Exhaust gas purification catalyst

Abstract

The purpose of the present invention is to provide an exhaust gas purification catalyst that improves exhaust gas purification performance after exposure to phosphorus (e.g., a phosphorus compound in engine oil). To achieve this purpose, provided is an exhaust gas purification catalyst (1) comprising a metal honeycomb substrate (10), a first catalyst layer (20) provided on the metal honeycomb substrate (10), and a second catalyst layer (30) provided on the first catalyst layer (20), wherein the first catalyst layer (20) contains Pd and Ce, the second catalyst layer (30) contains Rh, the mass content of Ce in the first catalyst layer (20) in terms of CeO2 is 25 mass% or greater, the WC amount of the second catalyst layer (30) is 30-90 g / L, the value of X × Y is 10 or greater when X [g / L] is the WC amount of the second catalyst layer (30) and Y × 102 (%) is the ratio of the total area of all pores to the area of a cross section of the second catalyst layer (30), and the ratio of the total area of pores having an area of 10 μm2 or less to the area of the cross section of the second catalyst layer (30) is 2% or greater.

Description

Exhaust gas purification catalyst

[0001] This invention relates to a catalyst for exhaust gas purification.

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

[0003] Pd is susceptible to poisoning by phosphorus (for example, phosphorus compounds in engine oil). In view of this problem, Patent Document 1 discloses an exhaust gas purification catalyst that can suppress the decrease in exhaust gas purification performance of Pd due to phosphorus poisoning. Specifically, Patent Document 1 discloses an exhaust gas purification catalyst comprising a substrate and a catalyst layer provided on the substrate, wherein the catalyst has a first section located upstream along the flow direction of the exhaust gas and a second section located downstream of the first section, the catalyst layer of the first section comprises a first catalyst layer containing palladium and a second catalyst layer containing rhodium covering the first catalyst layer, the pore diameter measured by the mercury intrusion method is 0.06 μm or more and 30.0 μm or less, the pore volume ratio, which is the ratio of the total volume of pores in the substrate and catalyst layer of the first section to the total volume of the first section, is 12% or more and less than 18%, and the wash coat amount, which is the mass per unit volume of the catalyst layer of the first section relative to the volume of the substrate present in the first section, is 100 g / L or more and 190 g / L or less.

[0004] On the other hand, motorcycles tend to have relatively higher SV conditions compared to four-wheeled vehicles, and increased emissions due to unpurified exhaust gases blowing through are a problem. In view of this problem, Patent Document 2 discloses an exhaust gas purification catalyst with improved exhaust gas diffusion to the entire catalyst layer. Specifically, Patent Document 2 discloses an exhaust gas purification catalyst that is placed in the exhaust path of an internal combustion engine and purifies the exhaust gas discharged from the internal combustion engine, comprising a base material and a catalyst layer placed on the base material and including a catalyst metal and a supporting material that supports the catalyst metal, wherein the catalyst layer has the following conditions: (1) In the pore distribution curve measured by a mercury porosimeter, the peak of the pore volume is largest in the range of pore diameters of 1 μm to 10 μm; (2) In an electron microscope observation image of the surface of the catalyst layer (observation magnification 1000x), when the area of ​​each of the multiple voids included in the electron microscope observation image is calculated, the standard deviation of the area of ​​the multiple voids is 30 μm. ２ A catalyst for exhaust gas purification is disclosed that satisfies all of the following conditions:

[0005] International Publication No. 2020 / 241248 Pamphlet, Japanese Patent Publication No. 2021-53604

[0006] The present invention aims to provide an exhaust gas purification catalyst with improved exhaust gas purification performance after exposure to phosphorus (for example, phosphorus compounds in engine oil).

[0007] To solve the above problems, the present invention provides the following exhaust gas purification catalyst: [1] An exhaust gas purification catalyst comprising a metal honeycomb substrate, a first catalyst layer provided on the metal honeycomb substrate, and a second catalyst layer provided on the first catalyst layer, wherein the first catalyst layer contains Pd and Ce, the second catalyst layer contains Rh, and the Ce in the first catalyst layer contains CeO ２ The converted mass content is 25% by mass or more, the mass of the second catalyst layer per unit volume of the portion of the metal honeycomb substrate on which the second catalyst layer is provided (hereinafter referred to as "WC amount of the second catalyst layer") is 30 g / L or more and 90 g / L or less, and the WC amount of the second catalyst layer is X [g / L], and the ratio of the total area of ​​all pores to the area of ​​the cross-sectional area of ​​the second catalyst layer is Y × 10 ２When expressed as [%], the value of X × Y is 10 or more, and the area of ​​the cross-sectional area of ​​the second catalyst layer is 10 μm ２ The exhaust gas purification catalyst wherein the proportion of the total area of ​​the pores is 2% or more. [2] The exhaust gas purification catalyst according to [1] wherein the value of Y is 0.12 or more and 0.25 or less. [3] CeO of Ce in the second catalyst layer ２ A catalytic converter for exhaust gas purification according to [1] or [2], wherein the converted mass content is 8% by mass or less. [4] A catalytic converter for exhaust gas purification according to any one of [1] to [3], wherein the value of X × Y is 12 or more, and the amount of WC in the second catalytic layer is 70 g / L or less.

[0008] According to the present invention, an exhaust gas purification catalyst is provided that has improved exhaust gas purification performance after exposure to phosphorus.

[0009] Figure 1 is a partial end view showing an exhaust gas purification catalyst according to one embodiment of the present invention arranged in the exhaust path of an internal combustion engine. Figure 2 is an end view of the line A-A in Figure 1. Figure 3 is an enlarged view of the region indicated by the symbol R in Figure 2. Figure 4 is an end view of the line B-B in Figure 1.

[0010] <<Explanation of Terms>> The following terms used in this specification are explained below. Unless otherwise specified, the following explanations apply to the entirety of this specification.

[0011] <Metallic Elements> The term "metallic elements" also includes metalloid elements such as Si and B.

[0012] <Rare Earth Elements> The "rare earth elements" include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0013] <Precious Metal Elements> "Precious metal elements" include Pt, Pd, Rh, Ru, Os, Ir, Au, and Ag.

[0014] <Oxides> The meaning of "oxides" for metallic elements is as follows: Oxides of rare earth elements other than Ce, Pr, and Tb are sesquioxides (M ２ O ３ ,M represents rare earth elements other than Ce, Pr, and Tb.) The oxide of Ce is CeO ２For Pr oxide, it is Pr ６ O １１ For Tb oxide, it is Tb ４ O ７ For Al oxide, it is Al ２ O ３ For Zr oxide, it is ZrO ２ For Si oxide, it is SiO ２ For B oxide, it is B ２ O ３ For Cr oxide, it is Cr ２ O ３ For Mg oxide, it is MgO, for Ca oxide, it is CaO, for Sr oxide, it is SrO, for Ba oxide, it is BaO, for Fe oxide, it is Fe ３ O ４ For Mn oxide, it is Mn ３ O ４ For Ni oxide, it is NiO, for Ti oxide, it is TiO ２ For Zn oxide, it is ZnO, for Sn oxide, it is SnO ２ means.

[0015] <Mass of the catalyst layer> "Mass of the catalyst layer" means classifying all metal elements contained in the catalyst layer into noble metal elements and metal elements other than noble metal elements, obtaining the mass in terms of metal for noble metal elements and the mass in terms of oxide for metal elements other than noble metal elements, and summing them up. That is, "Mass of the catalyst layer" means the calculated mass obtained by summing the mass in terms of metal of noble metal elements contained in the catalyst layer and the mass in terms of oxide of metal elements other than noble metal elements contained in the catalyst layer.

[0016] <Mass content ratio of metal elements in the catalyst layer in terms of metal or oxide> Regarding a certain metal element in a certain catalyst layer, the mass content ratio of the metal element in terms of metal in the catalyst layer is defined by the formula: mass content ratio of the metal element in terms of metal in the catalyst layer = (mass of the metal element in terms of metal in the catalyst layer) / (mass of the catalyst layer) × 100. "Mass of the metal element in terms of metal" means the mass of the metal obtained by assuming that the metal element exists as a metal composed of the metal element. The meaning of "Mass of the catalyst layer" is as described above.

[0017] With respect to a certain metal element in a catalyst layer, the mass content of that metal element in terms of oxide in the catalyst layer is defined by the formula: Mass content of the metal element in terms of oxide in the catalyst layer = (Mass of the metal element in terms of oxide in the catalyst layer) / (Mass of the catalyst layer) × 100. "Mass of the metal element in terms of oxide" means the mass of the oxide obtained by assuming that the metal element exists as an oxide of that metal element. The meaning of "oxide" of the metal element and the meaning of "mass of the catalyst layer" are as described above.

[0018] If information about the raw materials used to form the catalyst layer (e.g., composition, quantity, etc.) is known, the mass content of metal elements in the catalyst layer, either in terms of metal or oxide, can be determined from the raw material information.

[0019] If information on the raw materials used to form the catalyst layer is unknown, the amount of metal elements in the catalyst layer (in terms of metal equivalent or oxide equivalent) can be determined by conventional methods such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma emission spectroscopy (ICP-AES), and X-ray fluorescence spectroscopy (XRF). Specifically, these methods are as follows:

[0020] Elemental analysis of the catalyst layer is performed using conventional methods such as SEM-EDX, ICP-AES, and XRF to identify the types of constituent elements in the catalyst layer and determine the molar content (mol%) of each identified metal element. For example, in the case of SEM-EDX, the molar content (mol%) of each metal element is determined for each of the multiple fields of view of the SEM (e.g., 10 fields of view), and the average value of the molar content (mol%) of each metal element in the multiple fields of view is taken as the molar content (mol%) of each metal element in the catalyst layer.

[0021] The V value for each precious metal element in the catalyst layer can be calculated using the following formula: V value for each precious metal element = (molar content of each precious metal element in the catalyst layer) × (molar mass of each precious metal element)

[0022] The W value for each metal element other than the noble metal element in the catalyst layer can be calculated using the following formula: W value for each metal element other than the noble metal element = (Molar content of each metal element other than the noble metal element in the catalyst layer) × (Molar mass of oxide of each metal element)

[0023] The mass content (mass%) of each precious metal element in the catalyst layer, expressed as metal equivalent, can be calculated using the following formula: Mass content (mass%) of each precious metal element in the catalyst layer = (V value for each precious metal element) / {(Sum of V values ​​for all precious metal elements) + (Sum of W values ​​for all non-precious metal elements)} × 100

[0024] The mass content (mass%) of each metal element other than the noble metal elements in the catalyst layer, in terms of oxides, can be calculated using the following formula: Mass content (mass%) of each metal element other than the noble metal elements in the catalyst layer, in terms of oxides = (W value for each metal element other than the noble metal elements) / {(Sum of V values ​​for all noble metal elements) + (Sum of W values ​​for all metal elements other than the noble metal elements)} × 100

[0025] <Al-based oxides> "Al-based oxides" refer to oxides containing Al, in which Al is the most abundant metal element by mass among the metal elements constituting the oxide. However, those falling under the Ce-Zr composite oxides described later are not considered Al-based oxides. Al-based oxides are described below. Two or more of the characteristics of Al-based oxides described below can be combined.

