Catalyst for exhaust gas purification

Abstract

The present invention aims to provide an exhaust gas purification catalyst capable of improving the exhaust gas purification performance of the Rh-containing layer and the exhaust gas purification performance of the portion below the Rh-containing layer. To achieve such an object, there is provided an exhaust gas purification catalyst (1) comprising a substrate (10), a first catalyst layer (20) provided on the substrate (10), a second catalyst layer (30) provided on the first catalyst layer (20), and a third catalyst layer (40) provided on the second catalyst layer (30). 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 7% by mass or more, the content of Ce in terms of CeO2 in the third catalyst layer (40) is less than 7% by mass, and the average thickness of the third catalyst layer (40) is 10 μm or less. An exhaust gas purification catalyst (1) is provided.

Description

Technical Field

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

Background Art

[0002] Exhaust gas discharged from internal combustion engines such as automobiles and motorcycles contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). For the purpose of purifying and detoxifying these harmful components, noble metal elements such as Pd and Rh are used as catalyst active components of the exhaust gas purification catalyst.

[0003] Since Pd and Rh are expensive, it is required to enhance the exhaust gas purification performance as much as possible while using a limited amount 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 including a substrate, a lower layer containing Pd provided on the substrate, and an upper layer provided on the lower layer, wherein an Rh-containing layer is formed in the surface layer portion of the upper layer. In the exhaust gas purification catalyst described in Patent Document 1, since the Rh-containing layer is formed in the surface layer portion of the upper layer that is likely to come into contact with the exhaust gas, the contact property between Rh and the exhaust gas can be improved.

[0005] Catalyst active components such as Rh aggregate and the number of active sites decreases when exposed to a high-temperature environment. This phenomenon is more likely to occur as the concentration of the catalyst active component is higher. In this specification, the difficulty of aggregation of the catalyst active component contained in the catalyst layer when the catalyst layer is exposed to a high-temperature environment is referred to as "heat resistance of the catalyst layer". Further, in this specification, "high temperature" means 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 for improving the heat resistance of the Rh-containing layer (hereinafter sometimes referred to as "heat resistance components").

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2022-030627 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] In Patent Document 1, the Rh-containing layer contains a heat-resistant component, so it is advantageous in terms of heat resistance. However, since it does not contain Ce, which is a component having an oxygen storage capacity (OSC: Oxygen Storage Capacity), the OSC is low and there is a problem that the exhaust gas purification performance is insufficient.

[0009] When Ce is incorporated into the Rh-containing layer to solve the above problems, the following problems occur. If the amount of Ce in the Rh-containing layer is increased and accordingly the amount of the heat-resistant component in the Rh-containing layer is decreased, the OSC of the Rh-containing layer is improved, but the heat resistance of the Rh-containing layer is decreased, and the exhaust gas purification performance of the Rh-containing layer (especially the exhaust gas purification performance of the Rh-containing layer after being exposed to a high-temperature environment) is decreased. Conversely, if the amount of the heat-resistant component in the Rh-containing layer is increased and accordingly the amount of Ce in the Rh-containing layer is decreased, the heat resistance of the Rh-containing layer is improved, but the OSC of the Rh-containing layer is decreased, and the exhaust gas purification performance of the Rh-containing layer is decreased. On the other hand, if the amount of Ce and the amount of the heat-resistant component in the Rh-containing layer are increased, the heat resistance and OSC of the Rh-containing layer can be improved, but the Rh-containing layer becomes thicker, and the contact property between the catalytic active component contained in the portion below the Rh-containing layer and the exhaust gas deteriorates, and the exhaust gas purification performance of the portion below the Rh-containing layer is decreased.

[0010] Therefore, in the prior art, it is difficult to 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.

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

Means for Solving the Problem

[0012] In an exhaust gas purification catalyst including a substrate, a Pd-containing layer (first catalyst layer) provided on the substrate, and a Rh-containing layer provided on the Pd-containing layer, the present inventors divided the Rh-containing layer into a lower layer (second catalyst layer) and an upper layer (third catalyst layer), increased the Rh concentration in the third catalyst layer to be higher than the Rh concentration in the second catalyst layer, made the second catalyst layer contain Ce, made the third catalyst layer contain Al and / or Zr, adjusted the percentage of the mass of Ce in terms of CeO2 in the second catalyst layer to the mass of the second catalyst layer to be 7% by mass or more, adjusted the percentage of the mass of Ce in terms of CeO2 in the third catalyst layer to the mass of the third catalyst layer to be less than 7% by mass, and adjusted the average thickness of the third catalyst layer to be 10 μm or less, thereby realizing an improvement in the contact property between Rh contained in the third catalyst layer and exhaust gas, an improvement in the heat resistance of the third catalyst layer, an improvement in the OSC of the second catalyst layer, and an improvement in the contact property between the catalyst active component contained in the portion below the third catalyst layer and exhaust gas. As a result, the present inventors found that it is possible to improve the exhaust gas purification performance of the second catalyst layer, the exhaust gas purification performance of the third catalyst layer, and the exhaust gas purification performance of the portion below the third catalyst layer, and completed the present invention.

[0013] That is, the present invention provides the following exhaust gas purification catalyst. [1] An exhaust gas purification catalyst including 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 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 to the mass of the second catalyst layer.] and satisfy The percentage of the mass of Ce in terms of CeO2 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 Ce in terms of CeO2 in the third catalyst layer with respect to the mass of the third catalyst layer is less than 7% by mass, the exhaust gas purifying catalyst, wherein the average thickness of the third catalyst layer is 10 μm or less. [2] The exhaust gas purifying catalyst according to [1], wherein the percentage of the total of the mass of Al in terms of Al2O3 and the mass of Zr in terms of ZrO2 in the third catalyst layer with respect to the mass of the third catalyst layer is 80% by mass or more. [3] The exhaust gas purifying catalyst according to [1] or [2], wherein the ratio a / b of a to b is 2 or more and 10 or less. [4] The exhaust gas purifying catalyst according to [3], wherein b is 0.01% by mass or more and 5% by mass or less. [5] The exhaust gas purifying catalyst 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.

Advantages of the Invention

[0014] According to the present invention, there is provided an exhaust gas purifying catalyst capable of improving the exhaust gas purifying performance of the Rh-containing layer and the exhaust gas purifying performance of the portion below the Rh-containing layer.

Brief Description of the Drawings

[0015]

Figure 1

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DETAILED DESCRIPTION OF THE INVENTION

[0016] ≪Explanation of Terms≫ Hereinafter, the terms used in this specification will be explained. The following explanations apply throughout this specification unless otherwise specified.

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

[0018] <Metal Elements> "Metal elements" include semi-metal 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] <Oxides> The meaning of "oxides" is as follows. Oxides of rare earth elements excluding Ce, Pr, and Tb are sesquioxides (M2O3, where M represents rare earth elements other than Ce, Pr, and Tb), the oxide of Ce is CeO2, and the oxide of Pr is Pr6O 11means that the oxide of Tb is Tb4O7, the oxide of Al is Al2O3, the oxide of Zr is ZrO2, the oxide of Si is SiO2, the oxide of B is B2O3, the oxide of Cr is Cr2O3, the oxide of Mg is MgO, the oxide of Ca is CaO, the oxide of Sr is SrO, the oxide of Ba is BaO, the oxide of Fe is Fe3O4, the oxide of Mn is Mn3O4, the oxide of Ni is NiO, the oxide of Ti is TiO2, the oxide of Zn is ZnO, and the oxide of Sn is SnO2.

[0022] <Mass of the metal element in terms of metal "Mass of the metal element in terms of metal" means the mass of the metal obtained by assuming that the metal element exists as a metal composed of the metal element.

[0023] <Mass of the metal element in terms of its oxide "Mass of the metal element in terms of its oxide" means the mass of the oxide of the metal element obtained by assuming that the metal element exists as the oxide of the metal element.

[0024] <Mass of the catalyst layer "Mass of the catalyst layer" means classifying all the metal elements contained in the catalyst layer into noble metal elements and metal elements other than noble metal elements, obtaining the mass in terms of metal for the noble metal elements and the mass in terms of oxide for the metal elements other than noble metal elements, and summing these. That is, "Mass of the catalyst layer" means the calculated mass obtained by summing the mass of the noble metal elements contained in the catalyst layer in terms of metal and the mass of the metal elements other than noble metal elements contained in the catalyst layer in terms of oxide.

[0025] When information on the raw materials used in the production of the catalyst layer (such as composition, amount, etc.) is known, the mass of the catalyst layer can be determined from the information on the raw materials used in the production of the catalyst layer.

[0026] <Content rate of the metal element in the catalyst layer in terms of metal or oxide "The content rate of the metal element in the catalyst layer in terms of metal" is defined by the formula: content rate of the metal element in the catalyst layer in terms of metal (mass%) = (mass of the metal element in the catalyst layer in terms of metal) / (mass of the catalyst layer) × 100.

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

[0028] When information on the raw materials used for forming the catalyst layer (such as composition, amount, etc.) is known, the content rate (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 on the raw materials used for forming the catalyst layer is unknown, the content rate (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, it is as follows.

[0030] Perform elemental analysis of the catalyst layer using a conventional method such as SEM-EDX to identify the types of constituent elements of the catalyst layer and determine the mol% of each identified metal element. For each of the 10 fields of view of the SEM, determine the mol% of each metal element, and take the average value of the mol% of each metal element in the 10 fields of view as the mol% of each metal element in the catalyst layer.

[0031] Determine the V value of each noble metal element in the catalyst layer from the following formula. V value of each noble metal element = (mol% of each noble metal element in the catalyst layer) × (molar mass of each noble metal element)

[0032] Determine the W value of each metal element other than the noble metal element in the catalyst layer from the following formula. W value of each metal element = (mol% of each metal element in the catalyst layer) × (molar mass of the oxide of each metal element)

[0033] Determine the content rate (mass%) of each noble metal element in the catalyst layer in terms of metal from the following formula. Content rate (mass %) of each noble metal element in terms of metal in the catalyst layer = (V value of each noble metal element) / {(sum of V values of all noble metal elements)+(sum of W values of all metal elements other than noble metal elements)}×100

[0034] From the following formula, the content rate (mass %) of each metal element other than noble metal elements in the catalyst layer in terms of oxide is obtained. Content rate (mass %) of each metal element other than noble metal elements in the catalyst layer in terms of oxide = (W value of each metal element other than noble metal elements) / {(sum of V values of all noble metal elements)+(sum of W values of all metal elements other than noble metal elements)}×100

[0035] <Metal oxide> "Metal oxide" means an oxide containing one or more metal elements. Examples of metal oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, Ce-Zr-based composite oxides, etc.

[0036] <Mass of metal oxide> "Mass of metal oxide" means the total mass of the metal element oxides obtained by assuming that the metal elements in the metal oxide exist as oxides respectively.

