalytic layer 30 is ２ O ３ The percentage of the converted mass of Al in the second catalytic layer 30 relative to the mass of the second catalytic layer 30 (referred to herein as "Al of the second catalytic layer 30"). ２ O ３ The total content (referred to as "content in terms of total mass") is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0178] When the second catalytic layer 30 contains Al, the second catalytic layer 30 contains one or more Al sources. The description of the Al source is the same as above.

[0179] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the second catalytic layer 30 preferably contains an Al-based oxide as the Al source. The second catalytic layer 30 may contain, as the Al source, one or more other Al sources in addition to the Al-based oxide.

[0180] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the percentage of the mass of the Al-based oxide in the second catalytic layer 30 relative to the mass of the second catalytic layer 30 (referred to herein as the "content of Al-based oxide in the second catalytic layer 30") is preferably 10 mass% or more, more preferably 15 mass% or more, and even more preferably 20 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with cost, the contents of other components, and the like. The upper limit is preferably 45 mass% or less, more preferably 40 mass% or less, and even more preferably 35 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0181] The content of Al-based oxide in the second catalytic layer 30 can be determined in the same manner as the content of Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0182] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Al content of the Al in the second catalytic layer 30 is ２ O ３ Of the converted mass, Al derived from Al-based oxides ２ O ３ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0183] When the second catalytic layer 30 contains an Al-based oxide, the average particle size of the Al-based oxide contained in the second catalytic layer 30 is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0184] From the viewpoint of improving the heat resistance of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Zr in the second catalytic layer 30 is preferably ZrO ２ The percentage of the converted mass of Zr in the second catalyst layer 30 to the mass of the second catalyst layer 30 (in this specification, "ZrO ２The total content (referred to as "content in terms of total mass") is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 65% ​​by mass or less, more preferably 60% by mass or less, and even more preferably 55% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0185] When the second catalytic layer 30 contains Zr, the second catalytic layer 30 contains one or more Zr sources. The explanation regarding the Zr sources is the same as above.

[0186] From the viewpoint of improving the heat resistance and OSC of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the second catalytic layer 30 preferably contains a Ce—Zr-based composite oxide as a Zr source. The second catalytic layer 30 may contain, as a Zr source, one or more other Zr sources in addition to the Ce—Zr-based composite oxide.

[0187] The content of the Ce—Zr-based composite oxide in the second catalytic layer 30 is the same as that described above.

[0188] From the viewpoint of improving the heat resistance and OSC of the second catalytic layer 30 and thereby improving the exhaust gas purification performance of the second catalytic layer 30, the Zr in the second catalytic layer 30 is converted to ZrO ２ Of the converted mass, ZrO derived from Ce-Zr composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0189] The second catalyst layer 30 may contain other components such as a binder, a stabilizer, etc. The explanation regarding the binder and the stabilizer is the same as above.

[0190] From the viewpoint of improving the contact between the catalytically active component contained in the portion below the second catalytic layer 30 (e.g., Pd contained in the first catalytic layer 20) and the exhaust gas, thereby improving the exhaust gas purification performance of the first catalytic layer 20, the average thickness of the second catalytic layer 30 is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less. The lower limit can be adjusted appropriately taking into account the OSC required for the second catalytic layer 30, etc. The lower limit is preferably 20 μm or more, more preferably 25 μm or more, and even more preferably 30 μm or more. Each of the above lower limits may be combined with any of the above upper limits.

[0191] The method for calculating the average thickness of the second catalyst layer 30 will be described later.

[0192] <Third Catalyst Layer> The third catalyst layer 40 will now be described.

[0193] As shown in FIGS. 3 and 4, the third catalyst layer 40 is disposed on the second catalyst layer 30 .

[0194] The phrase "the third catalytic layer 40 is provided on the second catalytic layer 30" means that part or all of the third catalytic layer 40 is present on one of the two main surfaces of the second catalytic layer 30 opposite the main surface on the first catalytic layer 20 side. The "main surface of the second catalytic layer 30" means the outer surface of the second catalytic layer 30 extending in the exhaust gas flow direction X. The third catalytic layer 40 may be provided directly on the main surface of the second catalytic layer 30 or via another layer, but is usually provided directly on the main surface of the second catalytic layer 30. The third catalytic layer 40 may be provided so as to cover part of the main surface of the second catalytic layer 30 or so as to cover the entire main surface of the second catalytic layer 30. The "third catalytic layer 40 provided on the second catalytic layer 30" includes an embodiment in which the third catalytic layer 40 is provided directly on the main surface of the second catalytic layer 30, and an embodiment in which the third catalytic layer 40 is provided on the main surface of the second catalytic layer 30 via another layer.

[0195] Another catalyst layer may be provided on the third catalyst layer 40. However, from the viewpoint of more effectively improving the contact between the Rh contained in the third catalyst layer 40 and the exhaust gas, it is preferable that another catalyst layer is not provided on the third catalyst layer 40.

[0196] 4 , the third catalytic layer 40 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side. The third catalytic layer 40 may extend along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, or may extend in the direction opposite to the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas outlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0197] From the viewpoints of achieving a good balance between exhaust gas purification performance and cost and improving the contact between the catalytically active components contained in the portion below the third catalytic layer 40 (e.g., Pd contained in the first catalytic layer 20 and Rh contained in the second catalytic layer 30) and the exhaust gas, the mass of the third catalytic layer 40 per unit volume of the portion of the substrate 10 on which the third catalytic layer 40 is formed is preferably 10 g / L or more and 70 g / L or less, more preferably 12 g / L or more and 60 g / L or less, and even more preferably 15 g / L or more and 50 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0198] The mass of the third catalytic layer 40 per unit volume of the portion of the substrate 10 on which the third catalytic layer 40 is formed is calculated from the formula: (mass of the third catalytic layer 40) / ((volume of the substrate 10)×(average length L40 of the third catalytic layer 40 / length L10 of the substrate 10)).

[0199] The above description regarding the method for measuring the average length L20 of the first catalytic layer 20 also applies to the third catalytic layer 40. When applied, the "first catalytic layer 20" is replaced with the "third catalytic layer 40," and the "average length L20" is replaced with the "average length L40."

[0200] The third catalytic layer 40 contains Rh as a catalytically active component. Rh is contained in the third catalytic layer 40 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 a particulate form.