[0026] Al-based oxides are, for example, particulate. Al-based oxides are used as carriers for catalytically active components. From the viewpoint of improving the support capacity of catalytically active components, it is preferable that Al-based oxides are porous.

[0027] Al-based oxides are distinguished from alumina (alumina binder), which is used as a binder, by having a particle size suitable for use as a carrier for catalytically active components. The average particle size of Al-based oxides is preferably 3 μm to 25 μm, more preferably 4 μm to 20 μm, and even more preferably 5 μm to 15 μm. The lower limit above may be combined with any of the upper limits above. The method for measuring the average particle size of Al-based oxides is as follows: A sample containing Al-based oxides is observed using a scanning electron microscope (SEM), and the directional diameter (Ferret diameter) of 100 Al-based oxides arbitrarily selected from the field of view is measured, and the average value is taken as the average particle size of the Al-based oxides.

[0028] Al-based oxides may contain one or more metal elements other than Al (hereinafter referred to as "additional element M1"). 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.

[0029] In Al-based oxides, the additional element M1 is a solid solution phase (for example, Al ２ O ３ The additional element M1 may form a solid solution phase with an oxide of the additional element M1, or it may form a single phase that is either crystalline or amorphous (for example, an oxide phase of the additional element M1), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of the additional element M1 forms a solid solution phase.

[0030] Examples of Al-based oxides include alumina (Al ２ O ３ Examples include oxides obtained by modifying the surface of alumina with an additional element M1 or its oxide, and oxides obtained by solid-solving the additional element M1 or its oxide in alumina. Examples of Al-based oxides containing the additional element M1 include alumina-silica, alumina-zirconia, alumina-chromia, alumina-celia, and alumina-lantana.

[0031] From the viewpoint of improving the heat resistance of Al-based oxides, ２ O ３ The converted mass content (hereinafter referred to as "content C1") is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The upper limit is 100% by mass.

[0032] The content C1 is given by the formula: Content C1 = (Al content in Al-based oxides) ２ O ３It is defined as (Converted Mass) / (Mass of Al-based Oxide) × 100. "Mass of Al-based Oxide" refers to the total mass of oxides of the metal elements, calculated 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.

[0033] If the composition of the Al-based oxide is known, the content C1 can be determined from the composition of the Al-based oxide.

[0034] If the composition of the Al-based oxide is unknown, the content C1 can be determined by analyzing the sample containing the Al-based oxide using energy-dispersive X-ray spectroscopy (EDX), and combining the resulting elemental mapping with the EDX elemental analysis of the specified Al-based oxide. Specifically, by qualitatively identifying (color-coding) the Al-based oxide and other particles using elemental mapping, and then performing a compositional analysis (elemental analysis) on the specified Al-based oxide, the content C1 for the specified Al-based oxide can be determined.

[0035] <Ce-based oxides> "Ce-based oxides" refer to oxides containing Ce, in which Ce is the most abundant metal element by mass among the metal elements constituting the oxide. However, those falling under the Ce-Zr composite oxides described later are not considered Ce-based oxides. Ce-based oxides are described below. Two or more of the characteristics of Ce-based oxides described below can be combined.

[0036] Ce-based oxides are, for example, particulate. Ce-based oxides are used as carriers for catalytically active components. From the viewpoint of improving the support capacity of catalytically active components, it is preferable that the Ce-based oxide is porous.

[0037] Ce-based oxides are distinguished from ceria (ceria binders) used as binders by having a particle size suitable for use as a carrier for catalytically active components. The average particle size of Ce-based oxides is preferably 2.5 μm to 15 μm, more preferably 3.0 μm to 12 μm. The lower limit above may be combined with any of the upper limits above. The method for measuring the average particle size of Ce-based oxides is the same as the method for measuring the average particle size of Al-based oxides.

[0038] Ce-based oxides possess OSC (Oxygen Storage Capacity), meaning they absorb oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration is low. Therefore, when the catalyst layer contains Ce-based oxides, fluctuations in the oxygen concentration in the exhaust gas are mitigated, and the operating window of the catalytically active components in the catalyst layer is expanded.

[0039] Ce-based oxides may contain one or more metallic elements other than Ce (hereinafter referred to as "additional element M2"). Additional element M2 can be selected from, for example, rare earth elements other than Ce (e.g., Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0040] In Ce-based oxides, the additional element M2 is a solid solution phase (e.g., CeO ２ The additional element M2 may form a solid solution phase with an oxide of the additional element M2, or it may form a single phase that is either crystalline or amorphous (for example, an oxide phase of the additional element M2), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of the additional element M2 forms a solid solution phase.

[0041] Examples of Ce-based oxides include ceria (CeO ２ Examples include oxides obtained by modifying the surface of ceria with an additional element M2 or its oxide, and oxides obtained by solid-solving the additional element M2 or its oxide in ceria.

[0042] From the perspective of improving the OSC of Ce-based oxides, CeO ２ The converted mass content (hereinafter referred to as "content C2") is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. The upper limit is 100% by mass.

[0043] The content C2 is given by the formula: Content C2 = (CeO ２It is defined as (Converted Mass) / (Mass of Ce-based Oxide) × 100. "Mass of Ce-based Oxide" means the total mass of oxides of the metal elements, calculated assuming that each metal element in the Ce-based oxide exists as an oxide. The meaning of "oxide" of a metal element is as described above. The content C2 can be calculated in the same way as the content C1.

[0044] <Zr-based oxides> "Zr-based oxides" refer to oxides containing Zr, in which Zr is the most abundant metal element by mass among the metal elements constituting the oxide. However, those falling under the Ce-Zr composite oxides described later are not considered Zr-based oxides. Zr-based oxides are described below. Two or more of the characteristics of Zr-based oxides described below can be combined.

[0045] Zr oxides are, for example, particulate. Zr oxides are used as carriers for catalytically active components. From the viewpoint of improving the support capacity of catalytically active components, it is preferable that Zr oxides are porous.

[0046] Zr-based oxides are distinguished from zirconia (zirconia binders), which are used as binders, by having a particle size suitable for use as a carrier for catalytically active components. The average particle size of Zr-based oxides is preferably 3 μm to 25 μm, more preferably 4 μm to 20 μm, and even more preferably 5 μm to 15 μm. Each of the above lower limits may be combined with any of the above upper limits. The method for measuring the average particle size of Zr-based oxides is the same as the method for measuring the average particle size of Al-based oxides.

[0047] Zr-based oxides may contain one or more metal elements other than Zr (hereinafter referred to as "additional element M3"). Additional element M3 can be selected from, for example, rare earth elements (e.g., Ce, Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), B, Si, Al, Cr, etc.

[0048] In Zr-based oxides, the additional element M3 is a solid solution phase (for example, ZrO ２The additional element M3 may form a solid solution phase with an oxide of the additional element M3, or it may form a single phase that is either crystalline or amorphous (for example, an oxide phase of the additional element M3), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of the additional element M3 forms a solid solution phase.

[0049] Examples of Zr-based oxides include zirconia (ZrO ２ Examples include oxides obtained by modifying the surface of zirconia with the additional element M3 or its oxide, and oxides obtained by solid-solving the additional element M3 or its oxide in zirconia.

[0050] From the viewpoint of improving the heat resistance of Zr-based oxides, Zr in Zr-based oxides is ZrO ２ The converted mass content (hereinafter referred to as "content C3") is preferably 40% by mass or more, more preferably 50% by mass or more. The upper limit is 100% by mass.

[0051] The content C3 is given by the formula: Content C3 = (Zr in Zr-based oxides) ２ It is defined as (Converted Mass) / (Mass of Zr-based Oxide) × 100. "Mass of Zr-based Oxide" means the total mass of oxides of the metal elements, calculated assuming that each metal element in the Zr-based oxide exists as an oxide. The meaning of "oxide" of a metal element is as described above. The content C3 can be determined in the same way as the content C1.

[0052] From the viewpoint of improving the heat resistance of Zr-based oxides, it is preferable that Zr-based oxides contain one or more rare earth elements other than Ce. The one or more rare earth elements other than Ce are preferably selected from La, Y, Pr, and Nd.

[0053] From the viewpoint of improving the heat resistance of Zr-based oxides, the mass content of rare earth elements other than Ce in Zr-based oxides, in terms of oxides (hereinafter referred to as "content C4"), is preferably 5% by mass or more and 48% by mass or less, more preferably 15% by mass or more and 47% by mass or less, and even more preferably 25% by mass or more and 46% by mass or less. The lower limit above may be combined with any of the upper limits above. "Mass content of rare earth elements other than Ce in Zr-based oxides, in terms of oxides" means the mass content of one rare earth element other than Ce when the Zr-based oxide contains that one rare earth element, and the total mass content of two or more rare earth elements in terms of oxides when the Zr-based oxide contains two or more rare earth elements other than Ce.

[0054] The content C4 is defined by the formula: Content C4 = (mass of rare earth elements other than Ce in Zr oxides, converted to oxides) / (mass of Zr oxides) × 100. The meaning of "mass of Zr oxides" and "oxides" of metal elements is as described above. Content C4 can be determined in the same way as content C1.

[0055] <Ce-Zr composite oxide> A "Ce-Zr composite oxide" is a composite oxide containing Ce and Zr, wherein the Ce in the composite oxide is CeO ２ The converted mass content (hereinafter referred to as "content C5") is 5% by mass or more and 95% by mass or less, and the Zr in the composite oxide is ZrO ２ This refers to oxides with a converted mass content (hereinafter referred to as "content C6") of 5% by mass or more and 95% by mass or less. The following describes Ce-Zr composite oxides. Two or more of the characteristics of the Ce-Zr composite oxides described below can be combined.

[0056] Ce-Zr composite oxides are, for example, particulate. Ce-Zr composite oxides are used as carriers for catalytically active components. From the viewpoint of improving the support capacity of catalytically active components, it is preferable that Ce-Zr composite oxides are porous.

[0057] Ce-Zr composite oxides have particle sizes suitable for use as carriers for catalytically active components. The average particle size of Ce-Zr composite oxides is preferably 2.5 μm to 15 μm, more preferably 3 μm to 12 μm. The lower limit above may be combined with any of the upper limits above. The method for measuring the average particle size of Ce-Zr composite oxides is the same as the method for measuring the average particle size of Al-based oxides.

