Exhaust gas purification catalyst

Abstract

The purpose of the present invention is to provide an exhaust gas purification catalyst in which Rh and La are used in combination, the exhaust gas purification catalyst being capable of preventing any reduction in Rh exhaust gas purification performance (particularly NOx purification performance) resulting from the La while effectively manifesting the action of the La. In order to achieve this purpose, provided is an exhaust gas purification catalyst (1) comprising a base material (10), a first catalyst layer (20) provided on the upstream side of the base material (10), and a second catalyst layer (30) provided on the downstream side of the base material (10), wherein: the first catalyst layer (20) is provided with a lower layer (21) containing Pd, and an upper layer (22) containing Rh; the second catalyst layer (30) is provided with a lower layer (31) containing Pt, and an upper layer (32) containing Rh; the La content of the first catalyst layer (20) in terms of La2O3 is 5 mass% or higher with reference to the mass of the first catalyst layer (20); and the La content of the second catalyst layer (30) in terms of La2O3 is 3 mass% or lower with reference to the mass of the second catalyst layer (30).

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, and the like contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Exhaust gas purification catalysts containing precious metal elements such as Pt, Pd, and Rh are used to purify and neutralize these harmful components. Pt and Pd are primarily involved in the oxidation purification of HC and CO, while Rh is primarily involved in the reduction purification of NOx.

[0003] La is used as a component of the catalyst layer of an exhaust gas purification catalyst (for example, Patent Documents 1 and 2).

[0004] Due to the increasing demand for exhaust gas purification performance in recent years, there is a demand for improved exhaust gas purification performance of exhaust gas purification catalysts.

[0005] JP 2011-167631 A JP 2011-183317 A

[0006] La has the effect of improving heat resistance, so by increasing the La content, it is possible to improve heat resistance.

[0007] Since the exhaust gas purification performance (particularly, NOx purification performance) of Rh is reduced by the oxidation of Rh, it is preferable to use Rh under a reducing atmosphere. However, La has the effect of promoting the oxidation of precious metal elements. Therefore, in an exhaust gas purification catalyst using Rh and La in combination, if the La content is increased, there is a concern that Rh will be oxidized by La, and the exhaust gas purification performance (particularly, NOx purification performance) of Rh will be reduced.

[0008] An object of the present invention is to provide an exhaust gas purification catalyst that uses Rh and La in combination, and that can prevent a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh due to La while effectively exhibiting the action of La.

[0009] Another object of the present invention is to provide an exhaust gas purification system that uses Rh and La in combination, which can effectively exhibit the action of La while preventing a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh due to La.

[0010] The present invention provides the following inventions: [1] A catalyst for purifying exhaust gas, comprising: a substrate; a first catalytic layer provided on the upstream side of the substrate; and a second catalytic layer provided on the downstream side of the substrate, wherein the first catalytic layer comprises a lower layer containing Pd and an upper layer containing Rh, the second catalytic layer comprises a lower layer containing Pt and an upper layer containing Rh, and the La in the first catalytic layer is La. ２ O ３ the La content in the second catalytic layer is 5% by mass or more based on the mass of the first catalytic layer, ２ O ３ [2] A catalyst for purifying exhaust gas, wherein the content of precious metal elements other than Pt in the lower layer of the second catalytic layer, in terms of metal, is 1 mass % or less, based on the mass of the lower layer of the second catalytic layer, and the content of La in the lower layer of the second catalytic layer is 1 mass % or less, based on the mass of the lower layer of the second catalytic layer. ２ O ３ [3] The catalyst for purifying exhaust gas according to [1], wherein the La content in the upper layer of the first catalyst layer is 2 mass % or less based on the mass of the lower layer in the second catalyst layer. ２ O ３ the La content in the upper layer of the second catalytic layer is 5 mass % or more based on the mass of the upper layer of the first catalytic layer, ２ O ３[4] An exhaust gas purifying system comprising: an exhaust passage through which exhaust gas flows; a first exhaust gas purifying catalyst provided on the upstream side of the exhaust passage; and a second exhaust gas purifying catalyst provided on the downstream side of the exhaust passage, wherein the first exhaust gas purifying catalyst comprises a first substrate and a first catalyst layer provided on the first substrate, the second exhaust gas purifying catalyst comprises a second substrate and a second catalyst layer provided on the second substrate, the first catalyst layer comprises a lower layer containing Pd and an upper layer containing Rh, the second catalyst layer comprises a lower layer containing Pt and an upper layer containing Rh, and the La content in the first catalyst layer is 4% by mass or less, based on the mass of the upper layer of the second catalyst layer. ２ O ３ the La content in the second catalytic layer is 5% by mass or more based on the mass of the first catalytic layer, ２ O ３ [5] An exhaust gas purification system, wherein the content of precious metal elements other than Pt in the lower layer of the second catalytic layer, in terms of metal, is 3 mass% or less based on the mass of the second catalytic layer. ２ O ３ [6] The exhaust gas purification system according to [4], wherein the La content in the upper layer of the first catalyst layer is 2 mass % or less based on the mass of the lower layer in the second catalyst layer. ２ O ３ the La content in the upper layer of the second catalytic layer is 5 mass % or more based on the mass of the upper layer of the first catalytic layer, ２ O ３ The exhaust gas purification system according to [4] or [5], wherein the converted content is 4 mass% or less based on the mass of the upper layer of the second catalyst layer.

[0011] The exhaust gas purification catalyst and exhaust gas purification system of the present invention can effectively exhibit the action of La, while preventing a decrease in the exhaust gas purification performance (particularly NOx purification performance) of Rh due to La. Therefore, the exhaust gas purification catalyst and exhaust gas purification system of the present invention can effectively achieve an improvement in exhaust gas purification performance.

[0012] Fig. 1 is a partial end view showing a state in which an exhaust gas purification catalyst according to one embodiment of the present invention is 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 a plan view of an exhaust gas purification system according to one embodiment of the present invention. Fig. 8 is an end view taken along line D-D in Fig. 7. Fig. 9 is an end view taken along line E-E in Fig. 7.

[0013] <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 catalyst layers apply to all catalyst layers (e.g., the first catalyst layer 20, the second catalyst layer 30, the lower layer 21, the upper layer 22, the lower layer 31, the upper layer 32, etc., which will be described later).

[0014] <Metallic Elements> The term "metallic elements" also includes metalloid elements such as Si and B.

[0015] <Rare Earth Elements> "Rare earth elements" include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0016] <Noble Metal Elements> "Noble metal elements" include Pt, Pd, Rh, Ru, Os, Ir, Au, and Ag.

[0017] <Oxide> The meaning of "oxide" of a metal element 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 ６O １１ The oxide of Tb is Tb ４ O ７ The oxide of Al is Al ２ O ３ Zr oxide is ZrO ２ The oxide of Si is SiO ２ The oxide of B is B ２ O ３ Cr oxide is Cr ２ O ３ , Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe ３ O ４ Mn oxide is Mn ３ O ４ Ni oxide is NiO, Ti oxide is TiO ２ Zn oxide is ZnO, Sn oxide is SnO ２ means.

[0018] <Mass of catalytic layer> The "mass of catalytic layer" refers to the sum of all metal elements contained in the catalytic layer, classified into precious metal elements and metal elements other than precious metal elements, the mass of precious metal elements calculated in metal equivalent, and the mass of metal elements other than precious metal elements calculated in oxide equivalent. In other words, the "mass of catalytic layer" refers to the calculated mass calculated by summing the mass of precious metal elements contained in the catalytic layer calculated in metal equivalent and the mass of metal elements other than precious metal elements contained in the catalytic layer calculated in oxide equivalent.

[0019] <Concentration of precious metal elements in catalytic layer in terms of metal and concentration of metal elements other than precious metal elements in catalytic layer in terms of oxide> The "concentration of precious metal elements in catalytic layer in terms of metal" (mass%) is calculated from the formula: (mass of the precious metal elements in catalytic layer in terms of metal) / (mass of catalytic layer) × 100. The "mass of precious metal elements in terms of metal" refers to the mass of the metal calculated assuming that the precious metal element exists as a metal composed of the precious metal element. For example, the "mass of Pd in ​​terms of metal, the mass of Pt in terms of metal, and the mass of Rh in terms of metal refer to the mass of metallic Pd, the mass of metallic Pt, and the mass of metallic Rh, respectively. The meaning of the "mass of catalytic layer" is as described above.

[0020] The "content of a metal element in a catalytic layer in terms of its oxide" (mass %) is calculated from the formula: (mass of the metal element in the catalytic layer in terms of its oxide) / (mass of the catalytic layer) x 100. The "mass of a metal element other than a noble metal element in terms of its oxide" refers to the mass of the oxide calculated on the assumption that a metal element other than a noble metal element exists as an oxide of the metal element. The meanings of "oxide" of a metal element and "mass of the catalytic layer" are as described above.

[0021] For example, "La in the catalyst layer ２ O ３ The La content (mass%) in the catalyst layer is calculated by the formula: ２ O ３ The La content can be calculated by dividing the La content by the mass of the catalyst layer by the converted La content of the catalyst layer. ２ O ３ The "converted mass" is La to La ２ O ３ La is calculated assuming that it exists as ２ O ３ means the mass of

[0022] When information (e.g., composition, amount, etc.) of the raw materials used to form the catalytic layer is known, the metal-equivalent content (mass %) of the precious metal elements in the catalytic layer and the oxide-equivalent content (mass %) of the metal elements other than the precious metal elements in the catalytic layer can be determined from the information on the raw materials.

[0023] When information on the raw materials used to form the catalytic layer is unknown, the metal-equivalent content (mass %) of the precious metal elements in the catalytic layer and the oxide-equivalent content (mass %) of the metal elements other than the precious metal elements in the catalytic layer can be determined by a common method such as scanning electron microscope-energy dispersive X-ray analysis (SEM-EDX).

[0024] 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.

[0025] For each precious metal element in the catalyst layer, the V value is calculated using the following formula: V value = (mol % of each precious metal element in the catalyst layer) x (molar mass of each precious metal element)

[0026] For each metal element other than the noble metal element in the catalyst layer, the W value is calculated using the following formula: W value = (mol % of each metal element other than the noble metal element in the catalyst layer) x (molar mass of the oxide of each metal element)

[0027] The metal-equivalent content (mass%) of each precious metal element in the catalyst layer is calculated using the following formula: Metal-equivalent content (mass%) of each precious metal element in the catalyst layer = (V value for each precious metal element) / {(total V values ​​for all precious metal elements) + (total W values ​​for all metal elements other than precious metal elements)} × 100

[0028] The content (mass %) of each metal element other than precious metal elements in the catalyst layer in terms of oxide is calculated by the following formula: Content (mass %) of each metal element other than precious metal elements in the catalyst layer in terms of oxide = (W value of each metal element other than precious metal elements) / {(total V values ​​of all precious metal elements) + (total W values ​​of all metal elements other than precious metal elements)} × 100

[0029] <Al-based oxide> Al-based oxide refers to an oxide containing Al, in which Al is the metal element with the highest content by mass of the metal elements constituting the oxide. However, Ce-Zr-based composite oxides are not considered to be Al-based oxides. Ce-Zr-based composite oxides will be described later. Al-based oxides are distinguished from alumina used as a binder. In this specification, alumina used as a binder may be referred to as an "alumina binder."

[0030] 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.

[0031] Al-based oxides generally have higher heat resistance than other inorganic oxides (e.g., Ce-based oxides, Ce—Zr-based composite oxides, etc.), 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.

[0032] 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.

[0033] In the Al-based oxide, the additional element M1 is a solid solution phase (e.g., Al ２ O ３ The additional element M1 may form a solid solution phase (a solid solution phase of the additional element M1 and an oxide of the additional element M1), or may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of the additional element M1), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of the additional element M1 forms a solid solution phase.

[0034] Examples of methods for preparing an Al-based oxide containing a solid solution phase include a coprecipitation method and a solid phase method.

[0035] Examples of Al-based oxides include alumina (oxide composed of Al and O), oxides obtained by modifying the surface of alumina with an additional element M1 or an oxide thereof, and oxides obtained by dissolving the additional element M1 or an oxide thereof in alumina. Examples of Al-based oxides containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-ceria, and alumina-lanthana.

[0036] 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.

[0037] From the viewpoint of improving the heat resistance of the Al-based oxide, the Al-based oxide preferably contains La.

[0038] In the Al-based oxide containing La, at least a part of La is in a solid solution phase (for example, Al ２ O ３ and La ２ O ３ Although the detailed mechanism is not clear, it is believed that the formation of a solid solution phase of at least a part of La increases crystal strain, thereby improving the heat resistance of the Al-based oxide.

[0039] From the viewpoint of improving the heat resistance of the Al-based oxide, the La of La forming a solid solution phase in the Al-based oxide is ２ O ３ The converted content is preferably 0.1 mass % or more and 20 mass % or less, more preferably 0.5 mass % or more and 15 mass % or less, and even more preferably 1 mass % or more and 10 mass % or less, based on the mass of the Al-based oxide.

[0040] The content of each metal element in the Al-based oxide in terms of oxide is calculated from the formula: (mass of each metal element in the Al-based oxide in terms of oxide) / (mass of the Al-based oxide) x 100. The mass of the Al-based oxide means the total mass of oxides of the metal elements 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.

