Exhaust gas purification catalyst

Abstract

In order to provide an exhaust gas purification catalyst comprising a substrate, a first catalyst layer provided to the substrate, and a second catalyst layer provided to the upper side of the first catalyst layer, the first catalyst layer containing Pd, and the second catalyst layer containing Rh, wherein it is possible to prevent any reduction in exhaust gas purification performance after exposure to high-temperature environments and to prevent any reduction in exhaust gas purification performance after exposure to phosphorus, the present invention provides an exhaust gas purification catalyst (1) comprising a substrate (10), a first catalyst layer (20) provided to the substrate (10), and a second catalyst layer (30) provided to the upper side of the first catalyst layer (20), wherein: the first catalyst layer (20) contains Pd; the second catalyst layer (30) contains Rh and a mixed oxide that includes Ce, Zr, and Al; and the exhaust gas purification catalyst (1) satisfies at least one condition from among (A) the first catalyst layer (20) containing Sr, and the Sr content of the first catalyst layer (20) in terms of metal being 0.1 mass% or greater with reference to the mass of the first catalyst layer (20), and (B) the second catalyst layer (30) containing Sr, and the Sr content of the second catalyst layer (30) in terms of metal being 0.1 mass% or greater with reference to the mass of the second catalyst layer (30).

Description

Exhaust gas purification catalyst

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

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

[0003] Pd is susceptible to poisoning by phosphorus (e.g., phosphorus in engine oil). Known exhaust gas purification catalysts that can suppress phosphorus poisoning of Pd include a substrate, a first catalytic layer provided on the substrate, and a second catalytic layer provided on the upper side of the first catalytic layer, in which the first catalytic layer contains Pd and the second catalytic layer contains Rh (e.g., Patent Documents 1 and 2).

[0004] JP 2011-136319 A JP 2016-185531 A

[0005] Exhaust gas purification catalysts are required to prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine), and to prevent a decrease in exhaust gas purification performance after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, among the exhaust gas purification performance after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine). In this specification, "low temperature" means a temperature preferably not higher than 500°C, more preferably not higher than 400°C, and even more preferably not higher than 300°C, and "high temperature" means a temperature preferably not lower than 700°C, more preferably not lower than 800°C, and even more preferably not lower than 900°C.

[0006] The present invention aims to provide an exhaust gas purification catalyst that includes a substrate, a first catalytic layer provided on the substrate, and a second catalytic layer provided above the first catalytic layer, wherein the first catalytic layer contains Pd and the second catalytic layer contains Rh, and that can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine) and a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, among the exhaust gas purification performance after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine).

[0007] The present invention provides the following: [1] A catalyst for purifying exhaust gases, comprising a substrate, a first catalytic layer provided on the substrate, and a second catalytic layer provided on the upper side of the first catalytic layer, wherein the first catalytic layer contains Pd, and the second catalytic layer contains a composite oxide containing Rh, Ce, Zr, and Al, and the catalyst for purifying exhaust gases satisfies at least one of the following conditions A and B: (A) the first catalytic layer contains Sr, and the content of Sr in the first catalytic layer, converted into metal, is 0.1% by mass or more based on the mass of the first catalytic layer; or (B) the second catalytic layer contains Sr, and the content of Sr in the second catalytic layer, converted into metal, is 0.1% by mass or more based on the mass of the second catalytic layer. [2] The exhaust gas purifying catalyst according to [1], wherein under condition A, the metal-equivalent content of Sr in the first catalytic layer is 0.1 mass% or more and 7.0 mass% or less, based on the mass of the first catalytic layer, and under condition B, the metal-equivalent content of Sr in the second catalytic layer is 0.1 mass% or more and 7.0 mass% or less, based on the mass of the second catalytic layer. [3] The exhaust gas purifying catalyst according to [2], under condition A, the metal-equivalent content of Sr in the first catalytic layer is 0.1 mass% or more and 4.0 mass% or less, based on the mass of the first catalytic layer, and under condition B, the metal-equivalent content of Sr in the second catalytic layer is 0.1 mass% or more and 4.0 mass% or less, based on the mass of the second catalytic layer. [4] The composite oxide contains La, and the La in the composite oxide is ２ O ３[5] The exhaust gas purifying catalyst according to any of [1] to [4], wherein, when the exhaust gas purifying catalyst satisfies condition A, the exhaust gas purifying catalyst satisfies the following condition C: (C) a ratio of a metal-converted content of Sr in the first catalytic layer, based on the mass of the first catalytic layer, to a content of the complex oxide in the second catalytic layer, based on the mass of the second catalytic layer, is 0.0010 or more and 0.0800 or less, and when the exhaust gas purifying catalyst satisfies condition B, the exhaust gas purifying catalyst satisfies the following condition D: (D) a ratio of a metal-converted content of Sr in the second catalytic layer, based on the mass of the second catalytic layer, to a content of the complex oxide in the second catalytic layer, based on the mass of the second catalytic layer, is 0.0010 or more and 0.0800 or less. [6] The catalyst for purifying exhaust gas according to any one of [1] to [5], wherein the content of the composite oxide in the second catalytic layer is 80.0 mass % or more based on the mass of the second catalytic layer. ２ O ３ The catalyst for purifying exhaust gas according to any one of [1] to [6], wherein the converted content is 30.0 mass % or more and 60.0 mass % or less, based on the mass of the composite oxide. [8] The catalyst for purifying exhaust gas according to any one of [1] to [7], wherein the catalyst for purifying exhaust gas satisfies at least condition A of conditions A and B.

[0008] According to the present invention, there is provided an exhaust gas purification catalyst comprising a substrate, a first catalytic layer provided on the substrate, and a second catalytic layer provided above the first catalytic layer, wherein the first catalytic layer contains Pd and the second catalytic layer contains Rh, and which can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine) and can prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, among the exhaust gas purification performance after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine).

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

[0010] <<Catalyst for Purifying Exhaust Gas>> The catalyst for purifying exhaust gas of the present invention will be described below.

[0011] An exhaust gas purifying catalyst 1 (hereinafter sometimes referred to as "catalyst 1") according to one embodiment of the present invention will be described below with reference to FIGS.

[0012] As shown in Figure 1, a catalyst 1 is disposed in an exhaust passage in an exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine. Exhaust gas emitted from the internal combustion engine flows through the exhaust passage in the exhaust pipe P from one end to the other end of the exhaust pipe P and is purified by the catalyst 1 provided in the exhaust pipe P. In the drawing, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas inflow side" or "upstream side," and the downstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas outflow side" or "downstream side."

[0013] In the exhaust passage in the exhaust pipe P, other exhaust gas purifying catalysts may be arranged upstream and / or downstream of the catalyst 1 .

[0014] As shown in FIGS. 2 to 4, 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 .

[0015] <Substrate> The substrate 10 will be described below.

[0016] The material constituting the substrate 10 can be appropriately selected from known materials. Examples of materials constituting the substrate 10 include ceramic materials and metal materials, with ceramic materials being preferred. Examples of ceramic materials include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; and oxide ceramics such as alumina, zirconia, cordierite, mullite, zircon, aluminum titanate, and magnesium titanate. Examples of metal materials include alloys such as stainless steel.

[0017] 2 to 4, the substrate 10 has a cylindrical portion 11, partition walls 12 provided in the cylindrical portion 11, and cells 13 separated by the partition walls 12. The substrate 10 is preferably a honeycomb structure.

[0018] As shown in Fig. 2, the tubular portion 11 defines the outer shape of the substrate 10, and the axial direction of the tubular portion 11 coincides with the axial direction of the substrate 10. As shown in Fig. 2, the shape of the tubular portion 11 is cylindrical, but it may be other shapes such as an elliptical cylinder or a polygonal cylinder.

[0019] 2 to 4 , partition walls 12 are present between adjacent cells 13, and the adjacent cells 13 are separated by the partition walls 12. The partition walls 12 may have a porous structure that allows exhaust gas to pass through. The thickness of the partition walls 12 is, for example, 20 μm or more and 1500 μm or less.

[0020] As shown in FIG. 4, the cell 13 extends in the exhaust gas flow direction X, and has an end on the exhaust gas inlet side and an end on the exhaust gas outlet side.

[0021] As shown in Fig. 4, both the end on the exhaust gas inlet side and the end on the exhaust gas outlet side of the cell 13 are open. Therefore, the exhaust gas that flows in from the end (opening) on ​​the exhaust gas inlet side of the cell 13 flows out from the end (opening) on ​​the exhaust gas outlet side of the cell 13. This type of configuration is called a flow-through type.