[0201] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the mass of Rh in the third catalytic layer 40 in terms of metal relative to the mass of the third catalytic layer 40 (i.e., the above-mentioned a) is preferably 0.02 mass% or more and 10 mass% or less, more preferably 0.05 mass% or more and 8 mass% or less, and even more preferably 0.1 mass% or more and 6 mass% or less. Each of the above-mentioned lower limits may be combined with any of the above-mentioned upper limits.

[0202] The third catalytic layer 40 may contain one or more precious metal elements other than Rh as catalytically active components. The precious metal elements other than Rh can be selected from, for example, Pt, Pd, Ru, Os, Ir, Au, Ag, etc. The precious metal elements other than Rh are contained in the third catalytic layer 40 in a form that can function as a catalytically active component, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (e.g., an oxide of the precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the precious metal element other than Rh is preferably in a particulate form.

[0203] When the third catalytic layer 40 contains Rh and a precious metal element other than Rh, the Rh and the precious metal element other than Rh may form an alloy, which may reduce the active sites of Rh that contribute to exhaust gas purification performance. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Rh in the third catalytic layer 40 in terms of metal relative to the mass of the third catalytic layer 40 (referred to herein as the "content of the precious metal elements other than Rh in the third catalytic layer 40 in terms of metal") is small. Specifically, the content of the precious metal elements other than Rh in the third catalytic layer 40 in terms of metal is preferably 0.01% by mass or less, more preferably 0.005% by mass or less, and even more preferably 0.003% by mass or less. The lower limit is 0% by mass. The "metal-equivalent content of precious metal elements other than Rh in the third catalytic layer 40" means the metal-equivalent content of one precious metal element when the third catalytic layer 40 contains one precious metal element other than Rh, and means the metal-equivalent total content of the two or more precious metal elements when the third catalytic layer 40 contains two or more precious metal elements other than Rh.

[0204] The third catalytic layer 40 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers. The carrier may be selected from, for example, metal oxides. The description of the metal oxides is the same as above.

[0205] From the viewpoint of improving the heat resistance and / or OSC of the third catalytic layer 40, and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the support is preferably selected from an Al-based oxide, a Ce-based oxide, and a Ce—Zr-based composite oxide, and more preferably selected from an Al-based oxide and a Ce—Zr-based composite oxide. In one embodiment, the third catalytic layer 40 contains an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0206] The third catalytic layer 40 contains Al and / or Zr, which improves the heat resistance of the third catalytic layer 40 and enables the exhaust gas purification performance of the third catalytic layer 40 to be improved.

[0207] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the Al content of the Al in the third catalytic layer 40 is ２ O３ The converted mass of Zr in the third catalyst layer 40 and ZrO ２ The percentage of the total mass of the third catalyst layer 40, including the converted mass, is preferably 80 mass% or more, more preferably 85 mass% or more, and even more preferably 90 mass% or more. The upper limit can be adjusted appropriately taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 99 mass% or less, more preferably 97 mass% or less, and even more preferably 95 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0208] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the Al content of the Al in the third catalytic layer 40 is ２ O ３ The percentage of the converted mass of Al in the third catalytic layer 40 to the mass of the third catalytic layer 40 (referred to herein as "Al of the third catalytic layer 40") ２ O ３ The content (converted to "content in terms of mass") is preferably 60% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0209] When the third catalytic layer 40 contains Al, the third catalytic layer 40 contains one or more Al sources. The description of the Al source is the same as above.

[0210] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the third catalytic layer 40 preferably contains an Al-based oxide as the Al source. The third catalytic layer 40 may contain, as the Al source, one or more other Al sources in addition to the Al-based oxide.

[0211] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the percentage of the mass of the Al-based oxide in the third catalytic layer 40 relative to the mass of the third catalytic layer 40 (referred to herein as the "content of Al-based oxide in the third catalytic layer 40") is preferably 65 mass% or more, more preferably 70 mass% or more, and even more preferably 75 mass% or more. The upper limit can be appropriately adjusted taking into consideration the balance with cost, the contents of other components, and the like. The upper limit is preferably 99 mass% or less, more preferably 95 mass% or less, and even more preferably 93 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0212] The content of Al-based oxide in the third catalytic layer 40 can be determined in the same manner as the content of Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0213] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the Al content of Al in the third catalytic layer 40 is ２ O ３ Of the converted mass, Al derived from Al-based oxides ２ O ３ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0214] When the third catalytic layer 40 contains an Al-based oxide, the average particle size of the Al-based oxide contained in the third catalytic layer 40 is preferably 0.1 μm or more and 9 μm or less, and more preferably 0.1 μm or more and 7 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0215] From the viewpoint of improving the heat resistance of the third catalytic layer 40 and thereby improving the exhaust gas purification performance of the third catalytic layer 40, the Zr in the third catalytic layer 40 is converted to ZrO ２ The percentage of the converted mass of Zr in the third catalyst layer 40 to the mass of the third catalyst layer 40 (in this specification, "ZrO ２The total content (referred to as "content in terms of mass equivalent") is preferably 10% by mass or more, more preferably 11% by mass or more, and even more preferably 15% by mass or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 30% by mass or less, more preferably 27% by mass or less, and even more preferably 25% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0216] When the third catalytic layer 40 contains Zr, the third catalytic layer 40 contains one or more Zr sources. The description of the Zr sources is the same as above. In one embodiment, the third catalytic layer 40 contains a Ce—Zr-based composite oxide as the Zr source.

[0217] CeO of Ce in the third catalyst layer 40 ２ The percentage of the converted mass of Ce in the third catalytic layer 40 to the mass of the third catalytic layer 40 (referred to herein as "CeO ２ The content of the third catalyst layer 40 (converted to "content in terms of carbon black") is less than 7 mass %. This improves the heat resistance of the third catalyst layer 40, and the exhaust gas purification performance of the third catalyst layer 40.

[0218] From the viewpoint of more effectively improving the heat resistance of the third catalytic layer 40 and thereby more effectively improving the exhaust gas purification performance of the third catalytic layer 40, the Ce in the third catalytic layer 40 is ２ The converted content is preferably 6% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less. The lower limit is 0% by mass.

[0219] The third catalytic layer 40 may contain Ce. When the third catalytic layer 40 contains Ce, the CeO ２ The converted content may be, for example, 0.5% by mass or more, 1% by mass or more, or 1.5% by mass or more. Each of the above lower limits may be combined with any of the above upper limits.