[0041] When the composition of the Al-based oxide is known, the content of each element in the Al-based oxide calculated as an oxide can be determined from the composition of the Al-based oxide.

[0042] 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 particles are subjected to composition analysis (elemental analysis), thereby determining the content of each metal element in the designated particles in terms of oxide.

[0043] Whether or not at least a portion of La in an Al-based oxide containing La forms a solid solution phase can be determined as follows. First, EDX elemental analysis is performed on the target Al-based oxide to confirm the presence of La in the target Al-based oxide. Next, an XRD (X-ray diffraction measurement device) is used to measure the XRD diffraction pattern of the target Al-based oxide. Next, the obtained XRD diffraction pattern is compared with reference data on the crystal structure of the target Al-based oxide (e.g., γ-alumina, θ-alumina, δ-alumina, α-alumina, etc.) recorded in the ICSD (International Crystal Structure Database). At this time, attention is paid to the maximum peak position of the reference data, and if there is a peak shift to a lower angle side relative to the maximum peak position in the XRD diffraction pattern of the target Al-based oxide, it can be determined that at least a portion of La forms a solid solution phase in the target Al-based oxide. If all of La forms a solid solution phase in the target Al-based oxide, La ２ O ３ Therefore, when determining whether or not all of La forms a solid solution phase in the target Al-based oxide, the peaks due to La ２ O ３ It is preferable to check whether a peak derived from appears in the XRD diffraction pattern.

[0044] Based on the amount of La present by EDX elemental analysis and the degree of the peak shift, the La content of La forming a solid solution phase in the target Al-based oxide is determined. ２ O ３Furthermore, XPS (X-ray photoelectron spectroscopy) measurement is performed on the target Al-based oxide to determine the LaAlO content. ３ It is also possible to obtain semi-quantitative values ​​by analyzing the peaks assigned to La. In this way, it is possible to determine the La content of La that forms a solid solution phase in the target Al-based oxide. ２ O ３ The converted content can be determined by comprehensively considering the results of analysis such as XRD, EDX, and XPS.

[0045] <Ce-based oxide> 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 Ce-Zr-based composite oxide is not considered to fall under the category of Ce-based oxide. Ce-Zr-based composite oxides will be described later. Ce-based oxides are distinguished from ceria used as a binder. In this specification, ceria used as a binder may be referred to as a "ceria binder."

[0046] 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.

[0047] Ce-based oxides have oxygen storage capacity and reduce fluctuations in oxygen concentration in exhaust gases, widening the operating window of catalytically active components. Therefore, by including Ce-based oxides in the catalyst layer, the exhaust gas purification performance of the catalyst layer is improved.

[0048] 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, alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0049] 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.

[0050] Examples of methods for preparing a Ce-based oxide containing a solid solution phase include a coprecipitation method and a solid phase method.

[0051] Examples of Ce-based oxides include ceria (an oxide composed of Ce and O), oxides obtained by modifying the surface of ceria with an additional element M2 or an oxide thereof, and oxides obtained by dissolving an additional element M2 or an oxide thereof in ceria.

[0052] <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.

[0053] 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.

[0054] 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 ability of the catalyst layer is improved.

[0055] The Ce—Zr-based composite oxide may contain one or more metal elements other than Ce and Zr (hereinafter referred to as “additional element M3”). The additional element M3 can be selected from, for example, rare earth elements other than Ce, alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0056] 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.

[0057] 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.

[0058] When the Ce—Zr-based composite oxide contains the additional element M3, the additional element M3 forms a solid solution phase (for example, CeO ２ and a solid solution phase of the oxide of the additional element M3, ZrO ２ and a solid solution phase of the oxide of the additional element M3, CeO ２ and ZrO ２ The additional element M3 may form a solid solution phase (e.g., 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 (e.g., a single phase of an oxide 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.

[0059] Examples of methods for preparing a Ce-Zr based composite oxide containing a solid solution phase include a coprecipitation method and a solid phase method.

[0060] Examples of Ce-Zr based composite oxides include CeO ２ -ZrO ２ solid solution, CeO ２ -ZrO ２ An oxide obtained by modifying the surface of the solid solution with an additional element M3 or its oxide, CeO ２ -ZrO ２Examples of the oxide include an oxide obtained by dissolving the additional element M3 or its oxide in a solid solution.

[0061] 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 5% by mass or more and 80% by mass or less, more preferably 10% by mass or more and 60% by mass or less, and even more preferably 10% by mass or more and 50% by mass or less, based on the mass of the Ce-Zr based composite oxide.

[0062] 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 10% by mass or more and 95% by mass or less, more preferably 30% by mass or more and 90% by mass or less, and even more preferably 40% by mass or more and 90% by mass or less, based on the mass of the Ce-Zr based composite oxide.

[0063] 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 ２ Conversion rate and ZrO ２ The total content in terms of these elements is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the mass of the Ce—Zr-based composite oxide, with the upper limit being 100% by mass.

[0064] From the viewpoint of improving the heat resistance and oxygen storage capacity of the Ce—Zr based composite oxide, the Ce—Zr based composite oxide preferably contains La.

[0065] In the Ce—Zr-based composite oxide containing La, at least a part of La is in a solid solution phase (for example, CeO ２ and La ２ O ３ Solid solution phase with ZrO ２ and La ２ O ３ Solid solution phase with CeO ２ and ZrO ２ and La ２ O ３Although the detailed mechanism is not clear, it is believed that the formation of a solid solution phase of at least a portion of La increases crystal distortion, improving the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide.

[0066] From the viewpoint of improving the heat resistance and oxygen storage capacity of the Ce-Zr based composite oxide, the La that forms a solid solution phase in the Ce-Zr based composite oxide is ２ O ３ The converted content is preferably 0.1 mass % or more and 20 mass % or less, more preferably 0.5 mass % or more and 15 mass % or less, and even more preferably 1 mass % or more and 10 mass % or less, based on the mass of the Ce—Zr-based composite oxide.

[0067] The content of each metal element in the Ce—Zr-based composite oxide in terms of oxide is calculated from the formula: (mass of each metal element in the Ce—Zr-based composite oxide in terms of oxide) / (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 found on the assumption that each metal element in the Ce—Zr-based composite oxide exists as an oxide. The meaning of the "oxide" of the metal element is as described above.

[0068] 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.

[0069] Whether or not at least a part of La forms a solid solution phase in a Ce—Zr-based composite oxide containing La can be determined in the same manner as whether or not at least a part of La forms a solid solution phase in an Al-based oxide containing La. When comparing the XRD diffraction pattern of the target Ce—Zr-based composite oxide with the reference data for the crystal structure of the target Ce—Zr-based composite oxide, the fluorite-type CeO recorded in ICSD as the reference data for the crystal structure of the target Ce—Zr-based composite oxide can be used. ２ -ZrO ２ , pyrochlore-type CeO ２ -ZrO ２The data of La forming a solid solution phase in Ce-Zr based composite oxides can be used. ２ O ３ The converted content is the La content of La that forms a solid solution phase in the Al-based oxide. ２ O ３ It can be calculated in the same manner as the converted content.

[0070] <<Catalyst for Purifying Exhaust Gas>> Hereinafter, a catalyst 1 for purifying exhaust gas (hereinafter referred to as "catalyst 1") according to one embodiment of the present invention will be described with reference to FIGS.

[0071] As shown in Figure 1, a catalyst 1 is disposed in an exhaust passage in an exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas emitted from the internal combustion engine flows through the exhaust passage in the exhaust pipe P from one end to the other end of the exhaust pipe P and is purified by the catalyst 1 provided in the exhaust pipe P. In the drawing, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas inflow side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outflow side" or "downstream side."

[0072] In the exhaust passage in the exhaust pipe P, other exhaust gas purifying catalysts may be arranged upstream or downstream of the catalyst 1 .

[0073] As shown in FIGS. 2 to 6, the catalyst 1 includes a substrate 10, a first catalyst layer 20 provided on the upstream side of the substrate 10, and a second catalyst layer 30 provided on the downstream side of the substrate 10.

[0074] 4 and 6 , the first catalytic layer 20 includes a lower layer 21 provided on the substrate 10 and an upper layer 22 provided on the lower layer 21. The lower layer 21 contains Pd, and the upper layer 22 contains Rh. The entire first catalytic layer 20 may be composed of the lower layer 21 and the upper layer 22, but it is not essential that the entire first catalytic layer 20 be composed of the lower layer 21 and the upper layer 22. A portion of the first catalytic layer 20 may be composed of either the lower layer 21 or the upper layer 22. Therefore, in addition to the portion composed of the lower layer 21 and the upper layer 22, the portion composed of either the lower layer 21 or the upper layer 22 also constitutes part of the first catalytic layer 20.

[0075] 5 and 6 , the second catalytic layer 30 includes a lower layer 31 provided on the substrate 10 and an upper layer 32 provided on the lower layer 31. The lower layer 31 contains Pt, and the upper layer 32 contains Rh. The entire second catalytic layer 30 may be composed of the lower layer 31 and the upper layer 32, but it is not essential that the entire second catalytic layer 30 be composed of the lower layer 31 and the upper layer 32. A portion of the second catalytic layer 30 may be composed of either the lower layer 31 or the upper layer 32. Therefore, in addition to the portion composed of the lower layer 31 and the upper layer 32, the portion composed of either the lower layer 31 or the upper layer 32 also constitutes part of the second catalytic layer 30.

[0076] In this specification, the lower layer means a layer located closer to the partition wall portion 12 than the upper layer.

[0077] Catalyst 1 is La in the first catalyst layer 20 ２ O ３ The La content in the second catalytic layer 30 is 5 mass % or more based on the mass of the first catalytic layer 20, and ２ O ３ The converted content is characterized by being 3 mass % or less based on the mass of the second catalyst layer 30.

[0078] In catalyst 1, the first catalytic layer 20 is provided on the upstream side of the substrate 10, and the second catalytic layer 30 is provided on the downstream side of the substrate 10. Therefore, the exhaust gas contacts the first catalytic layer 20 and then the second catalytic layer 30. Therefore, the temperature of the exhaust gas contacting the first catalytic layer 20 is higher than the temperature of the exhaust gas contacting the second catalytic layer 30. Therefore, the first catalytic layer 20 is required to have higher heat resistance than the second catalytic layer 30.

[0079] La has the effect of improving heat resistance. Therefore, the La content of La in the first catalyst layer 20 is ２ O ３ By increasing the converted content, the heat resistance of the first catalyst layer 20 can be improved.

[0080] Since the exhaust gas purification performance (especially NOx purification performance) of Rh is reduced by oxidation of Rh, it is preferable to use Rh in a reducing atmosphere. On the other hand, La has the effect of promoting the oxidation of precious metal elements. Therefore, the La content of La in the first catalytic layer 20 is ２ O ３ If the converted content is increased, there is a concern that the Rh in the first catalytic layer 20 will be oxidized by the La in the first catalytic layer 20, and the exhaust gas purification performance (particularly, NOx purification performance) of the Rh in the first catalytic layer 20 will decrease. However, since the first catalytic layer 20 is provided on the upstream side of the substrate 10, the amount of reducing components (e.g., hydrocarbons (HC), carbon monoxide (CO), etc.) in the exhaust gas that contacts the first catalytic layer 20 will fluctuate and temporarily increase due to fluctuations in the operating conditions of the internal combustion engine. Therefore, even if the Rh in the first catalytic layer 20 is oxidized by the La in the first catalytic layer 20, the oxidized Rh will be reduced by the temporarily increased reducing components. Therefore, the amount of La in the first catalytic layer 20 will be reduced. ２ O ３ Even if the converted content is increased, it is possible to prevent a decrease in the exhaust gas purification performance (particularly NOx purification performance) of Rh in the first catalyst layer 20 due to La in the first catalyst layer 20 .

[0081] Because the exhaust gas comes into contact with the second catalytic layer 30 after coming into contact with the first catalytic layer 20, the amount of reducing components in the exhaust gas that comes into contact with the second catalytic layer 30 is unlikely to fluctuate or temporarily increase. Therefore, when Rh in the second catalytic layer 30 is oxidized by La in the second catalytic layer 30, the oxidized Rh is unlikely to be reduced. On the other hand, because the exhaust gas comes into contact with the second catalytic layer 30 after coming into contact with the first catalytic layer 20, the temperature of the exhaust gas that comes into contact with the second catalytic layer 30 is lower than the temperature of the exhaust gas that comes into contact with the first catalytic layer 20. Therefore, the heat resistance of the second catalytic layer 30 may be lower than that of the first catalytic layer 20.

[0082] As described above, in the first catalytic layer 20, from the viewpoint of preventing a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh in the first catalytic layer 20 due to La in the first catalytic layer 20, ２ O ３From the viewpoint of improving the heat resistance of the first catalytic layer 20, rather than reducing the La content in terms of La converted from La, ２ O ３ In the second catalytic layer 30, from the viewpoint of improving the heat resistance of the second catalytic layer 30, it is necessary to increase the La content in terms of La. ２ O ３ Rather than increasing the content of La in the second catalytic layer 30 in terms of La, from the viewpoint of preventing a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh in the second catalytic layer 30 due to La in the second catalytic layer 30, ２ O ３ Therefore, the catalyst 1 is required to reduce the La content in the first catalyst layer 20. ２ O ３ The La content in the second catalytic layer 30 is 5 mass % or more based on the mass of the first catalytic layer 20, and ２ O ３ The catalyst 1 is characterized in that the content of La converted into La is 3 mass % or less based on the mass of the second catalyst layer 30. The catalyst 1 can effectively exhibit the action of La while preventing a decrease in the exhaust gas purification performance (particularly NOx purification performance) of Rh due to La, thereby achieving an improvement in exhaust gas purification performance.