[0022] 2 and 3, the end (opening) of the cell 13 on the exhaust gas inlet side has a rectangular shape in plan view, but may have other shapes such as a hexagon, an octagon, etc. The end (opening) of the cell 13 on the exhaust gas outlet side also has a similar shape in plan view.

[0023] The cell density per square inch of the substrate 10 is, for example, 100 cells or more and 1000 cells or less. The cell density per square inch of the substrate 10 means the total number of cells 13 per square inch in a cross section obtained by cutting the substrate 10 along a plane perpendicular to the exhaust gas flow direction X.

[0024] The volume of the substrate 10 is, for example, 0.1 L or more and 20 L or less. The volume of the substrate 10 means the apparent volume of the substrate 10. For example, when the substrate 10 is cylindrical, the outer diameter of the substrate 10 is 2r, and the length of the substrate 10 is L. １０ Then, the volume of the substrate 10 is calculated by the formula: Volume of the substrate 10 = π × r ２ ×L １０ It is expressed as:

[0025] <First catalytic layer> The first catalytic layer 20 will now be described.

[0026] 3 and 4 , the first catalytic layer 20 is provided on the cell 13 side surface of the partition wall section 12. The "cell 13 side surface of the partition wall section 12" means the outer surface of the partition wall section 12 extending in the exhaust gas flow direction X. The first catalytic layer 20 may be provided directly on the cell 13 side surface of the partition wall section 12 or may be provided via another layer; however, the first catalytic layer 20 is usually provided directly on the cell 13 side surface of the partition wall section 12.

[0027] The first catalytic layer 20 may be composed of a portion (hereinafter referred to as a "protruding portion") that protrudes from the cell 13-side surface of the partition wall section 12 toward the cell 13, or may be composed of a portion (hereinafter referred to as an "internal portion") that exists inside the partition wall section 12, or may have both a protruding portion and an internal portion. The "first catalytic layer 20 provided on the substrate 10" includes any of an embodiment in which the first catalytic layer 20 is composed of a protruding portion, an embodiment in which the first catalytic layer 20 is composed of an internal portion, and an embodiment in which the first catalytic layer 20 has both a protruding portion and an internal portion.

[0028] 4 , the first catalytic layer 20 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side. The first catalytic layer 20 may extend along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, or may extend in the direction opposite to the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas outlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0029] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalytic layer 20 per unit volume of the portion of the substrate on which the first catalytic layer 20 is formed (mass after calcination) is preferably 50 g / L or more and 230 g / L or less, more preferably 70 g / L or more and 180 g / L or less, and even more preferably 80 g / L or more and 150 g / L or less. The mass of the first catalytic layer 20 per unit volume of the portion of the substrate 10 on which the first catalytic layer 20 is formed is calculated by the formula: (mass of the first catalytic layer 20) / ((volume of the substrate 10)×(average length L of the first catalytic layer 20) ２０ / Length L of the substrate 10 １０ In this specification, the term "length" refers to the dimension of the substrate 10 in the axial direction, unless otherwise specified.

[0030] In this specification, the "mass of the first catalytic layer 20" refers to the sum of the mass of precious metal elements in metal equivalent form and the mass of metal elements other than precious metal elements in oxide equivalent form, among all the metal elements contained in the first catalytic layer 20. In other words, the "mass of the first catalytic layer 20" refers to the calculated mass obtained by summing the mass of precious metal elements contained in the first catalytic layer 20 in metal equivalent form and the mass of metal elements other than precious metal elements contained in the first catalytic layer 20 in oxide equivalent form. Note that the "metal elements" also include metalloid elements such as Si and B.

[0031] In this specification, the term "precious metal element" includes Pt (platinum), Pd (palladium), Rh (rhodium), Ru (ruthenium), Os (osmium), Ir (iridium), Au (gold), and Ag (silver).

[0032] In this specification, oxides of rare earth elements other than Ce, Pr, and Tb are referred to as sesquioxides (M ２ O ３ , M represents a rare earth element other than Ce, Pr, and Tb), and the oxide of Ce is CeO ２ Pr oxide is Pr ６ O １１ The oxide of Tb is Tb ４ O ７ The oxide of Al is Al ２ O ３ Zr oxide is ZrO ２ The oxide of Si is SiO ２ The oxide of B is B ２ O ３ Cr oxide is Cr ２ O ３ , Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe ３ O ４ Mn oxide is Mn ３ O ４ Ni oxide is NiO, Ti oxide is TiO ２ Zn oxide is ZnO, Sn oxide is SnO ２ means.

[0033] Average length L of the first catalyst layer 20 ２０ An example of the measurement method is as follows.

[0034] Extending from the catalyst 1 in the axial direction of the substrate 10, the length L of the substrate 10 １０A sample having the same length as the sample is cut out. The sample is, for example, cylindrical with a diameter of 25.4 mm. The diameter of the sample can be changed as needed. When the first catalyst layer 20 extends from the end of the partition wall 12 on the exhaust gas inlet side along the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas inlet side. When the first catalyst layer 20 extends from the end of the partition wall 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X, the sample is cut at 5 mm intervals along a plane perpendicular to the axial direction of the substrate 10, and a first cut piece, a second cut piece, ..., an n-th cut piece is obtained in order from the end of the sample on the exhaust gas outlet side. In either case, the length of the cut pieces is 5 mm. The composition of the cut pieces is analyzed using an X-ray fluorescence analyzer (XRF) (for example, an energy dispersive X-ray analyzer (EDX), a wavelength dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma atomic emission spectrometer (ICP-AES), a scanning electron microscope-energy dispersive X-ray analysis method (SEM-EDX), etc., and it is confirmed based on the composition of the cut pieces whether or not the cut pieces contain a part of the first catalyst layer 20.

[0035] It is not necessary to perform composition analysis on a cut piece that is clearly containing a portion of the first catalytic layer 20. For example, the cut surface can be observed using a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA), or the like to confirm whether the cut piece contains a portion of the first catalytic layer 20. When observing the cut surface, elemental mapping of the cut surface may be performed.

[0036] After confirming whether the cut pieces include a part of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is calculated based on the following formula: Length of the first catalytic layer 20 included in the sample = 5 mm × (number of cut pieces including a part of the first catalytic layer 20)

[0037] For example, if the first to kth cut pieces include a portion of the first catalytic layer 20, but the (k+1)th to nth cut pieces do not include a portion of the first catalytic layer 20, the length of the first catalytic layer 20 included in the sample is (5×k) mm.

[0038] An example of a more detailed method for measuring the length of the first catalytic layer 20 included in a sample is as follows. A k-th piece (a piece obtained from the exhaust gas outlet side of the sample among the cut pieces including a part of the first catalytic layer 20 when the first catalytic layer 20 extends from the end of the partition wall 12 on the exhaust gas inlet side along the exhaust gas flow direction X, or a piece obtained from the exhaust gas inlet side of the sample among the cut pieces including a part of the first catalytic layer 20 when the first catalytic layer 20 extends from the end of the partition wall 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X) is cut in the axial direction of the substrate 10, and the part of the first catalytic layer 20 present on the cut surface is observed using an SEM, an EPMA, or the like, thereby measuring the length of the part of the first catalytic layer 20 in the k-th piece. Then, the length of the part of the first catalytic layer 20 included in the sample is calculated based on the following formula: Length of the first catalytic layer 20 included in the sample = (5 mm × (k - 1)) + (length of the part of the first catalytic layer 20 included in the k-th piece)

[0039] For 8 to 16 samples arbitrarily cut out from the catalyst 1, the length of the first catalyst layer 20 included in each sample was measured, and the average value thereof was taken as the average length L of the first catalyst layer 20. ２０ Let's say.

[0040] The first catalytic layer 20 contains Pd as a catalytically active component. Pd is contained in the first catalytic layer 20 in a form that can function as a catalytically active component, such as metallic Pd, an alloy containing Pd, or a compound containing Pd (e.g., an oxide of Pd). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing Pd is preferably in a particulate form.

[0041] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the metal-equivalent Pd content in the first catalytic layer 20 is preferably 0.010 mass % or more and 20 mass % or less, more preferably 0.050 mass % or more and 15 mass % or less, and even more preferably 0.10 mass % or more and 10 mass % or less, based on the mass of the first catalytic layer 20.

