Exhaust gas purification catalyst

Abstract

The present invention addresses the problem of providing an exhaust gas purification catalyst that exhibits an improved methane purification performance during the high-speed operation of an internal combustion engine. In order to solve this problem, the present invention provides an exhaust gas purification catalyst (1A) comprising: a first catalyst layer (20) containing Pd; a second catalyst layer (30) containing Pt, Ce, and an alkaline earth metal element; and an Rh-containing third catalyst layer (40) and / or an Rh-containing fourth catalyst layer (50). The mass as metal of Pt in the second catalyst layer (30) is at least 1.0 g / L. In the second catalyst layer (30), the ratio of the mass as CeO2 of Ce to the mass as metal of Pt is 6.0-20.0. Also in the second catalyst layer (30), the ratio of the mass as CeO2 of Ce to the mass as the oxide of the alkaline earth metal element is 2.0-18.0.

Description

Exhaust gas purification catalyst

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

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

[0003] HC emitted from internal combustion engines includes methane (CH ４ As an exhaust gas purification catalyst suitable for methane purification, Patent Document 1 describes an exhaust gas purification catalyst including a substrate and a catalytic layer provided on the substrate, in which the catalytic layer includes a Pd layer provided on the upstream side, a Pt layer provided on the downstream side, and a Rh layer laminated on both the Pd layer and the Pt layer, and the Pt layer does not substantially contain alkaline earth metal elements.

[0004] Patent No. 6775052

[0005] It is desirable to control the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine near the theoretical air-fuel ratio (stoichiometry). However, the actual air-fuel ratio fluctuates between a rich state (excess fuel atmosphere) and a lean state (lean fuel atmosphere) around the stoichiometry depending on the operating conditions of the internal combustion engine, and the exhaust gas atmosphere similarly fluctuates between a rich state and a lean state. When the exhaust gas is in a lean state, Pt is oxidized, and its methane purification performance deteriorates. When the exhaust gas switches from a lean state to a rich state, the oxidized Pt is reduced, and its methane purification performance is restored. Since alkaline earth metal elements have electron donating properties, when alkaline earth metal elements are present near Pt, the alkaline earth metal elements donate electrons to Pt. Therefore, when the Pt layer contains alkaline earth metal elements, Pt easily adsorbs oxygen and becomes stable as an oxide. In other words, if the Pt layer contains alkaline earth metal elements, the methane purification performance of the Pt is difficult to recover even when the exhaust gas switches from a lean state to a rich state. For this reason, Patent Document 1 specifies that the Pt layer is substantially free of alkaline earth metal elements.

[0006] Patent Document 1 does not consider methane purification performance during high-speed operation of an internal combustion engine. The present inventors have conducted a study on methane purification performance during high-speed operation of an internal combustion engine and found that the exhaust gas purification catalyst described in Patent Document 1 does not have sufficient methane purification performance during high-speed operation of an internal combustion engine.

[0007] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst that has improved methane purification performance during high-speed operation of an internal combustion engine.

[0008] In order to solve the above problems, the present invention provides the following exhaust gas purification catalyst: [1] An exhaust gas purification catalyst comprising: a substrate extending in an exhaust gas flow direction, the substrate having a first substrate portion located upstream in the exhaust gas flow direction and a second substrate portion located downstream in the exhaust gas flow direction, a first catalytic layer provided on the first substrate portion, a second catalytic layer provided on the second substrate portion, and a third catalytic layer provided above the first catalytic layer and / or a fourth catalytic layer provided above the second catalytic layer, wherein the first catalytic layer contains Pd, the second catalytic layer contains Pt, Ce, and an alkaline earth metal element, the third catalytic layer and the fourth catalytic layer each contain Rh, the mass of Pt in the second catalytic layer in terms of metal is 1.0 g / L or more based on the volume of the portion of the second substrate portion where the second catalytic layer is provided, and the ratio of Ce to the mass of Pt in the second catalytic layer is CeO ２ a ratio of CeO to the mass of alkaline earth metal elements in terms of oxides in the second catalytic layer is 6.0 or more and 20.0 or less; ２ [2] The exhaust gas purifying catalyst according to [1], wherein the second catalyst layer contains a Ce—Zr-based composite oxide as a Ce source. [3] The Ce in the Ce—Zr-based composite oxide is converted into CeO ２ [4] The exhaust gas purifying catalyst according to [2], wherein the content of Ce in the Ce-Zr-based composite oxide is 18 mass % or more and 70 mass % or less, calculated as CeO ２The catalyst for purifying exhaust gas according to [3], wherein the content of the Ce—Zr-based composite oxide in the second catalytic layer is 25 mass % or more and 63 mass % or less, based on the mass of the Ce—Zr-based composite oxide. [5] The catalyst for purifying exhaust gas according to any one of [2] to [4], wherein the content of the Ce—Zr-based composite oxide in the second catalytic layer is 20 mass % or more and 80 mass % or less, based on the mass of the second catalytic layer. [6] The catalyst for purifying exhaust gas according to any one of [2] to [5], wherein at least a portion of the Pt in the second catalytic layer is supported on the Ce—Zr-based composite oxide in the second catalytic layer. [7] The catalyst for purifying exhaust gas according to any one of [1] to [6], wherein the second catalytic layer contains an alkaline earth metal compound as an alkaline earth metal element source, and the alkaline earth metal compound is selected from oxides, carbonates, and sulfates. [8] The catalyst for purifying exhaust gas according to any one of [1] to [7], wherein the second catalyst layer contains one or more alkaline earth metal elements selected from Ca, Sr, and Ba. [9] The catalyst for purifying exhaust gas according to any one of [1] to [8], wherein the third catalyst layer and the fourth catalyst layer each contain Pt.

[10] The catalyst for purifying exhaust gas according to [9], wherein the third catalyst layer and the fourth catalyst layer each contain Ce.

[11] The catalyst for purifying exhaust gas according to

[10] , wherein the third catalyst layer and the fourth catalyst layer each contain a Ce—Zr-based composite oxide as a Ce source.

[12] The catalyst for purifying exhaust gas according to

[11] , wherein at least a portion of the Pt in the third catalyst layer is supported on the Ce—Zr-based composite oxide in the third catalyst layer, and at least a portion of the Pt in the fourth catalyst layer is supported on the Ce—Zr-based composite oxide in the fourth catalyst layer.

[0009] According to the present invention, there is provided an exhaust gas purification catalyst having improved methane purification performance during high-speed operation of an internal combustion engine.

[0010] Fig. 1 is a partial end view showing a state in which an exhaust gas purification catalyst according to a first embodiment of the present invention is arranged in an exhaust passage of an internal combustion engine. Fig. 2 is an end view taken along line A-A in Fig. 1. Fig. 3 is an end view taken along line B-B in Fig. 1. Fig. 4 is an enlarged view of the area indicated by reference symbol R1 in Fig. 2. Fig. 5 is an enlarged view of the area indicated by reference symbol R2 in Fig. 3. Fig. 6 is an end view taken along line C-C in Fig. 1. Fig. 7 is an end view (corresponding to Fig. 6) of an exhaust gas purification catalyst according to a second embodiment of the present invention.

[0011] <Explanation of Terms> The terms used in this specification are explained below. The following explanations apply to the entire specification unless otherwise specified. Note that the following explanations regarding the catalyst layers apply to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, the third catalyst layer 40, and the fourth catalyst layer 50).

[0012] The term "metallic element" also includes metalloid elements such as Si and B.

[0013] Rare Earth Elements The term "rare earth elements" includes Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0014] <Noble Metal Elements> The term "noble metal elements" includes Pt, Pd, Rh, Ru, Os, Ir, Au, and Ag.

[0015] <Oxide> The meaning of the term "oxide" is as follows: Oxides of rare earth elements other than Ce, Pr and Tb are called sesquioxides (M ２ O ３ , M represents a rare earth element other than Ce, Pr, and Tb), and the oxide of Ce is CeO ２ Pr oxide is Pr ６ O １１ The oxide of Tb is Tb ４ O ７ The oxide of Al is Al ２ O ３ Zr oxide is ZrO ２ The oxide of Si is SiO ２ The oxide of B is B ２ O ３ Cr oxide is Cr ２ O３ , Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe ３ O ４ Mn oxide is Mn ３ O ４ Ni oxide is NiO, Ti oxide is TiO ２ Zn oxide is ZnO, Sn oxide is SnO ２ means.

[0016] <Mass of Metal Element in Metal Equivalent> The term "mass of metal element in metal equivalent" means the mass of the metal that is determined on the assumption that the metal element exists as a metal composed of the metal element.

[0017] <Mass of Metal Element in Equivalent to Oxide> The term "mass of metal element in equivalent to oxide" means the mass of the oxide calculated on the assumption that the metal element exists as an oxide of the metal element. The meaning of "oxide" of the metal element is as described above.

[0018] <Mass of Metal Element in Carbonate Equivalent> The term "mass of metal element in carbonate equivalent" means the mass of the carbonate calculated on the assumption that the metal element exists as a carbonate of the metal element.

[0019] <Mass of Metal Element in Sulfate Equivalent> The term "mass of metal element in sulfate equivalent" means the mass of the sulfate obtained on the assumption that the metal element exists as the sulfate of the metal element.

[0020] <Mass of catalytic layer> The term "mass of catalytic layer" refers to the sum of the mass of precious metal elements contained in the catalytic layer, calculated on a metal basis, the mass of alkaline earth metal elements in the case where hydroxides, acetates, nitrates, or carbonates of alkaline earth metal elements are used as the raw material for the catalytic layer, the mass of sulfates in the case where sulfates of alkaline earth metal elements are used as the raw material for the catalytic layer, and the mass of oxides of metal elements other than precious metal elements and alkaline earth metal elements. In other words, the term "mass of catalytic layer" refers to the calculated mass obtained by summing the mass of precious metal elements contained in the catalytic layer in metal equivalent, the mass of alkaline earth metal elements contained in the catalytic layer in carbonate or sulfate equivalent, and the mass of metal elements other than precious metal elements contained in the catalytic layer in oxide equivalent.

[0021] When information (e.g., composition, amount, etc.) of the raw materials used in manufacturing the catalyst layer is known, the mass of the catalyst layer can be determined from the information of the raw materials used in manufacturing the catalyst layer.

[0022] <Content of Metal Element in Catalyst Layer in Metal Equivalent or Oxide Equivalent> The term "content of metal element in catalyst layer in metal equivalent" is defined by the formula: content of metal element in catalyst layer in metal equivalent (mass %) = (mass of metal element in catalyst layer in metal equivalent) / (mass of catalyst layer) × 100. The meanings of "mass of metal element in metal equivalent" and "mass of catalyst layer" are as described above.

[0023] The term "content of the metal element in the catalytic layer in terms of oxide" is defined by the formula: content of the metal element in the catalytic layer in terms of oxide (mass %) = (mass of the metal element in the catalytic layer in terms of oxide) / (mass of the catalytic layer) × 100. The meanings of "mass of the metal element in terms of oxide" and "mass of the catalytic layer" are as described above.

[0024] When information (e.g., composition, amount, etc.) of the raw materials used to form the catalyst layer is known, the content (mass %) of the metal element in the catalyst layer in terms of metal or oxide can be determined from the information on the raw materials.

[0025] When information about the raw materials used to form the catalyst layer is unknown, the content (mass %) of the metal element in the catalyst layer in terms of metal or oxide can be determined by a common method such as scanning electron microscope-energy dispersive X-ray analysis (SEM-EDX).

[0026] The catalyst layer is subjected to elemental analysis using a conventional method such as SEM-EDX to identify the types of constituent elements of the catalyst layer and determine the mole percentage of each identified metal element. The mole percentage of each metal element is determined for each of 10 SEM fields, and the average mole percentage of each metal element in the 10 fields is taken as the mole percentage of each metal element in the catalyst layer.

[0027] The U value for each precious metal element in the catalyst layer can be calculated using the following formula: U value for each precious metal element = (mol % of each precious metal element in the catalyst layer) x (molar mass of each precious metal element).

[0028] The V1 value for each alkaline earth metal element in the catalyst layer can be calculated using the following formula: V1 value for each alkaline earth metal element = (mol % of each alkaline earth metal element in the catalyst layer) x (molar mass of the oxide of each alkaline earth metal element)

[0029] The V2 value for each alkaline earth metal element in the catalyst layer can be calculated using the following formula: V2 value for each alkaline earth metal element = (mol % of each alkaline earth metal element in the catalyst layer) x (molar mass of carbonate or sulfate of each alkaline earth metal element)

[0030] The "molar mass of the carbonate or sulfate of an alkaline earth metal element" means the molar mass of the carbonate of the alkaline earth metal element when the hydroxide, acetate, nitrate, or carbonate of the alkaline earth metal element is used as a raw material for the catalytic layer, and means the molar mass of the sulfate of the alkaline earth metal element when the sulfate of the alkaline earth metal element is used as a raw material for the catalytic layer.

[0031] For each metal element other than the noble metal elements and alkaline earth metal elements in the catalyst layer, the W value can be calculated using the following formula: W value for each metal element = (mol % of each metal element in the catalyst layer) x (molar mass of the oxide of each metal element)

[0032] The content (mass%) of each precious metal element in the catalyst layer in terms of metal can be calculated using the following formula: Content (mass%) of each precious metal element in the catalyst layer in terms of metal = (U value for each precious metal element) / {(total U values ​​for all precious metal elements) + (total V2 values ​​for all alkaline earth metal elements) + (total W values ​​for all metal elements other than precious metal elements and alkaline earth metal elements)} × 100

[0033] The content (mass %) of each alkaline earth metal element in the catalyst layer in terms of oxide can be calculated using the following formula: Content (mass %) of each alkaline earth metal element in the catalyst layer in terms of oxide = (V1 value for each alkaline earth metal element) / {(total of U values ​​for all precious metal elements) + (total of V2 values ​​for all alkaline earth metal elements) + (total of W values ​​for all metal elements other than precious metal elements and alkaline earth metal elements)} × 100

[0034] The content (mass %) of each metal element other than precious metal elements and alkaline earth metal elements in the catalyst layer, converted into an oxide, can be calculated using the following formula: Content (mass %) of each metal element other than precious metal elements and alkaline earth metal elements in the catalyst layer, converted into an oxide = (W value for each metal element other than precious metal elements and alkaline earth metal elements) / {(total U values ​​for all precious metal elements) + (total V2 values ​​for all alkaline earth metal elements) + (total W values ​​for all metal elements other than precious metal elements and alkaline earth metal elements)} × 100

[0035] <Al-based oxide> The term "Al-based oxide" refers to an oxide containing Al, in which Al is the metal element with the highest content by mass of the metal elements constituting the oxide. However, Ce-Zr-based composite oxides are not considered to be Al-based oxides. Ce-Zr-based composite oxides will be described later. Al-based oxides are distinguished from alumina used as a binder. In this specification, alumina used as a binder may be referred to as an "alumina binder."

[0036] The Al-based oxide is, for example, in the form of particles. The Al-based oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Al-based oxide is preferably porous.

[0037] The Al-based oxide preferably has a particle size suitable for use as a support for a catalytically active component. The average particle size of the Al-based oxide is preferably 1 μm or more and 35 μm or less, more preferably 2 μm or more and 25 μm or less, and even more preferably 3 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits. The average particle size of the Al-based oxide is measured as follows: A sample containing Al-based oxide is observed using a scanning electron microscope, and the unidirectional diameters (Feret diameters) of 100 Al-based oxide particles randomly selected from within the field of view are measured, and the average value is taken as the average particle size of the Al-based oxide.

[0038] Al-based oxides generally have higher heat resistance than other inorganic oxides, and therefore, when the catalyst layer contains an Al-based oxide, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0039] The Al-based oxide may contain one or more metal elements other than Al (hereinafter referred to as "additional element M1"). The additional element M1 can be selected from, for example, rare earth elements (e.g., Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), B, Si, Zr, Cr, etc.

[0040] In the Al-based oxide, the additional element M1 is a solid solution phase (e.g., Al ２ O３ The additional element M1 may form a solid solution phase (a solid solution phase of the additional element M1 and an oxide of the additional element M1), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M1), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M1 forms a solid solution phase.

[0041] Examples of Al-based oxides include alumina (Al ２ O ３ Examples of the Al-based oxide containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-ceria, and alumina-lanthana.

[0042] From the viewpoint of improving the heat resistance of the Al-based oxide, the Al content of the Al in the Al-based oxide is ２ O ３ The converted content is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, based on the mass of the Al-based oxide, with the upper limit being 100% by mass.

[0043] Al in Al-based oxides ２ O ３ The converted content of Al in Al-based oxides is calculated using the formula: ２ O ３ The "mass of Al-based oxide" is defined as (mass of converted Al-based oxide) / (mass of Al-based oxide) x 100. The "mass of Al-based oxide" means the total mass of oxides of metal elements that are obtained by assuming that each metal element in the Al-based oxide exists as an oxide. The meaning of "oxide" of a metal element is as described above.

[0044] From the viewpoint of improving the heat resistance of the Al-based oxide, the Al-based oxide preferably contains one or more rare earth elements. The rare earth elements can be selected from, for example, La, Y, Nd, Pr, Sm, Eu, Gd, etc. In one embodiment, the Al-based oxide contains La. The Al-based oxide may contain, in addition to La, one or more other rare earth elements.

[0045] From the viewpoint of improving the heat resistance of the Al-based oxide, the content of the rare earth element in the Al-based oxide, calculated as oxide, is preferably 0.1 mass % or more and 20 mass % or less, more preferably 0.3 mass % or more and 15 mass % or less, and even more preferably 0.5 mass % or more and 10 mass % or less, based on the mass of the Al-based oxide. Each of the above lower limits may be combined with any of the above upper limits. When the Al-based oxide contains one rare earth element, the "content of the rare earth element in the Al-based oxide, calculated as oxide" means the content of the one rare earth element, calculated as oxide; when the Al-based oxide contains two or more rare earth elements, it means the total content of the two or more rare earth elements, calculated as oxide.

[0046] The content of the rare earth element in the Al-based oxide in terms of oxide is defined by the formula: (mass of the rare earth element in the Al-based oxide in terms of oxide) / (mass of the Al-based oxide) × 100. The meanings of the "mass of the Al-based oxide" and the "oxide" of the metal element are as described above.

[0047] When the composition of the Al-based oxide is known, the content of each metal element in the Al-based oxide calculated as an oxide can be determined from the composition of the Al-based oxide.

[0048] When the composition of the Al-based oxide is unknown, the content of each metal element in the Al-based oxide in terms of oxide can be determined by analyzing a sample containing the Al-based oxide by energy dispersive X-ray spectroscopy (EDX), obtaining elemental mapping, and performing EDX elemental analysis on the designated particles. Specifically, the Al-based oxide particles and other particles are qualitatively distinguished (color-coded) by elemental mapping, and the designated Al-based oxide particles are subjected to composition analysis (elemental analysis), thereby determining the content of each metal element in the designated Al-based oxide particles in terms of oxide.