[0037] <Content rate of metal elements in metal oxide in terms of oxide> "Content rate of metal elements in metal oxide in terms of oxide" is defined by the formula: content rate of metal elements in metal oxide (mass %) = (mass of metal elements in metal oxide in terms of oxide) / (mass of metal oxide)×100.

[0038] When the composition of the metal oxide is known, the content rate (mass %) of the metal elements in the metal oxide in terms of oxide can be obtained from the composition of the metal oxide.

[0039] When the composition of the metal oxide is unknown, the content rate (mass %) of the metal elements in the metal oxide in terms of oxide can be obtained by conventional methods such as SEM-EDX. Specifically, it is as follows.

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

[0041] <Average particle diameter of the metal oxide> The "average particle diameter of the metal oxide" means the average value of the defined-direction diameters (Feret diameters) of 100 metal oxides, which is obtained by observing a sample containing the metal oxide with a scanning electron microscope and measuring the defined-direction diameters (Feret diameters) of 100 metal oxides arbitrarily selected from within the field of view.

[0042] <Al-based oxide> The "Al-based oxide" is an oxide containing Al, and means an oxide in which the metal element having the largest content rate on a mass basis among the metal elements constituting the oxide is Al. However, those corresponding to Ce-Zr composite oxides shall not be regarded as corresponding to Al-based oxides. Ce-Zr 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" in some cases.

[0043] Al-based oxides are, for example, particulate. Al-based oxides are used as carriers for catalytic active components. From the viewpoint of improving the supportability of catalytic active components, it is preferable that Al-based oxides are porous.

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

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

[0046] In the Al-based oxide, the additional element M1 may form a solid solution phase (for example, a solid solution phase of Al2O3 and the oxide of the additional element M1), or may form a single phase (for example, the oxide phase of the additional element M1) that is a crystalline phase or an amorphous phase, or may form both a solid solution phase and a single phase. However, it is preferable that at least a part of the additional element M1 forms a solid solution phase.

[0047] Examples of the Al-based oxide include alumina (Al2O3), an oxide obtained by modifying the surface of alumina with the additional element M1, an oxide obtained by dissolving the additional element M1 in alumina, and the like. Examples of the Al-based oxide containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-ceria, alumina-lanthana, and the like.

[0048] From the viewpoint of improving the heat resistance of the Al-based oxide, the content of Al in the Al-based oxide in terms of Al2O3 is preferably 50% by mass or more, more preferably 75% by mass or more, still more preferably 95% by mass or more, based on the mass of the Al-based oxide. The upper limit is 100% by mass.

[0049] <Ce-based oxide> The "Ce-based oxide" is an oxide containing Ce, and means an oxide in which Ce is the element having the largest content rate in terms of mass among the metal elements constituting the oxide. However, those corresponding to Ce-Zr composite oxides shall not be regarded as Ce-based oxides. Ce-Zr 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 "ceria binder".

[0050] The Ce-based oxide is, for example, in a particulate form. The Ce-based oxide is used as a carrier for a catalytic active component. From the viewpoint of improving the loading property of the catalytic active component, the Ce-based oxide is preferably porous.

[0051] The Ce-based oxide has OSC (the ability to store oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low), and alleviates the fluctuation of the oxygen concentration in the exhaust gas to expand the operating window of the catalytic active component. Therefore, when the catalyst layer contains the Ce-based oxide, the exhaust gas purification performance of the catalyst layer is improved.

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

[0053] In the Ce-based oxide, the additional element M2 may form a solid solution phase (for example, a solid solution phase of CeO2 and the oxide of the additional element M2), or a single phase that is a crystal phase or an amorphous phase (for example, the oxide phase of the additional element M2), or may form both a solid solution phase and a single phase, but at least a part of the additional element M2 preferably forms a solid solution phase.

[0054] Examples of the Ce-based oxide include ceria (CeO2), 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 content of Ce in the Ce-based oxide in terms of CeO2 is preferably 50% by mass or more, more preferably 70% by mass or more, still more preferably 90% by mass or more, based on the mass of the Ce-based oxide. The upper limit is 100% by mass.

[0056] <Zr-based oxide> "Zr-based oxide" means an oxide containing Zr, and among the metal elements constituting the oxide, the element with the largest content by mass is Zr. However, those corresponding to Ce-Zr composite oxides shall not be regarded as Zr-based oxides. Ce-Zr 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 "zirconia binder".

[0057] Zr-based oxides are, for example, particulate. Zr-based oxides are used as carriers for catalytic active components. From the viewpoint of improving the loading property of catalytic active components, Zr-based oxides are preferably porous.

[0058] Zr-based oxides have high heat resistance. Therefore, when the catalyst layer contains Zr-based oxides, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

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

[0060] In Zr-based oxides, the additional element M3 may form a solid solution phase (for example, a solid solution phase of ZrO2 and an oxide of the additional element M3), or may form a single phase in the crystalline phase or 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 at least a part of the additional element M3 preferably forms a solid solution phase.

[0061] Examples of Zr-based oxides include zirconia (ZrO2), oxides obtained by modifying the surface of zirconia with the additional element M3, oxides 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, the content of Zr in the Zr-based oxide in terms of ZrO2 is preferably 50% by mass or more, more preferably 65% by mass or more, still more preferably 80% by mass or more, based on the mass of the Zr-based oxide. The upper limit is 100% by mass.

[0063] <Ce-Zr composite oxide> The Ce-Zr composite oxide is a composite oxide containing Ce and Zr, wherein the content of Ce in the composite oxide in terms of CeO2 is 5% by mass or more and 95% by mass or less, and the content of Zr in the composite oxide in terms of ZrO2 is 5% by mass or more and 95% by mass or less, based on the mass of the composite oxide.

[0064] The Ce-Zr composite oxide is, for example, particulate. The Ce-Zr composite oxide is used as a carrier for the catalytic active component. From the viewpoint of improving the loading property of the catalytic active component, the Ce-Zr composite oxide is preferably porous.

[0065] The Ce-Zr composite oxide has OSC, alleviates the fluctuation of the oxygen concentration in the exhaust gas, and expands the operating window of the catalytic active component. Therefore, when the catalyst layer contains the Ce-Zr composite oxide, the exhaust gas purification performance of the catalyst layer is improved.

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

[0067] In the Ce-Zr composite oxide, Ce may form a solid solution phase (e.g., a solid solution phase of CeO2 and ZrO2), a single phase that is a crystalline phase or an amorphous phase (e.g., a single CeO2 phase), or both a solid solution phase and a single phase. However, it is preferable that at least a part of Ce forms a solid solution phase.

[0068] In the Ce-Zr composite oxide, Zr may form a solid solution phase (e.g., a solid solution phase of CeO2 and ZrO2), a single phase that is a crystalline phase or an amorphous phase (e.g., a single ZrO2 phase), or both a solid solution phase and a single phase. However, it is preferable that at least a part of Zr forms a solid solution phase.

[0069] When the Ce-Zr composite oxide contains an additional element M4, the additional element M4 may form a solid solution phase (e.g., a solid solution phase of CeO2 and an oxide of the additional element M4, a solid solution phase of ZrO2 and an oxide of the additional element M4, a solid solution phase of CeO2, ZrO2, and an oxide of the additional element M4), 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 both a solid solution phase and a single phase. However, 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 composite oxide, the content of Ce in terms of CeO2 in the Ce-Zr composite oxide 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 composite oxide. The upper limit can be appropriately adjusted considering heat resistance, structural stability, the content of other components, etc. 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 composite oxide, the content of Zr in the Ce-Zr composite oxide in terms of ZrO2 is preferably 25% by mass or more, more preferably 30% by mass or more, still more preferably 35% by mass or more, based on the mass of the Ce-Zr composite oxide. The upper limit can be appropriately adjusted in consideration of the oxygen storage capacity, structural stability, content of other components, etc. The upper limit is preferably 80% by mass or less, more preferably 75% by mass or less, still 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 viewpoints of improving the heat resistance and OSC of the Ce-Zr composite oxide, the total content of Ce in the Ce-Zr composite oxide in terms of CeO2 and the content of Zr in terms of ZrO2 is preferably 50% by mass or more, more preferably 65% by mass or more, still more preferably 80% by mass or more, based on the mass of the Ce-Zr composite oxide. The upper limit is 100% by mass.

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

[0074] From the viewpoint of improving the heat resistance of the Ce-Zr composite oxide, the content of the rare earth element in the Ce-Zr composite oxide in terms of 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, still more preferably 9% by mass or more and 20% by mass or less, based on the mass of the Ce-Zr composite oxide. Each of the above lower limits may be combined with any of the above upper limits. The "content of the rare earth element in the Ce-Zr composite oxide in terms of oxide" means the content of the oxide of the single rare earth element when the Ce-Zr composite oxide contains one kind of rare earth element, and means the total content of the oxides of the two or more rare earth elements when the Ce-Zr composite oxide contains two or more rare earth elements.

[0075] <<Exhaust gas purification catalyst>> Hereinafter, the catalyst for purifying exhaust gas of the present invention will be described.

[0076] Hereinafter, based on FIGS. 1 to 4, the exhaust gas purification catalyst 1 (hereinafter referred to as "catalyst 1") according to an embodiment of the present invention will be described.

[0077] As shown in FIG. 1, the catalyst 1 is disposed in the exhaust passage in the exhaust pipe P of the internal combustion engine. The internal combustion engine is, for example, a gasoline engine or the like. The exhaust gas discharged 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 drawings, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side in the exhaust gas flow direction X may be referred to as the "exhaust gas inflow side" or the "upstream side", and the downstream side in the exhaust gas flow direction X may be referred to as the "exhaust gas outflow side" or the "downstream side".

[0078] Other exhaust gas purification catalysts may be disposed on the upstream side and / or the downstream side of the catalyst 1 in the exhaust passage in the exhaust pipe P.

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

[0080] In the catalyst 1, 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, and the second catalyst layer 30 and the third catalyst layer 40 satisfy the following formula: a>b [In the formula, a represents the percentage of the mass of Rh in terms of metal in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40, and b represents the percentage of the mass of Rh in terms of metal in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30.] satisfies the condition that the percentage of the mass of Ce in terms of CeO₂ in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 is 7% by mass or more, the percentage of the mass of Ce in terms of CeO₂ in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 is less than 7% by mass, and the average thickness of the third catalyst layer 40 is 10 μm or less.

[0081] Hereinafter, the effects of Catalyst 1 will be described.

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

[0083] From the viewpoint of more effectively improving the contact between Rh contained in the third catalyst layer 40 and the exhaust gas, the ratio a / b of a to 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. The above lower limits may be combined with any of the above upper limits.

