Catalyst for purification of exhaust gas

Abstract

The present invention aims to provide a catalyst for exhaust gas purification that can improve the exhaust gas purification performance of the Rh layer and the exhaust gas purification performance of the portion below the Rh layer. To achieve this objective, an exhaust gas purification catalyst (1) is provided, comprising: a substrate (10), a first catalyst layer (20) disposed on the substrate (10), a second catalyst layer (30) disposed on the first catalyst layer (20), and a third catalyst layer (40) disposed on the second catalyst layer (30). In the exhaust gas purification 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 contains Al and / or Zr, the metal equivalent content of Rh in the third catalyst layer (40) is greater than the metal equivalent content of Rh in the second catalyst layer (30), the Ce O2 equivalent content of Ce in the second catalyst layer (30) is more than 7% by mass, the Ce O2 equivalent content of Ce in the third catalyst layer (40) is less than 7% by mass, and the average thickness of the third catalyst layer (40) is less than 10 μm.

Description

Technical Field

[0001] This invention relates to catalysts for waste gas purification. Background Technology

[0002] Exhaust gases from internal combustion engines in automobiles, motorcycles, and other vehicles contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). To purify and neutralize these harmful components, precious metal elements such as Pd and Rh are used as the catalytic active components in catalysts for exhaust gas purification.

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

[0004] As an example of a catalyst for exhaust gas purification that meets these requirements, Patent Document 1 discloses a catalyst for exhaust gas purification comprising: a substrate, a lower layer containing Pd disposed on the substrate, and an upper layer disposed on the lower layer, wherein an Rh-containing layer is formed on the surface portion of the upper layer of the catalyst for exhaust gas purification. In the catalyst for exhaust gas purification disclosed in Patent Document 1, the Rh-containing layer is formed on the surface portion of the upper layer, which is easily in contact with exhaust gas, thereby improving the contact between Rh and exhaust gas.

[0005] Catalytically active components such as Rh will aggregate and reduce their active sites when exposed to high temperatures. The higher the concentration of the catalytically active component, the more likely this phenomenon will occur. It should be noted that in this specification, the ease with which the catalytically active components contained in the catalyst layer aggregate when exposed to high temperatures is referred to as the "heat resistance of the catalyst layer." Furthermore, in this specification, "high temperature" refers to temperatures above 850°C, particularly above 900°C.

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

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2022-030627 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] The Rh-containing layer in Patent Document 1 is advantageous in terms of heat resistance because it contains heat-resistant components, but it suffers from problems such as low OSC and insufficient exhaust gas purification performance because it does not contain Ce, which is a component with oxygen storage capacity (OSC).

[0012] When the Rh-containing layer contains Ce in an attempt to solve the aforementioned problems, the following issues arise. If the amount of Ce in the Rh-containing layer is increased, the amount of heat-resistant components in the Rh-containing layer decreases accordingly. This increases the OSC of the Rh-containing layer, but decreases its heat resistance, resulting in reduced exhaust gas purification performance (especially after exposure to high-temperature environments). Conversely, if the amount of heat-resistant components in the Rh-containing layer is increased, and the amount of Ce is decreased accordingly, the heat resistance of the Rh-containing layer increases, but the OSC decreases, resulting in reduced exhaust gas purification performance. On the other hand, increasing both the amount of Ce and the amount of heat-resistant components in the Rh-containing layer can increase both its heat resistance and OSC, but the Rh-containing layer becomes thicker. This deteriorates the contact between the catalytically active components contained in the portion lower than the Rh-containing layer and the exhaust gas, resulting in reduced exhaust gas purification performance in the portion lower than the Rh-containing layer.

[0013] Therefore, in the existing technology, it is difficult to improve the exhaust gas purification performance of the Rh layer and the exhaust gas purification performance of the portion lower than the Rh layer.

[0014] Therefore, the object of the present invention is to provide a catalyst for purifying exhaust gas that can improve the exhaust gas purification performance of the Rh layer and the exhaust gas purification performance of the portion below the Rh layer.

[0015] Solution for solving the problem

[0016] The inventors have discovered that in a catalyst for purifying exhaust gas comprising a substrate, a Pd-containing layer (first catalyst layer) disposed on the substrate, and an Rh-containing layer disposed on the Pd-containing layer, the Rh-containing layer is divided into a lower layer (second catalyst layer) and an upper layer (third catalyst layer), and the Rh concentration in the third catalyst layer is higher than the Rh concentration in the second catalyst layer. The second catalyst layer contains Ce, and the third catalyst layer contains Al and / or Zr. The percentage of Ce in the second catalyst layer (calculated as CeO2) relative to the mass of the second catalyst layer is adjusted to 7% by mass or more, and the percentage of Ce in the third catalyst layer (calculated as CeO2) relative to the mass of the second catalyst layer is adjusted to 7% by mass or more. By adjusting the mass percentage of the third catalyst layer to less than 7% by mass and adjusting the average thickness of the third catalyst layer to less than 10 μm, it is possible to improve the contact between the Rh contained in the third catalyst layer and the exhaust gas, improve the heat resistance of the third catalyst layer, improve the OSC of the second catalyst layer, and improve the contact between the catalytically active components contained in the lower part of the third catalyst layer and the exhaust gas. This results in improved exhaust gas purification performance of the second catalyst layer, improved exhaust gas purification performance of the third catalyst layer, and improved exhaust gas purification performance of the lower part of the third catalyst layer, thus completing the present invention.

[0017] That is, the present invention provides the following catalyst for exhaust gas purification.

[0018] [1] A catalyst for purifying waste gas, comprising: a substrate, a first catalyst layer disposed on the substrate, a second catalyst layer disposed on the first catalyst layer, and a third catalyst layer disposed on the second catalyst layer.

[0019] In the catalyst for waste gas purification,

[0020] The first catalyst layer contains Pd.

[0021] The second catalyst layer contains Rh and Ce.

[0022] The third catalyst layer contains Rh and also contains Al and / or Zr.

[0023] The second catalyst layer and the third catalyst layer satisfy the following formula:

[0024] a>b

[0025] In the formula, a represents the percentage of the metal-converted mass of Rh in the third catalyst layer relative to the mass of the third catalyst layer, and b represents the percentage of the metal-converted mass of Rh in the second catalyst layer relative to the mass of the second catalyst layer.

[0026] The percentage of CeO2 equivalent mass of Ce in the second catalyst layer relative to the mass of the second catalyst layer is 7% by mass or more.

[0027] The percentage of CeO2 equivalent mass of Ce in the third catalyst layer relative to the mass of the third catalyst layer is less than 7% by mass.

[0028] The average thickness of the third catalyst layer is less than 10 μm.

[0029] [2] According to the catalyst for purifying waste gas described in [1], the sum of the mass of Al in the third catalyst layer converted to Al2O3 and the mass of Zr in the third catalyst layer converted to ZrO2 is 80% or more by mass relative to the mass of the third catalyst layer.

[0030] [3] The catalyst for purifying exhaust gas according to [1] or [2], wherein the ratio of a to b, a / b, is 2 or more and 10 or less.

[0031] [4] According to the catalyst for purifying waste gas described in [3], b is 0.01% by mass or more and 5% by mass or less.

[0032] [5] The catalyst for purifying exhaust gas according to any one of [1] to [4], wherein the average thickness of the third catalyst layer is 0.5 μm or more and 5 μm or less.

[0033] The effects of the invention

[0034] According to the present invention, a catalyst for purifying exhaust gas is provided, which can improve the exhaust gas purification performance of the Rh layer and the exhaust gas purification performance of the portion lower than the Rh layer. Attached Figure Description

[0035] Figure 1 This is a partial cross-sectional view showing the state of a catalyst for exhaust gas purification disposed in the exhaust path of an internal combustion engine according to one embodiment of the present invention.

[0036] Figure 2 yes Figure 1 AA-line cross-section view.

[0037] Figure 3 yes Figure 2 An enlarged view of the region indicated by the symbol R in the diagram.

[0038] Figure 4 yes Figure 1 BB line cross-section.

[0039] Figure 5 This is a graph showing the BED observation image of the catalyst used for exhaust gas purification in Comparative Example 2.

[0040] Figure 6 This is a graph showing the elemental mapping of the catalyst used for exhaust gas purification in Comparative Example 2. Detailed Implementation

[0041] Glossary of Terms

[0042] The following describes the terminology used in this specification. Unless otherwise specified, the following description applies to the entire contents of this specification.

[0043] <abbreviation>

[0044] “SEM” stands for Scanning Electron Microscopy, “EDX” stands for Energy Dispersive X-ray Spectroscopy, “SEM-EDX” stands for Scanning Electron Microscopy-Energy Dispersive X-ray Analysis, “EPMA” stands for Electron Probe Microscopy, “XRF” stands for X-ray Fluorescence Analysis, “WDX” stands for Wavelength Dispersive X-ray Analysis, and “ICP-AES” stands for Inductively Coupled Plasma Atomic Emission Spectroscopy.

[0045] <Metallic Elements>

[0046] "Metallic elements" also include half-metallic elements such as Si and B.

[0047] Rare Earth Elements

[0048] Rare earth elements include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0049] <Precious Metal Elements>

[0050] "Noble metal elements" include Pt, Pd, Rh, Ru, Os, Ir, Au, and Ag.

[0051] <Oxides>

[0052] The meaning of "oxide" is as follows: Oxides of rare earth elements other than Ce, Pr, and Tb refer to sesquioxides (M₂O₃, where M represents a rare earth element other than Ce, Pr, and Tb). Ce oxide refers to CeO₂, and Pr oxide refers to Pr₆O₃. 11 The oxides of Tb are Tb4O7, Al is Al2O3, Zr is ZrO2, Si is SiO2, B is B2O3, Cr is Cr2O3, Mg is MgO, Ca is CaO, Sr is SrO, Ba is BaO, Fe is Fe3O4, Mn is Mn3O4, Ni is NiO, Ti is TiO2, Zn is ZnO, and Sn is SnO2.

[0053] <Metal Conversion Mass of Metallic Elements>

[0054] "Metal equivalent mass of a metallic element" refers to the mass of a metal calculated assuming that the metallic element exists as a metal composed of metallic elements.

[0055] <Mass converted from oxides of metallic elements>

[0056] "The mass of the oxide of a metal element" refers to the mass of the oxide of a metal element calculated assuming that the metal element exists as an oxide of the metal element.

[0057] <Material layer quality>

[0058] The "mass of the catalyst layer" refers to the calculated mass obtained by classifying all the metal elements contained in the catalyst layer into precious metal elements and non-precious metal elements, calculating the metal-converted mass for precious metal elements, and calculating the oxide-converted mass for non-precious metal elements, and then summing these values. In other words, the "mass of the catalyst layer" is the calculated mass obtained by adding the metal-converted mass of the precious metal elements in the catalyst layer to the oxide-converted mass of the non-precious metal elements in the catalyst layer.

[0059] When the information (e.g., composition, quantity, etc.) of the raw materials used to manufacture the catalyst layer is known, the quality of the catalyst layer can be determined from the information of the raw materials used in the manufacture of the catalyst layer.

[0060] <Content of metal elements in the catalyst layer, calculated using metal or oxide conversions>

[0061] The "metal content of metal elements in the catalyst layer" is defined by the following formula: Metal content of metal elements in the catalyst layer (mass%) = (mass of metal elements in the catalyst layer) / (mass of the catalyst layer) × 100.

[0062] The "content of metal elements in the catalyst layer as equivalent to oxides" is defined by the following formula: Content of metal elements in the catalyst layer as equivalent to oxides (mass%) = (mass of metal elements in the catalyst layer as equivalent to oxides) / (mass of the catalyst layer) × 100.

[0063] When the information (e.g., composition, quantity, etc.) of the raw materials used to form the catalyst layer is known, the content (mass %) of the metal elements in the catalyst layer, whether converted to metal or oxide, can be determined from the information of the raw materials.

[0064] When the information about the raw materials used to form the catalyst layer is unclear, the content (mass %) of metal elements in the catalyst layer, calculated using metal or oxide conversions, can be determined using conventional methods such as SEM-EDX. Specifically, as described below.

