Exhaust gas purification catalyst

Abstract

The purpose of the present invention is to provide an exhaust gas purification catalyst provided with a base material, a first catalyst layer provided on the base material, and a second catalyst layer provided on the upper side of the first catalyst layer, the first catalyst layer containing Pd, and the second catalyst layer containing Rh, in order to provide an exhaust gas purification catalyst (1) capable of preventing a decrease in exhaust gas purification performance after exposure to a high-temperature environment and a decrease in exhaust gas purification performance after exposure to phosphorus, the present invention provides an exhaust gas purification catalyst (1) provided with a base material (10), a first catalyst layer (20) provided on the base material (10), and a second catalyst layer (30) provided on the upper side of the first catalyst layer (20), the first catalyst layer (20) contains Pd, the second catalyst layer (30) contains Rh and a composite oxide containing Ce, Zr, and Al, and the exhaust gas purification catalyst (1) satisfies at least one of the following conditions A and B: (A) the first catalyst layer (20) contains Sr, and the metal content of Sr in the first catalyst layer (20) is 0.1 mass% or more based on the mass of the first catalyst layer (20); (B) The second catalyst layer (30) contains Sr, and the content of Sr in the second catalyst layer (30) in terms of metal is 0.1 mass% or more based on the mass of the second catalyst layer (30).

Description

Technical Field

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

[0002] Exhaust gases emitted from internal combustion engines in automobiles and motorcycles contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Catalysts containing precious metal elements such as Pt, Pd, and Rh are used to purify these harmful components and render them harmless. Pt and Pd primarily participate in the oxidation and purification of HC and CO, while Rh primarily participates in the reduction and purification of NOx.

[0003] Pd is susceptible to poisoning by phosphorus (e.g., phosphorus in engine oil). As an exhaust gas purification catalyst capable of suppressing phosphorus poisoning by Pd, the following exhaust gas purification catalyst is known, comprising a substrate, a first catalyst layer disposed on the substrate, and a second catalyst layer disposed on top of the first catalyst layer, wherein the first catalyst layer contains Pd and the second catalyst layer contains Rh (e.g., Patent Documents 1 and 2).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2011-136319

[0007] Patent Document 2: Japanese Patent Application Publication No. 2016-185531 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] Exhaust gas purification catalysts are required to prevent a decrease in exhaust gas purification performance after exposure to high-temperature environments (particularly, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine, within the exhaust gas purification performance after exposure to high-temperature environments). Furthermore, they are required to prevent a decrease in exhaust gas purification performance after exposure to phosphorus (e.g., phosphorus in engine oil, within the exhaust gas purification performance after exposure to phosphorus, within the exhaust gas purification performance after exposure to phosphorus, within the exhaust gas purification performance after exposure to phosphorus, within the exhaust gas purification performance after starting an internal combustion engine, within the exhaust gas purification performance after exposure to phosphorus). It should be noted that, in this specification, "low temperature" refers to a temperature preferably of 500°C or less, more preferably of 400°C or less, and even more preferably of 300°C or less, and "high temperature" refers to a temperature preferably of 700°C or more, more preferably of 800°C or more, and even more preferably of 900°C or more.

[0010] An object of the present invention is to provide an exhaust gas purification catalyst comprising a substrate, a first catalyst layer arranged on the substrate, and a second catalyst layer arranged on the upper side of the first catalyst layer, the first catalyst layer containing Pd and the second catalyst layer containing Rh, the exhaust gas purification catalyst being able to prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, the exhaust gas purification performance in a low-temperature environment immediately after the internal combustion engine is started, among the exhaust gas purification performance after exposure to a high-temperature environment) and prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, the exhaust gas purification performance in a low-temperature environment immediately after the internal combustion engine is started, among the exhaust gas purification performance after exposure to phosphorus).

[0011] Solutions for solving problems

[0012] The present invention provides the following inventions.

[0013] [1] An exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided on the upper side of the first catalyst layer.

[0014] The first catalyst layer contains Pd,

[0015] The second catalyst layer contains Rh and a composite oxide containing Ce, Zr and Al.

[0016] The exhaust gas purification catalyst satisfies at least one of the following conditions A and B:

[0017] (A) the first catalyst layer contains Sr, and the metal conversion content of Sr in the first catalyst layer is 0.1 mass% or more based on the mass of the first catalyst layer;

[0018] (B) The second catalyst layer contains Sr, and the metal conversion content of Sr in the second catalyst layer is 0.1% by mass or more based on the mass of the second catalyst layer.

[0019] [2] The exhaust gas purification catalyst according to [1], wherein

[0020] In condition A, the metal conversion content of Sr in the first catalyst layer is 0.1 mass % or more and 7.0 mass % or less based on the mass of the first catalyst layer.

[0021] In Condition B, the metal-equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 7.0% by mass or less based on the mass of the second catalyst layer.

[0022] [3] The exhaust gas purification catalyst according to [2], wherein

[0023] In condition A, the metal conversion content of Sr in the first catalyst layer is 0.1 mass % or more and 4.0 mass % or less based on the mass of the first catalyst layer.

[0024] In the condition B, the metal-equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 4.0% by mass or less based on the mass of the second catalyst layer.

[0025] [4] The exhaust gas purification catalyst according to any one of [1] to [3], wherein the composite oxide contains La, and the La content in the composite oxide as La2O3 is 1.0% by mass or more based on the mass of the composite oxide.

[0026] [5] The exhaust gas purification catalyst according to any one of [1] to [4], wherein

[0027] When the exhaust gas purifying catalyst satisfies condition A, the exhaust gas purifying catalyst satisfies the following condition C:

[0028] (C) a ratio of the metal-equivalent content of Sr in the first catalyst layer based on the mass of the first catalyst layer to the content of the composite oxide in the second catalyst layer based on the mass of the second catalyst layer is 0.0010 or more and 0.0800 or less;

[0029] When the exhaust gas purifying catalyst satisfies condition B, the exhaust gas purifying catalyst satisfies the following condition D:

[0030] (D) The ratio of the metal-equivalent content of Sr in the second catalyst layer based on the mass of the second catalyst layer to the content of the composite oxide in the second catalyst layer based on the mass of the second catalyst layer is 0.0010 or more and 0.0800 or less.

[0031] [6] The exhaust gas purification catalyst according to any one of [1] to [5], wherein the content of the composite oxide in the second catalyst layer is 80.0% by mass or more based on the mass of the second catalyst layer.

[0032] [7] The exhaust gas purification catalyst according to any one of [1] to [6], wherein the Al content in the composite oxide as calculated as Al2O3 is 30.0% by mass or more and 60.0% by mass or less, based on the mass of the composite oxide.

[0033] [8] The exhaust gas purification catalyst according to any one of [1] to [7], wherein the exhaust gas purification catalyst satisfies at least condition A among conditions A and B.

[0034] Effects of the Invention

[0035] According to the present invention, a catalyst for exhaust gas purification is provided, which comprises a substrate, a first catalyst layer arranged on the substrate, and a second catalyst layer arranged on the upper side of the first catalyst layer, the first catalyst layer containing Pd and the second catalyst layer containing Rh, and the exhaust gas purification catalyst is capable of preventing a decrease in exhaust gas purification performance after exposure to a high-temperature environment (especially the exhaust gas purification performance in a low-temperature environment just after the internal combustion engine is started in the exhaust gas purification performance after exposure to a high-temperature environment) and preventing a decrease in exhaust gas purification performance after exposure to phosphorus (especially the exhaust gas purification performance in a low-temperature environment just after the internal combustion engine is started in the exhaust gas purification performance after exposure to phosphorus). BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a partial cross-sectional view showing a state in which an exhaust gas-purifying catalyst according to one embodiment of the present invention is arranged in an exhaust passage of an internal combustion engine.

[0037] Figure 2 for Figure 1 AA line cross-section diagram.

[0038] Figure 3 for Figure 2 An enlarged view of the area indicated by symbol R in FIG.

[0039] Figure 4 for Figure 1 BB line cross-section diagram. DETAILED DESCRIPTION

[0040] Catalysts for exhaust gas purification

[0041] Hereinafter, the exhaust gas-purifying catalyst of the present invention will be described.

[0042] The following is based on Figures 1 to 4 , an exhaust gas-purifying catalyst 1 according to one embodiment of the present invention (hereinafter sometimes referred to as “catalyst 1”) will be described.

[0043] like Figure 1As shown, catalyst 1 is disposed in an exhaust passage within an exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas emitted from the internal combustion engine flows from one end of exhaust pipe P to the other through the exhaust passage within exhaust pipe P, where it is purified by catalyst 1 disposed within 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 is sometimes referred to as the "exhaust gas inflow side" or "upstream side," and the downstream side of the exhaust gas flow direction X is sometimes referred to as the "exhaust gas outflow side" or "downstream side."

[0044] In the exhaust passage within the exhaust pipe P, other exhaust gas purification catalysts may be arranged on the upstream side and / or downstream side of the catalyst 1 .

[0045] like Figures 2-4 As shown, the catalyst 1 includes a substrate 10 , a first catalyst layer 20 provided on the substrate 10 , and a second catalyst layer 30 provided on the upper side of the first catalyst layer 20 .

[0046] <Base material>

[0047] Hereinafter, the substrate 10 will be described.