[0083] In addition to improving heat resistance, La has the effect of trapping phosphorus components to prevent phosphorus poisoning of precious metal elements. Furthermore, when the Ce—Zr-based composite oxide contains La, the La in the Ce—Zr-based composite oxide has the effect of improving the oxygen storage capacity of the Ce—Zr-based composite oxide. These effects of La also contribute to improving the exhaust gas purification performance of the catalyst 1.

[0084] <Substrate> The substrate 10 will be described below.

[0085] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of materials constituting the substrate 10 include ceramic materials and metal materials, with ceramic materials being preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as alumina, zirconia, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metal materials include alloys such as stainless steel.

[0086] As shown in FIG. 1, the catalyst 1 is disposed in the exhaust path of an internal combustion engine so that the axial direction of the substrate 10 coincides or substantially coincides with the exhaust gas flow direction X.

[0087] 2 to 6, the substrate 10 has a cylindrical portion 11, partition walls 12 provided in the cylindrical portion 11, and cells 13 separated by the partition walls 12. The substrate 10 is preferably a honeycomb structure.

[0088] 2 and 3, the tubular portion 11 defines the outer shape of the substrate 10, and the axial direction of the tubular portion 11 coincides with the axial direction of the substrate 10. As shown in Figures 2 and 3, the shape of the tubular portion 11 is cylindrical, but it may be other shapes such as an elliptical cylinder or a polygonal cylinder.

[0089] As shown in Figures 2 and 3, the partition wall 12 is provided inside the cylindrical part 11. As shown in Figures 2 to 6, the partition wall 12 is present between adjacent cells 13, and the adjacent cells 13 are separated by the partition wall 12. The partition wall 12 may have a porous structure that allows exhaust gas to pass through. The thickness of the partition wall 12 is, for example, 20 µm or more and 1500 µm or less.

[0090] As shown in FIG. 6, the cell 13 extends in the exhaust gas flow direction X, and has an end on the exhaust gas inlet side and an end on the exhaust gas outlet side.

[0091] 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 cell 13 are open. Therefore, the exhaust gas that flows in from the end (opening) on ​​the exhaust gas inlet side of the cell 13 flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13. This type of configuration is called a flow-through type.

[0092] 2 to 5, the end (opening) of the cell 13 on the exhaust gas inlet side has a rectangular shape in plan view, but may have other shapes such as a hexagon, an octagon, etc. The end (opening) of the cell 13 on the exhaust gas outlet side also has a similar shape in plan view.

[0093] The cell density per square inch of the substrate 10 is, for example, 100 cells or more and 1000 cells or less. The cell density per square inch of the substrate 10 means the total number of cells 13 per square inch in a cross section obtained by cutting the substrate 10 along a plane perpendicular to the exhaust gas flow direction X.

[0094] The volume of the substrate 10 is, for example, 0.1 L or more and 20 L or less. The volume of the substrate 10 means the apparent volume of the substrate 10. For example, when the substrate 10 is cylindrical, if the outer diameter of the substrate 10 is 2r and the length of the substrate 10 is L, the volume of the substrate 10 can be calculated by the formula: Volume of substrate 10 = π × r ２ × L. In this specification, the term "length" refers to the dimension of the substrate 10 in the axial direction.

[0095] <First catalytic layer> The first catalytic layer 20 will now be described.

[0096] 4 and 6 , the first catalytic layer 20 is provided in an upstream region of the cell 13-side surface of the partition wall section 12. The "cell 13-side surface of the partition wall section 12" refers to the outer surface of the partition wall section 12 extending in the exhaust gas flow direction X. The "upstream region" refers to a region extending from the exhaust gas inlet-side end of the partition wall section 12 along the exhaust gas flow direction X so as not to reach the exhaust gas outlet-side end of the partition wall section 12. The first catalytic layer 20 may be provided directly on the cell 13-side surface of the partition wall section 12 or via another layer; however, it is usually provided directly on the cell 13-side surface of the partition wall section 12.

[0097] As shown in FIG. 6 , the first catalyst layer 20 extends from the end of the partition wall 12 on the exhaust gas inlet side along the exhaust gas flow direction X so as not to reach the end of the partition wall 12 on the exhaust gas outlet side.

[0098] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalytic layer 20 per unit volume of the portion of the substrate 10 on which the first catalytic layer 20 is formed (mass after calcination) is preferably 70 g / L or more and 380 g / L or less, more preferably 120 g / L or more and 300 g / L or less, and even more preferably 150 g / L or more and 250 g / L or less.

[0099] The mass of the first catalytic layer 20 per unit volume of the portion of the substrate 10 on which the first catalytic layer 20 is formed is calculated by the formula: (mass of the first catalytic layer 20) / ((volume of the substrate 10)×(average length L of the first catalytic layer 20) ２０ / Length L of the substrate 10 １０ )) is calculated.

[0100] Average length L of the first catalyst layer 20 ２０ An example of the measurement method is as follows.

[0101] Extending from the catalyst 1 in the axial direction of the substrate 10, the length L of the substrate 10 １０ A sample having the same length as the first catalyst layer 20 is cut out. The sample is, for example, cylindrical with a diameter of 25.4 mm. The diameter of the sample can be changed as needed. The sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and a first cut piece, a second cut piece, ..., an nth cut piece are obtained in order from the end of the sample on the exhaust gas inlet side. The length of the cut pieces is 5 mm. The composition of the cut pieces is analyzed using an X-ray fluorescence analyzer (XRF) (e.g., 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.

[0102] 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.

[0103] 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)

[0104] 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.

[0105] An example of a more detailed method for measuring the length of the first catalytic layer 20 included in the sample is as follows. The kth cut piece (i.e., the cut piece obtained from the exhaust gas outflow side of the sample among the cut pieces including a portion of the first catalytic layer 20) is cut in the axial direction of the substrate 10, and the portion 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 portion of the first catalytic layer 20 in the kth cut piece. Then, the length of the portion 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 portion of the first catalytic layer 20 included in the kth cut piece)

[0106] For 8 to 16 samples arbitrarily cut out from the catalyst 1, the length of the first catalyst layer 20 included in each sample was measured, and the average value thereof was taken as the average length L of the first catalyst layer 20. ２０ Let's say.

[0107] Average length L of the first catalyst layer 20 ２０ The larger the average length L of the first catalyst layer 20, the longer the contact time between the first catalyst layer 20 and the exhaust gas. ２０ When the other conditions are the same, the average length L of the first catalyst layer 20２０ The larger the value, the more the reactivity between the first catalytic layer 20 and exhaust gas improves, and the less reducing exhaust gas reaches the second catalytic layer 30. This creates an environment in which reduction of Rh in the second catalytic layer 30 is less likely to occur, so that the reduction action of Rh can be more effectively exerted in the second catalytic layer 30, which contains less La, which promotes the oxidation of Rh, and a decrease in exhaust gas purification performance (particularly NOx purification performance) can be prevented.

[0108] Length L of the substrate 10 １０ The average length L of the first catalyst layer 20 ２０ Percentage of (L ２０ / L １０ × 100) is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The upper limit can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0109] Length L of the substrate 10 １０ The average length L of the lower layer 21 of the first catalyst layer 20 ２１ Percentage of (L ２１ / L １０ × 100) and the length L of the substrate 10 １０ The average length L of the upper layer 22 of the first catalyst layer 20 ２２ Percentage of (L ２２ / L １０ × 100) is the length L of the substrate 10 １０ The average length L of the first catalyst layer 20 ２０ Percentage of (L ２０ / L １０ × 100). The length L of the substrate 10 １０ The average length L of the lower layer 21 of the first catalyst layer 20 ２１ and the length L of the substrate 10 １０ The average length L of the upper layer 22 of the first catalyst layer 20 ２２ may be the same or different.

[0110] The first catalytic layer 20 contains La. This improves the heat resistance of the first catalytic layer 20. Furthermore, phosphorus poisoning of the precious metal elements in the first catalytic layer 20 is prevented. Furthermore, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 is improved through the oxidation of Pd by La. Furthermore, when the Ce—Zr-based composite oxide in the first catalytic layer 20 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the first catalytic layer 20 are improved.

[0111] At least one of the lower layer 21 and the upper layer 22 contains La. In one embodiment, one of the lower layer 21 and the upper layer 22 contains La, and the other does not contain La. In another embodiment, both the lower layer 21 and the upper layer 22 contain La. From the viewpoint of effectively exerting the effects of La, it is preferable that both the lower layer 21 and the upper layer 22 contain La.

[0112] The first catalytic layer 20 contains one or more La sources. The La source is an oxide containing La. The La source can be selected from, for example, an Al-based oxide containing La, a Ce-based oxide containing La, or a Ce—Zr-based composite oxide containing La.

[0113] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the first catalytic layer 20, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. In one embodiment, the first catalytic layer 20 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. The Al-based oxide containing La contributes to improving the heat resistance of the first catalytic layer 20, and the Ce—Zr-based composite oxide containing La contributes to improving the heat resistance and oxygen storage capacity of the first catalytic layer 20.

[0114] In order to effectively exert the function of La, the La content of La in the first catalyst layer 20 is ２ O ３The converted content is preferably 5.5 mass% or more, and more preferably 6 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 exhaust gas purification performance and cost, the contents of other components, and the like. The upper limit is preferably 20 mass% or less, more preferably 15 mass% or less, and even more preferably 10 mass% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0115] Regarding a certain catalyst layer, "La in the catalyst layer" ２ O ３ When the catalyst layer contains one kind of La source, the "converted content" is the La content of La derived from the one kind of La source. ２ O ３ When the catalyst layer contains two or more La sources, the La content of La derived from the two or more La sources is ２ O ３ This definition applies to all catalyst layers (e.g., first catalyst layer 20, second catalyst layer 30, lower layer 21, upper layer 22, lower layer 31, upper layer 32, etc.).

[0116] La in the first catalytic layer 20 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based composite oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0117] Regarding a certain catalyst layer, "La derived from an Al-based oxide containing La and a Ce-Zr-based composite oxide containing La" ２ O ３ When the catalyst layer contains an Al-based oxide containing La and does not contain a Ce—Zr-based composite oxide containing La, the "converted mass" is the La of La derived from the Al-based oxide containing La. ２ O ３When the catalyst layer contains a Ce-Zr-based composite oxide containing La and does not contain an Al-based oxide containing La, the La content of La derived from the Ce-Zr-based composite oxide containing La is ２ O ３ When the catalyst layer contains an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La, the La content of La derived from the Al-based oxide containing La is ２ O ３ The mass of La derived from Ce-Zr based composite oxide containing La is ２ O ３ This definition applies to all catalyst layers (e.g., first catalyst layer 20, second catalyst layer 30, lower layer 21, upper layer 22, lower layer 31, upper layer 32, etc.).

[0118] <Lower Layer of First Catalyst Layer> The lower layer 21 of the first catalyst layer 20 will now be described.

[0119] 4 and 6 , the lower layer 21 is provided in the upstream region of the cell 13 side surface of the partition wall portion 12. The definitions of "the cell 13 side surface of the partition wall portion 12" and "upstream region" are as described above. The lower layer 21 may be provided directly on the cell 13 side surface of the partition wall portion 12 or may be provided via another layer, but is usually provided directly on the cell 13 side surface of the partition wall portion 12.

[0120] The lower layer 21 may be composed of a portion (hereinafter referred to as a "protruding portion") that protrudes from the cell 13-side surface of the partition wall portion 12 toward the cells 13, or may be composed of a portion (hereinafter referred to as an "internal portion") that exists inside the partition wall portion 12, or may have both a protruding portion and an internal portion. The present invention includes an embodiment in which the lower layer 21 is composed of a protruding portion, an embodiment in which the lower layer 21 is composed of an internal portion, and an embodiment in which the lower layer 21 has both a protruding portion and an internal portion.

[0121] 4 and 6, the lower layer 21 preferably has raised portions, which improves the contact between the lower layer 21 and exhaust gases, thereby improving the exhaust gas purification performance.

[0122] As shown in FIG. 6 , the lower layer 21 extends from the end of the partition wall 12 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 on the exhaust gas outlet side.

[0123] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the lower layer 21 per unit volume of the portion of the substrate 10 on which the lower layer 21 is formed (mass after firing) is preferably 50 g / L or more and 230 g / L or less, more preferably 70 g / L or more and 180 g / L or less, and even more preferably 80 g / L or more and 150 g / L or less.

[0124] The mass of the lower layer 21 per unit volume of the portion of the substrate 10 on which the lower layer 21 is formed is calculated by the formula: (mass of the lower layer 21) / ((volume of the substrate 10)×(average length L of the lower layer 21) ２１ / Length L of the substrate 10 １０ )) is calculated.

[0125] Average length L of the first catalyst layer 20 ２０ The above description of the average length L of the lower layer 21 ２１ When applied, the "first catalyst layer 20" is attached to the "lower layer 21" and has an average length L ２０ " is "average length L ２１ " can be read as ".