[0065] Elemental analysis of the catalyst layer was performed using conventional methods such as SEM-EDX to determine the constituent elements and calculate the molar percentage of each metal element. For each of the 10 SEM fields, the molar percentage of each metal element was calculated, and the average of the molar percentages of each metal element across the 10 fields was taken as the molar percentage of each metal element in the catalyst layer.

[0066] The V value of each noble metal element in the catalyst layer can be determined using the following formula.

[0067] The V value of each precious metal element = (molar percentage of each precious metal element in the catalyst layer) × (molar mass of each precious metal element)

[0068] The W values ​​of each metal element in the catalyst layer, excluding the noble metal element, are determined using the following formula.

[0069] The W value of each metal element = (molar percentage of each metal element in the catalyst layer) × (molar mass of the oxide of each metal element)

[0070] The metal content (mass %) of each noble metal element in the catalyst layer can be calculated using the following formula.

[0071] The metal content (mass %) of each noble metal element in the catalyst layer is calculated as follows: (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

[0072] The content (mass %) of oxides of each metal element other than noble metal elements in the catalyst layer can be calculated using the following formula.

[0073] The content (mass %) of oxides of all metal elements other than precious metal elements in the catalyst layer is calculated as follows: (W value of all metal elements other than precious metal elements) / {(sum of V values ​​of all precious metal elements) + (sum of W values ​​of all metal elements other than precious metal elements)} × 100

[0074] <Metal Oxides>

[0075] "Metal oxides" refer to oxides containing one or more metallic elements. Examples of metal oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, and Ce-Zr composite oxides.

[0076] <Mass of metal oxides>

[0077] "Mass of metal oxides" refers to the total mass of oxides of metal elements, calculated assuming that each metal element exists as an oxide.

[0078] <Content of metal elements in metal oxides converted from oxides>

[0079] The "content of metal elements in metal oxides as oxides" is defined by the following formula: Content of metal elements in metal oxides as oxides (mass%) = (mass of metal elements in metal oxides as oxides) / (mass of metal oxides) × 100.

[0080] When the composition of the metal oxide is clear, the content rate (mass %) in terms of the oxide of the metal element in the metal oxide can be determined from the composition of the metal oxide.

[0081] When the composition of the metal oxide is not clear, the content rate (mass %) in terms of the oxide of the metal element in the metal oxide can be determined by conventional methods such as SEM-EDX. Specifically, as described below.

[0082] Perform elemental analysis of the metal oxide using conventional methods such as SEM-EDX to determine the types of constituent elements of the metal oxide, and determine the content rate (mass %) in terms of the oxide of each determined metal element.

[0083] <Average particle size of metal oxide>

[0084] The "average particle size of the metal oxide" refers to the average value of the Feret diameters of 100 metal oxides randomly selected from the field of view by observing a sample containing the metal oxide with a scanning electron microscope.

[0085] <Al-based oxide>

[0086] "Al-based oxide" refers to an oxide containing Al, and is an oxide in which the metal element constituting the oxide has the largest content rate on a mass basis as Al. However, substances belonging to the Ce-Zr composite oxide are considered not to belong to the Al-based oxide. The Ce-Zr composite oxide will be described later. The Al-based oxide is different from the alumina used as a binder. In this specification, the alumina used as a binder is sometimes referred to as "alumina binder".

[0087] The Al-based oxide is, for example, granular. The Al-based 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 Al-based oxide is preferably porous.

[0088] The Al-based oxide has high heat resistance. Therefore, by including the Al-based oxide in the catalyst layer, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0089] The Al-based oxide may contain one or more metal elements (hereinafter referred to as "additional element M1") other than Al and O. The additional element M1 can be selected from, for example, rare earth elements (such as Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (such as Mg, Ca, Sr, Ba, etc.), B, Si, Zr, Cr, etc.

[0090] In the Al-based oxide, the additional element M1 can form a solid solution phase (e.g., a solid solution phase of Al2O3 and the oxide of the additional element M1), can form a separate phase as a crystalline phase or an amorphous phase (e.g., the oxide phase of the additional element M1), or can form both a solid solution phase and a separate phase. However, it is preferred that at least a part of the additional element M1 forms a solid solution phase.

[0091] 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 solid-solving the additional element M1 in alumina, etc. Examples of the Al-based oxide containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-ceria, alumina-lanthana, etc.

[0092] From the viewpoint of improving the heat resistance of the Al-based oxide, based on the mass of the Al-based oxide, the content of Al in terms of Al2O3 in the Al-based oxide is preferably 50% by mass or more, more preferably 75% by mass or more, and still more preferably 95% by mass or more. The upper limit is 100% by mass.

[0093] <Ce-based oxide>

[0094] "Ce-based oxide" refers to an oxide containing Ce, and is an oxide in which the element with the largest content rate based on mass among the metal elements constituting the oxide is Ce. However, substances belonging to the Ce-Zr-based composite oxide are considered not to belong to the Ce-based oxide. The Ce-Zr-based composite oxide will be described later. The Ce-based oxide is different from ceria used as a binder. In this specification, ceria used as a binder is sometimes referred to as "ceria binder".

[0095] The Ce-based oxide is, for example, granular. The Ce-based 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-based oxide is preferably porous.

[0096] The Ce-based oxide has OSC (the ability to absorb and store oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low), and moderates the change in the oxygen concentration in the exhaust gas to widen the working window of the catalytic active component. Therefore, by including the Ce-based oxide in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0097] The Ce-based oxide may contain one or more than two metal elements (hereinafter referred to as "additional element M2") other than Ce and O. The additional element M2 may be selected from, for example, rare earth elements other than Ce (such as Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (such as Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0098] 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), may also form a separate phase as a crystalline phase or an amorphous phase (for example, the oxide phase of the additional element M2), and may also form both a solid solution phase and a separate phase, but it is preferred that at least a part of the additional element M2 forms a solid solution phase.

[0099] Examples of the Ce-based oxide include cerium dioxide (CeO2), an oxide obtained by modifying the surface of cerium dioxide with the additional element M2, an oxide obtained by dissolving the additional element M2 in cerium dioxide, and the like.

[0100] From the viewpoint of improving the OSC of the Ce-based oxide, based on the mass of the Ce-based oxide, the CeO2-converted content of Ce in the Ce-based oxide is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more. The upper limit is 100% by mass.

[0101] <Zr-based oxide>

[0102] "Zr-based oxide" refers to an oxide containing Zr, and refers to an oxide in which the element with the largest content rate based on mass among the metal elements constituting the oxide is Zr. However, substances belonging to the Ce-Zr composite oxide are considered not to belong to the Zr-based oxide. The Ce-Zr composite oxide will be described later. The Zr-based oxide is different from zirconia used as a binder. In this specification, zirconia used as a binder is sometimes referred to as "zirconia binder".

[0103] The Zr-based oxide is, for example, granular. The Zr-based 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 Zr-based oxide is preferably porous.

[0104] The Zr-based oxide has high heat resistance. Therefore, by including the Zr-based oxide in the catalyst layer, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0105] The Zr-based oxide may contain one or more than two metal elements (hereinafter referred to as "additional element M3") other than Zr and O. The additional element M3 may be selected from, for example, 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.

[0106] In the Zr-based oxide, the additional element M3 may form a solid solution phase (for example, a solid solution phase of ZrO2 and the oxide of the additional element M3), may form a separate phase as a crystalline phase or an amorphous phase (for example, the oxide phase of the additional element M3), or may form both a solid solution phase and a separate phase. However, it is preferred that at least a part of the additional element M3 forms a solid solution phase.

[0107] Examples of the Zr-based oxide include zirconia (ZrO2), an oxide obtained by modifying the surface of zirconia with the additional element M3, an oxide obtained by dissolving the additional element M3 in zirconia, and the like.

[0108] From the viewpoint of improving the heat resistance of the Zr-based oxide, based on the mass of the Zr-based oxide, the ZrO2-converted content of Zr in the Zr-based oxide is preferably 50% by mass or more, more preferably 65% by mass or more, and still more preferably 80% by mass or more. The upper limit is 100% by mass.

[0109] <Ce-Zr Composite Oxide>

[0110] The Ce-Zr composite oxide refers to a composite oxide containing Ce and Zr. Based on the mass of the composite oxide, the CeO2-converted content of Ce in the composite oxide is 5% by mass or more and 95% by mass or less, and based on the mass of the composite oxide, the ZrO2-converted content of Zr in the composite oxide is 5% by mass or more and 95% by mass or less.

[0111] The Ce-Zr composite oxide is, for example, granular. 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.

[0112] The Ce-Zr composite oxide has OSC, which alleviates the variation in the oxygen concentration in the exhaust gas and expands the working window of the catalytic active component. Therefore, by including the Ce-Zr composite oxide in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0113] Ce-Zr composite oxides may contain one or more metallic elements other than Ce, Zr, and O (hereinafter referred to as "additional element M4"). The additional element M4 may be selected from rare earth elements other than Ce (e.g., Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0114] In Ce-Zr composite oxides, Ce can form a solid solution phase (e.g., solid solutions of CeO2 and ZrO2), or a single phase as a crystalline or amorphous phase (e.g., a single CeO2 phase), or both a solid solution phase and a single phase, but preferably at least a portion of Ce forms a solid solution phase.

[0115] In Ce-Zr composite oxides, Zr can form a solid solution phase (e.g., solid solutions of CeO2 and ZrO2), or a single phase as a crystalline or amorphous phase (e.g., a single ZrO2 phase), or both a solid solution phase and a single phase, but preferably at least a portion of Zr forms a solid solution phase.

[0116] When the Ce-Zr composite oxide contains the additional element M4, the additional element M4 can form a solid solution phase (e.g., a solid solution phase of CeO2 and the oxide of the additional element M4, a solid solution phase of ZrO2 and the oxide of the additional element M4, a solid solution phase of CeO2 and ZrO2 and the oxide of the additional element M4, etc.), or it can form a separate phase as a crystalline phase or an amorphous phase (e.g., a separate phase of the oxide of the additional element M4), or both a solid solution phase and a separate phase, but it is preferred that at least a portion of the additional element M4 forms a solid solution phase.

[0117] From the viewpoint of improving the OSC of Ce-Zr composite oxides, based on the mass of the Ce-Zr composite oxide, the Ce content of Ce in the Ce-Zr composite oxide, calculated as CeO2, is preferably 7% by mass or more, more preferably 10% by mass or more, and even more preferably 13% by mass or more. The upper limit can be appropriately adjusted considering factors such as heat resistance, structural stability, and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0118] From the viewpoint of improving the heat resistance of Ce-Zr composite oxides, based on the mass of the Ce-Zr composite oxide, the Zr content in the Ce-Zr composite oxide, calculated as ZrO2, is preferably 25% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more. The upper limit can be appropriately adjusted considering factors such as oxygen storage capacity, structural stability, and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0119] From the viewpoint of improving the heat resistance and OSC of Ce-Zr composite oxides, based on the mass of the Ce-Zr composite oxide, the sum of the Ce content converted from CeO2 and the Zr content converted from Zr in the Ce-Zr composite oxide is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. The upper limit is 100% by mass.

[0120] From the perspective of improving the heat resistance of Ce-Zr composite oxides, Ce-Zr composite oxides preferably contain one or more rare earth elements. Rare earth elements can be selected from, for example, Y, Pr, La, Nd, Sm, Eu, Gd, etc.

[0121] From the viewpoint of improving the heat resistance of Ce-Zr composite oxides, based on the mass of the Ce-Zr composite oxide, the oxide content of rare earth elements in the Ce-Zr composite oxide is preferably 5% by mass or more and 30% by mass or less, more preferably 7% by mass or more and 25% by mass or less, and even more preferably 9% by mass or more and 20% by mass or less. The lower limit mentioned above can be combined with any of the upper limits mentioned above. Regarding "oxide content of rare earth elements in Ce-Zr composite oxides," when the Ce-Zr composite oxide contains one rare earth element, it refers to the oxide content of that one rare earth element; when the Ce-Zr composite oxide contains two or more rare earth elements, it refers to the total oxide content of those two or more rare earth elements.

[0122] Catalysts for Waste Gas Purification

[0123] The catalyst for purifying waste gas according to the present invention will be described below.

[0124] The following is based on Figures 1-4 A catalyst 1 for purifying exhaust gas (hereinafter referred to as "catalyst 1") according to one embodiment of the present invention will be described.