[0048] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of the material constituting the substrate 10 include ceramic materials and metal materials, with ceramic materials being preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as aluminum oxide, zirconium oxide, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metal materials include alloys such as stainless steel.

[0049] like Figures 2-4 As shown, the substrate 10 includes a cylindrical portion 11, a partition wall portion 12 provided in the cylindrical portion 11, and cells 13 partitioned by the partition wall portion 12. The substrate 10 is preferably a honeycomb structure.

[0050] like Figure 2 As shown, the cylindrical portion 11 defines the outer shape of the substrate 10, and the axial direction of the cylindrical portion 11 is consistent with the axial direction of the substrate 10. Figure 2 As shown, the cylindrical portion 11 is cylindrical, but may be in other shapes such as an elliptical cylinder or a polygonal cylinder.

[0051] like Figures 2-4 As shown, partition walls 12 are present between adjacent chambers 13, and adjacent chambers 13 are partitioned by partition walls 12. Partition walls 12 may have a porous structure that allows exhaust gas to pass through. The thickness of partition walls 12 is, for example, 20 μm to 1500 μm.

[0052] like Figure 4As shown, the chamber 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.

[0053] like Figure 4 As shown, both the exhaust gas inlet and outlet ends of chamber 13 are open. Therefore, exhaust gas flowing into chamber 13 from the exhaust gas inlet end (opening) flows out from the exhaust gas outlet end (opening). This design is called a flow-through type.

[0054] like Figure 2 and 3 As shown, the end (opening) of the chamber 13 on the exhaust gas inlet side is a quadrilateral in plan view, but may be other shapes such as hexagonal or octagonal. The same applies to the end (opening) on the exhaust gas outlet side of the chamber 13 in plan view.

[0055] 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 of the substrate 10 obtained by cutting the substrate 10 along a plane perpendicular to the exhaust gas flow direction X.

[0056] The volume of the substrate 10 is, for example, 0.1 L or more and 20 L or less. The volume of the substrate 10 refers to the apparent volume of the substrate 10. For example, when the substrate 10 is cylindrical, the outer diameter of the substrate 10 is 2r, and the length of the substrate 10 is L. 10 The volume of the substrate 10 is calculated using the formula: Volume of substrate 10 = π × r 2 ×L 10 express.

[0057] <First catalyst layer>

[0058] Next, the first catalyst layer 20 will be described.

[0059] like Figure 3 and 4 As shown, the first catalyst layer 20 is provided on the chamber 13-side surface of the partition wall 12. The "chamber 13-side surface of the partition wall 12" refers to the outer surface of the partition wall 12 extending in the exhaust gas flow direction X. The first catalyst layer 20 can be provided directly on the chamber 13-side surface of the partition wall 12 or through another layer. Typically, it is provided directly on the chamber 13-side surface of the partition wall 12.

[0060] The first catalyst layer 20 may be composed of a portion that protrudes from the chamber 13-side surface of the partition wall 12 toward the chamber 13 (hereinafter referred to as the "protruding portion"), or may be composed of a portion that exists within the partition wall 12 (hereinafter referred to as the "internal portion"), or may include both a protruding portion and an internal portion. Embodiments in which the first catalyst layer 20 is composed of a protruding portion, embodiments in which the first catalyst layer 20 is composed of an internal portion, and embodiments in which the first catalyst layer 20 has both a protruding portion and an internal portion are all included in the "first catalyst layer 20 provided on the substrate 10."

[0061] like Figure 4 As shown, the first catalyst layer 20 extends from the end of the partition wall portion 12 on the exhaust gas inlet side to the end of the partition wall portion 12 on the exhaust gas outlet side along the exhaust gas flow direction X. The first catalyst layer 20 may extend from the end of the partition wall portion 12 on the exhaust gas inlet side along the exhaust gas flow direction X but not reach the end of the partition wall portion 12 on the exhaust gas outlet side; or may extend from the end of the partition wall portion 12 on the exhaust gas outlet side in a direction opposite to the exhaust gas flow direction X but not reach the end of the partition wall portion 12 on the exhaust gas inlet side.

[0062] In order to achieve a good balance between exhaust gas purification performance and cost, the mass of the first catalyst layer 20 per unit volume of the portion of the substrate where the first catalyst layer 20 is formed (mass after firing) is preferably 50 g / L or more and 230 g / L or less, more preferably 70 g / L or more and 180 g / L or less, and further preferably 80 g / L or more and 150 g / L or less. The mass of the first catalyst layer 20 per unit volume of the portion of the substrate 10 where the first catalyst layer 20 is formed is calculated according to the formula: (mass of the first catalyst layer 20) / ((volume of the substrate 10)×(average length L of the first catalyst layer 20) 20 / length L of substrate 10 10 It should be noted that, in this specification, “length” refers to the dimension of the substrate 10 in the axial direction, unless otherwise specified.

[0063] In this specification, the "mass of the first catalyst layer 20" refers to the mass obtained by summing the metal-converted mass of the precious metal elements and the oxide-converted mass of the metal elements other than the precious metal elements, among all the metal elements contained in the first catalyst layer 20. In other words, the "mass of the first catalyst layer 20" refers to the calculated mass obtained by summing the metal-converted mass of the precious metal elements contained in the first catalyst layer 20 and the oxide-converted mass of the metal elements other than the precious metal elements contained in the first catalyst layer 20. It should be noted that "metal elements" also include semi-metallic elements such as Si and B.

[0064] In this specification, the “precious metal element” includes Pt (platinum), Pd (palladium), Rh (rhodium), Ru (ruthenium), Os (osmium), Ir (iridium), Au (gold), and Ag (silver).

[0065] In this specification, oxides of rare earth elements other than Ce, Pr and Tb refer to sesquioxides (M2O3, where M represents a rare earth element other than Ce, Pr and Tb), oxides of Ce refer to CeO2, and oxides of Pr refer to Pr6O3. 11 The oxide of Tb refers to Tb4O7, the oxide of Al refers to Al2O3, the oxide of Zr refers to ZrO2, the oxide of Si refers to SiO2, the oxide of B refers to B2O3, the oxide of Cr refers to Cr2O3, the oxide of Mg refers to MgO, the oxide of Ca refers to CaO, the oxide of Sr refers to SrO, the oxide of Ba refers to BaO, the oxide of Fe refers to Fe3O4, the oxide of Mn refers to Mn3O4, the oxide of Ni refers to NiO, the oxide of Ti refers to TiO2, the oxide of Zn refers to ZnO, and the oxide of Sn refers to SnO2.

[0066] The average length L of the first catalyst layer 20 20 An example of a determination method is as follows.

[0067] A portion extending in the axial direction of the substrate 10 and having a length L from the substrate 10 is cut out from the catalyst 1 . 10 Samples of the same length. The sample is, for example, cylindrical with a diameter of 25.4 mm. It should be noted that the value of the sample diameter can be changed as needed. When the first catalyst layer 20 extends from the end of the exhaust gas inlet side of the partition wall portion 12 along the exhaust gas flow direction X, the sample is cut at intervals of 5 mm through a plane perpendicular to the axial direction of the substrate 10, and the first slice, the second slice,..., and the nth slice are obtained in sequence from the end side of the exhaust gas inlet side of the sample. When the first catalyst layer 20 extends from the end of the exhaust gas outlet side of the partition wall portion 12 in a direction opposite to the exhaust gas flow direction X, the sample is cut at intervals of 5 mm through a plane perpendicular to the axial direction of the substrate 10, and the first slice, the second slice,..., and the nth slice are obtained in sequence from the end side of the exhaust gas outlet side of the sample. In either case, the length of the slice is 5 mm. The composition of the slice is analyzed using a fluorescent X-ray analyzer (XRF) (such as an energy dispersive X-ray analyzer (EDX), a wavelength dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma emission spectrometer (ICP-AES), a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX), etc., and it is confirmed based on the composition of the slice whether the slice contains part of the first catalyst layer 20.

[0068] For slices that clearly contain a portion of the first catalyst layer 20, composition analysis is not necessarily required. For example, the cross section can be observed using a scanning electron microscope (SEM) or electron probe microanalyzer (EPMA) to confirm whether the slice contains a portion of the first catalyst layer 20. When observing the cross section, elemental mapping of the cross section can be performed.

[0069] After confirming whether the slice includes a portion of the first catalyst layer 20 , the length of the first catalyst layer 20 included in the sample is calculated using the following formula.

[0070] Length of the first catalyst layer 20 contained in the sample = 5 mm × (number of slices containing part of the first catalyst layer 20)

[0071] For example, when the 1st to kth slices partially include the first catalyst layer 20 and the (k+1)th to nth slices partially do not include the first catalyst layer 20 , the length of the first catalyst layer 20 included in the sample is (5×k) mm.

[0072] An example of a more detailed method for measuring the length of the first catalyst layer 20 contained in a sample is as follows.

[0073] The length of the portion of the first catalyst layer 20 in the kth slice (the kth slice, when the first catalyst layer 20 extends from the end of the partition wall portion 12 on the exhaust gas inlet side along the exhaust gas flow direction X, is the slice obtained from the sample closest to the exhaust gas outflow side among the slices containing the portion of the first catalyst layer 20; when the first catalyst layer 20 extends from the end of the partition wall portion 12 on the exhaust gas outflow side in a direction opposite to the exhaust gas flow direction X, is the slice obtained from the sample closest to the exhaust gas inflow side among the slices containing the portion of the first catalyst layer 20) is cut along the axial direction of the substrate 10 and the portion of the first catalyst layer 20 present in the cross section is observed using SEM, EPMA, or the like. The length of the portion of the first catalyst layer 20 in the kth slice is then calculated using the following formula.