[0126] The lower layer 21 contains Pd.

[0127] The lower layer 21 is covered with the upper layer 22. Therefore, the precious metal element in the lower layer 21 is not easily poisoned by phosphorus, whereas the precious metal element in the upper layer 22 is easily poisoned by phosphorus. On the other hand, Pd is easily poisoned by phosphorus, whereas Rh is not easily poisoned by phosphorus. Therefore, Pd is suitable as the precious metal element contained in the lower layer 21, and Rh is suitable as the precious metal element contained in the upper layer 22.

[0128] Since the first catalytic layer 20 is provided upstream of the second catalytic layer 30, the temperature of the exhaust gas contacting the first catalytic layer 20 is higher than the temperature of the exhaust gas contacting the second catalytic layer 30. On the other hand, Pd has better heat resistance than Pt. Therefore, Pd is more suitable than Pt as the precious metal element to be contained in the lower layer 21.

[0129] La in the first catalytic layer 20 ２ O ３ Since the converted content is 5 mass % or more based on the mass of the first catalytic layer 20, the precious metal elements in the first catalytic layer 20 are easily oxidized by La in the first catalytic layer 20. On the other hand, when Pt is oxidized, the exhaust gas purification performance decreases, whereas when Pd is oxidized, the exhaust gas purification performance improves. Therefore, Pd is more suitable than Pt as the precious metal element to be contained in the lower layer 21.

[0130] Pd is contained in the lower layer 21 in a form capable of functioning 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 improving exhaust gas purification performance, the catalytically active component containing Pd is preferably in the form of particles.

[0131] From the viewpoint of effectively exhibiting the exhaust gas purification performance of Pd, the content of Pd in ​​the lower layer 21 in terms of metal is preferably 0.1 mass % or more, more preferably 0.5 mass % or more, and even more preferably 1 mass % or more, based on the mass of the lower layer 21. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 20 mass % or less, more preferably 15 mass % or less, and even more preferably 10 mass % or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0132] The lower layer 21 may contain one or more noble metal elements other than Pd.

[0133] The precious metal element other than Pd can be selected from, for example, Rh, Pt, Ru, Ir, Os, etc. The precious metal element other than Pd is contained in the lower layer 21 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 improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Pd is preferably in a particulate form.

[0134] If the lower layer 21 contains Pd and a precious metal element other than Pd, the Pd and the precious metal element other than Pd may form an alloy, potentially reducing the active sites of Pd, which contributes to exhaust gas purification performance. Therefore, when the lower layer 21 contains Pd, it is preferable that the content of the precious metal element other than Pd in ​​the lower layer 21 in terms of metal equivalent is small. Specifically, the content of the precious metal element other than Pd in ​​the lower layer 21 in terms of metal equivalent is preferably 0.05 mass% or less, and more preferably 0.01 mass% or less, based on the mass of the lower layer 21. The lower limit is 0 mass%. The "content of the precious metal element other than Pd in ​​the lower layer 21 in terms of metal equivalent" refers to the content of the one precious metal element in terms of metal equivalent when the lower layer 21 contains one precious metal element other than Pd. If the lower layer 21 contains two or more precious metal elements other than Pd, it refers to the total content of the two or more precious metal elements in terms of metal equivalent.

[0135] The lower layer 21 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.

[0136] 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 pore surfaces of the support. This definition applies to all catalyst layers (e.g., lower layer 21, upper layer 22, lower layer 31, upper layer 32, etc.).

[0137] 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.

[0138] 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).

[0139] 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.

[0140] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the lower layer 21, the carrier 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 lower layer 21 contains an Al-based oxide and a Ce—Zr-based composite oxide as the carrier.

[0141] The lower layer 21 preferably contains La. This improves the heat resistance of the lower layer 21. Furthermore, phosphorus poisoning of the precious metal elements in the lower layer 21 is prevented. Furthermore, the exhaust gas purification performance of Pd in ​​the lower layer 21 is improved through the oxidation of Pd by La. Furthermore, when the Ce—Zr-based composite oxide in the lower layer 21 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the lower layer 21 are improved.

[0142] When the lower layer 21 contains La, the lower layer 21 contains one or more La sources. The explanation regarding the La source is the same as above.

[0143] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the lower layer 21, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. In one embodiment, the lower layer 21 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. The Al-based oxide containing La contributes to improving the heat resistance of the lower layer 21, and the Ce—Zr-based composite oxide containing La contributes to improving the heat resistance and oxygen storage capacity of the lower layer 21.

[0144] From the viewpoint of effectively exerting the effect of La, the La content of La in the lower layer 21 is ２ O ３ The converted content is preferably 5% by mass or more, more preferably 6% by mass or more, and even more preferably 7% by mass or more, based on the mass of the lower layer 21. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0145] La in the lower layer 21 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based composite oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0146] From the viewpoint of improving the heat resistance of the lower layer 21, the lower layer 21 preferably contains Al.

[0147] When the lower layer 21 contains Al, the lower layer 21 contains one or more Al sources. The Al source is an oxide containing Al. The Al source can be selected from, for example, an Al-based oxide, a Ce-based oxide containing Al, a Ce-Zr-based composite oxide containing Al, an alumina binder, etc. In one embodiment, the lower layer 21 contains an Al-based oxide and an alumina binder as the Al source.

[0148] From the viewpoint of improving the heat resistance of the lower layer 21, the Al content of the lower layer 21 is ２ O ３ The content in terms of the converted amount is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more, based on the mass of the lower layer 21. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0149] Regarding a certain catalyst layer, "Al in the catalyst layer" ２ O ３ When the catalyst layer contains one type of Al source, the "concentration of Al converted from Al derived from the one type of Al source" is ２ O ３ When the catalyst layer contains two or more Al sources, the content of Al derived from the two or more Al sources is expressed as the Al content in terms of Al. ２ O ３ This definition applies to all catalyst layers (e.g., lower layer 21, upper layer 22, lower layer 31, upper layer 32, etc.).

[0150] From the viewpoint of improving the oxygen storage capacity of the lower layer 21, the lower layer 21 preferably contains Ce.

[0151] When the lower layer 21 contains Ce, the lower layer 21 includes one or more Ce sources. The Ce source is an oxide containing Ce. The Ce source can be selected from, for example, an Al-based oxide containing Ce, a Ce-based oxide, a Ce—Zr-based composite oxide, a ceria binder, etc. In one embodiment, the lower layer 21 includes a Ce—Zr-based composite oxide as the Ce source.

[0152] From the viewpoint of improving the heat resistance and oxygen storage capacity of the lower layer 21, the lower layer 21 preferably contains Zr.

[0153] When the lower layer 21 contains Zr, the lower layer 21 contains one or more Zr sources. The description of the Zr sources is the same as above. In one embodiment, the lower layer 21 contains a Ce—Zr-based composite oxide as the Zr source.

[0154] From the viewpoint of improving the heat resistance and oxygen storage capacity of the lower layer 21, the Zr in the lower layer 21 is converted to ZrO ２ The converted content is preferably 10% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, based on the mass of the lower layer 21. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0155] Regarding a certain catalyst layer, "ZrO of Zr in the catalyst layer" ２ When the catalyst layer contains one type of Zr source, the "content of ZrO converted from Zr derived from the one type of Zr source" is ２ When the catalyst layer contains two or more Zr sources, the ZrO content of Zr derived from the two or more Zr sources is ２ This definition applies to all catalyst layers (e.g., lower layer 21, upper layer 22, lower layer 31, upper layer 32, etc.).

[0156] The lower layer 21 may contain components such as a binder and a stabilizer. Examples of the binder include inorganic oxide binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol. Examples of the stabilizer include carbonates, oxides, and sulfates of alkaline earth metal elements (e.g., Sr, Ba, etc.).

[0157] <Upper Layer of First Catalyst Layer> The upper layer 22 of the first catalyst layer 20 will now be described.

[0158] As shown in FIGS. 4 and 6, the upper layer 22 is disposed above the lower layer 21 .

[0159] The phrase "the upper layer 22 is provided above the lower layer 21" means that part or all of the upper layer 22 is present on one of the two main surfaces of the lower layer 21 opposite the main surface on the partition wall portion 12 side. The "main surface of the lower layer 21" means the outer surface of the lower layer 21 extending in the exhaust gas flow direction X. The upper layer 22 may be provided directly on the main surface of the lower layer 21 or may be provided via another layer, but is typically provided directly on the main surface of the lower layer 21. The upper layer 22 may be provided so as to cover part of the main surface of the lower layer 21 or may be provided so as to cover the entire main surface of the lower layer 21. The phrase "the upper layer 22 is provided above the lower layer 21" encompasses both an embodiment in which the upper layer 22 is provided directly on the main surface of the lower layer 21 and an embodiment in which the upper layer 22 is provided on the main surface of the lower layer 21 via another layer.

[0160] As shown in FIG. 6 , the upper layer 22 extends from the end of the partition wall 12 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 on the exhaust gas outlet side.

[0161] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the upper layer 22 per unit volume of the portion of the substrate 10 on which the upper layer 22 is formed (mass after firing) is preferably 20 g / L or more and 150 g / L or less, more preferably 50 g / L or more and 120 g / L or less, and even more preferably 70 g / L or more and 100 g / L or less.

[0162] The mass of the upper layer 22 per unit volume of the portion of the substrate 10 on which the upper layer 22 is formed is calculated by the formula: (mass of the upper layer 22) / ((volume of the substrate 10)×(average length L of the upper layer 22) ２２ / Length L of the substrate 10 １０ )) is calculated.

[0163] Average length L of the first catalyst layer 20 ２０ The above description of the average length L of the upper layer 22 ２２ In this application, the "first catalyst layer 20" is defined as the "upper layer 22" and the "average length L ２０ " is "average length L ２２ " can be read as ".

[0164] The upper layer 22 contains Rh.

[0165] Rh is contained in the upper layer 22 in a form capable of functioning as a catalytically active component, for example, in the form of a catalytically active component containing Rh, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Rh is preferably in a particulate form.

[0166] From the viewpoint of effectively exhibiting the exhaust gas purification performance of Rh, the content of Rh in the upper layer 22 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 upper layer 22. The upper limit can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 2 mass% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0167] The upper layer 22 may contain, in addition to Rh, one or more noble metal elements other than Rh.

[0168] The precious metal element other than Rh can be selected from, for example, Pd, Pt, Ru, Ir, Os, etc. The precious metal element other than Rh is contained in the upper layer 22 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 improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Rh is preferably in a particulate form.

[0169] If the upper layer 22 contains Rh and a precious metal element other than Rh, the Rh and the precious metal element other than Rh may form an alloy, which may reduce the active sites of Rh, which contributes to exhaust gas purification performance. Therefore, it is preferable that the content of the precious metal element other than Rh in the upper layer 22 in terms of metal equivalent is small. Specifically, the content of the precious metal element other than Rh in the upper layer 22 in terms of metal equivalent is preferably 0.05 mass% or less, and more preferably 0.01 mass% or less, based on the mass of the upper layer 22. The lower limit is 0 mass%. The "content of the precious metal element other than Rh in the upper layer 22 in terms of metal equivalent" refers to the content of the one precious metal element in terms of metal equivalent when the upper layer 22 contains one precious metal element other than Rh, or refers to the total content of the two or more precious metal elements in terms of metal equivalent when the upper layer 22 contains two or more precious metal elements other than Rh.

[0170] The upper layer 22 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.

[0171] The support can be selected from, for example, inorganic oxides, the explanation of which is the same as above.

[0172] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the upper layer 22, the carrier 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 upper layer 22 contains an Al-based oxide and a Ce—Zr-based composite oxide as the carrier.

[0173] The upper layer 22 preferably contains La. This improves the heat resistance of the upper layer 22. It also prevents phosphorus poisoning of the precious metal elements in the upper layer 22. Furthermore, when the Ce—Zr-based composite oxide in the upper layer 22 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the upper layer 22 are improved.

[0174] When the upper layer 22 contains La, the upper layer 22 contains one or more La sources. The explanation regarding the La source is the same as above.

[0175] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the upper layer 22, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. In one embodiment, the upper layer 22 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. The Al-based oxide containing La contributes to improving the heat resistance of the upper layer 22, and the Ce—Zr-based composite oxide containing La contributes to improving the heat resistance and oxygen storage capacity of the upper layer 22.

[0176] From the viewpoint of effectively exerting the effect of La, the La content of La in the upper layer 22 is ２ O ３ The converted content is preferably 5% by mass or more, more preferably 6% by mass or more, and even more preferably 7% by mass or more, based on the mass of the upper layer 22. The upper limit can be adjusted as appropriate, taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, and the like. The upper limit is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0177] La in the upper layer 22 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based composite oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0178] From the viewpoint of improving the heat resistance of the upper layer 22, the upper layer 22 preferably contains Al.

[0179] When the upper layer 22 contains Al, the upper layer 22 contains one or more Al sources. The description of the Al sources is the same as above. In one embodiment, the upper layer 22 contains an Al-based oxide and an alumina binder as the Al sources.

[0180] In order to improve the heat resistance of the upper layer 22, the Al content of the Al in the upper layer 22 is ２ O ３The converted content is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, based on the mass of the upper layer 22. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0181] From the viewpoint of improving the oxygen storage capacity of the upper layer 22, the upper layer 22 preferably contains Ce.