[0125] like Figure 1As shown, catalyst 1 is disposed in the exhaust passage within the exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas discharged from the internal combustion engine flows from one end of the exhaust pipe P to the other end in the exhaust passage within the exhaust pipe P, and is purified by catalyst 1 disposed within the exhaust pipe P. In the accompanying drawings, the direction of exhaust gas flow is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X is sometimes referred to as the "exhaust gas inflow side" or "upstream side," and the downstream side of the exhaust gas flow direction X is sometimes referred to as the "exhaust gas outflow side" or "downstream side."

[0126] Other exhaust gas purification catalysts can also be configured in the exhaust passage of the exhaust pipe P on the upstream and / or downstream side of the catalyst 1.

[0127] like Figures 2-4 As shown, the catalyst 1 includes a substrate 10, a first catalyst layer 20 disposed on the substrate 10, a second catalyst layer 30 disposed on the first catalyst layer 20, and a third catalyst layer 40 disposed on the second catalyst layer 30.

[0128] Catalyst 1 is characterized in that the first catalyst layer 20 contains Pd, the second catalyst layer 30 contains Rh and Ce, and the third catalyst layer 40 contains Rh and contains Al and / or Zr, wherein the second catalyst layer 30 and the third catalyst layer 40 satisfy the following formula:

[0129] a>b

[0130] [In the formula, a represents the percentage of the metal equivalent mass of Rh in the third catalyst layer 40 relative to the mass of the third catalyst layer 40, and b represents the percentage of the metal equivalent mass of Rh in the second catalyst layer 30 relative to the mass of the second catalyst layer 30.]

[0131] The mass percentage of CeO2 converted from Ce in the second catalyst layer 30 relative to the mass of the second catalyst layer 30 is 7% by mass or more, the mass percentage of CeO2 converted from Ce in the third catalyst layer 40 relative 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 less than 10 μm.

[0132] The effects of catalyst 1 will be explained below.

[0133] The second catalyst layer 30 and the third catalyst layer 40 disposed on the second catalyst layer 30 satisfy the equation: a>b. Therefore, the catalyst 1 can improve the contact between Rh contained in the third catalyst layer 40 and the exhaust gas, thereby improving the exhaust gas purification performance of the third catalyst layer 40.

[0134] From the viewpoint of more effectively improving the contact between Rh contained in the third catalyst layer 40 and the exhaust gas, the ratio of a to b, a / b, is preferably 2 or more and 10 or less, more preferably 3 or more and 6 or less, and even more preferably 4 or more and 6 or less. The lower limit described above can be combined with any of the upper limits described above.

[0135] The third catalyst layer 40 contains Al and / or Zr, and the percentage of CeO2 equivalent mass of Ce in the third catalyst layer 40 relative 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, thereby improving the exhaust gas purification performance of the third catalyst layer 40 (especially the exhaust gas purification performance of the third catalyst layer 40 after exposure to high temperature environment).

[0136] The second catalyst layer 30 contains Ce, and the mass percentage of CeO2 in the second catalyst layer 30 relative 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, thereby improving the exhaust gas purification performance of the second catalyst layer 30.

[0137] The average thickness of the third catalyst layer 40 is less than 10 μm. Therefore, the catalyst 1 can improve the contact between the catalytically active components (e.g., Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) contained in the portion lower than 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.

[0138] As described above, 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 catalytically active components contained in the lower part of the third catalyst layer 40 and the exhaust gas. These effects complement each other and can achieve excellent exhaust gas purification performance.

[0139] <Substrate>

[0140] The substrate 10 will be described below.

[0141] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of materials constituting the substrate 10 include ceramic materials and metallic materials, but ceramic materials are preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as alumina, zirconium oxide, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metallic materials include alloys such as stainless steel.

[0142] like Figures 2-4 As shown, the substrate 10 has a cylindrical portion 11, a partition wall portion 12 disposed within the cylindrical portion 11, and a chamber 13 partitioned by the partition wall portion 12. The substrate 10 is preferably a honeycomb structure.

[0143] like Figure 2 As shown, the cylindrical portion 11 defines the shape of the substrate 10, and the axial direction of the cylindrical portion 11 is aligned with the axial direction of the substrate 10. For example... Figure 2 As shown, the cylindrical part 11 is cylindrical, but it can also be other shapes such as elliptical cylindrical or polygonal cylindrical.

[0144] like Figures 2-4 As shown, a partition wall 12 exists between adjacent chambers 13, separating them. The partition wall 12 may also have a porous structure that allows exhaust gas to pass through. The thickness of the partition wall 12 is, for example, 20 μm or more and 1500 μm or less.

[0145] like Figure 4 As shown, chamber 13 extends along 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.

[0146] like Figure 4 As shown, both the exhaust gas inlet end and the exhaust gas outlet end of chamber 13 are open. Therefore, exhaust gas flowing in from the exhaust gas inlet end (opening) of chamber 13 flows out from the exhaust gas outlet end (opening) of chamber 13. This configuration is called a flow-through type.

[0147] like Figure 2 and Figure 3 As shown, the top view shape of the exhaust gas inflow end (opening) of chamber 13 is quadrilateral, but it can also be hexagonal, octagonal, or other shapes. The top view shape of the exhaust gas outflow end (opening) of chamber 13 is the same.

[0148] The chamber density of substrate 10 is, for example, more than 100 chambers and less than 1200 chambers per square inch. The chamber density of substrate 10 per square inch refers to the total number of chambers 13 per square inch in a cross section obtained by cutting substrate 10 with a plane perpendicular to the exhaust gas flow direction X.

[0149] The volume of substrate 10 is, for example, 0.1L or more and 20L or less. The volume of substrate 10 refers to its apparent volume. For example, if substrate 10 is cylindrical, and its outer diameter is set to 2r, and its length to L10, then the volume of substrate 10 is given by the formula: Volume of substrate 10 = π × r 2 ×L10 indicates the length. In this specification, unless otherwise specified, "length" refers to the axial dimension of the substrate 10.

[0150] <First Catalyst Layer>

[0151] The first catalyst layer 20 will be described below.

[0152] like Figure 3 and Figure 4 As shown, the first catalyst layer 20 is disposed on the substrate 10. Specifically, the first catalyst layer 20 is disposed on the chamber 13 side surface of the partition wall portion 12. "The chamber 13 side surface of the partition wall portion 12" refers to the outer surface of the partition wall portion 12 that extends along the exhaust gas flow direction X and contacts the chamber 13. The first catalyst layer 20 may be disposed directly on the chamber 13 side surface of the partition wall portion 12, or it may be disposed in between other layers, but it is generally disposed directly on the chamber 13 side surface of the partition wall portion 12. "The first catalyst layer 20 disposed on the substrate 10" includes embodiments where the first catalyst layer 20 is disposed directly on the chamber 13 side surface of the partition wall portion 12, and embodiments where the first catalyst layer 20 is disposed in between other layers on the chamber 13 side surface of the partition wall portion 12.

[0153] The first catalyst layer 20 may be composed of a portion protruding from the chamber 13 side surface of the partition wall portion 12 toward the chamber 13 side (hereinafter referred to as "protruding portion"), or it may be composed of a portion existing inside the partition wall portion 12 (hereinafter referred to as "inner portion"), or it may have both a protruding portion and an inner portion. "The first catalyst layer 20 disposed on the substrate 10" includes embodiments in which the first catalyst layer 20 is composed of a protruding portion, embodiments in which the first catalyst layer 20 is composed of an inner portion, and embodiments in which the first catalyst layer 20 has both a protruding portion and an inner portion.

[0154] like Figure 4 As shown, the first catalyst layer 20 extends from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X to the end of the partition wall portion 12 on the exhaust gas outflow side. The first catalyst layer 20 may extend from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas outflow side, or it may extend from the end of the partition wall portion 12 on the exhaust gas outflow side in a direction opposite to the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas inflow side.

[0155] 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. The lower limit described above can be combined with any of the upper limits described above.

[0156] The mass of the first catalyst layer 20 per unit volume of the portion of the substrate 10 in which the first catalyst layer 20 is formed is calculated by the following 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)).

[0157] An example of a method for determining the average length L20 of the first catalyst layer 20 is as follows.

[0158] A sample extending axially along the substrate 10 and having the same length L10 as the substrate 10 is cut from the catalyst 1. The sample is, for example, a cylinder with a diameter of 25.4 mm. 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 of the partition wall portion 12 on the exhaust gas inflow side, the sample is cut at 5 mm intervals using a plane perpendicular to the axial direction of the substrate 10, and the first slice, second slice, ..., nth slice are obtained sequentially from the end side of the sample on the exhaust gas inflow side. When the first catalyst layer 20 extends in the opposite direction to the exhaust gas flow direction X from the end of the partition wall portion 12 on the exhaust gas outflow side, the sample is cut at 5 mm intervals using a plane perpendicular to the axial direction of the substrate 10, and the first slice, second slice, ..., nth slice are obtained sequentially from the end side of the sample on the exhaust gas outflow side. In either case, the length of the slice is 5 mm. The composition of the slices was analyzed using XRF (e.g., EDX, WDX), ICP-AES, SEM-EDX, etc., and based on the composition of the slices, it was confirmed whether the slices contained a portion of the first catalyst layer 20.

[0159] For slices that clearly contain a portion of the first catalyst layer 20, compositional analysis may not be necessary. For example, SEM or EPMA can be used to observe the cut surface to confirm whether the slice contains a portion of the first catalyst layer 20. Elemental mapping of the cut surface can also be performed during the observation of the cut surface.

[0160] After confirming whether the slice contains a portion of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula.

[0161] The length of the first catalyst layer 20 contained in the sample is 5 mm × (the number of slices containing a portion of the first catalyst layer 20).

[0162] For example, if slices 1 to k contain a portion of the first catalyst layer 20, but slices (k+1) to n do not contain a portion of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is (5×k) mm.

[0163] A more detailed method for determining the length of the first catalyst layer 20 contained in the sample is as follows.

[0164] The k-th slice is cut axially along the substrate 10, and a portion of the first catalyst layer 20 present on the cut surface is observed using SEM, EPMA, etc., thereby determining the length of the portion of the first catalyst layer 20 in the k-th slice. Then, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula. It should be noted that 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, the k-th slice is the slice obtained from the exhaust gas outflow side of the sample that contains a portion of the first catalyst layer 20. When the first catalyst layer 20 extends in the opposite direction to the exhaust gas flow direction X from the end of the partition wall portion 12 on the exhaust gas outflow side, the k-th slice is the slice obtained from the exhaust gas inflow side of the sample that contains a portion of the first catalyst layer 20.

[0165] The length of the first catalyst layer 20 contained in the sample = (5mm × (k-1)) + (the length of a portion of the first catalyst layer 20 contained in the k-th slice)

[0166] For 8 to 16 samples arbitrarily cut from catalyst 1, the length of the first catalyst layer 20 contained in each sample was measured, and their average value was taken as the average length L20 of the first catalyst layer 20.

[0167] The first catalyst layer 20 contains Pd as a catalytically active component. Pd is contained in the first catalyst layer 20 in forms capable of functioning as a catalytically active component, such as metallic Pd, Pd-containing alloys, Pd-containing compounds (e.g., Pd oxides), etc. From the viewpoint of improving exhaust gas purification performance, the Pd-containing catalytically active component is preferably in particulate form.

[0168] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the metal-converted mass of Pd in ​​the first catalyst layer 20 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the metal-converted content of Pd in ​​the first catalyst layer 20") 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 lower limit described above can be combined with any of the upper limits described above.

[0169] The first catalyst layer 20 may also contain one or more noble metal elements other than Pd as catalytic active components. The noble metal elements other than Pd can be selected from, for example, Pt, Rh, Ru, Os, Ir, Au, Ag, etc. The noble metal elements other than Pd are contained in the first catalyst layer 20 in forms capable of functioning as catalytic active components, such as metals, alloys containing noble metal elements, compounds containing noble metal elements (e.g., oxides of noble metal elements), etc. From the viewpoint of improving exhaust gas purification performance, the catalytic active components containing noble metal elements other than Pd are preferably in particulate form.