[0074] 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 slice)

[0075] The length of the first catalyst layer 20 included in each of 8 to 16 samples randomly cut out from the catalyst 1 is measured, and the average value thereof is defined as the average length L of the first catalyst layer 20. 20 .

[0076] The first catalyst layer 20 contains Pd as a catalytically active component. Pd is contained in the first catalyst layer 20 in a form capable of functioning as a catalytically active component, such as metallic Pd, a Pd-containing alloy, or a Pd-containing compound (e.g., a Pd oxide). To improve exhaust gas purification performance, the Pd-containing catalytically active component is preferably in a granular form.

[0077] In order to achieve a good balance between exhaust gas purification performance and cost, based on the mass of the first catalyst layer 20, the metal conversion content of Pd in the first catalyst layer 20 is preferably greater than 0.010 mass% and less than 20 mass%, more preferably greater than 0.050 mass% and less than 15 mass%, and further preferably greater than 0.10 mass% and less than 10 mass%.

[0078] The first catalyst layer 20 may further include precious metal elements other than Pd as catalytically active components. Examples of precious metal elements other than Pd include Pt, Rh, Ir, Ru, Os, Au, and Ag. Precious 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 metals, alloys containing precious metal elements, compounds containing precious metal elements (e.g., oxides of precious metal elements), and the like, in the form of catalytically active components containing precious metal elements other than Pd. In order to improve exhaust gas purification performance, the catalytically active components containing precious metal elements other than Pd are preferably in a granular form.

[0079] When the first catalyst layer 20 contains Pd and a noble metal element other than Pd, Pd and the noble metal element other than Pd form an alloy, which may reduce the number of Pd active sites that contribute to exhaust gas purification performance. Therefore, the first catalyst layer 20 preferably contains substantially no noble metal element other than Pd.

[0080] “The first catalyst layer 20 does not substantially contain any precious metal elements other than Pd” means that, based on the mass of the first catalyst layer 20, the metal conversion content of precious metal elements other than Pd in the first catalyst layer 20 is preferably 0.050 mass% or less, and more preferably 0.010 mass% or less. The lower limit is 0. With respect to the “metal conversion content of precious metal elements other than Pd in the first catalyst layer 20”, when the first catalyst layer 20 contains one precious metal element other than Pd, it refers to the metal conversion content of the one precious metal element; when the first catalyst layer 20 contains two or more precious metal elements other than Pd, it refers to the sum of the metal conversion contents of the two or more precious metal elements.

[0081] The metal-equivalent content of each metal element in the first catalyst layer 20 is calculated using the formula: (metal-equivalent mass of each metal element in the first catalyst layer 20) / (mass of the first catalyst layer 20)×100. For example, the metal-equivalent content of Pd in the first catalyst layer 20 is calculated using the formula: (metal-equivalent mass of Pd in the first catalyst layer 20) / (mass of the first catalyst layer 20)×100.

[0082] When the composition of the raw material for forming the first catalyst layer 20 is known, the metal-equivalent content of each metal element contained in the first catalyst layer 20 can be determined based on the composition of the raw material for forming the first catalyst layer 20 .

[0083] When the composition of the raw material for forming the first catalyst layer 20 is unknown, the metal conversion content of each metal element contained in the first catalyst layer 20 can be determined using conventional methods such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX).

[0084] The sample obtained from the first catalyst layer 20 is subjected to elemental analysis using conventional methods such as SEM-EDX to determine the types of constituent elements in the entire sample and calculate the content (mass %) of each identified metal element. The content (mass %) of each metal element is calculated for each of the 10 fields of view of the SEM, and the average of the content (mass %) of each metal element in the 10 fields of view is defined as the content (mass %) of each metal element in the first catalyst layer 20.

[0085] Preferably, the first catalyst layer 20 includes one or more carriers, and at least a portion of the catalytically active components is supported on the one or more carriers.

[0086] "At least a portion of the catalytically active component is supported on the carrier" means that at least a portion of the catalytically active component is physically or chemically adsorbed or retained on the outer surface and / or pore inner surface of the carrier. The fact that at least a portion of the catalytically active component is supported on the carrier can be confirmed, for example, using SEM-EDX. Specifically, when elemental mapping obtained by analyzing a cross section of the first catalyst layer 20 using SEM-EDX reveals that at least a portion of the catalytically active component and the carrier are present in the same region, it can be determined that at least a portion of the catalytically active component is supported on the carrier.

[0087] The carrier can be selected from, for example, inorganic oxides. The inorganic oxide is, for example, in a granular form. In order to improve the loading capacity of the catalytically active component, the inorganic oxide is preferably porous. The inorganic oxide may or may not have oxygen storage capacity (OSC: Oxygen Storage Capacity). In this specification, inorganic oxides having OSC are sometimes referred to as "OSC materials". Inorganic oxides used as carriers are distinguished from inorganic oxides used as binders (for example, inorganic oxide-based binders such as alumina binders, zirconia binders, titania binders, and silica binders).

[0088] As inorganic oxides, there can be listed, for example, Al-based oxides, Ce-based oxides, Ce-Zr-based composite oxides, oxides of rare earth elements other than Ce, zirconium oxide (ZrO2), silicon dioxide (SiO2), titanium dioxide (TiO2), zeolite (aluminosilicate), oxides based on MgO, ZnO, SnO2, etc.

[0089] Al-based oxides are oxides containing Al, wherein Al is the element with the highest mass content among the elements constituting the oxide other than O. However, Ce-Zr-based composite oxides are not Al-based oxides.

[0090] Ce-based oxides are oxides containing Ce, wherein Ce is the element with the highest mass content among the elements constituting the oxide other than O. However, Ce-Zr-based composite oxides are not included in Ce-based oxides.

[0091] Ce-Zr composite oxides refer to composite oxides containing Ce and Zr, wherein, based on the mass of the composite oxide, the CeO2 conversion content of Ce in the composite oxide is greater than 5.0 mass% and less than 95.0 mass%, and, based on the mass of the composite oxide, the ZrO2 conversion content of Zr in the composite oxide is greater than 5.0 mass% and less than 95.0 mass%, and, based on the mass of the composite oxide, the Al2O3 conversion content of Al in the composite oxide is less than 5.0 mass%.

[0092] The oxide-equivalent content of each element in the Al-based oxide, Ce-based oxide, or Ce-Zr-based composite oxide can be determined in the same manner as the oxide-equivalent content of each element in the Ce-Zr-Al-based composite oxide described below.

[0093] The first catalyst layer 20 may further contain other components such as a binder and a stabilizer. Examples of the binder include inorganic oxide-based binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol.

[0094] <Second catalyst layer>

[0095] Next, the second catalyst layer 30 will be described.

[0096] like Figure 3 and 4 As shown, the second catalyst layer 30 is provided on the upper side of the first catalyst layer 20 .

[0097] “The second catalyst layer 30 is provided on the upper side of the first catalyst layer 20” means that, of the two main surfaces of the first catalyst layer 20, part or all of the second catalyst layer 30 exists on the main surface on the opposite side to the main surface on the partition wall portion 12 side. “The main surface of the first catalyst layer 20” refers to the outer surface of the first catalyst layer 20 extending in the exhaust gas flow direction X. The second catalyst layer 30 can be provided directly on the main surface of the first catalyst layer 20 or through other layers. Usually, it is provided directly on the main surface of the first catalyst layer 20. The second catalyst layer 30 can be provided in a manner covering a portion of the main surface of the first catalyst layer 20 or in a manner covering the entire main surface of the first catalyst layer 20. Both the embodiment in which the second catalyst layer 30 is provided directly on the main surface of the first catalyst layer 20 and the embodiment in which the second catalyst layer 30 is provided on the main surface of the first catalyst layer 20 through other layers are included in the “second catalyst layer 30 provided on the upper side of the first catalyst layer 20”.

[0098] like Figure 4 As shown, the second catalyst layer 30 extends from the end of the partition wall portion 12 on the exhaust gas inlet side to the end of the partition wall portion 12 on the exhaust gas outlet side along the exhaust gas flow direction X. The second catalyst layer 30 may extend from the end of the partition wall portion 12 on the exhaust gas inlet side along the exhaust gas flow direction X but not reach the end of the partition wall portion 12 on the exhaust gas outlet side; or may extend from the end of the partition wall portion 12 on the exhaust gas outlet side in a direction opposite to the exhaust gas flow direction X but not reach the end of the partition wall portion 12 on the exhaust gas inlet side.

[0099] In order to achieve a good balance between exhaust gas purification performance and cost, the mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 where the second catalyst layer 30 is formed (mass after firing) is preferably 20 g / L or more and 150 g / L or less, more preferably 50 g / L or more and 120 g / L or less, and even more preferably 70 g / L or more and 100 g / L or less. The mass of the second catalyst layer 30 per unit volume of the portion of the substrate 10 where the second catalyst layer 30 is formed is calculated according to the formula: (mass of the second catalyst layer 30) / ((volume of the substrate 10)×(average length L of the second catalyst layer 30) 30 / length L of substrate 10 10 )) figure it out.