[0182] When the upper layer 22 contains Ce, the upper layer 22 contains one or more Ce sources. The description of the Ce sources is the same as above. In one embodiment, the upper layer 22 contains a Ce—Zr-based composite oxide as the Ce source.

[0183] From the viewpoint of improving the heat resistance and oxygen storage capacity of the upper layer 22, the upper layer 22 preferably contains Zr.

[0184] When the upper layer 22 contains Zr, the upper layer 22 contains one or more Zr sources. The description of the Zr sources is the same as above. In one embodiment, the upper layer 22 contains a Ce—Zr-based composite oxide as the Zr source.

[0185] From the viewpoint of improving the heat resistance and oxygen storage capacity of the upper layer 22, the Zr in the upper layer 22 is converted into ZrO ２ The converted content is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, based on the mass of the upper layer 22. The upper limit can be adjusted as appropriate, taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, and the like. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0186] The upper layer 22 may contain components such as a binder, a stabilizer, etc. The explanation regarding the binder and the stabilizer is the same as above.

[0187] <Second catalytic layer> The second catalytic layer 30 will now be described.

[0188] 5 and 6 , the second catalyst layer 30 is provided in a downstream region of the cell 13-side surface of the partition wall section 12. The definition of "the cell 13-side surface of the partition wall section 12" is the same as above. The "downstream region" refers to a region that extends from the end of the exhaust gas outlet side of the partition wall section 12 along the direction opposite to the exhaust gas flow direction X so as not to reach the end of the partition wall section 12 on the exhaust gas inlet side. The second catalyst layer 30 may be provided directly on the cell 13-side surface of the partition wall section 12 or via another layer; however, it is usually provided directly on the cell 13-side surface of the partition wall section 12.

[0189] As shown in FIG. 6 , the second catalyst layer 30 extends from the end of the partition wall 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0190] 6 , the second catalytic layer 30 is provided downstream of the first catalytic layer 20. "The second catalytic layer 30 is provided downstream of the first catalytic layer 20" means that at least a part of the second catalytic layer 30 is present in a region of the cell 13-side surface of the partition wall section 12 that is downstream in the exhaust gas flow direction X from the region where the first catalytic layer 20 is provided.

[0191] 6 , the end of the second catalytic layer 30 on the exhaust gas inlet side is in contact with the end of the first catalytic layer 20 on the exhaust gas outlet side at a boundary surface S. However, the end of the second catalytic layer 30 on the exhaust gas inlet side may have a portion overlapping the end of the first catalytic layer 20 on the exhaust gas outlet side (i.e., a portion located above the end of the first catalytic layer 20 on the exhaust gas outlet side and covering the end of the first catalytic layer 20 on the exhaust gas outlet side). Furthermore, the end of the first catalytic layer 20 on the exhaust gas outlet side may have a portion overlapping the end of the second catalytic layer 30 on the exhaust gas inlet side (i.e., a portion located above the end of the second catalytic layer 30 on the exhaust gas inlet side and covering the end of the second catalytic layer 30 on the exhaust gas inlet side). These cases are also included in the phrase "the second catalytic layer 30 being provided downstream of the first catalytic layer 20."

[0192] The end of the lower layer 31 of the second catalytic layer 30 on the exhaust gas inlet side may have a portion overlapping with the end of the lower layer 21 of the first catalytic layer 20 on the exhaust gas outlet side (i.e., a portion located above the end of the lower layer 21 of the first catalytic layer 20 on the exhaust gas outlet side and covering the end of the lower layer 21 of the first catalytic layer 20 on the exhaust gas outlet side). Furthermore, the end of the lower layer 21 of the first catalytic layer 20 on the exhaust gas outlet side may have a portion overlapping with the end of the lower layer 31 of the second catalytic layer 30 on the exhaust gas inlet side (i.e., a portion located above the end of the lower layer 31 of the second catalytic layer 30 on the exhaust gas inlet side and covering the end of the lower layer 31 of the second catalytic layer 30 on the exhaust gas inlet side). These cases are also included in the phrase "the second catalytic layer 30 being provided downstream of the first catalytic layer 20."

[0193] The exhaust gas inlet side end of the upper layer 32 of the second catalytic layer 30 may have a portion overlapping with the exhaust gas outlet side end of the upper layer 22 of the first catalytic layer 20 (i.e., a portion located above the exhaust gas outlet side end of the upper layer 22 of the first catalytic layer 20 and covering the exhaust gas outlet side end of the upper layer 22 of the first catalytic layer 20). Furthermore, the exhaust gas outlet side end of the upper layer 22 of the first catalytic layer 20 may have a portion overlapping with the exhaust gas inlet side end of the upper layer 32 of the second catalytic layer 30 (i.e., a portion located above the exhaust gas inlet side end of the upper layer 32 of the second catalytic layer 30 and covering the exhaust gas inlet side end of the upper layer 32 of the second catalytic layer 30). These cases are also included in the phrase "the second catalytic layer 30 is provided downstream of the first catalytic layer 20."

[0194] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the second catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed (mass after calcination) is preferably 70 g / L or more and 380 g / L or less, more preferably 150 g / L or more and 300 g / L or less, and even more preferably 200 g / L or more and 260 g / L or less.

[0195] The mass of the second catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed is calculated by the formula: (mass of the second catalytic layer 30) / ((volume of the substrate 10)×(average length L of the second catalytic layer 30) ３０ / Length L of the substrate 10 １０ )) is calculated.

[0196] Average length L of the first catalyst layer 20 ２０ The above description of the method for measuring the average length L of the second catalyst layer 30 ３０ When applying this method, the "first catalyst layer 20" is referred to as the "second catalyst layer 30" and the "average length L ２０ " is "average length L ３０ However, the average length L of the second catalyst layer 30 ３０ In the measurement method, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and the first cut piece, the second cut piece, ..., the nth cut piece are obtained in order from the end of the sample on the exhaust gas outflow side.

[0197] Average length L of the second catalyst layer 30 ３０ is the balance between the exhaust gas purification performance and the cost, and the average length L of the first catalyst layer 20 ２０ The amount of the saturation can be adjusted appropriately taking into consideration the above.

[0198] The second catalytic layer 30 may or may not contain La. When the second catalytic layer 30 contains La, the heat resistance of the second catalytic layer 30 is improved. Furthermore, when the Ce—Zr-based composite oxide in the second catalytic layer 30 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the second catalytic layer 30 are improved.

[0199] In one embodiment in which the second catalytic layer 30 contains La, one of the lower layer 31 and the upper layer 32 contains La, and the other does not contain La. In another embodiment in which the second catalytic layer 30 contains La, both the lower layer 31 and the upper layer 32 contain La. From the viewpoint of effectively exerting the effects of La, it is preferable that both the lower layer 31 and the upper layer 32 contain La.

[0200] When the second catalytic layer 30 contains La, the second catalytic layer 30 contains one or more La sources. The explanation regarding the La source is the same as above.

[0201] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the second catalytic layer 30, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. In one embodiment, the second catalytic layer 30 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. The Al-based oxide containing La contributes to improving the heat resistance of the second catalytic layer 30, and the Ce—Zr-based composite oxide containing La contributes to improving the heat resistance and oxygen storage capacity of the second catalytic layer 30.

[0202] From the viewpoint of preventing a decrease in the exhaust gas purification performance of the precious metal elements (particularly Rh) in the second catalytic layer 30 due to La in the second catalytic layer 30, ２ O ３ The converted content is preferably 2.7 mass% or less, and more preferably 2.5 mass% or less, based on the mass of the second catalytic layer 30. The lower limit may be 0 mass% or more than 0 mass%. When the second catalytic layer 30 contains La, from the viewpoint of effectively exerting the effects of La, the lower limit is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.1 mass% or more. Each of these lower limits may be combined with any of the above-mentioned upper limits.

[0203] La in the second catalytic layer 30 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based composite oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0204] <Lower Layer of Second Catalyst Layer> The lower layer 31 of the second catalyst layer 30 will now be described.

[0205] 5 and 6 , the lower layer 31 is provided in a downstream region of the cell 13 side surface of the partition wall portion 12. The definitions of "the cell 13 side surface of the partition wall portion 12" and "downstream region" are as described above. The lower layer 31 may be provided directly on the cell 13 side surface of the partition wall portion 12 or may be provided via another layer; however, the lower layer 31 is usually provided directly on the cell 13 side surface of the partition wall portion 12.

[0206] The lower layer 31 may be composed of a portion (hereinafter referred to as a "protruding portion") that protrudes from the cell 13-side surface of the partition wall portion 12 toward the cells 13, or may be composed of a portion (hereinafter referred to as an "internal portion") that exists inside the partition wall portion 12, or may have both a protruding portion and an internal portion. The present invention includes an embodiment in which the lower layer 31 is composed of a protruding portion, an embodiment in which the lower layer 31 is composed of an internal portion, and an embodiment in which the lower layer 31 has both a protruding portion and an internal portion.

[0207] 5 and 6, the lower layer 31 preferably has raised portions, which improves the contact between the lower layer 31 and exhaust gases, thereby improving the exhaust gas purification performance.

[0208] As shown in FIG. 6 , the lower layer 31 extends from the end of the partition portion 12 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 portion 12 on the exhaust gas inlet side.

[0209] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the lower layer 31 per unit volume of the portion of the substrate 10 on which the lower layer 31 is formed (mass after firing) is preferably 50 g / L or more and 230 g / L or less, more preferably 100 g / L or more and 180 g / L or less, and even more preferably 130 g / L or more and 160 g / L or less.

[0210] The mass of the lower layer 31 per unit volume of the portion of the substrate 10 on which the lower layer 31 is formed is calculated by the formula: (mass of the lower layer 31) / ((volume of the substrate 10)×(average length L of the lower layer 31) ３１ / Length L of the substrate 10 １０ )) is calculated.

[0211] Average length L of the second catalyst layer 30 ３０The above description of the average length L of the lower layer 31 ３１ When applied, the "second catalyst layer 30" is attached to the "lower layer 31" and has an average length L ３０ " is "average length L ３１ " can be read as ".

[0212] The lower layer 31 contains Pt, which is less expensive than Rh and Pd and is therefore used from the viewpoint of cost reduction.

[0213] Because the lower layer 31 is covered by the upper layer 32, the exhaust gas that comes into contact with the lower layer 31 has a more stable atmosphere than the exhaust gas that comes into contact with the upper layer 32. More specifically, the exhaust gas that comes into contact with the lower layer 31 is less likely to momentarily become an oxygen-rich atmosphere or an atmosphere rich in reducing substances (CO / HC). Rh has excellent purification properties in environments with large atmospheric fluctuations, while Pt has excellent purification properties in environments with a stable atmosphere. Therefore, Pt is suitable as the precious metal element to be contained in the lower layer 31, and Rh is suitable as the precious metal element to be contained in the upper layer 32.

[0214] Since the second catalytic layer 30 is provided downstream of the first catalytic layer 20, the temperature of the exhaust gas contacting the second catalytic layer 30 is lower than the temperature of the exhaust gas contacting the first catalytic layer 20. On the other hand, Pt has poorer heat resistance than Pd. Therefore, Pt is more suitable than Pd as the precious metal element to be contained in the lower layer 31.

[0215] La in the second catalytic layer 30 ２ O ３ Since the converted content is 3 mass% or less based on the mass of the second catalytic layer 30, the precious metal elements in the second catalytic layer 30 are less likely to be oxidized by La in the second catalytic layer 30. On the other hand, when Pd is oxidized, the exhaust gas purification performance improves, whereas when Pt is oxidized, the exhaust gas purification performance decreases. Therefore, Pt is more suitable than Pd as the precious metal element to be contained in the lower layer 31.

[0216] Pt is contained in the lower layer 31 in a form capable of functioning 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 improving exhaust gas purification performance, the catalytically active component containing Pt is preferably in a particulate form.

[0217] From the viewpoint of effectively exhibiting the exhaust gas purification performance of Pt, the content of Pt in the lower layer 31 in terms of metal, based on the mass of the lower layer 31, is preferably 0.05 mass% or more, more preferably 0.1 mass% or more, and even more preferably 0.3 mass% or more. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 1 mass% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0218] The lower layer 31 may contain one or more kinds of noble metal elements other than Pt.

[0219] The precious metal element other than Pt can be selected from, for example, Rh, Pd, Ru, Ir, Os, etc. The precious metal element other than Pt is contained in the lower layer 31 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 improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Pt is preferably in a particulate form.

[0220] If the lower layer 31 contains Pt and a precious metal element other than Pt, the Pt and the precious metal element other than Pt may form an alloy, potentially reducing the active sites of Pt, which contributes to exhaust gas purification performance. Therefore, when the lower layer 31 contains Pt, it is preferable that the content of the precious metal element other than Pt in the lower layer 31, in terms of metal equivalent, is small. Specifically, the content of the precious metal element other than Pt in the lower layer 31, in terms of metal equivalent, is preferably 1 mass% or less, more preferably 0.3 mass% or less, and even more preferably 0.05 mass% or less, based on the mass of the lower layer 31. The lower limit is 0 mass%. The "content of the precious metal element other than Pt in the lower layer 31, in terms of metal equivalent," when the lower layer 31 contains one precious metal element other than Pt, refers to the content of that one precious metal element in terms of metal equivalent. When the lower layer 31 contains two or more precious metal elements other than Pt, it refers to the total content of the two or more precious metal elements in terms of metal equivalent.