[0058] Ce-Zr composite oxides have an OSC (Oxygen Saturation Cell). Therefore, when the catalyst layer contains a Ce-Zr composite oxide, fluctuations in the oxygen concentration in the exhaust gas are mitigated, and the operating window of the catalytically active components in the catalyst layer is expanded.

[0059] Ce-Zr composite oxides may contain one or more metal elements other than Ce and Zr (hereinafter referred to as "additional element M4"). Additional element M4 can be selected from, for example, rare earth elements other than Ce (e.g., Y, Pr, La, Nd, Sm, Eu, Gd, etc.), alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0060] In Ce-Zr composite oxides, Ce is a solid solution phase (for example, CeO ２ and ZrO ２ It may form a solid solution phase with, or a single phase that is a crystalline or amorphous phase (for example, CeO ２ It may form a single phase, or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Ce forms a solid solution phase.

[0061] In Ce-Zr composite oxides, Zr is a solid solution phase (e.g., CeO ２ and ZrO ２ It may form a solid solution phase with (for example, ZrO), or it may form a single phase that is a crystalline phase or an amorphous phase (for example, ZrO ２ It may form a single phase, or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of Zr forms a solid solution phase.

[0062] In Ce-Zr composite oxides, the additional element M4 is a solid solution phase (e.g., CeO ２A solid solution phase with an oxide of the additional element M4, ZrO ２ A solid solution phase with an oxide of the additional element M4, CeO ２ and ZrO ２ It may form a solid solution phase (such as a solid solution phase with an oxide of the additional element M4), or it may form a single phase that is a crystalline or amorphous phase (for example, a single oxide phase of the additional element M4), or it may form both a solid solution phase and a single phase, but it is preferable that at least a portion of the additional element M4 forms a solid solution phase.

[0063] From the viewpoint of improving the OSC of the Ce-Zr composite oxide, the content of C5 is preferably 7% by mass or more, more preferably 10% by mass or more, and even more preferably 30% by mass or more. The upper limit can be adjusted as appropriate considering heat resistance, structural stability, the content of other components, etc. The upper limit is preferably 93% by mass or less, more preferably 90% by mass or less, and even more preferably 70% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0064] From the viewpoint of improving the heat resistance of the Ce-Zr composite oxide, the C6 content is preferably 7% by mass or more, more preferably 10% by mass or more, and even more preferably 30% by mass or more. The upper limit can be adjusted as appropriate considering heat resistance, structural stability, the content of other components, etc. The upper limit is preferably 93% by mass or less, more preferably 90% by mass or less, and even more preferably 70% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0065] From the viewpoint of improving the heat resistance and OSC of the Ce-Zr composite oxide, the sum of the C5 and C6 content is preferably 70% by mass or more, more preferably 75% by mass or more, even more preferably 80% by mass or more, and even more preferably 85% by mass or more. The upper limit is 100% by mass.

[0066] The content of C5 and C6 are given by the formula: Content C5 = (CeO of Ce in Ce-Zr composite oxide) ２ Conversion mass) / (mass of Ce-Zr composite oxide) × 100 and formula: Content C6 = (Zr in Ce-Zr composite oxide) ２It is defined as (Converted Mass) / (Mass of Ce-Zr Composite Oxide) × 100. The "Mass of Ce-Zr Composite Oxide" refers to the total mass of oxides of the metal elements, calculated assuming that each metal element in the Ce-Zr composite oxide exists as an oxide. The meaning of "oxide" of the metal elements is as described above. The content of C5 and C6 can be determined in the same way as the content of C1.

[0067] From the viewpoint of improving the heat resistance and / or OSC of Ce-Zr composite oxides, it is preferable that Ce-Zr composite oxides contain one or more rare earth elements other than Ce. The one or more rare earth elements other than Ce are preferably selected from La, Pr, Y, and Nd.

[0068] From the viewpoint of improving the heat resistance and / or OSC of Ce-Zr composite oxides, the mass content of rare earth elements other than Ce in the Ce-Zr composite oxide in terms of oxides (hereinafter referred to as "content C7") is preferably 4% by mass or more and 40% by mass or less, more preferably 6% by mass or more and 30% by mass or less, and even more preferably 8% by mass or more and 20% by mass or less. The lower limit above may be combined with any of the upper limits above. "Mass content of rare earth elements other than Ce in the Ce-Zr composite oxide in terms of oxides" means the mass content of one rare earth element other than Ce when the Ce-Zr composite oxide contains that one rare earth element, and the total mass content of two or more rare earth elements in terms of oxides when the Ce-Zr composite oxide contains two or more rare earth elements other than Ce.

[0069] The content C7 is defined by the formula: Content C7 = (mass of rare earth elements other than Ce in the Ce-Zr composite oxide in oxide form) / (mass of the Ce-Zr composite oxide) × 100. The meaning of "mass of the Ce-Zr composite oxide" and "oxide" of the metal element is as described above. The content C7 can be determined in the same way as the content C1.

[0070] <<Exhaust Gas Purification Catalyst>> The exhaust gas purification catalyst of the present invention will be described below.

[0071] Below, an exhaust gas purification catalyst 1 (hereinafter sometimes referred to as "catalyst 1") according to one embodiment of the present invention will be described based on Figures 1 to 4. Two or more of the features of catalyst 1 described below can be combined.

[0072] As shown in Figure 1, the catalyst 1 is located in the exhaust passage within the exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas discharged from the internal combustion engine flows through the exhaust passage within the exhaust pipe P from one end to the other and is purified by the catalyst 1 installed in the exhaust pipe P. In the drawings, the direction of exhaust gas flow is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas inlet side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outlet side" or "downstream side."

[0073] Other exhaust gas purification catalysts may be placed in the exhaust passage within the exhaust pipe P, upstream and / or downstream of catalyst 1.

[0074] As shown in Figures 2 to 4, the catalyst 1 comprises a metal honeycomb substrate 10 (hereinafter referred to as "substrate 10"), a first catalyst layer 20 provided on the substrate 10, and a second catalyst layer 30 provided on the first catalyst layer 20.

[0075] Catalyst 1 comprises a first catalyst layer 20 containing Pd and Ce, a second catalyst layer 30 containing Rh, and CeO2 in the first catalyst layer 20. ２ The converted mass content is 25% by mass or more, the mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 in which the second catalyst layer 30 is provided (hereinafter referred to as "WC amount of the second catalyst layer 30") is 30 g / L or more and 90 g / L or less, and the WC amount of the second catalyst layer 30 is X [g / L], and the ratio of the total area of ​​all pores to the area of ​​the cross-sectional area of ​​the second catalyst layer 30 is Y × 10 ２ When expressed as [%], the value of X × Y is 10 or more, and the area of ​​the cross-sectional area of ​​the second catalyst layer 30 is 10 μm ２ The following characteristics are characterized by the fact that the total area of ​​the pores accounts for 2% or more of the total area.

[0076] The effects of catalyst 1 will be explained below.

[0077] Effects of the second catalyst layer 30 being provided on the first catalyst layer 20: Because the second catalyst layer 30 is provided on the first catalyst layer 20, the exhaust gas passes through the second catalyst layer 30 before reaching the first catalyst layer 20. As the exhaust gas passes through the second catalyst layer 30, the second catalyst layer 30 captures phosphorus compounds in the exhaust gas. In other words, the second catalyst layer 30 has phosphorus-capturing ability. Due to the phosphorus-capturing ability of the second catalyst layer 30, the reduction in the exhaust gas purification performance of the precious metal elements in the first catalyst layer 20 due to phosphorus poisoning is suppressed.

[0078] Effects of the first catalyst layer 20 containing Pd and the second catalyst layer 30 containing Rh: Because the second catalyst layer 30 is provided on the first catalyst layer 20, the noble metal elements in the first catalyst layer 20 are less susceptible to phosphorus poisoning by phosphorus compounds in the exhaust gas, whereas the noble metal elements in the second catalyst layer 30 are more susceptible to phosphorus poisoning by phosphorus compounds in the exhaust gas. On the other hand, the exhaust gas purification performance of Pd is easily reduced by phosphorus poisoning, while the exhaust gas purification performance of Rh is less susceptible to reduction by phosphorus poisoning. Therefore, Pd is suitable as a noble metal element in the first catalyst layer 20, and Rh is suitable as a noble metal element in the second catalyst layer 30.

[0079] Since the exhaust gas purification performance of Rh decreases due to Rh oxidation, it is preferable to use Rh in an environment where Rh reduction is likely to occur. Because the second catalyst layer 30 is provided on the first catalyst layer 20, the amount of reducing components in the exhaust gas that come into contact with the second catalyst layer 30 (e.g., hydrocarbons (HC), carbon monoxide (CO), etc.) fluctuates with changes in the operating conditions of the internal combustion engine and temporarily increases. On the other hand, since the exhaust gas comes into contact with the first catalyst layer 20 after coming into contact with the second catalyst layer 30, the amount of reducing components in the exhaust gas that comes into contact with the first catalyst layer 20 is less likely to fluctuate and less likely to temporarily increase. Therefore, the second catalyst layer 30 is in an environment where Rh reduction is more likely to occur than in the first catalyst layer 20.

[0080] Based on the above, the exhaust gas purification performance of Pd and Rh is effectively realized by having the first catalyst layer 20 contain Pd and the second catalyst layer 30 contain Rh.

[0081] Effects of having a WC content of 30 g / L or more and 90 g / L or less in the second catalyst layer 30: The greater the WC content of the second catalyst layer 30 (i.e., the greater the thickness of the second catalyst layer 30), the greater the phosphorus scavenging ability of the second catalyst layer 30. By having a WC content of 30 g / L or more in the second catalyst layer 30, the phosphorus scavenging ability of the second catalyst layer 30 is improved.

[0082] If the amount of WC in the second catalyst layer 30 is too low, peeling of the second catalyst layer 30 from the substrate 10 is likely to occur. If the amount of WC in the second catalyst layer 30 is 30 g / L or more, peeling of the second catalyst layer 30 from the substrate 10 is suppressed.

[0083] There is a demand for reducing the amount of Rh used. Assuming that the concentration of Rh in the second catalyst layer 30 (i.e., the mass content of Rh in the second catalyst layer 30 in terms of metal) is constant, the less WC (Waste Clearing) in the second catalyst layer 30 there is, the less Rh is used in the second catalyst layer 30. By keeping the WC amount of the second catalyst layer 30 at 90 g / L or less, the demand for reducing the amount of Rh used can be met. On the other hand, assuming that the amount of Rh used in the second catalyst layer is constant, the less WC in the second catalyst layer 30 there is, the more Rh is concentrated on the surface of the second catalyst layer 30. By keeping the WC amount of the second catalyst layer 30 at 90 g / L or less, even under conditions where exhaust gas only diffuses to the surface of the second catalyst layer 30, such as during high-speed operation, the exhaust gas can more easily come into contact with Rh, and the exhaust gas purification performance of Rh can be effectively demonstrated.