[0100] The above description about the mass of the first catalyst layer 20 also applies to the second catalyst layer 30. Where applicable, "first catalyst layer 20" is replaced with "second catalyst layer 30."

[0101] Regarding the average length L of the first catalyst layer 20 20 The above description of the measuring method also applies to the average length L of the second catalyst layer 30. 30 When applicable, replace "1st catalyst layer 20" with "2nd catalyst layer 30" and "average length L 20 " is replaced by "average length L 30 ”.

[0102] The second catalyst layer 30 contains Rh as a catalytically active component. Rh is contained in the second catalyst layer 30 in a form capable of functioning as a catalytically active component, such as metallic Rh, an Rh-containing alloy, or an Rh-containing compound (e.g., an Rh oxide). To improve exhaust gas purification performance, the Rh-containing catalytically active component is preferably in a granular form.

[0103] In order to achieve a good balance between exhaust gas purification performance and cost, based on the mass of the second catalyst layer 30, the metal conversion content of Rh in the second catalyst layer 30 is preferably greater than 0.010 mass% and less than 5 mass%, more preferably greater than 0.050 mass% and less than 3 mass%, and further preferably greater than 0.10 mass% and less than 2 mass%.

[0104] The second catalyst layer 30 may further include precious metal elements other than Rh as catalytically active components. Examples of precious metal elements other than Rh include Pt, Pd, Ir, Ru, Os, Au, and Ag. Precious 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 metals, alloys containing precious metal elements, compounds containing precious metal elements (e.g., oxides of precious metal elements), and other forms of catalytically active components containing precious metal elements other than Rh. In order to improve exhaust gas purification performance, the catalytically active components containing precious metal elements other than Rh are preferably in a granular form.

[0105] When the second catalyst layer 30 contains Rh and a precious metal element other than Rh, Rh and the precious metal element other than Rh form an alloy, which may reduce the number of Rh active sites that contribute to exhaust gas purification performance. Therefore, the second catalyst layer 30 preferably contains substantially no precious metal element other than Rh.

[0106] “The second catalyst layer 30 does not substantially contain any precious metal elements other than Rh” means that, based on the mass of the second catalyst layer 30, the metal conversion content of precious metal elements other than Rh in the second catalyst layer 30 is preferably 0.050 mass% or less, and more preferably 0.010 mass% or less. The lower limit is 0. Regarding the “metal conversion content of precious metal elements other than Rh in the second catalyst layer 30”, when the second catalyst layer 30 contains one precious metal element other than Rh, it refers to the metal conversion content of the one precious metal element; when the second catalyst layer 30 contains two or more precious metal elements other than Rh, it refers to the sum of the metal conversion contents of the two or more precious metal elements.

[0107] The metal-equivalent content of each metal element in the second catalyst layer 30 is calculated using the formula: (metal-equivalent mass of each metal element in the second catalyst layer 30) / (mass of the second catalyst layer 30)×100. For example, the metal-equivalent content of Rh in the second catalyst layer 30 is calculated using the formula: (metal-equivalent mass of Rh in the second catalyst layer 30) / (mass of the second catalyst layer 30)×100.

[0108] The metal-equivalent content of each metal element in the second catalyst layer 30 can be determined in the same manner as the metal-equivalent content of each metal element in the first catalyst layer 20 .

[0109] The second catalyst layer 30 includes a composite oxide containing Ce, Zr, and Al (hereinafter sometimes referred to as a “Ce—Zr—Al-based composite oxide”).

[0110] Whether the second catalyst layer 30 contains a Ce-Zr-Al composite oxide can be determined by conventional methods such as SEM-EDX. Specifically, by observing the second catalyst layer 30 using SEM-EDX, if Ce, Zr, and Al are present in the same location in the second catalyst layer 30, it can be determined that the second catalyst layer 30 contains a Ce-Zr-Al composite oxide.

[0111] The air / fuel ratio (air-fuel ratio) supplied to the internal combustion engine is expected to be controlled near the theoretical air-fuel ratio (stoichiometric ratio). However, the actual air-fuel ratio will fluctuate toward the rich (fuel-rich atmosphere) side or the lean (fuel-lean atmosphere) side with the stoichiometric ratio as the center, depending on the driving conditions of the vehicle, and therefore, the air-fuel ratio of the exhaust gas will also fluctuate toward the rich side or the lean side. Since Ce-Zr-Al based composite oxides have oxygen storage capacity, the use of Ce-Zr-Al based composite oxides can alleviate the fluctuation of oxygen concentration in the exhaust gas and expand the operating window of the catalyst. In addition, since Ce-Zr-Al based composite oxides have heat resistance, they can prevent Rh burial and Rh sintering caused by the aggregation of Ce-Zr-Al based composite oxides, the disappearance of pores of Ce-Zr-Al based composite oxides (i.e., reduction in specific surface area), etc. in a high temperature environment. Therefore, by making the second catalyst layer 30 contain Ce-Zr-Al based composite oxide, it is possible to prevent the exhaust gas purification performance of Rh after exposure to a high temperature environment (especially the exhaust gas purification performance in a low temperature environment immediately after the internal combustion engine is started among the exhaust gas purification performance after exposure to a high temperature environment) from decreasing.

[0112] Ce-Zr-Al composite oxide refers to a composite oxide containing Ce, Zr and Al, wherein, based on the mass of the composite oxide, the CeO2 conversion content of Ce in the composite oxide is greater than 1.0 mass% and less than 94.0 mass%, and based on the mass of the composite oxide, the ZrO2 conversion content of Zr in the composite oxide is greater than 1.0 mass% and less than 94.0 mass%, and based on the mass of the composite oxide, the Al2O3 conversion content of Al in the composite oxide is greater than 5.0 mass% and less than 98.0 mass%.

[0113] The Ce-Zr-Al composite oxide is, for example, in a granular form. The Ce-Zr-Al composite oxide is used as a carrier of a catalytically active component. To improve the loading capacity of the catalytically active component, the Ce-Zr-Al composite oxide is preferably porous.

[0114] Preferably, at least a portion of the catalytically active component is supported on the Ce-Zr-Al composite oxide in the second catalyst layer 30. The meaning and confirmation method of the support are the same as those described in <First Catalyst Layer 20>.

[0115] In order to improve the oxygen storage capacity and heat resistance, based on the mass of the Ce-Zr-Al composite oxide, the CeO2 conversion content of Ce in the Ce-Zr-Al composite oxide is preferably greater than 2.0 mass% and less than 40.0 mass%, more preferably greater than 3.0 mass% and less than 20.0 mass%, and further preferably greater than 5.0 mass% and less than 15.0 mass%.

[0116] In order to improve the oxygen storage capacity and heat resistance, based on the mass of the Ce-Zr-Al composite oxide, the ZrO2 conversion content of Zr in the Ce-Zr-Al composite oxide is preferably greater than 10.0 mass% and less than 70.0 mass%, more preferably greater than 15.0 mass% and less than 60.0 mass%, and further preferably greater than 20.0 mass% and less than 50.0 mass%.

[0117] In order to improve the oxygen storage capacity and heat resistance, based on the mass of the Ce-Zr-Al composite oxide, the Al2O3 conversion content of Al in the Ce-Zr-Al composite oxide is preferably greater than 30.0 mass% and less than 60.0 mass%, more preferably greater than 35.0 mass% and less than 55.0 mass%, and further preferably greater than 40.0 mass% and less than 50.0 mass%.

[0118] In order to improve oxygen storage capacity and heat resistance, the sum of the Ce content in the Ce-Zr-Al composite oxide as CeO2, the Zr content as ZrO2, and the Al content as Al2O3, based on the mass of the Ce-Zr-Al composite oxide, is preferably 70.0% by mass or more, more preferably 80.0% by mass or more, and even more preferably 90.0% by mass or more. The upper limit is 100% by mass.

[0119] In order to improve the oxygen storage capacity and heat resistance, the ratio of the sum of the CeO2 conversion content of Ce and the ZrO2 conversion content of Zr in the Ce-Zr-Al composite oxide to the Al2O3 conversion content of Al in the Ce-Zr-Al composite oxide is preferably greater than 0.5 and less than 1.2, more preferably greater than 0.6 and less than 1.1, and further preferably greater than 0.7 and less than 1.0.

[0120] The Ce-Zr-Al composite oxide may further contain one or more metal elements other than Ce, Zr, and Al. Examples of the metal elements other than Ce, Zr, and Al include rare earth elements other than Ce. Examples of rare earth elements other than Ce include Y, Pr, Sc, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0121] The oxide-converted content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al composite oxide can be appropriately adjusted by considering the CeO2-converted content of Ce, the ZrO2-converted content of Zr, and the Al2O3-converted content of Al in the Ce-Zr-Al composite oxide. In order to improve oxygen storage capacity and heat resistance, the oxide-converted content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al composite oxide is preferably 30.0% by mass or less, more preferably 20.0% by mass or less, and further preferably 15.0% by mass or less. The lower limit is 0. Regarding the “oxide-converted content of metal elements other than Ce, Zr and Al in the Ce-Zr-Al composite oxide”, when the Ce-Zr-Al composite oxide contains one metal element other than Ce, Zr and Al, it refers to the oxide-converted content of the one metal element; when the Ce-Zr-Al composite oxide contains two or more metal elements other than Ce, Zr and Al, it refers to the sum of the oxide-converted contents of the two or more metal elements.