[0221] The lower layer 31 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.

[0222] The support can be selected from, for example, inorganic oxides, the explanation of which is the same as above.

[0223] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the lower layer 31, the carrier 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 lower layer 31 contains an Al-based oxide and a Ce—Zr-based composite oxide as the carrier.

[0224] The lower layer 31 may or may not contain La. When the lower layer 31 contains La, the heat resistance of the lower layer 31 is improved. Furthermore, when the Ce—Zr-based composite oxide in the lower layer 31 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the lower layer 31 are improved.

[0225] When the lower layer 31 contains La, the lower layer 31 contains one or more La sources. The explanation regarding the La source is the same as above.

[0226] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the lower layer 31, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La. In one embodiment, the lower layer 31 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La.

[0227] From the viewpoint of preventing a decrease in the exhaust gas purification performance of the precious metal elements (particularly Pt) in the lower layer 31 due to La in the lower layer 31, ２ O ３ The content in terms of La is preferably 2% by mass or less, more preferably 1.9% by mass or less, and even more preferably 1.8% by mass or less, based on the mass of the lower layer 31. The lower limit may be 0% by mass or more, or may be greater than 0% by mass. When the lower layer 31 contains La, in order to effectively exert the effects of La, the lower limit is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. Each of these lower limits may be combined with any of the above-mentioned upper limits.

[0228] La in the lower layer 31 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based composite oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0229] When the metal-equivalent content of precious metal elements other than Pt in the lower layer 31 is small, the metal-equivalent content of Pt in the lower layer 31 becomes relatively large, and the effect of Pt on the exhaust gas purification performance of the lower layer 31 becomes large. In this case, the Pt in the lower layer 31 is oxidized by La in the lower layer 31, which reduces the exhaust gas purification performance of Pt in the lower layer 31, resulting in a significant reduction in the exhaust gas purification performance of the lower layer 31. Therefore, when the metal-equivalent content of precious metal elements other than Pt in the lower layer 31 is small, the La content of La in the lower layer 31 ２ O ３It is preferable that the content in terms of metal equivalent is small. This makes it possible to effectively prevent a decrease in the exhaust gas purification performance of the lower layer 31. Specifically, from the viewpoint of effectively preventing a decrease in the exhaust gas purification performance of the lower layer 31, the content in terms of metal equivalent of the precious metal elements other than Pt in the lower layer 31 is preferably 1 mass % or less, more preferably 0.3 mass % or less, and even more preferably 0.05 mass % or less based on the mass of the lower layer 31; ２ O ３ It is preferable to combine this with the embodiment in which the converted content is preferably 2% by mass or less, more preferably 1.9% by mass or less, and even more preferably 1.8% by mass or less, based on the mass of the lower layer 31. The former and latter embodiments have been described above.

[0230] From the viewpoint of improving the heat resistance of the lower layer 31, the lower layer 31 preferably contains Al.

[0231] When the lower layer 31 contains Al, the lower layer 31 contains one or more Al sources. The description of the Al sources is the same as above. In one embodiment, the lower layer 31 contains an Al-based oxide and an alumina binder as the Al sources.

[0232] In order to improve the heat resistance of the lower layer 31, the Al content of the lower layer 31 is ２ O ３ The converted content is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, based on the mass of the lower layer 31. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0233] From the viewpoint of improving the oxygen storage capacity of the lower layer 31, the lower layer 31 preferably contains Ce.

[0234] When the lower layer 31 contains Ce, the lower layer 31 contains one or more Ce sources. The Ce sources are the same as those described above. In one embodiment, the lower layer 31 contains a Ce—Zr-based composite oxide as the Ce source.

[0235] From the viewpoint of improving the heat resistance and oxygen storage capacity of the lower layer 31, the lower layer 31 preferably contains Zr.

[0236] When the lower layer 31 contains Zr, the lower layer 31 contains one or more Zr sources. The description of the Zr sources is the same as above. In one embodiment, the lower layer 31 contains a Ce—Zr-based composite oxide as the Zr source.

[0237] From the viewpoint of improving the heat resistance and oxygen storage capacity of the lower layer 31, the Zr in the lower layer 31 is changed to ZrO ２ The converted content is preferably 10% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, based on the mass of the lower layer 31. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, etc. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0238] The lower layer 31 may contain components such as a binder, a stabilizer, etc. The explanation regarding the binder and the stabilizer is the same as above.

[0239] <Upper Layer of Second Catalyst Layer> The upper layer 32 of the second catalyst layer 30 will now be described.

[0240] As shown in FIGS. 5 and 6, the upper layer 32 is disposed above the lower layer 31 .

[0241] The phrase "the upper layer 32 is provided above the lower layer 31" means that part or all of the upper layer 32 is present on one of the two main surfaces of the lower layer 31 opposite the main surface on the partition wall portion 12 side. The "main surface of the lower layer 31" means the outer surface of the lower layer 31 extending in the exhaust gas flow direction X. The upper layer 32 may be provided directly on the main surface of the lower layer 31 or may be provided via another layer, but is typically provided directly on the main surface of the lower layer 31. The upper layer 32 may be provided so as to cover part of the main surface of the lower layer 31 or may be provided so as to cover the entire main surface of the lower layer 31. The phrase "the upper layer 32 is provided above the lower layer 31" encompasses both an embodiment in which the upper layer 32 is provided directly on the main surface of the lower layer 31 and an embodiment in which the upper layer 32 is provided on the main surface of the lower layer 31 via another layer.

[0242] As shown in FIG. 6 , the upper layer 32 extends from the end of the partition portion 12 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 portion 12 on the exhaust gas inlet side.

[0243] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the upper layer 32 per unit volume of the portion of the substrate 10 on which the upper layer 32 is formed (mass after firing) is preferably 20 g / L or more and 150 g / L or less, more preferably 50 g / L or more and 120 g / L or less, and even more preferably 70 g / L or more and 100 g / L or less.

[0244] The mass of the upper layer 32 per unit volume of the portion of the substrate 10 on which the upper layer 32 is formed is calculated by the formula: (mass of the upper layer 32) / ((volume of the substrate 10)×(average length L of the upper layer 32) ３２ / Length L of the substrate 10 １０ )) is calculated.

[0245] Average length L of the second catalyst layer 30 ３０ The above description of the average length L of the upper layer 32 ３２ When applied, the "second catalyst layer 30" is attached to the "upper layer 32" and has an average length L ３０ " is "average length L ３２ " can be read as ".

[0246] The upper layer 32 contains Rh.

[0247] Rh is contained in the upper layer 32 in a form that can function as a catalytically active component, for example, in the form of a catalytically active component containing Rh, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Rh is preferably in a particulate form.

[0248] From the viewpoint of effectively exhibiting the exhaust gas purification performance of Rh, the content of Rh in the upper layer 32 in terms of metal, based on the mass of the upper layer 32, is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.1 mass% or more. The upper limit can be appropriately adjusted taking into consideration the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 1 mass% or less, more preferably 0.5 mass% or less, and even more preferably 0.3 mass% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0249] The upper layer 32 may contain, in addition to Rh, one or more noble metal elements other than Rh.

[0250] The precious metal element other than Rh can be selected from, for example, Pd, Pt, Ru, Ir, Os, etc. The precious metal element other than Rh is contained in the upper layer 32 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 improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Rh is preferably in a particulate form.

[0251] If the upper layer 32 contains Rh and a precious metal element other than Rh, the Rh and the precious metal element other than Rh may form an alloy, which may reduce the active sites of Rh, which contributes to exhaust gas purification performance. Therefore, it is preferable that the content of the precious metal element other than Rh in the upper layer 32 in terms of metal equivalent is small. Specifically, the content of the precious metal element other than Rh in the upper layer 32 in terms of metal equivalent is preferably 0.05 mass% or less, and more preferably 0.01 mass% or less, based on the mass of the upper layer 32. The lower limit is 0 mass%. The "content of the precious metal element other than Rh in the upper layer 32 in terms of metal equivalent" refers to the content of the one precious metal element in terms of metal equivalent when the upper layer 32 contains one precious metal element other than Rh, or refers to the total content of the two or more precious metal elements in terms of metal equivalent when the upper layer 32 contains two or more precious metal elements other than Rh.

[0252] The upper layer 32 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.

[0253] The support can be selected from, for example, inorganic oxides, the explanation of which is the same as above.

[0254] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the upper layer 32, the carrier 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 upper layer 32 contains an Al-based oxide and a Ce—Zr-based composite oxide as the carrier.

[0255] The upper layer 32 may or may not contain La. When the upper layer 32 contains La, the heat resistance of the upper layer 32 is improved. Furthermore, when the Ce—Zr-based composite oxide in the upper layer 32 contains La, the heat resistance and oxygen storage capacity of the Ce—Zr-based composite oxide in the upper layer 32 are improved.

[0256] When the upper layer 32 contains La, the upper layer 32 contains one or more La sources. The explanation regarding the La source is the same as above.

[0257] From the viewpoint of improving the heat resistance and / or oxygen storage capacity of the upper layer 32, the La source is preferably selected from an Al-based oxide containing La and a Ce—Zr-based oxide containing La. In one embodiment, the upper layer 32 contains, as the La source, an Al-based oxide containing La and a Ce—Zr-based composite oxide containing La.

[0258] From the viewpoint of preventing a decrease in the exhaust gas purification performance of the precious metal elements (particularly Rh) in the upper layer 32 due to La in the upper layer 32, ２ O ３ The content, calculated based on the mass of the upper layer 32, is preferably 4% by mass or less, more preferably 3.5% by mass or less, and even more preferably 3% by mass or less. The lower limit may be 0% by mass or more, or may be greater than 0% by mass. When the upper layer 32 contains La, in order to effectively exert the effects of La, the lower limit is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. Each of these lower limits may be combined with any of the above-mentioned upper limits.

[0259] La in the upper layer 32 ２ O ３ Of the converted mass, La derived from Al-based oxides containing La and Ce-Zr-based oxides containing La ２ O ３ The proportion of the converted mass is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

[0260] From the viewpoint of improving the heat resistance of the upper layer 32, the upper layer 32 preferably contains Al.

[0261] When the upper layer 32 contains Al, the upper layer 32 contains one or more Al sources. The description of the Al sources is the same as above. In one embodiment, the upper layer 32 contains an Al-based oxide and an alumina binder as the Al sources.

[0262] In order to improve the heat resistance of the upper layer 32, the Al content of the Al in the upper layer 32 is ２ O ３The converted content is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, based on the mass of the upper layer 32. The upper limit can be adjusted as appropriate, taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, and the like. The upper limit is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0263] From the viewpoint of improving the oxygen storage capacity of the upper layer 32, the upper layer 32 preferably contains Ce.

[0264] When the upper layer 32 contains Ce, the upper layer 32 contains one or more Ce sources. The description of the Ce sources is the same as above. In one embodiment, the upper layer 32 contains a Ce—Zr-based composite oxide as the Ce source.

[0265] From the viewpoint of improving the heat resistance and oxygen storage capacity of the upper layer 32, the upper layer 32 preferably contains Zr.

[0266] When the upper layer 32 contains Zr, the upper layer 32 contains one or more Zr sources. The description of the Zr sources is the same as above. In one embodiment, the upper layer 32 contains a Ce—Zr-based composite oxide as the Zr source.

[0267] From the viewpoint of improving the heat resistance and oxygen storage capacity of the upper layer 32, the Zr in the upper layer 32 is converted into ZrO ２ The converted content is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, based on the mass of the upper layer 32. The upper limit can be adjusted as appropriate, taking into consideration the balance between exhaust gas purification performance and cost, the content of other components, and the like. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0268] The upper layer 32 may contain components such as a binder, a stabilizer, etc. The explanation regarding the binder and the stabilizer is the same as above.

[0269] <Production of Catalyst> The catalyst 1 can be produced by forming a lower layer 21 on a substrate 10, then forming an upper layer 22 on the lower layer 21, and by forming a lower layer 31 on the substrate 10, then forming an upper layer 32 on the lower layer 31. The lower layer 21 and the upper layer 22 may be formed on the substrate 10, and then the lower layer 31 and the upper layer 32 may be formed on the substrate 10, or the lower layer 31 and the upper layer 32 may be formed on the substrate 10, and then the lower layer 21 and the upper layer 22 may be formed on the substrate 10.

[0270] The lower layer 21 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Pd), optionally a source of La (e.g., an Al-based oxide containing La, a Ce-based oxide containing La, a Ce-Zr-based composite oxide containing La, etc.), and optionally other components (e.g., an inorganic oxide other than the source of La, a binder such as an alumina binder, a solvent, etc.) to prepare a first slurry, applying the first slurry onto the substrate 10, drying, and firing.

[0271] The upper layer 22 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Rh), optionally a source of La (e.g., an Al-based oxide containing La, a Ce-based oxide containing Al, a Ce-Zr-based composite oxide containing Al, etc.), and optionally other components (e.g., an inorganic oxide other than the source of La, a binder such as an alumina binder, a solvent, etc.) to prepare a second slurry, applying the second slurry onto the lower layer 21, drying, and firing.