[0084] The third catalyst layer 40 contains Al and / or Zr, and the percentage of the mass of Ce in terms of CeO₂ in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 is less than 7% by mass. Therefore, Catalyst 1 can improve the heat resistance of the third catalyst layer 40, and thereby improve the exhaust gas purification performance of the third catalyst layer 40 (particularly, the exhaust gas purification performance of the third catalyst layer 40 after being exposed to a high-temperature environment).

[0085] The second catalyst layer 30 contains Ce, and the percentage of the mass of Ce in terms of CeO₂ in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 is 7% by mass or more. Therefore, 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 catalyst layer 40 is 10 μm or less. Therefore, the catalyst 1 can improve the contact between the catalyst active components (for example, Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) contained in the portion below the third catalyst layer 40 and the exhaust gas, thereby improving the exhaust gas purification performance of the first catalyst layer 20 and the second catalyst layer 30.

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

[0088] <Substrate> Hereinafter, the substrate 10 will be described.

[0089] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of the material constituting the substrate 10 include ceramic materials and metal materials, and ceramic materials are preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; 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] As shown in FIGS. 2 to 4, the substrate 10 has a cylindrical portion 11, a partition portion 12 provided in the cylindrical portion 11, and cells 13 partitioned by the partition portion 12. The substrate 10 is preferably a honeycomb structure.

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

[0092] As shown in FIGS. 2 to 4, a partition portion 12 exists between adjacent cells 13, and the adjacent cells 13 are partitioned by the partition portion 12. The partition portion 12 may have a porous structure through which exhaust gas can pass. The thickness of the partition portion 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 inflow side and an end on the exhaust gas outflow side.

[0094] As shown in FIG. 4, both the end on the exhaust gas inflow side and the end on the exhaust gas outflow side of the cell 13 are open. Therefore, the exhaust gas flowing in from the end (opening) on the exhaust gas inflow side of the cell 13 flows out from the end (opening) on the exhaust gas outflow side of the cell 13. Such a mode is called a flow-through type.

[0095] As shown in FIGS. 2 and 3, the planar shape of the end (opening) on the exhaust gas inflow side of the cell 13 is a quadrangle, but it may be other shapes such as a hexagon or an octagon. The same applies to the planar shape of the end (opening) on the exhaust gas outflow side of the cell 13.

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

[0097] The volume of the base material 10 is, for example, 0.1 L or more and 20 L or less. The volume of the base material 10 means the apparent volume of the base material 10. For example, when the base material 10 is cylindrical, if the outer diameter of the base material 10 is 2r and the length of the base material 10 is L10, the volume of the base material 10 is given by the formula: Volume of base material 10 = π × r2 It is represented by ×L10. In this specification, "length" means the axial dimension of the base material 10 unless otherwise specified.

[0098] <The first catalyst layer> Hereinafter, the first catalyst layer 20 will be described.

[0099] As shown in FIGS. 3 and 4, the first catalyst layer 20 is provided on the base material 10. Specifically, the first catalyst layer 20 is provided on the surface of the partition portion 12 on the cell 13 side. The "surface of the partition portion 12 on the cell 13 side" means the outer surface of the partition portion 12 that extends in the exhaust gas flow direction X and is in contact with the cell 13. The first catalyst layer 20 may be provided directly on the surface of the partition portion 12 on the cell 13 side, or may be provided via another layer, but usually it is provided directly on the surface of the partition portion 12 on the cell 13 side. The "first catalyst layer 20 provided on the base material 10" includes an embodiment in which the first catalyst layer 20 is provided directly on the surface of the partition portion 12 on the cell 13 side, and an embodiment in which the first catalyst layer 20 is provided via another layer on the surface of the partition portion 12 on the cell 13 side.

[0100] The first catalyst layer 20 may be composed of a portion that bulges from the surface of the partition portion 12 on the cell 13 side toward the cell 13 side (hereinafter referred to as the "bulging portion"), or may be composed of a portion existing inside the partition portion 12 (hereinafter referred to as the "intrinsic portion"), or may have a bulging portion and an intrinsic portion. The "first catalyst layer 20 provided on the base material 10" includes an embodiment in which the first catalyst layer 20 is composed of a bulging portion, an embodiment in which the first catalyst layer 20 is composed of an intrinsic portion, and an embodiment in which the first catalyst layer 20 has a bulging portion and an intrinsic portion.

[0101] As shown in FIG. 4, the first catalyst layer 20 extends along the exhaust gas flow direction X from the end of the partition portion 12 on the exhaust gas inflow side to the end of the partition portion 12 on the exhaust gas outflow side. The first catalyst layer 20 may extend along the exhaust gas flow direction X from the end of the partition portion 12 on the exhaust gas inflow side without reaching the end of the partition portion 12 on the exhaust gas outflow side, or may extend along the direction opposite to the exhaust gas flow direction X from the end of the partition portion 12 on the exhaust gas outflow side without reaching the end of the partition portion 12 on the exhaust gas inflow side.

[0102] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalyst layer 20 per unit volume of the portion of the substrate 10 where the first catalyst 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 catalyst layer 20 per unit volume of the portion of the substrate 10 where the first catalyst layer 20 is formed is calculated from the formula: (mass of the first catalyst layer 20) / ((volume of the substrate 10)×(average length L20 of the first catalyst layer 20 / length L10 of the substrate 10)).

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

[0105] A sample that extends in the axial direction of the substrate 10 and has 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 value of the diameter of the sample can be changed as needed. When the first catalyst layer 20 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition wall portion 12, the sample is cut at 5 mm intervals by a plane perpendicular to the axial direction of the substrate 10, and in order from the end side on the exhaust gas inflow side of the sample, a first cut piece, a second cut piece, ···, a nth cut piece are obtained. When the first catalyst layer 20 extends along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition wall portion 12, the sample is cut at 5 mm intervals by a plane perpendicular to the axial direction of the substrate 10, and in order from the end side on the exhaust gas outflow side of the sample, a first cut piece, a second cut piece, ···, a nth cut piece are obtained. In either case, the length of the cut piece is 5 mm. The composition of the cut piece is analyzed using XRF (for example, EDX, WDX, etc.), ICP - AES, SEM - EDX, etc., and based on the composition of the cut piece, it is confirmed whether the cut piece contains a part of the first catalyst layer 20.

[0106] Regarding the cut pieces that are clearly found to contain a part of the first catalyst layer 20, it is not always necessary to perform a composition analysis. For example, the cut surface can be observed using SEM, EPMA, etc., to confirm whether the cut piece contains a part of the first catalyst layer 20. When observing the cut surface, elemental mapping of the cut surface may be performed.

[0107] After confirming whether the cut piece contains a part of the first catalyst layer 20, based on the following formula, the length of the first catalyst layer 20 contained in the sample is calculated. Length of the first catalyst layer 20 contained in the sample = 5 mm × (Number of cut pieces containing a part of the first catalyst layer 20)

[0108] For example, when the first cut piece to the kth cut piece contain a part of the first catalyst layer 20, but the (k + 1)th to the nth cut pieces do not contain a part of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is (5 × k) mm.

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

[0110] By cutting the k-th cut piece in the axial direction of the substrate 10 and observing a part of the first catalyst layer 20 present on the cut surface using SEM, EPMA, etc., the length of a part of the first catalyst layer 20 in the k-th cut piece is measured. Then, based on the following formula, the length of the first catalyst layer 20 included in the sample is calculated. When the first catalyst layer 20 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition wall portion 12, the k-th cut piece is the cut piece obtained from the most exhaust gas outflow side of the sample among the cut pieces including a part of the first catalyst layer 20. When the first catalyst layer 20 extends along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition wall portion 12, the k-th cut piece is the cut piece obtained from the most exhaust gas inflow side of the sample among the cut pieces including a part of the first catalyst layer 20. Length of the first catalyst layer 20 included in the sample = (5 mm × (k - 1)) + (Length of a part of the first catalyst layer 20 included in the k-th cut piece)

[0111] Regarding 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 defined as the average length L20 of the first catalyst layer 20.

[0112] The first catalyst layer 20 contains Pd as a catalytic active component. Pd is included in the first catalyst layer 20 in the form of a catalytic active component containing Pd, such as metallic Pd, an alloy containing Pd, a compound containing Pd (for example, an oxide of Pd), etc. From the viewpoint of improving the exhaust gas purification performance, the catalytic active component containing Pd is preferably in particulate form.

[0113] From the perspective of achieving a good balance between exhaust gas purification performance and cost, the percentage of the mass of Pd in terms of metal in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the content of Pd in terms of metal in the first catalyst layer 20" in this specification) is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.2% by mass or more and 7% by mass or less, and even more preferably 0.3% by mass or more and 5% by mass or less. The above lower limits may be combined with any of the above upper limits respectively.

[0114] The first catalyst layer 20 may contain one or more noble metal elements other than Pd as catalyst active components. Noble metal elements other than Pd can be selected from, for example, Pt, Rh, Ru, Os, Ir, Au, Ag, etc. Noble metal elements other than Pd are included in the first catalyst layer 20 in the form of catalyst active components containing noble metal elements, such as metals, alloys containing noble metal elements, compounds containing noble metal elements (e.g., oxides of noble metal elements), etc., which can function as catalyst active components. From the perspective of improving exhaust gas purification performance, the catalyst active component containing noble metal elements other than Pd is preferably in particulate form.

[0115] When the first catalyst layer 20 contains Pd and noble metal elements other than Pd, there is a possibility that Pd and noble metal elements other than Pd form an alloy, resulting in a decrease in the active sites of Pd involved in exhaust gas purification performance. Therefore, the percentage of the mass of noble metal elements other than Pd in terms of metal in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the content of noble metal elements other than Pd in terms of metal in the first catalyst layer 20" in this specification) is preferably small. Specifically, the content of noble metal elements other than Pd in terms of metal in the first catalyst layer 20 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 content of noble metal elements other than Pd in terms of metal in the first catalyst layer 20" means the content of the metal in terms of the single noble metal element when the first catalyst layer 20 contains one noble metal element other than Pd, and means the total content of the metal in terms of the two or more noble metal elements when the first catalyst layer 20 contains two or more noble metal elements other than Pd.

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

[0117] "At least a part of the catalyst active component is supported on the carrier" means a state in which at least a part of the catalyst active component is physically or chemically adsorbed or held on the outer surface and / or the inner surface of the pores 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 catalyst active component is supported on the carrier in a certain catalyst layer can be confirmed, for example, by using SEM-EDX or the like. Specifically, in the elemental mapping obtained by analyzing the cross-section of the catalyst layer by SEM-EDX, if at least a part of the catalyst active component and the carrier are present in the same region, it can be determined that at least a part of the catalyst active component is supported on the carrier.