[0122] For the purpose of improving heat resistance, the Ce-Zr-Al composite oxide preferably contains La. For the purpose of improving heat resistance, based on the mass of the Ce-Zr-Al composite oxide, the La2O3 conversion content of La in the Ce-Zr-Al composite oxide is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and further preferably 4.0% by mass or more. The upper limit of the La2O3 conversion content of La in the Ce-Zr-Al composite oxide can be appropriately adjusted by considering the CeO2 conversion content of Ce, the ZrO2 conversion content of Zr, and the Al2O3 conversion content of Al in the Ce-Zr-Al composite oxide. The La2O3 conversion content of La in the Ce-Zr-Al composite oxide is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, and further preferably 8.0% by mass or less. These upper limits can be combined with any one of the above lower limits, respectively.

[0123] In order to improve heat resistance, the Ce-Zr-Al composite oxide may contain rare earth elements other than Ce and La. In order to improve heat resistance, based on the mass of the Ce-Zr-Al composite oxide, the oxide-converted content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and further preferably 4.0% by mass or more. The upper limit of the oxide-converted content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide can be appropriately adjusted by considering the CeO2-converted content of Ce, the ZrO2-converted content of Zr, and the Al2O3-converted content of Al in the Ce-Zr-Al composite oxide. The oxide-converted content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, and further preferably 8.0% by mass or less. These upper limits can be combined with any one of the above lower limits. Regarding the “oxide-converted content of rare earth elements other than Ce and La in Ce-Zr-Al composite oxides,” when the Ce-Zr-Al composite oxides contain one rare earth element other than Ce and La, it refers to the oxide-converted content of the one rare earth element; when the Ce-Zr-Al composite oxides contain two or more rare earth elements other than Ce and La, it refers to the sum of the oxide-converted contents of the two or more rare earth elements.

[0124] When the composition of the Ce-Zr-Al based composite oxide is known, the oxide-equivalent content of each element in the Ce-Zr-Al based composite oxide can be determined based on the composition of the Ce-Zr-Al based composite oxide.

[0125] When the composition of the Ce-Zr-Al composite oxide is unknown, the oxide-equivalent content of each element in the Ce-Zr-Al composite oxide can be measured based on elemental mapping obtained by analyzing a sample obtained from the second catalyst layer 30 using energy dispersive X-ray spectroscopy (EDX) and EDX elemental analysis of a specific particle. Specifically, elemental mapping can be used to qualitatively identify Ce-Zr-Al composite oxide particles from other particles (color coding), and composition analysis (elemental analysis) can be performed on the specific particle to determine the oxide-equivalent content of each element in the specific particle.

[0126] In Ce-Zr-Al composite oxides, Ce can form a solid solution phase (for example, a solid solution phase of CeO2 and ZrO2, a solid solution phase of CeO2 and Al2O3, a solid solution phase of CeO2, ZrO2 and Al2O3, etc.), or a single phase of a crystalline phase or an amorphous phase (for example, a CeO2 single phase), or both a solid solution phase and a single phase. It is preferred that at least a portion of Ce forms a solid solution phase.

[0127] In Ce-Zr-Al composite oxides, Zr can form a solid solution phase (for example, a solid solution phase of CeO2 and ZrO2, a solid solution phase of ZrO2 and Al2O3, a solid solution phase of CeO2, ZrO2 and Al2O3, etc.), or a single phase of a crystalline phase or an amorphous phase (for example, a ZrO2 single phase), or both a solid solution phase and a single phase. It is preferred that at least a portion of Zr forms a solid solution phase.

[0128] In Ce-Zr-Al composite oxides, Al can form a solid solution phase (for example, the solid solution phase of CeO2 and Al2O3, the solid solution phase of ZrO2 and Al2O3, the solid solution phase of CeO2, ZrO2 and Al2O3, etc.), or it can form a single phase of crystalline phase or amorphous phase (Al2O3 single phase).

[0129] When the Ce-Zr-Al composite oxide contains one or more metal elements other than Ce, Zr and Al, the metal elements other than Ce, Zr and Al can form a solid solution phase (for example, the solid solution phase of CeO2 and the oxides of metal elements other than Ce, Zr and Al; the solid solution phase of ZrO2 and the oxides of metal elements other than Ce, Zr and Al; the solid solution phase of CeO2, ZrO2 and the oxides of metal elements other than Ce, Zr and Al, etc.), or form a single phase of crystalline phase or amorphous phase (a single phase of oxides of metal elements other than Ce, Zr and Al). Preferably, at least a part of the metal elements other than Ce, Zr and Al form a solid solution phase.

[0130] In order to more effectively prevent the decline in the exhaust gas purification performance of Rh after exposure to a high temperature environment (particularly the exhaust gas purification performance in a low temperature environment immediately after the internal combustion engine is started among the exhaust gas purification performance after exposure to a high temperature environment), the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 is preferably 80.0% by mass or more, more preferably 85.0% by mass or more, and further preferably 88.0% by mass or more, based on the mass of the second catalyst layer 30. The upper limit of the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 can be appropriately adjusted in consideration of the content of other components (such as Rh) in the second catalyst layer 30. Based on the mass of the second catalyst layer 30, the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 is preferably 98.0% by mass or less, more preferably 97.0% by mass or less, and further preferably 95.0% by mass or less. These upper limits can be combined with any of the above lower limits.

[0131] When the composition of the raw material for forming the second catalyst layer 30 is known, the content of the Ce—Zr—Al-based composite oxide in the second catalyst layer 30 can be determined based on the composition of the raw material for forming the second catalyst layer 30 .

[0132] When the composition of the raw material for forming the second catalyst layer 30 is unknown, the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 can be determined by a conventional method such as SEM-EDX.

[0133] (1) The sample obtained from the second catalyst layer 30 is subjected to elemental analysis using a conventional method such as SEM-EDX to identify the types of constituent elements of the entire sample and determine the content (mass %) of each identified element.

[0134] (2) The sample obtained from the second catalyst layer 30 is subjected to element mapping using conventional methods such as SEM-EDX to identify the types of particles contained in the sample (eg, Ce-Zr-Al composite oxide particles and optional other particles).

[0135] (3) For each type of particle, perform elemental analysis on a plurality of randomly selected particles (e.g., 50 particles) using SEM-EDX to determine the types of constituent elements of the particles and calculate the content (mass %) of each element. For each type of particle, calculate the average content (mass %) of each element.

[0136] (4) By establishing and solving an equation that represents the relationship between the content (mass %) of each element in the sample, the content (mass %) of each element in various particles, and the content (mass %) of various particles in the sample, the content (mass %) of various particles in the sample is calculated and set as the content (mass %) of various particles in the second catalyst layer 30.

[0137] The second catalyst layer 30 may further include one or more supports other than the Ce-Zr-Al composite oxide (hereinafter sometimes referred to as "other supports"). When the second catalyst layer 30 includes another support, it is preferred that at least a portion of the catalytically active component be supported on the other support. The description of the support (including the description of the inorganic oxide), as well as the meaning and confirmation method of the support, are the same as those described in <First Catalyst Layer 20>.

[0138] <Conditions A and B>

[0139] Catalyst 1 satisfies at least one of the following conditions A and B:

[0140] (A) The first catalyst layer 20 contains Sr, and the metal conversion content of Sr in the first catalyst layer 20 is 0.1 mass % or more based on the mass of the first catalyst layer 20;

[0141] (B) The second catalyst layer 30 contains Sr, and the metal conversion content of Sr in the second catalyst layer 30 is 0.1 mass % or more based on the mass of the second catalyst layer 30 .

[0142] Since the Pd in the first catalyst layer 20 is easily poisoned by phosphorus (e.g., phosphorus in engine oil), the exhaust gas purification performance of the Pd in the first catalyst layer 20 (particularly the exhaust gas purification performance of the Pd in the first catalyst layer 20 in a low-temperature environment immediately after the internal combustion engine is started) is likely to decrease after exposure to phosphorus. In contrast, when the catalyst 1 satisfies at least one of conditions A and B, the Sr in the first catalyst layer 20 and / or the second catalyst layer 30 captures phosphorus, thereby preventing the exhaust gas purification performance of the Pd in the first catalyst layer 20 after exposure to phosphorus (particularly the exhaust gas purification performance of the Pd in the first catalyst layer 20 in a low-temperature environment immediately after the internal combustion engine is started) from decreasing. It should be noted that the inventors of the present invention have studied various alkaline earth metal elements in order to suppress phosphorus poisoning, and found that: Mg and Ca are more likely to combine with Al2O3 that may exist in the first catalyst layer 20 or the second catalyst layer 30 than Sr, thereby easily reducing the heat resistance; Ba is more likely to combine with Al2O3 that may exist in the first catalyst layer 20 or the second catalyst layer 30 than Sr, thereby easily reducing the heat resistance and easily causing a reduction in the exhaust gas purification performance of Rh; and Sr is less likely to combine with Al2O3 that may exist in the first catalyst layer 20 or the second catalyst layer 30 than Mg, Ca and Ba, thereby less likely to reduce the heat resistance and less likely to cause a reduction in the exhaust gas purification performance of Rh and Pd. Based on these insights, Sr was selected.