[0220] When the third catalytic layer 40 contains Ce, the third catalytic layer 40 contains one or more Ce sources. The explanation regarding the Ce source is the same as above.

[0221] When the third catalytic layer 40 contains Ce, it is preferable that the third catalytic layer 40 contains a Ce—Zr-based composite oxide as a Ce source. The third catalytic layer 40 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide.

[0222] When the third catalytic layer 40 contains Ce, the percentage of the mass of the Ce—Zr-based composite oxide in the third catalytic layer 40 relative to the mass of the third catalytic layer 40 (referred to herein as the "content of the Ce—Zr-based composite oxide in the third catalytic layer 40") is preferably 5 mass% or more, more preferably 10 mass% or more, and even more preferably 15 mass% or more. The upper limit can be adjusted as appropriate, taking into consideration the balance with costs, the contents of other components, and the like. The upper limit is preferably 40 mass% or less, more preferably 37 mass% or less, and even more preferably 35 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0223] The content of the Ce—Zr-based composite oxide in the third catalytic layer 40 can be determined in the same manner as the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0224] When the third catalytic layer 40 contains Ce, the CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0225] When the third catalytic layer 40 contains a Ce—Zr-based composite oxide, the average particle size of the Ce—Zr-based composite oxide contained in the third catalytic layer 40 is preferably 0.1 μm or more and 9 μm or less, and more preferably 0.1 μm or more and 7 μm or less. Each of the above lower limits may be combined with any of the above upper limits.

[0226] The third catalyst layer 40 may contain other components such as a binder, a stabilizer, etc. The explanation regarding the binder and the stabilizer is the same as above.

[0227] The average thickness of the third catalytic layer 40 is 10 μm or less. This improves the contact between the catalytically active components contained in the portion below the third catalytic layer 40 (e.g., Pd contained in the first catalytic layer 20 and Rh contained in the second catalytic layer 30) and the exhaust gas, thereby improving the exhaust gas purification performance of the portion below the third catalytic layer 40 (e.g., the first catalytic layer 20 and the second catalytic layer 30).

[0228] From the viewpoint of more effectively improving the contact between the catalytically active components contained in the portion below the third catalytic layer 40 and the exhaust gas, the average thickness of the third catalytic layer 40 is preferably 7 μm or less, more preferably 5 μm or less, and even more preferably 4 μm or less. The lower limit can be adjusted appropriately taking into consideration the heat resistance required of the third catalytic layer 40, etc. The lower limit is preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 1.5 μm or more. Each of the above lower limits may be combined with any of the above upper limits.

[0229] An example of a method for calculating the average thickness of the second catalytic layer 30 and the average thickness of the third catalytic layer 40 is as follows.

[0230] Catalyst 1 (when the first catalyst layer 20 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side, for example, a position 30 mm away from the end of the partition wall 12 on the exhaust gas inlet side in the exhaust gas flow direction X. When the first catalyst layer 20 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, for example, a position 10 mm away from the end of the partition wall 12 on the exhaust gas inlet side in the exhaust gas flow direction X. When the first catalyst layer 20 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side, for example, a position 10 mm away from the end of the partition wall 12 on the exhaust gas inlet side in the exhaust gas flow direction X. In the case where the partition wall 12 extends in the direction opposite to the exhaust gas flow direction X from the end of the exhaust gas outflow side of the partition wall 12, for example, a point 10 mm away from the end of the exhaust gas outflow side of the partition wall 12 in the direction opposite to the exhaust gas flow direction X. The substrate 10 is cut along a plane perpendicular to the axial direction of the substrate 10, and a backscattered electron detector (BED) in an SEM or EPMA is used to observe the first catalytic layer 20, the second catalytic layer 30, and the third catalytic layer 40 present in one cell 13 arbitrarily selected from the cut surface, thereby identifying the region where the first catalytic layer 20 is present, the region where the second catalytic layer 30 is present, and the region where the third catalytic layer 40 is present. In observing the cut surface with SEM or BED, the magnification of the field of view is, for example, 500 times, and the field of view width (length) is, for example, 100 to 200 μm. The region observed with SEM or BED is set so as not to include the corners of the cells 13. This is because the corners of the cells 13 are easily affected by the cell shape. The region where the first catalytic layer 20 exists, the region where the second catalytic layer 30 exists, and the region where the third catalytic layer 40 exists can be identified based on the differences in morphology, composition, and the like between the first catalytic layer 20, the second catalytic layer 30, and the third catalytic layer 40. At this time, elemental mapping of the cross section may be performed. Elemental mapping can be performed, for example, by combining observation of the cross section using SEM or BED with composition analysis of the cross section. Elemental mapping can be performed, for example, using SEM-EDX, EPMA, or the like. By elemental mapping of the cross section, the region where the first catalytic layer 20 exists, the region where the second catalytic layer 30 exists, and the region where the third catalytic layer 40 exists can be identified based on the differences in morphology and composition between the first catalytic layer 20, the second catalytic layer 30, and the third catalytic layer 40.

[0231] In the SEM or BED observation image, first to Nth grid lines parallel to the thickness direction of the partition wall portion 12 of the substrate 10 are drawn at 15 μm intervals, starting from the left end or right end. The intersections of the grid lines with the outline of the region where the first catalytic layer 20 is present are connected with straight lines to identify the surface position of the first catalytic layer 20. N is, for example, an integer between 5 and 10. Similarly, the intersections of the grid lines with the outline of the region where the second catalytic layer 30 is present are connected with straight lines to identify the surface position of the second catalytic layer 30. Similarly, the intersections of the grid lines with the outline of the region where the third catalytic layer 40 is present are connected with straight lines to identify the surface position of the third catalytic layer 40. If the change in thickness direction from a certain intersection point P1 to an intersection point P2 adjacent to the intersection point P1 exceeds the grid line spacing (15 μm), it is preferable not to use the intersection point P2 to identify the surface position (i.e., to exclude the intersection point P2 from the intersection points connected with straight lines). The change in thickness direction from a certain intersection point P1 to an intersection point P2 adjacent to the intersection point P1 refers to the distance between a straight line passing through the intersection point P1 and perpendicular to the thickness direction of the partition wall portion 12 of the substrate 10, and a straight line passing through the intersection point P2 and perpendicular to the thickness direction of the partition wall portion 12 of the substrate 10. If the change in thickness direction from the intersection point P1 to the intersection point P2 adjacent to the intersection point P1 exceeds the grid line spacing (15 μm), and the change in thickness direction from the intersection point P1 to the intersection point P3 adjacent to the intersection point P2 also exceeds the grid line spacing (15 μm), it is preferable not to use the intersection point P3 in addition to the intersection point P2 to identify the surface position (i.e., to exclude the intersection points P2 and P3 from the intersection points connected by the straight lines). In this case, if five consecutive intersections are excluded from the intersection points connected by the straight lines, it is preferable not to measure the thickness of the SEM or BED image.