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

[0120] Examples of the metal oxide include oxides based on Al-based oxides, Ce-based oxides, Zr-based oxides, Ce-Zr-based composite oxides, oxides of rare earth elements other than Ce, silica (SiO2), titania (TiO2), zeolite (aluminosilicate), MgO, ZnO, SnO2, etc.

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

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

[0123] From the viewpoint of improving the OSC of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the mass of Ce in terms of CeO2 in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the CeO2-equivalent content of Ce in the first catalyst layer 20" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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, "the CeO2-equivalent content of Ce in the catalyst layer" means the CeO2-equivalent content of Ce derived from the one kind of Ce source when the catalyst layer contains one kind of Ce source, and means the total CeO2-equivalent content of Ce derived from the two or more kinds of Ce sources when the catalyst layer contains two or more kinds of Ce sources. 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 catalyst layer 20 contains Ce, the first catalyst layer 20 contains one kind or two or more kinds of Ce sources.

[0126] Examples of the Ce source include oxides containing Ce. Examples of the oxides containing Ce include Al-based oxides containing Ce, Ce-based oxides, Zr-based oxides containing Ce, Ce-Zr composite oxides, and ceria binders.

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

[0128] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the mass of the Ce-Zr composite oxide in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the content rate of the Ce-Zr composite oxide in the first catalyst layer 20" in this specification) is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rates 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.

[0129] When information on the raw materials used for forming the first catalyst layer 20 (such as composition, amount, etc.) is known, the content rate of the Ce-Zr composite oxide in the first catalyst layer 20 can be determined from the information on the raw materials used for forming the first catalyst layer 20.

[0130] When information on the raw materials used for forming the first catalyst layer 20 is not known, the content rate of the Ce-Zr composite oxide in the first catalyst layer 20 can be determined by a conventional method such as SEM-EDX. Specifically, it is as follows.

[0131] (1) For the sample obtained from the first catalyst layer 20, elemental analysis is performed using a conventional method such as SEM-EDX to identify the types of constituent elements of the entire sample, and to determine the content rate (mass%) in terms of oxide of each identified metal element. (2) For the sample obtained from the first catalyst layer 20, elemental mapping is performed using a conventional method such as SEM-EDX to identify the types of particles contained in the sample (for example, Al-based oxides, Ce-based oxides, Ce-Zr composite oxides, etc.). (3) For each type of particle, a plurality of arbitrarily selected particles (for example, 50 particles) are subjected to elemental analysis by SEM-EDX to identify the types of constituent elements of the particles, and to determine the content rate (mass%) in terms of oxide of each identified metal element. For each type of particle, the average value of the content rate (mass%) in terms of oxide of each metal element is determined, and this is taken as the content rate (mass%) in terms of oxide of each metal element in each type of particle. (4) By creating and solving an equation representing the relationship between the content rate (mass%) in terms of oxide of each metal element in the sample, the content rate (mass%) in terms of oxide of each metal element in each type of particle, and the content rate (mass%) of each type of particle in the sample, the content rate (mass%) of each type of particle in the sample is calculated, and this is taken as the content rate (mass%) of each type of particle in the first catalyst layer 20.

[0132] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the proportion of the mass of Ce in terms of CeO2 derived from the Ce-Zr composite oxide in the mass of Ce in terms of CeO2 in the first catalyst layer 20 is preferably 80 mass% or more, more preferably 85 mass% or more, and even more preferably 90 mass% or more. The upper limit is 100 mass%.

[0133] When the first catalyst layer 20 contains a Ce-Zr composite oxide, the average particle diameter of the Ce-Zr composite oxide contained in the first catalyst 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 catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the first catalyst layer 20 preferably contains Al and / or Zr.

[0135] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the total of the mass of Al in terms of Al2O3 and the mass of Zr in terms of ZrO2 in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content of other components, etc. The upper limit is preferably 80% by mass or less, more preferably 78% by mass or less, and even more preferably 76% by 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 catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the mass of Al in terms of Al2O3 in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the content of Al in terms of Al2O3 in the first catalyst layer 20" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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, "the content of Al in terms of Al2O3 in the catalyst layer" means the content of Al in terms of Al2O3 derived from the one kind of Al source when the catalyst layer contains one kind of Al source, and means the total content of Al in terms of Al2O3 derived from the two or more kinds of Al sources when the catalyst layer contains two or more kinds of Al sources. 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 catalyst layer 20 contains Al, the first catalyst layer 20 contains one or more Al sources.

[0139] Examples of the Al source include oxides containing Al. Examples of the oxides containing Al include, for example, Al-based oxides, Ce-based oxides containing Al, Zr-based oxides containing Al, Ce-Zr-based composite oxides containing Al, alumina binders, and the like.

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

[0141] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the mass of the Al-based oxide in the first catalyst layer 20 with respect to the mass of the first catalyst layer 20 (referred to as "the content rate of the Al-based oxide in the first catalyst layer 20" in this specification) is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rates of other components, and the like. The upper limit is preferably 70% by mass or less, more preferably 65% by mass or less, and even more preferably 60% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

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

[0143] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, among the mass of Al in the first catalyst layer 20 in terms of Al2O3, the proportion of the mass of Al derived from the Al-based oxide in terms of Al2O3 is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

[0144] When the first catalyst layer 20 contains an Al-based oxide, the average particle diameter of the Al-based oxide contained in the first catalyst layer 20 is preferably 3 μm or more and 20 μm or less, 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 catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the percentage of the mass of Zr in the first catalyst layer 20 in terms of ZrO2 with respect to the mass of the first catalyst layer 20 (referred to as "the content of Zr in the first catalyst layer 20 in terms of ZrO2" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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, "the content of Zr in the catalyst layer in terms of ZrO2" means the content of Zr in terms of ZrO2 derived from the one kind of Zr source when the catalyst layer contains one kind of Zr source, and means the total content of Zr in terms of ZrO2 derived from the two or more kinds of Zr sources when the catalyst layer contains two or more kinds of Zr sources. This definition applies to all catalyst layers (that is, the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40).

[0147] When the first catalyst layer 20 contains Zr, the first catalyst layer 20 contains one kind or two or more kinds of Zr sources.

[0148] Examples of the Zr source include oxides containing Zr. Examples of the oxides containing Zr include Al-based oxides containing Zr, Ce-based oxides containing Zr, Zr-based oxides, Ce-Zr composite oxides, zirconia binders, and the like.

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

[0150] The description regarding the content ratio of the Ce-Zr composite oxide in the first catalyst layer 20 is the same as above.

[0151] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the proportion of the mass of Zr in terms of ZrO2 derived from the Ce-Zr composite oxide in the mass of Zr in terms of ZrO2 in the first catalyst layer 20 is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 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-based binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol. Examples of the stabilizer include nitrates, carbonates, oxides, sulfates, etc. of alkaline earth metal elements (for example, Sr, Ba, etc.).

[0153] <The second catalyst layer> Hereinafter, the second catalyst layer 30 will be described.

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

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

[0156] As shown in FIG. 4, the second catalyst layer 30 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition portion 12 to the end on the exhaust gas outflow side of the partition portion 12. The second catalyst layer 30 may extend along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition portion 12 so as not to reach the end on the exhaust gas outflow side of the partition portion 12, or may extend along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition portion 12 so as not to reach the end on the exhaust gas inflow side of the partition portion 12.

[0157] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost and improving the contact between the catalyst active component (for example, Pd contained in the first catalyst layer 20) contained in the portion below the second catalyst layer 30 and the exhaust gas, the mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 where the second catalyst 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 per unit volume of the second catalyst layer 30 in the portion of the substrate 10 where the second catalyst layer 30 is formed is calculated by the formula: (mass of the second catalyst layer 30) / ((volume of the substrate 10)×(average length L30 of the second catalyst layer 30 / length L10 of the substrate 10)).

[0159] The above description regarding the method for measuring the average length L20 of the first catalyst layer 20 is also applicable to the second catalyst layer 30. When applying, "the first catalyst layer 20" is read as "the second catalyst layer 30", and "average length L20" is read as "average length L30".

[0160] The second catalyst layer 30 contains Rh as a catalytic active component. Rh is contained in the second catalyst layer 30 in the form of a catalytic active component containing Rh, such as metallic Rh, an alloy containing Rh, a compound containing Rh (e.g., an oxide of Rh), etc. From the viewpoint of improving the exhaust gas purification performance, the catalytic active component containing Rh is preferably in 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 terms of metal in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (i.e., the above b) is preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.02% by mass or more and 4% by mass or less, and even more preferably 0.03% by mass or more and 3% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

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

[0163] When the second catalyst layer 30 contains Rh and a noble metal element other than Rh, there is a risk that Rh and the noble metal element other than Rh form an alloy, reducing the active sites of Rh involved in exhaust gas purification performance. Therefore, the percentage of the mass in terms of metal of the noble metal element other than Rh in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content rate in terms of metal of the noble metal element other than Rh in the second catalyst layer 30" in this specification) is preferably small. Specifically, the content rate in terms of metal of the noble metal element other than Rh in the second catalyst layer 30 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 "content rate in terms of metal of the noble metal element other than Rh in the second catalyst layer 30" means the content rate in terms of metal of the one noble metal element when the second catalyst layer 30 contains one noble metal element other than Rh, and means the total content rate in terms of metal of the two or more noble metal elements when the second catalyst layer 30 contains two or more noble metal elements other than Rh.

[0164] The content rate in terms of metal of a predetermined noble metal element in the second catalyst layer 30 can be determined in the same manner as the content rate in terms of metal of the predetermined noble metal element in the first catalyst layer 20.

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

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

[0167] The second catalyst layer 30 contains Ce. The percentage of the mass of Ce in terms of CeO2 in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content rate of Ce in terms of CeO2 in the second catalyst layer 30" in this specification) is 7% by mass or more. Thereby, the OSC of the second catalyst layer 30 can be improved, and the exhaust gas purification performance of the second catalyst layer 30 can be improved.

[0168] From the viewpoint of more effectively improving the OSC of the second catalyst layer 30 and thereby more effectively improving the exhaust gas purification performance of the second catalyst layer 30, the content rate of Ce in terms of CeO2 in the second catalyst layer 30 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 in consideration of the balance with cost, the content rate 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 description regarding the Ce source is the same as above.

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

[0171] From the viewpoint of improving the heat resistance and OSC of the second catalyst layer 30, and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the percentage of the mass of the Ce-Zr composite oxide in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content rate of the Ce-Zr composite oxide in the second catalyst layer 30" in this specification) is preferably 20% by mass or more, more preferably 25% by mass or more, still more preferably 30% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rate of other components, etc. The upper limit is preferably 80% by mass or less, more preferably 75% by mass or less, still more preferably 70% by mass or less. Any of the above lower limits may be combined with any of the above upper limits.