[0143] The Ce-Zr-Al composite oxide has a large volume density, that is, the apparent volume per unit mass is small. Therefore, when the second catalyst layer 30 is exposed to phosphorus, the physical barrier properties of the Ce-Zr-Al composite oxide in the second catalyst layer 30 to phosphorus intrusion are low. Therefore, the more the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 increases, the easier it is for phosphorus to pass through the second catalyst layer 30 and reach the first catalyst layer 20, and the more likely the Pd in the first catalyst layer 20 is to be poisoned by phosphorus. Therefore, when the content of the Ce-Zr-Al composite oxide in the second catalyst layer 30 is 80.0% by mass or more, the effect brought about by the catalyst 1 satisfying at least one of conditions A and B is significant.

[0144] In order to more effectively prevent the decline of the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (especially the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus in a low-temperature environment just after the internal combustion engine is started), under condition A, based on the mass of the first catalyst layer 20, the metal conversion content of Sr in the first catalyst layer 20 is preferably 0.3 mass% or more, and more preferably 0.5 mass% or more.

[0145] In order to more effectively prevent the decline in the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (especially the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus in a low-temperature environment just after the internal combustion engine is started), under condition B, based on the mass of the second catalyst layer 30, the metal conversion content of Sr in the second catalyst layer 30 is preferably 0.3 mass% or more, and more preferably 0.5 mass% or more.

[0146] The air / fuel ratio (air-fuel ratio) supplied to the internal combustion engine is expected to be controlled near the theoretical air-fuel ratio (stoichiometric ratio). However, the actual air-fuel ratio will fluctuate toward the rich (fuel-rich atmosphere) side or the lean (fuel-lean atmosphere) side with the stoichiometric ratio as the center, depending on the driving conditions of the vehicle, etc. Therefore, the air-fuel ratio of the exhaust gas will also fluctuate toward the rich side or the lean side. In addition to having the property of capturing phosphorus, Sr also has the property of adsorbing NOx when the exhaust gas is in a lean state and releasing NOx when the exhaust gas is in a rich state. Therefore, if the amount of Sr is too much, when the exhaust gas switches from a lean state to a rich state, a large amount of NOx will be released by Sr at one time, and the released NOx may not all be purified by the catalyst 1. It should be noted that the NOx released by Sr when the exhaust gas switches from a lean state to a rich state is sometimes referred to as "transient NOx" below. For this reason, it is preferable to appropriately limit the upper limit of the Sr amount to prevent excessive increase in the transient NOx amount and control the transient NOx amount within the range that the catalyst 1 can purify.

[0147] In order to more effectively prevent an excessive increase in transient NOx levels, under condition A, the Sr metal content in the first catalyst layer 20 is preferably 7.0 mass% or less, more preferably 4.0 mass% or less, and even more preferably 3.0 mass% or less, based on the mass of the first catalyst layer 20. These upper limits may be combined with any of the above lower limits.

[0148] In order to more effectively prevent an excessive increase in transient NOx, under condition B, the Sr metal content in the second catalyst layer 30 is preferably 7.0 mass% or less, more preferably 4.0 mass% or less, and even more preferably 3.0 mass% or less, based on the mass of the second catalyst layer 30. These upper limits may be combined with any of the above lower limits.

[0149] In a preferred embodiment, the Sr metal content in the first catalyst layer 20 is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the first catalyst layer 20, and / or the Sr metal content in the second catalyst layer 30 is 0.1% by mass or more and 7.0% by mass or less, based on the mass of the second catalyst layer 30. This can more effectively prevent a decrease in the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (particularly, the exhaust gas purification performance of Pd in the first catalyst layer 20 under a low-temperature environment immediately after engine startup after exposure to phosphorus), and can more effectively prevent an excessive increase in the transient NOx amount.

[0150] In a more preferred embodiment, the Sr metal content in the first catalyst layer 20 is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the first catalyst layer 20, and / or the Sr metal content in the second catalyst layer 30 is 0.1% by mass or more and 4.0% by mass or less, based on the mass of the second catalyst layer 30. This can more effectively prevent a decrease in the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (particularly, the exhaust gas purification performance of Pd in the first catalyst layer 20 under a low-temperature environment immediately after engine startup after exposure to phosphorus), and can more effectively prevent an excessive increase in the transient NOx amount.

[0151] When catalyst 1 satisfies condition A, catalyst 1 may or may not satisfy condition B. "Catalyst 1 does not satisfy condition B" includes embodiments in which the second catalyst layer 30 does not contain Sr, and embodiments in which the second catalyst layer 30 contains Sr, and the metal-equivalent content of Sr in the second catalyst layer 30 is less than 0.1% by mass, based on the mass of the second catalyst layer 30. In the latter embodiment, the metal-equivalent content of Sr in the second catalyst layer 30 is preferably 0.05% by mass or less, more preferably 0.03% by mass or less, and even more preferably 0.01% by mass or less, based on the mass of the second catalyst layer 30.

[0152] When catalyst 1 satisfies condition B, catalyst 1 may or may not satisfy condition A. "Catalyst 1 does not satisfy condition A" includes embodiments in which the first catalyst layer 20 does not contain Sr, and embodiments in which the first catalyst layer 20 contains Sr, and the metal-converted Sr content in the first catalyst layer 20 is less than 0.1% by mass, based on the mass of the first catalyst layer 20. In the latter embodiment, the metal-converted Sr content in the first catalyst layer 20 is preferably 0.05% by mass or less, more preferably 0.03% by mass or less, and even more preferably 0.01% by mass or less, based on the mass of the first catalyst layer 20.

[0153] (a) An embodiment in which the catalyst 1 satisfies conditions A and B, (b) an embodiment in which the catalyst 1 satisfies condition A but not condition B, and (c) an embodiment in which the catalyst 1 satisfies condition B but not condition A are included in “the catalyst 1 satisfies at least one of conditions A and B”.

[0154] When the catalyst 1 satisfies condition B, the Sr in the second catalyst layer 30 can capture phosphorus contained in the exhaust gas before it reaches the first catalyst layer 20. This provides an advantage in that phosphorus poisoning of the Pd in the first catalyst layer 20 can be more effectively prevented compared to when the first catalyst layer 20 satisfies condition A. On the other hand, when the catalyst 1 satisfies condition B, there is a disadvantage in that the Sr in the second catalyst layer 30 degrades the exhaust gas purification performance of the Rh in the second catalyst layer 30. In contrast, when the catalyst 1 satisfies condition A, while the effectiveness of preventing phosphorus poisoning of the Pd in the first catalyst layer 20 is less than when the catalyst 1 satisfies condition B, there is no disadvantage in that the Sr in the first catalyst layer 20 degrades the exhaust gas purification performance of the Pd in the first catalyst layer 20. Therefore, in order to achieve a good balance between preventing Pd from being poisoned by phosphorus using Sr and preventing the decline in exhaust gas purification performance of precious metal elements caused by Sr, among embodiments a to c, embodiments a and b are preferred (i.e., catalyst 1 satisfies at least condition A among conditions A and B), and embodiment b is more preferred (i.e., catalyst 1 satisfies condition A and does not satisfy condition B).

[0155] The metal content of Sr in the first catalyst layer 20 and the metal content of Sr in the second catalyst layer 30 can be determined in the same manner as the metal-equivalent content of each metal element in the first catalyst layer 20 .

[0156] When the catalyst 1 satisfies condition A, the first catalyst layer 20 contains one or more Sr sources. When the catalyst 1 satisfies condition B, the second catalyst layer 30 contains one or more Sr sources.

[0157] The Sr source is not particularly limited as long as it is a compound containing Sr. The form of Sr is not particularly limited. Sr can exist in one or more forms selected from, for example, oxides, carbonates, and sulfates. In order to more effectively exert the effects of Sr, Sr is preferably present in one or two forms selected from SrO and SrCO3.

[0158] When catalyst 1 satisfies condition A, catalyst 1 preferably satisfies the following condition C:

[0159] (C) The ratio of the metal conversion content (mass %) of Sr in the first catalyst layer 20 based on the mass of the first catalyst layer 20 to the content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30 based on the mass of the second catalyst layer 30 (metal conversion content (mass %) of Sr in the first catalyst layer 20 / content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30) is greater than 0.0010 and less than 0.0800.

[0160] The high bulk density of the Ce-Zr-Al composite oxide makes it easy for the Pd in the first catalyst layer 20 to be poisoned by phosphorus, which is particularly pronounced when a large amount of the Ce-Zr-Al composite oxide is included in the second catalyst layer 30. Therefore, by including an amount of Sr in the first catalyst layer 20 corresponding to the amount of the Ce-Zr-Al composite oxide in the second catalyst layer 30, that is, by ensuring that the catalyst 1 satisfies Condition C, phosphorus poisoning of the Pd in the first catalyst layer 20 can be more effectively prevented.

[0161] In order to more effectively prevent phosphorus poisoning of Pd in the first catalyst layer 20, the ratio of the metal conversion content (mass %) of Sr in the first catalyst layer 20 based on the mass of the first catalyst layer 20 to the content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30 based on the mass of the second catalyst layer 30 is preferably greater than 0.0010 and less than 0.0800, more preferably greater than 0.0020 and less than 0.0500, and further preferably greater than 0.0040 and less than 0.0130.