[0232] After identifying the positions of the surfaces of the first catalytic layer 20, the second catalytic layer 30, and the third catalytic layer 40, image analysis software is used to determine the area of ​​a first region surrounded by the second grid line, the (N-1)th grid line, the surface of the first catalytic layer 20, and the surface of the second catalytic layer 30. Similarly, the area of ​​a second region surrounded by the second grid line, the (N-1)th grid line, the surface of the second catalytic layer 30, and the surface of the third catalytic layer 40 is determined. Examples of image analysis software that can be used include AreaQ (manufactured by Estec Co., Ltd.), ImageJ (public domain), and Photoshop (Adobe Systems Inc.). Note that the first grid line and the Nth grid line are not used because both ends of the image tend to be blurred, making it difficult to identify the positions of the surfaces of the first catalytic layer 20, the second catalytic layer 30, and the third catalytic layer 40.

[0233] After determining the areas of the first and second regions, the thickness of each region is calculated based on the following formula: Thickness of each region = Area of ​​each region / (Grid line spacing × Number of grid line spacings) The grid line spacing is 15 μm, and the number of grid line spacings is (N-3).

[0234] The thickness of the first region is calculated for 20 cells 13 randomly selected from the cut surface, and the average value thereof is set as the average thickness of the second catalytic layer 30. The thickness of the second region is calculated for 20 cells 13 randomly selected from the cut surface, and the average value thereof is set as the average thickness of the third catalytic layer 40.

[0235] <Production of catalyst> Catalyst 1 can be produced by forming a first catalytic layer 20 on a substrate 10, then forming a second catalytic layer 30 on the first catalytic layer 20, and then forming a third catalytic layer 40 on the second catalytic layer 30.

[0236] The first catalyst layer 20 can be formed by mixing a Pd source (e.g., a Pd salt) and optionally other components (e.g., a metal oxide, a binder, a stabilizer, a solvent, etc.) to prepare a first slurry, applying the first slurry onto the substrate 10, drying, and firing.

[0237] The second catalytic layer 30 can be formed by mixing a Rh source (e.g., an Rh salt), a Ce source (e.g., a Ce-Zr composite oxide), and optionally other components (e.g., a metal oxide other than the Ce source, a binder, a stabilizer, a solvent, etc.) to prepare a second slurry, applying the second slurry onto the first catalytic layer 20, drying, and firing.

[0238] The third catalytic layer 40 can be formed by mixing a Rh source (e.g., an Rh salt), an Al source (e.g., an Al-based oxide), and / or a Zr source (e.g., a Ce-Zr-based composite oxide), and optionally other components (e.g., metal oxides other than the Al source and the Zr source, a binder, a stabilizer, a solvent, etc.) to prepare a third slurry, applying the third slurry onto the second catalytic layer 30, drying, and firing.

[0239] Examples of Pd salts and Rh salts include nitrates, ammine complex salts, acetates, 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.

[0240] The drying temperature is, for example, 60° C. or higher and 150° C. or lower, and the drying time is, for example, 0.1 hour or higher and 1 hour or lower. The firing temperature is, for example, 300° C. or higher and 700° C. or lower, and the firing time is, for example, 1 hour or higher and 10 hours or lower. The firing can be carried out, for example, in an air atmosphere.

[0241] Example 1 (1) Preparation of Slurry for Forming Lower Layer In a mixing vessel, an aqueous solution of palladium nitrate, Ce—Zr-based composite oxide (CeO ２ Conversion content: 13% by mass or more and 50% by mass or less of ZrO ２ content of one or more rare earth elements other than Ce in terms of oxides: 9% by mass to 20% by mass), Al-based oxides (Al in terms of Al ２ O ３The amount of each component in the slurry for forming the lower layer was 2.0 mass % in terms of Pd and 1.0 mass % in terms of metal, and Ce was 1.0 mass % in terms of metal, based on the mass of the lower layer after firing (100 mass %). ２ converted to 18.0 mass %, Zr is ZrO ２ 18.0 mass% in terms of Al, ２ O ３ The amount of rare earth elements other than Ce was adjusted to 57.5% by mass in terms of oxides.

[0242] (2) Formation of the lower layer As a flow-through type substrate, cells extending in the axial direction and partitioned by partition walls having a thickness of 50 to 70 μm were formed at a density of 600 cells / inch in a plane perpendicular to the axial direction. ２ A flow-through substrate having a density of 1.0 L and a volume of 1.0 L was prepared.

[0243] The flow-through type substrate was immersed in the underlayer forming slurry, and the flow-through type substrate coated with the underlayer forming slurry was dried at 150° C. for 0.5 hours and then baked at 500° C. for 1 hour to form a underlayer on the flow-through type substrate. The mass of the underlayer per unit volume of the portion of the flow-through type substrate on which the underlayer was formed was 100 g / L.

[0244] (3) Preparation of slurry for forming middle layer A mixing vessel was charged with an aqueous solution of rhodium nitrate, Ce—Zr-based composite oxide (CeO ２ Conversion content: 13% by mass or more and 50% by mass or less of ZrO ２ content of one or more rare earth elements other than Ce in terms of oxides: 9% by mass to 20% by mass), Al-based oxides (Al in terms of Al ２ O ３ The amount of each component in the slurry for forming the intermediate layer was 0.1 mass % in terms of Rh and 0.1 mass % in terms of CeO, with the mass of the fired intermediate layer being 100 mass %. ２ 12.0 mass% in terms of ZrO ２42.0 mass% in terms of Al, ２ O ３ The amount of rare earth elements other than Ce was adjusted to 39.6% by mass in terms of oxides, and 6.3% by mass in terms of oxides.

[0245] (4) Formation of Middle Layer The flow-through type substrate on which the lower layer had been formed was immersed in the middle layer forming slurry, and the flow-through type substrate coated with the middle layer forming slurry was dried at 150° C. for 0.5 hours and then fired at 500° C. for 1 hour to form a middle layer on the lower layer. The mass of the middle layer per unit volume of the portion of the flow-through type substrate on which the middle layer had been formed was 100 g / L.