[0084] Effects of X×Y value being 10 or greater As described above, the larger the amount of WC in the second catalyst layer 30 (i.e., the greater the thickness of the second catalyst layer 30), the greater the phosphorus capture capacity of the second catalyst layer 30. However, it becomes more difficult for exhaust gas to reach the first catalyst layer 20, and the exhaust gas purification performance of the first catalyst layer 20 becomes less effective. When the exhaust gas purification performance of the first catalyst layer 20 becomes less effective, the exhaust gas is more likely to pass through catalyst 1 unpurified. The value of X×Y is an index that represents the absolute amount of voids in the second catalyst layer 30, that is, the ease with which exhaust gas can be transferred from the second catalyst layer 30 to the first catalyst layer 20. When the value of X×Y is 10 or greater, the ease with which exhaust gas can be transferred from the second catalyst layer 30 to the first catalyst layer 20 is improved, and the exhaust gas purification performance of the first catalyst layer 20 is effectively demonstrated.

[0085] The area of ​​the cross-sectional area of ​​the second catalyst layer 30 is 10 μm ２ The following effects occur when the total area of ​​the pores accounts for 2% or more of the total area: If the value of X × Y is 10 or more, exhaust gas can more easily reach the first catalyst layer 20, but phosphorus compounds in the exhaust gas can also more easily reach the first catalyst layer 20. When phosphorus compounds in the exhaust gas can more easily reach the first catalyst layer 20, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 is more likely to decrease due to phosphorus poisoning. When phosphorus compounds in the exhaust gas pass through the pores, they come into contact with the surface of the pores and are captured on the surface of the pores. The smaller the diameter of the pores, the more easily phosphorus compounds in the exhaust gas come into contact with the surface of the pores and are captured on the surface of the pores. Therefore, the area of ​​the cross-sectional area of ​​the second catalyst layer 30 is 10 μm ２ The percentage of the total pore area shown below is an indicator of the phosphorus scavenging ability of the second catalyst layer 30. When this percentage is 2% or more, the phosphorus scavenging ability of the second catalyst layer 30 is improved, and the reduction in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 due to phosphorus poisoning is effectively suppressed.

[0086] The first catalyst layer 20 contains Ce, and the CeO in the first catalyst layer 20 ２ Effect of having a converted mass content of 25% by mass or more: Ce has OSC. If the value of X × Y is 10 or more, the exhaust gas is more likely to reach the first catalyst layer 20. Under these conditions, CeO of Ce in the first catalyst layer 20 ２ By having a converted mass content of 25% by mass or more, the OSC is effectively utilized, and the exhaust gas purification performance of the first catalyst layer 20 is improved.

[0087] As a result of the combined effects described above, the exhaust gas purification performance of catalyst 1 after exposure to phosphorus is improved. Therefore, catalyst 1 can exhibit excellent exhaust gas purification performance even after exposure to phosphorus.

[0088] <Base Material> The base material 10 will be described below. Two or more of the features of the base material 10 described below can be combined.

[0089] The metal honeycomb substrate 10 is a honeycomb structure made of metal material.

[0090] The metal material constituting the metal honeycomb substrate 10 can be appropriately selected from known metal materials. Examples of materials constituting the metal honeycomb substrate 10 include alloys such as stainless steel.

[0091] As shown in Figures 2 to 4, the base material 10 has a cylindrical portion 11, a partition wall portion 12 provided inside the cylindrical portion 11, and cells 13 separated by the partition wall portion 12.

[0092] As shown in Figure 2, the cylindrical portion 11 defines the outer shape of the base material 10, and the axial direction of the cylindrical portion 11 coincides with the axial direction of the base material 10. As shown in Figure 2, the shape of the cylindrical portion 11 is cylindrical, but it may also be other shapes such as an elliptical cylinder or a polygonal cylinder.

[0093] As shown in Figures 2 to 4, a partition wall 12 exists between adjacent cells 13, and adjacent cells 13 are separated by the partition wall 12. The thickness of the partition wall 12 is, for example, 20 μm or more and 1500 μm or less.

[0094] As shown in Figure 4, 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.

[0095] As shown in Figure 4, both the exhaust gas inlet and exhaust gas outlet ends of cell 13 are open. Therefore, exhaust gas that flows in from the exhaust gas inlet end (opening) of cell 13 flows out from the exhaust gas outlet end (opening) of cell 13. This type of configuration is called a flow-through type.

[0096] As shown in Figures 2 and 3, the plan view shape of the exhaust gas inlet end (opening) of cell 13 is a rectangle, but it may be a hexagon, octagon, or other shape. The same applies to the plan view shape of the exhaust gas outlet end (opening) of cell 13.

[0097] 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 refers to the total number of cells 13 per square inch in a cross-section obtained by cutting the substrate 10 with a plane perpendicular to the exhaust gas flow direction X.

[0098] The volume of the base material 10 is, for example, 0.1 L or more and 20 L or less. The volume of the base material 10 refers to the apparent volume of the base material 10. For example, if the base material 10 is cylindrical, and the outer diameter of the base material 10 is 2r and the length of the base material 10 is L10, then the volume of the base material 10 is given by the formula: Volume of base material 10 = π × r ２ It is represented by ×L10.

[0099] <First Catalyst Layer> The first catalyst layer 20 will be described below. Two or more of the features of the first catalyst layer 20 described below can be combined.

[0100] As shown in Figures 3 and 4, the first catalyst layer 20 is provided on the substrate 10. Specifically, the first catalyst layer 20 is provided on the cell 13 side surface of the partition wall 12. The "cell 13 side surface of the partition wall 12" means the outer surface of the partition wall 12 that extends in the exhaust gas flow direction X. The first catalyst layer 20 may be provided directly on the cell 13 side surface of the partition wall 12 or via another layer, but is usually provided directly on the cell 13 side surface of the partition wall 12. The "first catalyst layer 20 provided on the substrate 10" includes both embodiments in which the first catalyst layer 20 is provided directly on the cell 13 side surface of the partition wall 12 and embodiments in which the first catalyst layer 20 is provided on the cell 13 side surface of the partition wall 12 via another layer.

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

[0102] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalyst layer 20 per unit volume of the portion of the base material 10 in which the first catalyst layer 20 is provided (hereinafter referred to as "WC amount of the first catalyst layer 20") is preferably 50 g / L or more and 200 g / L or less, more preferably 80 g / L or more and 150 g / L or less, and even more preferably 90 g / L or more and 130 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0103] The amount of WC in the first catalyst layer 20 is calculated using the formula: (mass of the first catalyst layer 20) / ((volume of the substrate 10) × (length L20 of the first catalyst layer 20 / length L10 of the substrate 10)). In this specification, unless otherwise specified, "length" refers to the axial dimension of the substrate 10.

[0104] The mass of the first catalyst layer 20 can be determined by the method described later.

[0105] An example of a method for measuring the length L20 of the first catalyst layer 20 is as follows.

[0106] A sample is cut from the catalyst 1 that extends in the axial direction of the substrate 10 and has the same length as the length L10 of the substrate 10. The sample is, for example, cylindrical with a diameter of 25.4 mm. The diameter of the sample can be changed as needed. If the first catalyst layer 20 extends from the exhaust gas inlet side end of the partition wall 12 along the exhaust gas flow direction X, the sample is cut at 5 mm intervals using a plane perpendicular to the axial direction of the substrate 10, and the first cut piece, second cut piece, ..., nth cut piece are obtained in order from the exhaust gas inlet side end of the sample. If the first catalyst layer 20 extends from the exhaust gas outlet side end of the partition wall 12 in the direction opposite to the exhaust gas flow direction X, the sample is cut at 5 mm intervals using a plane perpendicular to the axial direction of the substrate 10, and the first cut piece, second cut piece, ..., nth cut piece are obtained in order from the exhaust gas outlet side end of the sample. In either case, the length of the cut piece is 5 mm. The composition of the cut piece is analyzed using an X-ray fluorescence analyzer (XRF) (e.g., energy-dispersive X-ray analyzer (EDX), wavelength-dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma emission spectrometer (ICP-AES), a scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDX), etc., and based on the composition of the cut piece, it is confirmed whether or not the cut piece contains a part of the first catalyst layer 20.

[0107] For cut pieces that clearly contain a portion of the first catalyst layer 20, compositional analysis is not necessarily required. For example, the cut surface can be observed using a scanning electron microscope (SEM), electron beam microanalyzer (EPMA), etc., to confirm whether or not the cut piece contains a portion of the first catalyst layer 20. Elemental mapping of the cut surface may also be performed when observing the cut surface.

[0108] After confirming whether the cut piece contains a portion of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula: Length of the first catalyst layer 20 contained in the sample = 5 mm × (Number of cut pieces containing a portion of the first catalyst layer 20)

[0109] For example, if the first to kth sections contain a portion of the first catalyst layer 20, but the (k+1) to nth sections do not, the total length of the first catalyst layer 20 contained in the sample is (5 × k) mm.

[0110] An example of a more detailed method for measuring the length of the first catalyst layer 20 contained in the sample is as follows.

[0111] The kth cut is cut along the axial direction of the substrate 10, and the length of the portion of the first catalyst layer 20 present on the cut surface is observed using SEM, EPMA, etc., to measure the length of the portion of the first catalyst layer 20 in the kth cut. Then, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula. Note that if the first catalyst layer 20 extends from the exhaust gas inlet end of the partition wall 12 along the exhaust gas flow direction X, the kth cut is the cut piece containing a portion of the first catalyst layer 20 obtained from the exhaust gas outlet side of the sample. If the first catalyst layer 20 extends from the exhaust gas outlet end of the partition wall 12 in the direction opposite to the exhaust gas flow direction X, the kth cut is the cut piece containing a portion of the first catalyst layer 20 obtained from the exhaust gas inlet side of the sample. Length of the first catalyst layer 20 contained in the sample = (5 mm × (k - 1)) + (Length of the portion of the first catalyst layer 20 contained in the kth cut)

[0112] The length of the catalyst layer 20 contained in one sample may be used as the length L20 of the catalyst layer 20, or the average value of the lengths of the catalyst layers 20 contained in multiple samples may be used as the length L20 of the catalyst layer 20, but the latter is preferred. In the latter case, it is preferable to arbitrarily cut out 8 to 16 samples from the catalyst 1, measure the length of the first catalyst layer 20 contained in each sample, and use the average value of these measurements as the length L20 of the first catalyst layer 20.

[0113] The first catalyst layer 20 contains Pd as a catalytically active component. The Pd is included in the first catalyst layer 20 in a form that can function as a catalytically active component, such as metallic Pd, alloys containing Pd, or compounds containing Pd (e.g., oxides of Pd). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Pd is preferably in particulate form.