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

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

[0051] The Ce-based oxide preferably has a particle size suitable for use as a support for a catalytically active component. The average particle size of the Ce-based oxide is preferably 1 μm or more and 35 μm or less, more preferably 2 μm or more and 25 μm or less, and even more preferably 3 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits. The method for measuring the average particle size of the Ce-based oxide is the same as the method for measuring the average particle size of the Al-based oxide.

[0052] Ce-based oxides have oxygen storage capacity (the ability to store oxygen when the oxygen concentration in exhaust gas is high and to release oxygen when the oxygen concentration in exhaust gas is low), which alleviates fluctuations in the oxygen concentration in exhaust gas and widens the operating window of the catalytically active components. Therefore, by including Ce-based oxides in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0053] The Ce-based oxide may contain one or more metal elements other than Ce (hereinafter referred to as "additional element M2"). The additional element M2 can be selected from, for example, rare earth elements other than Ce (e.g., La, Y, Nd, Pr, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0054] In the Ce-based oxide, the additional element M2 forms a solid solution phase (e.g., CeO ２The additional element M2 may form a solid solution phase (a solid solution phase of the additional element M2 and an oxide of the additional element M2), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M2), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M2 forms a solid solution phase.

[0055] Examples of Ce-based oxides include ceria (CeO ２ ), an oxide obtained by modifying the surface of ceria with the additional element M2, an oxide obtained by dissolving the additional element M2 in ceria, and the like.

[0056] From the viewpoint of improving the oxygen storage capacity of the Ce-based oxide, the Ce in the Ce-based oxide is ２ The content in terms of the mass of the Ce-based oxide is preferably 80 mass % or more, more preferably 85 mass % or more, and even more preferably 90 mass % or more, with the upper limit being 100 mass %.

[0057] CeO of Ce in Ce-based oxides ２ The converted content is expressed by the formula: (CeO ２ The "mass of Ce-based oxide" is defined as (mass of converted Ce-based oxide) / (mass of Ce-based oxide) x 100. The "mass of Ce-based oxide" means the total mass of oxides of metal elements that are determined on the assumption that each metal element in the Ce-based oxide exists as an oxide. The meaning of "oxide" of a metal element is as described above.

[0058] The content of each metal element in the Ce-based oxide in terms of oxide can be determined in the same manner as the content of each metal element in the Al-based oxide in terms of oxide.

[0059] <Zr-based oxide> The term "Zr-based oxide" refers to an oxide containing Zr, in which Zr is the metal element with the largest content by mass among the metal elements constituting the oxide. However, Ce-Zr-based composite oxides do not fall under the category of Zr-based oxides. Ce-Zr-based composite oxides will be described later. Zr-based oxides are distinguished from zirconia used as a binder. In this specification, zirconia used as a binder may be referred to as a "zirconia binder."

[0060] The Zr-based oxide is, for example, in a particulate form. The Zr-based oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Zr-based oxide is preferably porous.

[0061] The Zr-based oxide preferably has a particle size suitable for use as a support for a catalytically active component. The average particle size of the Zr-based oxide is preferably 0.1 μm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. Each of the above lower limits may be combined with any of the above upper limits. The method for measuring the average particle size of the Zr-based oxide is the same as the method for measuring the average particle size of the Al-based oxide.

[0062] Zr-based oxides have high heat resistance, and therefore, by including Zr-based oxides in the catalyst layer, the heat resistance of the catalyst layer is improved, and the exhaust gas purification performance of the catalyst layer is improved.

[0063] The Zr-based oxide may contain one or more metal elements other than Zr (hereinafter referred to as "additional element M3"). The additional element M3 can be selected from, for example, rare earth elements (e.g., Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), B, Si, Al, Cr, etc.

[0064] In the Zr-based oxide, the additional element M3 forms a solid solution phase (e.g., ZrO ２ The additional element M3 may form a solid solution phase (a solid solution phase of the additional element M3 and an oxide of the additional element M3), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M3), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M3 forms a solid solution phase.

[0065] Examples of Zr-based oxides include zirconia (ZrO ２ ), an oxide obtained by modifying the surface of zirconia with the additional element M3, an oxide obtained by dissolving the additional element M3 in zirconia, and the like.

[0066] From the viewpoint of improving the heat resistance of the Zr-based oxide, Zr in the Zr-based oxide is ２The converted content is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the mass of the Zr-based oxide, with the upper limit being 100% by mass.

[0067] Zr in Zr-based oxides: ZrO ２ The converted content is expressed by the formula: (ZrO ２ The "mass of Zr-based oxide" is defined as (mass of converted Zr-based oxide) / (mass of Zr-based oxide) x 100. The "mass of Zr-based oxide" means the total mass of oxides of metal elements that are obtained by assuming that each metal element in the Zr-based oxide exists as an oxide. The meaning of "oxide" of a metal element is as described above.

[0068] The content of each metal element in the Zr-based oxide calculated as an oxide can be determined in the same manner as the content of each metal element in the Al-based oxide calculated as an oxide.

[0069] <Ce—Zr-based composite oxide> The Ce—Zr-based composite oxide is a composite oxide containing Ce and Zr, and the Ce in the composite oxide is represented by CeO ２ The content of Zr in the composite oxide in terms of ZrO is 5 mass % or more and 95 mass % or less based on the mass of the composite oxide. ２ The term "oxide" refers to an oxide whose content, calculated based on the mass of the composite oxide, is 5 mass % or more and 95 mass % or less.

[0070] CeO of Ce in Ce-Zr based composite oxide ２ The converted content is expressed by the formula: (CeO ２ The mass of Zr in the Ce-Zr-based composite oxide is defined as ZrO ２ The converted content is calculated by the formula: (ZrO ２ The "mass of the Ce-Zr-based composite oxide" is defined as (mass of the Ce-Zr-based composite oxide converted from the original mass) / (mass of the Ce-Zr-based composite oxide)×100. The "mass of the Ce-Zr-based composite oxide" means the total mass of oxides of the metal elements that are determined on the assumption that each of the metal elements in the Ce-Zr-based composite oxide exists as an oxide. The meaning of the "oxide" of the metal element is as described above.

[0071] The content of each metal element in the Ce-Zr based composite oxide in terms of its oxide can be determined in the same manner as the content of each metal element in the Al based oxide in terms of its oxide.

[0072] The Ce—Zr-based composite oxide is, for example, in the form of particles. The Ce—Zr-based composite oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Ce—Zr-based composite oxide is preferably porous.

[0073] The Ce—Zr-based composite oxide preferably has a particle size suitable for use as a support for a catalytically active component. The average particle size of the Ce—Zr-based composite oxide is preferably 1 μm or more and 35 μm or less, more preferably 3 μm or more and 15 μm or less. Each of the above lower limits may be combined with any of the above upper limits. The method for measuring the average particle size of the Ce—Zr-based composite oxide is the same as the method for measuring the average particle size of the Al-based oxide.

[0074] The Ce—Zr-based composite oxide has oxygen storage capacity, which mitigates fluctuations in the oxygen concentration in exhaust gas and widens the operating window of the catalytically active component. Therefore, by including the Ce—Zr-based composite oxide in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0075] The Ce—Zr-based composite oxide may contain one or more metal elements other than Ce and Zr (hereinafter referred to as “additional element M4”). The additional element M4 can be selected from, for example, rare earth elements other than Ce (e.g., La, Y, Nd, Pr, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0076] In the Ce-Zr based composite oxide, Ce exists in a solid solution phase (e.g., CeO ２ and ZrO ２ or a single phase that is a crystalline phase or an amorphous phase (e.g., a solid solution phase with CeO ２ Although Ce may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Ce forms a solid solution phase.

[0077] In the Ce-Zr based composite oxide, Zr exists in a solid solution phase (e.g., CeO ２ and ZrO ２ or a single phase that is a crystalline phase or an amorphous phase (e.g., ZrO ２ Although Zr may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Zr forms a solid solution phase.

[0078] When the Ce—Zr-based composite oxide contains the additional element M4, the additional element M4 forms a solid solution phase (for example, CeO ２ and a solid solution phase of the oxide of the additional element M4, ZrO ２ and a solid solution phase of the oxide of the additional element M4, CeO ２ and ZrO ２ The additional element M4 may form a solid solution phase (e.g., a solid solution phase of the additional element M4 and an oxide of the additional element M4), or may form a single phase that is a crystalline phase or an amorphous phase (e.g., a single phase of an oxide of the additional element M4), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M4 forms a solid solution phase.

[0079] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the Ce—Zr-based composite oxide preferably contains one or more rare earth elements selected from, for example, La, Y, Nd, Pr, Sm, Eu, Gd, etc.

[0080] First Embodiment An exhaust gas purifying catalyst 1A (hereinafter referred to as "catalyst 1A") according to a first embodiment will be described below with reference to FIGS.

[0081] As shown in FIG. 1 , a catalyst 1A is disposed in an exhaust passage in an exhaust pipe P of an internal combustion engine. The internal combustion engine may be a gasoline-fueled internal combustion engine (e.g., a gasoline engine) or a natural gas-fueled internal combustion engine (e.g., a natural gas engine). Exhaust gas emitted from a gasoline-fueled internal combustion engine and a natural gas-fueled internal combustion engine both contains methane. The exhaust gas emitted from the internal combustion engine flows through an exhaust passage in the exhaust pipe P from one end to the other end and is purified by the catalyst 1A provided in the exhaust pipe P. In the drawings, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas inlet side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outlet side" or "downstream side." In this specification, "length" refers to the dimension in the exhaust gas flow direction X, unless otherwise specified.

[0082] In the exhaust passage in the exhaust pipe P, other exhaust gas purifying catalysts may be arranged upstream and / or downstream of the catalyst 1A.

[0083] As shown in FIGS. 2 to 6 , the catalyst 1A includes a substrate 10 having a first substrate portion 10a located on the upstream side and a second substrate portion 10b located on the downstream side, a first catalytic layer 20 provided on the first substrate portion 10a, a second catalytic layer 30 provided on the second substrate portion 10b, a third catalytic layer 40 provided above the first catalytic layer 20, and a fourth catalytic layer 50 provided above the second catalytic layer 30.

[0084] Either the third catalytic layer 40 or the fourth catalytic layer 50 can be omitted. Therefore, the present invention encompasses both an embodiment having the third catalytic layer 40 but not the fourth catalytic layer 50, and an embodiment having the fourth catalytic layer 50 but not the third catalytic layer 40. However, from the viewpoint of further improving the methane purification performance of catalyst 1A, it is preferable that catalyst 1A have both the third catalytic layer 40 and the fourth catalytic layer 50.

[0085] In the catalyst 1A, the first catalytic layer 20 contains Pd, the second catalytic layer 30 contains Pt, Ce, and an alkaline earth metal element, the third catalytic layer 40 and the fourth catalytic layer 50 each contain Rh, the mass of Pt in the second catalytic layer 30 in terms of metal is 1.0 g / L or more based on the volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided, and the ratio of Ce to the mass of Pt in the second catalytic layer 30 is 1.0 g / L or more based on the volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided. ２ The ratio of the mass of Ce to the mass of alkaline earth metal elements in terms of oxides is 6.0 or more and 20.0 or less, and in the second catalytic layer 30, ２ The ratio of the converted mass is 2.0 or more and 18.0 or less.

[0086] The effects of the catalyst 1A will be described below.

[0087] It is desirable to control the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine near the theoretical air-fuel ratio (stoichiometry). However, the actual air-fuel ratio fluctuates between a rich state (excess fuel atmosphere) and a lean state (lean fuel atmosphere) around the stoichiometry depending on the operating conditions of the internal combustion engine, and the exhaust gas atmosphere similarly fluctuates between a rich state and a lean state. When the exhaust gas is in a lean state, Pt is oxidized, and its methane purification performance deteriorates. When the exhaust gas switches from a lean state to a rich state, the oxidized Pt is reduced, and its methane purification performance is restored. Since alkaline earth metal elements have electron donating properties, when alkaline earth metal elements are present near Pt, the alkaline earth metal elements donate electrons to Pt. Therefore, when the Pt layer contains alkaline earth metal elements, Pt easily adsorbs oxygen and becomes stable as an oxide. In other words, if the Pt layer contains alkaline earth metal elements, the methane purification performance of the Pt is difficult to recover even when the exhaust gas switches from a lean state to a rich state. For this reason, Patent Document 1 specifies that the Pt layer is substantially free of alkaline earth metal elements.

[0088] When the catalyst is placed in an atmosphere where the catalyst temperature fluctuates drastically from lean to rich and vice versa, as described in Patent Document 1, it is preferable that the Pt layer contains substantially no alkaline earth metal elements in order to prevent Pt from stabilizing as an oxide.

[0089] However, during high-speed operation of the internal combustion engine, the atmosphere in which the catalyst is placed is almost constant (rich or stoichiometric), so there is no need to limit the content of alkaline earth metal elements in the Pt layer in order to prevent Pt from stabilizing as an oxide.

[0090] In rich and stoichiometric conditions, the methane purification reaction by Pt proceeds mainly through the steam reforming reactions shown in the following formulas 1 to 3. Because the steam reforming reaction is an equilibrium reaction, the concentrations of reactants and products in the reaction system are important for the progress of the steam reforming reaction.

[0091] Formula 1: CH ４ +H ２ O ⇔ CO + 3H ２ Equation 2: CO + H ２ O ⇔ CO ２ +H ２ Formula 3: CH ４ +2H ２ O ⇔ CO ２ +4H ２

[0092] Effect of the First Catalytic Layer 20 When the reaction of Equation 1 proceeds to the right, CO is produced. Therefore, CO in the exhaust gas that contacts the second catalytic layer 30 inhibits the reaction of Equation 1 caused by Pt in the second catalytic layer 30 from proceeding to the right. Because the first catalytic layer 20 is located upstream of the second catalytic layer 30, the exhaust gas contacts the first catalytic layer 20 and then the second catalytic layer 30. When the exhaust gas contacts the first catalytic layer 20, Pd in ​​the first catalytic layer 20 purifies and removes CO from the exhaust gas, so the CO concentration in the exhaust gas after contacting the first catalytic layer 20 (i.e., the exhaust gas that contacts the second catalytic layer 30) decreases. The decrease in the CO concentration in the exhaust gas that contacts the second catalytic layer 30 makes it easier for the reaction of Equation 1 caused by Pt in the second catalytic layer 30 to proceed to the right. Therefore, when the first catalyst layer 20 contains Pd, the methane purification performance of the Pt in the second catalyst layer 30 is improved during high-speed operation of the internal combustion engine.

[0093] Effects of the second catalytic layer 30 Pt has higher catalytic activity for the steam reforming reaction than other precious metal elements. Therefore, the greater the amount of Pt used, the higher the methane purification activity obtained. Therefore, when the second catalytic layer 30 contains Pt and the metal-equivalent mass of Pt in the second catalytic layer 30 is 1.0 g / L or more, based on the volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided, the methane purification performance of the Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine is improved.

[0094] Ce has oxygen storage capacity, and O ２ The H produced by the reactions of Equations 1, 2, and 3 with Pt in the second catalyst layer 30 is released. ２ and O released due to the oxygen storage capacity of Ce in the second catalyst layer 30. ２ The reaction is H ２ When O is generated, H on the left side of Equations 1, 2, and 3 ２ As O increases, H on the right side of Equations 1, 2 and 3 ２ decreases, and the reactions of formulas 1, 2, and 3 caused by Pt in the second catalytic layer 30 tend to proceed to the right. This promotes methane purification by Pt in the second catalytic layer 30. Therefore, when the second catalytic layer 30 contains Ce, the methane purification performance by Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine is improved.

[0095] The alkaline earth metal element is CO ２ The alkaline earth metal elements in the second catalyst layer 30 trap CO ２ When CO is captured, the CO on the right side of Eqs. 2 and 3 ２ decreases, and the reactions of formulas 2 and 3 caused by Pt in the second catalytic layer 30 tend to proceed to the right. This promotes the purification of methane by Pt in the second catalytic layer 30. Therefore, the inclusion of an alkaline earth metal element in the second catalytic layer 30 improves the methane purification performance of Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine.

[0096] As mentioned above, the oxygen storage capacity of Ce allows H ２ decreases, and alkaline earth metal elements cause CO ２ decreases. ２ or H ２ Even if the reduction in one of the above is sufficient, if the reduction in the other is insufficient, the reactions of formulas 2 and 3 caused by Pt in the second catalytic layer 30 are unlikely to proceed to the right. Therefore, in order to effectively promote methane purification by Pt in the second catalytic layer 30, it is important to balance the content of alkaline earth metal elements in the second catalytic layer 30 and the content of Ce in the second catalytic layer 30. Therefore, in the second catalytic layer 30, the ratio of CeO to the mass of alkaline earth metal elements converted into oxides is ２ The ratio of the converted masses is adjusted to be equal to or greater than 2.0 and equal to or less than 18.0, thereby improving the methane purification performance of Pt in the second catalyst layer 30 during high-speed operation of the internal combustion engine.

[0097] As described above, Ce in the second catalytic layer 30 contributes to improving the methane purification performance during high-speed operation of the internal combustion engine. However, Ce in the second catalytic layer 30 promotes the oxidation of Pt in the second catalytic layer 30, thereby reducing the methane purification performance of Pt in the second catalytic layer 30. Therefore, if the relative amount of Ce to Pt in the second catalytic layer 30 is excessive, the methane purification performance of Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine will be reduced. Therefore, in the second catalytic layer 30, the CeO ratio of Ce to the metal-equivalent mass of Pt is ２The ratio of the converted masses is adjusted to be equal to or greater than 6.0 and equal to or less than 20.0, thereby improving the methane purification performance of Pt in the second catalyst layer 30 during high-speed operation of the internal combustion engine.

[0098] Effects of the third catalytic layer 40 and the fourth catalytic layer 50 When the reaction in formula 1 proceeds to the right, CO is produced. Therefore, CO in the exhaust gas that contacts the second catalytic layer 30 inhibits the reaction in formula 1 from proceeding to the right due to Pt in the second catalytic layer 30. Furthermore, NOx in the exhaust gas that contacts the second catalytic layer 30 promotes the deactivation of alkaline earth metal elements in the second catalytic layer 30 by forming nitrates. Because the third catalytic layer 40 is located upstream of the second catalytic layer 30 and the fourth catalytic layer 50 is located above the second catalytic layer 30, the exhaust gas contacts the third catalytic layer 40 and / or the fourth catalytic layer 50 before contacting the second catalytic layer 30. When the exhaust gas comes into contact with the third catalytic layer 40 and / or the fourth catalytic layer 50, the Rh in the third catalytic layer 40 and / or the Rh in the fourth catalytic layer 50 purifies and removes CO and NOx in the exhaust gas, so that the CO concentration and NOx concentration in the exhaust gas after contact with the third catalytic layer 40 and / or the fourth catalytic layer 50 (i.e., the exhaust gas coming into contact with the second catalytic layer 30) decrease. The decrease in the CO concentration in the exhaust gas coming into contact with the second catalytic layer 30 makes it easier for the reaction of formula 1 caused by Pt in the second catalytic layer 30 to proceed to the right. Furthermore, the decrease in the NOx concentration in the exhaust gas coming into contact with the second catalytic layer 30 prevents the alkaline earth metal elements in the second catalytic layer 30 from forming nitrates and becoming inactivated, and the CO of the alkaline earth metal elements in the second catalytic layer 30 decreases. ２ The trapping ability is effectively exerted. As a result, the reactions of formulas 2 and 3 caused by Pt in the second catalytic layer 30 are more likely to proceed to the right. Therefore, when the third catalytic layer 40 and the fourth catalytic layer 50 each contain Rh, the methane purification performance of Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine is improved.