[0162] When catalyst 1 satisfies condition B, catalyst 1 preferably satisfies the following condition D:

[0163] (D) The ratio of the metal conversion content (mass %) of Sr in the second catalyst layer 30 based on the mass of the second catalyst layer 30 to the content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30 based on the mass of the second catalyst layer 30 (metal conversion content (mass %) of Sr in the second catalyst layer 30 / content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30) is greater than 0.0010 and less than 0.0800.

[0164] The high bulk density of the Ce-Zr-Al composite oxide makes it easy for the Pd in the first catalyst layer 20 to be poisoned by phosphorus, which is particularly pronounced when a large amount of the Ce-Zr-Al composite oxide is included in the second catalyst layer 30. Therefore, by including an amount of Sr in the second catalyst layer 30 corresponding to the amount of the Ce-Zr-Al composite oxide in the second catalyst layer 30, that is, by ensuring that the catalyst 1 satisfies Condition D, phosphorus poisoning of the Pd in the first catalyst layer 20 can be more effectively prevented.

[0165] In order to more effectively prevent phosphorus poisoning of Pd in the first catalyst layer 20, the ratio of the metal conversion content (mass %) of Sr in the second catalyst layer 30 based on the mass of the second catalyst layer 30 to the content (mass %) of the Ce-Zr-Al composite oxide in the second catalyst layer 30 based on the mass of the second catalyst layer 30 is preferably greater than 0.0010 and less than 0.0800, more preferably greater than 0.0020 and less than 0.0500, and further preferably greater than 0.0040 and less than 0.0130.

[0166] <Function of Catalyst>

[0167] In catalyst 1, the second catalyst layer 30 is provided above the first catalyst layer 20. Therefore, exhaust gas flowing in from the end (opening) on the exhaust gas inlet side of chamber 13 contacts the second catalyst layer 30 before contacting the first catalyst layer 20. Exhaust gas contacting the second catalyst layer 30 is purified by the catalytically active components (e.g., Rh) in the second catalyst layer 30, while exhaust gas contacting the first catalyst layer 20 is purified by the catalytically active components (e.g., Pd) in the first catalyst layer 20. By including a Ce-Zr-Al composite oxide in the second catalyst layer 30, it is possible to prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly, the exhaust gas purification performance in a low-temperature environment immediately after the internal combustion engine is started, among the exhaust gas purification performance after exposure to a high-temperature environment). Furthermore, by ensuring that the catalyst 1 satisfies at least one of conditions A and B, it is possible to prevent a decrease in the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus, in a low-temperature environment immediately after the internal combustion engine is started). Therefore, the catalyst 1 can prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly, the exhaust gas purification performance in a low-temperature environment immediately after the internal combustion engine is started) and prevent a decrease in the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, the exhaust gas purification performance of Pd in the first catalyst layer 20 after exposure to phosphorus, in a low-temperature environment immediately after the internal combustion engine is started).

[0168] Catalyst Production

[0169] The catalyst 1 can be manufactured by forming the first catalyst layer 20 on the substrate 10 and then forming the second catalyst layer 30 on the upper side of the first catalyst layer 20 .

[0170] The first catalyst layer 20 can be formed by mixing a Pd supply source (e.g., Pd salt), an optional Sr supply source (e.g., Sr salt), and optional other components (e.g., inorganic oxides, binders, solvents, etc.) to prepare a first slurry, applying the first slurry on the substrate 10, and drying and sintering to form it.

[0171] The second catalyst layer 30 can be formed by mixing a Rh supply source (e.g., Rh salt), a Ce-Zr-Al composite oxide, an optional Sr supply source (e.g., Sr salt), and other optional components (e.g., inorganic oxides other than the Ce-Zr-Al composite oxide, binders, solvents, etc.) to prepare a second slurry, coating the second slurry on the first catalyst layer 20, and drying and calcining to form it.

[0172] Examples of Pd sources include Pd salts, such as nitrates, ammonia complex salts, acetates, and chlorides. Examples of Rh sources include Rh salts, such as nitrates, ammonia complex salts, acetates, and chlorides. Examples of Sr sources include Sr salts, such as nitrates, acetates, carbonates, and chlorides. Examples of binders include alumina sols, zirconia sols, titania sols, silica sols, and ceria sols. Examples of solvents include water and organic solvents.

[0173] The drying temperature is, for example, 70°C to 150°C, and the drying time is, for example, 5 minutes to 1 hour. The firing temperature is, for example, 200°C to 700°C, and the firing time is, for example, 0.5 hours to 5 hours. Firing can be performed, for example, in an air atmosphere.

[0174] Example

[0175] [Example 1]

[0176] (1) Preparation of slurry for lower layer formation

[0177] An OSC material (Ce-Zr based composite oxide, hereinafter sometimes referred to as "CZ") having the following composition was prepared.

[0178] Ce content as CeO2: 40.0 mass%, Zr content as ZrO2: 50.0 mass%, rare earth element oxide content other than Ce: 10.0 mass%

[0179] A palladium nitrate aqueous solution, a Ce-Zr composite oxide, La2O3-modified alumina (La2O3 modification: 1 mass%), strontium nitrate, an alumina sol, and water were added to a mixing container, mixed, and stirred to prepare a slurry for forming a lower layer. The amounts of the components in the slurry for forming the lower layer were adjusted so that, based on the mass of the fired lower layer (100 mass%), the following contents were: 4.0 mass% of Pd (metal equivalent), 40.0 mass% of Ce-Zr composite oxide, 45.4 mass% of La2O3-modified alumina, 0.5 mass% of Sr (metal equivalent) (0.6 mass% as SrO), and the solid content of the alumina sol was 10.0 mass%. It should be noted that the strontium nitrate was converted to strontium oxide during firing.

[0180] (2) Formation of the lower layer

[0181] As a flow-through substrate, a flow-through substrate was prepared in which a flow-through substrate was formed at a surface perpendicular to the axial direction at 600 cells / inch. 2The density has axially extending chambers separated by partition walls having a thickness of 50 to 70 μm, and the volume of the substrate is 1.0 L.

[0182] The flow-through substrate was immersed in the slurry for forming the lower layer. The flow-through substrate coated with the slurry was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form the lower layer. The mass of the lower layer per unit volume of the portion of the flow-through substrate where the lower layer was formed was 100 g / L.

[0183] (3) Preparation of slurry for upper layer formation

[0184] An OSC material (Ce-Zr-Al composite oxide, hereinafter sometimes referred to as "CZA") having the following composition was prepared.

[0185] Ce content as CeO2: 10.0 mass%, Zr content as ZrO2: 35.0 mass%, Al content as Al2O3: 45.0 mass%, La content as La2O3: 5.0 mass%, rare earth element oxide content other than Ce and La: 5.0 mass%

[0186] An aqueous rhodium nitrate solution, a Ce-Zr-Al composite oxide, an alumina sol, and water were added to a mixing container, mixed, and stirred to prepare a slurry for forming an upper layer. The amounts of the components in the slurry for forming the upper layer were adjusted so that, based on the mass of the fired upper layer (100 mass%), the metal content of Rh was 1.0 mass%, the Ce-Zr-Al composite oxide was 89.0 mass%, and the solid content of the alumina sol was 10.0 mass%.

[0187] (4) Formation of the upper layer

[0188] The flow-through substrate with the lower layer formed thereon was immersed in the slurry for forming the upper layer. The flow-through substrate coated with the slurry for forming the upper layer was dried at 150°C for 0.5 hours and then fired at 500°C for 1 hour to form the upper layer on the lower layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate where the upper layer was formed was 80 g / L.

[0189] As described above, an exhaust gas-purifying catalyst is manufactured, which includes a lower layer formed on a flow-through substrate and an upper layer formed on the lower layer.

[0190] [Examples 2 to 4]

[0191] In the slurry for forming the lower layer, the amount of strontium nitrate added was adjusted so that the Sr content in terms of metal was 1.0% by mass (Example 2), 2.5% by mass (Example 3), or 5.0% by mass (Example 4) based on the mass of the calcined lower layer (100% by mass), and the amount of La2O3-modified alumina added was reduced accordingly (corresponding to the increase in the amount of strontium nitrate added). Except for this, the exhaust gas purification catalysts of Examples 2 to 4 were manufactured in the same manner as in Example 1.

[0192] [Comparative Example 1]

[0193] The preparation of the lower layer forming slurry and the formation of the lower layer were carried out in the same manner as in Example 2.

[0194] An OSC material (Ce-Zr based composite oxide, hereinafter sometimes referred to as "CZ") having the following composition was prepared.

[0195] Ce content as CeO2: 15.0 mass %, Zr content as ZrO2: 75.0 mass %, rare earth element oxide content other than Ce: 10.0 mass %

[0196] A rhodium nitrate aqueous solution, Ce-Zr composite oxide, La2O3-modified alumina (La2O3 modification: 1 mass%), alumina sol, and water were added to a mixing container, mixed, and stirred to prepare a slurry for forming the upper layer. The amounts of the components in the slurry for forming the upper layer were adjusted so that, based on the mass of the fired upper layer (100 mass%), the metal content of Rh was 1.0 mass%, the Ce-Zr composite oxide was 50.0 mass%, the La2O3-modified alumina was 39.0 mass%, and the solid content of the alumina sol was 10.0 mass%. The upper layer was formed in the same manner as in Example 2, except that this slurry for the upper layer was used.