[0114] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of Pd in ​​the first catalyst layer 20 in terms of metal equivalent per unit volume of the portion of the substrate 10 in which the first catalyst layer 20 is provided is preferably 0.2 g / L or more and 2.5 g / L or less, more preferably 0.3 g / L or more and 1.5 g / L or less, and even more preferably 0.4 g / L or more and 1.0 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0115] The metallic mass of Pd in ​​the first catalyst layer 20 per unit volume of the portion of the substrate 10 in which the first catalyst layer 20 is provided can be calculated using the formula: (metallic mass content of Pd in ​​the first catalyst layer 20) × (WC amount in the first catalyst layer 20).

[0116] The first catalyst layer 20 may contain one or more noble metal elements other than Pd as catalytically active components. The noble metal elements other than Pd are included in the first catalyst layer 20 in a form that can function as catalytically active components, such as a metal composed of the noble metal element, an alloy containing the noble metal element, or a compound containing the noble metal element (for example, an oxide of the noble metal element). From the viewpoint of improving exhaust gas purification performance, it is preferable that the catalytically active components containing the noble metal elements other than Pd are in particulate form.

[0117] If the first catalyst layer 20 contains Pd and other precious metal elements, the Pd and other precious metal elements may form an alloy, potentially reducing the number of active sites of Pd that are involved in exhaust gas purification performance. Therefore, it is preferable that the first catalyst layer 20 substantially does not contain any precious metal elements other than Pd.

[0118] "The first catalyst layer 20 substantially does not contain any noble metal elements other than Pd" means that the mass content of noble metal elements other than Pd in ​​the first catalyst layer 20, in terms of metal, is preferably 1% by mass or less, more preferably 0.5% by mass or less, based on the mass of the first catalyst layer 20. The lower limit is 0% by mass. "Mass content of noble metal elements other than Pd in ​​the first catalyst layer 20, in terms of metal" means the mass content of one noble metal element other than Pd if the first catalyst layer 20 contains that one noble metal element, and the sum of the mass content of two or more noble metal elements in terms of metal if the first catalyst layer 20 contains two or more noble metal elements other than Pd.

[0119] The first catalyst layer 20 preferably comprises one or more types of carriers, and at least a portion of the catalytically active components is supported on one or more types of carriers. The "catalystically active components" include catalytically active components containing Pd and catalytically active components containing noble metal elements other than Pd.

[0120] In this specification, "at least a portion of the catalytically active component is supported on a carrier" means a state in which at least a portion of the catalytically active component is physically or chemically adsorbed or retained on the outer surface and / or the inner surface of the pores of the carrier.

[0121] The fact that at least a portion of the catalytically active components in a given catalyst layer are supported on a support can be confirmed, for example, using SEM-EDX. Specifically, if, in the elemental mapping obtained by analyzing a cross-section of the catalyst layer with SEM-EDX, at least a portion of the catalytically active components and the support are located in the same region, it can be determined that at least a portion of the catalytically active components are supported on a support.

[0122] The support material can be selected from, for example, inorganic oxides. Inorganic oxides are, for example, particulate. From the viewpoint of improving the support of catalytically active components, it is preferable that the inorganic oxide be porous. Inorganic oxides may or may not have OSCs. Examples of inorganic oxides include Al-based oxides, Ce-based oxides, Zr-based oxides, Ce-Zr composite oxides, oxides of rare earth elements other than Ce, and silica (SiO2). ２), Titania (TiO ２ ), zeolite (aluminosilicate), MgO, ZnO, SnO ２ Examples include oxides based on the above.

[0123] From the viewpoint of improving the heat resistance and / or OSC of the first catalyst layer 20, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the support is preferably selected from Al-based oxides, Ce-based oxides, Zr-based oxides, and Ce-Zr-based composite oxides, and more preferably selected from Al-based oxides and Ce-Zr-based composite oxides. In one embodiment, the first catalyst layer 20 includes Al-based oxides and Ce-Zr-based composite oxides as support.

[0124] The first catalyst layer 20 contains Ce, and the CeO in the first catalyst layer 20 ２ The converted mass content (hereinafter referred to as "content C8") is 25% by mass or more.

[0125] From the viewpoint of making OSC more effective and further improving the exhaust gas purification performance of the first catalyst layer 20, the content of C8 is preferably 27% by mass or more, more preferably 30% by mass or more. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 60% by mass or less, more preferably 50% by mass or less. The lower limits above may be combined with any of the upper limits above. If the first catalyst layer 20 contains one type of Ce source, the content of C8 is CeO derived from that one type of Ce source. ２ This refers to the converted mass content, and if the first catalyst layer 20 contains two or more Ce sources, then CeO2 is the Ce derived from those two or more Ce sources. ２ This refers to the sum of the converted mass content.

[0126] The first catalyst layer 20 contains one or more Ce sources.

[0127] Examples of Ce sources include Al oxides containing Ce, Ce oxides, Zr oxides containing Ce, Ce-Zr composite oxides, and ceria binders.

[0128] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, it is preferable that the first catalyst layer 20 contains a Ce-Zr composite oxide as a Ce source. In addition to the Ce-Zr composite oxide, the first catalyst layer 20 may also contain one or more other Ce sources.

[0129] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the mass content of Ce-Zr composite oxide in the first catalyst layer 20 (hereinafter referred to as "content C9") is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0130] The content C9 is defined by the formula: Content C9 = (Mass of Ce-Zr composite oxide in the first catalyst layer 20) / (Mass of the first catalyst layer 20) × 100.

[0131] If information about the raw materials used to form the first catalyst layer 20 (e.g., composition, quantity, etc.) is known, the content C9 can be determined from the raw material information.

[0132] If information on the raw materials used to form the first catalyst layer 20 is not available, the content C9 can be determined by conventional methods such as SEM-EDX. Specifically, it is as follows.

[0133] (1) Perform elemental analysis of the first catalyst layer 20 using SEM-EDX to identify the types of constituent elements of the first catalyst layer 20 and determine the mass content (mass%) of each identified metal element in terms of metal. (2) Perform elemental mapping of the first catalyst layer 20 using SEM-EDX to identify the types of particles contained in the first catalyst layer 20 (e.g., Al-based oxides, Ce-Zr-based composite oxides, etc.). (3) For each type of particle, perform elemental analysis of a number of arbitrarily selected particles (e.g., 50 particles) using SEM-EDX to identify the types of constituent elements of the particles and determine the mass content (mass%) of each identified metal element in terms of metal. For each type of particle, calculate the average value of the mass content (mass%) of each metal element in terms of metal. (4) The mass content of each type of particle in the sample is calculated by creating and solving an equation that expresses the relationship between the mass content of each metal element in the sample on a metal basis (mass%), the mass content of each type of particle in each type of particle on a metal basis (mass%), and the mass content of each type of particle in the sample (mass%), and this is taken as the mass content of each type of particle in the first catalyst layer 20.

[0134] From the viewpoint of improving the heat resistance and OSC of the first catalyst layer 20, and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the CeO2 in the first catalyst layer 20 ２ Of the converted mass, CeO from Ce-Zr composite oxides ２ The proportion of the converted mass is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more. The upper limit is 100% by mass.

[0135] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, it is preferable that the first catalyst layer 20 contains Al.

[0136] If the first catalyst layer 20 contains Al, the first catalyst layer 20 contains one or more Al sources.

[0137] Examples of Al sources include Al-based oxides, Al-containing Ce-based oxides, Al-containing Zr-based oxides, Al-containing Ce-Zr composite oxides, and alumina binders.

[0138] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, it is preferable that the first catalyst layer 20 contains an Al-based oxide as an Al source. In addition to the Al-based oxide, the first catalyst layer 20 may also contain one or more other Al sources as an Al source.

[0139] From the viewpoint of improving the heat resistance of the first catalyst layer 20 and thereby improving the exhaust gas purification performance of the first catalyst layer 20, the mass content of Al-based oxides in the first catalyst layer 20 (hereinafter referred to as "content C10") is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less. The lower limits above may be combined with any of the upper limits above.

[0140] The content C10 is defined by the formula: Content C10 = (Mass of Al-based oxide in the first catalyst layer 20) / (Mass of the first catalyst layer 20) × 100. The content C10 can be determined in the same manner as the content C9.

[0141] The first catalyst layer 20 may contain components such as a binder and a stabilizer. Examples of binders include inorganic oxide binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol. Examples of stabilizers include nitrates, carbonates, oxides, and sulfates of alkaline earth metal elements (e.g., Sr, Ba, etc.).

[0142] <Second Catalyst Layer> The second catalyst layer 30 will be described below. Two or more of the features of the second catalyst layer 30 described below can be combined.

[0143] As shown in Figures 3 and 4, the second catalyst layer 30 is provided on the first catalyst layer 20.

[0144] "The second catalyst layer 30 is provided on the first catalyst layer 20" means that part or all of the second catalyst layer 30 is located on the main surface of the first catalyst layer 20 that is opposite to the main surface on the partition wall portion 12 side. "Main surface of the first catalyst layer 20" means the outer surface of the first catalyst layer 20 that extends in the exhaust gas flow direction X. The second catalyst layer 30 may be provided directly on the main surface of the first catalyst layer 20 or via another layer, but is usually provided directly on the main surface of the first catalyst layer 20. The second catalyst layer 30 may be provided so as to cover a part of the main surface of the first catalyst layer 20, or so as to cover the entire main surface of the first catalyst layer 20. The phrase "second catalyst layer 30 provided on the first catalyst layer 20" includes both embodiments in which the second catalyst layer 30 is directly provided on the main surface of the first catalyst layer 20, and embodiments in which the second catalyst layer 30 is provided on the main surface of the first catalyst layer 20 via another layer.

[0145] As shown in Figure 4, the second catalyst layer 30 extends along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 to the exhaust gas outlet end of the partition wall 12. The second catalyst layer 30 may extend along the exhaust gas flow direction X from the exhaust gas inlet end of the partition wall 12 so as not to reach the exhaust gas outlet end of the partition wall 12, or it may extend along the direction opposite to the exhaust gas flow direction X from the exhaust gas outlet end of the partition wall 12 so as not to reach the exhaust gas inlet end of the partition wall 12.

[0146] The mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 in which the second catalyst layer 30 is provided (WC amount of the second catalyst layer 30) is 30 g / L or more and 90 g / L or less.

[0147] From the viewpoint of further improving the phosphorus scavenging ability of the second catalyst layer 30, the amount of WC in the second catalyst layer 30 is preferably 35 g / L or more, more preferably 40 g / L or more, and even more preferably 50 g / L or more. From the viewpoint of meeting the demand for reducing the amount of Rh used and from the viewpoint of concentrating Rh on the surface of the second catalyst layer 30, the amount of WC in the second catalyst layer 30 is preferably 85 g / L or less, more preferably 80 g / L or less, and even more preferably 70 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0148] The amount of WC in the second catalyst layer 30 can be calculated using the formula: (mass of the second catalyst layer 30) / ((volume of the substrate 10) × (length L30 of the second catalyst layer 30 / length L10 of the substrate 10)).