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

[0173] From the viewpoint of improving the heat resistance and OSC of the second catalyst layer 30, and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the ratio that the mass of Ce in terms of CeO2 derived from the Ce-Zr composite oxide occupies among the mass of Ce in terms of CeO2 in the second catalyst layer 30 is preferably 80% by mass or more, more preferably 85% by mass or more, still more preferably 90% by mass or more. The upper limit is 100% by mass.

[0174] When the second catalyst layer 30 contains a Ce-Zr composite oxide, the average particle diameter of the Ce-Zr composite oxide contained in the second catalyst layer 30 is preferably 2 μm or more and 20 μm or less, more preferably 5 μm or more and 15 μm or less. Any 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 catalyst layer 30, and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the second catalyst layer 30 preferably contains Al and / or Zr.

[0176] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the percentage of the total of the mass of Al in terms of Al2O3 and the mass of Zr in terms of ZrO2 in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 is preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 65% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content of other components, etc. The upper limit is preferably 96% by mass or less, more preferably 93% by mass or less, and even more preferably 90% by 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 catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the percentage of the mass of Al in terms of Al2O3 in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content of Al in terms of Al2O3 in the second catalyst layer 30" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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 catalyst layer 30 contains Al, the second catalyst layer 30 contains one or more Al sources. The description regarding the Al source is the same as above.

[0179] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the second catalyst layer 30 preferably contains an Al-based oxide as an Al source. The second catalyst layer 30 may contain, as an 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 catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the percentage of the mass of the Al-based oxide in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content rate of the Al-based oxide in the second catalyst layer 30" in this specification) is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rate of other components, etc. 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. Any of the above lower limits may be combined with any of the above upper limits.

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

[0182] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the ratio of the mass of Al in terms of Al2O3 derived from the Al-based oxide to the mass of Al in terms of Al2O3 in the second catalyst layer 30 is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

[0183] When the second catalyst layer 30 contains an Al-based oxide, the average particle diameter of the Al-based oxide contained in the second catalyst 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. Any of the above lower limits may be combined with any of the above upper limits.

[0184] From the perspective of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the percentage of the mass of Zr in terms of ZrO2 in the second catalyst layer 30 with respect to the mass of the second catalyst layer 30 (referred to as "the content rate of Zr in terms of ZrO2 in the second catalyst layer 30" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content rates of other components, etc. 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 catalyst layer 30 contains Zr, the second catalyst layer 30 contains one or more Zr sources. The description regarding the Zr source is the same as above.

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

[0187] The description regarding the content rate of the Ce-Zr composite oxide in the second catalyst layer 30 is the same as above.

[0188] From the perspective of improving the heat resistance and OSC of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the proportion of the mass of Zr in terms of ZrO2 in the second catalyst layer 30 that is accounted for by the mass of Zr in terms of ZrO2 derived from the Ce-Zr composite oxide is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

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

[0190] From the viewpoint of improving the contact between the catalyst active component (for example, Pd contained in the first catalyst layer 20) contained in the portion below the second catalyst layer 30 and the exhaust gas, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the average thickness of the second catalyst 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 appropriately adjusted in consideration of the OSC etc. required for the second catalyst layer 30. 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] A method for calculating the average thickness of the second catalyst layer 30 will be described later.

[0192] <The third catalyst layer> Hereinafter, the third catalyst layer 40 will be described.

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

[0194] "The third catalyst layer 40 is provided on the second catalyst layer 30" means that a part or all of the third catalyst layer 40 exists on the main surface of the second catalyst layer 30 opposite to the main surface on the first catalyst layer 20 side among the two main surfaces of the second catalyst layer 30. The "main surface of the second catalyst layer 30" means the outer surface of the second catalyst layer 30 extending in the exhaust gas flow direction X. The third catalyst layer 40 may be provided directly on the main surface of the second catalyst layer 30, or may be provided via another layer, but usually it is provided directly on the main surface of the second catalyst layer 30. The third catalyst layer 40 may be provided so as to cover a part of the main surface of the second catalyst layer 30, or may be provided so as to cover the entire main surface of the second catalyst layer 30. The "third catalyst layer 40 provided on the second catalyst layer 30" includes an embodiment in which the third catalyst layer 40 is provided directly on the main surface of the second catalyst layer 30, and an embodiment in which the third catalyst layer 40 is provided on the main surface of the second catalyst 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 Rh contained in the third catalyst layer 40 and the exhaust gas, it is preferable that no other catalyst layer is provided on the third catalyst layer 40.

[0196] As shown in FIG. 4, the third catalyst layer 40 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition wall portion 12 to the end on the exhaust gas outflow side of the partition wall portion 12. The third catalyst layer 40 may extend along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition wall portion 12 so as not to reach the end on the exhaust gas outflow side of the partition wall portion 12, or may extend along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition wall portion 12 so as not to reach the end on the exhaust gas inflow side of the partition wall portion 12.

[0197] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost and improving the contact between the catalyst active components (for example, Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) contained in the portion below the third catalyst layer 40 and the exhaust gas, the mass of the third catalyst layer 40 per unit volume of the portion of the substrate 10 where the third catalyst 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. Any of the above lower limits may be combined with any of the above upper limits.

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

[0199] The above description regarding the measurement method of the average length L20 of the first catalyst layer 20 is also applicable to the third catalyst layer 40. When applying, "the first catalyst layer 20" is read as "the third catalyst layer 40", and "the average length L20" is read as "the average length L40".

[0200] The third catalyst layer 40 contains Rh as a catalytic active component. Rh is contained in the third catalyst layer 40 in the form of a catalytic active component capable of functioning as a catalytic active component, for example, metallic Rh, an alloy containing Rh, a compound containing Rh (for example, an oxide of Rh), etc. From the viewpoint of improving the exhaust gas purification performance, the catalytic active component containing Rh is preferably in particulate form.

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

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

[0203] When the third catalyst layer 40 contains Rh and a noble metal element other than Rh, there is a possibility that Rh and the noble metal element other than Rh form an alloy, and the active sites of Rh involved in the exhaust gas purification performance may decrease. Therefore, the percentage of the mass in terms of metal of the noble metal element other than Rh in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 (referred to as "the content rate in terms of metal of the noble metal element other than Rh in the third catalyst layer 40" in this specification) is preferably small. Specifically, the content rate in terms of metal of the noble metal element other than Rh in the third catalyst layer 40 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 "content rate in terms of metal of the noble metal element other than Rh in the third catalyst layer 40" means the content rate in terms of metal of the one noble metal element when the third catalyst layer 40 contains one noble metal element other than Rh, and means the total content rate in terms of metal of the two or more noble metal elements when the third catalyst layer 40 contains two or more noble metal elements other than Rh.

[0204] The third catalyst layer 40 preferably contains one or two or more carriers, and at least a part of the catalyst active component is supported on one or two or more carriers. The carrier can be selected from, for example, metal oxides. The description of the metal oxide is the same as above.

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

[0206] The third catalyst layer 40 contains Al and / or Zr. Thereby, the heat resistance of the third catalyst layer 40 can be improved, and the exhaust gas purification performance of the third catalyst layer 40 can be improved.

[0207] From the viewpoint of improving the heat resistance of the third catalyst layer 40 and thereby improving the exhaust gas purification performance of the third catalyst layer 40, the percentage of the total mass of Al in terms of Al2O3 and the mass of Zr in terms of ZrO2 in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content of other components, etc. The upper limit is preferably 99% by mass or less, more preferably 97% by mass or less, and even more preferably 95% by mass or less. Each of the above lower limits can be combined with any of the above upper limits.

[0208] From the viewpoint of improving the heat resistance of the third catalyst layer 40 and thereby improving the exhaust gas purification performance of the third catalyst layer 40, the percentage of the mass of Al in terms of Al2O3 in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 (referred to as "the content of Al in terms of Al2O3 in the third catalyst layer 40" in this specification) 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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 can be combined with any of the above upper limits.

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

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

[0211] From the perspective of improving the heat resistance of the third catalyst layer 40 and thereby improving the exhaust gas purification performance of the third catalyst layer 40, the percentage of the mass of the Al-based oxide in the third catalyst layer 40 with respect to the mass of the third catalyst layer 40 (referred to as "the content rate of the Al-based oxide in the third catalyst layer 40" in this specification) is preferably 65% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rates of other components, etc. The upper limit is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 93% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

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

[0213] From the perspective of improving the heat resistance of the third catalyst layer 40 and thereby improving the exhaust gas purification performance of the third catalyst layer 40, the ratio occupied by the mass of Al in terms of Al2O3 derived from the Al-based oxide among the mass of Al in terms of Al2O3 in the third catalyst layer 40 is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

[0214] When the third catalyst layer 40 contains an Al-based oxide, the average particle diameter of the Al-based oxide contained in the third catalyst layer 40 is preferably 0.1 μm or more and 9 μm or less, 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 perspective of improving the heat resistance of the third catalyst layer 40 and thereby improving the exhaust gas purification performance of the third catalyst layer 40, the percentage of the mass of Zr in the third catalyst layer 40 in terms of ZrO2 (referred to as "the content of Zr in the third catalyst layer 40 in terms of ZrO2" in this specification) relative to the mass of the third catalyst layer 40 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 appropriately adjusted in consideration of the balance with cost, the content of other components, etc. 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 can be combined with any of the above upper limits.

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

[0217] The percentage of the mass of Ce in the third catalyst layer 40 in terms of CeO2 (referred to as "the content of Ce in the third catalyst layer 40 in terms of CeO2" in this specification) relative to the mass of the third catalyst layer 40 is less than 7% by mass. Thereby, the heat resistance of the third catalyst layer 40 can be improved, and the exhaust gas purification performance of the third catalyst layer 40 can be improved.

[0218] From the perspective of more effectively improving the heat resistance of the third catalyst layer 40 and thereby more effectively improving the exhaust gas purification performance of the third catalyst layer 40, the content of Ce in the third catalyst layer 40 in terms of CeO2 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 catalyst layer 40 may contain Ce. When the third catalyst layer 40 contains Ce, the content of Ce in the third catalyst layer 40 in terms of CeO2 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 can be combined with any of the above upper limits.

[0220] When the third catalyst layer 40 contains Ce, the third catalyst layer 40 contains one or more Ce sources. The description of the Ce source is the same as above.

[0221] When the third catalyst layer 40 contains Ce, the third catalyst layer 40 preferably contains a Ce-Zr composite oxide as a Ce source. In addition to the Ce-Zr composite oxide, the third catalyst layer 40 may contain one or more other Ce sources.