[0042] The first catalytic layer 20 may contain a precious metal element other than Pd as a catalytically active component. Examples of the precious metal element other than Pd include Pt, Rh, Ir, Ru, Os, Au, and Ag. The precious metal element other than Pd is contained in the first catalytic layer 20 in a form that can function as a catalytically active component, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (e.g., an oxide of the precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the precious metal element other than Pd is preferably in a particulate form.

[0043] If the first catalytic layer 20 contains Pd and a precious metal element other than Pd, the Pd and the precious metal element other than Pd may form an alloy, which may reduce the active sites of Pd that contribute to the exhaust gas purification performance. Therefore, it is preferable that the first catalytic layer 20 does not substantially contain any precious metal element other than Pd.

[0044] The phrase "the first catalytic layer 20 is substantially free of precious metal elements other than Pd" means that the metal-equivalent content of precious metal elements other than Pd in ​​the first catalytic layer 20 is preferably 0.050 mass% or less, and more preferably 0.010 mass% or less, based on the mass of the first catalytic layer 20. The lower limit is zero. The phrase "the metal-equivalent content of precious metal elements other than Pd in ​​the first catalytic layer 20" means the metal-equivalent content of one precious metal element when the first catalytic layer 20 contains one precious metal element other than Pd, and means the sum of the metal-equivalent contents of the two or more precious metal elements when the first catalytic layer 20 contains two or more precious metal elements other than Pd.

[0045] The metal-equivalent content of each metal element in the first catalytic layer 20 can be calculated from the formula: (metal-equivalent mass of each metal element in the first catalytic layer 20) / (mass of the first catalytic layer 20) × 100. For example, the metal-equivalent content of Pd in ​​the first catalytic layer 20 can be calculated from the formula: (metal-equivalent mass of Pd in ​​the first catalytic layer 20) / (mass of the first catalytic layer 20) × 100.

[0046] If the composition of the raw materials used to form the first catalytic layer 20 is known, the metal equivalent content of each metal element contained in the first catalytic layer 20 can be determined from the composition of the raw materials used to form the first catalytic layer 20.

[0047] When the composition of the raw materials used to form the first catalytic layer 20 is unknown, the metal-equivalent content of each metal element contained in the first catalytic layer 20 can be determined using a common method such as scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). Specifically, this is as follows.

[0048] The sample obtained from the first catalytic layer 20 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 to determine the content (mass%) of each identified metal element. The content (mass%) of each metal element is determined for each of 10 SEM fields of view, and the average value of the content (mass%) of each metal element in the 10 fields of view is taken as the content (mass%) of each metal element in the first catalytic layer 20.

[0049] The first catalyst layer 20 preferably contains one or more types of carriers, and at least a portion of the catalytically active component is preferably supported on one or more types of carriers.

[0050] The phrase "at least a portion of the catalytically active component is supported on a 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 the inner surface of the pores of the carrier. Whether at least a portion of the catalytically active component is supported on the carrier can be confirmed, for example, using SEM-EDX or the like. Specifically, when elemental mapping obtained by analyzing a cross section of the first catalytic layer 20 with SEM-EDX shows 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.

[0051] The support can be selected from, for example, inorganic oxides. The inorganic oxide is, for example, particulate. From the viewpoint of improving the supportability of the catalytically active component, the inorganic oxide is preferably porous. The inorganic oxide may or may not have oxygen storage capacity (OSC). In this specification, an inorganic oxide having OSC may be referred to as an "OSC material." The inorganic oxide used as a support is distinguished from the inorganic oxide used as a binder (e.g., inorganic oxide-based binders such as alumina binder, zirconia binder, titania binder, and silica binder).

[0052] Examples of inorganic oxides include Al-based oxides, Ce-based oxides, Ce-Zr-based composite oxides, oxides of rare earth elements other than Ce, and zirconia (ZrO ２ ), silica (SiO ２ ), titania (TiO ２ ), zeolite (aluminosilicate), MgO, ZnO, SnO ２ and the like.

[0053] The Al-based oxide refers to an oxide containing Al, in which Al is the element with the highest content by mass among the elements constituting the oxide other than O. However, an oxide that falls under the category of a Ce-Zr-based composite oxide does not fall under the category of an Al-based oxide.

[0054] The Ce-based oxide refers to an oxide containing Ce, in which Ce is the element with the highest content by mass among elements constituting the oxide other than O. However, an oxide that falls under the category of a Ce-Zr-based composite oxide is not considered to be a Ce-based oxide.

[0055] The Ce-Zr composite oxide is a composite oxide containing Ce and Zr, and the Ce in the composite oxide is represented by CeO ２ The converted content is 5.0 mass % or more and 95.0 mass % or less based on the mass of the composite oxide, and Zr in the composite oxide is ZrO ２ The converted content of Al in the composite oxide is 5.0 mass % or more and 95.0 mass % or less based on the mass of the composite oxide, and ２O ３ It means an oxide whose converted content is less than 5.0 mass % based on the mass of the composite oxide.

[0056] 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.

[0057] The first catalyst layer 20 may contain other components such as a binder, a stabilizer, etc. Examples of the binder include inorganic oxide binders such as alumina sol, ceria sol, zirconia sol, titania sol, and silica sol.

[0058] <Second catalytic layer> The second catalytic layer 30 will now be described.

[0059] As shown in FIGS. 3 and 4, the second catalyst layer 30 is provided above the first catalyst layer 20 .

[0060] The phrase "the second catalytic layer 30 is provided on the upper side of the first catalytic layer 20" means that part or all of the second catalytic layer 30 is present on one of the two main surfaces of the first catalytic layer 20 opposite the main surface on the partition wall section 12 side. The "main surface of the first catalytic layer 20" means the outer surface of the first catalytic layer 20 extending in the exhaust gas flow direction X. The second catalytic layer 30 may be provided directly on the main surface of the first catalytic layer 20 or via another layer, but is usually provided directly on the main surface of the first catalytic layer 20. The second catalytic layer 30 may be provided so as to cover part of the main surface of the first catalytic layer 20 or so as to cover the entire main surface of the first catalytic layer 20. The "second catalytic layer 30 provided on the upper side of the first catalytic layer 20" includes both an embodiment in which the second catalytic layer 30 is provided directly on the main surface of the first catalytic layer 20, and an embodiment in which the second catalytic layer 30 is provided on the main surface of the first catalytic layer 20 via another layer.

[0061] 4 , the second catalytic layer 30 extends along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side to the end of the partition wall 12 on the exhaust gas outlet side. The second catalytic layer 30 may extend along the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas inlet side so as not to reach the end of the partition wall 12 on the exhaust gas outlet side, or may extend in the direction opposite to the exhaust gas flow direction X from the end of the partition wall 12 on the exhaust gas outlet side so as not to reach the end of the partition wall 12 on the exhaust gas inlet side.

[0062] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the second catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed (mass after calcination) is preferably 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 catalytic layer 30 per unit volume of the portion of the substrate 10 on which the second catalytic layer 30 is formed is calculated by the formula: (mass of the second catalytic layer 30) / ((volume of the substrate 10)×(average length L of the second catalytic layer 30) ３０ / Length L of the substrate 10 １０ )) is calculated.

[0063] The above description regarding the mass of the first catalytic layer 20 also applies to the second catalytic layer 30. When applied, the "first catalytic layer 20" is read as the "second catalytic layer 30."

[0064] Average length L of the first catalyst layer 20 ２０ The above description of the method for measuring the average length L of the second catalyst layer 30 ３０ When applying this method, the "first catalyst layer 20" is referred to as the "second catalyst layer 30" and the "average length L ２０ " is "average length L ３０ " can be read as ".

[0065] The second catalytic layer 30 contains Rh as a catalytically active component. Rh is contained in the second catalytic 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 a particulate form.

[0066] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the metal-equivalent content of Rh in the second catalytic layer 30 is preferably 0.010 mass % or more and 5 mass % or less, more preferably 0.050 mass % or more and 3 mass % or less, and even more preferably 0.10 mass % or more and 2 mass % or less, based on the mass of the second catalytic layer 30.

[0067] The second catalytic layer 30 may contain a precious metal element other than Rh as a catalytically active component. Examples of the precious metal element other than Rh include Pt, Pd, Ir, Ru, Os, Au, and Ag. The precious metal element other than Rh is contained in the second catalytic layer 30 in a form that can function as a catalytically active component, such as a metal, an alloy containing the precious metal element, or a compound containing the precious metal element (e.g., an oxide of the precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing the precious metal element other than Rh is preferably in a particulate form.