[0272] The lower layer 31 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Pt), optionally a source of La (e.g., an Al-based oxide containing La, a Ce-based oxide containing La, a Ce-Zr-based composite oxide containing La, etc.), and optionally other components (e.g., an inorganic oxide other than the source of La, a binder such as an alumina binder, a solvent, etc.) to prepare a third slurry, applying the third slurry onto the substrate 10, drying, and firing.

[0273] The upper layer 32 can be formed by mixing a source of a precious metal element (e.g., a salt of a precious metal element such as Rh), optionally a source of La (e.g., an Al-based oxide containing La, a Ce-based oxide containing Al, a Ce-Zr-based composite oxide containing Al, etc.), and optionally other components (e.g., an inorganic oxide other than the source of La, a binder such as an alumina binder, a solvent, etc.) to prepare a fourth slurry, applying the fourth slurry onto the lower layer 31, drying, and firing.

[0274] Examples of salts of precious metal elements include nitrates, ammine complex salts, acetates, and chlorides. Examples of binders include alumina sol, zirconia sol, titania sol, silica sol, and ceria sol. Examples of solvents include water and organic solvents.

[0275] 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.

[0276] <Exhaust Gas Purification System> Hereinafter, the exhaust gas purification system 100 will be described with reference to FIGS.

[0277] As shown in FIG. 7 , the exhaust gas purification system 100 includes an exhaust pipe P, a first exhaust gas purification catalyst 1B (hereinafter referred to as “catalyst 1B”) provided upstream in the exhaust passage of the exhaust pipe P, and a second exhaust gas purification catalyst 1C (hereinafter referred to as “catalyst 1C”) provided downstream in the exhaust passage of the exhaust pipe P.

[0278] One end P1 of the exhaust pipe P is connected to an internal combustion engine (e.g., a gasoline engine), and exhaust gas emitted from the internal combustion engine flows through the exhaust pipe P from one end P1 to the other end P2 of the exhaust pipe P. In other words, the exhaust pipe P forms an exhaust passage through which the exhaust gas flows. In the drawings, the exhaust gas flow direction is indicated by the symbol X. The exhaust gas flowing through the exhaust pipe P is treated by a catalyst 1B provided upstream in the exhaust passage of the exhaust pipe P, and the exhaust gas that has passed through the catalyst 1B is treated by a catalyst 1C provided downstream in the exhaust passage of the exhaust pipe P.

[0279] Catalyst 1B will be described below with reference to Fig. 8. In catalyst 1B, the same components as those in catalyst 1 are designated by the same reference numerals as those in catalyst 1. Unless otherwise specified below, the above description of catalyst 1 also applies to catalyst 1B.

[0280] As shown in FIG. 8, the catalyst 1B includes a first substrate 10B and a first catalyst layer 20B provided on the first substrate 10B.

[0281] 8, the first catalyst layer 20B includes a lower layer 21B provided on a first substrate 10B and an upper layer 22B provided on the lower layer 21B. The lower layer 21B contains Pd, and the upper layer 22B contains Rh.

[0282] The above description regarding the substrate 10 also applies to the first substrate 10B. When applying, "substrate 10" is read as "first substrate 10B."

[0283] The above description of the first catalytic layer 20 also applies to the first catalytic layer 20B, unless otherwise specified below. When applied, "first catalytic layer 20" is read as "first catalytic layer 20B."

[0284] The above description of the lower layer 21 and upper layer 22 of the first catalytic layer 20 also applies to the lower layer 21B and upper layer 22B of the first catalytic layer 20B, unless otherwise specified below. When applying, "lower layer 21" is replaced with "lower layer 21B," and "upper layer 22" is replaced with "upper layer 22B."

[0285] 8 , the first catalytic layer 20B, the lower layer 21B, and the upper layer 22B extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall section 12 to the exhaust gas outlet-side end of the partition wall section 12. The first catalytic layer 20B, the lower layer 21B, and the upper layer 22B may extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall section 12 so as not to reach the exhaust gas outlet-side end of the partition wall section 12, or may extend in the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet-side end of the partition wall section 12 so as not to reach the exhaust gas inlet-side end of the partition wall section 12.

[0286] Catalyst 1C will be described below with reference to Fig. 9. In catalyst 1C, the same components as those in catalyst 1 are designated by the same reference numerals as those in catalyst 1. Unless otherwise specified below, the above description of catalyst 1 also applies to catalyst 1C.

[0287] As shown in FIG. 9, a catalyst 1C includes a second substrate 10C and a second catalyst layer 30C provided on the second substrate 10C.

[0288] 9, the second catalyst layer 30C includes a lower layer 31C provided on the second substrate 10C and an upper layer 32C provided on the lower layer 31C. The lower layer 31C contains Pt, and the upper layer 32C contains Rh.

[0289] The above description regarding the substrate 10 also applies to the second substrate 10C. When applying, "substrate 10" is read as "second substrate 10C."

[0290] The above description of the second catalytic layer 30 also applies to the second catalytic layer 30C, unless otherwise specified below. When applied, "second catalytic layer 30" is read as "second catalytic layer 30C."

[0291] The above description of the lower layer 31 and upper layer 32 of the second catalytic layer 30 also applies to the lower layer 31C and upper layer 32C of the second catalytic layer 30C, unless otherwise specified below. When applying, "lower layer 31" should be read as "lower layer 31C" and "upper layer 32" should be read as "upper layer 32C."

[0292] 9 , the second catalytic layer 30C, the lower layer 31C, and the upper layer 32C extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall section 12 to the exhaust gas outlet-side end of the partition wall section 12. The second catalytic layer 30C, the lower layer 31C, and the upper layer 32C may extend along the exhaust gas flow direction X from the exhaust gas inlet-side end of the partition wall section 12 so as not to reach the exhaust gas outlet-side end of the partition wall section 12, or may extend in the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet-side end of the partition wall section 12 so as not to reach the exhaust gas inlet-side end of the partition wall section 12.

[0293] For the same reasons as those explained for catalyst 1, in the first catalytic layer 20B, from the viewpoint of preventing a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh in the first catalytic layer 20B due to La in the first catalytic layer 20B, ２ O ３ From the viewpoint of improving the heat resistance of the first catalytic layer 20B rather than reducing the La content in terms of La converted from La, ２ O ３ In the second catalytic layer 30C, it is necessary to increase the La content in terms of La ... ２ O ３ Rather than increasing the content of La in the second catalytic layer 30C in terms of La, from the viewpoint of preventing a decrease in the exhaust gas purification performance (particularly, NOx purification performance) of Rh in the second catalytic layer 30C due to La in the second catalytic layer 30C, ２ O ３ Therefore, the exhaust gas purification system 100 is required to reduce the La content in the first catalyst layer 20B. ２ O ３ The La content in the second catalytic layer 30C is 5 mass % or more based on the mass of the first catalytic layer 20B. ２ O ３ The exhaust gas purification system 100 is characterized in that the content of La converted into Rh is 3 mass % or less based on the mass of the second catalyst layer 30C. The exhaust gas purification system 100 can effectively exhibit the action of La while preventing a decrease in the exhaust gas purification performance (particularly NOx purification performance) of Rh due to La, thereby achieving an improvement in exhaust gas purification performance.

[0294] In addition to improving heat resistance, La has the effect of trapping phosphorus components to prevent phosphorus poisoning of precious metal elements. Furthermore, when the Ce—Zr-based composite oxide contains La, the La in the Ce—Zr-based composite oxide has the effect of improving the oxygen storage capacity of the Ce—Zr-based composite oxide. These effects of La also contribute to improving the exhaust gas purification performance of the exhaust gas purification system 100.

[0295] Average length L of the first catalytic layer 20B ２０ＢThe larger the average length L of the first catalytic layer 20B, the longer the contact time between the first catalytic layer 20B and the exhaust gas. ２０Ｂ When the other conditions are the same, the average length L of the first catalytic layer 20B ２０Ｂ The larger the value, the more the reactivity between the first catalytic layer 20B and the exhaust gas improves, and the less reducing exhaust gas reaches the second catalytic layer 30C. This creates an environment in which the reduction of Rh in the second catalytic layer 30C is less likely to occur, so that the reduction action of Rh can be more effectively exerted in the second catalytic layer 30C, which contains less La, which promotes the oxidation of Rh, and a decrease in the exhaust gas purification performance (particularly, NOx purification performance) can be prevented.

[0296] Length L of the first base material 10B １０Ｂ and the length L of the second substrate 10C １０ｃ The average length L of the first catalyst layer 20B relative to the total length of ２０Ｂ Percentage of (L ２０Ｂ / (L １０Ｂ +L １０ｃ ) × 100) is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The upper limit can be appropriately adjusted in consideration of the balance between exhaust gas purification performance and cost, etc. The upper limit is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0297] In the catalyst 1, the first catalyst layer 20 and the second catalyst layer 30 are provided on one substrate, so the average length L of the first catalyst layer 20 is ２０ There is a limit to the average length L of the first catalyst layer 20. ２０ In contrast, in the exhaust gas purification system 100, the first catalyst layer 20B and the second catalyst layer 30C are provided on two substrates, respectively, and therefore there is no such limitation, which is advantageous.

[0298] The present invention will be described below based on examples and comparative examples.

[0299] In the following examples and comparative examples, it is understood that all of La forms a solid solution phase in the OSC material. ２ O ３The peaks due to the above do not appear in the XRD diffraction pattern of the OSC material, and the XRD diffraction pattern of the OSC material and the crystal structure of the OSC material (fluorite-type CeO ２ -ZrO ２ ) and confirmed based on the fact that the XRD diffraction pattern of the OSC material has a peak shift to the lower angle side relative to the maximum peak position of the standard data. In addition, EDX elemental analysis of the OSC material was performed to confirm the La content of La forming a solid solution phase in the OSC material. ２ O ３ The converted content was calculated.

[0300] In the following examples and comparative examples, La ２ O ３ The fact that all of La forms a solid solution phase in the modified alumina is ２ O ３ The peak derived from La ２ O ３ The absence of La in the XRD diffraction pattern of the modified alumina ２ O ３ XRD diffraction pattern of modified alumina and La ２ O ３ The crystal structure of modified alumina (θ-alumina) was compared with the standard data, and La ２ O ３ This was confirmed based on the fact that the XRD diffraction pattern of the modified alumina had a peak shift to the lower angle side relative to the maximum peak position of the reference data. ２ O ３ EDX elemental analysis of the modified alumina was performed to determine La ２ O ３ La forming a solid solution phase in modified alumina ２ O ３ The converted content was calculated.

[0301] Example 1 (1) Production of First Exhaust Gas Purification Catalyst A first exhaust gas purification catalyst including a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer was produced as follows.

[0302] (1-1) Preparation of Slurry for Forming Lower Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 40.0 mass% ZrO ２ Conversion rate: 51.0 mass% La ２ O ３ Converted content: 9.0 mass% (all of La forms a solid solution phase)

[0303] In a mixing vessel, a palladium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the lower layer was 5.0 mass% in terms of Pd, 40.0 mass% in terms of the OSC material, and 8.0 mass% in terms of Pd (the entire La formed a solid solution phase), in terms of metal, based on the mass of the lower layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 50.0% by mass, and the solid content of the alumina sol was adjusted to 5.0% by mass.

[0304] (1-2) Formation of Lower Layer As a flow-through type substrate, cells extending in the axial direction and partitioned by partition walls having a thickness of 50 to 70 μm were formed at a density of 600 cells / inch in a plane perpendicular to the axial direction. ２ A flow-through substrate having a density of 1.0 L, an axial length of 100 mm, and a volume of 1.0 L was prepared.

[0305] A flow-through substrate was immersed in the lower layer forming slurry, and the flow-through substrate coated with the lower layer forming slurry was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form a lower layer. The mass of the lower layer per unit volume of the portion of the flow-through substrate on which the lower layer was formed was 100 g / L. The length of the lower layer was 100 mm.

[0306] (1-3) Preparation of Slurry for Forming Upper Layer An OSC material with the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO ２ Conversion rate: 81.0 mass% La ２O ３ Converted content: 9.0 mass% (all of La forms a solid solution phase)

[0307] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the upper layer was 1.0 mass% in terms of metal Rh, 40.0 mass% in terms of OSC material, and 1.0 mass% in terms of metal La, based on the mass of the upper layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 50.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0308] (1-4) Formation of Upper Layer The flow-through substrate on which the lower layer was formed was immersed in the upper layer forming slurry, and the flow-through substrate coated with the upper layer forming slurry was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form an upper layer on the lower layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate on which the upper layer was formed was 80 g / L. The length of the upper layer was 100 mm.

[0309] In this manner, a first exhaust gas purifying catalyst was produced, which had a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer.

[0310] (2) Production of Second Exhaust Gas Purification Catalyst A second exhaust gas purification catalyst having a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer was produced as follows.