[0222] When the third catalyst layer 40 contains Ce, the percentage of the mass of the Ce-Zr composite oxide in the third catalyst layer 40 to the mass of the third catalyst layer 40 (referred to as "the content rate of the Ce-Zr composite oxide in the third catalyst layer 40" in this specification) is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. The upper limit can be appropriately adjusted in consideration of the balance with cost, the content rate of other components, etc. The upper limit is preferably 40% by mass or less, more preferably 37% by mass or less, and even more preferably 35% by mass or less. Any of the above lower limits may be combined with any of the above upper limits.

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

[0224] When the third catalyst layer 40 contains Ce, the ratio of the mass of Ce in terms of CeO2 in the third catalyst layer 40 that is occupied by the mass of Ce in terms of CeO2 derived from the Ce-Zr composite oxide is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

[0225] When the third catalyst layer 40 contains a Ce-Zr composite oxide, the average particle diameter of the Ce-Zr composite oxide contained in the third catalyst layer 40 is preferably 0.1 μm or more and 9 μm or less, more preferably 0.1 μm or more and 7 μm or less. Any 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 and a stabilizer. The descriptions of the binder and the stabilizer are the same as above.

[0227] The average thickness of the third catalyst layer 40 is 10 μm or less. Thereby, the contact property between the catalyst active components (for example, Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) contained in the portion below the third catalyst layer 40 and the exhaust gas can be improved, and the exhaust gas purification performance of the portion below the third catalyst layer 40 (for example, the first catalyst layer 20 and the second catalyst layer 30) can be improved.

[0228] From the viewpoint of more effectively realizing the improvement of the contact property between the catalyst active components contained in the portion below the third catalyst layer 40 and the exhaust gas, the average thickness of the third catalyst 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 appropriately adjusted in consideration of the required heat resistance and the like in the third catalyst layer 40. 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. Any of the above lower limits may be combined with any of the above upper limits.

[0229] An example of the calculation method of the average thickness of the second catalyst layer 30 and the average thickness of the third catalyst layer 40 is as follows.

[0230] When the catalyst 1 (for example, in the case where the first catalyst layer 20 extends along the exhaust gas flow direction X from the end of the partition wall portion 12 on the exhaust gas inflow side to the end of the partition wall portion 12 on the exhaust gas outflow side, for example, a location 30 mm away from the end of the partition wall portion 12 on the exhaust gas inflow 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 portion 12 on the exhaust gas inflow side so as not to reach the end of the partition wall portion 12 on the exhaust gas outflow side, for example, a location 10 mm away from the end of the partition wall portion 12 on the exhaust gas inflow side in the exhaust gas flow direction X. When the first catalyst layer 20 extends along the direction opposite to the exhaust gas flow direction X from the end of the partition wall portion 12 on the exhaust gas outflow side so as not to reach the end of the partition wall portion 12 on the exhaust gas inflow side, for example, a location 10 mm away from the end of the partition wall portion 12 on the exhaust gas outflow side in the direction opposite to the exhaust gas flow direction X.) is cut in a plane perpendicular to the axial direction of the base material 10, and the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 existing in one arbitrarily selected cell 13 from the cut surface are observed using a backscattered electron detector (BED) in SEM or EPMA, and the regions where the first catalyst layer 20 exists, the regions where the second catalyst layer 30 exists, and the regions where the third catalyst layer 40 exists are specified. In the observation of the cut surface by SEM or BED, the field magnification is, for example, 500 times, and the field width (length) is, for example, 100 to 200 μm. The region observed by SEM or BED is set so as not to include the corners of the cell 13. This is because the corners of the cell 13 are easily affected by the cell shape. The regions where the first catalyst layer 20 exists, the regions where the second catalyst layer 30 exists, and the regions where the third catalyst layer 40 exists can be specified based on differences in morphology, composition, etc. between the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40. At this time, elemental mapping of the cut surface may be performed. Elemental mapping can be performed, for example, by using a combination of the observation of the cut surface by SEM or BED and the composition analysis of the cut surface. Elemental mapping can be performed, for example, using SEM-EDX, EPMA, etc. Based on the differences in morphology and composition between the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 by elemental mapping of the cut surface, the regions where the first catalyst layer 20 exists, the regions where the second catalyst layer 30 exists, and the regions where the third catalyst layer 40 exists can be specified.

[0231] In the SEM or BED observation image, draw the first to Nth grid lines parallel to the thickness direction of the partition portion 12 of the base material 10 at intervals of 15 μm in order from the left end side or the right end side, and connect the intersections of the contour line of the region where the first catalyst layer 20 exists and each grid line with a straight line to specify the position of the surface of the first catalyst layer 20. N is an integer of 5 to 10, for example. Similarly, connect the intersections of the contour line of the region where the second catalyst layer 30 exists and each grid line with a straight line to specify the position of the surface of the second catalyst layer 30. Similarly, connect the intersections of the contour line of the region where the third catalyst layer 40 exists and each grid line with a straight line to specify the position of the surface of the third catalyst layer 40. When the change amount in the thickness direction from a certain intersection P1 to the adjacent intersection P2 exceeds the interval of the grid lines (15 μm), it is preferable not to use the intersection P2 for specifying the position of the surface (that is, exclude the intersection P2 from the intersections connected by a straight line). The change amount in the thickness direction from a certain intersection P1 to the adjacent intersection P2 means the distance between a straight line passing through the intersection P1 and perpendicular to the thickness direction of the partition portion 12 of the base material 10 and a straight line passing through the intersection P2 and perpendicular to the thickness direction of the partition portion 12 of the base material 10. When the change amount in the thickness direction from the intersection P1 to the adjacent intersection P2 exceeds the interval of the grid lines (15 μm) and the change amount in the thickness direction from the intersection P1 to the intersection P3 adjacent to the intersection P2 also exceeds the interval of the grid lines (15 μm), it is preferable not to use the intersection P3 in addition to the intersection P2 for specifying the position of the surface (that is, exclude the intersections P2 and P3 from the intersections connected by a straight line). When excluding five consecutive intersections from the intersections connected by a straight line in this way, it is preferable not to measure the thickness for the SEM or BED image.

[0232] After identifying the positions of the surfaces of the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40, the area of the first region surrounded by the second grid line, the (N - 1)th grid line, the surface of the first catalyst layer 20, and the surface of the second catalyst layer 30 is determined using image analysis software. Similarly, the area of the second region surrounded by the second grid line, the (N - 1)th grid line, the surface of the second catalyst layer 30, and the surface of the third catalyst layer 40 is determined. As the image analysis software, for example, AreaQ (manufactured by Estec Corporation), ImageJ (public domain), Photoshop (Adobe Systems Incorporated), etc. can be used. Note that both ends of the image tend to be unclear and it is difficult to identify the positions of the surfaces of the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40, so the first grid line and the Nth grid line are not used.

[0233] After determining the areas of the first region and the second region, the thickness of each region is calculated based on the following formula. Thickness of each region = Area of each region / (Interval between grid lines × Number of intervals between grid lines) Note that the interval between grid lines is 15 μm and the number of intervals between grid lines is (N - 3).

[0234] Regarding 20 cells 13 arbitrarily selected from the cross-section, the thickness of the first region is calculated, and their average value is taken as the average thickness of the second catalyst layer 30. Regarding 20 cells 13 arbitrarily selected from the cross-section, the thickness of the second region is calculated, and their average value is taken as the average thickness of the third catalyst layer 40.

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

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

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

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

[0239] Examples of the Pd salt and Rh salt include nitrates, ammine complex salts, acetates, chlorides, etc. Examples of the binder include alumina sol, zirconia sol, titania sol, silica sol, ceria sol, etc. Examples of the solvent include water, organic solvents, etc.

[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 longer and 1 hour or shorter. 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 longer and 10 hours or shorter. The firing can be carried out, for example, in an air atmosphere.

Example

[0241] 〔Example 1〕 (1) Preparation of the slurry for forming the lower layer To a blending container, an aqueous palladium nitrate solution, a Ce-Zr composite oxide (content of Ce in terms of CeO2: 13 mass% or more and 50 mass% or less, content of Zr in terms of ZrO2: 35 mass% or more and 70 mass% or less, content of oxides of one or more rare earth elements other than Ce in terms of oxides: 9 mass% or more and 20 mass% or less), an Al-based oxide (content of Al in terms of Al2O3: 95 mass% or more), an alumina binder, a zirconia binder and water were added, mixed and stirred to prepare a slurry for forming the lower layer. The amounts of the respective components in the slurry for forming the lower layer were adjusted such that, based on the mass of the lower layer after firing (100 mass%), Pd was 2.0 mass% in terms of metal, Ce was 18.0 mass% in terms of CeO2, Zr was 18.0 mass% in terms of ZrO2, Al was 57.5 mass% in terms of Al2O3, and rare earth elements other than Ce were 4.5 mass% in terms of oxides.

[0242] (2) Formation of the lower layer As a flow-through type substrate, a flow-through type substrate having cells extending in the axial direction partitioned by partition walls with a thickness of 50 to 70 μm and having a density of 600 cells / inch on a plane orthogonal to the axial direction and a volume of 1.0 L was prepared. 2 The flow-through type substrate was immersed in the slurry for forming the lower layer, and after drying the flow-through type substrate coated with the slurry for forming the lower layer at 150 °C for 0.5 hour, it was fired at 500 °C for 1 hour to form a lower layer on the flow-through type substrate. The mass of the lower layer per unit volume of the portion of the flow-through type substrate where the lower layer was formed was 100 g / L.

[0243]

[0244] (3) Preparation of the slurry for forming the middle layer ​To a blending container, an aqueous rhodium nitrate solution, a Ce-Zr composite oxide (content of Ce in terms of CeO2: 13 mass% or more and 50 mass% or less, content of Zr in terms of ZrO2: 35 mass% or more and 70 mass% or less, content of one or more rare earth elements other than Ce in terms of oxide: 9 mass% or more and 20 mass% or less), an Al-based oxide (content of Al in terms of Al2O3: 95 mass% or more), an alumina binder, a zirconia binder, and water were added, mixed, and stirred to prepare a slurry for forming the middle layer. The amounts of each component in the slurry for forming the middle layer were adjusted such that, based on the mass of the middle layer after firing (100 mass%), Rh was 0.1 mass% in terms of metal, Ce was 12.0 mass% in terms of CeO2, Zr was 42.0 mass% in terms of ZrO2, Al was 39.6 mass% in terms of Al2O3, and rare earth elements other than Ce were 6.3 mass% in terms of oxide.

[0245] (4) Formation of the middle layer The flow-through type substrate with the lower layer formed was immersed in the slurry for forming the middle layer, and after drying the flow-through type substrate coated with the slurry for forming the middle layer at 150 °C for 0.5 hour, it was 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 where the middle layer was formed was 100 g / L.