[0170] When the first catalyst layer 20 contains Pd and other precious metal elements, Pd may form alloys with these elements, potentially reducing the number of active sites of Pd involved in exhaust gas purification. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Pd in ​​the first catalyst layer 20 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the percentage of the content of the precious metal elements other than Pd in ​​the first catalyst layer 20") is small. Specifically, the percentage of the content of the precious metal elements other than Pd 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 metal conversion content of precious metal elements other than Pd in ​​the first catalyst layer 20" refers to the metal conversion content of that one precious metal element when the first catalyst layer 20 contains one precious metal element other than Pd, and refers to the total metal conversion content of the two or more precious metal elements when the first catalyst layer 20 contains two or more precious metal elements other than Pd.

[0171] Preferably, the first catalyst layer 20 comprises one or more supports, and at least a portion of the catalytically active component is loaded onto one or more supports.

[0172] "At least a portion of the catalytically active component is supported on the support" means that at least a portion of the catalytically active component is physically or chemically adsorbed or retained on the outer surface and / or the inner surface of the pores of the support. 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).

[0173] In a catalyst layer, at least a portion of the catalytically active component is loaded onto a support, which can be confirmed, for example, using SEM-EDX. Specifically, in the elemental mapping obtained by analyzing the cross-section of the catalyst layer using SEM-EDX, if at least a portion of the catalytically active component and the support exist in the same region, it can be determined that at least a portion of the catalytically active component is loaded onto the support.

[0174] The support can be selected from, for example, metal oxides. The metal oxide, for example, is particulate. From the viewpoint of improving the loading of the catalytically active component, the metal oxide is preferably porous. The metal oxide may or may not have an OSC (osmotically porous concrete). The metal oxide used as a support differs from the metal oxide used as a binder (e.g., alumina binders, zirconium oxide binders, titanium dioxide binders, silica binders, and other metal oxide-based binders).

[0175] Examples of metal oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, Ce-Zr composite oxides, oxides of rare earth elements other than Ce, and oxides based on silicon dioxide (SiO2), titanium dioxide (TiO2), zeolite (aluminosilicate), MgO, ZnO, SnO2, etc.

[0176] From the viewpoint of improving the heat resistance and / or OSC of the first catalyst layer 20, thereby improving the exhaust gas purification performance of the first catalyst layer 20, the support is preferably selected from Al-based oxides, Ce-based oxides, and Ce-Zr composite oxides, more preferably from Al-based oxides and Ce-Zr composite oxides. In one embodiment, the first catalyst layer 20 comprises Al-based oxides and Ce-Zr composite oxides as supports.

[0177] 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.

[0178] 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 CeO2 equivalent mass of Ce in the first catalyst layer 20 relative to the mass of the first catalyst layer 20 (referred to in this specification as "CeO2 equivalent content of Ce in the first catalyst layer 20") 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 considering cost balance, 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. The lower limit described above can be combined with any of the upper limits described above.

[0179] Regarding a catalyst layer, "the CeO2 content of Ce in the catalyst layer" refers to the CeO2 content of Ce originating from that single Ce source when the catalyst layer contains one Ce source, and to the total CeO2 content of Ce originating from two or more Ce sources when the catalyst layer contains two or more 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).

[0180] When the first catalyst layer 20 contains Ce, the first catalyst layer 20 contains one or more Ce sources.

[0181] Examples of Ce sources include oxides containing Ce. Examples of Ce-containing oxides include Al-based oxides, Ce-based oxides, Zr-based oxides containing Ce, Ce-Zr composite oxides, and cerium dioxide binders.

[0182] 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 a Ce source. In addition to the Ce-Zr composite oxide, the first catalyst layer 20 may also contain one or more other Ce sources.

[0183] 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 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the content of Ce-Zr composite oxide in the first catalyst layer 20") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0184] When the information (e.g., composition, amount, etc.) of the raw materials used to form the first catalyst layer 20 is known, the content of Ce-Zr composite oxide in the first catalyst layer 20 can be determined from the information of the raw materials used to form the first catalyst layer 20.

[0185] When the information regarding the raw materials used to form the first catalyst layer 20 is unclear, the content of Ce-Zr composite oxides in the first catalyst layer 20 can be determined using conventional methods such as SEM-EDX. Specifically, as described below.

[0186] (1) For the sample obtained from the first catalyst layer 20, elemental analysis was performed using conventional methods such as SEM-EDX to determine the types of constituent elements of the sample as a whole, and the content (mass%) of the oxides of each metal element was calculated.

[0187] (2) For the sample obtained from the first catalyst layer 20, elemental mapping was performed using conventional methods such as SEM-EDX to determine the types of particles contained in the sample (e.g., Al-based oxides, Ce-based oxides, Ce-Zr composite oxides, etc.).

[0188] (3) For each type of particle, elemental analysis was performed on a randomly selected number (e.g., 50) of particles using SEM-EDX to determine the types of constituent elements of the particles and to calculate the oxide content (mass %) of each metal element. For each type of particle, the average value of the oxide content (mass %) of each metal element was calculated and used as the oxide content (mass %) of each metal element in each type of particle.

[0189] (4) By creating and solving equations that represent the relationship between the content (mass%) of oxides of each metal element in the sample, the content (mass%) of oxides of each metal element in each particle and the content (mass%) of each particle in the sample, the content (mass%) of each particle in the sample is calculated and used as the content (mass%) of each particle in the first catalyst layer 20.

[0190] 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 O2 derived from Ce-Zr composite oxides 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.

[0191] When the first catalyst layer 20 contains Ce-Zr composite oxides, the average particle size of the Ce-Zr composite oxides contained in the first catalyst layer 20 is preferably 2 μm or more and 20 μm or less, more preferably 5 μm or more and 15 μm or less. The lower limit described above can be combined with any of the upper limits described above.

[0192] 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.

[0193] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving its exhaust gas purification performance, the percentage of the sum of the mass of Al converted to Al2O3 and the mass of Zr converted to ZrO2 in the first catalyst layer 20 relative 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 considering cost balance, 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. The lower limit described above can be combined with any of the upper limits described above.

[0194] 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 the first catalyst layer 20 converted to Al2O3 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the content of Al in the first catalyst layer 20 converted to Al2O3") 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 considering 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. The lower limit described above can be combined with any of the upper limits described above.

[0195] Regarding a catalyst layer, "the Al2O3 content of Al in the catalyst layer" refers to the Al2O3 content of Al originating from that single Al source when the catalyst layer contains one Al source, and to the total Al2O3 content of Al originating from two or more Al sources when the catalyst layer contains two or more 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).

[0196] When the first catalyst layer 20 contains Al, the first catalyst layer 20 contains one or more Al sources.

[0197] Examples of Al sources include Al-containing oxides. Examples of Al-containing oxides include Al-based oxides, Al-containing Ce-based oxides, Al-containing Zr-based oxides, Al-containing Ce-Zr composite oxides, and alumina binders.

[0198] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving its exhaust gas purification performance, the first catalyst layer 20 preferably contains Al-based oxides as an Al source. In addition to Al-based oxides, the first catalyst layer 20 may also contain one or more other Al sources as Al sources.

[0199] 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-based oxides in the first catalyst layer 20 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the content of Al-based oxides in the first catalyst layer 20") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0200] The content of Al-based oxides in the first catalyst layer 20 can be calculated in the same way as the content of Ce-Zr composite oxides in the first catalyst layer 20.

[0201] 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 proportion of the mass of Al2O3 derived from Al oxides 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.

[0202] When the first catalyst layer 20 contains Al-based oxides, the average particle size of the Al-based oxides 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. The lower limit described above can be combined with any of the upper limits described above.

[0203] 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 Zr in the first catalyst layer 20 by mass equivalent to ZrO2 relative to the mass of the first catalyst layer 20 (referred to in this specification as "the content of Zr in the first catalyst layer 20 by ZrO2 equivalent") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0204] Regarding a catalyst layer, "the ZrO2 content of Zr in the catalyst layer" refers to the ZrO2 content of Zr derived from that single Zr source when the catalyst layer contains one Zr source, and to the total ZrO2 content of Zr derived from two or more Zr sources when the catalyst layer contains two or more Zr 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).

[0205] When the first catalyst layer 20 contains Zr, the first catalyst layer 20 contains one or more Zr sources.

[0206] Examples of Zr-containing oxides include Zr-based oxides, Zr-based oxides, Zr-based oxides, Ce-Zr composite oxides, and zirconium oxide binders.

[0207] 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 a Zr source. In addition to the Ce-Zr composite oxide, the first catalyst layer 20 may also contain one or more other Zr sources as Zr sources.

[0208] The description of the content of Ce-Zr composite oxide in the first catalyst layer 20 is the same as above.

[0209] 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 ZrO2 equivalent mass of Zr in the first catalyst layer 20, which is derived from Ce-Zr composite oxides, 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.

[0210] The first catalyst layer 20 may also contain other components such as binders and stabilizers. Examples of binders include metal oxide-based binders such as alumina sol, cerium dioxide sol, zirconium oxide sol, titanium dioxide sol, and silica sol. Examples of stabilizers include nitrates, carbonates, oxides, and sulfates of alkaline earth metal elements (such as Sr and Ba).

[0211] <Second Catalyst Layer>

[0212] The second catalyst layer 30 will be described below.

[0213] like Figure 3 and Figure 4 As shown, the second catalyst layer 30 is disposed on the first catalyst layer 20.

[0214] "The second catalyst layer 30 is disposed on the first catalyst layer 20" means that a portion or all of the second catalyst layer 30 exists on the main surface of the first catalyst layer 20 opposite to the main surface of the partition wall portion 12. "The main surface of the first catalyst layer 20" refers to the outer surface of the first catalyst layer 20 extending along the exhaust gas flow direction X. The second catalyst layer 30 can be disposed directly on the main surface of the first catalyst layer 20, or it can be disposed in between other layers, but it is usually disposed directly on the main surface of the first catalyst layer 20. The second catalyst layer 30 can be configured to cover a portion of the main surface of the first catalyst layer 20, or it can be configured to cover the entire main surface of the first catalyst layer 20. "The second catalyst layer 30 disposed on the first catalyst layer 20" includes embodiments where the second catalyst layer 30 is disposed directly on the main surface of the first catalyst layer 20, and embodiments where the second catalyst layer 30 is disposed in between other layers.

[0215] like Figure 4 As shown, the second catalyst layer 30 extends from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X to the end of the partition wall portion 12 on the exhaust gas outflow side. The second catalyst layer 30 may extend from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas outflow side, or it may extend from the end of the partition wall portion 12 on the exhaust gas outflow side in a direction opposite to the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas inflow side.

[0216] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, and improving the contact between the catalytically active components (e.g., Pd contained in the first catalyst layer 20) contained in the portion lower than 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 in which 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. The lower limit described above can be combined with any of the upper limits described above.

[0217] The mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 in which the second catalyst layer 30 is formed is calculated by the following 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)).

[0218] The above description of the method for determining the average length L20 of the first catalyst layer 20 also applies to the second catalyst layer 30. In application, "first catalyst layer 20" is replaced with "second catalyst layer 30", and "average length L20" is replaced with "average length L30".

[0219] The second catalyst layer 30 contains Rh as a catalytically active component. Rh is contained in the second catalyst layer 30 in a form capable of functioning as a catalytically active component, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of improving exhaust gas purification performance, the Rh-containing catalytically active component is preferably in particulate form.

[0220] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the metal equivalent mass of Rh in the second catalyst layer 30 relative to the mass of the second catalyst layer 30 (i.e., b above) 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. The lower limit described above can be combined with any of the upper limits described above.

[0221] The second catalyst layer 30 may also contain one or more noble metal elements other than Rh as catalytic active components. The noble metal elements other than Rh may be selected from, for example, Pt, Pd, Ru, Os, Ir, Au, Ag, etc. The noble metal elements other than Rh may be contained in the second catalyst layer 30 in a form capable of functioning as catalytic active components, such as metals, alloys containing noble metal elements, compounds containing noble metal elements (e.g., oxides of noble metal elements), etc. From the viewpoint of improving exhaust gas purification performance, the catalytic active components containing noble metal elements other than Rh are preferably in particulate form.

[0222] When the second catalyst layer 30 contains Rh and other precious metal elements, Rh may alloy with these elements, potentially reducing the number of active sites of Rh involved in waste gas purification. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Rh in the second catalyst layer 30 relative to the mass of the second catalyst layer 30 (referred to in this specification as "the percentage of the content of the precious metal elements other than Rh in the second catalyst layer 30") is small. Specifically, the percentage of the content of the precious metal elements 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 metal conversion content of precious metal elements other than Rh in the second catalyst layer 30" refers to the metal conversion content of that one precious metal element when the second catalyst layer 30 contains one precious metal element other than Rh, and to the total metal conversion content of the two or more precious metal elements when the second catalyst layer 30 contains two or more precious metal elements other than Rh.