[0068] If the second catalytic layer 30 contains Rh and a precious metal element other than Rh, the Rh and the precious metal element other than Rh may form an alloy, which may reduce the active sites of Rh that contribute to the exhaust gas purification performance. Therefore, it is preferable that the second catalytic layer 30 does not substantially contain any precious metal element other than Rh.

[0069] The phrase "the second catalytic layer 30 is substantially free of precious metal elements other than Rh" means that the metal-equivalent content of precious metal elements other than Rh in the second catalytic layer 30 is preferably 0.050 mass% or less, and more preferably 0.010 mass% or less, based on the mass of the second catalytic layer 30. The lower limit is zero. The phrase "the metal-equivalent content of precious metal elements other than Rh in the second catalytic layer 30" means the metal-equivalent content of one precious metal element when the second catalytic layer 30 contains one precious metal element other than Rh, or means the sum of the metal-equivalent contents of the two or more precious metal elements when the second catalytic layer 30 contains two or more precious metal elements other than Rh.

[0070] The metal-equivalent content of each metal element in the second catalytic layer 30 can be calculated from the formula: (metal-equivalent mass of each metal element in the second catalytic layer 30) / (mass of the second catalytic layer 30) × 100. For example, the metal-equivalent content of Rh in the second catalytic layer 30 can be calculated from the formula: (metal-equivalent mass of Rh in the second catalytic layer 30) / (mass of the second catalytic layer 30) × 100.

[0071] The metal-equivalent content of each metal element in the second catalytic layer 30 can be determined in the same manner as the metal-equivalent content of each metal element in the first catalytic layer 20 .

[0072] The second catalytic layer 30 contains a composite oxide containing Ce, Zr, and Al (hereinafter, may be referred to as a "Ce-Zr-Al based composite oxide").

[0073] Whether or not the second catalytic layer 30 contains a Ce—Zr—Al-based composite oxide can be determined by a common method such as SEM-EDX. Specifically, when the second catalytic layer 30 is observed with SEM-EDX and Ce, Zr, and Al are present in the same location in the second catalytic layer 30, it can be determined that the second catalytic layer 30 contains a Ce—Zr—Al-based composite oxide.

[0074] It is desirable to control the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine to be close to the theoretical air-fuel ratio (stoichiometry). However, the actual air-fuel ratio fluctuates from stoichiometry to the rich (excess fuel atmosphere) or lean (lean fuel atmosphere) side, and the air-fuel ratio of the exhaust gas also fluctuates to the rich or lean side depending on the vehicle's running conditions, etc. Ce—Zr—Al-based composite oxides have oxygen storage capacity, so by using Ce—Zr—Al-based composite oxides, fluctuations in the oxygen concentration in exhaust gas can be alleviated and the operating window of the catalyst can be expanded. Furthermore, because Ce—Zr—Al-based composite oxides are heat-resistant, they can prevent aggregation of Ce—Zr—Al-based composite oxides, burial of Rh and sintering of Rh that are associated with the disappearance of pores in Ce—Zr—Al-based composite oxides (i.e., a decrease in specific surface area) in high-temperature environments. Therefore, by including a Ce-Zr-Al composite oxide in the second catalytic 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, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine).

[0075] The Ce-Zr-Al composite oxide is a composite oxide containing Ce, Zr, and Al, and the Ce in the composite oxide is represented by CeO ２ The converted content is 1.0 mass % or more and 94.0 mass % or less based on the mass of the composite oxide, and Zr in the composite oxide is ZrO ２ The converted content of Al in the composite oxide is 1.0 mass % or more and 94.0 mass % or less based on the mass of the composite oxide, and ２ O ３ This refers to a composite oxide whose converted content is 5.0 mass % or more and 98.0 mass % or less based on the mass of the composite oxide.

[0076] The Ce—Zr—Al-based composite oxide is, for example, in the form of particles. The Ce—Zr—Al-based composite oxide is used as a support for a catalytically active component. From the viewpoint of improving the supportability of the catalytically active component, the Ce—Zr—Al-based composite oxide is preferably porous.

[0077] It is preferable that at least a part of the catalytically active component is supported on a Ce—Zr—Al-based composite oxide in the second catalytic layer 30. The meaning of the support and the method for confirming it are the same as those described in <First catalytic layer 20>.

[0078] From the viewpoint of improving oxygen storage capacity and heat resistance, CeO of Ce in Ce-Zr-Al based composite oxide ２ The converted content is preferably 2.0 mass % or more and 40.0 mass % or less, more preferably 3.0 mass % or more and 20.0 mass % or less, and even more preferably 5.0 mass % or more and 15.0 mass % or less, based on the mass of the Ce-Zr-Al-based composite oxide.

[0079] From the viewpoint of improving oxygen storage capacity and heat resistance, Zr in the Ce-Zr-Al composite oxide is converted to ZrO ２ The converted content is preferably 10.0 mass % or more and 70.0 mass % or less, more preferably 15.0 mass % or more and 60.0 mass % or less, and even more preferably 20.0 mass % or more and 50.0 mass % or less, based on the mass of the Ce-Zr-Al-based composite oxide.

[0080] From the viewpoint of improving oxygen storage capacity and heat resistance, the Al content of the Al in the Ce-Zr-Al composite oxide is ２ O ３ The converted content is preferably 30.0 mass % or more and 60.0 mass % or less, more preferably 35.0 mass % or more and 55.0 mass % or less, and even more preferably 40.0 mass % or more and 50.0 mass % or less, based on the mass of the Ce-Zr-Al-based composite oxide.

[0081] From the viewpoint of improving oxygen storage capacity and heat resistance, CeO of Ce in Ce-Zr-Al based composite oxide ２ Conversion ratio of Zr to ZrO ２ Conversion rate and Al content ２ O ３ The total converted content is preferably 70.0 mass % or more, more preferably 80.0 mass % or more, and even more preferably 90.0 mass % or more, based on the mass of the Ce—Zr—Al-based composite oxide, with the upper limit being 100 mass %.

[0082] From the viewpoint of improving oxygen storage capacity and heat resistance, CeO of Ce in Ce-Zr-Al based composite oxide ２ Conversion rate and ZrO ２ The total converted content of Al in Ce-Zr-Al composite oxides ２ O ３ The ratio to the converted content is preferably 0.5 or more and 1.2 or less, more preferably 0.6 or more and 1.1 or less, and even more preferably 0.7 or more and 1.0 or less.

[0083] The Ce-Zr-Al composite oxide may contain one or more metal elements other than Ce, Zr, and Al. Examples of 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.

[0084] The oxide-equivalent content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al composite oxide is CeO ２ Conversion ratio of Zr to ZrO ２ Conversion rate and Al content ２ O ３ The "oxide-equivalent content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al-based composite oxide" can be appropriately adjusted taking into account the oxide-equivalent content. From the viewpoint of improving oxygen storage capacity and heat resistance, the oxide-equivalent content of metal elements other than Ce, Zr, and Al in the Ce-Zr-Al-based composite oxide is preferably 30.0 mass% or less, more preferably 20.0 mass% or less, and even more preferably 15.0 mass% or less. The lower limit is zero. When the Ce-Zr-Al-based composite oxide contains one metal element other than Ce, Zr, and Al, the "oxide-equivalent content of metal elements other than Ce, Zr, and Al" refers to the oxide-equivalent content of the one metal element, and when the Ce-Zr-Al-based composite oxide contains two or more metal elements other than Ce, Zr, and Al, the "oxide-equivalent content of metal elements other than Ce, Zr, and Al" refers to the sum of the oxide-equivalent contents of the two or more metal elements.