[0099] The above-described effects of the first catalytic layer 20, the second catalytic layer 30, the third catalytic layer 40, and the fourth catalytic layer 50 are combined to enable the catalyst 1A to exhibit improved methane purification performance during high-speed operation of the internal combustion engine.

[0100] From the viewpoint of further improving the methane purification performance of catalyst 1A, it is preferable that second catalytic layer 30 contains a Ce—Zr-based composite oxide as a Ce source, and that at least a portion of Pt in second catalytic layer 30 is supported on the Ce—Zr-based composite oxide in second catalytic layer 30.

[0101] When Pt is supported on a Ce-Zr composite oxide, H generated by the reactions of Equations 1, 2, and 3 with Pt is ２ and O released due to the oxygen storage capacity of the Ce-Zr composite oxide. ２ Since the Ce-Zr-based composite oxide reacts with the Ce-based composite oxide in the second catalytic layer 30 rapidly, the reactions of formulas 1, 2, and 3 caused by Pt tend to proceed to the right. This promotes methane purification by Pt. Therefore, since the second catalytic layer 30 contains a Ce—Zr-based composite oxide as a Ce source and at least a portion of the Pt in the second catalytic layer 30 is supported on the Ce—Zr-based composite oxide in the second catalytic layer 30, the methane purification performance of the Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine is improved.

[0102] From the viewpoint of further improving the methane purification performance of the catalyst 1A, it is preferable that the third catalyst layer 40 and the fourth catalyst layer 50 each contain Pt in addition to Rh.

[0103] The exhaust gas in contact with the third catalytic layer 40 and the exhaust gas in contact with the fourth catalytic layer 50 have a higher CO concentration than the exhaust gas in contact with the second catalytic layer 30. When the CO concentration in the exhaust gas is relatively high, Pt mainly catalyzes the reaction of formula 2 among the reactions of formulas 1, 2, and 3. Therefore, Pt in the third catalytic layer 40 and Pt in the fourth catalytic layer 50 mainly catalyze the reaction of formula 2 among the reactions of formulas 1, 2, and 3. Pt in the third catalytic layer 40 and Pt in the fourth catalytic layer 50 catalyze the reaction of formula 2 to remove CO from the exhaust gas, thereby reducing the CO concentration in the exhaust gas after contact with the third catalytic layer 40 and / or the fourth catalytic layer 50 (i.e., the exhaust gas in contact with the second catalytic layer 30). The reduced CO concentration in the exhaust gas in contact with the second catalytic layer 30 makes the reaction of formula 1 caused by Pt in the second catalytic layer 30 more likely to proceed to the right. Therefore, by each of the third catalyst layer 40 and the fourth catalyst layer 50 containing Pt in addition to Rh, the methane purification performance of the Pt in the second catalyst layer 30 is improved during high-speed operation of the internal combustion engine.

[0104] H produced by the reaction of Equation 2 with Pt in the third catalyst layer 40 ２ promotes the reduction and purification of NOx by Rh in the third catalyst layer 40, and promotes the reduction and purification of H produced by the reaction of Equation 2 by Pt in the fourth catalyst layer 50. ２ Since the reduction and purification of NOx by Rh in the fourth catalytic layer 50 is promoted, the NOx concentration in the exhaust gas after contact with the third catalytic layer 40 and / or the fourth catalytic layer 50 (i.e., the exhaust gas in contact with the second catalytic layer 30) decreases. The reduction in the NOx concentration in the exhaust gas in contact with the second catalytic layer 30 prevents the alkaline earth metal elements in the second catalytic layer 30 from forming nitrates and becoming inactivated, and the alkaline earth metal elements in the second catalytic layer 30 are prevented from converting to CO ２ The trapping ability is effectively exerted. As a result, the reactions of formulas 2 and 3 caused by Pt in the second catalytic layer 30 are more likely to proceed to the right. Therefore, by each of the third catalytic layer 40 and the fourth catalytic layer 50 containing Pt in addition to Rh, the methane purification performance of Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine is improved.

[0105] From the viewpoint of further improving the methane purification performance of the catalyst 1A, it is preferable that the third catalyst layer 40 and the fourth catalyst layer 50 each contain Ce in addition to Rh and Pt.

[0106] H produced by the reaction of Equation 2 with Pt in the third catalyst layer 40 ２ A part of the H remains without being consumed in the reduction and purification of NOx by Rh in the third catalyst layer 40. ２ hinders the reaction of formula 2 caused by Pt in the third catalyst layer 40 from proceeding to the right. ２ and O released due to the oxygen storage capacity of Ce in the third catalyst layer 40. ２ The reaction is H ２ When O is generated, H on the left side of Eq. ２ As O increases, H on the right side of Equation 2 ２is reduced, and the reaction of formula 2 caused by Pt in the third catalytic layer 40 is more likely to proceed to the right. This promotes the reaction of formula 2 caused by Pt in the third catalytic layer 40. The above explanation regarding the third catalytic layer 40 also applies to the fourth catalytic layer 50. Therefore, when the third catalytic layer 40 and the fourth catalytic layer 50 each contain Ce in addition to Rh and Pt, the CO concentration in the exhaust gas after contact with the third catalytic layer 40 and / or the fourth catalytic layer 50 (i.e., the exhaust gas contacting the second catalytic layer 30) is reduced, and the reaction of formula 1 caused by Pt in the second catalytic layer 30 is more likely to proceed to the right, thereby improving the methane purification performance of Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine.

[0107] From the viewpoint of further improving the methane purification performance of catalyst 1A, it is preferred that the third catalytic layer 40 and the fourth catalytic layer 50 each contain a Ce—Zr-based composite oxide as a Ce source, that at least a portion of the Pt in the third catalytic layer 40 be supported on the Ce—Zr-based composite oxide in the third catalytic layer 40, and that at least a portion of the Pt in the fourth catalytic layer 50 be supported on the Ce—Zr-based composite oxide in the fourth catalytic layer 50.

[0108] When Pt is supported on a Ce-Zr composite oxide, H generated by the reaction of Equation 2 with Pt is ２ and O released due to the oxygen storage capacity of the Ce-Zr composite oxide. ２ and react quickly, so the reactions of formulas 1 to 3 caused by Pt tend to proceed to the right. This promotes the reaction of formula 2 caused by Pt. Therefore, since the third catalytic layer 40 and the fourth catalytic layer 50 each contain a Ce—Zr-based composite oxide as a Ce source, and at least a portion of the Pt in the third catalytic layer 40 is supported on the Ce—Zr-based composite oxide in the third catalytic layer 40, and at least a portion of the Pt in the fourth catalytic layer 50 is supported on the Ce—Zr-based composite oxide in the fourth catalytic layer 50, the CO concentration in the exhaust gas after contact with the third catalytic layer 40 and / or the fourth catalytic layer 50 (i.e., the exhaust gas contacting the second catalytic layer 30) decreases, and the reaction of formula 1 caused by Pt in the second catalytic layer 30 tends to proceed to the right, thereby improving the methane purification performance of the Pt in the second catalytic layer 30 during high-speed operation of the internal combustion engine.

[0109] <Substrate> As shown in FIG. 1, the catalyst 1A includes a substrate 10 .

[0110] As shown in FIGS. 1 and 6, the substrate 10 extends in the exhaust gas flow direction X.

[0111] As shown in FIGS. 1 and 6, the substrate 10 is disposed in the exhaust passage in the exhaust pipe P so that the axial direction of the substrate 10 coincides or substantially coincides with the exhaust gas flow direction X.

[0112] As shown in FIGS. 1 to 6, the substrate 10 has a first substrate portion 10a located upstream in the exhaust gas flow direction X and a second substrate portion 10b located downstream in the exhaust gas flow direction X.

[0113] As shown in FIGS. 1 and 6, the first substrate portion 10a and the second substrate portion 10b each extend in the exhaust gas flow direction X, and have an end portion on the exhaust gas inlet side and an end portion on the exhaust gas outlet side.

[0114] As shown in Figures 1 and 6, the first substrate portion 10a and the second substrate portion 10b are each arranged in the exhaust passage within the exhaust pipe P so that the axial directions of the first substrate portion 10a and the second substrate portion 10b coincide or approximately coincide with the exhaust gas flow direction X.

[0115] As shown in FIGS. 1 and 6, it is preferable that the axial direction of the first substrate portion 10a and the axial direction of the second substrate portion 10b coincide or approximately coincide.

[0116] The first substrate portion 10a and the second substrate portion 10b are integral, and the end of the first substrate portion 10a on the exhaust gas outlet side is continuous with the end of the second substrate portion 10b on the exhaust gas inlet side. In the drawings, the boundary between the end of the first substrate portion 10a on the exhaust gas outlet side and the end of the second substrate portion 10b on the exhaust gas inlet side is indicated by the symbol S.

[0117] Hereinafter, commonalities between the first substrate portion 10a and the second substrate portion 10b will be described together with the description of the first substrate portion 10a and the second substrate portion 10b. In this regard, expressions such as "substrate portion 10a, 10b," "cylindrical portion 11a, 11b," "partition portion 12a, 12b," "cell 13a, 13a," and "length L10a, L10b" will be used. These expressions refer to the "substrate portion 10a," "cylindrical portion 11a," "partition portion 12a," "cell 13a," and "length L10a" for the first substrate portion 10a, and to the "substrate portion 10b," "cylindrical portion 11b," "partition portion 12b," "cell 13b," and "length L10b" for the second substrate portion 10b.

[0118] The material constituting the substrate portions 10a, 10b can be appropriately selected from known materials. Examples of materials constituting the substrate portions 10a, 10b include ceramic materials and metal materials, with ceramic materials being preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as alumina, zirconia, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metal materials include alloys such as stainless steel.

[0119] The material constituting the substrate portion 10a and the material constituting the substrate portion 10b are usually the same.

[0120] 2 to 6, the base members 10a and 10b each have a cylindrical portion 11a and 11b, partition walls 12a and 12b provided in the cylindrical portions 11a and 11b, and cells 13a and 13b separated by the partition walls 12a and 12b. The base members 10a and 10b are preferably honeycomb structures.

[0121] 2 and 3, the cylindrical portions 11a and 11b define the outer shapes of the base portions 10a and 10b, and the axial directions of the cylindrical portions 11a and 11b coincide with the axial directions of the base portions 10a and 10b. As shown in Figures 2 and 3, the cylindrical portions 11a and 11b have a cylindrical shape, but may have other shapes such as an elliptical cylindrical shape or a polygonal cylindrical shape.

[0122] As shown in Figures 2 and 3, the partition walls 12a, 12b are provided inside the cylindrical parts 11a, 11b. As shown in Figures 2 to 6, the partition walls 12a, 12b are present between adjacent cells 13a, 13b, and the adjacent cells 13a, 13b are separated by the partition walls 12a, 12b. The partition walls 12a, 12b may have a porous structure that allows exhaust gas to pass through. The thickness of the partition walls 12a, 12b is, for example, 20 µm or more and 1500 µm or less.

[0123] As shown in Fig. 6, the cells 13a and 13b extend in the exhaust gas flow direction X and have an end on an exhaust gas inlet side and an end on an exhaust gas outlet side. As shown in Fig. 6, both the end on the exhaust gas inlet side and the end on the exhaust gas outlet side of the cells 13a and 13b are open.

[0124] As shown in FIG. 6 , the end (opening) of the cell 13 a on the exhaust gas outflow side and the end (opening) of the cell 13 b on the exhaust gas inflow side are continuous so that one cell 13 a and one cell 13 b form one exhaust gas flow path extending in the exhaust gas flow direction X.

[0125] The exhaust gas that flows into the cell 13a from the end (opening) on ​​the exhaust gas inlet side of the cell 13a flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13a. The exhaust gas that flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13a flows into the cell 13b from the end (opening) on ​​the exhaust gas inlet side of the cell 13b, and flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13b. This type of configuration is called a flow-through type.

[0126] 2 to 5, the end portions (openings) of the cells 13a and 13b on the exhaust gas inlet side have a rectangular shape in plan view, but may have other shapes such as a hexagon, an octagon, etc. The end portions (openings) of the cells 13a and 13b on the exhaust gas outlet side also have the same shape in plan view.

[0127] The cell density per square inch of the substrate parts 10a, 10b is, for example, 100 cells or more and 1000 cells or less. The cell density per square inch of the substrate parts 10a, 10b means the total number of cells 13a, 13b per square inch in a cross section obtained by cutting the substrate parts 10a, 10b along a plane perpendicular to the exhaust gas flow direction X.

[0128] The substrate portions 10a and 10b have lengths L10a and L10b.

[0129] The volume of the substrate parts 10a, 10b is, for example, 0.1 L or more and 20 L or less. The volume of the substrate parts 10a, 10b means the apparent volume of the substrate parts 10a, 10b. For example, if the substrate parts 10a, 10b are cylindrical and the outer diameter of the substrate parts 10a, 10b is 2r, the volume of the substrate parts 10a, 10b can be calculated by the formula: Volume of the substrate parts 10a, 10b = π × r ２ ×(length L10a, L10b).

[0130] The percentage (L10a / (L10a+L10b)×100) of the length L10a of the first substrate portion 10a to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 15% or more and 80% or less, more preferably 18% or more and 70% or less, and even more preferably 20% or more and 60% or less. Each of the above lower limits may be combined with any of the above upper limits. In one example, the percentage (L10a / (L10a+L10b)×100) of the length L10a of the first substrate portion 10a to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is 50%.

[0131] The percentage (L10b / (L10a+L10b)×100) of the length L10b of the second substrate portion 10b to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 20% or more and 85% or less, more preferably 30% or more and 82% or less, and even more preferably 40% or more and 80% or less. Each of the above lower limits may be combined with any of the above upper limits. In one example, the percentage (L10b / (L10a+L10b)×100) of the length L10b of the first substrate portion 10b to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is 50%.

[0132] <First Catalyst Layer> As shown in FIGS. 4 and 6, the catalyst 1A includes a first catalyst layer 20 provided on a first substrate portion 10a.

[0133] 4 and 6 , the first catalytic layer 20 is provided on the cell 13a-side surface of the partition wall portion 12a. The "cell 13a-side surface of the partition wall portion 12a" refers to the outer surface of the partition wall portion 12a that extends in the exhaust gas flow direction X and is in contact with the cells 13a. The first catalytic layer 20 may be provided directly on the cell 13a-side surface of the partition wall portion 12a, or may be provided via another layer. However, the first catalytic layer 20 is typically provided directly on the cell 13a-side surface of the partition wall portion 12a. The "first catalytic layer 20 provided on the first substrate portion 10a" encompasses both an embodiment in which the first catalytic layer 20 is provided directly on the cell 13a-side surface of the partition wall portion 12a, and an embodiment in which the first catalytic layer 20 is provided via another layer on the cell 13a-side surface of the partition wall portion 12a.

[0134] The first catalytic layer 20 may be composed of a portion that protrudes from the cell 13a-side surface of the partition wall portion 12a toward the cell 13a (hereinafter referred to as a "protruding portion"), or may be composed of a portion that exists inside the partition wall portion 12a (hereinafter referred to as an "internal portion"), or may have both a protruding portion and an internal portion. The "first catalytic layer 20 provided on the first substrate portion 10a" includes any of an embodiment in which the first catalytic layer 20 is composed of a protruding portion, an embodiment in which the first catalytic layer 20 is composed of an internal portion, and an embodiment in which the first catalytic layer 20 has both a protruding portion and an internal portion.

[0135] 6 , the first catalytic layer 20 extends from the exhaust gas inlet-side end of the partition wall 12 a to the exhaust gas outlet-side end of the partition wall 12 a along the exhaust gas flow direction X. The first catalytic layer 20 may extend from the exhaust gas inlet-side end of the partition wall 12 a along the exhaust gas flow direction X so as not to reach the exhaust gas outlet-side end of the partition wall 12 a, or may extend from the exhaust gas outlet-side end of the partition wall 12 a in the direction opposite to the exhaust gas flow direction X so as not to reach the exhaust gas inlet-side end of the partition wall 12 a.

[0136] From the viewpoint of achieving a good balance between improving the CO purification performance of the first catalytic layer 20 and suppressing pressure loss due to the first catalytic layer 20, the mass of the first catalytic layer 20 per unit volume of the portion of the first substrate portion 10a where the first catalytic layer 20 is provided (mass after calcination) is preferably 80 g / L or more and 250 g / L or less, more preferably 90 g / L or more and 220 g / L or less, and even more preferably 100 g / L or more and 200 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0137] The mass of the first catalytic layer 20 per unit volume of the portion of the first substrate portion 10a where the first catalytic layer 20 is provided is calculated from the formula: (mass of the first catalytic layer 20) / ((volume of the first substrate portion 10a) × (average length L20 of the first catalytic layer 20 / length L10a of the first substrate portion 10a)).

[0138] An example of a method for measuring the average length L20 of the first catalyst layer 20 is as follows.

[0139] A sample extending in the axial direction of the first substrate portion 10a and having the same length as the length L10a of the first substrate portion 10a is cut out from the first substrate portion 10a. The sample is, for example, cylindrical with a diameter of 25.4 mm. The diameter of the sample can be changed as needed. When the first catalyst layer 20 extends from the end of the partition wall portion 12a on the exhaust gas inlet side along the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the first substrate portion 10a, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas inlet side. When the first catalyst layer 20 extends from the end of the partition wall portion 12a on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the first substrate portion 10a, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas outlet side. In either case, the length of the cut pieces is 5 mm. The composition of the cut pieces is analyzed using an X-ray fluorescence analyzer (XRF) (for example, an energy dispersive X-ray analyzer (EDX), a wavelength dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma atomic emission spectrometer (ICP-AES), a scanning electron microscope-energy dispersive X-ray analysis method (SEM-EDX), etc., and it is confirmed based on the composition of the cut pieces whether or not the cut pieces contain a part of the first catalyst layer 20.