[0223] The metal conversion content of the specified precious metal element in the second catalyst layer 30 can be calculated in the same way as the metal conversion content of the specified precious metal element in the first catalyst layer 20.

[0224] Preferably, the second catalyst layer 30 comprises one or more supports, and at least a portion of the catalytically active component is supported on one or more supports. The supports may be selected, for example, metal oxides. The description of metal oxides is the same as above.

[0225] From the viewpoint 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 support is preferably selected from Al-based oxides, Ce-based oxides, and Ce-Zr composite oxides, more preferably from Al-based oxides and Ce-Zr composite oxides. In one embodiment, the second catalyst layer 30 comprises Al-based oxides and Ce-Zr composite oxides as supports.

[0226] The second catalyst layer 30 contains Ce. The percentage of Ce in the second catalyst layer 30 by mass equivalent to CeO2 relative to the mass of the second catalyst layer 30 (referred to in this specification as "CeO2 content of Ce in the second catalyst layer 30") is 7% by mass or more. This improves the OSC of the second catalyst layer 30 and enhances its exhaust gas purification performance.

[0227] From the viewpoint of more effectively improving the OSC of the second catalyst layer 30, and thus more effectively improving the exhaust gas purification performance of the second catalyst layer 30, the CeO2 content of Ce 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 considering cost balance, the content of other components, etc. The upper limit is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less. The lower limit described above can be combined with any of the upper limits described above.

[0228] The second catalyst layer 30 contains one or more Ce sources. The description of the Ce sources is the same as above.

[0229] 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 a Ce source. In addition to the Ce-Zr composite oxide, the second catalyst layer 30 may also contain one or more other Ce sources.

[0230] 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 relative to the mass of the second catalyst layer 30 (referred to in this specification as "the content of Ce-Zr composite oxide in the second catalyst layer 30") is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more. The upper limit can be appropriately adjusted considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0231] The content of Ce-Zr composite oxides in the second catalyst layer 30 can be calculated in the same way as the content of Ce-Zr composite oxides in the first catalyst layer 20.

[0232] 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 proportion of the mass of Ce O2 derived from Ce-Zr composite oxides 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.

[0233] When the second catalyst layer 30 contains Ce-Zr composite oxides, the average particle size of the Ce-Zr composite oxides 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. The lower limit described above can be combined with any of the upper limits described above.

[0234] 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.

[0235] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving its exhaust gas purification performance, the percentage of the sum of the mass of Al converted to Al2O3 and the mass of Zr converted to ZrO2 in the second catalyst layer 30 relative 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 considering cost balance, 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. The lower limit described above can be combined with any of the upper limits described above.

[0236] 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 the second catalyst layer 30 converted to Al2O3 relative to the mass of the second catalyst layer 30 (referred to in this specification as "the content of Al in the second catalyst layer 30 converted to Al2O3") 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 considering 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. The lower limit described above can be combined with any of the upper limits described above.

[0237] When the second catalyst layer 30 contains Al, the second catalyst layer 30 contains one or more Al sources. The description of the Al sources is the same as above.

[0238] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving its exhaust gas purification performance, the second catalyst layer 30 preferably contains Al-based oxides as an Al source. In addition to Al-based oxides, the second catalyst layer 30 may also contain one or more other Al sources as Al sources.

[0239] 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-based oxides in the second catalyst layer 30 relative to the mass of the second catalyst layer 30 (referred to in this specification as "the content of Al-based oxides in the second catalyst layer 30") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0240] The content of Al-based oxides in the second catalyst layer 30 can be calculated in the same way as the content of Ce-Zr composite oxides in the first catalyst layer 20.

[0241] 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 proportion of the mass of Al2O3 derived from Al oxides 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.

[0242] When the second catalyst layer 30 contains Al-based oxides, the average particle size of the Al-based oxides contained in the second catalyst layer 30 is preferably 3 μm or more and 20 μm or less, more preferably 5 μm or more and 15 μm or less. The lower limit described above can be combined with any of the upper limits described above.

[0243] 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 Zr in the second catalyst layer 30 by mass equivalent to ZrO2 relative to the mass of the second catalyst layer 30 (referred to in this specification as "the content of Zr in the second catalyst layer 30 by ZrO2 equivalent") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0244] When the second catalyst layer 30 contains Zr, the second catalyst layer 30 contains one or more Zr sources. The description of the Zr source is the same as above.

[0245] 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 second catalyst layer 30 preferably contains a Ce-Zr composite oxide as a Zr source. In addition to the Ce-Zr composite oxide, the second catalyst layer 30 may also contain one or more other Zr sources as Zr sources.

[0246] The description of the content of Ce-Zr composite oxide in the second catalyst layer 30 is the same as above.

[0247] 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 proportion of ZrO2 equivalent mass of Zr in the second catalyst layer 30, which is derived from Ce-Zr composite oxides, 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.

[0248] The second catalyst layer 30 may also contain other components such as binders and stabilizers. The descriptions of binders and stabilizers are the same as above.

[0249] From the viewpoint of improving the contact between the catalytically active components (e.g., Pd contained in the first catalyst layer 20) in the portion lower than the second catalyst layer 30 and the exhaust gas, 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 considering the OSC or similar requirements in 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. The aforementioned lower limits can be combined with any of the aforementioned upper limits.

[0250] The method for calculating the average thickness of the second catalyst layer 30 will be described later.

[0251] <Third Catalyst Layer>

[0252] The third catalyst layer 40 will be described below.

[0253] like Figure 3 and Figure 4 As shown, the third catalyst layer 40 is disposed on the second catalyst layer 30.

[0254] "The third catalyst layer 40 is disposed on the second catalyst layer 30" means that a portion or all of the third catalyst layer 40 exists on the main surface of the second catalyst layer 30 opposite to the main surface of the first catalyst layer 20. "The main surface of the second catalyst layer 30" refers to the outer surface of the second catalyst layer 30 extending along the exhaust gas flow direction X. The third catalyst layer 40 can be directly disposed on the main surface of the second catalyst layer 30, or it can be disposed in between other layers, but it is usually directly disposed on the main surface of the second catalyst layer 30. The third catalyst layer 40 can be configured to cover a portion of the main surface of the second catalyst layer 30, or it can be configured to cover the entire main surface of the second catalyst layer 30. "The third catalyst layer 40 disposed on the second catalyst layer 30" includes embodiments where the third catalyst layer 40 is directly disposed on the main surface of the second catalyst layer 30, and embodiments where the third catalyst layer 40 is disposed in between other layers.

[0255] Other catalyst layers may be provided on the third catalyst layer 40, but from the viewpoint of more effectively improving the contact between the Rh contained in the third catalyst layer 40 and the exhaust gas, it is preferable not to provide other catalyst layers on the third catalyst layer 40.

[0256] like Figure 4 As shown, the third catalyst layer 40 extends from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X to the end of the partition wall portion 12 on the exhaust gas outflow side. The third catalyst layer 40 may extend from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas outflow side, or it may extend from the end of the partition wall portion 12 on the exhaust gas outflow side in a direction opposite to the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas inflow side.

[0257] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, and improving the contact between the catalytically active components contained in the portion lower than the third catalyst layer 40 (e.g., Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) and the exhaust gas, the mass of the third catalyst layer 40 per unit volume of the portion of the substrate 10 in which 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. The lower limit described above can be combined with any of the upper limits described above.

[0258] The mass of the third catalyst layer 40 per unit volume of the portion of the substrate 10 in which the third catalyst layer 40 is formed is calculated by the following 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)).

[0259] The above description of the method for determining the average length L20 of the first catalyst layer 20 also applies to the third catalyst layer 40. In application, "first catalyst layer 20" is replaced with "third catalyst layer 40", and "average length L20" is replaced with "average length L40".

[0260] The third catalyst layer 40 contains Rh as a catalytically active component. Rh is contained in the third catalyst layer 40 in forms capable of functioning as a catalytically active component, such as metallic Rh, alloys containing Rh, compounds containing Rh (e.g., oxides of Rh), etc. From the viewpoint of improving exhaust gas purification performance, the Rh-containing catalytically active component is preferably in particulate form.

[0261] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the percentage of the metal equivalent mass of Rh in the third catalyst layer 40 relative to the mass of the third catalyst layer 40 (i.e., a above) 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. The lower limit described above can be combined with any of the upper limits described above.

[0262] The third catalyst layer 40 may also contain one or more noble metal elements other than Rh as catalytic active components. The noble metal elements other than Rh may be selected from, for example, Pt, Pd, Ru, Os, Ir, Au, Ag, etc. The noble metal elements other than Rh may be contained in the third catalyst layer 40 in a form capable of functioning as catalytic active components, such as metals, alloys containing noble metal elements, compounds containing noble metal elements (e.g., oxides of noble metal elements), etc. From the viewpoint of improving exhaust gas purification performance, the catalytic active components containing noble metal elements other than Rh are preferably in particulate form.

[0263] When the third catalyst layer 40 contains Rh and other precious metal elements, Rh may form alloys with these elements, potentially reducing the number of active sites of Rh involved in the exhaust gas purification process. Therefore, it is preferable that the percentage of the mass of the precious metal elements other than Rh in the third catalyst layer 40 relative to the mass of the third catalyst layer 40 (referred to in this specification as "the percentage of the content of the precious metal elements other than Rh in the third catalyst layer 40") is small. Specifically, the percentage of the content of the precious metal elements 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 metal conversion content of precious metal elements other than Rh in the third catalyst layer 40" refers to the metal conversion content of that one precious metal element when the third catalyst layer 40 contains one precious metal element other than Rh, and to the total metal conversion content of the two or more precious metal elements when the third catalyst layer 40 contains two or more precious metal elements other than Rh.

[0264] Preferably, the third catalyst layer 40 comprises one or more supports, and at least a portion of the catalytically active component is loaded onto one or more supports. The supports may be selected, for example, metal oxides. The description of metal oxides is the same as above.

[0265] From the viewpoint of improving the heat resistance and / or OSC of the third catalyst layer 40, thereby improving the exhaust gas purification performance of the third catalyst layer 40, the support is preferably selected from Al-based oxides, Ce-based oxides, and Ce-Zr composite oxides, more preferably from Al-based oxides and Ce-Zr composite oxides. In one embodiment, the third catalyst layer 40 comprises Al-based oxides and Ce-Zr composite oxides as supports.

[0266] The third catalyst layer 40 contains Al and / or Zr. This improves the heat resistance of the third catalyst layer 40 and enhances its exhaust gas purification performance.

[0267] From the viewpoint of improving the heat resistance of the third catalyst layer 40 and thereby improving its exhaust gas purification performance, the percentage of the sum of the mass of Al converted to Al2O3 and the mass of Zr converted to ZrO2 in the third catalyst layer 40 relative 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 considering cost balance, 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. The lower limit described above can be combined with any of the upper limits described above.

[0268] 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 the third catalyst layer 40 converted to Al2O3 relative to the mass of the third catalyst layer 40 (referred to in this specification as "the content of Al in the third catalyst layer 40 converted to Al2O3") 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 considering 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. The lower limit described above can be combined with any of the upper limits described above.

[0269] When the third catalyst layer 40 contains Al, the third catalyst layer 40 contains one or more Al sources. The description of the Al source is the same as above.

[0270] From the viewpoint of improving the heat resistance of the third catalyst layer 40 and thereby improving its exhaust gas purification performance, the third catalyst layer 40 preferably contains Al-based oxides as an Al source. In addition to Al-based oxides, the third catalyst layer 40 may also contain one or more other Al sources as Al sources.

[0271] 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-based oxides in the third catalyst layer 40 relative to the mass of the third catalyst layer 40 (referred to in this specification as "the content of Al-based oxides in the third catalyst layer 40") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0272] The content of Al-based oxides in the third catalyst layer 40 can be calculated in the same way as the content of Ce-Zr composite oxides in the first catalyst layer 20.

[0273] 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 proportion of the mass of Al2O3 derived from Al oxides 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.