[0149] The above explanation regarding the method for measuring the length L20 of the first catalyst layer 20 also applies to the second catalyst layer 30. When applying this, "first catalyst layer 20" is replaced with "second catalyst layer 30," and "length L20" is replaced with "length L30."

[0150] The mass of the first catalyst layer 20 and the mass of the second catalyst layer 30 can be determined by the following method.

[0151] If information on the raw materials used to form the first catalyst layer 20 and the second catalyst layer 30 (e.g., composition, quantity, etc.) is known, the mass of the first catalyst layer 20 and the mass of the second catalyst layer 30 can be determined from the raw material information.

[0152] If information on the raw materials used to form the first catalyst layer 20 and the second catalyst layer 30 is not available, the masses of the first catalyst layer 20 and the second catalyst layer 30 can be measured by conventional methods such as SEM-EDX. Specifically, the methods are as follows.

[0153] (1) Identify the region in catalyst 1 where the first catalyst layer 20 and the second catalyst layer 30 are stacked. (2) Use SEM-EDX to perform elemental analysis of the first catalyst layer 20 and the second catalyst layer 30 in the region identified in (1) above, identify the types of constituent elements of the first catalyst layer 20 and the second catalyst layer 30, and determine the mass content (mass%) of each identified metal element in terms of metal. The identified metal elements are numbered as metal element 1, metal element 2, ..., metal element N-1, metal element N, etc. If the identified metal elements are Pd, Rh, Al, Ce, Zr, La, etc., they are numbered as follows: for example, Pd = metal element 1, Rh = metal element 2, Al = metal element 3, Ce = metal element 4, Zr = metal element 5, La = metal element 6. Hereinafter, metal element 1, metal element 2, ..., metal element N-1, and metal element N will be referred to as "metal element i" (i = 1, 2, ..., N-1, N), and the mass content (mass%) of metal element i in the first catalyst layer 20 measured by SEM-EDX will be referred to as "aｉ ", the mass content (mass%) of metal element i in the second catalyst layer 30 measured by SEM-EDX is "b ｉ This is written as . Metal element i only needs to be contained in one or more of the first catalyst layer 20 and the second catalyst layer 30, and does not need to be contained in all of the first catalyst layer 20 and the second catalyst layer 30. If the first catalyst layer 20 does not contain metal element i, a ｉ = 0. If the second catalyst layer 30 does not contain metal element i, b ｉ = 0. (3) The entire catalyst layer (first catalyst layer 20 and second catalyst layer 30) is scraped from catalyst 1, and the metal equivalent mass (g) of metal element i in the entire catalyst layer is determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Hereinafter, the metal equivalent mass (g) of metal element i in the entire catalyst layer will be denoted as "Pi". (4) The variable that minimizes the objective function under the following conditions is found. ・Objective function: • Variables: x (mass of the first catalyst layer 20 [g]), y (mass of the second catalyst layer 30 [g]) • Constraint: x + y = total mass of all catalyst layers • Constraint: 0 ≤ variable P ｉ = Mass of metal element i in the entire catalyst layer measured by ICP-AES [g] ・a ｉ = Mass content of metal element i in the first catalyst layer 20 as measured by SEM-EDX [mass%] ・b ｉ = Mass content of metal element i in the second catalyst layer 30 as measured by SEM-EDX [mass%] • Total mass of all catalyst layers = Total mass of all catalyst layers (first catalyst layer 20 and second catalyst layer 30) scraped from catalyst 1 [g]

[0154] The second catalyst layer 30 contains Rh as a catalytically active component. Rh is included in the second catalyst layer 30 in a form that can function as a catalytically active component, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (e.g., an oxide of Rh). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Rh is preferably in particulate form.

[0155] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the metallic mass of Rh in the second catalyst layer 30 per unit volume of the portion of the base material 10 in which the second catalyst layer 30 is provided is preferably 0.01 g / L or more and 1.0 g / L or less, more preferably 0.05 g / L or more and 0.5 g / L or less, and even more preferably 0.06 g / L or more and 0.3 g / L or less. Each of the above lower limits may be combined with any of the above upper limits.

[0156] The metallic mass of Rh in the second catalyst layer 30 per unit volume of the portion of the substrate 10 in which the second catalyst layer 30 is provided can be calculated using the formula: (metallic mass content of Rh in the second catalyst layer 30) × (WC amount in the second catalyst layer 30).

[0157] The second catalyst layer 30 may contain one or more noble metal elements other than Rh as catalytically active components. The noble metal elements other than Rh are included in the second catalyst layer 30 in a form that can function as catalytically active components, such as a metal composed of the noble metal element, an alloy containing the noble metal element, or a compound containing the noble metal element (for example, an oxide of the noble metal element). From the viewpoint of improving exhaust gas purification performance, it is preferable that the catalytically active components containing the noble metal elements other than Rh are in particulate form.

[0158] If the second catalyst layer 30 contains Rh and other precious metal elements, the Rh and other precious metal elements may form an alloy, potentially reducing the number of Rh active sites involved in exhaust gas purification performance. Therefore, it is preferable that the second catalyst layer 30 substantially does not contain any precious metal elements other than Rh.

[0159] "The second catalyst layer 30 substantially does not contain any noble metal elements other than Rh" means that the mass content of noble metal elements other than Rh in the second catalyst layer 30, in terms of metal, is preferably 1% by mass or less, more preferably 0.5% by mass or less, based on the mass of the second catalyst layer 30. The lower limit is 0% by mass. "Mass content of noble metal elements other than Rh in the second catalyst layer 30, in terms of metal" means the mass content of one noble metal element other than Rh if the second catalyst layer 30 contains that one noble metal element, and the sum of the mass content of two or more noble metal elements in terms of metal if the second catalyst layer 30 contains two or more noble metal elements other than Rh.

[0160] The second catalyst layer 30 preferably includes one or more types of carriers, and at least a portion of the catalytically active components is supported on one or more types of carriers. The "catalystically active components" include catalytically active components containing Rh and catalytically active components containing noble metal elements other than Rh. The meaning and method of confirming that "at least a portion of the catalytically active components is supported on carriers" is as described above. The carriers can be selected from, for example, inorganic oxides. The explanation regarding inorganic oxides is the same as described above.

[0161] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, the support is preferably selected from Al-based oxides, Zr-based oxides, and Ce-Zr-based composite oxides, and more preferably selected from Al-based oxides and Zr-based oxides. In one embodiment, the second catalyst layer 30 contains Al-based oxides and Zr-based oxides as the support.

[0162] The second catalyst layer 30 may contain Ce. However, if the second catalyst layer 30 contains Ce, the Rh in the second catalyst layer 30 may be oxidized by Ce, which may reduce the exhaust gas purification performance of Rh. From the viewpoint of effectively suppressing the reduction in exhaust gas purification performance of Rh due to Ce, the CeO2 in the second catalyst layer 30 ２The converted mass content (hereinafter referred to as "content C11") is preferably 8% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. The lower limit is 0% by mass. When the second catalyst layer 30 contains one type of Ce source, content C11 is the CeO of Ce derived from that one type of Ce source. ２ This refers to the converted mass content, and if the second catalyst layer 30 contains two or more Ce sources, then CeO2 is the amount of Ce derived from those two or more Ce sources. ２ This refers to the sum of the converted mass content.

[0163] If the second catalyst layer 30 contains Ce, the second catalyst layer 30 contains one or more Ce sources. The explanation regarding the Ce sources is the same as above.

[0164] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, it is preferable that the second catalyst layer 30 contains Al and / or Zr.

[0165] If the second catalyst layer 30 contains Al, the second catalyst layer 30 contains one or more Al sources. The explanation regarding the Al sources is the same as above.

[0166] From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, it is preferable that the second catalyst layer 30 contains an Al-based oxide as an Al source. In addition to the Al-based oxide, the second catalyst layer 30 may also contain one or more other Al sources as an Al source.

[0167] If the second catalyst layer 30 contains Zr, the second catalyst layer 30 contains one or more Zr sources. Examples of Zr sources include Al oxides containing Zr, Ce oxides containing Zr, Zr oxides, Ce-Zr composite oxides, zirconia binders, etc. From the viewpoint of improving the heat resistance of the second catalyst layer 30 and thereby improving the exhaust gas purification performance of the second catalyst layer 30, it is preferable that the second catalyst layer 30 contains a Zr oxide as a Zr source. In addition to a Zr oxide, the second catalyst layer 30 may also contain one or more other Zr sources.

[0168] The second catalyst layer 30 may contain components such as a binder and a stabilizer. The explanation regarding the binder and stabilizer is the same as described above.

[0169] In the second catalyst layer 30, the value of X × Y is 10 or greater. The larger the value of X × Y, the easier it is for exhaust gas to transfer from the second catalyst layer 30 to the first catalyst layer 20, and the more effectively the exhaust gas purification performance of the first catalyst layer 20 is demonstrated. Therefore, the value of X × Y is preferably 12 or greater, more preferably 12.2 or greater, and even more preferably 13.5 or greater. The upper limit can be adjusted as appropriate, taking into consideration the need to ensure the peeling strength of the second catalyst layer 30. The upper limit is preferably 20 or less, more preferably 17 or less, and even more preferably 15 or less. Each of the above lower limits may be combined with any of the above upper limits.

[0170] The X × Y value can be adjusted by increasing the Y value when the X value is small, and by decreasing the Y value when the X value is large.

[0171] From the viewpoint of improving the ease of exhaust gas transfer from the second catalyst layer 30 to the first catalyst layer 20 and effectively demonstrating the exhaust gas purification performance of the first catalyst layer 20, it is preferable that the value of X × Y is large and the value of X is small. In one embodiment, the value of X × Y is 12 or more, and the value of X (WC amount of the second catalyst layer 30) is 70 g / L or less.

[0172] The larger the value of Y, the easier it is for exhaust gas to transfer from the first catalyst layer 20 to the second catalyst layer 30, and the more effectively the exhaust gas purification performance of the first catalyst layer 20 is demonstrated. Also, the larger the value of Y, the easier it is for exhaust gas to diffuse within the second catalyst layer 30, and the more effectively the exhaust gas purification performance of the second catalyst layer 30 is demonstrated. Therefore, the value of Y is preferably 0.12 or higher, more preferably 0.13 or higher, and even more preferably 0.14 or higher. On the other hand, if the value of Y is too high, the strength of the second catalyst layer 30 decreases, and it becomes difficult to maintain the structure of the second catalyst layer 30. Therefore, the value of Y is preferably 0.25 or lower, more preferably 0.20 or lower, and even more preferably 0.17 or lower. Each of the above lower limits may be combined with any of the above upper limits.