[0246] (5) Preparation of Slurry for Forming Upper Layer In a mixing vessel, an aqueous solution of rhodium nitrate, Ce—Zr-based composite oxide (CeO ２ Conversion content: 13% by mass or more and 50% by mass or less of ZrO ２ The content of one or more rare earth elements other than Ce in terms of oxides is 35% by mass or more and 70% by mass or less, and the content of one or more rare earth elements other than Ce in terms of oxides is 9% by mass or more and 20% by mass or less), Al-based oxides (Al ２ O ３ The amount of each component in the slurry for forming the upper layer was 0.7 mass % in terms of metal for Rh, 0.7 mass % in terms of metal for Ce, and 0.5 mass % in terms of metal for CeO, based on the mass of the upper layer after firing (100 mass %). ２ 6.7 mass% in terms of ZrO ２ 23.3 mass% in terms of Al, ２ O ３ The amount of rare earth elements other than Ce was adjusted to 66.0 mass % in terms of oxides.

[0247] (6) Formation of Upper Layer The flow-through substrate on which the middle layer had been formed was immersed in the upper layer forming slurry, and the flow-through substrate coated with the upper layer forming slurry was dried at 150° C. for 0.5 hours and then fired at 500° C. for 1 hour to form an upper layer on the middle layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate on which the upper layer had been formed was 15 g / L.

[0248] In this manner, an exhaust gas purifying catalyst of Example 1 was produced, which included a lower layer formed on a flow-through type substrate, a middle layer formed on the lower layer, and an upper layer formed on the middle layer. The lower layer, middle layer, and upper layer in the exhaust gas purifying catalyst of Example 1 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0249] The average thickness of the second catalyst layer and the average thickness of the third catalyst layer in the exhaust gas purifying catalyst of Example 1 were calculated by the above-mentioned method. Specifically, the calculations are as follows.

[0250] The exhaust gas purification catalyst of Example 1 (at a location 30 mm away from the end of the substrate on the exhaust gas inlet side in the axial direction of the substrate) was cut along a plane perpendicular to the axial direction of the substrate, and a backscattered electron detector (BED) in an EPMA was used to observe the first catalyst layer, the second catalyst layer, and the third catalyst layer present in one cell arbitrarily selected from the cut surface, and the region where the first catalyst layer was present, the region where the second catalyst layer was present, and the region where the third catalyst layer was present were identified. In observing the cut surface with BED, the field of view magnification was 500 times, and the field of view width was 100 to 200 μm. The region observed with BED was set so as not to include the corners of the cells. The region where the first catalyst layer was present, the region where the second catalyst layer was present, and the region where the third catalyst layer was present were identified by elemental mapping of the cut surface using EPMA.

[0251] In the BED observation image, starting from the left end, first to ninth grid lines parallel to the thickness direction of the partition wall portion of the substrate were drawn at 15 μm intervals, and the intersections of the grid lines with the outline of the region where the first catalytic layer was present were connected with straight lines to identify the surface position of the first catalytic layer. Similarly, the intersections of the grid lines with the outline of the region where the second catalytic layer was present were connected with straight lines to identify the surface position of the second catalytic layer. Similarly, the intersections of the grid lines with the outline of the region where the third catalytic layer was present were connected with straight lines to identify the surface position of the third catalytic layer. If the change in thickness direction from a certain intersection P1 to an intersection P2 adjacent to the intersection P1 exceeded the grid line spacing (15 μm), the intersection P2 was not used to identify the surface position (i.e., the intersection P2 was excluded from the intersections connected with straight lines). Furthermore, if the change in thickness direction from intersection P1 to intersection P2 adjacent to intersection P1 exceeds the grid line spacing (15 μm), and if the change in thickness direction from intersection P1 to intersection P3 adjacent to intersection P2 also exceeds the grid line spacing (15 μm), intersection P3 as well as intersection P2 were not used to identify the surface position (i.e., intersections P2 and P3 were excluded from the intersections connected by straight lines). In this way, if five consecutive intersections were excluded from the intersections connected by straight lines, the BED image was not used for thickness measurement.

[0252] After identifying the positions of the surfaces of the first catalytic layer, the second catalytic layer, and the third catalytic layer, image analysis software was used to determine the area of ​​the first region enclosed by the second grid line, the eighth grid line, the surface of the first catalytic layer, and the surface of the second catalytic layer. Similarly, the area of ​​the second region enclosed by the second grid line, the eighth grid line, the surface of the second catalytic layer, and the surface of the third catalytic layer was determined. AreaQ (manufactured by Estec Co., Ltd.) was used as the image analysis software. Note that the first grid line and the ninth grid line were not used because the edges of the image tend to be blurred, making it difficult to identify the positions of the surfaces of the first catalytic layer, the second catalytic layer, and the third catalytic layer.

[0253] After determining the areas of the first and second regions, the thickness of each region was calculated based on the following formula: Thickness of each region = Area of ​​each region / (Spacing of grid lines × Number of spacings of grid lines) The spacing of the grid lines was 15 μm, and the number of spacings of the grid lines was 6.

[0254] The thicknesses of the first region were calculated for 20 cells randomly selected from the cross section, and their average value was used as the average thickness of the second catalytic layer. Also, the thicknesses of the second region were calculated for 20 cells randomly selected from the cross section, and their average value was used as the average thickness of the third catalytic layer.

[0255] The average thickness of the second catalyst layer was 40 μm, and the average thickness of the third catalyst layer was 3.3 μm. Table 1 shows the characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purifying catalyst of Example 1.

[0256] In Table 1, “Rh”, “CeO ２ ", "Al ２ O ３ ", "ZrO ２ ", "Al ２ O ３ + ZrO ２ The meanings of "," "average thickness" and "a / b" are as follows.

[0257] [Second catalytic layer] Rh: percentage (mass %) of the mass of Rh in the second catalytic layer in terms of metal relative to the mass of the second catalytic layer CeO ２ : CeO of Ce in the second catalyst layer ２ Percentage (mass%) of the converted mass to the mass of the second catalyst layer Al ２ O ３ : Al of Al in the second catalyst layer ２ O ３ Percentage (mass%) of converted mass to the mass of the second catalyst layer ZrO ２ : ZrO of Zr in the second catalyst layer ２ Percentage (mass%) of the converted mass to the mass of the second catalyst layer Al ２ O ３ + ZrO ２ : Al of Al in the second catalyst layer ２ O ３ The converted mass of Zr in the second catalyst layer and ZrO ２The percentage (mass %) of the total mass of the second catalyst layer and the converted mass of the second catalyst layer. Average thickness: the average thickness (μm) of the second catalyst layer.