[0274] When the third catalyst layer 40 contains Al-based oxides, the average particle size of the Al-based oxides 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. The lower limit described above can be combined with any of the upper limits described above.

[0275] 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 Zr in the third catalyst layer 40 by mass equivalent to ZrO2 relative to the mass of the third catalyst layer 40 (referred to in this specification as "the content of Zr in the third catalyst layer 40 by ZrO2 equivalent") is preferably 10% by mass or more, more preferably 11% by mass or more, and even more preferably 15% by mass or more. The upper limit can be appropriately adjusted considering cost balance, 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. The lower limit described above can be combined with any of the upper limits described above.

[0276] When the third catalyst layer 40 contains Zr, the third catalyst layer 40 contains one or more Zr sources. The description of 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.

[0277] The percentage of Ce in the third catalyst layer 40, calculated as CeO2, relative to the mass of the third catalyst layer 40 (referred to in this specification as "CeO2 content in the third catalyst layer 40") is less than 7% by mass. This improves the heat resistance of the third catalyst layer 40 and enhances its exhaust gas purification performance.

[0278] From the viewpoint 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 Ce content of Ce in the third catalyst layer 40, calculated as 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.

[0279] The third catalyst layer 40 may also contain Ce. When the third catalyst layer 40 contains Ce, the CeO2 content of Ce in the third catalyst layer 40 may be, for example, 0.5% by mass or more, 1% by mass or more, or 1.5% by mass or more. The lower limit described above may be combined with any of the upper limits described above.

[0280] 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.

[0281] When the third catalyst layer 40 contains Ce, it preferably contains a Ce-Zr composite oxide as the Ce source. In addition to the Ce-Zr composite oxide, the third catalyst layer 40 may also contain one or more other Ce sources.

[0282] 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 relative to the mass of the third catalyst layer 40 (referred to in this specification as "the content of the Ce-Zr composite oxide in the third catalyst layer 40") 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 considering factors such as cost balance and the content of other components. 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. The lower limit described above can be combined with any of the upper limits described above.

[0283] The content of Ce-Zr composite oxides in the third catalyst layer 40 can be calculated in the same way as the content of Ce-Zr composite oxides in the first catalyst layer 20.

[0284] When the third catalyst layer 40 contains Ce, the proportion of the mass of Ce derived from Ce-Zr composite oxides in the CeO2 equivalent of Ce 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.

[0285] When the third catalyst layer 40 contains Ce-Zr composite oxides, the average particle size of the Ce-Zr composite oxides 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. The lower limit described above can be combined with any of the upper limits described above.

[0286] The third catalyst layer 40 may also contain other components such as binders and stabilizers. The descriptions of binders and stabilizers are the same as above.

[0287] The average thickness of the third catalyst layer 40 is less than 10 μm. As a result, the contact between the catalytically active components contained in the portion lower than the third catalyst layer 40 (e.g., Pd contained in the first catalyst layer 20 and Rh contained in the second catalyst layer 30) and the exhaust gas can be improved, thereby improving the exhaust gas purification performance of the portion lower than the third catalyst layer 40 (e.g., the first catalyst layer 20 and the second catalyst layer 30).

[0288] From the viewpoint of more effectively improving the contact between the catalytically active components contained in the portion lower than 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 considering factors such as the required heat resistance 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. The above-mentioned lower limits can be combined with any of the above-mentioned upper limits.

[0289] An example of the calculation method for the average thickness of the second catalyst layer 30 and the average thickness of the third catalyst layer 40 is as follows.

[0290] When the first catalyst layer 20 extends 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 along the exhaust gas flow direction X, for example, a portion 30 mm away from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X. When the first catalyst layer 20 extends from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X without reaching the end of the partition wall portion 12 on the exhaust gas outflow side, for example, a portion 10 mm away from the end of the partition wall portion 12 on the exhaust gas inflow side along the exhaust gas flow direction X. When the first catalyst layer 20 extends from the partition wall portion 12 in a direction opposite to the exhaust gas flow direction X... When the partition wall 12 extends from the end of the exhaust gas outlet side without reaching the end of the exhaust gas inflow side, for example, at a distance of 10 mm from the end of the partition wall 12 on the exhaust gas outlet side in the direction opposite to the exhaust gas flow direction X, it is cut with a plane perpendicular to the axis of the substrate 10. Using a backscattered electron detector (BED) in SEM or EPMA, the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 present in one randomly selected chamber 13 from the cut surface are observed to determine the area where the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 are present. In the observation of the cut surface based on SEM or BED, the field of view magnification is, for example, 500x, and the field of view width (length) is, for example, 100~200μm. The area observed based on SEM or BED is set to exclude the corners of the chamber 13. This is because the corners of the chamber 13 are easily affected by the shape of the chamber. The regions containing the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 can be determined based on the differences in morphology, composition, etc., among the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40. Elemental mapping of the cross-section can also be performed. Elemental mapping can be performed, for example, by observing and analyzing the composition of the cross-section using SEM or BED. Elemental mapping can be performed using, for example, SEM-EDX, EPMA, etc. By performing elemental mapping of the cross-section, the regions containing the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40 can be determined based on the differences in morphology and composition among the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40.

[0291] In SEM or BED images, starting from the left or right end, first to Nth grid lines parallel to the thickness direction of the partition wall portion 12 of the substrate 10 are drawn at 15 μm intervals. The intersections of the outline of the region where the first catalyst layer 20 exists with each grid line are connected by straight lines to determine the position of the surface of the first catalyst layer 20. N is, for example, an integer from 5 to 10. Similarly, the intersections of the outline of the region where the second catalyst layer 30 exists with each grid line are connected by straight lines to determine the position of the surface of the second catalyst layer 30. Similarly, the intersections of the outline of the region where the third catalyst layer 40 exists with each grid line are connected by straight lines to determine the position of the surface of the third catalyst layer 40. If the change in thickness direction from a certain intersection point P1 to the intersection point P2 adjacent to that intersection point P1 exceeds the grid line interval (15 μm), it is preferable not to use intersection point P2 for determining the position of the surface (i.e., exclude intersection point P2 from the intersection points connected by straight lines). The change in thickness from a certain intersection point P1 to the adjacent intersection point P2 refers to the distance between a straight line passing through intersection point P1 and perpendicular to the thickness direction of the partition wall portion 12 of the substrate 10, and a straight line passing through intersection point P2 and perpendicular to the thickness direction of the partition wall portion 12 of the substrate 10. If the change in thickness from intersection point P1 to the adjacent intersection point P2 exceeds the grid line interval (15 μm), and the change in thickness from intersection point P1 to the adjacent intersection point P3 also exceeds the grid line interval (15 μm), it is preferable not to use intersection point P3, except for intersection point P2, for determining the surface position (i.e., excluding intersection points P2 and P3 from the intersection points connected by straight lines). If five intersection points are consecutively excluded from the intersection points connected by straight lines, it is preferable not to measure the thickness of the SEM or BED image.

[0292] After determining the positions of the surfaces of the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40, image analysis software is used to calculate the area of ​​the first region enclosed 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. Similarly, the area of ​​the second region enclosed 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 calculated. Image analysis software such as AreaQ (manufactured by Estec Corporation), ImageJ (free and open-source), and Photoshop (Adobe Systems Inc.) can be used. It should be noted that the edges of the image tend to become blurry, making it difficult to determine the positions of the surfaces of the first catalyst layer 20, the second catalyst layer 30, and the third catalyst layer 40; therefore, the first and Nth grid lines are not used.

[0293] After determining the areas of region 1 and region 2, the thickness of each region is calculated based on the following formula.

[0294] Thickness of each region = Area of ​​each region / (Grid line spacing × Number of grid line spacings)

[0295] It should be noted that the grid line spacing is 15μm, and the number of grid line spacings is (N-3).

[0296] For 20 chambers 13 randomly selected from the cut surface, the thickness of the first region is calculated, and their average value is taken as the average thickness of the second catalyst layer 30. For 20 chambers 13 randomly selected from the cut surface, the thickness of the second region is calculated, and their average value is taken as the average thickness of the third catalyst layer 40.

[0297] <Catalyst Manufacturing>

[0298] Catalyst 1 can be manufactured by forming a first catalyst layer 20 on a substrate 10, then forming a second catalyst layer 30 on the first catalyst layer 20, and then forming a third catalyst layer 40 on the second catalyst layer 30.

[0299] The first catalyst layer 20 can be formed by mixing a Pd supply source (e.g., Pd salt) and other components (e.g., metal oxides, binders, stabilizers, solvents, etc.) as appropriate, applying the first slurry onto the substrate 10, and then drying and firing it.

[0300] The second catalyst layer 30 can be formed by mixing a Rh supply source (e.g., Rh salt), a Ce supply source (e.g., Ce-Zr composite oxide) and other components as appropriate (e.g., metal oxides other than Ce supply source, binders, stabilizers, solvents, etc.), coating the second slurry onto the first catalyst layer 20, and then drying and firing it.

[0301] The third catalyst layer 40 can be formed by mixing a Rh supply source (e.g., Rh salt), an Al supply source (e.g., Al-based oxides), and / or a Zr supply source (e.g., Ce-Zr composite oxides) and other components as appropriate (e.g., metal oxides other than Al and Zr supply sources, binders, stabilizers, solvents, etc.), coating the third slurry onto the second catalyst layer 30, and then drying and firing it.

[0302] Examples of Pd and Rh salts include nitrates, ammonium complex salts, acetates, and chlorides. Examples of binders include alumina sol, zirconium oxide sol, titanium dioxide sol, silica sol, and cerium dioxide sol. Examples of solvents include water and organic solvents.

[0303] The drying temperature is, for example, 60°C or higher and 150°C or lower, and the drying time is, for example, 0.1 hours or higher and 1 hour or lower. The firing temperature is, for example, 300°C or higher and 700°C or lower, and the firing time is, for example, 1 hour or higher and 10 hours or lower. Firing can be carried out, for example, in an atmospheric atmosphere.

[0304] Example

[0305] [Example 1]

[0306] (1) Preparation of slurry for lower layer formation

[0307] Palladium nitrate aqueous solution, Ce-Zr composite oxide (Ce content of CeO2 equivalent: ≥13% by mass and ≤50% by mass, Zr content of ZrO2 equivalent: ≥35% by mass and ≤70% by mass, and oxide equivalent of one or more rare earth elements other than Ce content: ≥9% by mass and ≤20% by mass), Al oxide (Al content of Al2O3 equivalent: ≥95% by mass), alumina binder, zirconium oxide binder, and water are added to a mixing container, and the mixture is mixed and stirred to prepare a slurry for lower layer formation. The amounts of each component in the slurry used to form the lower layer are adjusted so that, based on the mass of the lower layer after firing (100% by mass), Pd is 2.0% by mass (converted to metal), Ce is 18.0% by mass (converted to CeO2), Zr is 18.0% by mass (converted to ZrO2), Al is 57.5% by mass (converted to Al2O3), and rare earth elements other than Ce are 4.5% by mass (converted to oxides).

[0308] (2) Formation of the lower layer

[0309] As a flow-through substrate, it is prepared to be applied at 600 cells / inch on a surface orthogonal to the axial direction. 2 The density is a flow-through substrate having axially extending chambers divided by partition walls with a thickness of 50~70μm and a volume of 1.0L.

[0310] A flowable substrate is impregnated in a slurry for forming a lower layer. The flowable substrate coated with the slurry is dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form a lower layer on the flowable substrate. The mass of the lower layer per unit volume of the portion of the flowable substrate in which the lower layer is formed is 100 g / L.

[0311] (3) Preparation of slurry for intermediate layer formation

[0312] In a mixing container, add rhodium nitrate aqueous solution, Ce-Zr composite oxide (Ce content converted to CeO2: ≥13% by mass and ≤50% by mass, Zr content converted to ZrO2: ≥35% by mass and ≤70% by mass, and oxide content converted to one or more rare earth elements other than Ce: ≥9% by mass and ≤20% by mass), Al oxide (Al content converted to Al2O3: ≥95% by mass), alumina binder, zirconium oxide binder, and water, mix and stir to prepare a slurry for intermediate layer formation. The amounts of each component in the slurry for forming the intermediate layer were adjusted so that, based on the mass of the fired intermediate layer (100% by mass), Rh was 0.1% by mass (converted to metal), Ce was 12.0% by mass (converted to CeO2), Zr was 42.0% by mass (converted to ZrO2), Al was 39.6% by mass (converted to Al2O3), and rare earth elements other than Ce were 6.3% by mass (converted to oxides).