[0173] The value of Y, that is, the ratio of the total area of ​​all pores to the area of ​​the cross-sectional area of ​​the second catalyst layer 30 (Y × 10 ２ [%]) can be measured by the method described in the examples.

[0174] The area of ​​the cross-sectional area of ​​the second catalyst layer 30 is 10 μm ２ The proportion of the total area of ​​the pores (hereinafter referred to as "proportion Z") is 2% or more. The larger proportion Z is, the better the phosphorus capture ability of the second catalyst layer 30, and the more effectively the reduction in the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 due to phosphorus poisoning is suppressed. Therefore, proportion Z is preferably 2.1% or more, more preferably 2.2% or more, and even more preferably 2.3% or more. On the other hand, if proportion Z is too large, the proportion of voids in the second catalyst layer 30 that are blocked by captured phosphorus increases, making it difficult for exhaust gas to reach the first catalyst layer 20. Therefore, proportion Z is preferably 3.5% or less, more preferably 3% or less, and even more preferably 2.5% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0175] The ratio Z can be measured by the method described in the examples.

[0176] The value of Y and the ratio Z can be adjusted to a desired range by adjusting the particle size, content, etc., of the carrier. The particle size of the carrier may be adjusted, for example, using a known grinding method such as a ball mill, or it may be adjusted using a granulation method such as spray drying during the production of the carrier.

[0177] In one embodiment, either the first inorganic oxide or the second inorganic oxide is used alone as a raw material for the second catalyst layer 30 in order to adjust the value of Y and the ratio Z to a desired range. In another embodiment, the first inorganic oxide and the second inorganic oxide are used in combination as raw materials for the second catalyst layer 30 in order to adjust the value of Y and the ratio Z to a desired range. From the viewpoint of effectively achieving the desired range of Y value and ratio Z, it is preferable that the first inorganic oxide and the second inorganic oxide are used in combination.

[0178] The first inorganic oxide is, for example, a Zr-based oxide. From the viewpoint of effectively realizing a desired range of Y values ​​and proportion Z, it is preferable that the Zr-based oxide has high heat resistance, and from the viewpoint of improving the heat resistance of the Zr-based oxide, ZrO ２ The converted mass content (content C3) is preferably within the above range.

[0179] From the viewpoint of effectively achieving a desired range of Y values ​​and proportion Z, the D50 of the first inorganic oxide used as a raw material is preferably 3 μm to 25 μm, more preferably 4 μm to 20 μm, and even more preferably 5 μm to 15 μm. Each of the above lower limits may be combined with any of the above upper limits. From the same viewpoint, the D90 of the first inorganic oxide used as a raw material is preferably 7 μm to 30 μm, more preferably 12 μm to 27 μm, and even more preferably 18 μm to 24 μm. Each of the above lower limits may be combined with any of the above upper limits. The D50 and D90 of the first inorganic oxide used as raw materials are usually maintained even after the production of the second catalyst layer 30. Therefore, if the D50 and D90 of the first inorganic oxide used as raw materials are within the above ranges, then the D50 and D90 of the first inorganic oxide in the second catalyst layer 30 are usually within the above ranges. By controlling D50 and D90 of the first inorganic oxide, it becomes easier to adjust the value of Y and the ratio Z to a desired range.

[0180] The second inorganic oxide is, for example, an Al-based oxide. From the viewpoint of effectively achieving a desired range of Y values ​​and proportion Z, it is preferable that the Al-based oxide has high heat resistance, and from the viewpoint of improving the heat resistance of the Al-based oxide, the Al in the Al-based oxide is ２ O ３ The converted mass content (content C1) is preferably within the above range.

[0181] From the viewpoint of effectively achieving a desired range of Y values ​​and proportion Z, the D50 of the second inorganic oxide used as a raw material is preferably 3 μm to 13 μm, more preferably 4 μm to 10 μm, and even more preferably 5 μm to 7 μm. Each of the above lower limits may be combined with any of the above upper limits. From the same viewpoint, the D90 of the second inorganic oxide used as a raw material is preferably 11 μm to 30 μm, more preferably 13 μm to 28 μm, and even more preferably 15 μm to 26 μm. Each of the above lower limits may be combined with any of the above upper limits. The D50 and D90 of the second inorganic oxide used as raw materials are usually maintained even after the manufacture of the second catalyst layer 30. Therefore, if the D50 and D90 of the second inorganic oxide used as raw materials are within the above ranges, then the D50 and D90 of the second inorganic oxide in the second catalyst layer 30 are usually within the above ranges. By controlling D50 and D90 of the second inorganic oxide, it becomes easier to adjust the value of Y and the ratio Z to a desired range.

[0182] In this specification, "D50" and "D90" refer to particle sizes at which the cumulative volume accounts for 50% and 90% of the volume-based particle size distribution measured by laser diffraction scattering particle size distribution analysis, respectively. D50 and D90 can be measured by the method described in the examples.

[0183] When the first inorganic oxide and the second inorganic oxide are used in combination as raw materials for the second catalyst layer 30, the mass content of the first inorganic oxide in the second catalyst layer 30 (hereinafter referred to as "content C12") is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of effectively achieving a desired range of Y value and proportion Z. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 48% by mass or less, more preferably 44% by mass or less, and even more preferably 40% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0184] When the first inorganic oxide and the second inorganic oxide are used in combination as raw materials for the second catalyst layer 30, the mass content of the second inorganic oxide in the second catalyst layer 30 (hereinafter referred to as "content C13") is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of effectively achieving a desired range of Y value and proportion Z. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 48% by mass or less, more preferably 44% by mass or less, and even more preferably 40% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0185] When the first inorganic oxide and the second inorganic oxide are used in combination as raw materials for the second catalyst layer 30, the sum of the content C12 and the content C13 is preferably 20% by mass or more, more preferably 40% by mass or more, and even more preferably 60% by mass or more, from the viewpoint of effectively achieving a desired range of Y value and proportion Z. The upper limit can be adjusted as appropriate considering the balance with cost, the content of other components, etc. The upper limit is preferably 96% by mass or less, more preferably 88% by mass or less, and even more preferably 80% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0186] When the first inorganic oxide is used alone as a raw material for the second catalyst layer 30 (i.e., the first inorganic oxide is used, but the second inorganic oxide is not), the content C12 is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, from the viewpoint of effectively achieving a desired range of Y values ​​and proportion Z. The upper limit can be adjusted as appropriate considering the balance with 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 the above lower limits may be combined with any of the above upper limits.

[0187] When the second inorganic oxide is used alone as a raw material for the second catalyst layer 30 (i.e., the second inorganic oxide is used, but the first inorganic oxide is not), the content C13 is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, from the viewpoint of effectively achieving a desired range of Y values ​​and proportion Z. The upper limit can be adjusted as appropriate considering the balance with 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 the above lower limits may be combined with any of the above upper limits.

[0188] The content ratios C12 and C13 are defined by the following formulas: Content ratio C12 = (Mass of the first inorganic oxide in the second catalyst layer 30) / (Mass of the second catalyst layer 30) × 100 and Content ratio C13 = (Mass of the second inorganic oxide in the second catalyst layer 30) / (Mass of the second catalyst layer 30) × 100, respectively. The content ratios C12 and C13 can be determined in the same manner as the content ratio C9.

[0189] <Manufacturing of Catalyst> Catalyst 1 can be manufactured by forming a first catalyst layer 20 on a substrate 10, and then forming a second catalyst layer 30 on the first catalyst layer 20.

[0190] The first catalyst layer 20 can be formed by preparing a first slurry by mixing a Pd source (e.g., a Pd salt), a Ce source (e.g., a Ce-Zr composite oxide), and other components (e.g., inorganic oxides other than the Ce source, a binder, a stabilizer, a solvent, etc.), coating the first slurry onto the substrate 10, drying it, and firing it.

[0191] The second catalyst layer 30 can be formed by preparing a second slurry by mixing a Rh source (e.g., Rh salt) and other components (e.g., inorganic oxides, binders, stabilizers, solvents, etc.), coating the second slurry onto the first catalyst layer 20, drying it, and firing it.

[0192] Examples of Pd and Rh salts include nitrates, ammine complex salts, acetates, and chlorides. Examples of binders include alumina sol, zirconia sol, titania sol, silica sol, and ceria sol. Examples of solvents include water and organic solvents.

[0193] The drying temperature is, for example, 60°C to 150°C, and the drying time is, for example, 0.1 hours to 1 hour. The firing temperature is, for example, 300°C to 700°C, and the firing time is, for example, 1 hour to 10 hours. Firing can be carried out, for example, in an air atmosphere.

[0194] [Example 1] (1) Preparation of the lower layer slurry In a container containing pure water, add palladium nitrate solution, Ce-Zr composite oxide (CeO ２ Conversion content: 40% by mass, ZrO ２ The following were added in order: a converted content of 50% by mass, an oxide-converted content of rare earth elements other than Ce (La and Pr) of 10% by mass, alumina, and binders (alumina binder and zirconia binder). The mixture was thoroughly stirred and mixed to obtain a slurry for the lower layer. The composition of the lower layer slurry was adjusted so that the composition of the lower layer after drying and calcination would be: Ce-Zr composite oxide: 80.1% by mass, alumina: 11.1% by mass, binder: 8% by mass in terms of solid content, and Pd: 0.8% by mass in terms of metal.

[0195] (2) Formation of the lower layer The entire through-flow type stainless metal honeycomb substrate (diameter: 40 mm, length: 120 mm, cell density: 400 cells / square inch, volume: 151 mL) was immersed in the slurry for the lower layer, excess slurry was removed, and the slurry for the lower layer was applied onto the inner wall surface of the substrate. After drying the substrate coated with the slurry for the lower layer at 90°C for 1 hour, it was fired at 500°C for 4 hours to form a lower layer on the inner wall surface of the substrate. The mass of the lower layer (mass after drying and firing), the mass of Pd in terms of metal in the lower layer, the mass of the Ce-Zr based composite oxide in the lower layer, and the mass of alumina in the lower layer per unit volume of the portion of the substrate where the lower layer was provided were 100 g / L, 0.8 g / L, 80.1 g / L, and 11.1 g / L, respectively. The lower layer was formed on the entire inner wall surface of the substrate. In this case, the mass of the lower layer (mass after drying and firing), the mass of Pd in terms of metal in the lower layer, the mass of the Ce-Zr based composite oxide in the lower layer, and the mass of alumina in the lower layer per unit volume of the portion of the substrate where the lower layer was provided were equal to the mass of the lower layer (mass after drying and firing), the mass of Pd in terms of metal in the lower layer, the mass of the Ce-Zr based composite oxide in the lower layer, and the mass of alumina in the lower layer per unit volume of the substrate, respectively.