[0085] From the viewpoint of improving heat resistance, the Ce—Zr—Al based composite oxide preferably contains La. From the viewpoint of improving heat resistance, the La in the Ce—Zr—Al based composite oxide ２ O ３ The converted content of La in the Ce-Zr-Al composite oxide is preferably 1.0 mass % or more, more preferably 2.0 mass % or more, and even more preferably 4.0 mass % or more, based on the mass of the Ce-Zr-Al composite oxide. ２ O ３ The upper limit of the converted content is CeO of Ce in the Ce-Zr-Al based composite oxide. ２ Conversion ratio of Zr to ZrO ２ Conversion rate and Al content ２ O ３ The La content in the Ce—Zr—Al composite oxide can be adjusted appropriately in consideration of the converted content. ２ O ３ The converted content is preferably 15.0% by mass or less, more preferably 10.0% by mass or less, and even more preferably 8.0% by mass or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0086] From the viewpoint of improving heat resistance, the Ce—Zr—Al composite oxide may contain rare earth elements other than Ce and La. From the viewpoint of improving heat resistance, the oxide-equivalent content of the rare earth elements other than Ce and La in the Ce—Zr—Al composite oxide is preferably 1.0 mass % or more, more preferably 2.0 mass % or more, and even more preferably 4.0 mass % or more, based on the mass of the Ce—Zr—Al composite oxide. The upper limit of the oxide-equivalent content of the rare earth elements other than Ce and La in the Ce—Zr—Al composite oxide is the CeO of Ce in the Ce—Zr—Al composite oxide. ２ Conversion ratio of Zr to ZrO ２ Conversion rate and Al content ２ O ３The oxide-equivalent content can be appropriately adjusted taking into account the oxide-equivalent content. The oxide-equivalent content of rare earth elements other than Ce and La in the Ce—Zr—Al-based composite oxide is preferably 15.0 mass % or less, more preferably 10.0 mass % or less, and even more preferably 8.0 mass % or less. Each of these upper limits may be combined with any of the above-mentioned lower limits. The "oxide-equivalent content of rare earth elements other than Ce and La in the Ce—Zr—Al-based composite oxide" refers to the oxide-equivalent content of the one rare earth element when the Ce—Zr—Al-based composite oxide contains one rare earth element other than Ce and La, and refers to the sum of the oxide-equivalent contents of the two or more rare earth elements when the Ce—Zr—Al-based composite oxide contains two or more rare earth elements other than Ce and La.

[0087] When the composition of the Ce—Zr—Al-based composite oxide is known, the content of each element in the Ce—Zr—Al-based composite oxide in terms of oxide can be determined from the composition of the Ce—Zr—Al-based composite oxide.

[0088] When the composition of the Ce-Zr-Al-based composite oxide is unknown, the oxide-equivalent content of each element in the Ce-Zr-Al-based composite oxide can be measured from the element mapping obtained by analyzing a sample obtained from the second catalytic layer 30 by energy dispersive X-ray spectroscopy (EDX) and EDX elemental analysis of the specified particles. Specifically, the Ce-Zr-Al-based composite oxide particles and other particles are qualitatively identified (color-coded) by elemental mapping, and the specified particles are subjected to composition analysis (elemental analysis), thereby measuring the oxide-equivalent content of each element in the specified particles.

[0089] In the Ce-Zr-Al composite oxide, Ce exists in a solid solution phase (e.g., CeO ２ and ZrO ２ Solid solution phase with CeO ２ and Al ２ O ３ Solid solution phase with CeO ２ and ZrO ２ and Al ２ O ３ or a single phase that is a crystalline phase or an amorphous phase (e.g., a solid solution phase with CeO ２Although Ce may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Ce forms a solid solution phase.

[0090] In the Ce-Zr-Al composite oxide, Zr exists in a solid solution phase (e.g., CeO ２ and ZrO ２ Solid solution phase with ZrO ２ and Al ２ O ３ Solid solution phase with CeO ２ and ZrO ２ and Al ２ O ３ or a single phase that is a crystalline phase or an amorphous phase (e.g., ZrO ２ Although Zr may form a solid solution phase or both a solid solution phase and a single phase, it is preferable that at least a part of Zr forms a solid solution phase.

[0091] In the Ce-Zr-Al composite oxide, Al exists in a solid solution phase (e.g., CeO ２ and Al ２ O ３ Solid solution phase with ZrO ２ and Al ２ O ３ Solid solution phase with CeO ２ and ZrO ２ and Al ２ O ３ A solid solution phase with Al may be formed, or a single phase (such as a crystalline phase or an amorphous phase) may be formed. ２ O ３ It may form a single phase.

[0092] When the Ce—Zr—Al based composite oxide contains one or more metal elements other than Ce, Zr, and Al, the metal elements other than Ce, Zr, and Al are in a solid solution phase (for example, CeO ２ and a solid solution phase of oxides of metal elements other than Ce, Zr, and Al, ZrO ２ and a solid solution phase of oxides of metal elements other than Ce, Zr, and Al, CeO ２ and ZrO ２The metal elements other than Ce, Zr, and Al may form a solid solution phase (a solid solution phase of Ce and an oxide of a metal element other than Ce, Zr, and Al, etc.), or may form a single phase that is a crystalline phase or an amorphous phase (a single phase of an oxide of a metal element other than Ce, Zr, and Al), but it is preferable that at least a part of the metal elements other than Ce, Zr, and Al form a solid solution phase.

[0093] From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine), the content of the Ce—Zr—Al-based composite oxide in the second catalytic layer 30 is preferably 80.0 mass% or more, more preferably 85.0 mass% or more, and even more preferably 88.0 mass% or more, based on the mass of the second catalytic layer 30. The upper limit of the content of the Ce—Zr—Al-based composite oxide in the second catalytic layer 30 can be appropriately adjusted taking into account the contents of other components (e.g., Rh) in the second catalytic layer 30. The content of the Ce—Zr—Al-based composite oxide in the second catalytic layer 30 is preferably 98.0 mass% or less, more preferably 97.0 mass% or less, and even more preferably 95.0 mass% or less, based on the mass of the second catalytic layer 30. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0094] When the composition of the raw materials used to form the second catalytic layer 30 is known, the content of the Ce-Zr-Al-based composite oxide in the second catalytic layer 30 can be determined from the composition of the raw materials used to form the second catalytic layer 30.

[0095] When the composition of the raw materials used to form the second catalytic layer 30 is unknown, the content of the Ce—Zr—Al-based composite oxide in the second catalytic layer 30 can be determined by a conventional method such as SEM-EDX. Specifically, this is as follows.

[0096] (1) Elemental analysis is performed on the sample obtained from the second catalytic layer 30 using a standard method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the content (mass%) of each identified element. (2) Elemental mapping is performed on the sample obtained from the second catalytic layer 30 using a standard method such as SEM-EDX to identify the types of particles contained in the sample (e.g., Ce-Zr-Al composite oxide particles and, in some cases, other particles). (3) For each type of particle, elemental analysis is performed on a number of arbitrarily selected particles (e.g., 50 particles) using SEM-EDX to identify the types of constituent elements of the particles and to determine the content (mass%) of each identified element. The average content (mass%) of each element is determined for each type of particle. (4) An equation is created and solved to represent the relationship between the content (mass%) of each element in the sample, the content (mass%) of each element in each type of particle, and the content (mass%) of each type of particle in the sample, thereby calculating the content (mass%) of each type of particle in the sample, and this is used as the content (mass%) of each type of particle in the second catalyst layer 30.

[0097] The second catalytic layer 30 may contain one or more types of carriers other than the Ce-Zr-Al composite oxide (hereinafter, sometimes referred to as "other carriers"). When the second catalytic layer 30 contains the other carriers, it is preferable that at least a portion of the catalytically active component is supported on the other carriers. The explanation regarding the carriers (including the explanation regarding inorganic oxides), as well as the meaning and confirmation method of the support, are the same as those in the explanation for the first catalytic layer 20.

[0098] <Conditions A and B> The catalyst 1 satisfies at least one of the following conditions A and B: (A) the first catalytic layer 20 contains Sr, and the metal-equivalent content of Sr in the first catalytic layer 20 is 0.1 mass % or more based on the mass of the first catalytic layer 20; (B) the second catalytic layer 30 contains Sr, and the metal-equivalent content of Sr in the second catalytic layer 30 is 0.1 mass % or more based on the mass of the second catalytic layer 30.

[0099] Since Pd in ​​the first catalytic layer 20 is susceptible to poisoning by phosphorus (e.g., phosphorus in engine oil), the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 (particularly, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine) is likely to decrease after exposure to phosphorus. In contrast, when the catalyst 1 satisfies at least one of conditions A and B, Sr in the first catalytic layer 20 and / or the second catalytic layer 30 captures phosphorus, thereby preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (particularly, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus). Note that the present inventors have investigated various alkaline earth metal elements for suppressing phosphorus poisoning and have found that Mg and Ca are more effective than Sr in suppressing phosphorus poisoning than Al, which may be present in the first catalytic layer 20 or the second catalytic layer 30. ２ O ３ Ba is more likely to react with Al, which may be present in the first catalytic layer 20 or the second catalytic layer 30, than Sr. ２ O ３ Sr is easily reacted with Al, which may be present in the first catalytic layer 20 or the second catalytic layer 30, and this may reduce the heat resistance and the exhaust gas purification performance of Rh. ２ O ３ It was found that Sr is unlikely to react with Rh and Pd, which would not decrease the heat resistance and would not decrease the exhaust gas purification performance of Rh and Pd. Based on these findings, Sr was selected.