[0140] It is not necessary to perform composition analysis on a cut piece that is clearly containing a portion of the first catalytic layer 20. For example, the cut surface can be observed using a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA), or the like to confirm whether the cut piece contains a portion of the first catalytic layer 20. When observing the cut surface, elemental mapping of the cut surface may be performed.

[0141] After confirming whether the cut pieces include a part of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is calculated based on the following formula: Length of the first catalytic layer 20 included in the sample = 5 mm × (number of cut pieces including a part of the first catalytic layer 20)

[0142] For example, if the first to kth cut pieces include a portion of the first catalytic layer 20, but the (k+1)th to nth cut pieces do not include a portion of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is (5×k) mm.

[0143] A more detailed example of a method for measuring the length of the first catalyst layer 20 included in the sample is as follows.

[0144] The k-th cut piece (of cut pieces including a part of the first catalytic layer 20, the cut piece obtained from the nearest exhaust gas outlet side of the sample when the first catalytic layer 20 extends from the end of the partition wall 12a on the exhaust gas inlet side along the exhaust gas flow direction X, and of cut pieces including a part of the first catalytic layer 20, the cut piece obtained from the nearest exhaust gas inlet side of the sample when the first catalytic layer 20 extends from the end of the partition wall 12a on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X) is cut in the axial direction of the first substrate unit 10a, and the part of the first catalytic layer 20 present on the cut surface is observed using an SEM, an EPMA, or the like, to measure the length of the part of the first catalytic layer 20 in the k-th cut piece. Then, the length of the first catalytic layer 20 included in the sample is calculated based on the following formula: Length of the first catalytic layer 20 included in the sample = (5 mm × (k - 1)) + (Length of the part of the first catalytic layer 20 included in the k-th cut piece)

[0145] For 8 to 16 samples arbitrarily cut out from the catalyst 1A, the length of the first catalyst layer 20 included in each sample is measured, and the average value thereof is taken as the average length L20 of the first catalyst layer 20.

[0146] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20, the percentage (L20 / L10a×100) of the average length L20 of the first catalytic layer 20 to the length L10a of the first substrate portion 10a is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As will be described later, when the exhaust gas outlet side portion of the first catalytic layer 20 overlaps with the exhaust gas inlet side portion of the second catalytic layer 30, the percentage exceeds 100%. When the percentage exceeds 100%, the upper limit of the percentage is preferably 600% or less, more preferably 400% or less, and even more preferably 300% or less. Each of the above lower limits may be combined with any of the above upper limits. In addition, when the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side are in contact, this percentage can exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side, this percentage does not exceed 100%.

[0147] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the percentage (L20 / (L10a+L10b)×100) of the average length L20 of the first catalytic layer 20 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 15% or more and 80% or less, more preferably 18% or more and 70% or less, and even more preferably 20% or more and 60% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0148] The first catalytic layer 20 contains Pd.

[0149] Pd is contained in the first catalytic layer 20 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing Pd, such as metallic Pd, an alloy containing Pd, or a compound containing Pd (e.g., an oxide of Pd). From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the catalytically active component containing Pd is preferably in a particulate form.

[0150] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the mass of Pd in ​​the first catalytic layer 20 in terms of metal is preferably 0.5 g / L or more, more preferably 0.7 g / L or more, and even more preferably 1.0 g / L or more, based on the volume of the portion of the first substrate portion 10a on which the first catalytic layer 20 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 7.0 g / L or less, more preferably 6.0 g / L or less, and even more preferably 5.0 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0151] The metal-equivalent mass of Pd in ​​the first catalytic layer 20, based on the volume of the portion of the first substrate portion 10a where the first catalytic layer 20 is provided, is calculated using the formula: (mass of the first catalytic layer 20 per unit volume of the portion of the first substrate portion 10a where the first catalytic layer 20 is provided) x (metal-equivalent Pd content in the first catalytic layer 20).

[0152] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the content of Pd in ​​the first catalytic layer 20 in terms of metal is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.10 mass% or more, based on the mass of the first catalytic layer 20. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 15 mass% or less, more preferably 12 mass% or less, and even more preferably 10 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0153] The first catalytic layer 20 may contain one or more kinds of noble metal elements other than Pd.

[0154] The precious metal element other than Pd can be selected from, for example, Pt, Rh, Ru, Os, Ir, Au, Ag, etc. The precious metal element other than Pd is contained in the first catalytic layer 20 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing a precious metal element other than Pd, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (for example, an oxide of the precious metal element). From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the catalytically active component containing a precious metal element other than Pd is preferably in a particulate form.

[0155] If the first catalytic layer 20 contains Pd and a precious metal element other than Pd, the Pd and the precious metal element other than Pd may form an alloy, which may reduce the active sites of Pd that contribute to CO purification performance. Therefore, it is preferable that the content of the precious metal element other than Pd in ​​the first catalytic layer 20 in terms of metal is small. Specifically, the content of the precious metal element other than Pd in ​​the first catalytic layer 20 in terms of metal is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total mass of all the precious metal elements in the first catalytic layer 20 in terms of metal. The lower limit is 0% by mass.

[0156] The "metal-equivalent content of precious metal elements other than Pd in ​​the first catalytic layer 20" means, when the first catalytic layer 20 contains one type of precious metal element other than Pd, the metal-equivalent content of that one type of precious metal element, and when the first catalytic layer 20 contains two or more types of precious metal elements other than Pd, the metal-equivalent content of the two or more types of precious metal elements.

[0157] The first catalyst layer 20 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers.

[0158] The phrase "at least a portion of the catalytically active component is supported on a 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 surfaces 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, the third catalyst layer 40, and the fourth catalyst layer 50).

[0159] Whether at least a part of the catalytically active component in a certain catalyst layer is supported on the carrier can be confirmed, for example, by using SEM-EDX etc. Specifically, when at least a part of the catalytically active component and the carrier are present in the same region in elemental mapping obtained by analyzing a cross section of the catalyst layer with SEM-EDX, it can be determined that at least a part of the catalytically active component is supported on the carrier.

[0160] The support can be selected from, for example, inorganic oxides. The inorganic oxide is, for example, particulate. From the viewpoint of improving the supportability of the catalytically active component, the inorganic oxide is preferably porous. The inorganic oxide may or may not have oxygen storage capacity (OSC). The inorganic oxide used as a support is distinguished from the inorganic oxide used as a binder (e.g., inorganic oxide-based binders such as alumina binder, zirconia binder, titania binder, and silica binder).

[0161] Examples of inorganic oxides include Al-based oxides, Ce-based oxides, Ce-Zr-based composite oxides, oxides of rare earth elements other than Ce, and zirconia (ZrO ２ ), silica (SiO ２ ), titania (TiO ２ ), zeolite (aluminosilicate), MgO, ZnO, SnO ２ and the like.

[0162] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the first catalytic layer 20 and thereby further improving the CO conversion performance of the first catalytic layer 20, the support is preferably selected from an Al-based oxide, a Ce-based oxide, and a Ce—Zr-based composite oxide, and more preferably selected from an Al-based oxide and a Ce—Zr-based composite oxide. In one embodiment, the first catalytic layer 20 contains an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0163] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains Ce.

[0164] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20, the Ce in the first catalytic layer 20 is preferably CeO ２ The converted content is preferably 10 mass% or more, more preferably 12 mass% or more, and even more preferably 15 mass% or more, based on the mass of the first catalyst layer 20. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 40 mass% or less, more preferably 35 mass% or less, and even more preferably 30 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0165] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, it is preferable that the first catalytic layer 20 has a large oxygen storage capacity. On the other hand, in order to improve the oxygen storage capacity of the first catalytic layer 20, it is preferable to convert Ce in the first catalytic layer 20 into CeO ２ Increasing the converted Pd content makes the precious metal elements in the first catalytic layer 20 more susceptible to oxidation. Compared to other precious metal elements such as Rh and Pt, Pd has the property of being less susceptible to degradation in CO purification performance even when oxidized. Therefore, by including Pd as a catalytically active component in the first catalytic layer 20, a large oxygen storage capacity can be effectively achieved without deteriorating CO purification performance. Furthermore, when the first catalytic layer 20 has a large oxygen storage capacity, the effect is exerted not only on the first catalytic layer 20 but also on the second catalytic layer 30 located downstream of the first catalytic layer 20, making the exhaust gas that comes into contact with the second catalytic layer 30 suitable for methane purification by the second catalytic layer 30.

[0166] Regarding a certain catalyst layer, "CeO of Ce in the catalyst layer" ２ When the catalyst layer contains one kind of Ce source, the "concentration of CeO converted from Ce derived from the one kind of Ce source" is ２ When the catalyst layer contains two or more Ce sources, the CeO content of Ce derived from the two or more Ce sources is ２ This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, the third catalyst layer 40, and the fourth catalyst layer 50).

[0167] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the ratio of Ce to Pd in ​​terms of metal conversion is set to CeO ２ The ratio of the converted mass is preferably 1 to 100, more preferably 2 to 90, and even more preferably 3 to 80. Each of the above lower limits may be combined with any of the above upper limits.

[0168] When the first catalytic layer 20 contains Ce, the first catalytic layer 20 contains one or more Ce sources.

[0169] Examples of Ce sources include oxides containing Ce, such as Ce-based oxides, Ce-Zr-based composite oxides, Al-based oxides containing Ce, Zr-based oxides containing Ce, and ceria binders.

[0170] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20, the first catalytic layer 20 preferably contains a Ce—Zr-based composite oxide as a Ce source. The first catalytic layer 20 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide.

[0171] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 is preferably 20 mass % or more and 80 mass % or less, more preferably 25 mass % or more and 70 mass % or less, and even more preferably 30 mass % or more and 60 mass % or less, based on the mass of the first catalytic layer 20. Each of the above lower limits may be combined with any of the above upper limits.

[0172] The content of the Ce—Zr-based composite oxide in the first catalytic layer 20 is defined by the formula: (mass of the Ce—Zr-based composite oxide in the first catalytic layer 20) / (mass of the first catalytic layer 20)×100.

[0173] When information (e.g., composition, amount, etc.) about the raw materials used to form the first catalytic layer 20 is known, the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 can be determined from the information about the raw materials used to form the first catalytic layer 20.

[0174] When information about the raw materials used to form the first catalytic layer 20 is unknown, the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 can be determined by a conventional method such as SEM-EDX. Specifically, this is as follows.

[0175] (1) Elemental analysis is performed on the sample obtained from the first catalytic layer 20 using a standard method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the oxide-equivalent content (mass%) of each identified metal element. (2) Elemental mapping is performed on the sample obtained from the first catalytic layer 20 using a standard method such as SEM-EDX to identify the type of particles contained in the sample (e.g., Ce-Zr composite oxide, Al-based oxide, etc.). (3) For each type of particle, elemental analysis is performed on a number of arbitrarily selected particles (e.g., 50 particles) using SEM-EDX to identify the types of constituent elements of the particles and to determine the oxide-equivalent content (mass%) of each identified metal element. For each type of particle, the average oxide-equivalent content (mass%) of each metal element is calculated, and this is defined as the oxide-equivalent content (mass%) of each metal element in each type of particle. (4) An equation is created and solved to represent the relationship between the oxide-equivalent content (mass %) of each metal element in the sample, the oxide-equivalent content (mass %) of each metal element in each type of particle, and the content (mass %) of each type of particle in the sample, thereby calculating the content (mass %) of each type of particle in the sample, and this is set as the content (mass %) of each type of particle in the first catalytic layer 20.

[0176] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20, the Ce in the first catalytic layer 20 is preferably CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0177] Hereinafter, preferred embodiments of the Ce—Zr-based composite oxide that can be contained in the first catalytic layer 20 will be described.

[0178] From the viewpoint of improving the oxygen storage capacity of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２ The converted content is preferably 7% by mass or more, and more preferably 10% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be appropriately adjusted taking into consideration heat resistance, structural stability, the contents of other components, etc. The upper limit is preferably 90% by mass or less, and more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0179] From the viewpoint of improving the heat resistance of the Ce-Zr based composite oxide, the Zr in the Ce-Zr based composite oxide is converted to ZrO. ２ The converted content is preferably 7% by mass or more, and more preferably 10% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be appropriately adjusted taking into consideration the oxygen storage capacity, structural stability, the contents of other components, etc. The upper limit is preferably 90% by mass or less, and more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0180] From the viewpoint of improving the oxygen storage capacity and heat resistance of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２ Conversion rate and ZrO ２ The total content in terms of these elements is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the mass of the Ce—Zr-based composite oxide, with the upper limit being 100% by mass.

[0181] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide, is preferably 0.1 mass % or more and 30 mass % or less, more preferably 0.5 mass % or more and 25 mass % or less, and even more preferably 1 mass % or more and 20 mass % or less, based on the mass of the Ce—Zr-based composite oxide. Each of the above lower limits may be combined with any of the above upper limits.

[0182] The "content of rare earth elements in the Ce—Zr-based composite oxide, in terms of oxides," when the Ce—Zr-based composite oxide contains one rare earth element, means the content of that one rare earth element, in terms of oxides, and when the Ce—Zr-based composite oxide contains two or more rare earth elements, means the total content of those two or more rare earth elements, in terms of oxides. This definition applies to the Ce—Zr-based composite oxide contained in all catalytic layers (i.e., the first catalytic layer 20, the second catalytic layer 30, the third catalytic layer 40, and the fourth catalytic layer 50).

[0183] The first catalytic layer 20 preferably contains an alkaline earth metal element from the viewpoint of improving the catalytic activity of Pd in ​​the first catalytic layer 20 and thereby further improving the CO purification performance of the first catalytic layer 20. The first catalytic layer 20 may contain one type of alkaline earth metal element, or may contain two or more types of alkaline earth metal elements.

[0184] The alkaline earth metal element can be selected from, for example, Mg, Ca, Sr, Ba, etc., but is preferably selected from Ca, Sr, and Ba, and more preferably selected from Sr and Ba.

[0185] From the viewpoint of further improving the CO purification performance of the first catalytic layer 20, the content of alkaline earth metal elements in the first catalytic layer 20, calculated as oxides, is preferably 0.1 mass % or more, more preferably 0.3 mass % or more, and even more preferably 0.5 mass % or more, based on the mass of the first catalytic layer 20. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 20 mass % or less, more preferably 18 mass % or less, and even more preferably 15 mass % or less. Each of the above lower limits may be combined with any of the above upper limits.

[0186] With respect to a certain catalyst layer, the "content of alkaline earth metal elements in the catalyst layer, in terms of oxides," when the catalyst layer contains one alkaline earth metal element, means the content of the one alkaline earth metal element, in terms of oxides, and when the catalyst layer contains two or more alkaline earth metal elements, means the total content of the two or more alkaline earth metal elements, in terms of oxides. This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, the third catalyst layer 40, and the fourth catalyst layer 50).

[0187] With respect to a certain catalyst layer, the "content of alkaline earth metal elements in the catalyst layer, in terms of oxides," when the catalyst layer contains one alkaline earth metal source, means the content of alkaline earth metal elements, in terms of oxides, derived from that one alkaline earth metal source, and when the catalyst layer contains two or more alkaline earth metal sources, means the total content of alkaline earth metal elements, in terms of oxides, derived from the two or more alkaline earth metal sources. This definition applies to all catalyst layers (i.e., the first catalyst layer 20, the second catalyst layer 30, the third catalyst layer 40, and the fourth catalyst layer 50).

[0188] When the first catalyst layer 20 contains an alkaline earth metal element, the first catalyst layer 20 contains one or more alkaline earth metal element sources.

[0189] Examples of alkaline earth metal element sources include alkaline earth metal compounds. The alkaline earth metal compounds are compounds containing alkaline earth metal elements. The alkaline earth metal compounds are preferably selected from oxides, carbonates, and sulfates, and more preferably from oxides and carbonates.

[0190] In element mapping obtained by analyzing the cross section of a catalytic layer with SEM-EDX, if an alkaline earth metal element and an O element are present in the same region, but an alkaline earth metal element and a C element or an S element are not present in the same region, it can be determined that the alkaline earth metal element is present in the form of an oxide in the catalytic layer; if an alkaline earth metal element and an O element and a C element are present in the same region, it can be determined that the alkaline earth metal element is present in the form of a carbonate in the catalytic layer; and if an alkaline earth metal element and an O element and an S element are present in the same region, it can be determined that the alkaline earth metal element is present in the form of a sulfate in the catalytic layer.

[0191] When acetates, nitrates, or hydroxides of alkaline earth metal elements are used as raw materials for the catalyst layer, the acetates, nitrates, or hydroxides are converted into oxides by the calcination process performed to form the catalyst layer, and the oxides are converted into oxides by the CO ２ When carbonates and sulfates of alkaline earth metal elements are used as raw materials for the catalyst layer, the carbonates and sulfates remain as carbonates and sulfates, respectively, even after calcination.

[0192] The first catalyst layer 20 may contain other components such as a binder. Examples of the binder include inorganic oxide binders such as an alumina binder, a ceria binder, a zirconia binder, a titania binder, and a silica binder.

[0193] <Second Catalyst Layer> As shown in FIGS. 5 and 6, the catalyst 1A includes a second catalyst layer 30 provided on the second substrate portion 10b.

[0194] 5 and 6 , the second catalytic layer 30 is provided on the cell 13b-side surface of the partition wall portion 12b. The "cell 13b-side surface of the partition wall portion 12b" refers to the outer surface of the partition wall portion 12b that extends in the exhaust gas flow direction X and is in contact with the cells 13b. The second catalytic layer 30 may be provided directly on the cell 13b-side surface of the partition wall portion 12b, or may be provided via another layer. However, the second catalytic layer 30 is typically provided directly on the cell 13b-side surface of the partition wall portion 12b. The "second catalytic layer 30 provided on the second substrate portion 10b" encompasses both an embodiment in which the second catalytic layer 30 is provided directly on the cell 13b-side surface of the partition wall portion 12b, and an embodiment in which the second catalytic layer 30 is provided via another layer on the cell 13b-side surface of the partition wall portion 12b.

[0195] The second catalytic layer 30 may be composed of a portion that protrudes from the cell 13b-side surface of the partition wall portion 12b toward the cell 13b (hereinafter referred to as a "protruding portion"), or may be composed of a portion that exists inside the partition wall portion 12b (hereinafter referred to as an "internal portion"), or may have both a protruding portion and an internal portion. The "second catalytic layer 30 provided on the second substrate portion 10b" includes any of an embodiment in which the second catalytic layer 30 is composed of a protruding portion, an embodiment in which the second catalytic layer 30 is composed of an internal portion, and an embodiment in which the second catalytic layer 30 has both a protruding portion and an internal portion.