[0246] (5) Preparation of the slurry for forming the upper layer To a blending container, an aqueous rhodium nitrate solution, a Ce-Zr composite oxide (content of Ce in terms of CeO2: 13 mass% or more and 50 mass% or less, content of Zr in terms of ZrO2: 35 mass% or more and 70 mass% or less, content of one or more rare earth elements other than Ce in terms of oxide: 9 mass% or more and 20 mass% or less), an Al-based oxide (content of Al in terms of Al2O3: 95 mass% or more), an alumina binder, and water were added, mixed, and stirred to prepare a slurry for forming the upper layer. The amounts of each component in the slurry for forming the upper layer were adjusted such that, based on the mass of the upper layer after firing (100 mass%), Rh was 0.7 mass% in terms of metal, Ce was 6.7 mass% in terms of CeO2, Zr was 23.3 mass% in terms of ZrO2, Al was 66.0 mass% in terms of Al2O3, and rare earth elements other than Ce were 3.3 mass% in terms of oxide.

[0247] (6) Formation of the upper layer The flow-through type substrate with the middle layer formed thereon was immersed in the slurry for forming the upper layer, and after the flow-through type substrate coated with the slurry for forming the upper layer was dried at 150°C for 0.5 hour, it was fired at 500°C for 1 hour to form the upper layer on the middle layer. The mass of the upper layer per unit volume of the portion of the flow-through type substrate where the upper layer was formed was 15 g / L.

[0248] As described above, the exhaust gas purification catalyst of Example 1 was manufactured, which includes the lower layer formed on the flow-through type substrate, the middle layer formed on the lower layer, and the upper layer formed on the middle layer. The lower layer, the middle layer, and the upper layer in the exhaust gas purification 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 purification catalyst of Example 1 were calculated by the above-described method. Specifically, it is as follows.

[0250] The exhaust gas purification catalyst of Example 1 (a location 30 mm away from the end of the substrate on the exhaust gas inflow side in the axial direction of the substrate) was cut with a plane perpendicular to the axial direction of the substrate, and using the backscattered electron detector (BED) in EPMA, the first catalyst layer, the second catalyst layer, and the third catalyst layer existing in one cell arbitrarily selected from the cut surface were observed to identify the regions where the first catalyst layer exists, the region where the second catalyst layer exists, and the region where the third catalyst layer exists. In the observation of the cut surface by BED, the field magnification was set to 500 times, and the field width was set to 100 - 200 μm. The region observed by BED was set so as not to include the corners of the cell. The regions where the first catalyst layer exists, the region where the second catalyst layer exists, and the region where the third catalyst layer exists were performed by elemental mapping of the cut surface using EPMA.

[0251] In the BED observation image, from the left end side in order, the 1st to 9th grid lines parallel to the thickness direction of the partition wall portion of the base material were drawn at 15-μm intervals, and the intersections between the contour line of the region where the 1st catalyst layer exists and each grid line were connected by straight lines to specify the position of the surface of the 1st catalyst layer. Similarly, the intersections between the contour line of the region where the 2nd catalyst layer exists and each grid line were connected by straight lines to specify the position of the surface of the 2nd catalyst layer. Similarly, the intersections between the contour line of the region where the 3rd catalyst layer exists and each grid line were connected by straight lines to specify the position of the surface of the 3rd catalyst layer. When the amount of change in the thickness direction from a certain intersection P1 to an adjacent intersection P2 exceeded the interval of the grid lines (15 μm), intersection P2 was not used for specifying the position of the surface (that is, intersection P2 was excluded from the intersections connected by straight lines). Further, when the amount of change in the thickness direction from intersection P1 to an adjacent intersection P2 exceeded the interval of the grid lines (15 μm) and the amount of change in the thickness direction from intersection P1 to an intersection P3 adjacent to intersection P2 also exceeded the interval of the grid lines (15 μm), in addition to intersection P2, intersection P3 was also not used for specifying the position of the surface (that is, intersection P2 and intersection P3 were excluded from the intersections connected by straight lines). When continuously excluding five intersections from the intersections connected by straight lines in this way, the BED image was not used for measuring the thickness.

[0252] After specifying the positions of the surfaces of the 1st catalyst layer, the 2nd catalyst layer, and the 3rd catalyst layer, using image analysis software, the area of the 1st region surrounded by the 2nd grid line, the 8th grid line, the surface of the 1st catalyst layer, and the surface of the 2nd catalyst layer was obtained. Similarly, the area of the 2nd region surrounded by the 2nd grid line, the 8th grid line, the surface of the 2nd catalyst layer, and the surface of the 3rd catalyst layer was obtained. As the image analysis software, AreaQ (manufactured by Estec Co., Ltd.) was used. Note that both ends of the image tend to be unclear and it is difficult to specify the positions of the surfaces of the 1st catalyst layer, the 2nd catalyst layer, and the 3rd catalyst layer, so the 1st grid line and the 9th grid line were not used.

[0253] After obtaining the areas of the 1st region and the 2nd region, based on the following formula, the thickness of each region was calculated. Thickness of each region = Area of each region / (Interval between grid lines × Number of grid lines) Note that the interval between grid lines is 15 μm, and the number of grid lines is 6.

[0254] Regarding 20 cells arbitrarily selected from the cross-section, the thickness of the first region was calculated, and the average value thereof was taken as the average thickness of the second catalyst layer. Also, regarding 20 cells arbitrarily selected from the cross-section, the thickness of the second region was calculated, and the average value thereof was taken as the average thickness of the third catalyst 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. The characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purification catalyst of Example 1 are shown in Table 1.

[0256] In Table 1, the meanings of "Rh", "CeO2", "Al2O3", "ZrO2", "Al2O3 + ZrO2", "Average thickness" and "a / b" are as follows.

[0257] [Second catalyst layer] Rh: Percentage (mass%) of the mass of Rh in the second catalyst layer in terms of metal to the mass of the second catalyst layer CeO2: Percentage (mass%) of the mass of Ce in the second catalyst layer in terms of CeO2 to the mass of the second catalyst layer Al2O3: Percentage (mass%) of the mass of Al in the second catalyst layer in terms of Al2O3 to the mass of the second catalyst layer ZrO2: Percentage (mass%) of the mass of Zr in the second catalyst layer in terms of ZrO2 to the mass of the second catalyst layer Al2O3 + ZrO2: Percentage (mass%) of the sum of the mass of Al in the second catalyst layer in terms of Al2O3 and the mass of Zr in the second catalyst layer in terms of ZrO2 to the mass of the second catalyst layer Average thickness: Average thickness of the second catalyst layer (μm)

[0258] [Third catalyst layer] Rh: Percentage (mass%) of the mass of Rh in the third catalyst layer in terms of metal to the mass of the third catalyst layer CeO2: The percentage (mass %) of the mass of Ce in CeO2 conversion in the third catalyst layer with respect to the mass of the third catalyst layer Al2O3: The percentage (mass %) of the mass of Al in Al2O3 conversion in the third catalyst layer with respect to the mass of the third catalyst layer ZrO2: The percentage (mass %) of the mass of Zr in ZrO2 conversion in the third catalyst layer with respect to the mass of the third catalyst layer Al2O3 + ZrO2: The percentage (mass %) of the total mass of Al in Al2O3 conversion and Zr in ZrO2 conversion in the third catalyst layer with respect to the mass of the third catalyst layer Average thickness: The average thickness (μm) of the third catalyst layer

[0259] [a / b] a: The percentage (mass %) of the mass of Rh in metal conversion in the third catalyst layer with respect to the mass of the third catalyst layer b: The percentage (mass %) of the mass of Rh in metal conversion in the second catalyst layer with respect to the mass of the second catalyst layer

[0260] [Example 2] The amounts of each component in the upper layer forming slurry were adjusted so that, based on the mass of the upper layer after firing (100 mass %), Rh was 0.5 mass % in metal conversion, Ce was 5.0 mass % in CeO2 conversion, Zr was 17.5 mass % in ZrO2 conversion, Al was 74.5 mass % in Al2O3 conversion, and rare earth elements other than Ce were 2.5 mass % in oxide conversion. In addition, except that the mass of the upper layer per unit volume of the portion where the upper layer was formed in the flow-through type substrate was changed to 20 g / L, the exhaust gas purifying catalyst of Example 2 was produced in the same manner as in Example 1. The lower layer, middle layer, and upper layer in the exhaust gas purifying catalyst of Example 2 corresponded to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

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

[0262] [Example 3] Except that the amounts of the respective components in the upper layer forming slurry were adjusted such that, based on the mass of the upper layer after firing (100% by mass), Rh was 0.3% by mass in terms of metal, Ce was 3.3% by mass in terms of CeO2, Zr was 11.7% by mass in terms of ZrO2, Al was 83.0% by mass in terms of Al2O3, and rare earth elements other than Ce were 1.7% by mass in terms of oxide, and the mass of the upper layer per unit volume of the portion of the flow-through type substrate where the upper layer was formed was changed to 30 g / L, the exhaust gas purifying catalyst of Example 3 was produced in the same manner as in Example 1. The lower layer, the middle layer, and the 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] When 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 3 were calculated in the same manner as in Example 1, the average thickness of the second catalyst layer was 40 μm and the average thickness of the third catalyst layer was 6.6 μm. The characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purifying catalyst of Example 3 are shown in Table 1.

[0264] [Comparative Example 1] Except that the amounts of the respective components in the middle layer forming slurry were adjusted such that, based on the mass of the middle layer after firing (100% by mass), Rh was 0.2% by mass in terms of metal, Ce was 6.0% by mass in terms of CeO2, Zr was 21.0% by mass in terms of ZrO2, Al was 69.2% by mass in terms of Al2O3, and rare earth elements other than Ce were 3.6% by mass in terms of oxide, and the upper layer was not formed, the exhaust gas purifying catalyst of Comparative Example 1 was produced in the same manner as in Example 1. 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 regarded as 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 exhaust gas purification catalyst of Comparative Example 2 was produced in the same manner as in Comparative Example 1, except that the amounts of the respective components in the slurry for forming the middle layer were adjusted so that, based on the mass of the middle layer after firing (100% by mass), Rh was 0.2% by mass in terms of metal, Ce was 12.0% by mass in terms of CeO2, Zr was 42.0% by mass in terms of ZrO2, Al was 39.5% by mass in terms of Al2O3, and rare earth elements other than Ce were 6.3% by mass in terms of oxide. The lower layer in the exhaust gas purification catalyst of Comparative Example 2 corresponds to the first catalyst layer. The middle layer in the exhaust gas purification catalyst of Comparative Example 2 was regarded as 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 purification catalyst of Comparative Example 2 are shown in Table 1.

[0266] The BED observation image and the element mapping image regarding the exhaust gas purification catalyst of Comparative Example 2 obtained by the same method as in Example 1 are shown in FIGS. 5 and 6, respectively. Note that FIG. 6 is a mapping image regarding Pd.