[0100] The Ce—Zr—Al composite oxide has a high bulk density, i.e., a small apparent volume per mass. Therefore, when the second catalytic layer 30 is exposed to phosphorus, the Ce—Zr—Al composite oxide in the second catalytic layer 30 has low physical blocking properties against phosphorus penetration. Therefore, as the content of Ce—Zr—Al composite oxide in the second catalytic layer 30 increases, phosphorus passes through the second catalytic layer 30 more easily to reach the first catalytic layer 20, and Pd in ​​the first catalytic layer 20 becomes more susceptible to poisoning by phosphorus. Therefore, when the content of Ce—Zr—Al composite oxide in the second catalytic layer 30 is 80.0 mass% or more, the effect of catalyst 1 satisfying at least one of conditions A and B is significant.

[0101] From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine), under condition A, the metal-equivalent content of Sr in the first catalytic layer 20 is preferably 0.3 mass% or more, and more preferably 0.5 mass% or more, based on the mass of the first catalytic layer 20.

[0102] From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine), under condition B, the metal-equivalent content of Sr in the second catalytic layer 30 is preferably 0.3 mass% or more, and more preferably 0.5 mass% or more, based on the mass of the second catalytic layer 30.

[0103] It is desirable to control the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine to be close to the theoretical air-fuel ratio (stoichiometry). However, the actual air-fuel ratio fluctuates from stoichiometry to the rich (excess fuel atmosphere) or lean (lean fuel atmosphere) side, and the air-fuel ratio of the exhaust gas also fluctuates to the rich or lean side. In addition to its ability to capture phosphorus, Sr 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 excessive, when the exhaust gas switches from a lean state to a rich state, a large amount of NOx may be released from Sr all at once, and the released NOx may not be completely purified by the catalyst 1. Note that NOx released from Sr when the exhaust gas switches from a lean state to a rich state may be referred to as "transient NOx" hereinafter. Therefore, it is preferable to appropriately limit the upper limit of the Sr amount to prevent an excessive increase in the amount of transient NOx and to keep the amount of transient NOx within the range that the catalyst 1 can purify.

[0104] From the viewpoint of more effectively preventing an excessive increase in the transient NOx amount, under condition A, the metal-equivalent content of Sr in the first catalytic 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 catalytic layer 20. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0105] From the viewpoint of more effectively preventing an excessive increase in the transient NOx amount, under condition B, the metal-equivalent content of Sr in the second catalytic 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 catalytic layer 30. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0106] In a preferred embodiment, the metal-equivalent content of Sr in the first catalytic layer 20 is 0.1 mass % or more and 7.0 mass % or less, based on the mass of the first catalytic layer 20, and / or the metal-equivalent content of Sr in the second catalytic layer 30 is 0.1 mass % or more and 7.0 mass % or less, based on the mass of the second catalytic layer 30. This makes it possible to more effectively prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after startup of an internal combustion engine), and also makes it possible to more effectively prevent an excessive increase in the transient NOx amount.

[0107] In a more preferred embodiment, the metal-equivalent content of Sr in the first catalytic layer 20 is 0.1 mass % or more and 4.0 mass % or less, based on the mass of the first catalytic layer 20, and / or the metal-equivalent content of Sr in the second catalytic layer 30 is 0.1 mass % or more and 4.0 mass % or less, based on the mass of the second catalytic layer 30. This makes it possible to more effectively prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after startup of an internal combustion engine), and also makes it possible to more effectively prevent an excessive increase in the transient NOx amount.

[0108] When catalyst 1 satisfies condition A, catalyst 1 may or may not satisfy condition B. "Catalyst 1 does not satisfy condition B" encompasses an embodiment in which second catalytic layer 30 does not contain Sr, and an embodiment in which second catalytic layer 30 contains Sr and the metal-equivalent content of Sr in second catalytic layer 30 is less than 0.1 mass% based on the mass of second catalytic layer 30. In the latter embodiment, the metal-equivalent content of Sr in second catalytic layer 30 is preferably 0.05 mass% or less, more preferably 0.03 mass% or less, and even more preferably 0.01 mass% or less, based on the mass of second catalytic layer 30.

[0109] When catalyst 1 satisfies condition B, catalyst 1 may or may not satisfy condition A. "Catalyst 1 does not satisfy condition A" encompasses an embodiment in which first catalytic layer 20 does not contain Sr, and an embodiment in which first catalytic layer 20 contains Sr and the metal-equivalent content of Sr in first catalytic layer 20 is less than 0.1 mass% based on the mass of first catalytic layer 20. In the latter embodiment, the metal-equivalent content of Sr in first catalytic layer 20 is preferably 0.05 mass% or less, more preferably 0.03 mass% or less, and even more preferably 0.01 mass% or less, based on the mass of first catalytic layer 20.

[0110] "Catalyst 1 satisfies at least one of conditions A and B" includes (a) embodiments in which catalyst 1 satisfies conditions A and B, (b) embodiments in which catalyst 1 satisfies condition A but not condition B, and (c) embodiments in which catalyst 1 satisfies condition B but not condition A.

[0111] When catalyst 1 satisfies condition B, Sr in the second catalytic layer 30 can capture phosphorus contained in exhaust gas before it reaches the first catalytic layer 20, which has the advantage of more effectively preventing phosphorus poisoning of Pd in ​​the first catalytic layer 20 compared to when first catalytic layer 20 satisfies condition A. On the other hand, when catalyst 1 satisfies condition B, there is a disadvantage that Sr in the second catalytic layer 30 reduces the exhaust gas purification performance of Rh in the second catalytic layer 30. In contrast, when catalyst 1 satisfies condition A, the effect of preventing phosphorus poisoning of Pd in ​​the first catalytic layer 20 is inferior compared to when catalyst 1 satisfies condition B, but there is no disadvantage that Sr in the first catalytic layer 20 reduces the exhaust gas purification performance of Pd in ​​the first catalytic layer 20. Therefore, from the viewpoint of achieving a good balance between preventing phosphorus poisoning of Pd by Sr and preventing a decrease in the exhaust gas purification performance of precious metal elements by Sr, among embodiments a to c, embodiments a and b (i.e., catalyst 1 satisfies at least condition A of conditions A and B) are preferred, and embodiment b (i.e., catalyst 1 satisfies condition A but does not satisfy condition B) is more preferred.

[0112] The metal content of Sr in the first catalytic layer 20 and the metal content of Sr in the second catalytic layer 30 can each be determined in the same manner as the metal-equivalent content of each metal element in the first catalytic layer 20.

[0113] When catalyst 1 satisfies condition A, first catalyst layer 20 contains one or more Sr sources, and when catalyst 1 satisfies condition B, second catalyst layer 30 contains one or more Sr sources.

[0114] 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 may exist in one or more forms selected from, for example, oxides, carbonates, and sulfates. From the viewpoint of more effectively exerting the effects of Sr, Sr may exist in the form of SrO and SrCO. ３ It is preferred that the compound is present in one or two forms selected from the following:

[0115] When catalyst 1 satisfies condition A, catalyst 1 preferably satisfies the following condition C: (C) the ratio of the metal-equivalent content (mass %) of Sr in the first catalytic layer 20, based on the mass of the first catalytic layer 20, to the content (mass %) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30, based on the mass of the second catalytic layer 30 (metal-equivalent content (mass %) of Sr in the first catalytic layer 20 / content (mass %) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30) is 0.0010 or more and 0.0800 or less. The phenomenon in which Pd in ​​the first catalytic layer 20 becomes susceptible to phosphorus poisoning due to the high bulk density of the Ce—Zr—Al-based composite oxide is particularly noticeable when the second catalytic layer 30 contains a large amount of Ce—Zr—Al-based composite oxide. Therefore, by containing in the first catalytic layer 20 an amount of Sr that corresponds to the amount of Ce-Zr-Al-based composite oxide in the second catalytic layer 30, that is, by making the catalyst 1 satisfy the condition C, it is possible to more effectively prevent phosphorus poisoning of the Pd in ​​the first catalytic layer 20.

[0116] From the viewpoint of more effectively preventing phosphorus poisoning of Pd in ​​the first catalytic layer 20, the ratio of the metal-equivalent content (mass%) of Sr in the first catalytic layer 20 based on the mass of the first catalytic layer 20 to the content (mass%) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30 based on the mass of the second catalytic layer 30 is preferably 0.0010 or more and 0.0800 or less, more preferably 0.0020 or more and 0.0500 or less, and even more preferably 0.0040 or more and 0.0130 or less.