[0196] 6 , the second catalytic layer 30 extends from the exhaust gas outlet end of the partition wall 12b to the exhaust gas inlet end of the partition wall 12b in the direction opposite to the exhaust gas flow direction X. The second catalytic layer 30 may extend from the exhaust gas outlet end of the partition wall 12b in the direction opposite to the exhaust gas flow direction X so as not to reach the exhaust gas inlet end of the partition wall 12b, or may extend from the exhaust gas inlet end of the partition wall 12b in the exhaust gas flow direction X so as not to reach the exhaust gas outlet end of the partition wall 12b.

[0197] As shown in Fig. 6, the end of the first catalytic layer 20 on the exhaust gas outlet side and the end of the second catalytic layer 30 on the exhaust gas inlet side are in contact at a boundary S. As shown in Fig. 6, the boundary S between the end of the first catalytic layer 20 on the exhaust gas outlet side and the end of the second catalytic layer 30 on the exhaust gas inlet side coincides with the boundary S between the end of the first substrate piece 10a on the exhaust gas outlet side and the end of the second substrate piece 10b on the exhaust gas inlet side.

[0198] The exhaust gas outflow side portion of the first catalytic layer 20 may overlap with the exhaust gas inflow side portion of the second catalytic layer 30 (i.e., the exhaust gas outflow side portion of the first catalytic layer 20 may be located above the exhaust gas inflow side portion of the second catalytic layer 30 and cover the exhaust gas inflow side portion of the second catalytic layer 30). Alternatively, the exhaust gas inflow side portion of the second catalytic layer 30 may overlap with the exhaust gas outflow side portion of the first catalytic layer 20 (i.e., the exhaust gas inflow side portion of the second catalytic layer 30 may be located above the exhaust gas outflow side portion of the first catalytic layer 20 and cover the exhaust gas outflow side portion of the first catalytic layer 20). The "exhaust gas outflow side portion of the first catalytic layer 20" refers to a portion of the first catalytic layer 20 that extends from the end of the exhaust gas outflow side along the direction opposite to the exhaust gas flow direction X. The "exhaust gas inflow side portion of the second catalytic layer 30" refers to a portion of the second catalytic layer 30 that extends from the end of the exhaust gas inflow side along the exhaust gas flow direction X.

[0199] The end of the first catalytic layer 20 on the exhaust gas outlet side and the end of the second catalytic layer 30 on the exhaust gas inlet side do not have to be in contact with each other. That is, a gap may be formed between the end of the first catalytic layer 20 on the exhaust gas outlet side and the end of the second catalytic layer 30 on the exhaust gas inlet side.

[0200] From the viewpoint of achieving a good balance between improving the methane purification performance of the second catalytic layer 30 and suppressing pressure loss due to the second catalytic layer 30, the mass of the second catalytic layer 30 per unit volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided (mass after calcination) is preferably 80 g / L or more and 250 g / L or less, more preferably 90 g / L or more and 220 g / L or less, and even more preferably 100 g / L or more and 200 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0201] The mass of the second catalytic layer 30 per unit volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided is calculated using the formula: (mass of the second catalytic layer 30) / ((volume of the second substrate portion 10b) × (average length L30 of the second catalytic layer 30 / length L10b of the second substrate portion 10b)).

[0202] The above description of the method for measuring the average length L20 of the first catalytic layer 20 also applies to the method for measuring the average length L30 of the second catalytic layer 30. When applied, the "first catalytic layer 20" is replaced with the "second catalytic layer 30," and the "average length L20" is replaced with the "average length L30."

[0203] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the percentage (L30 / L10b × 100) of the average length L30 of the second catalytic layer 30 to the length L10b of the second substrate portion 10b is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As described above, when the exhaust gas inlet side portion of the second catalytic layer 30 overlaps with the exhaust gas outlet side portion of the first catalytic layer 20, the percentage exceeds 100%. When the percentage exceeds 100%, the upper limit of the percentage is preferably 500% or less, more preferably 350% or less, and even more preferably 200% or less. Each of the above lower limits may be combined with any of the above upper limits. In addition, when the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side are in contact, this percentage can exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side, this percentage does not exceed 100%.

[0204] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the percentage (L30 / (L10a+L10b)×100) of the average length L30 of the second catalytic layer 30 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 20% or more and 95% or less, more preferably 30% or more and 92% or less, and even more preferably 40% or more and 90% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0205] From the viewpoint of further improving the CO conversion performance of the first catalytic layer 20 and the methane conversion performance of the second catalytic layer 30, the percentage ((L20 + L30) / (L10a + L10b) × 100) of the sum of the average length L20 of the first catalytic layer 20 and the average length L30 of the second catalytic layer 30 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As described above, when the exhaust gas outflow side portion of the first catalytic layer 20 overlaps with the exhaust gas inflow side portion of the second catalytic layer 30, and when the exhaust gas inflow side portion of the second catalytic layer 30 overlaps with the exhaust gas outflow side portion of the first catalytic layer 20, the percentage exceeds 100%. The upper limit of this percentage is preferably 175% or less, more preferably 162% or less, and even more preferably 150% or less. Each of the above lower limits may be combined with any of the above upper limits. When the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b are in contact, this percentage may exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b, this percentage does not exceed 100%.

[0206] The second catalytic layer 30 contains Pt.

[0207] Pt is contained in the second catalytic layer 30 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing Pt, such as metallic Pt, an alloy containing Pt, or a compound containing Pt (e.g., an oxide of Pt). From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the catalytically active component containing Pt is preferably in a particulate form.

[0208] The metal-equivalent mass of Pt in the second catalytic layer 30 is 1.0 g / L or more, based on the volume of the portion of the second substrate 10b where the second catalytic layer 30 is provided. From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the metal-equivalent mass of Pt in the second catalytic layer 30 is preferably 1.1 g / L or more, more preferably 1.2 g / L or more, based on the volume of the portion of the second substrate 10b where the second catalytic layer 30 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 7.0 g / L or less, more preferably 6.0 g / L or less, and even more preferably 5.0 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0209] The mass of Pt in the second catalytic layer 30 in terms of metal, based on the volume of the portion of the second substrate 10b where the second catalytic layer 30 is provided, is calculated from the formula: (mass of the second catalytic layer 30 per unit volume of the portion of the second substrate 10b where the second catalytic layer 30 is provided) × (content of Pt in terms of metal in the second catalytic layer 30). The masses of the other metal elements in the second catalytic layer 30 in terms of oxide, based on the volume of the portion of the second substrate 10b where the second catalytic layer 30 is provided, are calculated in a similar manner.

[0210] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the content of Pt in the second catalytic layer 30 in terms of metal is preferably 0.40 mass% or more, more preferably 0.45 mass% or more, and even more preferably 0.50 mass% or more, based on the mass of the second catalytic layer 30. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 9 mass% or less, more preferably 8 mass% or less, and even more preferably 7 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0211] The second catalytic layer 30 may contain one or more kinds of precious metal elements other than Pt.

[0212] The precious metal element other than Pt can be selected from, for example, Pd, Rh, Ru, Os, Ir, Au, Ag, etc. The precious metal element other than Pt is contained in the second catalytic layer 30 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing a precious metal element other than Pt, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (for example, an oxide of the precious metal element). From the viewpoint of further improving methane purification performance, the catalytically active component containing a precious metal element other than Pt is preferably in a particulate form.

[0213] When the second catalytic layer 30 contains Pt and a precious metal element other than Pt, the Pt and the precious metal element other than Pt may form an alloy, which may reduce the active sites of Pt that contribute to methane purification performance. Therefore, it is preferable that the content of the precious metal element other than Pt in the second catalytic layer 30 in terms of metal is small. Specifically, the content of the precious metal element other than Pt in the second catalytic layer 30 in terms of metal is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total mass of all the precious metal elements in the second catalytic layer 30 in terms of metal. The lower limit is 0% by mass.

[0214] The "metal-equivalent content of precious metal elements other than Pt in the second catalytic layer 30" means the metal-equivalent content of one precious metal element when the second catalytic layer 30 contains one precious metal element other than Pt, and means the metal-equivalent total content of the two or more precious metal elements when the second catalytic layer 30 contains two or more precious metal elements other than Pt.

[0215] The second catalyst layer 30 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers. The carrier may be selected from, for example, inorganic oxides. The description of the inorganic oxide is the same as above.

[0216] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the second catalytic layer 30 and thereby further improving the methane purification performance of the second catalytic layer 30, the support is preferably selected from an Al-based oxide, a Ce-based oxide, and a Ce—Zr-based composite oxide, and more preferably selected from an Al-based oxide and a Ce—Zr-based composite oxide. In one embodiment, the second catalytic layer 30 contains an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0217] The second catalytic layer 30 contains Ce.

[0218] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the Ce in the second catalytic layer 30 is preferably CeO ２ The converted mass is preferably 5 g / L or more, more preferably 8 g / L or more, and even more preferably 10 g / L or more, based on the volume of the portion of the second substrate portion 10b where the second catalytic layer 30 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 90 g / L or less, more preferably 80 g / L or less, and even more preferably 70 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0219] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the Ce in the second catalytic layer 30 is preferably CeO ２The converted content is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more, based on the mass of the second catalyst layer 30. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0220] The second catalyst layer 30 contains one or more Ce sources. The Ce sources are the same as those described above.

[0221] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the second catalytic layer 30 preferably contains a Ce—Zr-based composite oxide as a Ce source. The second catalytic layer 30 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide. In one embodiment, the second catalytic layer 30 contains a Ce—Zr-based composite oxide and a ceria binder as the Ce source.

[0222] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, it is preferable that the second catalytic layer 30 contains a Ce—Zr-based composite oxide as a Ce source, and that at least a portion of the Pt in the second catalytic layer 30 is supported on the Ce—Zr-based composite oxide in the second catalytic layer 30.

[0223] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the percentage of the mass of Pt, in terms of metal, supported on the Ce—Zr-based composite oxide in the second catalytic layer 30 relative to the total mass of all Pt, in terms of metal, in the second catalytic layer 30 (hereinafter referred to as "percentage Q1") is preferably 20 mass% or more, more preferably 30 mass% or more, and even more preferably 40 mass% or more, with the upper limit being 100 mass%.

[0224] The percentage Q1 is calculated from the formula: Q11 / Q12 × 100 (wherein Q11 represents the percentage (mass %) of the mass of Pt supported on the Ce—Zr-based composite oxide in the second catalytic layer 30 in terms of oxide relative to the mass of the second catalytic layer 30, and Q12 represents the percentage (mass %) of the total mass of all Pt in the second catalytic layer 30 in terms of metal relative to the mass of the second catalytic layer 30).

[0225] The method for determining Q11 will be explained below.

[0226] Q11 can be calculated from the following formula: Q11 = content (mass %) of Ce-Zr based composite oxide in the second catalyst layer 30 × ratio A

[0227] The content of the Ce—Zr-based composite oxide in the second catalytic layer 30 can be determined in the same manner as the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0228] The ratio A is determined as follows. (1) Elemental mapping is performed on the second catalytic layer 30 using a standard method such as SEM-EDX to identify the type of particles contained in the second catalytic layer 30 (e.g., Ce—Zr-based composite oxide, a support other than Ce—Zr-based composite oxide, etc.). (2) Elemental analysis is performed on a number of arbitrarily selected Ce—Zr-based composite oxides (e.g., 50 particles) using SEM-EDX, including the metal elements supported on the Ce—Zr-based composite oxide (hereinafter referred to as "supported elements"), to identify the constituent elements of the Ce—Zr-based composite oxide and the type of supported elements, and to determine the oxide-equivalent or metal-equivalent content (mass %) of each identified metal element. Specifically, for precious metal elements, the metal-equivalent content (mass %) of the precious metal element is determined, and for metal elements other than precious metal elements, the oxide-equivalent content (mass %) of the metal element is determined. From the analysis results of a plurality of (for example, 50) Ce—Zr-based composite oxides and supported elements, the average content (mass %) of each metal element in terms of oxide or metal is calculated, and the ratio A is calculated using the following formula: Ratio A = (average content of Pt in terms of metal) / (total average content of all constituent elements in terms of oxide of the Ce—Zr-based composite oxide).

[0229] The method for determining Q12 will be explained below.

[0230] Q12 can be calculated from the following formula: Q12=(total mass of all Pt in metal equivalent) / (mass of second catalyst layer 30)×100

[0231] Q12 can be determined in the same manner as the content of Pt in the second catalyst layer 30 in terms of metal.

[0232] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the mass of the Ce—Zr-based composite oxide in the second catalytic layer 30 is preferably 30 g / L or more, more preferably 35 g / L or more, and even more preferably 40 g / L or more, based on the volume of the portion of the second substrate portion 10b on which the second catalytic layer 30 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 130 g / L or less, more preferably 125 g / L or less, and even more preferably 120 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0233] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the content of the Ce—Zr-based composite oxide in the second catalytic layer 30 is preferably 20 mass % or more and 80 mass % or less, more preferably 25 mass % or more and 70 mass % or less, and even more preferably 30 mass % or more and 60 mass % or less, based on the mass of the second catalytic layer 30. Each of the above lower limits may be combined with any of the above upper limits.

[0234] The content of the Ce—Zr-based composite oxide in the second catalytic layer 30 can be determined in the same manner as the content of the Ce—Zr-based composite oxide in the first catalytic layer 20 .

[0235] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the Ce in the second catalytic layer 30 is preferably CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion in terms of mass is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, with the upper limit being 100% by mass.

[0236] Hereinafter, preferred embodiments of the Ce—Zr-based composite oxide that can be contained in the second catalytic layer 30 will be described.

[0237] From the viewpoint of improving the oxygen storage capacity of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２The converted content is preferably 18% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be adjusted appropriately taking into consideration heat resistance, structural stability, the contents of other components, and the like. The upper limit is preferably 70% by mass or less, more preferably 65% ​​by mass or less, even more preferably 63% by mass or less, and even more preferably 58% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0238] From the viewpoint of improving the heat resistance of the Ce-Zr based composite oxide, the Zr in the Ce-Zr based composite oxide is converted to ZrO. ２ The converted content is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 32% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be adjusted appropriately taking into consideration the oxygen storage capacity, structural stability, the contents of other components, etc. The upper limit is preferably 70% by mass or less, more preferably 65% ​​by mass or less, and even more preferably 62% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0239] From the viewpoint of improving the oxygen storage capacity and heat resistance of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２ Conversion rate and ZrO ２ The total content in terms of these elements is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the mass of the Ce—Zr-based composite oxide, with the upper limit being 100% by mass.

[0240] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide, is preferably 0.1 mass % or more and 30 mass % or less, more preferably 0.5 mass % or more and 25 mass % or less, and even more preferably 1 mass % or more and 20 mass % or less, based on the mass of the Ce—Zr-based composite oxide. Each of the above lower limits may be combined with any of the above upper limits.

[0241] The second catalytic layer 30 contains an alkaline earth metal element. The second catalytic layer 30 may contain one type of alkaline earth metal element, or may contain two or more types of alkaline earth metal elements.

[0242] The alkaline earth metal element can be selected from, for example, Mg, Ca, Sr, Ba, etc., but is preferably selected from Ca, Sr, and Ba, and more preferably selected from Sr and Ba.

[0243] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the mass of the alkaline earth metal element in the second catalytic layer 30, calculated as an oxide, is preferably 2.5 g / L or more, more preferably 3.0 g / L or more, and even more preferably 3.5 g / L or more, based on the volume of the portion of the second substrate portion 10b on which the second catalytic layer 30 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 30 g / L or less, more preferably 25 g / L or less, and even more preferably 20 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0244] From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the content of alkaline earth metal elements in the second catalytic layer 30, calculated as oxides, is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, and even more preferably 0.5 mass% or more, based on the mass of the second catalytic layer 30. The upper limit can be adjusted as appropriate, taking into consideration the balance between methane purification performance and cost, etc. The upper limit is preferably 20 mass% or less, more preferably 18 mass% or less, and even more preferably 15 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0245] The second catalyst layer 30 contains one or more alkaline earth metal element sources. The alkaline earth metal element sources are the same as those described above.

[0246] In the second catalytic layer 30, the ratio of Ce to the mass of Pt converted into metal is CeO ２The ratio of the converted mass is 6.0 or more and 20.0 or less. From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the ratio is preferably 7.0 or more and 19.0 or less, and more preferably 8.0 or more and 18.0 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0247] In the second catalytic layer 30, the ratio of Ce to the mass of the alkaline earth metal element converted into oxide is CeO ２ The ratio of the converted mass is 2.0 or more and 18.0 or less. From the viewpoint of further improving the methane purification performance of the second catalytic layer 30, the ratio is preferably 2.5 or more, and more preferably 3.0 or more. From the same viewpoint, the ratio is preferably 17.0 or less, even more preferably 16.0 or less, even more preferably 15.0 or less, even more preferably 14.0 or less, even more preferably 13.0 or less, and even more preferably 12.0 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0248] The second catalyst layer 30 may contain other components such as a binder. Examples of the binder include inorganic oxide binders such as an alumina binder, a ceria binder, a zirconia binder, a titania binder, and a silica binder.

[0249] <Third and Fourth Catalyst Layers> As shown in FIGS. 4 and 6 , the catalyst 1A includes a third catalyst layer 40 provided on the upper side of the first catalyst layer 20 .

[0250] 6 , the third catalytic layer 40 extends from the end of the partition wall 12 a on the exhaust gas inlet side to the end of the partition wall 12 a on the exhaust gas outlet side along the exhaust gas flow direction X. The third catalytic layer 40 may extend from the end of the partition wall 12 a on the exhaust gas inlet side along the exhaust gas flow direction X so as not to reach the end of the partition wall 12 a on the exhaust gas outlet side, or may extend from the end of the partition wall 12 a on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X so as not to reach the end of the partition wall 12 a on the exhaust gas inlet side.

[0251] The phrase "the third catalytic layer 40 is provided on the upper side of the first catalytic layer 20" means that part or all of the third catalytic layer 40 is present on one of the two main surfaces of the first catalytic layer 20 opposite the main surface on the partition wall section 12a side. The "main surface of the first catalytic layer 20" means the outer surface of the first catalytic layer 20 extending in the exhaust gas flow direction X. The third catalytic layer 40 may be provided directly on the main surface of the first catalytic layer 20 or via another layer, but is usually provided directly on the main surface of the first catalytic layer 20. The third catalytic layer 40 may be provided so as to cover part of the main surface of the first catalytic layer 20 or so as to cover the entire main surface of the first catalytic layer 20. The "third catalytic layer 40 provided on the upper side of the first catalytic layer 20" includes both an embodiment in which the third catalytic layer 40 is provided directly on the main surface of the first catalytic layer 20 and an embodiment in which the third catalytic layer 40 is provided on the main surface of the first catalytic layer 20 via another layer.