[0311] (2-1) Preparation of Slurry for Forming Lower Layer An OSC material with the following composition was prepared: CeO ２ Conversion rate: 40.0 mass% ZrO ２ Conversion rate: 58.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0312] In a mixing vessel, platinum nitrate solution, OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the lower layer was 1.0 mass % in terms of metal for Pt, 60.0 mass % of the OSC material, and 1.0 mass % of La, based on the mass of the lower layer after firing (100 mass %). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0313] (2-2) Formation of Lower Layer As a flow-through type substrate, cells extending in the axial direction and partitioned by partition walls having a thickness of 50 to 70 μm were formed at a density of 600 cells / inch in a plane perpendicular to the axial direction. ２ A flow-through substrate having a density of 1.0 L, an axial length of 100 mm, and a volume of 1.0 L was prepared.

[0314] A flow-through substrate was immersed in the lower layer forming slurry, and the flow-through substrate coated with the lower layer forming slurry was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form a lower layer. The mass of the lower layer per unit volume of the portion of the flow-through substrate on which the lower layer was formed was 140 g / L. The length of the lower layer was 100 mm.

[0315] (2-3) Preparation of Slurry for Forming Upper Layer An OSC material with the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO ２ Conversion rate: 86.0 mass% La ２ O ３ Converted content: 4.0 mass% (all of La forms a solid solution phase)

[0316] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３The amount of each component in the slurry for forming the upper layer was 0.2 mass% in terms of metal Rh, 60.0 mass% in terms of OSC material, and 100.0 mass% in terms of La, based on the mass of the upper layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.8% by mass.

[0317] (2-4) Formation of Upper Layer The flow-through substrate on which the lower layer was formed was immersed in the upper layer forming slurry, and the flow-through substrate coated with the upper layer forming slurry was dried at 150°C for 0.5 hours and then baked at 500°C for 1 hour to form an upper layer on the lower layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate on which the upper layer was formed was 80 g / L. The length of the upper layer was 100 mm.

[0318] In this manner, a second exhaust gas purifying catalyst was produced, which had a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer.

[0319] (3) Evaluation of exhaust gas purification performance The manufactured first and second exhaust gas purification catalysts were installed on the upstream and downstream sides of the exhaust passage of an engine, respectively, and durability treatment was carried out under the following durability conditions as a deterioration treatment assuming driving 100,000 to 200,000 kilometers. <Durability conditions> Durability engine: NA 2L passenger gasoline engine Gasoline used: Commercially available regular gasoline Treatment temperature: 900°C Treatment time: 100 hours

[0320] The first and second exhaust gas purification catalysts after durability testing were installed upstream and downstream of the exhaust passage of a vehicle (a passenger car equipped with a 1.5L direct-injection turbo engine), and the vehicle was operated under the driving conditions of the World Wide Harmonized Exhaust Gas Test (WLTC). The emissions of non-methane hydrocarbons (NMHCs) and nitrogen oxides (NOx) in the exhaust gas passing through the first and second exhaust gas purification catalysts were measured during low-temperature operation from 589 seconds after the start of operation, medium-speed operation from 589 seconds to 1022 seconds after the start of operation, high-speed operation from 1022 seconds to 1477 seconds after the start of operation, and ultra-high-speed operation from 1477 seconds to 1800 seconds after the start of operation, and the total emissions of NMHCs and NOx per unit driving distance were calculated. Note that certification test fuel was used as the gasoline, and an exhaust gas measurement device manufactured by Horiba, Ltd. was used as the exhaust gas measurement device.

[0321] [Example 2] In the production of the first exhaust gas purifying catalyst, La in the lower layer forming slurry ２ O ３ La modified alumina ２ O ３ First and second exhaust gas purifying catalysts were produced in the same manner as in Example 1, except that the modification amount was changed to 2.0 mass %, and the exhaust gas purifying performance was evaluated.

[0322] Example 3 An OSC material having the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO ２ Conversion rate: 88.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0323] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the upper layer was 1.0 mass% in terms of metal Rh, 40.0 mass% in terms of OSC material, and 1.0 mass% in terms of metal La, based on the mass of the upper layer after firing (100 mass%). ２ O３ The modified alumina was adjusted to 50.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0324] In the production of the first exhaust gas purifying catalyst, the first and second exhaust gas purifying catalysts were produced in the same manner as in Example 1, except that the upper layer forming slurry prepared above was used, and the exhaust gas purifying performance was evaluated.

[0325] Comparative Example 1 (A) Preparation of Slurry for Forming Lower Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 40.0 mass% ZrO ２ Conversion rate: 51.0 mass% La ２ O ３ Converted content: 9.0 mass% (all of La forms a solid solution phase)

[0326] In a mixing vessel, platinum nitrate solution, OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the lower layer was 1.0 mass % in terms of metal for Pt, 60.0 mass % of the OSC material, and 1.0 mass % of La, based on the mass of the lower layer after firing (100 mass %). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0327] (B) Preparation of Slurry for Forming Upper Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO ２ Conversion rate: 86.0 mass% La ２ O ３ Converted content: 4.0 mass% (all of La forms a solid solution phase)

[0328] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O３ The amount of each component in the slurry for forming the upper layer was 0.2 mass% in terms of metal Rh, 60.0 mass% in terms of OSC material, and 100.0 mass% in terms of La, based on the mass of the upper layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.8% by mass.

[0329] In producing the second exhaust gas purification catalyst, the first and second exhaust gas purification catalysts were produced in the same manner as in Example 1, except that the lower layer forming slurry prepared in (A) above and the upper layer forming slurry prepared in (B) above were used, and the exhaust gas purification performance was evaluated.

[0330] Comparative Example 2 (C) Preparation of Slurry for Forming Lower Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 40.0 mass% ZrO ２ Conversion rate: 58.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0331] In a mixing vessel, a palladium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the lower layer was 5.0 mass % in terms of Pd, 40.0 mass % in terms of the OSC material, and 2.0 mass % in terms of La (the entire La formed a solid solution phase), in terms of metal, based on the mass of the lower layer after firing (100 mass %). ２ O ３ The modified alumina was adjusted to 50.0% by mass, and the solid content of the alumina sol was adjusted to 5.0% by mass.

[0332] (D) Preparation of Slurry for Forming Upper Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO２ Conversion rate: 88.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0333] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the upper layer was 1.0 mass% in terms of metal Rh, 40.0 mass% in terms of OSC material, and 1.0 mass% in terms of metal La, based on the mass of the upper layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 50.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0334] (E) Preparation of Slurry for Forming Lower Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 40.0 mass% ZrO ２ Conversion rate: 58.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0335] In a mixing vessel, platinum nitrate solution, OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the lower layer was 1.0 mass % in terms of metal for Pt, 60.0 mass % of the OSC material, and 1.0 mass % of La, based on the mass of the lower layer after firing (100 mass %). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.0% by mass.

[0336] (F) Preparation of Slurry for Forming Upper Layer An OSC material having the following composition was prepared: CeO ２ Conversion rate: 10.0 mass% ZrO ２ Conversion rate: 88.0 mass% La ２ O ３ Converted content: 2.0 mass% (all of La forms a solid solution phase)

[0337] In a mixing vessel, a rhodium nitrate solution, an OSC material, La ２ O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the upper layer was 0.2 mass% in terms of metal Rh, 60.0 mass% in terms of OSC material, and 100.0 mass% in terms of La, based on the mass of the upper layer after firing (100 mass%). ２ O ３ The modified alumina was adjusted to 30.0% by mass, and the solid content of the alumina sol was adjusted to 9.8% by mass.

[0338] The first and second exhaust gas purification catalysts were produced in the same manner as in Example 1, except that the lower layer forming slurry prepared in (C) above and the upper layer forming slurry prepared in (D) above were used in the production of the first exhaust gas purification catalyst, and the lower layer forming slurry prepared in (E) above and the upper layer forming slurry prepared in (F) above were used in the production of the second exhaust gas purification catalyst, and the exhaust gas purification performance was evaluated.

[0339] Comparative Example 3 The first and second exhaust gas purification catalysts were produced in the same manner as in Example 1, except that the lower layer forming slurry prepared in Comparative Example 2 (C) and the upper layer forming slurry prepared in Comparative Example 2 (D) were used in the production of the first exhaust gas purification catalyst, and the lower layer forming slurry prepared in Comparative Example 1 (A) and the upper layer forming slurry prepared in Comparative Example 1 (B) were used in the production of the second exhaust gas purification catalyst, and the first and second exhaust gas purification catalysts were evaluated for their exhaust gas purification performance.

[0340] The results are shown in Tables 1 and 2. In Table 2, the total amount of NMHC and NOx emissions is expressed as a relative value when the value of Comparative Example 3 is set to 1.

[0341]

[0342]

[0343] The first exhaust gas purifying catalysts of Examples 1 to 3 and Comparative Examples 1 to 3 include a first substrate and a first catalyst layer (lower layer and upper layer) provided on the first substrate, and the first catalyst layer contains Rh. The second exhaust gas purifying catalysts of Examples 1 to 3 and Comparative Examples 1 to 3 include a second substrate and a second catalyst layer (lower layer and upper layer) provided on the second substrate, and the second catalyst layer contains Rh.

[0344] The first and second exhaust gas purifying catalysts of Examples 1 to 3 satisfy the following conditions: ２ O ３ The La content in the second catalytic layer is 5% by mass or more based on the mass of the first catalytic layer. ２ O ３ The converted content is 3 mass % or less based on the mass of the second catalyst layer.

[0345] The first and second exhaust gas purifying catalysts of Comparative Examples 1 to 3 do not satisfy one or more of the above conditions.

[0346] As shown in Table 2, when a first exhaust gas purification catalyst that satisfies the above conditions is installed upstream of the exhaust passage and a second exhaust gas purification catalyst that satisfies the above conditions is installed downstream of the exhaust passage (Examples 1 to 3), the exhaust gas purification performance is higher than when a first exhaust gas purification catalyst that does not satisfy one or more of the above conditions is installed upstream of the exhaust passage and a second exhaust gas purification catalyst that does not satisfy one or more of the above conditions is installed downstream of the exhaust passage (Comparative Examples 1 to 3).

[0347] P...Exhaust pipe of internal combustion engine 1, 1B, 1C...Catalyst for purifying exhaust gas 10, 10B, 10C...Substrate 11...Cylindrical portion 12...Partition portion 13...Cell 20, 20B...First catalyst layer 21, 21B...Lower layer of first catalyst layer 22, 22B...Upper layer of first catalyst layer 30, 30C...Second catalyst layer 31, 31C...Lower layer of second catalyst layer 32, 32C...Upper layer of second catalyst layer 100...Exhaust gas purification system

Claims

1. A catalyst for purifying exhaust gas comprising a substrate, a first catalytic layer provided on the upstream side of the substrate, and a second catalytic layer provided on the downstream side of the substrate, wherein the first catalytic layer comprises a lower layer containing Pd and an upper layer containing Rh, the second catalytic layer comprises a lower layer containing Pt and an upper layer containing Rh, and the La content of the La in the first catalytic layer is 1.0 to 1.0 times that of the La in the first catalytic layer. ２ O ３ the La content in the second catalytic layer is 5% by mass or more based on the mass of the first catalytic layer, ２ O ３ The exhaust gas purifying catalyst has a converted content of 3 mass% or less based on the mass of the second catalyst layer.

2. The content of precious metal elements other than Pt in the lower layer of the second catalytic layer is 1 mass% or less based on the mass of the lower layer of the second catalytic layer, and the content of La in the lower layer of the second catalytic layer is 1 mass% or less. ２ O ３ 2. The exhaust gas purifying catalyst according to claim 1, wherein the converted content is 2 mass % or less based on the mass of the lower layer in the second catalyst layer.

3. The La content of La in the upper layer of the first catalyst layer ２ O ３ the La content in the upper layer of the second catalytic layer is 5 mass% or more based on the mass of the upper layer of the first catalytic layer, ２ O ３ 3. The exhaust gas purifying catalyst according to claim 1, wherein the converted content is 4 mass % or less based on the mass of the upper layer of the second catalyst layer.

4. An exhaust gas purification system comprising: an exhaust passage through which exhaust gas flows; a first exhaust gas purification catalyst provided on the upstream side of the exhaust passage; and a second exhaust gas purification catalyst provided on the downstream side of the exhaust passage, wherein the first exhaust gas purification catalyst comprises a first substrate and a first catalyst layer provided on the first substrate, the second exhaust gas purification catalyst comprises a second substrate and a second catalyst layer provided on the second substrate, the first catalyst layer comprises a lower layer containing Pd and an upper layer containing Rh, the second catalyst layer comprises a lower layer containing Pt and an upper layer containing Rh, and the La in the first catalyst layer is preferably La. ２ O ３ the La content in the second catalytic layer is 5% by mass or more based on the mass of the first catalytic layer, ２ O ３ The converted content is 3 mass% or less based on the mass of the second catalyst layer.

5. The content of precious metal elements other than Pt in the lower layer of the second catalytic layer is 1 mass% or less based on the mass of the lower layer of the second catalytic layer, and the content of La in the lower layer of the second catalytic layer is 1 mass% or less. ２ O ３ The exhaust gas purification system according to claim 4 , wherein the converted content is 2 mass % or less based on the mass of the lower layer in the second catalyst layer.

6. The La content of La in the upper layer of the first catalyst layer ２ O ３ the La content in the upper layer of the second catalytic layer is 5 mass% or more based on the mass of the upper layer of the first catalytic layer, ２ O ３ The exhaust gas purification system according to claim 4 or 5, wherein the converted content is 4 mass % or less based on the mass of the upper layer of the second catalyst layer.

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