[0117] When catalyst 1 satisfies condition B, catalyst 1 preferably satisfies the following condition D: (D) the ratio of the metal-equivalent content (mass %) of Sr in the second catalytic layer 30, based on the mass of the second catalytic layer 30, to the content (mass %) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30, based on the mass of the second catalytic layer 30 (metal-equivalent content (mass %) of Sr in the second catalytic layer 30 / content (mass %) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30) is 0.0010 or more and 0.0800 or less. The phenomenon in which Pd in ​​the first catalytic layer 20 becomes susceptible to phosphorus poisoning due to the high bulk density of the Ce—Zr—Al-based composite oxide is particularly noticeable when the second catalytic layer 30 contains a large amount of Ce—Zr—Al-based composite oxide. Therefore, by containing an amount of Sr in the second catalytic layer 30 that corresponds to the amount of Ce-Zr-Al-based composite oxide in the second catalytic layer 30, that is, by making the catalyst 1 satisfy the condition D, it is possible to more effectively prevent phosphorus poisoning of the Pd in ​​the first catalytic layer 20.

[0118] From the viewpoint of more effectively preventing phosphorus poisoning of Pd in ​​the first catalytic layer 20, the ratio of the metal-equivalent content (mass%) of Sr in the second catalytic layer 30 based on the mass of the second catalytic layer 30 to the content (mass%) of Ce—Zr—Al-based composite oxide in the second catalytic layer 30 based on the mass of the second catalytic layer 30 is preferably 0.0010 or more and 0.0800 or less, more preferably 0.0020 or more and 0.0500 or less, and even more preferably 0.0040 or more and 0.0130 or less.

[0119] <Catalyst Action> In the catalyst 1, the second catalytic layer 30 is provided above the first catalytic layer 20. Therefore, exhaust gas flowing in from the exhaust gas inlet end (opening) of the cell 13 first comes into contact with the second catalytic layer 30 and then with the first catalytic layer 20. The exhaust gas that comes into contact with the second catalytic layer 30 is purified by the catalytically active component (e.g., Rh) in the second catalytic layer 30, and the exhaust gas that comes into contact with the first catalytic layer 20 is purified by the catalytically active component (e.g., Pd) in the first catalytic layer 20. The inclusion of a Ce—Zr—Al-based composite oxide in the second catalytic layer 30 can prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine). Furthermore, by making the catalyst 1 satisfy 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 catalytic layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine). Therefore, the catalyst 1 can prevent a decrease in the exhaust gas purification performance of Rh after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine), and prevent a decrease in the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, among the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalytic layer 20 in a low-temperature environment immediately after the start of an internal combustion engine).

[0120] <Production of Catalyst> The catalyst 1 can be produced by forming a first catalyst layer 20 on a substrate 10 and then forming a second catalyst layer 30 on the upper side of the first catalyst layer 20 .

[0121] The first catalyst layer 20 can be formed by mixing a Pd source (e.g., a Pd salt), optionally a Sr source (e.g., an Sr salt), and optionally other components (e.g., an inorganic oxide, a binder, a solvent, etc.) to prepare a first slurry, applying the first slurry onto the substrate 10, drying, and firing.

[0122] The second catalytic layer 30 can be formed by mixing a Rh source (e.g., an Rh salt), a Ce-Zr-Al composite oxide, optionally a Sr source (e.g., an Sr salt), and optionally other components (e.g., an inorganic oxide other than the Ce-Zr-Al composite oxide, a binder, a solvent, etc.) to prepare a second slurry, applying the second slurry onto the first catalytic layer 20, drying, and firing.

[0123] Examples of Pd sources include Pd salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of Rh sources include Rh salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of Sr sources include Sr salts, such as nitrates, acetates, carbonates, 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.

[0124] The drying temperature is, for example, 70° C. to 150° C., and the drying time is, for example, 5 minutes to 1 hour. The firing temperature is, for example, 200° C. to 700° C., and the firing time is, for example, 0.5 hours to 5 hours. The firing can be carried out, for example, in the air.

[0125] Example 1 (1) Preparation of Slurry for Forming Lower Layer An OSC material (Ce—Zr-based composite oxide, hereinafter sometimes referred to as “CZ”) having the following composition was prepared: CeO ２ Converted content: 40.0 mass% ZrO ２ Concentration rate in terms of oxides of rare earth elements other than Ce: 10.0% by mass

[0126] In a mixing vessel, an aqueous solution of palladium nitrate, a Ce-Zr composite oxide, La ２ O ３ Modified alumina (La ２ O ３The amount of Pd in ​​the slurry for forming the lower layer was 4.0 mass % in terms of metal, the amount of Ce—Zr composite oxide was 40.0 mass %, the amount of La was 4.0 mass %, and the amount of La was 100 mass %. ２ O ３ The modified alumina was adjusted to 45.4 mass%, the Sr metal content was 0.5 mass% (0.6 mass% in terms of SrO), and the solid content of the alumina sol was adjusted to 10.0 mass%. Note that strontium nitrate is converted to strontium oxide by calcination.

[0127] (2) Formation of the Lower Layer As a flow-through type substrate, cells extending in the axial direction and partitioned by partition walls having a thickness of 50 to 70 μm were formed at a density of 600 cells / inch in a plane perpendicular to the axial direction. ２ A flow-through substrate having a density of 1.0 L and a volume of 1.0 L was prepared.

[0128] A flow-through type substrate was immersed in the lower layer forming slurry, and the flow-through type substrate coated with the lower layer forming slurry was dried at 150° C. for 0.5 hours and then fired at 500° C. for 1 hour to form a lower layer. The mass of the lower layer per unit volume of the portion of the flow-through type substrate on which the lower layer was formed was 100 g / L.

[0129] (3) Preparation of Slurry for Forming Upper Layer An OSC material (Ce—Zr—Al composite oxide, hereinafter sometimes referred to as “CZA”) having the following composition was prepared: CeO ２ Converted content: 10.0 mass% ZrO ２ Conversion content: 35.0 mass% of Al ２ O ３ Converted content: 45.0 mass%, La ２ O ３ Concentration rate in terms of oxides of rare earth elements other than Ce and La: 5.0% by mass

[0130] An aqueous rhodium nitrate solution, a Ce—Zr—Al composite oxide, an alumina sol, and water were added to a mixing vessel, and the mixture was mixed and stirred to prepare a slurry for forming an upper layer. The amounts of the components in the slurry for forming an upper layer were adjusted so that, based on the mass of the fired upper layer (100 mass%), Rh was 1.0 mass% in terms of metal, the Ce—Zr—Al composite oxide was 89.0 mass%, and the solids content of the alumina sol was 10.0 mass%.

[0131] (4) Formation of Upper Layer The flow-through substrate on which the lower layer had been formed was immersed in the upper layer forming slurry, and the flow-through substrate coated with the upper layer forming slurry was dried at 150° C. for 0.5 hours and then baked at 500° C. for 1 hour to form an upper layer on the lower layer. The mass of the upper layer per unit volume of the portion of the flow-through substrate on which the upper layer had been formed was 80 g / L.

[0132] In this manner, an exhaust gas purifying catalyst was produced, which had a lower layer formed on a flow-through type substrate and an upper layer formed on the lower layer.

[0133] [Examples 2 to 4] The amount of strontium nitrate added to the lower layer forming slurry was adjusted so that Sr was 1.0 mass % (Example 2), 2.5 mass % (Example 3), or 5.0 mass % (Example 4) in terms of metal, with the mass of the lower layer after firing being taken as the reference (100 mass %). ２ O ３ Exhaust gas purifying catalysts of Examples 2 to 4 were produced in the same manner as in Example 1, except that the amount of modified alumina added was reduced.

[0134] Comparative Example 1 Preparation of a slurry for forming a lower layer and formation of a lower layer were carried out in the same manner as in Example 2.

[0135] An OSC material (Ce—Zr composite oxide, hereinafter sometimes referred to as “CZ”) having the following composition was prepared: CeO ２ Converted content: 15.0 mass% ZrO ２ Concentration rate in terms of oxides of rare earth elements other than Ce: 75.0% by mass, Concentration rate in terms of oxides of rare earth elements other than Ce: 10.0% by mass

[0136] In a mixing vessel, add rhodium nitrate solution, Ce-Zr composite oxide, La ２O ３ Modified alumina (La ２ O ３ The amount of each component in the slurry for forming the upper layer was 1.0 mass % in terms of metal, Rh, 50.0 mass % in terms of metal, Ce—Zr-based composite oxide, La, based on the mass of the upper layer after firing (100 mass %). ２ O ３ The modified alumina was adjusted to 39.0 mass % and the solid content of the alumina sol was adjusted to 10.0 mass %. An upper layer was formed in the same manner as in Example 2, except that this upper layer slurry was used.