[0252] From the viewpoint of achieving a good balance between improving the CO and NOx purification performance of the third catalytic layer 40 and suppressing pressure loss due to the third catalytic layer 40, the mass of the third catalytic layer 40 per unit volume of the portion of the first substrate portion 10a where the third catalytic layer 40 is provided (mass after calcination) is preferably 20 g / L or more and 200 g / L or less, more preferably 30 g / L or more and 150 g / L or less, and even more preferably 40 g / L or more and 120 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0253] The mass of the third catalytic layer 40 per unit volume of the portion of the first substrate portion 10a where the third catalytic layer 40 is provided is calculated using the formula: (mass of the third catalytic layer 40) / ((volume of the first substrate portion 10a) × (average length L40 of the third catalytic layer 40 / length L10a of the first substrate portion 10a)).

[0254] The above description of the method for measuring the average length L20 of the first catalytic layer 20 also applies to the method for measuring the average length L40 of the third catalytic layer 40. When applied, the "first catalytic layer 20" is replaced with the "third catalytic layer 40," and the "average length L20" is replaced with the "average length L40."

[0255] From the viewpoint of further improving the CO and NOx purification performance of the third catalytic layer 40, the percentage (L40 / L10a×100) of the average length L40 of the third catalytic layer 40 relative to the length L10a of the first substrate portion 10a is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As will be described later, when the exhaust gas outlet side portion of the third catalytic layer 40 overlaps with the exhaust gas inlet side portion of the fourth catalytic layer 50, the percentage exceeds 100%. When the percentage exceeds 100%, the upper limit of the percentage is preferably 600% or less, more preferably 400% or less, and even more preferably 300% or less. Each of the above lower limits may be combined with any of the above upper limits. In addition, when the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side are in contact, this percentage can exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side, this percentage does not exceed 100%.

[0256] From the viewpoint of further improving the CO and NOx purification performance of the third catalytic layer 40, the percentage (L40 / (L10a+L10b)×100) of the average length L40 of the third catalytic layer 40 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 15% or more and 80% or less, more preferably 18% or more and 70% or less, and even more preferably 20% or more and 60% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0257] As shown in FIGS. 5 and 6, the catalyst 1A includes a fourth catalyst layer 50 provided on the upper side of the second catalyst layer 30.

[0258] The fourth catalytic layer 50 extends from the end of the partition wall 12b on the exhaust gas outlet side to the end of the partition wall 12b on the exhaust gas inlet side in the direction opposite to the exhaust gas flow direction X. The fourth catalytic layer 50 may extend from the end of the partition wall 12b on the exhaust gas outlet side in the direction opposite to the exhaust gas flow direction X so as not to reach the end of the partition wall 12b on the exhaust gas inlet side, or may extend from the end of the partition wall 12b on the exhaust gas inlet side in the exhaust gas flow direction X so as not to reach the end of the partition wall 12b on the exhaust gas outlet side.

[0259] The phrase "the fourth catalytic layer 50 is provided on the upper side of the second catalytic layer 30" means that part or all of the fourth catalytic layer 50 is present on one of the two main surfaces of the second catalytic layer 30 opposite the main surface on the partition wall section 12b side. The "main surface of the second catalytic layer 30" means the outer surface of the second catalytic layer 30 extending in the exhaust gas flow direction X. The fourth catalytic layer 50 may be provided directly on the main surface of the second catalytic layer 30 or via another layer, but is usually provided directly on the main surface of the second catalytic layer 30. The fourth catalytic layer 50 may be provided so as to cover part of the main surface of the second catalytic layer 30 or so as to cover the entire main surface of the second catalytic layer 30. The "fourth catalytic layer 50 provided on the upper side of the second catalytic layer 30" includes both an embodiment in which the fourth catalytic layer 50 is provided directly on the main surface of the second catalytic layer 30, and an embodiment in which the fourth catalytic layer 50 is provided on the main surface of the second catalytic layer 30 via another layer.

[0260] From the viewpoint of achieving a good balance between improving the CO and NOx purification performance of the fourth catalytic layer 50 and suppressing pressure loss due to the fourth catalytic layer 50, the mass of the fourth catalytic layer 50 per unit volume of the portion of the second substrate portion 10b where the fourth catalytic layer 50 is provided (mass after calcination) is preferably 20 g / L or more and 200 g / L or less, more preferably 30 g / L or more and 150 g / L or less, and even more preferably 40 g / L or more and 120 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0261] The mass of the fourth catalyst layer 50 per unit volume of the portion of the second substrate portion 10b where the fourth catalyst layer 50 is provided is calculated using the formula: (mass of the fourth catalyst layer 50) / ((volume of the second substrate portion 10b) × (average length L50 of the fourth catalyst layer 50 / length L10b of the second substrate portion 10b)).

[0262] The above description regarding the method for measuring the average length L20 of the first catalytic layer 20 also applies to the method for measuring the average length L50 of the fourth catalytic layer 50. When applied, the "first catalytic layer 20" is replaced with the "fourth catalytic layer 50," and the "average length L20" is replaced with the "average length L50."

[0263] From the viewpoint of further improving the CO and NOx purification performance of the fourth catalytic layer 50, the percentage (L50 / L10b × 100) of the average length L50 of the fourth catalytic layer 50 to the length L10b of the second substrate portion 10b is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As will be described later, when the exhaust gas inlet side portion of the fourth catalytic layer 50 overlaps with the exhaust gas outlet side portion of the third catalytic layer 40, the percentage exceeds 100%. When the percentage exceeds 100%, the upper limit of the percentage is preferably 500% or less, more preferably 350% or less, and even more preferably 200% or less. Each of the above lower limits may be combined with any of the above upper limits. In addition, when the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side are in contact, this percentage can exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the end of the first substrate part 10a on the exhaust gas outlet side and the end of the second substrate part 10b on the exhaust gas inlet side, this percentage does not exceed 100%.

[0264] From the viewpoint of further improving the CO and NOx purification performance of the fourth catalytic layer 50, the percentage (L50 / (L10a+L10b)×100) of the average length L50 of the fourth catalytic layer 50 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 20% or more and 95% or less, more preferably 30% or more and 92% or less, and even more preferably 40% or more and 90% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0265] As shown in Fig. 6, the exhaust gas outflow side end of the third catalytic layer 40 and the exhaust gas inflow side end of the fourth catalytic layer 50 are in contact at a boundary S. As shown in Fig. 6, the boundary S between the exhaust gas outflow side end of the third catalytic layer 40 and the exhaust gas inflow side end of the fourth catalytic layer 50 coincides with the boundary S between the exhaust gas outflow side end of the first substrate piece 10a and the exhaust gas inflow side end of the second substrate piece 10b.

[0266] The exhaust gas outlet side portion of the third catalytic layer 40 may overlap with the exhaust gas inlet side portion of the fourth catalytic layer 50 (i.e., the exhaust gas outlet side portion of the third catalytic layer 40 may be located above the exhaust gas inlet side portion of the fourth catalytic layer 50 and cover the exhaust gas inlet side portion of the fourth catalytic layer 50). Alternatively, the exhaust gas inlet side portion of the fourth catalytic layer 50 may overlap with the exhaust gas outlet side portion of the third catalytic layer 40 (i.e., the exhaust gas inlet side portion of the fourth catalytic layer 50 may be located above the exhaust gas outlet side portion of the third catalytic layer 40 and cover the exhaust gas outlet side portion of the third catalytic layer 40). The "exhaust gas outlet side portion of the third catalytic layer 40" refers to a portion of the third catalytic layer 40 that extends from the end of the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X. The "exhaust gas inlet side portion of the fourth catalytic layer 50" refers to a portion of the fourth catalytic layer 50 that extends from the end of the exhaust gas inlet side along the exhaust gas flow direction X.

[0267] When the exhaust gas outlet end of the third catalytic layer 40 and the exhaust gas inlet end of the fourth catalytic layer 50 are in contact with each other at the boundary S, the exhaust gas outlet end of the third catalytic layer 40 and the exhaust gas inlet end of the fourth catalytic layer 50 may be continuous. That is, the third catalytic layer 40 and the fourth catalytic layer 50 may be integrated to form a single catalytic layer.

[0268] The end of the third catalytic layer 40 on the exhaust gas outlet side and the end of the fourth catalytic layer 50 on the exhaust gas inlet side do not have to be in contact with each other. That is, a gap may be formed between the end of the third catalytic layer 40 on the exhaust gas outlet side and the end of the fourth catalytic layer 50 on the exhaust gas inlet side.

[0269] In one embodiment, the third catalytic layer 40 extends along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall 12a to the exhaust gas outlet-side end of the partition wall 12a, and the exhaust gas outlet-side end of the third catalytic layer 40 and the exhaust gas inlet-side end of the fourth catalytic layer 50 are in contact with each other at a boundary S. In this embodiment, the exhaust gas outlet-side end of the third catalytic layer 40 and the exhaust gas inlet-side end of the fourth catalytic layer 50 may be continuous. In this embodiment, the fourth catalytic layer 50 may extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall 12b to the exhaust gas outlet-side end of the partition wall 12b, or may extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall 12b so as not to reach the exhaust gas outlet-side end of the partition wall 12b.

[0270] In another embodiment, the fourth catalytic layer 50 extends in the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12b to the exhaust gas inlet end of the partition wall 12b, and the exhaust gas outlet end of the third catalytic layer 40 and the exhaust gas inlet end of the fourth catalytic layer 50 are in contact with each other at a boundary S. In this embodiment, the exhaust gas outlet end of the third catalytic layer 40 and the exhaust gas inlet end of the fourth catalytic layer 50 may be continuous. In this embodiment, the third catalytic layer 40 may extend in the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12a to the exhaust gas inlet end of the partition wall 12a, or may extend in the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12a so as not to reach the exhaust gas inlet end of the partition wall 12a.

[0271] From the viewpoint of further improving the CO and NOx purification performance of the third catalytic layer 40 and the fourth catalytic layer 50, the percentage ((L40 + L50) / (L10a + L10b) × 100) of the sum of the average length L40 of the third catalytic layer 40 and the average length L50 of the fourth catalytic layer 50 to the sum of the length L10a of the first substrate portion 10a and the length L10b of the second substrate portion 10b is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit may be 100% or more. As described above, when the exhaust gas outlet side portion of the third catalytic layer 40 overlaps with the exhaust gas inlet side portion of the fourth catalytic layer 50, and when the exhaust gas inlet side portion of the fourth catalytic layer 50 overlaps with the exhaust gas outlet side portion of the third catalytic layer 40, the percentage exceeds 100%. The upper limit of this percentage is preferably 175% or less, more preferably 162% or less, and even more preferably 150% or less. Each of the above lower limits may be combined with any of the above upper limits. When the first substrate part 10a and the second substrate part 10b are integral, or when the first substrate part 10a and the second substrate part 10b are separate and the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b are in contact, this percentage may exceed 100%. However, when the first substrate part 10a and the second substrate part 10b are separate and a gap is formed between the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b, this percentage does not exceed 100%.

[0272] Hereinafter, commonalities between the third catalytic layer 40 and the fourth catalytic layer 50 will be described together with the description of the third catalytic layer 40 and the fourth catalytic layer 50. In this regard, expressions such as "catalyst layer 40, 50," "lengths L40, L50," "substrate portions 10a, 10b," and "lengths L10a, L10b" will be used. These expressions mean "catalyst layer 40," "length L40," "substrate portion 10a," and "length L10a" for the third catalytic layer 40, and "catalyst layer 50," "length L50," "substrate portion 10b," and "length L10b" for the fourth catalytic layer 50.

[0273] The catalyst layers 40 and 50 contain Rh.

[0274] Rh is contained in the catalyst layers 40, 50 in a form that can function as a catalytically active component, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40, 50, the catalytically active component containing Rh is preferably in a particulate form.

[0275] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layer 40, 50, the metal-equivalent mass of Rh in the catalyst layer 40, 50 is preferably 0.01 g / L or more, more preferably 0.02 g / L or more, and even more preferably 0.05 g / L or more, based on the volume of the portion of the substrate 10a, 10b on which the catalyst layer 40, 50 is provided. The upper limit can be adjusted appropriately taking into consideration the balance between CO and NOx purification performance and cost, etc. The upper limit is preferably 2.0 g / L or less, more preferably 1.0 g / L or less, and even more preferably 0.5 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0276] The mass of Rh in the catalyst layer 40, 50 in terms of metal, based on the volume of the portion of the base member 10a, 10b where the catalyst layer 40, 50 is provided, is calculated using the formula: (mass of the catalyst layer 40, 50 per unit volume of the portion of the base member 10a, 10b where the catalyst layer 40, 50 is provided) × (content of Rh in terms of metal in the catalyst layer 40, 50).

[0277] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layer 40, 50, the metal-equivalent Rh content in the catalyst layer 40, 50 is preferably 0.005 mass% or more, more preferably 0.010 mass% or more, and even more preferably 0.025 mass% or more, based on the mass of the catalyst layer 40, 50. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO and NOx purification performance and cost, etc. The upper limit is preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 2.5 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0278] From the viewpoint of further improving the CO conversion performance of the catalyst layers 40, 50, the catalyst layers 40, 50 preferably contain Pt.

[0279] Pt is contained in the catalyst layers 40, 50 in a form that can function as a catalytically active component, such as metallic Pt, an alloy containing Pt, or a compound containing Pt (e.g., an oxide of Pt). From the viewpoint of further improving the CO purification performance of the catalyst layers 40, 50, the catalytically active component containing Pt is preferably in a particulate form.

[0280] From the viewpoint of further improving the CO purification performance of the catalyst layer 40, 50, the mass of Pt in the catalyst layer 40, 50 in terms of metal is preferably 0.01 g / L or more, more preferably 0.05 g / L or more, and even more preferably 0.1 g / L or more, based on the volume of the portion of the substrate 10a, 10b where the catalyst layer 40, 50 is provided. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 1.0 g / L or less, more preferably 0.5 g / L or less, and even more preferably 0.3 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0281] The mass of Pt in the catalyst layer 40, 50 in terms of metal, based on the volume of the portion of the substrate 10a, 10b where the catalyst layer 40, 50 is provided, is calculated using the formula: (mass of the catalyst layer 40, 50 per unit volume of the portion of the substrate 10a, 10b where the catalyst layer 40, 50 is provided) x (content of Pt in the catalyst layer 40, 50 in terms of metal).

[0282] From the viewpoint of further improving the CO purification performance of the catalyst layers 40, 50, the content of Pt in the catalyst layers 40, 50 in terms of metal is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.1 mass% or more, based on the mass of the catalyst layers 40, 50. The upper limit can be adjusted as appropriate, taking into consideration the balance between CO purification performance and cost, etc. The upper limit is preferably 1.0 mass% or less, more preferably 0.5 mass% or less, and even more preferably 0.3 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0283] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40, 50, the ratio of the mass of Pt, in terms of metal, to the mass of Rh, in terms of metal, in the catalyst layers 40, 50 is preferably 0.1 or more and 5 or less, more preferably 0.3 or more and 3 or less, and even more preferably 0.5 or more and 1 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0284] The catalyst layers 40, 50 may contain one or more kinds of precious metal elements other than Rh and Pt.

[0285] The precious metal element other than Rh and Pt can be selected from, for example, Pd, Ru, Ir, Os, etc. The precious metal element other than Rh and Pt is contained in the catalyst layer 40, 50 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing a precious metal element other than Rh, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (for example, an oxide of the precious metal element). From the viewpoint of further improving the CO and NOx purification performance of the catalyst layer 40, 50, the catalytically active component containing a precious metal element other than Rh and Pt is preferably in a particulate form.

[0286] The content of the precious metal elements other than Rh and Pt in the catalyst layers 40, 50 in terms of metal is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, based on the total mass of all the precious metal elements in terms of metal in the catalyst layers 40, 50. The lower limit is 0% by mass.

[0287] The "metal-equivalent content of precious metal elements other than Rh and Pt in the catalytic layer 40, 50" means the metal-equivalent content of one precious metal element when the catalytic layer 40, 50 contains one precious metal element other than Rh and Pt, and means the metal-equivalent total content of the two or more precious metal elements when the catalytic layer 40, 50 contains two or more precious metal elements other than Rh and Pt.

[0288] The catalyst layers 40, 50 preferably contain one or more types of carriers, and at least a portion of the catalytically active components is preferably supported on one or more types of carriers.

[0289] The support can be selected from, for example, inorganic oxides, the explanation of which is the same as above.

[0290] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the catalyst layers 40, 50 and thereby further improving the CO and NOx purification performance of the catalyst layers 40, 50, the support is preferably selected from Al-based oxides, Ce-based oxides, and Ce—Zr-based composite oxides, and more preferably selected from Al-based oxides and Ce—Zr-based composite oxides. In one embodiment, the catalyst layers 40, 50 contain an Al-based oxide and a Ce—Zr-based composite oxide as the support.

[0291] From the viewpoint of improving the oxygen storage capacity of the catalyst layers 40, 50 and thereby further improving the CO and NOx purification performance of the catalyst layers 40, 50, the catalyst layers 40, 50 preferably contain Ce.

[0292] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40 and 50, the Ce in the catalyst layers 40 and 50 is preferably CeO ２ The converted mass is preferably 1 g / L or more, more preferably 3 g / L or more, and even more preferably 5 g / L or more, based on the volume of the portion of the substrate portion 10a, 10b where the catalyst layer 40, 50 is provided. The upper limit can be adjusted appropriately taking into consideration the balance between CO and NOx purification performance and cost, etc. The upper limit is preferably 50 g / L or less, more preferably 40 g / L or less, and even more preferably 30 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0293] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40 and 50, the Ce in the catalyst layers 40 and 50 is preferably CeO ２ The converted content is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, and even more preferably 2.0 mass% or more, based on the mass of the catalyst layer 40, 50. The upper limit can be adjusted appropriately taking into consideration the balance between CO and NOx purification performance and cost, etc. The upper limit is preferably 35 mass% or less, more preferably 30 mass% or less, and even more preferably 25 mass% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0294] From the viewpoint of further improving the CO conversion performance of the catalyst layers 40 and 50, the ratio of Ce to the mass of Pt converted into metal in the catalyst layers 40 and 50 is set to CeO ２ The ratio of the converted mass is preferably 0.1 or more and 300 or less, preferably 0.5 or more and 250 or less, and more preferably 1 or more and 200 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0295] Ce has the effect of oxidizing Rh and reducing the CO and NOx purification performance of Rh. From the viewpoint of suppressing the reduction in the CO and NOx purification performance of Rh due to Ce, the CeO ２ The content of Ce in the second catalyst layer 30 converted to CeO ２ The Ce content in the catalyst layers 40 and 50 is preferably smaller than the CeO content. ２ The content of Ce in the second catalyst layer 30 converted into CeO ２ The ratio of the converted amount to the content is preferably 0.9 or less, preferably 0.7 or less, and preferably 0.4 or less. The lower limit can be adjusted appropriately taking into consideration the balance between CO and NOx purification performance and cost, etc. The lower limit is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.03 or more. Each of the above lower limits may be combined with any of the above upper limits.

[0296] When the catalyst layers 40, 50 contain Ce, the catalyst layers 40, 50 contain one or more Ce sources. The Ce sources are the same as those described above.