[0313] (4) Formation of the middle layer

[0314] A flowable substrate with a lower layer is impregnated in a slurry for forming an intermediate layer. The flowable substrate coated with the slurry is dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form an intermediate layer on top of the lower layer. The mass of the intermediate layer per unit volume in the portion of the flowable substrate where the intermediate layer is formed is 100 g / L.

[0315] (5) Preparation of slurry for upper layer formation

[0316] In a mixing container, add rhodium nitrate aqueous solution, Ce-Zr composite oxide (Ce content of CeO2 equivalent: ≥13% by mass and ≤50% by mass, Zr content of ZrO2 equivalent: ≥35% by mass and ≤70% by mass, and oxides of one or more rare earth elements other than rare earth element Ce equivalent: ≥9% by mass and ≤20% by mass), Al oxide (Al content of Al2O3 equivalent: ≥95% by mass), alumina binder, and water, mix and stir to prepare a slurry for upper layer formation. The amounts of each component in the slurry used to form the upper layer are adjusted so that, based on the mass of the fired upper layer (100% by mass), Rh is 0.7% by mass (converted to metal), Ce is 6.7% by mass (converted to CeO2), Zr is 23.3% by mass (converted to ZrO2), Al is 66.0% by mass (converted to Al2O3), and rare earth elements other than Ce are 3.3% by mass (converted to oxides).

[0317] (6) Formation of the upper layer

[0318] A flowable substrate with a middle layer is impregnated in a slurry for forming the upper layer. The flowable substrate coated with the slurry for forming the upper layer is dried at 150°C for 0.5 hours and then 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 flowable substrate in which the upper layer is formed is 15 g / L.

[0319] As described above, the catalyst for exhaust gas purification of Example 1 was manufactured, comprising a lower layer formed on a flowable substrate, a middle layer formed on the lower layer, and an upper layer formed on the middle layer. The lower layer, middle layer, and upper layer of the catalyst for exhaust gas purification of Example 1 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0320] 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 using the above method. Specifically, as described below.

[0321] The catalyst for exhaust gas purification in Example 1 (located 30 mm axially from the end of the substrate on the exhaust gas inflow side) was cut with a plane perpendicular to the substrate axis. Using a backscattered electron detector (BED) in an EPMA, the first, second, and third catalyst layers present in one randomly selected chamber were observed along the cut surface to determine the regions where the first, second, and third catalyst layers were present. In the BED-based cross-section observation, the field of view magnification was set to 500x, and the field of view width was set to 100–200 μm. The area observed based on the BED was set to exclude the corners of the chambers. The regions where the first, second, and third catalyst layers were present were determined using elemental mapping of the EPMA cross-section.

[0322] In the BED observation image, grid lines 1 to 9, parallel to the thickness direction of the partition wall portion of the substrate, are drawn sequentially at 15 μm intervals from the left end. The intersections of the outline of the region containing the first catalyst layer with each grid line are connected by straight lines to determine the position of the surface of the first catalyst layer. Similarly, the intersections of the outline of the region containing the second catalyst layer with each grid line are connected by straight lines to determine the position of the surface of the second catalyst layer. Likewise, the intersections of the outline of the region containing the third catalyst layer with each grid line are connected by straight lines to determine the position of the surface of the third catalyst layer. If the change in thickness direction from a certain intersection point P1 to the adjacent intersection point P2 exceeds the grid line interval (15 μm), intersection point P2 is not used to determine the position of the surface (i.e., intersection point P2 is excluded from the intersections connected by straight lines). Furthermore, if the change in thickness from intersection point P1 to the adjacent intersection point P2 exceeds the grid line interval (15 μm), and the change in thickness from intersection point P1 to the adjacent intersection point P3 also exceeds the grid line interval (15 μm), then intersection point P3, except for intersection point P2, is not used for determining the surface position (i.e., intersection points P2 and P3 are excluded from the intersection points connected by straight lines). If five intersection points are consecutively excluded from the intersection points connected by straight lines in this manner, the BED image is not used for thickness measurement.

[0323] After determining the positions of the surfaces of the first, second, and third catalyst layers, image analysis software was used to calculate the area of ​​the first region enclosed by the second grid line, the eighth grid line, the surfaces of the first and second catalyst layers. Similarly, the area of ​​the second region enclosed by the second grid line, the eighth grid line, the surfaces of the second and third catalyst layers was calculated. AreaQ (manufactured by Estek Co., Ltd.) was used as the image analysis software. It should be noted that the edges of the image tend to become blurry, making it difficult to determine the positions of the surfaces of the first, second, and third catalyst layers; therefore, the first and ninth grid lines were not used.

[0324] After determining the areas of region 1 and region 2, the thickness of each region is calculated based on the following formula.

[0325] Thickness of each region = Area of ​​each region / (Grid line spacing × Number of grid line spacings)

[0326] It should be noted that the grid lines are spaced 15 μm apart, and the number of grid lines is 6.

[0327] For 20 chambers randomly selected from the cut surface, the thickness of the first region was calculated, and their average value was used as the average thickness of the second catalyst layer. Similarly, for 20 chambers randomly selected from the cut surface, the thickness of the second region was calculated, and their average value was used as the average thickness of the third catalyst layer.

[0328] The average thickness of the second catalyst layer is 40 μm, and the average thickness of the third catalyst layer is 3.3 μm. The characteristics of the second and third catalyst layers in the exhaust gas purification catalyst of Example 1 are shown in Table 1.

[0329] In Table 1, the meanings of “Rh”, “CeO2”, “Al2O3”, “ZrO2”, “Al2O3+ZrO2”, “average thickness” and “a / b” are as follows.

[0330] [Second catalyst layer]

[0331] Rh: The percentage (mass %) of the metal equivalent mass of Rh in the second catalyst layer relative to the mass of the second catalyst layer.

[0332] CeO2: The percentage (mass %) of Ce in the second catalyst layer relative to the mass of the second catalyst layer, calculated as CeO2.

[0333] Al2O3: The percentage (mass %) of the mass of Al in the second catalyst layer relative to the mass of the second catalyst layer, calculated as Al2O3.

[0334] ZrO2: The percentage (mass %) of Zr in the second catalyst layer relative to the mass of the second catalyst layer, calculated as ZrO2.

[0335] Al₂O₃ + ZrO₂: The percentage (mass %) of the sum of the mass of Al (Al₂O₃ equivalent) and the mass of Zr (ZrO₂ equivalent) in the second catalyst layer relative to the mass of the second catalyst layer.

[0336] Average thickness: The average thickness (μm) of the second catalyst layer.

[0337] [Third catalyst layer]

[0338] Rh: The percentage (mass %) of the metal equivalent mass of Rh in the third catalyst layer relative to the mass of the third catalyst layer.

[0339] CeO2: The percentage (mass %) of Ce in the third catalyst layer relative to the mass of the third catalyst layer, calculated as CeO2.

[0340] Al2O3: The percentage (mass %) of the mass of Al in the third catalyst layer relative to the mass of the third catalyst layer, calculated as Al2O3.

[0341] ZrO2: The percentage (mass %) of Zr in the third catalyst layer relative to the mass of the third catalyst layer, calculated as ZrO2.

[0342] Al₂O₃ + ZrO₂: The percentage (mass %) of the sum of the mass of Al (Al₂O₃ equivalent) and the mass of Zr (ZrO₂ equivalent) in the third catalyst layer relative to the mass of the third catalyst layer.

[0343] Average thickness: The average thickness (μm) of the third catalyst layer.

[0344] [a / b]

[0345] a: The percentage (mass %) of the metal equivalent mass of Rh in the third catalyst layer relative to the mass of the third catalyst layer.

[0346] b: The percentage (mass %) of the metal equivalent mass of Rh in the second catalyst layer relative to the mass of the second catalyst layer.

[0347] [Example 2]

[0348] The amounts of each component in the slurry for forming the upper layer were adjusted so that, based on the mass of the fired upper layer (100% by mass), Rh was 0.5% by mass (converted to metal), Ce was 5.0% by mass (converted to CeO2), Zr was 17.5% by mass (converted to ZrO2), Al was 74.5% by mass (converted to Al2O3), and rare earth elements other than Ce were 2.5% by mass (converted to oxides). The mass of the upper layer per unit volume in the flowable substrate where the upper layer is formed was changed to 20 g / L. Otherwise, the same procedure as in Example 1 was followed to manufacture the catalyst for exhaust gas purification of Example 2. The lower, middle, and upper layers of the catalyst for exhaust gas purification in Example 2 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0349] Similar to Example 1, the average thickness of the second catalyst layer and the average thickness of the third catalyst layer in the catalyst for exhaust gas purification of Example 2 were calculated. The results showed that 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 catalyst for exhaust gas purification of Example 2 are shown in Table 1.

[0350] [Example 3]

[0351] The amounts of each component in the slurry for forming the upper layer were adjusted so that, based on the mass of the fired upper layer (100% by mass), Rh was 0.3% by mass (converted to metal), Ce was 3.3% by mass (converted to CeO2), Zr was 11.7% by mass (converted to ZrO2), Al was 83.0% by mass (converted to Al2O3), and rare earth elements other than Ce were 1.7% by mass (converted to oxides). The mass of the upper layer per unit volume in the flowable substrate where the upper layer is formed was changed to 30 g / L. Otherwise, the same procedure as in Example 1 was followed to manufacture the catalyst for exhaust gas purification of Example 3. The lower, middle, and upper layers of the catalyst for exhaust gas purification in Example 3 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively.

[0352] Similar to Example 1, 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 3 were calculated. The results showed that 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 purification catalyst of Example 3 are shown in Table 1.

[0353] [Comparative Example 1]

[0354] The amounts of each component in the slurry for forming the intermediate layer were adjusted so that, based on the mass of the fired intermediate layer (100% by mass), Rh was 0.2% by mass (converted to metal), Ce was 6.0% by mass (converted to CeO2), Zr was 21.0% by mass (converted to ZrO2), Al was 69.2% by mass (converted to Al2O3), rare earth elements other than Ce were 3.6% by mass (converted to oxides), and no upper layer was formed. Otherwise, the process was the same as in Example 1 to manufacture the catalyst for exhaust gas purification of Comparative Example 1. The lower layer in the catalyst for exhaust gas purification of Comparative Example 1 corresponds to the first catalyst layer. The intermediate layer in the catalyst for exhaust gas purification of Comparative Example 1 is considered as a stack 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 catalyst for exhaust gas purification of Comparative Example 1 are shown in Table 1.

[0355] [Comparative Example 2]

[0356] The amounts of each component in the slurry for forming the intermediate layer were adjusted so that, based on the mass of the fired intermediate layer (100% by mass), Rh was 0.2% by mass (converted to metal), Ce was 12.0% by mass (converted to CeO2), Zr was 42.0% by mass (converted to ZrO2), Al was 39.5% by mass (converted to Al2O3), and rare earth elements other than Ce were 6.3% by mass (converted to oxides). Otherwise, the same procedure as in Comparative Example 1 was followed to manufacture the catalyst for exhaust gas purification of Comparative Example 2. The lower layer of the catalyst for exhaust gas purification of Comparative Example 2 corresponds to the first catalyst layer. The intermediate layer of the catalyst for exhaust gas purification of Comparative Example 2 is considered as a stack of the second and third catalyst layers having the same composition. The characteristics of the second and third catalyst layers of the catalyst for exhaust gas purification of Comparative Example 2 are shown in Table 1.

[0357] BED observation images and elemental mapping images of the catalyst for exhaust gas purification in Comparative Example 2, obtained using the same method as in Example 1, are shown below. Figure 5 and Figure 6 .in addition, Figure 6 It is a mapping image of Pd.