[0258] [Third catalytic layer] Rh: percentage (mass %) of the mass of Rh in the third catalytic layer in terms of metal relative to the mass of the third catalytic layer ２ : CeO of Ce in the third catalyst layer ２ Percentage (mass%) of converted mass to the mass of the third catalyst layer Al ２ O ３ : Al of Al in the third catalyst layer ２ O ３ Percentage (mass %) of converted mass to the mass of the third catalyst layer ZrO ２ : ZrO of Zr in the third catalyst layer ２ Percentage (mass%) of converted mass to the mass of the third catalyst layer Al ２ O ３ + ZrO ２ : Al of Al in the third catalyst layer ２ O ３ The mass of Zr in the third catalyst layer is converted to ZrO. ２ The percentage (mass %) of the total mass of the converted catalyst layer and the mass of the third catalyst layer. Average thickness: the average thickness (μm) of the third catalyst layer.

[0259] [a / b] a: percentage (mass %) of the mass of Rh in the third catalytic layer in terms of metal relative to the mass of the third catalytic layer b: percentage (mass %) of the mass of Rh in the second catalytic layer in terms of metal relative to the mass of the second catalytic layer

[0260] Example 2 The amount of each component in the slurry for forming the upper layer was 0.5 mass % in terms of metal for Rh, 0.5 mass % in terms of metal for Ce, and 100 mass % in terms of CeO ２ 5.0 mass% in terms of ZrO ２ 17.5 mass% in terms of Al, ２ O ３The exhaust gas purifying catalyst of Example 2 was produced in the same manner as in Example 1, except that the amount of rare earth elements other than Ce was adjusted to 74.5 mass% in terms of oxides, and the amount of rare earth elements other than Ce was adjusted to 2.5 mass% in terms of oxides, and the mass of the upper layer per unit volume of the portion of the flow-through type substrate on which the upper layer was formed was changed to 20 g / L. The lower layer, middle layer, and upper layer in the exhaust gas purifying catalyst of Example 2 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0261] The average thickness of the second catalytic layer and the average thickness of the third catalytic layer in the exhaust gas purifying catalyst of Example 2 were calculated in the same manner as in Example 1, and the average thickness of the second catalytic layer was 40 μm and the average thickness of the third catalytic layer was 4.4 μm. The characteristics of the second catalytic layer and the third catalytic layer in the exhaust gas purifying catalyst of Example 2 are shown in Table 1.

[0262] [Example 3] The amount of each component in the slurry for forming the upper layer was 0.3 mass % in terms of metal for Rh, 0.3 mass % in terms of metal for Ce, and 100 mass % in terms of CeO ２ 3.3 mass% in terms of ZrO ２ 11.7 mass% in terms of Al, ２ O ３ The exhaust gas purifying catalyst of Example 3 was produced in the same manner as in Example 1, except that the content of rare earth elements other than Ce was adjusted to 83.0 mass% in terms of oxides, and the content of rare earth elements other than Ce was adjusted to 1.7 mass% in terms of oxides, and the mass of the upper layer per unit volume of the portion of the flow-through type substrate on which the upper layer was formed was changed to 30 g / L. The lower layer, middle layer, and upper layer in the exhaust gas purifying catalyst of Example 3 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0263] The average thickness of the second catalytic layer and the average thickness of the third catalytic layer in the exhaust gas purifying catalyst of Example 3 were calculated in the same manner as in Example 1, and the average thickness of the second catalytic layer was 40 μm and the average thickness of the third catalytic layer was 6.6 μm. The characteristics of the second catalytic layer and the third catalytic layer in the exhaust gas purifying catalyst of Example 3 are shown in Table 1.

[0264] Comparative Example 1 The amount of each component in the slurry for forming the intermediate layer was 0.2 mass % in terms of metal for Rh, 0.2 mass % in terms of metal for Ce, and 100 mass % in terms of CeO ２6.0 mass% in terms of ZrO ２ 21.0 mass% in terms of Al, ２ O ３ The exhaust gas purifying catalyst of Comparative Example 1 was produced in the same manner as in Example 1, except that the content of rare earth elements other than Ce was adjusted to 69.2 mass% in oxide equivalent, and the content of rare earth elements other than Ce was adjusted to 3.6 mass% in oxide equivalent, and that an upper layer was not formed. The lower layer in the exhaust gas purifying catalyst of Comparative Example 1 corresponds to the first catalyst layer. The middle layer in the exhaust gas purifying catalyst of Comparative Example 1 was considered to be a laminate of a second catalyst layer and a third catalyst layer having the same composition. The characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purifying catalyst of Comparative Example 1 are shown in Table 1.

[0265] Comparative Example 2 The amount of each component in the slurry for forming the intermediate layer was 0.2 mass % in terms of metal for Rh and 0.2 mass % in terms of CeO, based on the mass of the intermediate layer after firing (100 mass %). ２ 12.0 mass% in terms of ZrO ２ 42.0 mass% in terms of Al, ２ O ３ The exhaust gas purifying catalyst of Comparative Example 2 was produced in the same manner as in Comparative Example 1, except that the content of rare earth elements other than Ce was adjusted to 39.5 mass% in oxide equivalent, and the content of rare earth elements other than Ce was adjusted to 6.3 mass% in oxide equivalent. The lower layer in the exhaust gas purifying catalyst of Comparative Example 2 corresponds to the first catalyst layer. The middle layer in the exhaust gas purifying catalyst of Comparative Example 2 was regarded as a laminate of second and third catalyst layers having the same composition. The characteristics of the second and third catalyst layers in the exhaust gas purifying catalyst of Comparative Example 2 are shown in Table 1.

[0266] 5 and 6 show a BED observation image and an element mapping image, respectively, of the exhaust gas purifying catalyst of Comparative Example 2, which were obtained by the same method as in Example 1. Note that Fig. 6 is a mapping image of Pd.