[0297] The catalyst layers 40, 50 preferably contain a Ce—Zr-based composite oxide as a Ce source from the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40, 50. The catalyst layers 40, 50 may contain one or more other Ce sources in addition to the Ce—Zr-based composite oxide.

[0298] From the viewpoint of further improving the CO purification performance of the catalyst layers 40, 50, it is preferable that the catalyst layers 40, 50 contain a Ce—Zr-based composite oxide as a Ce source, and that at least a portion of the Pt in the catalyst layers 40, 50 be supported on the Ce—Zr-based composite oxide in the catalyst layers 40, 50.

[0299] From the viewpoint of further improving the CO purification performance of the catalyst layers 40, 50, the percentage of the mass of Pt, in terms of metal, supported on the Ce—Zr-based composite oxide in the catalyst layers 40, 50 relative to the mass of all Pt, in terms of metal, in the catalyst layers 40, 50 (hereinafter referred to as "percentage Q2") is preferably 10 mass% or more, more preferably 20 mass% or more, and even more preferably 30 mass% or more. The upper limit is 100 mass%. Percentage Q2 can be determined in the same manner as percentage Q1.

[0300] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40, 50, the content of the Ce—Zr-based composite oxide in the catalyst layers 40, 50 is preferably 5 mass % or more and 80 mass % or less, more preferably 10 mass % or more and 75 mass % or less, and even more preferably 15 mass % or more and 70 mass % or less, based on the mass of the catalyst layers 40, 50. Each of the above lower limits may be combined with any of the above upper limits.

[0301] The content of the Ce—Zr-based composite oxide in the catalyst layers 40 and 50 can be determined in the same manner as the content of the Ce—Zr-based composite oxide in the first catalyst layer 20 .

[0302] From the viewpoint of further improving the CO and NOx purification performance of the catalyst layers 40 and 50, the Ce in the catalyst layers 40 and 50 is preferably CeO ２ Of the converted mass, CeO of Ce derived from Ce-Zr based composite oxide ２ The proportion of the converted mass is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0303] Preferred embodiments of the Ce—Zr-based composite oxide that can be contained in the catalyst layers 40 and 50 will be described below.

[0304] From the viewpoint of improving the oxygen storage capacity of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２The converted content is preferably 7% by mass or more, and more preferably 10% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be appropriately adjusted taking into consideration heat resistance, structural stability, the contents of other components, etc. The upper limit is preferably 90% by mass or less, and more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0305] From the viewpoint of improving the heat resistance of the Ce-Zr based composite oxide, the Zr in the Ce-Zr based composite oxide is converted to ZrO. ２ The converted content is preferably 7% by mass or more, and more preferably 10% by mass or more, based on the mass of the Ce—Zr-based composite oxide. The upper limit can be appropriately adjusted taking into consideration the oxygen storage capacity, structural stability, the contents of other components, etc. The upper limit is preferably 90% by mass or less, and more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0306] From the viewpoint of improving the oxygen storage capacity and heat resistance of the Ce—Zr-based composite oxide, the Ce in the Ce—Zr-based composite oxide is converted to CeO ２ Conversion rate and ZrO ２ The total content in terms of these elements is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the mass of the Ce—Zr-based composite oxide, with the upper limit being 100% by mass.

[0307] From the viewpoint of improving the heat resistance of the Ce—Zr-based composite oxide, the content of the rare earth element in the Ce—Zr-based composite oxide, calculated as an oxide, is preferably 0.1 mass % or more and 30 mass % or less, more preferably 0.5 mass % or more and 25 mass % or less, and even more preferably 1 mass % or more and 20 mass % or less, based on the mass of the Ce—Zr-based composite oxide. Each of the above lower limits may be combined with any of the above upper limits.

[0308] The catalyst layers 40, 50 preferably contain an alkaline earth metal element from the viewpoint of improving the catalytic activity of Rh in the catalyst layers 40, 50 and thereby further improving the CO and NOx purification performance of the catalyst layers 40, 50. The catalyst layers 40, 50 may contain one type of alkaline earth metal element or two or more types of alkaline earth metal elements.

[0309] The alkaline earth metal element can be selected from, for example, Mg, Ca, Sr, Ba, etc., but is preferably selected from Ca, Sr, and Ba, and more preferably selected from Sr and Ba.

[0310] Alkaline earth metal elements have the effect of oxidizing Rh and reducing the CO and NOx purification performance of Rh. From the viewpoint of suppressing the reduction in the CO and NOx purification performance of Rh due to alkaline earth metal elements, the content of alkaline earth metal elements in the catalyst layer 40, 50, calculated as oxides, is preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 3 mass% or less, based on the mass of the catalyst layer 40, 50. The lower limit can be appropriately adjusted taking into consideration the balance between CO and NOx purification performance and cost, etc. The upper limit may be 0 mass% or more. The upper limit may be, for example, 0.05 mass% or more, 0.1 mass% or more, or 0.15 mass% or more. Each of the above lower limits may be combined with any of the above upper limits.

[0311] The oxide-equivalent content of alkaline earth metal elements in the catalyst layers 40 and 50 is CeO of Ce in the first catalyst layer 20. ２ It can be calculated in the same manner as the converted content.

[0312] When the catalyst layers 40, 50 contain alkaline earth metal elements, the catalyst layers 40, 50 contain one or more alkaline earth metal element sources. The alkaline earth metal element sources are the same as those described above.

[0313] The catalyst layers 40, 50 may contain other components such as a binder, for example, an inorganic oxide binder such as an alumina binder, a ceria binder, a zirconia binder, a titania binder, or a silica binder.

[0314] <Manufacture of catalyst> Catalyst 1A can be manufactured by forming a first catalytic layer 20 on the first substrate portion 10a, forming a second catalytic layer 30 on the second substrate portion 10b, forming a third catalytic layer 40 on the first catalytic layer 20, and forming a fourth catalytic layer 50 on the second catalytic layer 30. The first catalytic layer 20 and the second catalytic layer 30 may be formed on the first substrate portion 10a and the second substrate portion 10b, respectively, and then the third catalytic layer 40 and the fourth catalytic layer 50 may be formed on the first catalytic layer 20 and the second catalytic layer 30, respectively. Alternatively, the first catalytic layer 20 may be formed on the first substrate portion 10a, the third catalytic layer 40 may be formed on the first catalytic layer 20, and then the second catalytic layer 30 may be formed on the second substrate portion 10b, and then the fourth catalytic layer 50 may be formed on the second catalytic layer 30.

[0315] The first catalyst layer 20 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Pd) and other components (e.g., an inorganic oxide, a binder, a solvent, etc.) to prepare a first slurry, applying the first slurry onto the first substrate portion 10a, drying, and firing.

[0316] The second catalytic layer 30 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Pt), a source of Ce (e.g., a Ce-based oxide, a Ce-Zr-based composite oxide, a ceria binder, etc.), a source of an alkaline earth metal element (e.g., an acetate, a nitrate, a hydroxide, a carbonate, a sulfate, etc. of an alkaline earth metal element), and other components (e.g., an inorganic oxide other than the Ce source, a binder other than the Ce source, a solvent, etc.), preparing a second slurry, applying the second slurry to the second substrate portion 10b, drying, and firing.

[0317] The third catalytic layer 40 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Rh) and other components (e.g., an inorganic oxide, a binder, a solvent, etc.) to prepare a third slurry, applying the third slurry onto the first catalytic layer 20, drying, and firing.

[0318] The fourth catalytic layer 50 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Rh) and other components (e.g., an inorganic oxide, a binder, a solvent, etc.) to prepare a fourth slurry, applying the fourth slurry onto the second catalytic layer 30, drying, and firing.

[0319] When the end of the third catalytic layer 40 on the exhaust gas outlet side and the end of the fourth catalytic layer 50 on the exhaust gas inlet side are continuous (i.e., when the third catalytic layer 40 and the fourth catalytic layer 50 are integrated to form a single catalytic layer), the fifth slurry can be prepared by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Rh) and other components (e.g., an inorganic oxide, a binder, a solvent, etc.), and then applying the fifth slurry onto the first catalytic layer 20 and the second catalytic layer 30, drying, and firing the fifth slurry.

[0320] Examples of salts of precious metal elements include nitrates, ammine complex salts, acetates, and chlorides. Examples of binders include alumina binders, zirconia binders, titania binders, silica binders, and ceria binders. Examples of solvents include water and organic solvents.

[0321] The drying temperature is, for example, 70° C. to 150° C., and the drying time is, for example, 5 minutes to 1 hour. The firing temperature is, for example, 200° C. to 700° C., and the firing time is, for example, 0.5 hours to 5 hours. The firing can be carried out, for example, in the air.

[0322] Second Embodiment An exhaust gas purifying catalyst 1B (hereinafter referred to as "catalyst 1B") according to a second embodiment will be described below with reference to FIG.

[0323] In catalyst 1B, the same components as those in catalyst 1A are designated by the same reference numerals as those in catalyst 1A. Unless otherwise specified below, the above description of catalyst 1A also applies to catalyst 1B.

[0324] The catalyst 1B differs from the catalyst 1A in that the first substrate portion 10a and the second substrate portion 10b are separate bodies, whereas the catalyst 1A has the first substrate portion 10a and the second substrate portion 10b integrated together.

[0325] The present invention encompasses both an embodiment in which the substrate 10 is composed of one substrate, as in catalyst 1A, and an embodiment in which the substrate 10 is composed of two substrates, as in catalyst 1B.

[0326] As shown in Figure 7, the first substrate portion 10a and the second substrate portion 10b are each arranged in the exhaust passage within the exhaust pipe P so that the axial directions of the first substrate portion 10a and the second substrate portion 10b coincide or approximately coincide with the exhaust gas flow direction X.

[0327] As shown in FIG. 7, the axial direction of the first substrate portion 10a and the axial direction of the second substrate portion 10b may be the same or approximately the same.

[0328] 7, a gap is formed between the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b. The distance between the exhaust gas outlet end of the first substrate part 10a and the exhaust gas inlet end of the second substrate part 10b is, for example, 5 to 1000 mm.

[0329] "The substrate 10 extends in the exhaust gas flow direction X" also includes an embodiment in which a gap is formed between the end of the first substrate portion 10a on the exhaust gas outflow side and the end of the second substrate portion 10b on the exhaust gas inflow side, and in which the axial directions of the first substrate portion 10a and the second substrate portion 10b coincide or approximately coincide with the exhaust gas flow direction X.

[0330] The end of the first substrate part 10a on the exhaust gas outflow side and the end of the second substrate part 10b on the exhaust gas inflow side may be in contact with each other. In this case, the arrangement of the first substrate part 10a and the second substrate part 10b may be the same as that shown in FIG. 6, for example.

[0331] Example 1A (1) Preparation of Slurry for Forming First Catalyst Layer In pure water, 0.9 parts by mass of an aqueous solution of palladium nitrate in terms of Pd metal, 0.9 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), La ２ O ３ Modified alumina (La ２ O ３Modification amount: 44.0 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ The mixture was mixed and stirred to prepare a slurry for forming a first catalyst layer.

[0332] (2) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 20 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as the second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0333] (3) Formation of the first catalytic layer and the second catalytic layer As a substrate, a honeycomb porous substrate made of cordierite (manufactured by Nippon Gaishi Co., Ltd., axial length: 120 mm, outer diameter: 83 mm, volume: 0.65 L, cell density: 600 cells / inch) having cells extending in the axial direction and separated by partition walls with a thickness of 95 to 125 μm was used. ２ ) was prepared.

[0334] A portion of the substrate up to 40 mm from the upstream end in the exhaust gas flow direction X was immersed in the first catalyst layer forming slurry, and the first catalyst layer forming slurry was applied to the upstream side of the substrate. Next, a portion of the substrate up to 80 mm from the downstream end in the exhaust gas flow direction X was immersed in the second catalyst layer forming slurry, and the second catalyst layer forming slurry was applied to the downstream side of the substrate. The substrate coated with the first catalyst layer forming slurry and the second catalyst layer forming slurry was then dried at 90°C and fired at 450°C. This resulted in the formation of the first and second catalyst layers on the substrate without any gaps in the exhaust gas flow direction X (i.e., the exhaust gas outlet end of the first catalyst layer and the exhaust gas inlet end of the second catalyst layer were in contact). The mass of the first catalyst layer per unit volume of the portion of the substrate where the first catalyst layer was provided (washcoat amount) was 170 g / L. The mass of the second catalyst layer per unit volume of the substrate (washcoat amount) on which the second catalyst layer was provided was 170 g / L. The washcoat amount means the mass of the catalyst layer after calcination (the same applies below). Note that the barium hydroxide in the slurry was converted to barium oxide by calcination, and the barium oxide was converted to CO in the air. ２ It reacts with and is converted to barium carbonate.

[0335] (4) Preparation of Slurry for Forming Third and Fourth Catalyst Layers A rhodium nitrate aqueous solution (0.2 parts by mass in terms of Rh metal), a Ce-Zr composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 70% by mass of rare earth elements in terms of oxides thereof, 66.7 parts by mass of one or more rare earth elements other than Ce in terms of oxides thereof, 15% by mass of La ２ O ３ Modified alumina (La ２ O ３ Modification amount: 22.7 parts by mass of 1% by mass of barium hydroxide, 0.3 parts by mass (BaCO ３ The mixture was mixed and stirred to prepare a slurry for forming the third catalyst layer and the fourth catalyst layer.

[0336] (5) Formation of the Third and Fourth Catalytic Layers The substrate on which the first and second catalytic layers were formed was entirely immersed in the slurry for forming the third and fourth catalytic layers, and the slurry for forming the third and fourth catalytic layers was applied to the entire substrate on which the first and second catalytic layers were formed. The substrate on which the slurry for forming the third and fourth catalytic layers was applied was then dried at 90°C and then fired at 450°C. This resulted in the formation of the third and fourth catalytic layers on the first and second catalytic layers, respectively, without any gaps in the exhaust gas flow direction X (i.e., the exhaust gas outlet end of the third catalytic layer was continuous with the exhaust gas inlet end of the fourth catalytic layer, and the third catalytic layer 40 and the fourth catalytic layer 50 were integrated to form a single catalytic layer). The mass (washcoat amount) of the third catalytic layer per unit volume of the portion of the substrate on which the third catalytic layer was formed was 60 g / L. The mass (washcoat amount) of the fourth catalyst layer per unit volume of the portion of the substrate where the fourth catalyst layer was provided was 60 g / L.

[0337] In this manner, an exhaust gas purification catalyst of Example 1A was produced, which included a substrate, a first catalytic layer provided on the upstream side of the substrate, a second catalytic layer provided on the downstream side of the substrate, a third catalytic layer provided above the first catalytic layer, and a fourth catalytic layer provided above the second catalytic layer.

[0338] In the first catalyst layer of the exhaust gas purifying catalyst of Example 1A, the mass of Pd in ​​terms of metal was 1.5 g / L based on the volume of the portion of the substrate where the first catalyst layer was provided.

[0339] In the second catalyst layer of the exhaust gas purifying catalyst of Example 1A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 17.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 7.0. ２ The ratio of the converted masses was 2.2.

[0340] In the third catalyst layer of the exhaust gas purifying catalyst of Example 1A, the mass of Rh in terms of metal was 0.1 g / L based on the volume of the portion of the substrate where the third catalyst layer was provided.

[0341] In the fourth catalyst layer of the exhaust gas purifying catalyst of Example 1A, the mass of Rh in terms of metal was 0.1 g / L based on the volume of the portion of the substrate where the fourth catalyst layer was provided.

[0342] Example 2A (A) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 30 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0343] An exhaust gas purifying catalyst of Example 2A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (A) above was used.

[0344] In the second catalyst layer of the exhaust gas purifying catalyst of Example 2A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 24.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 9.8. ２ The ratio of the converted masses was 3.1.

[0345] [Example 3A] (B) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0346] An exhaust gas purifying catalyst of Example 3A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (B) above was used.

[0347] In the second catalyst layer of the exhaust gas purifying catalyst of Example 3A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 31.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ce to the mass of Ba in terms of BaO is 12.6. ２ The converted mass ratio was 4.0.

[0348] Example 4A (C) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 50 mass% ZrO ２40 mass% of rare earth elements in terms of oxides other than Ce, 41.2 mass parts of La as the second carrier, 10 mass% of rare earth elements in terms of oxides other than Ce, ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0349] An exhaust gas purifying catalyst of Example 4A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (C) above was used.

[0350] In the second catalyst layer of the exhaust gas purifying catalyst of Example 4A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 38.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 15.4. ２ The converted mass ratio was 4.9.

[0351] Example 5A (D) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 55 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0352] An exhaust gas purifying catalyst of Example 5A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (D) above was used.

[0353] In the second catalyst layer of the exhaust gas purifying catalyst of Example 5A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 41.9 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ce to the mass of Ba in terms of BaO is 16.8. ２ The converted mass ratio was 5.4.

[0354] Example 6A (E) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 60 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as the second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0355] An exhaust gas purifying catalyst of Example 6A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (E) above was used.

[0356] In the second catalyst layer of the exhaust gas purifying catalyst of Example 6A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ In the second catalytic layer of the exhaust gas purifying catalyst of Example 6A, the ratio of Ce to the mass of Pt in metal equivalent and the mass of Ba in BaO equivalent were 2.5 g / L, 45.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 18.2. ２ The converted mass ratio was 5.8.

[0357] [Example 7A] (F) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion rate: 65 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0358] An exhaust gas purifying catalyst of Example 7A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (F) above was used.

[0359] In the second catalyst layer of the exhaust gas purifying catalyst of Example 7A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 48.9 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ce to the mass of Ba in terms of BaO is 19.6. ２ The converted mass ratio was 6.3.

[0360] [Example 8A] (G) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 30 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium sulfate (BaSO ) ４ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0361] An exhaust gas purifying catalyst of Example 8A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (G) above was used.

[0362] In the second catalyst layer of the exhaust gas purifying catalyst of Example 8A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 24.4 g / L, and 6.6 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 9.8.２ The converted mass ratio was 3.7.

[0363] [Example 9A] (H) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 30.9 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 53.7 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0364] An exhaust gas purifying catalyst of Example 9A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (H) above was used.

[0365] In the second catalyst layer of the exhaust gas purifying catalyst of Example 9A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 24.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 9.8. ２ The ratio of the converted masses was 3.1.

[0366] Example 10A (I) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 40 mass% ZrO２ 50 mass% of rare earth elements in terms of oxides other than Ce, 58.8 mass% of rare earth elements in terms of oxides other than Ce, 10 mass% of rare earth elements in terms of oxides other than Ce, ２ O ３ Modified alumina (La ２ O ３ Modification amount: 25.8 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0367] An exhaust gas purifying catalyst of Example 10A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (I) above was used.