[0196] (3) Preparation of the slurry for the upper layer Into a container containing pure water, a rhodium nitrate solution, a Zr-based oxide (content in terms of ZrO ２ conversion: 55% by mass, content of oxides of rare earth elements other than Ce (La and Y) in terms of oxide conversion: 45% by mass), alumina, barium sulfate, and binders (a ceria binder and an alumina binder) were added in sequence and thoroughly stirred and mixed to obtain the slurry for the upper layer. The composition of the slurry for the upper layer was adjusted so that the composition of the upper layer after drying and firing was 39% by mass of the Zr-based oxide, 34.9% by mass of alumina, 3% by mass in terms of solid content of the ceria binder, 14% by mass in terms of solid content of the alumina binder, 0.1% by mass in terms of metal of Rh, and 9% by mass of barium sulfate.

[0197] (4) Formation of the upper layer The entire substrate with the lower layer formed on it was immersed in the upper layer slurry, excess slurry was removed, and the upper layer slurry was applied to the lower layer. The substrate coated with the upper layer slurry was dried at 90°C for 1 hour, and then fired at 500°C for 4 hours to form the upper layer on the lower layer. The mass of the upper layer (mass after drying and firing), the metallic mass of Rh in the upper layer, the mass of Zr oxides in the upper layer, and the mass of alumina in the upper layer per unit volume of the portion of the substrate with the upper layer were 60 g / L, 0.06 g / L, 23.4 g / L, and 21.0 g / L, respectively.

[0198] As described above, an exhaust gas purification catalyst comprising a lower layer provided on a substrate and an upper layer provided on the lower layer was manufactured. The lower layer and upper layer in the exhaust gas purification catalyst correspond to the first catalyst layer and the second catalyst layer, respectively.

[0199] The D50 and D90 values ​​of the Zr-based oxides and alumina used in the preparation of the upper layer slurry were measured before the preparation of the upper layer slurry using the following method. The results are shown in Table 1.

[0200] [Measurement Method for D50 and D90] Using an automatic sample feeder for laser diffraction particle size distribution analyzers ("Microtrac SDC" manufactured by Nikkiso Co., Ltd.), powder samples were placed in an aqueous solvent. After irradiating with 30W ultrasound at a flow rate of 40% for 360 seconds, the volume-based particle size distribution was measured using a laser diffraction particle size distribution analyzer "Microtrac MT3300II" manufactured by Nikkiso Co., Ltd. The particle size (μm) at which the cumulative volume reached 50% and 90% was measured from the volume-based particle size distribution. The measurement was performed twice, and the average value of the particle size (μm) at which the cumulative volume reached 50% was defined as D50 (μm), and the average value of the particle size (μm) at which the cumulative volume reached 90% was defined as D90 (μm). The measurement conditions were: particle refractive index 1.5, particle shape spherical, solvent refractive index 1.3, set zero 30 seconds, and measurement time 30 seconds.

[0201] (5) Imaging of SEM image The manufactured exhaust gas purification catalyst was cut by a plane perpendicular to the axial direction of the substrate. In this embodiment, the substrate was cut by a plane perpendicular to the axial direction of the substrate, but it may also be cut by a plane parallel to the axial direction of the substrate.

[0202] Five regions were arbitrarily selected from the upper cross-section, and SEM images of the five selected regions were acquired using a scanning electron microscope (SEM).

[0203] SEM images were acquired under the following conditions: • SEM: JEOL FE-SEM JCM-7000 • Detector: BED-C • Acceleration voltage: 15.0 kV • Irradiation current mode: high-PC

[0204] (6) Measurement of the ratio of the total area of ​​all pores to the area of ​​the upper layer cross-section After adjusting the brightness of each SEM image using the image analysis software (Image Pro Plus 6.3J), the measurement item of the image analysis software was set to "Area Ratio," and for each SEM image, the ratio R1 (R1 = total area of ​​all pores in the SEM image / total area of ​​the SEM image × 100) of the total area of ​​the SEM image was calculated. The average value of the ratio R1 for the five SEM images was used to calculate the ratio of the total area of ​​all pores to the area of ​​the upper layer cross-section (Y × 10 ２ The percentage was calculated as (%). The results are shown in Table 1.

[0205] (7) Of the area of ​​the cross-section of the upper layer, the area is 10 μm ２ Measurement of the proportion of the total area of ​​pores as follows: After the measurement in (6) above, "area" was added to the measurement items of the image analysis software, and for each SEM image, the proportion R2 (R2 = area of ​​each pore present in the SEM image / total area of ​​the SEM image × 100) of the total area of ​​the SEM image was calculated. Area 10 μm ２ The sum of the pore ratio R2 is calculated as follows, for the area of ​​the upper layer cross-section, where the area is 10 μm. ２ The percentage (%) of the total pore area was used for each of the following. The results are shown in Table 1.

[0206] [Examples 2-3 and Comparative Examples 1-2] For Examples 2 and Comparative Example 2, the same procedure as in Example 1 was followed, except that the D50 and D90 Zr oxides used in preparing the upper layer slurry were changed as shown in Table 1. For Examples 3 and Comparative Example 1, the D50 and D90 Zr oxides used in preparing the upper layer slurry, as well as the mass of the upper layer per unit volume of the portion of the substrate where the upper layer is provided, were changed as shown in Table 1. The results are shown in Table 1.

[0207] [Comparative Example 3] The composition of the Ce-Zr composite oxide used in the preparation of the lower layer slurry was changed to CeO ２ Conversion content: 30% by mass, ZrO ２ The same procedure as in Example 1 was followed, except that the converted content was changed to 60% by mass, the oxide-converted content of rare earth elements other than Ce (La and Pr) was changed to 10% by mass, and the D50 and D90 Zr-based oxides used in the preparation of the upper layer slurry were changed as shown in Table 1. The results are shown in Table 1.

[0208] [Test Examples] (1) Evaluation of exhaust gas purification performance before phosphorus poisoning endurance treatment The exhaust gas purification performance of each exhaust gas purification catalyst in Examples 1 to 3 and Comparative Examples 1 to 3 was evaluated as follows before phosphorus poisoning endurance treatment.

[0209] Each exhaust gas purification catalyst was heat-treated using an electric furnace at 900°C for 10 hours in an atmospheric environment, and then heat-treated again using an atmospheric furnace at 950°C for 10 hours in an atmosphere with nitrogen (3 L / min) flowing through it.

[0210] Each exhaust gas purification catalyst, after heat treatment, was incorporated into the muffler of a motorcycle, and the emissions (mg / km) of non-methane hydrocarbons (NMHC) and nitrogen oxides (NOx) were measured under the following measurement conditions. The results are shown in Table 2.

[0211] [Measurement Conditions] Vehicle used: Single-cylinder 125cc motorcycle Fuel: Unleaded gasoline Driving mode: WMTC Measurement method: In accordance with ISO 6460

[0212] (2) Evaluation of Exhaust Gas Purification Performance after Phosphorus Poisoning Durability Treatment For each exhaust gas purification catalyst of Examples 1 to 3 and Comparative Examples 1 to 3, the evaluation of the exhaust gas purification performance after phosphorus poisoning durability treatment was conducted as follows.

[0213] For each exhaust gas purification catalyst, after heat treatment in the same manner as in Test Example 1, a phosphorus poisoning durability treatment was performed. The phosphorus poisoning durability treatment was carried out as follows. The exhaust gas purification catalyst after heat treatment was mounted in the exhaust pipe, and a thermocouple was inserted into the center of the honeycomb substrate. This exhaust pipe was set in a gasoline engine (displacement: 2300 cc, fuel: gasoline with engine oil added), and the engine speed / torque, etc. were adjusted so that the temperature of the thermocouple reached 700°C, and the phosphorus poisoning durability treatment was performed by maintaining it at A / F (air / fuel) = 14.6 for 24 hours.

[0214] Each exhaust gas purification catalyst after phosphorus poisoning durability treatment was incorporated into the muffler of a motorcycle, and in the same manner as in Test Example 1, the emissions (mg / km) of non-methane hydrocarbons (NMHC) and nitrogen oxides (NOx) were measured. The results are shown in Table 2.

[0215] In Table 1, "CeO ２ mass content rate" is the mass content rate (mass%) of Ce in CeO in the lower layer, "X" is the mass (g / L) of the upper layer per unit volume of the part of the substrate where the upper layer is provided, "Y×10 ２ " is the ratio (%) of the total area of all pores in the cross-sectional area of the upper layer, and "Z" represents the ratio (%) of the total area of pores with an area of 10 μm ２ or less in the cross-sectional area of the upper layer. ２

[0216] In Table 2, the overall evaluation "A" represents an exhaust gas purification catalyst in which the emissions of NMHC and NOx after phosphorus poisoning durability treatment are both below 69 mg / km, the overall evaluation "B" represents an exhaust gas purification catalyst in which either one of the emissions of NMHC and NOx after phosphorus poisoning durability treatment exceeds 69 mg / km, and the overall evaluation "C" represents an exhaust gas purification catalyst in which the emissions of NMHC and NOx after phosphorus poisoning durability treatment both exceed 69 mg / km.

[0217]

[0218]

[0219] P... Exhaust pipe of an internal combustion engine 1... Catalyst for exhaust gas purification 10... Base material 11... Cylindrical section 12... Partition section 13... Cell 20... First catalyst layer 30... Second catalyst layer

Claims

1. An exhaust gas purification catalyst comprising a metal honeycomb substrate, a first catalyst layer provided on the metal honeycomb substrate, and a second catalyst layer provided on the first catalyst layer, wherein the first catalyst layer contains Pd and Ce, the second catalyst layer contains Rh, and the Ce in the first catalyst layer contains CeO ２ The converted mass content is 25% by mass or more, the mass of the second catalyst layer per unit volume of the portion of the metal honeycomb substrate on which the second catalyst layer is provided (hereinafter referred to as "WC amount of the second catalyst layer") is 30 g / L or more and 90 g / L or less, and the WC amount of the second catalyst layer is X [g / L], and the ratio of the total area of ​​all pores to the area of ​​the cross-sectional area of ​​the second catalyst layer is Y × 10 ２ When expressed as [%], the value of X × Y is 10 or more, and the area of ​​the cross-sectional area of ​​the second catalyst layer is 10 μm ２ The exhaust gas purification catalyst wherein the proportion of the total area of ​​the pores is 2% or more.

2. The exhaust gas purification catalyst according to claim 1, wherein the value of Y is 0.12 or more and 0.25 or less.

3. CeO of Ce in the second catalyst layer ２ The exhaust gas purification catalyst according to claim 1 or 2, wherein the converted mass content is 8% by mass or less.

4. The exhaust gas purification catalyst according to claim 1 or 2, wherein the value of X × Y is 12 or more, and the amount of WC in the second catalyst layer is 70 g / L or less.

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