[0267] Comparative Example 3 The flow-through type substrate having the lower layer and middle layer formed thereon obtained in Example 1 (4) was immersed in an aqueous rhodium nitrate solution at 25°C for 48 hours. After immersion, the flow-through type substrate was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form a noble metal-containing surface layer portion containing Rh on the entire surface of the middle layer. After firing, the middle layer contained 0.2 mass% Rh in metal equivalent and 0.5 mass% Ce in CeO, based on the mass of the middle layer (100 mass%). ２ 12.0 mass% in terms of ZrO ２ 42.0 mass% in terms of Al, ２ O ３ The intermediate layer contains 39.5 mass% of rare earth elements in terms of oxides, and 6.3 mass% of rare earth elements other than Ce in terms of oxides. Furthermore, Rh in the intermediate layer is not uniformly present in the thickness direction of the intermediate layer, but is unevenly distributed near the surface of the intermediate layer. The lower layer in the exhaust gas purifying catalyst of Comparative Example 3 corresponds to the first catalyst layer. Of the intermediate layer in the exhaust gas purifying catalyst of Comparative Example 3, the portion other than the precious metal-containing surface layer portion was regarded as the second catalyst layer, and the precious metal-containing surface layer portion was regarded as the third catalyst layer. The characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purifying catalyst of Comparative Example 3 are shown in Table 1. In Comparative Example 3, the "CeO ２ ", "Al ２ O ３ ", "ZrO ２ " and "Al ２ O ３ + ZrO ２ " in the second catalyst layer and the third catalyst layer, ２ ", "Al ２ O ３ ", "ZrO ２ " and "Al ２ O ３ + ZrO ２ " was considered.

[0268] Comparative Example 4 The amount of each component in the slurry for forming the upper layer was 0.2 mass % in terms of metal for Rh and 0.5 mass % for CeO, based on the mass of the upper layer after firing (100 mass %). ２ converted to 1.7 mass %, Zr is ZrO ２ 5.8 mass% in terms of Al, ２ O ３The exhaust gas purifying catalyst of Comparative Example 4 was produced in the same manner as in Example 1, except that the content of rare earth elements other than Ce was adjusted to 91.5 mass% in oxide equivalents, and the content of rare earth elements other than Ce was adjusted to 0.8 mass% in oxide equivalents, and the mass of the upper layer per unit volume of the portion of the flow-through substrate on which the upper layer was formed was set to 60 g / L. The lower layer, middle layer, and upper layer in the exhaust gas purifying catalyst of Comparative Example 4 correspond to the first catalytic layer, the second catalytic layer, and the third catalytic layer, respectively. The average thicknesses of the second catalytic layer and the third catalytic layer in the exhaust gas purifying catalyst of Comparative Example 4 were calculated using the method described above, and the average thickness of the second catalytic layer was 40 μm, and the average thickness of the third catalytic layer was 13.2 μm. The characteristics of the second catalytic layer and the third catalytic layer in the exhaust gas purifying catalyst of Comparative Example 4 are shown in Table 1.

[0269] [Test Example] After the catalysts for purifying exhaust gases of Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to durability treatment, the exhaust gas purification performance and OSC performance were evaluated as follows. ２ Gas 0.50%, water vapor 10%, balance gas N ２ The heat treatment was carried out at 1000° C. for 30 hours in an atmosphere in which HCl was circulated.

[0270] <Exhaust gas purification performance test> The purification performance of hydrocarbons (HC), which are representative of harmful components, was measured. A model gas with the following composition and an A / F ratio of 14.6 was added to the exhaust gas purification catalyst (catalyst volume 15 mL) after durability treatment, and the CO concentration and O concentration were measured while the A / F ratio was varied in the range of 14.4 to 14.8. ２ The gas was allowed to flow at 32 L / min while adjusting the concentration. The temperature of the gas flowing into the exhaust gas purification catalyst was gradually increased from room temperature at a predetermined temperature increase rate, and the amount of HC contained in the exhaust gas that passed through the catalyst was determined using the following device, and the HC purification rate was calculated based on the following formula. V represents the amount detected when no catalyst was installed, and W represents the amount detected after the catalyst was installed. HC purification rate (%) = (V - W) / V x 100

[0271] [Model gas (composition by volume)] CO: 0.3%, C ３ H ６ : 1000ppmC, NO: 500ppm, O ２ : 0.28%, CO ２ : 14%, H ２O: 10%, N ２ : Remainder [Temperature increase rate] 10°C / min [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by HORIBA, Ltd.

[0272] The inlet gas temperatures (°C) of the catalyst when the HC purification rate reached 50% and 80% were determined as light-off temperatures T50 and T80, respectively. T50 and T80 were measured during temperature rise. The measurement results of T50 and T80 are shown in Table 2. T50 is an index of heat resistance because it is greatly affected by the deterioration state of the catalyst due to durability treatment. The smaller the T50, the higher the heat resistance. T80 is an index of the contact between the catalyst and exhaust gas because it is greatly affected by the contact between the catalyst and exhaust gas rather than the deterioration state of the catalyst. The smaller the T80, the higher the contact between the catalyst and exhaust gas.

[0273] <OSC Performance Test> Model gases 1 and 2 having the following composition were passed alternately at 32 L / min every minute through the exhaust gas purification catalyst (catalyst volume 15 mL) after durability treatment. The temperature of the gas flowing into the exhaust gas purification catalyst was fixed at 500°C, and the time from when model gas 1 was switched to model gas 2 until the CO concentration contained in the exhaust gas that had passed through the catalyst reached 0.25% (hereinafter referred to as "delay time") was measured using the following device, and the difference in delay time (seconds) was calculated based on the following formula. X represents the delay time when the catalyst was not installed, and Y represents the delay time after the catalyst was installed. Delay time difference (seconds) = Y - X

[0274] [Model gas 1 (composition by volume)] O ２ : 0.5%, N ２ : balance [Model gas 2 (composition by volume)] CO: 0.5%, N ２ : Remainder [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by HORIBA, Ltd.

[0275] The difference in delay time serves as an index of OSC. The larger the difference in delay time, the larger the OSC. Therefore, the difference in delay time was calculated as a relative value when the value of Comparative Example 1 was set to 100, and this was taken as the OSC. The measurement results of OSC are shown in Table 2.