[0267] [Comparative Example 3] The flow-through type substrate with the lower layer and the middle layer formed in Example 1(4) was immersed in an aqueous rhodium nitrate solution at 25°C for 48 hours. After the immersed flow-through type substrate was dried at 150°C for 0.5 hours, it was calcined at 500°C for 1 hour to form a noble metal-containing surface layer portion containing Rh on the entire surface portion of the middle layer. In the middle layer after calcination, based on the mass of the middle layer (100% by mass), Rh is 0.2% by mass in terms of metal, Ce is 12.0% by mass in terms of CeO2, Zr is 42.0% by mass in terms of ZrO2, Al is 39.5% by mass in terms of Al2O3, and rare earth elements other than Ce are 6.3% by mass in terms of oxide. Further, Rh in the middle layer does not exist uniformly in the thickness direction of the middle layer but is unevenly distributed near the surface of the middle layer. The lower layer in the exhaust gas purification catalyst of Comparative Example 3 corresponds to the first catalyst layer. Among the middle layer in the exhaust gas purification catalyst of Comparative Example 3, the portion other than the noble metal-containing surface layer portion was regarded as the second catalyst layer, and the noble metal-containing surface layer portion was regarded as the third catalyst layer. Table 1 shows the characteristics of the second catalyst layer and the third catalyst layer in the exhaust gas purification catalyst of Comparative Example 3. In Comparative Example 3, "CeO2", "Al2O3", "ZrO2", and "Al2O3 + ZrO2" of the entire middle layer were regarded as "CeO2", "Al2O3", "ZrO2", and "Al2O3 + ZrO2" of the second catalyst layer and the third catalyst layer.

[0268] 〔Comparative Example 4〕 The amounts of the respective components in the slurry for forming the upper layer were adjusted such that, based on the mass of the upper layer after firing (100% by mass), Rh was 0.2% by mass in terms of metal, Ce was 1.7% by mass in terms of CeO₂, Zr was 5.8% by mass in terms of ZrO₂, Al was 91.5% by mass in terms of Al₂O₃, and rare earth elements other than Ce were 0.8% by mass in terms of oxide. Except that the mass of the upper layer per unit volume of the portion of the flow-through type substrate where the upper layer was formed was 60 g / L, a catalyst for purifying exhaust gas of Comparative Example 4 was produced in the same manner as in Example 1. In the catalyst for purifying exhaust gas of Comparative Example 4, the lower layer, the middle layer, and the upper layer corresponded to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively. When the average thickness of the second catalyst layer and the average thickness of the third catalyst layer in the catalyst for purifying exhaust gas of Comparative Example 4 were calculated by the above-described method, the average thickness of the second catalyst layer was 40 μm and the average thickness of the third catalyst layer was 13.2 μm. The characteristics of the second catalyst layer and the third catalyst layer in the catalyst for purifying exhaust gas of Comparative Example 4 are shown in Table 1.

[0269] 〔Test Example〕 After subjecting the catalysts for purifying exhaust gas of Examples 1 to 3 and Comparative Examples 1 to 4 to durability treatment, the exhaust gas purification performance and the OSC performance were evaluated as follows. The durability treatment was performed by heat treatment at 1000 °C for 30 hours in an atmosphere in which 0.50% of O₂ gas, 10% of water vapor, and N₂ as a balance gas were circulated.

[0270] <Exhaust Gas Purification Performance Test> Among the harmful components, the purification performance of hydrocarbons (HC) was measured as a representative. A model gas having the following composition with an A / F of 14.6 was passed through the exhaust gas purification catalyst (catalyst volume: 15 mL) after the durability treatment at 32 L / min while adjusting the CO concentration and the O₂ concentration so that the A / F varied in the range of 14.4 to 14.8. The temperature of the gas flowing into the exhaust gas purification catalyst was gradually increased from room temperature at a predetermined heating rate, and the amount of HC contained in the exhaust gas passing through the catalyst was determined by the following apparatus, and the HC purification rate was determined based on the following formula. Here, V represents the detected amount when no catalyst is installed, and W represents the detected amount after the catalyst is installed. HC purification rate (%) = (V - W) / V × 100

[0271] [Model gas (composition by volume)] CO: 0.3%, C3H6: 1000 ppmC, NO: 500 ppm, O2: 0.28%, CO2: 14%, H2O: 10%, N2: the balance [Temperature increase rate] 10 °C / min [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by Horiba, Ltd.

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

[0273] <OSC performance test> Model gases 1 and 2 with the following compositions were alternately passed through the exhaust gas purification catalyst (catalyst volume 15 mL) after the durability treatment at 32 L / min every minute. The gas temperature flowing into the exhaust gas purification catalyst was fixed at 500 °C, and the time (hereinafter referred to as "delay time") until the CO concentration in the exhaust gas passing through the catalyst reached 0.25% after switching from model gas 1 to model gas 2 was determined using the following device, and the difference (seconds) in the delay time was determined based on the following formula. Here, X represents the delay time when no catalyst is installed, and Y represents the delay time after the catalyst is installed. Difference in delay time (seconds) = Y - X

[0274] [Model gas 1 (composition by volume)] O2: 0.5%, N2: the balance [Model gas 2 (composition by volume)] CO: 0.5%, N2: the balance [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by Horiba, Ltd.

[0275] The difference in delay time serves as an indicator of OSC. The greater the difference in delay time, the greater the OSC. Therefore, the difference in delay time was determined as the relative value when the value of Comparative Example 1 was set to 100, and this was used as the OSC. Table 2 shows the measurement results of the OSC.

[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 sequence. While Model Gas 1 is flowing, the CO concentration detected by the gas concentration meter is zero. When no catalyst is installed, when switching from Model Gas 1 to Model Gas 2, the CO in Model Gas 2 is detected by the gas concentration meter without being consumed. When a catalyst is installed, when switching from Model Gas 1 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 kept low. The more oxygen stored in the catalyst, the longer the time the CO concentration detected by the gas concentration meter is kept low. Therefore, the difference in delay time serves as an indicator of the OSC ability.

[0277]

Table 1

[0278]

Table 2

[0279] The exhaust gas purification catalysts of Examples 1 to 3 include 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. The first catalyst layer contains Pd, the second catalyst layer contains Rh and Ce, and the third catalyst layer contains Rh and Al and / or Zr. The exhaust gas purification catalyst satisfies the following conditions (1) to (4). On the other hand, the exhaust gas purification catalysts of Comparative Examples 1 to 4 include 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. The first catalyst layer contains Pd, the second catalyst layer contains Rh and Ce, and the third catalyst layer contains Rh and Al and / or Zr. The exhaust gas purification catalyst does not satisfy any one or more of the following conditions (1) to (4). (1) The second catalyst layer and the third catalyst layer satisfy the following formula: a > b [In the formula, 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.] Satisfy it. (2) The percentage of the mass of Ce in terms of CeO2 in the second catalyst layer with respect to the mass of the second catalyst layer is 7% by mass or more. (3) The percentage of the mass of Ce in terms of CeO2 in the third catalyst layer with respect to the mass of the third catalyst layer is less than 7% by mass. (4) The average thickness of the third catalyst layer is 10 μm or less.

[0280] Since Comparative Example 1 does not satisfy condition (1), the contact property between Rh contained in the third catalyst layer and the exhaust gas cannot be improved. Therefore, T80 was large (that is, the contact property between the catalyst and the exhaust gas was low). Further, since Comparative Example 1 does not satisfy condition (2), the improvement of the OSC of the second catalyst layer cannot be realized. Therefore, the OSC was low.

[0281] Since Comparative Example 2 did not satisfy condition (1), the contact between Rh contained in the third catalyst layer and the exhaust gas could not be improved. Therefore, T80 was large (i.e., the contact between the catalyst and the exhaust gas was low). Further, since Comparative Example 2 did not satisfy condition (3), the heat resistance of the third catalyst layer could not be improved. Therefore, T50 was large (i.e., the heat resistance was low).

[0282] Since Comparative Example 3 did not satisfy condition (3), the heat resistance of the third catalyst layer could not be improved. Therefore, T50 was large (i.e., the heat resistance was low). Although Comparative Example 3 satisfied conditions (1) and (4), T80 was large (i.e., the contact between the catalyst and the exhaust gas was low). It is considered that since aggregation of Rh in the third catalyst layer occurred and the active sites decreased due to Comparative Example 3 not satisfying condition (3), although conditions (1) and (4) were satisfied, T80 was large.

[0283] Since Comparative Example 4 did not satisfy condition (4), the contact between the catalyst active component contained in the portion below the third catalyst layer and the exhaust gas could not be improved. Therefore, T80 was large (i.e., the contact between the catalyst and the exhaust gas was low).

[0284] Since Examples 1 to 3 satisfied condition (1), the contact between Rh contained in the third catalyst layer and the exhaust gas could be improved. Therefore, T80 was small (i.e., the contact between the catalyst and the exhaust gas was high).

[0285] Since Examples 1 to 3 satisfied condition (2), the improvement of the OSC of the second catalyst layer could be realized. Therefore, the OSC was high.

[0286] Since Examples 1 to 3 satisfied condition (3), the improvement of the heat resistance of the third catalyst layer could be realized. Therefore, T50 was small (i.e., the heat resistance was high).

[0287] Examples 1 to 3 satisfy condition (4), and thus, it is possible to improve the contact between the catalyst active component contained in the portion below the third catalyst layer and the exhaust gas. Therefore, T80 was small (i.e., the contact between the catalyst and the exhaust gas was high).

Explanation of Reference Numerals

[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. A catalyst for purifying exhaust gas, 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, The first catalyst layer contains Pd, the second catalyst layer contains Rh and Ce, the third catalytic layer contains Rh and Al and / or Zr; The second catalyst layer and the third catalyst layer are formed of a catalyst having the following formula: a>b [In the formula, 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.] Fulfilling CeO of Ce in the second catalyst layer ２ The percentage of the converted mass to the mass of the second catalyst layer is 7 mass% or more, CeO of Ce in the third catalyst layer ２ The percentage of the converted mass to the mass of the third catalyst layer is less than 7 mass%, The above exhaust gas purifying catalyst, wherein the average thickness of the third catalyst layer is 10 μm or less.

2. Al in the third catalyst layer ２ O ３ The mass of Zr in the third catalyst layer is calculated by the following equation: ２ 2. The exhaust gas purifying catalyst according to claim 1, wherein a percentage of the total of the converted mass of the first catalyst layer and the converted mass of the second catalyst layer to the mass of the third catalyst layer is 80 mass % or more.

3. 3. The exhaust gas purifying catalyst according to claim 1, wherein a ratio of a to b, a / b, is 2 or more and 10 or less.

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

5. 3. The exhaust gas purifying catalyst according to claim 1, wherein the third catalyst layer has an average thickness of 0.5 μm or more and 5 μm or less.

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