[0197] [Comparative Example 2]

[0198] The exhaust gas purifying catalyst of Comparative Example 2 was produced in the same manner as in Example 1 except that no strontium nitrate was added to the lower layer forming slurry and the amount of La2O3-modified alumina added was increased accordingly (corresponding to the reduction in the amount of strontium nitrate added).

[0199] [Test Example]

[0200] (1) Heat treatment of catalyst

[0201] A 30 mL core of the exhaust gas purifying catalyst was heat treated in a quartz tubular furnace at 1000° C. in a 10 vol % H 2 O atmosphere according to the following FC mode for 20 hours. The heat-treated catalyst was used as an evaluation sample.

[0202] FC mode: Simulated gas (3 L / min) and air (3 L / min) of the following composition flowed in alternately.

[0203] Simulated gas flow rates: C3H6 6 mL / min, O2 71 mL / min, N2 2923 mL / min (total 3 L / min).

[0204] 10% by volume of H2O is evaporated from the water tank and mixed into the simulated gas or air in the form of water vapor. The saturated water vapor pressure is adjusted according to the temperature to form the above volume % of water vapor.

[0205] (2) Treatment of phosphorus poisoning of catalyst

[0206] In a quartz tubular furnace, a solution containing zinc dialkyldithiophosphate was sprayed from the upstream side of the catalyst after the heat treatment in (1) above, and phosphorus poisoning treatment was carried out at 750°C in a 10% by volume H2O atmosphere under the following gas flow conditions for 20 hours.

[0207] The gas flow rate conditions were: N2 11.9 L / min, CO2 3.0 L / min, O2 0.1 L / min (total 15 L / min).

[0208] 10% by volume of H2O is evaporated from the water tank and mixed into the simulated gas or air in the form of water vapor. The saturated water vapor pressure is adjusted according to the temperature to form the above volume % of water vapor.

[0209] (3) Evaluation of exhaust gas purification performance (T50)

[0210] A catalyst treated with the above-mentioned (1) or (2) was placed in an exhaust passage, and an exhaust gas simulation gas (CO 0.50 vol%, H2 0.17 vol%, O2 0.50 vol%, NO 400 volppm, C3H6 1180 volppm, CO2 14 vol%, H2O 10 vol%, N2 balance) was circulated at a space velocity of 100,000 / h. The temperature was raised to 500°C at a rate of 20°C / min. The purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) were continuously measured, and the temperature (ignition temperature T50) (°C) at which the purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) reached 50% was determined. The ignition temperature T50 was determined during the temperature increase. The results are shown in Table 1.

[0211] (4) Evaluation of transient NOx emissions

[0212] The catalyst subjected to the treatment of (2) above was placed in the exhaust passage, and the atmosphere was switched in the order of the following lean condition, N2, and the following rich condition, and was allowed to flow at a space velocity of 100,000 / h at 400°C for 5 minutes each.

[0213] Dilute conditions: NO 1000volppm, O2 5000volppm, N2 balance

[0214] Concentrated conditions: CO 5000volppm, N2 balance

[0215] The total amount of NO and NO 2 emitted when switching to the rich condition was evaluated as the transient NO x emission amount (μmol).

[0216] [Table 1]

[0217]

[0218] The exhaust gas-purifying catalysts of Examples 1 to 4 include a substrate, a lower layer containing Pd, and an upper layer containing Rh and a Ce-Zr-Al composite oxide, and satisfy the following condition A:

[0219] (A) The lower layer contains Sr, and the metal conversion content of Sr in the lower layer is 0.1 mass% or more based on the mass of the lower layer.

[0220] The exhaust gas-purifying catalyst of Comparative Example 1 includes a substrate, a lower layer containing Pd, and an upper layer containing Rh, and satisfies the following condition A:

[0221] (A) The lower layer contains Sr, and the metal conversion content of Sr in the lower layer is 0.1 mass% or more based on the mass of the lower layer.

[0222] However, in the exhaust gas-purifying catalyst of Comparative Example 1, the upper layer contains the Ce-Zr-based composite oxide but does not contain the Ce-Zr-Al-based composite oxide.

[0223] The exhaust gas purifying catalyst of Comparative Example 2 comprises a substrate, a lower layer containing Pd, and an upper layer containing Rh and a Ce-Zr-Al composite oxide. However, the exhaust gas purifying catalyst of Comparative Example 2 does not satisfy the following condition A:

[0224] (A) The lower layer contains Sr, and the metal conversion content of Sr in the lower layer is 0.1 mass% or more based on the mass of the lower layer.

[0225] In addition, the exhaust gas purification catalyst of Comparative Example 2 also does not satisfy the following condition B:

[0226] (B) The upper layer contains Sr, and the metal conversion content of Sr in the upper layer is 0.1 mass% or more based on the mass of the upper layer.

[0227] As shown in Table 1, compared with the exhaust gas purification catalyst of Comparative Example 1, the exhaust gas purification catalysts of Examples 1 to 4 can more effectively prevent the decline in exhaust gas purification performance after exposure to a high-temperature environment (especially the exhaust gas purification performance in a low-temperature environment immediately after the internal combustion engine is started among the exhaust gas purification performance after exposure to a high-temperature environment).

[0228] As shown in Table 1, the exhaust gas purification catalysts of Examples 1 to 4 can more effectively prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, the exhaust gas purification performance in a low-temperature environment immediately after starting the internal combustion engine after exposure to phosphorus) compared to the exhaust gas purification catalyst of Comparative Example 2.

[0229] In this way, the exhaust gas purification catalysts of Examples 1 to 4 can prevent the decline of the exhaust gas purification performance after exposure to a high-temperature environment (especially the exhaust gas purification performance in a low-temperature environment just after the internal combustion engine is started among the exhaust gas purification performance after exposure to a high-temperature environment), and the decline of the exhaust gas purification performance after exposure to phosphorus (especially the exhaust gas purification performance in a low-temperature environment just after the internal combustion engine is started among the exhaust gas purification performance after exposure to phosphorus).

[0230] Description of Reference Numerals

[0231] P···Exhaust pipe of internal combustion engine

[0232] 1···Catalysts for exhaust gas purification

[0233] 10···Base material

[0234] 11···Tubular part

[0235] 12···Partition wall

[0236] Room 13

[0237] 20···1st catalyst layer

[0238] 30···Second catalyst layer

Claims

1. An exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided on the upper side of the first catalyst layer. The first catalyst layer contains Pd, The second catalyst layer contains Rh and a composite oxide containing Ce, Zr and Al. The exhaust gas purification catalyst satisfies at least one of the following conditions A and B: (A) the first catalyst layer contains Sr, and the metal conversion content of Sr in the first catalyst layer is 0.1% by mass or more based on the mass of the first catalyst layer; (B) The second catalyst layer contains Sr, and the metal conversion content of Sr in the second catalyst layer is 0.1% by mass or more based on the mass of the second catalyst layer.

2. The exhaust gas purification catalyst according to claim 1, wherein In condition A, the metal conversion content of Sr in the first catalyst layer is 0.1 mass % or more and 7.0 mass % or less based on the mass of the first catalyst layer. In Condition B, the metal-equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 7.0% by mass or less based on the mass of the second catalyst layer.

3. The exhaust gas purification catalyst according to claim 2, wherein In condition A, the metal conversion content of Sr in the first catalyst layer is 0.1 mass % or more and 4.0 mass % or less based on the mass of the first catalyst layer. In the condition B, the metal-equivalent content of Sr in the second catalyst layer is 0.1% by mass or more and 4.0% by mass or less based on the mass of the second catalyst layer.

4. The exhaust gas purification catalyst according to claim 1 or 2, wherein The composite oxide contains La, and the content of La in the composite oxide as La 2 O 3 is 1.0 mass % or more, based on the mass of the composite oxide.

5. The exhaust gas purification catalyst according to claim 1 or 2, wherein When the exhaust gas purifying catalyst satisfies condition A, the exhaust gas purifying catalyst satisfies the following condition C: (C) a ratio of the metal-equivalent content of Sr in the first catalyst layer based on the mass of the first catalyst layer to the content of the composite oxide in the second catalyst layer based on the mass of the second catalyst layer is 0.0010 or more and 0.0800 or less; When the exhaust gas purifying catalyst satisfies condition B, the exhaust gas purifying catalyst satisfies the following condition D: (D) The ratio of the metal-equivalent content of Sr in the second catalyst layer based on the mass of the second catalyst layer to the content of the composite oxide in the second catalyst layer based on the mass of the second catalyst layer is 0.0010 or more and 0.0800 or less.

6. The exhaust gas purification catalyst according to claim 1 or 2, wherein The content of the composite oxide in the second catalyst layer is 80.0% by mass or more based on the mass of the second catalyst layer.

7. The exhaust gas purification catalyst according to claim 1 or 2, wherein The Al 2 O 3 conversion content of Al in the composite oxide is 30.0 mass % or more and 60.0 mass % or less, based on the mass of the composite oxide.

8. The exhaust gas purification catalyst according to claim 1 or 2, wherein The exhaust gas-purifying catalyst satisfies at least condition A among conditions A and B.

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