[0276] The reason why the difference in delay time serves as an indicator of OSC is as follows: In the test system, a model gas supply section, a catalyst installation section, and a gas concentration meter installation section are arranged in this order. While model gas 1 is flowing, the CO concentration detected by the gas concentration meter is zero. If a catalyst is not installed, when model gas 1 is switched to model gas 2, the CO in model gas 2 is not consumed and is detected by the gas concentration meter. If a catalyst is installed, when model gas 1 is switched to model gas 2, the CO in model gas 2 is consumed by the oxygen stored in the catalyst, so the CO concentration detected by the gas concentration meter is maintained low. The more oxygen stored in the catalyst, the longer the CO concentration detected by the gas concentration meter is maintained low. Therefore, the difference in delay time serves as an indicator of OSC performance.

[0277]

[0278]

[0279] The exhaust gas purifying catalysts of Examples 1 to 3 are exhaust gas purifying catalysts comprising a substrate, a first catalytic layer provided on the substrate, a second catalytic layer provided on the first catalytic layer, and a third catalytic layer provided on the second catalytic layer, wherein the first catalytic layer contains Pd, the second catalytic layer contains Rh and Ce, and the third catalytic layer contains Rh and Al and / or Zr, and satisfy the following conditions (1) to (4): On the other hand, the exhaust gas purifying catalysts of Comparative Examples 1 to 4 are exhaust gas purifying catalysts comprising a substrate, a first catalytic layer provided on the substrate, a second catalytic layer provided on the first catalytic layer, and a third catalytic layer provided on the second catalytic layer, wherein the first catalytic layer contains Pd, the second catalytic layer contains Rh and Ce, and the third catalytic layer contains Rh and Al and / or Zr, but do not satisfy any one or more of the following conditions (1) to (4): (1) The second catalytic layer and the third catalytic layer satisfy the following formula: a>b (wherein a represents the percentage of the mass of Rh in the third catalytic layer in terms of metal relative to the mass of the third catalytic layer, and b represents the percentage of the mass of Rh in the second catalytic layer in terms of metal relative to the mass of the second catalytic layer). (2) The amount of Ce in the second catalytic layer is CeO ２The percentage of the mass of the second catalytic layer converted into CeO is 7 mass % or more. ２ The percentage of the converted mass to the mass of the third catalyst layer is less than 7 mass %. (4) The average thickness of the third catalyst layer is 10 μm or less.

[0280] Comparative Example 1 did not satisfy condition (1), and therefore could not improve the contact between the Rh contained in the third catalytic layer and the exhaust gas, resulting in a large T80 (i.e., poor contact between the catalyst and the exhaust gas).Comparative Example 1 also did not satisfy condition (2), and therefore could not improve the OSC of the second catalytic layer, resulting in a low OSC.

[0281] Comparative Example 2 did not satisfy condition (1), and therefore was unable to improve the contact between the Rh contained in the third catalytic layer and the exhaust gas, resulting in a large T80 (i.e., poor contact between the catalyst and the exhaust gas). Furthermore, Comparative Example 2 did not satisfy condition (3), and therefore was unable to improve the heat resistance of the third catalytic layer, resulting in a large T50 (i.e., poor heat resistance).

[0282] In Comparative Example 3, the condition (3) was not satisfied, and therefore the heat resistance of the third catalytic layer could not be improved, and therefore T50 was large (i.e., heat resistance was low). In Comparative Example 3, the conditions (1) and (4) were satisfied, but T80 was large (i.e., contact between the catalyst and exhaust gas was low). In Comparative Example 3, the condition (3) was not satisfied, and therefore Rh in the third catalytic layer aggregated, and the number of active sites decreased, which is thought to be the reason why T80 was large, although conditions (1) and (4) were satisfied.

[0283] Comparative Example 4 did not satisfy condition (4), and therefore was unable to improve the contact between the catalytically active components contained in the portion below the third catalyst layer and the exhaust gas, and therefore had a large T80 (i.e., low contact between the catalyst and the exhaust gas).

[0284] In Examples 1 to 3, the condition (1) was satisfied, and therefore the contactability between the Rh contained in the third catalyst layer and the exhaust gas was improved, and therefore the T80 was small (i.e., the contactability between the catalyst and the exhaust gas was high).

[0285] In Examples 1 to 3, the condition (2) was satisfied, and therefore the OSC of the second catalyst layer was improved, and therefore the OSC was high.

[0286] In Examples 1 to 3, the condition (3) was satisfied, and therefore the heat resistance of the third catalyst layer was improved, and therefore T50 was small (that is, the heat resistance was high).

[0287] In Examples 1 to 3, the condition (4) was satisfied, and therefore the contact between the catalytically active component contained in the portion below the third catalyst layer and the exhaust gas was improved, and therefore the T80 was small (i.e., the contact between the catalyst and the exhaust gas was high).

[0288] P: exhaust pipe of internal combustion engine 1: catalyst for purifying exhaust gas 10: substrate 11: cylindrical portion 12: partition portion 13: cell 20: first catalyst layer 30: second catalyst layer 40: third catalyst layer

Claims

1. An exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided on the substrate, a second catalyst layer provided on the first catalyst layer, and a third catalyst layer provided on the second catalyst layer, wherein: The first catalyst layer contains Pd; The second catalyst layer contains Rh and Ce; The third catalyst layer contains Rh and Al and / or Zr; The second catalyst layer and the third catalyst layer satisfy the following formula: a > b [wherein, a represents the percentage of the mass of Rh in terms of metal in the third catalyst layer with respect to the mass of the third catalyst layer, and b represents the percentage of the mass of Rh in terms of metal in the second catalyst layer with respect to the mass of the second catalyst layer.]; The percentage of the mass of CeO ２ in terms of conversion in the second catalyst layer with respect to the mass of the second catalyst layer is 7% by mass or more; The percentage of the mass of CeO ２ in terms of conversion in the third catalyst layer with respect to the mass of the third catalyst layer is less than 7% by mass; The average thickness of the third catalyst layer is 10 μm or less. The exhaust gas purification catalyst described above.

2. The Al in terms of mass of Al in the third catalyst layer ２ O ３ in terms of mass of ZrO of Zr in the third catalyst layer, the percentage of the total with respect to the mass of the third catalyst layer is 80% by mass or more. The exhaust gas purification catalyst according to claim 1. ２ ​ 3. The exhaust gas purification catalyst according to claim 1 or 2, wherein the ratio a / b of a to b is 2 or more and 10 or less.

4. The exhaust gas purification catalyst according to claim 3, wherein b is 0.01% by mass or more and 5% by mass or less.

5. The exhaust gas purification catalyst according to claim 1 or 2, wherein the average thickness of the third catalyst layer is 0.5 μm or more and 5 μm or less.

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