[0137] Comparative Example 2: In the lower layer forming slurry, strontium nitrate was not added, and the amount of La ２ O ３ An exhaust gas purifying catalyst of Comparative Example 2 was produced in the same manner as in Example 1, except that the amount of modified alumina added was increased.

[0138] [Test Example] (1) Heat Treatment of Catalyst A 30 mL core of the catalyst for exhaust gas purification was heated in a quartz tube furnace at 1000°C for 20 hours with 10% by volume H ２ The catalyst was subjected to heat treatment in an O atmosphere in the following FC mode. The catalyst after the heat treatment was used as an evaluation sample. FC mode: A model gas (3 L / min) having the following composition and air (3 L / min) were alternately flowed. Model gas flow rate: C ３ H ６ 6 mL / min, O ２ 71mL / min, N ２ 2923 mL / min (total 3 L / min). 10% by volume H ２ O was vaporized from a water tank and mixed as water vapor into the model gas or air. The saturated water vapor pressure was adjusted by the temperature to obtain the water vapor amount in the above volume percentage.

[0139] (2) Phosphorus poisoning treatment of catalyst In a quartz tube furnace, the catalyst after the heat treatment in (1) above was poisoned with 10% by volume of zinc dialkyldithiophosphate at 750°C for 20 hours while spraying the solution from the upstream side. ２ Phosphorus poisoning treatment was carried out under the following gas flow conditions in an O atmosphere.２ 11.9L / min, CO ２ 3.0 L / min, O ２ The flow rate was 0.1 L / min (total 15 L / min). ２ O was vaporized from a water tank and mixed as water vapor into the model gas or air. The saturated water vapor pressure was adjusted by the temperature to obtain the water vapor amount in the above volume percentage.

[0140] (3) Evaluation of exhaust gas purification performance (T50) The catalyst treated in (1) or (2) above was placed in the exhaust passage, and the catalyst was subjected to a test using a model exhaust gas (CO 0.50 vol%, H ２ 0.17vol%, O ２ 0.50vol%, NO 400volppm, C ３ H ６ 1180volppm, CO ２ 14 vol%, H ２ O 10 vol%, N ２ The temperature was raised to 500°C at a rate of 20°C / min while the remaining mixture was passed through the reactor at a space velocity of 100,000 / h, and the purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) were continuously measured to determine the temperature (light-off temperature T50) (°C) at which the purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) reached 50%. The light-off temperature T50 was determined during the temperature rise. The results are shown in Table 1.

[0141] (4) Evaluation of transient NOx emissions The catalyst treated in (2) above was placed in the exhaust passage, and the following lean conditions were used: ２ The atmosphere was switched in the following order of rich conditions, and the mixture was passed through at a space velocity of 100,000 / h and 400°C for 5 minutes each. Lean conditions: NO 1,000 vol ppm, O ２ 5000volppm, N ２ Remainder Rich condition: CO 5000 vol ppm, N ２ remainder

[0142] NO and NO emitted when switching to rich conditions ２ The total amount was evaluated as transient NOx emissions (μmol).

[0143]

[0144] The exhaust gas purifying catalysts of Examples 1 to 4 each comprise a substrate, a lower layer containing Pd, and an upper layer containing Rh and a Ce—Zr—Al-based composite oxide, and satisfy the following condition A: (A) the lower layer contains Sr, and the metal-equivalent content of Sr in the lower layer is 0.1 mass % or more based on the mass of the lower layer.

[0145] 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: (A) the lower layer contains Sr, and the metal-equivalent content of Sr in the lower layer is 0.1 mass% or more based on the mass of the lower layer. However, in the exhaust gas purifying catalyst of Comparative Example 1, the upper layer contains a Ce-Zr-based composite oxide but does not contain a Ce-Zr-Al-based composite oxide.

[0146] The exhaust gas purifying catalyst of Comparative Example 2 includes a substrate, a lower layer containing Pd, and an upper layer containing Rh and a Ce-Zr-Al-based composite oxide. However, the exhaust gas purifying catalyst of Comparative Example 2 does not satisfy the following condition A: (A) the lower layer contains Sr, and the content of Sr in the lower layer in terms of metal, based on the mass of the lower layer, is 0.1 mass% or more. Furthermore, the exhaust gas purifying catalyst of Comparative Example 2 does not satisfy the following condition B: (B) the upper layer contains Sr, and the content of Sr in the upper layer in terms of metal, based on the mass of the upper layer, is 0.1 mass% or more.

[0147] 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 a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after starting an internal combustion engine) compared to the exhaust gas purification catalyst of Comparative Example 1.

[0148] As shown in Table 1, the exhaust gas purification catalysts of Examples 1 to 4 can more effectively prevent a decrease in the exhaust gas purification performance after exposure to phosphorus (particularly, among the exhaust gas purification performance after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine) compared to the exhaust gas purification catalyst of Comparative Example 2.

[0149] In this way, the exhaust gas purification catalysts of Examples 1 to 4 can prevent a decrease in exhaust gas purification performance after exposure to a high-temperature environment (particularly, among the exhaust gas purification performance after exposure to a high-temperature environment, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine) and can prevent a decrease in exhaust gas purification performance after exposure to phosphorus (particularly, among the exhaust gas purification performance after exposure to phosphorus, the exhaust gas purification performance in a low-temperature environment immediately after the start of an internal combustion engine).

[0150] P: exhaust pipe of internal combustion engine 1: catalyst for purifying exhaust gas 10: substrate 11: cylindrical portion 12: partition portion 13: cell 20: first catalyst layer 30: second catalyst layer

Claims

1. A catalyst for purifying exhaust gas comprising a substrate, a first catalyst layer provided on the substrate, and a second catalyst layer provided on an upper side of the first catalyst layer, The first catalyst layer contains Pd, the second catalytic layer contains Rh and a composite oxide containing Ce, Zr and Al, the second catalytic layer is provided so as to cover the entire main surface of the first catalytic layer, The exhaust gas purifying catalyst satisfies the following conditions A and B: (A) the first catalytic layer contains Sr, and the content of Sr in the first catalytic layer in terms of metal is 0.1 mass% or more based on the mass of the first catalytic layer; (B) the second catalytic layer contains Sr, and the metal-equivalent content of Sr in the second catalytic layer is 0.1 mass% or more based on the mass of the second catalytic layer. The exhaust gas purifying catalyst as described above satisfies at least one of the above.

2. In the condition A, the content of Sr in the first catalytic layer in terms of metal is 0.1 mass% or more and 7.0 mass% or less based on the mass of the first catalytic layer, 2. The exhaust gas purifying catalyst according to claim 1, wherein under condition B, a content of Sr in the second catalytic layer in terms of metal is 0.1 mass % or more and 7.0 mass % or less based on the mass of the second catalytic layer.

3. In the condition A, the content of Sr in the first catalytic layer in terms of metal is 0.1 mass% or more and 4.0 mass% or less based on the mass of the first catalytic layer, 3. The exhaust gas purifying catalyst according to claim 2, wherein under condition B, a content of Sr in the second catalyst layer in terms of metal is 0.1 mass % or more and 4.0 mass % or less based on the mass of the second catalyst layer.

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

5. In the case where the exhaust gas purifying catalyst satisfies the condition A, the exhaust gas purifying catalyst also satisfies the following condition C: (C) a ratio of a metal converted content of Sr in the first catalytic layer based on the mass of the first catalytic layer to a content of the composite oxide in the second catalytic layer based on the mass of the second catalytic layer is 0.0010 or more and 0.0800 or less. Fulfilling In the case where the exhaust gas purifying catalyst satisfies the condition B, the exhaust gas purifying catalyst also satisfies the following condition D: (D) a ratio of a metal converted content of Sr in the second catalytic layer based on the mass of the second catalytic layer to a content of the composite oxide in the second catalytic layer based on the mass of the second catalytic layer is 0.0010 or more and 0.0800 or less. The exhaust gas purifying catalyst according to claim 1 or 2, which satisfies the above.

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

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

8. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the exhaust gas purifying catalyst satisfies at least condition A out of conditions A and B.