[0368] In the second catalyst layer of the exhaust gas purifying catalyst of Example 10A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 43.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 17.4. ２ The converted mass ratio was 5.6.

[0369] Example 11A (J) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 50 mass% of rare earth elements in terms of oxides other than Ce, 59.9 mass% of rare earth elements in terms of oxides other than Ce, 10 mass% of rare earth elements in terms of oxides other than Ce, ２ O ３ Modified alumina (La ２ O ３Modification amount: 26.3 parts by mass of 1% by mass of barium hydroxide, 3.3 parts by mass (BaCO ３ 4.2 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.1 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0370] An exhaust gas purifying catalyst of Example 11A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (J) above was used and the washcoat amount of the second catalyst layer was set to 167 g / L.

[0371] In the second catalyst layer of the exhaust gas purifying catalyst of Example 11A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 43.4 g / L, and 5.4 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 17.4. ２ The converted mass ratio was 8.0.

[0372] Example 12A (K) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 60.6 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 26.6 parts by mass of 1% by mass of barium hydroxide, 2.4 parts by mass (BaCO ３ 3.0 parts by mass in solids equivalent), 2.1 parts by mass of ceria binder in solids equivalent, and 6.2 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0373] An exhaust gas purifying catalyst of Example 12A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (K) above was used and the washcoat amount of the second catalyst layer was set to 165 g / L.

[0374] In the second catalyst layer of the exhaust gas purifying catalyst of Example 12A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ In the second catalytic layer of the exhaust gas purifying catalyst of Example 12A, the ratio of Ce to the mass of Pt in metal equivalent and the mass of Ba in BaO equivalent were 2.5 g / L, 43.4 g / L, and 3.9 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 17.4. ２ The converted mass ratio was 11.2.

[0375] Comparative Example 1A (L) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Zr-based oxide (ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0376] An exhaust gas purifying catalyst of Comparative Example 1A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (L) above was used.

[0377] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 1A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 3.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce to the mass of Ba in terms of BaO is 1.4. ２ The converted mass ratio was 0.4.

[0378] Comparative Example 2A (M) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0379] An exhaust gas purifying catalyst of Comparative Example 2A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (M) above was used.

[0380] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 2A, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 13.9 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 5.6.２ The converted mass ratio was 1.8.

[0381] Comparative Example 3A (N) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 80 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0382] An exhaust gas purifying catalyst of Comparative Example 3A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (N) above was used.

[0383] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 3A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of Pt metal and the mass of Ba in terms of BaO were 2.5 g / L, 59.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 23.8. ２ The ratio of the converted masses was 7.6.

[0384] Comparative Example 4A (O) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce-based oxide (CeO ２41.2 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 5% by mass; ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0385] An exhaust gas purifying catalyst of Comparative Example 4A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (O) above was used.

[0386] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 4A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 69.9 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 28.0. ２ The converted mass ratio was 9.0.

[0387] Comparative Example 5A (P) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce-based oxide (CeO ２ Concentration of La as the second carrier: 41.2 parts by mass (100% by mass) ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.4 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0388] An exhaust gas purifying catalyst of Comparative Example 5A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (P) above was used.

[0389] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 5A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of Pt metal and the mass of Ba in terms of BaO were 2.5 g / L, 73.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 29.4. ２ The converted mass ratio was 9.4.

[0390] [Comparative Example 6A] (Q) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.6 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.6 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 30 mass% ZrO ２ 43.8 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Amount of modification: 1% by mass: 46.1 parts by mass, 2.1 parts by mass of ceria binder in terms of solid matter, and 6.4 parts by mass of Ce-free binder in terms of solid matter were added, mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0391] An exhaust gas purifying catalyst of Comparative Example 6A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalytic layer produced in (Q) above was used and the washcoat amount of the second catalytic layer was set to 160 g / L.

[0392] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 6A, the mass of Pt converted into metal and the mass of CeO ２ The masses of Ce in terms of metal, based on the volume of the portion of the substrate where the second catalytic layer was provided, were 2.5 g / L and 24.4 g / L, respectively. ２ The converted mass ratio was 9.8.

[0393] Comparative Example 7A (R) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal and 1.5 parts by mass of La as the second carrier were dissolved. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 84.6 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass in terms of BaO (BaCO ３ The slurry for forming the second catalyst layer was prepared by adding 5.9 parts by mass, calculated as a solid content, 2.0 parts by mass of a ceria binder, and 6.0 parts by mass, calculated as a solid content, of a Ce-free binder, and mixing and stirring the mixture. Note that the slurry for forming the second catalyst layer does not contain any of the Ce-based oxides, Ce-Zr-based composite oxides, or Zr-based oxides that correspond to the first support.

[0394] An exhaust gas purifying catalyst of Comparative Example 7A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (R) above was used.

[0395] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 7A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 3.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce to the mass of Ba in terms of BaO is 1.4. ２ The converted mass ratio was 0.4.

[0396] Comparative Example 8A (S) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 15.4 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass; ２ O ３ Modified alumina (La ２ O ３ Modification amount: 69.2 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass of barium hydroxide (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0397] An exhaust gas purifying catalyst of Comparative Example 8A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (S) above was used.

[0398] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 8A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 13.9 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 5.6. ２ The converted mass ratio was 1.8.

[0399] Comparative Example 9A (T) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２82.4 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 2.2 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass of barium hydroxide (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0400] An exhaust gas purifying catalyst of Comparative Example 9A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in (T) above was used.

[0401] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 9A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 59.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate on which the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 23.8. ２ The ratio of the converted masses was 7.6.

[0402] Comparative Example 10A (U) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.5 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.5 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 61.3 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 27.0 parts by mass of 1% by mass of barium hydroxide, 1.4 parts by mass (BaCO ３1.8 parts by mass in solids equivalent), 2.1 parts by mass of ceria binder in solids equivalent, and 6.3 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0403] An exhaust gas purifying catalyst of Comparative Example 10A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalytic layer produced in (U) above was used and the washcoat amount of the second catalytic layer was set to 163 g / L.

[0404] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 10A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 43.4 g / L, and 2.3 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 17.4. ２ The converted mass ratio was 18.6.

[0405] Comparative Example 11A (V) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.6 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.6 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 62.1 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 27.3 parts by mass of 1% by mass of barium hydroxide, 0.5 parts by mass (BaCO ３ 0.6 parts by mass in solids equivalent), 2.1 parts by mass of ceria binder in solids equivalent, and 6.3 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0406] An exhaust gas purifying catalyst of Comparative Example 11A was produced in the same manner as in Example 1A, except that the slurry for forming the second catalytic layer produced in (V) above was used and the washcoat amount of the second catalytic layer was set to 161 g / L.

[0407] In the second catalyst layer of the exhaust gas purifying catalyst of Comparative Example 11A, the mass of Pt converted into metal, the mass of CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.5 g / L, 43.4 g / L, and 0.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ba in terms of BaO to the mass of Ce in terms of CeO is 17.4. ２ The converted mass ratio was 55.9.

[0408] Example 1B The same exhaust gas purifying catalyst as in Example 3A was used as the exhaust gas purifying catalyst in Example 1B.

[0409] [Example 2B] (W) Preparation of Slurry for Forming Second Catalyst Layer In pure water, 1.4 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 1.4 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 41.2 parts by mass of one or more rare earth elements other than Ce (in terms of oxides): 10% by mass), and La as a second carrier. ２ O ３ Modified alumina (La ２ O ３ Modification amount: 43.5 parts by mass of 1% by mass of barium hydroxide, 4.6 parts by mass (BaCO ３ 5.9 parts by mass in solids equivalent), 2.0 parts by mass of ceria binder in solids equivalent, and 6.0 parts by mass of Ce-free binder in solids equivalent were added, and the mixture was mixed and stirred to prepare a slurry for forming a second catalyst layer.

[0410] (X) Preparation of Slurry for Forming Third and Fourth Catalyst Layers In pure water, 0.2 parts by mass of a rhodium nitrate aqueous solution in terms of Rh metal, 0.2 parts by mass of a dinitrodiamine platinum (II) solution in terms of Pt metal, and 0.2 parts by mass of a Ce-Zr based composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 70% by mass of rare earth elements in terms of oxides thereof, 66.7 parts by mass of one or more rare earth elements other than Ce in terms of oxides thereof, 15% by mass of La ２ O ３ Modified alumina (La ２ O ３ Modification amount: 22.5 parts by mass of 1% by mass of barium hydroxide, 0.3 parts by mass (BaCO ３ The mixture was mixed and stirred to prepare a slurry for forming the third catalyst layer and the fourth catalyst layer.

[0411] An exhaust gas purifying catalyst of Example 2B was produced in the same manner as in Example 1A, except that the slurry for forming the second catalyst layer produced in the above (W) and the slurries for forming the third catalyst layer and the fourth catalyst layer produced in the above (X) were used.

[0412] In the second catalyst layer of the exhaust gas purifying catalyst of Example 2B, the mass of Pt converted into metal, the mass of Ce converted into CeO ２ The mass of Ce in terms of metal and the mass of Ba in terms of BaO were 2.4 g / L, 31.4 g / L, and 7.8 g / L, respectively, based on the volume of the portion of the substrate where the second catalytic layer was provided. ２ The ratio of the mass of Ce in terms of BaO to the mass of Ba in terms of CeO is 13.1. ２ The converted mass ratio was 4.0.

[0413] In the third catalytic layer of the exhaust gas purifying catalyst of Example 2B, the mass of Rh in terms of metal and the mass of Pt in terms of metal were 0.1 g / L and 0.1 g / L, respectively, based on the volume of the portion of the substrate on which the third catalytic layer was provided. The percentage Q2 in the third catalytic layer was 67.2 mass%.

[0414] In the fourth catalytic layer of the exhaust gas purifying catalyst of Example 2B, the mass of Rh in terms of metal and the mass of Pt in terms of metal were 0.1 g / L and 0.1 g / L, respectively, based on the volume of the portion of the substrate where the fourth catalytic layer was provided. The percentage Q2 in the fourth catalytic layer was 67.2 mass%.

[0415] Comparative Example 1B The same exhaust gas purifying catalyst as in Comparative Example 6A was used as the exhaust gas purifying catalyst in Comparative Example 1B.

[0416] The characteristics of the first catalytic layer, the third catalytic layer, and the fourth catalytic layer of each exhaust gas purification catalyst are shown in Tables 1A and 1B. In Tables 1A and 1B, "WC" represents the mass (washcoat amount) of each catalytic layer per unit volume of the portion of the substrate on which each catalytic layer is provided, "Pd content" represents the metal-equivalent content of Pd in ​​the first catalytic layer based on the mass of the first catalytic layer, "Rh content" represents the metal-equivalent content of Rh in the third catalytic layer or the fourth catalytic layer based on the mass of the third catalytic layer or the fourth catalytic layer, and "Pt content" represents the metal-equivalent content of Pt in the third catalytic layer or the fourth catalytic layer based on the mass of the third catalytic layer or the fourth catalytic layer.

[0417] The characteristics of the second catalytic layer of each exhaust gas purifying catalyst are shown in Tables 2A, B, and C. In Tables 2A, B, and C, "WC" represents the mass (wash coat amount) of the second catalytic layer per unit volume of the portion of the substrate where the second catalytic layer is provided, "Pt" represents the mass of Pt in the second catalytic layer converted into metal based on the volume of the portion of the substrate where the second catalytic layer is provided, and "CeO ２ " is the ratio of Ce in the second catalytic layer to CeO in the base material, based on the volume of the portion of the base material on which the second catalytic layer is provided. ２ The BaO-equivalent mass, "BaO", is the BaO-equivalent mass of Ba in the second catalyst layer, based on the volume of the portion of the substrate on which the second catalyst layer is provided, "CeO ２ / Pt" is the ratio of CeO of Ce in the second catalyst layer to the mass of Pt in the second catalyst layer in terms of metal. ２ The ratio of the converted masses, "CeO ２ / BaO" is the ratio of the mass of Ce in the second catalyst layer to the mass of Ba in the second catalyst layer converted into BaO. ２The ratio of the converted mass, the "type" of the "first support" refers to the type of oxide (Ce-Zr based composite oxide, Ce based oxide or Zr based oxide) used as the first support, and the "CeO ２ The "CeO content" is the percentage of Ce in the first support based on the mass of the first support. ２ The "quantity" of the "first support" represents the content of the first support in the second catalytic layer based on the mass of the second catalytic layer, and the "quantity" of the "ceria binder" represents the content of the ceria binder in the second catalytic layer based on the mass of the second catalytic layer. In the "type" of the "first support," "CZ" represents a Ce-Zr-based composite oxide, "Ce" represents a Ce-based oxide, and "Zr" represents a Zr-based oxide.

[0418] [Test Example 1] (1) Durability Treatment Each of the exhaust gas purification catalysts of Examples 1A to 12A and Comparative Examples 1A to 11A was mounted in an exhaust pipe, and this exhaust pipe was attached to a gasoline engine. The engine speed / torque, etc. were adjusted so that the catalyst temperature would reach 950°C, and a durability treatment was carried out for 50 hours.

[0419] (2) Evaluation of model gas purification performance After durability treatment, the exhaust gas purification catalyst was cut into a size of 12 cells x 12 cells x 120 mm, placed in a pipe, and subjected to an exhaust model gas (CO 0.26 vol%, O ２ 0.10vol%, NO 900volppm, CH ４ 700volppm, CO ２ 4.4 vol%, H ２ O 10 vol%, N ２ The mixture (the remainder) was circulated at a space velocity of 100,000 / h, while the temperature was raised to 600°C at a rate of 30°C / min and maintained at this temperature for 10 minutes. The amount of methane contained in the gas flowing out from the outlet of the pipe was detected using an engine exhaust gas measuring device MEXA7100 manufactured by Horiba, Ltd., and the methane purification rate was calculated based on the following formula. In the formula, X represents the amount of methane detected when no catalyst was installed, and Y represents the amount of methane detected when a catalyst was installed. Methane purification rate (%) = (X - Y) / X x 100

[0420] The measurement results of the methane purification rate are shown in Tables 2A and 2B.

[0421] Test Example 2 (1) Durability Treatment The exhaust gas purifying catalysts of Examples 1B to 2B and Comparative Example 1B were subjected to durability treatment in the same manner as in Test Example 1 (1).

[0422] (2) Evaluation of Purification Performance on Actual Vehicle The exhaust gas purification catalyst after durability treatment was installed in a vehicle. The vehicle was driven under the operating conditions of a Modified Indian Driving Cycle (MIDC). The emissions of methane and nitrogen oxides (NOx) in the exhaust gas that passed through the exhaust gas purification catalyst were measured, and the methane and NOx emissions per unit driving distance (mg / km) were calculated. A compressed natural gas (CNG) vehicle was used as the vehicle. The methane emissions (mg / km) and NOx emissions (mg / km) per unit driving distance during the test mode are shown in Table 2C.

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[0424]

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[0427]

[0428] DESCRIPTION OF SYMBOLS 1A, 1B... Catalyst for purifying exhaust gas 10... Substrate 10a... First substrate portion 10b... Second substrate portion 11a, 11b... Cylindrical portion 12a, 12b... Partition wall portion 13a, 13b... Cell 20... First catalyst layer 30... Second catalyst layer 40... Third catalyst layer 50... Fourth catalyst layer L10a... Length of first substrate portion L10b... Length of second substrate portion L20... Average length of first catalyst layer L30... Average length of second catalyst layer L40... Average length of third catalyst layer L50... Average length of fourth catalyst layer P... Exhaust pipe of internal combustion engine

Claims

1. An exhaust gas purifying catalyst comprising: a substrate extending in an exhaust gas flow direction, the substrate having a first substrate portion located upstream in the exhaust gas flow direction and a second substrate portion located downstream in the exhaust gas flow direction; a first catalyst layer provided on the first substrate portion; a second catalyst layer provided on the second substrate portion; a third catalyst layer provided above the first catalyst layer and / or a fourth catalyst layer provided above the second catalyst layer, wherein the first catalyst layer contains Pd, the second catalyst layer contains Pt, Ce and an alkaline earth metal element, the third catalyst layer and the fourth catalyst layer each contain Rh, the mass of Pt in the second catalyst layer in terms of metal is 1.0 g / L or more based on the volume of the portion of the second substrate portion where the second catalyst layer is provided, and the ratio of Ce to the mass of Pt in terms of metal in the second catalyst layer is CeO ２ The ratio of the mass of Ce to the mass of the alkaline earth metal element in terms of oxide is 6.0 or more and 20.0 or less, and in the second catalyst layer, ２ The exhaust gas purifying catalyst as described above, wherein the converted mass ratio is 2.0 or more and 18.0 or less.

2. The exhaust gas purifying catalyst according to claim 1, wherein the second catalyst layer contains a Ce-Zr based composite oxide as a Ce source.

3. CeO of Ce in the Ce-Zr based composite oxide ２ 3. The exhaust gas purifying catalyst according to claim 2, wherein the content in terms of the amount of the Ce-Zr based composite oxide is 18 mass % or more and 70 mass % or less based on the mass of the Ce-Zr based composite oxide.

4. CeO of Ce in the Ce-Zr based composite oxide ２ 4. The exhaust gas purifying catalyst according to claim 3, wherein the content in terms of the converted amount is 25% by mass or more and 63% by mass or less based on the mass of the Ce-Zr based composite oxide.

5. A catalyst for purifying exhaust gas as described in claim 2, wherein the content of the Ce-Zr based composite oxide in the second catalyst layer is 20 mass % or more and 80 mass % or less, based on the mass of the second catalyst layer.

6. The exhaust gas purifying catalyst according to claim 2, wherein at least a portion of the Pt in said second catalytic layer is supported on said Ce-Zr based composite oxide in said second catalytic layer.

7. The exhaust gas purification catalyst according to any one of claims 1 to 6, wherein the second catalyst layer contains an alkaline earth metal compound as an alkaline earth metal element source, and the alkaline earth metal compound is selected from the group consisting of oxides, carbonates and sulfates.

8. The exhaust gas purifying catalyst according to any one of claims 1 to 6, wherein the second catalyst layer contains one or more alkaline earth metal elements selected from the group consisting of Ca, Sr and Ba.

9. The exhaust gas purifying catalyst according to any one of claims 1 to 6, wherein the third catalyst layer and the fourth catalyst layer each contain Pt.

10. The exhaust gas purifying catalyst according to claim 9, wherein the third catalyst layer and the fourth catalyst layer each contain Ce.

11. The exhaust gas purifying catalyst according to claim 10, wherein the third catalyst layer and the fourth catalyst layer each contain a Ce-Zr based composite oxide as a Ce source.

12. An exhaust gas purifying catalyst according to claim 11, wherein at least a portion of the Pt in the third catalytic layer is supported on the Ce-Zr composite oxide in the third catalytic layer, and at least a portion of the Pt in the fourth catalytic layer is supported on the Ce-Zr composite oxide in the fourth catalytic layer.

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