[0358] [Comparative Example 3]

[0359] The flow-through substrate with a lower and middle layer obtained in Example 1 (4) was immersed in an aqueous solution of rhodium nitrate at 25°C for 48 hours. After drying the immersed flow-through substrate at 150°C for 0.5 hours, it was fired at 500°C for 1 hour to form a noble metal-containing surface layer containing Rh on the entire surface of the middle layer. In the fired middle layer, based on the mass of the middle layer (100% by mass), it contains 0.2% by mass of Rh (metal equivalent), 12.0% by mass of Ce (CeO2 equivalent), 42.0% by mass of Zr (ZrO2 equivalent), 39.5% by mass of Al (Al2O3 equivalent), and 6.3% by mass of rare earth elements other than Ce (oxide equivalent). In addition, the Rh in the middle layer is not uniformly distributed in the thickness direction of the middle layer, but is biased towards the surface of the middle layer. The lower layer in the catalyst for exhaust gas purification in Comparative Example 3 corresponds to the first catalyst layer. The portion of the middle layer of the catalyst for exhaust gas purification in Comparative Example 3, excluding the surface layer containing precious metals, is considered the second catalyst layer, and the surface layer containing precious metals is considered the third catalyst layer. The characteristics of the second and third catalyst layers in the catalyst for exhaust gas purification of Comparative Example 3 are shown in Table 1. It should be noted that in Comparative Example 3, the entire middle layer containing “CeO2,” “Al2O3,” “ZrO2,” and “Al2O3+ZrO2” is considered as “CeO2,” “Al2O3,” “ZrO2,” and “Al2O3+ZrO2” in the second and third catalyst layers, respectively.

[0360] [Comparative Example 4]

[0361] The amounts of each component in the slurry for forming the upper layer were adjusted so that, based on the mass of the fired upper layer (100% by mass), Rh was 0.2% by mass (converted to metal), Ce was 1.7% by mass (converted to CeO2), Zr was 5.8% by mass (converted to ZrO2), Al was 91.5% by mass (converted to Al2O3), and rare earth elements other than Ce were 0.8% by mass (converted to oxides). The mass of the upper layer per unit volume of the portion of the flowable substrate in which the upper layer is formed was set to 60 g / L. Otherwise, the same procedure as in Example 1 was followed to manufacture the catalyst for exhaust gas purification of Comparative Example 4. The lower, middle, and upper layers of the catalyst for exhaust gas purification in Comparative Example 4 correspond to the first catalyst layer, the second catalyst layer, and the third catalyst layer, respectively. The average thickness of the second catalyst layer and the average thickness of the third catalyst layer in the catalyst for exhaust gas purification of Comparative Example 4 were calculated using the above method. The results showed that 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 exhaust gas purification of Comparative Example 4 are shown in Table 1.

[0362] [Experimental Example]

[0363] After durability treatment of the catalysts for exhaust gas purification in Examples 1-3 and Comparative Examples 1-4, the exhaust gas purification performance and OSC performance were evaluated as follows. It should be noted that the durability treatment was performed by heat treatment at 1000°C for 30 hours in an atmosphere with 0.50% O2 gas flow, 10% water vapor flow, and N2 as the equilibrium gas flow.

[0364] <Exhaust Gas Purification Performance Test>

[0365] The purification performance of hydrocarbons (HC), a representative of harmful components, was determined. In a durable treated exhaust gas purification catalyst (catalyst volume 15 mL), a simulated gas with the following composition (A / F = 14.6) was circulated at a rate of 32 L / min while adjusting the CO and O2 concentrations by varying the A / F within the range of 14.4–14.8. The temperature of the gas flowing into the catalyst was gradually increased from room temperature at a specified rate. The amount of HC contained in the exhaust gas passing through the catalyst was determined using the following apparatus, and the HC purification rate was calculated based on the following formula. It should be noted that V represents the detection quantity without a catalyst, and W represents the detection quantity with a catalyst.

[0366] HC purification rate (%) = (VW) / V × 100

[0367] [Simulated gas (composition based on volume)]

[0368] CO: 0.3%, C3H6: 1000 ppmC, NO: 500 ppm, O2: 0.28%, CO2: 14%, H2O: 10%, N2: the balance

[0369] [Rate of temperature rise] 10 °C / minute

[0370] [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by Horiba, Ltd.

[0371] The inlet gas temperatures (°C) of the catalyst when the HC purification rate reaches 50% and 80% are determined as the light-off temperature T50 and T80 respectively. T50 and T80 are measured during the temperature rise. The measurement results of T50 and T80 are shown in Table 2. Since T50 is greatly affected by the deterioration state of the catalyst caused by the durability treatment, it becomes an index of heat resistance. The smaller the T50 value, the higher the heat resistance. Compared with the deterioration state of the catalyst, T80 is more affected by the contact between the catalyst and the exhaust gas, so it becomes an index of the contact between the catalyst and the exhaust gas. The smaller T80 is, the higher the contact between the catalyst and the exhaust gas.

[0372] <OSC performance test>

[0373] In the exhaust gas purification catalyst after the durability treatment (catalyst volume 15 mL), simulated gas 1 and simulated gas 2 with the following compositions are alternately passed through at 32 L / min every 1 minute. The gas temperature flowing into the exhaust gas purification catalyst is 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 after switching from simulated gas 1 to simulated gas 2 reaches 0.25% is obtained using the following device. The difference in delay time (seconds) is calculated based on the following formula. It should be noted that X represents the delay time when no catalyst is installed, and Y represents the delay time after the catalyst is installed.

[0374] Difference in delay time (seconds) = Y - X

[0375] [Simulated gas 1 (composition based on volume)]

[0376] O2: 0.5%, N2: the balance

[0377] [Simulated gas 2 (composition based on volume)]

[0378] CO: 0.5%, N2: the balance

[0379] [Evaluation device] MOTOR EXHAUST GAS ANALYZER MEXA7100 manufactured by Horiba, Ltd.

[0380] The difference in delay time becomes an indicator of OSC. The larger the difference in delay time, the larger the OSC. Therefore, the difference in delay time is calculated as a relative value when the value of Comparative Example 1 is set to 100, and this is used as the OSC. The measurement results of OSC are shown in Table 2.

[0381] The reason why the difference in latency is used as an indicator of OSC is as follows.

[0382] The test system is configured with a simulated gas supply unit, a catalyst setting unit, and a gas concentration meter setting unit in sequence. During the flow of simulated gas 1, the CO concentration detected by the gas concentration meter is zero. Without a catalyst, if switching from simulated gas 1 to simulated gas 2, the CO in simulated gas 2 is not consumed and is detected by the gas concentration meter. With a catalyst, if switching from simulated gas 1 to simulated gas 2, the CO in simulated gas 2 is consumed by the oxygen stored in the catalyst, thus maintaining a low CO concentration detected by the gas concentration meter. The more oxygen stored in the catalyst, the longer the CO concentration detected by the gas concentration meter remains low. Therefore, the difference in delay time becomes an indicator of OSC capability.

[0383] [Table 1]

[0384]

[0385] [Table 2]

[0386]

[0387] The catalysts for exhaust gas purification in Examples 1-3 include a substrate, a first catalyst layer disposed on the substrate, a second catalyst layer disposed on the first catalyst layer, and a third catalyst layer disposed 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 contains Al and / or Zr. The catalysts for exhaust gas purification satisfy the following conditions (1) to (4). On the other hand, the catalysts for exhaust gas purification in Comparative Examples 1-4 include a substrate, a first catalyst layer disposed on the substrate, a second catalyst layer disposed on the first catalyst layer, and a third catalyst layer disposed 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 contains Al and / or Zr. The catalysts for exhaust gas purification do not satisfy any one or more of the following conditions (1) to (4).

[0388] (1) The second catalyst layer and the third catalyst layer satisfy the following equation:

[0389] a>b

[0390] [In the formula, a represents the percentage of the metal equivalent mass of Rh in the third catalyst layer relative to the mass of the third catalyst layer, and b represents the percentage of the metal equivalent mass of Rh in the second catalyst layer relative to the mass of the second catalyst layer.]

[0391] (2) The percentage of CeO2 equivalent mass of Ce in the second catalyst layer relative to the mass of the second catalyst layer is 7% by mass or more.

[0392] (3) The percentage of CeO2 equivalent mass of Ce in the third catalyst layer relative to the mass of the third catalyst layer is less than 7% by mass.

[0393] (4) The average thickness of the third catalyst layer is less than 10 μm.

[0394] Comparative Example 1 fails to meet condition (1), thus it cannot achieve an improvement in the contact between Rh contained in the third catalyst layer and the exhaust gas, resulting in a large T80 (i.e., low contact between the catalyst and the exhaust gas). Furthermore, Comparative Example 1 fails to meet condition (2), thus it cannot achieve an improvement in the OSC of the second catalyst layer, resulting in a low OSC.

[0395] Comparative Example 2 fails to meet condition (1), thus it cannot improve the contact between Rh contained in the third catalyst layer and the exhaust gas. Therefore, T80 is large (i.e., the contact between the catalyst and the exhaust gas is low). In addition, Comparative Example 2 fails to meet condition (3), thus it cannot improve the heat resistance of the third catalyst layer. Therefore, T50 is large (i.e., the heat resistance is low).

[0396] Comparative Example 3 fails to meet condition (3), thus failing to improve the heat resistance of the third catalyst layer; therefore, T50 is high (i.e., low heat resistance). Although Comparative Example 3 meets conditions (1) and (4), T80 is high (i.e., low catalyst-exhaust gas contact). It is believed that Comparative Example 3 fails to meet condition (3), resulting in Rh aggregation in the third catalyst layer and a reduction in active sites; therefore, although conditions (1) and (4) are met, T80 is high.

[0397] Comparative Example 4 does not meet condition (4), so it is impossible to improve the contact between the catalytic active components contained in the part lower than the third catalyst layer and the exhaust gas. Therefore, T80 is large (i.e., the contact between the catalyst and the exhaust gas is low).

[0398] Since condition (1) is met, the contact between Rh contained in the third catalyst layer and the exhaust gas can be improved in Examples 1-3, and therefore T80 is small (i.e., the contact between the catalyst and the exhaust gas is high).

[0399] Since condition (2) is met, the OSC of the second catalyst layer in Examples 1-3 can be improved, and therefore the OSC is high.

[0400] Since condition (3) is met, the heat resistance of the third catalyst layer can be improved in Examples 1-3, and therefore, T50 is small (i.e., high heat resistance).

[0401] Since conditions (4) are met, the contact between the catalytic active components contained in the lower part of the third catalyst layer and the exhaust gas can be improved. Therefore, T80 is small (i.e., the contact between the catalyst and the exhaust gas is high).

[0402] Explanation of reference numerals in the attached figures

[0403] P...exhaust pipe of internal combustion engine

[0404] 1. Catalysts for exhaust gas purification

[0405] 10···Substrate

[0406] 11···Tubular part

[0407] 12···Separation wall section

[0408] Room 13

[0409] 20···First catalyst layer

[0410] 30···Second catalyst layer

[0411] 40···Third catalyst layer

Claims

1. A catalyst for purifying waste gas, comprising: a substrate, a first catalyst layer disposed on the substrate, a second catalyst layer disposed on the first catalyst layer, and a third catalyst layer disposed on the second catalyst layer. In the catalyst for waste gas purification, The first catalyst layer contains Pd. The second catalyst layer contains Rh and Ce. The third catalyst layer contains Rh and also contains Al and / or Zr. The second catalyst layer and the third catalyst layer satisfy the following formula: a>b In the formula, a represents the percentage of the metal-converted mass of Rh in the third catalyst layer relative to the mass of the third catalyst layer, and b represents the percentage of the metal-converted mass of Rh in the second catalyst layer relative to the mass of the second catalyst layer. The percentage of CeO2 equivalent mass of Ce in the second catalyst layer relative to the mass of the second catalyst layer is 7% by mass or more. The percentage of CeO2 equivalent mass of Ce in the third catalyst layer relative to the mass of the third catalyst layer is less than 7% by mass. The average thickness of the third catalyst layer is less than 10 μm.

2. The catalyst for purifying waste gas according to claim 1, wherein, The sum of the mass of Al in the third catalyst layer converted to Al2O3 and the mass of Zr in the third catalyst layer converted to ZrO2, relative to the mass of the third catalyst layer, is more than 80% by mass.

3. The catalyst for purifying waste gas according to claim 1 or 2, wherein, The ratio of a to b, a / b, is greater than 2 and less than 10.

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

5. The catalyst for purifying waste gas according to claim 1 or 2, wherein, The average thickness of the third catalyst layer is greater than 0.5 μm and less than 5 μm.

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