Catalyst for purifying exhaust gas

Abstract

To provide a catalyst for purifying an exhaust gas, which can realize prevention of degradation of exhaust gas purification performance after having been exposed to high temperature environment, and prevention of degradation of exhaust gas purification performance after having been exposed to phosphorus.SOLUTION: A catalyst 1 for purifying the exhaust gas includes a substrate 10, a first catalyst layer 20 provided on the substrate, and a second catalyst layer 30 provided on an upper side of the first catalyst layer, wherein the first catalyst layer contains Pd, the second catalyst layer contains Rh and a complex oxide containing Ce, Zr and Al, and the catalyst for purifying the exhaust gas satisfies at least one of the following conditions A and B: (A) the first catalyst layer contains Sr, and a percentage content of Sr in terms of metal in the first catalyst layer is 0.1% by mass or more based on the mass of the first catalyst layer; and (B) the second catalyst layer contains Sr, and a percentage content of Sr in terms of metal in the second catalyst layer is 0.1% by mass or more based on the mass of the second catalyst layer.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] Exhaust gas emitted from internal combustion engines of automobiles, motorcycles, etc. contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Catalysts, for example, catalysts containing precious metal elements such as Pt, Pd, and Rh, are used to purify and detoxify these harmful components. Pt and Pd are mainly involved in the oxidation purification of HC and CO, while Rh is mainly involved in the reduction purification of NOx.

[0003] Pd is easily poisoned by phosphorus (e.g., phosphorus in engine oil). As an exhaust gas purification catalyst capable of suppressing phosphorus poisoning of Pd, there is known 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, in which the first catalyst layer contains Pd and the second catalyst layer contains Rh (e.g., Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-136319 A [Patent Document 2] JP 2016-185531 A Summary of the Invention [Problem to be solved by the invention]

[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 of preferably 500°C or lower, more preferably 400°C or lower, and even more preferably 300°C or lower, and "high temperature" means a temperature of preferably 700°C or higher, more preferably 800°C or higher, and even more preferably 900°C or higher.

[0006] The present invention has an object to provide an exhaust gas purification catalyst 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, the first catalytic layer containing Pd and the second catalytic layer containing Rh, 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 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). [Means for solving the problem]

[0007] The present invention provides the following inventions. [1] A catalyst for purifying exhaust gas comprising a substrate, a first catalytic layer provided on the substrate, and a second catalytic layer provided on an upper side of the first catalytic layer, the first catalyst layer contains Pd, the second catalytic layer contains Rh and a composite oxide containing Ce, Zr and Al, The exhaust gas purification 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] Under 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; In the condition B, the content of Sr in the second catalyst layer in terms of metal is 0.1 mass % or more and 7.0 mass % or less, based on the mass of the second catalyst layer. [3] Under 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; In the condition B, the 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 exhaust gas purifying catalyst according to any one of [1] to [3], wherein the composite oxide contains La, and the La content in the composite oxide, calculated as La2O3, is 1.0 mass% or more based on the mass of the composite oxide. [5] When the exhaust gas purifying catalyst satisfies the 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 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 a 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 catalyst for purifying exhaust gas according to any one of [1] to [4], which satisfies the above. [6] The exhaust gas purifying catalyst according to any one of [1] to [5], wherein a 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. [7] The exhaust gas purifying catalyst according to any one of [1] to [6], wherein the content of Al in the composite oxide, calculated as Al2O3, 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], which satisfies at least condition A among conditions A and B. Effect of the Invention

[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 on the upper side of the first catalytic layer, the first catalytic layer containing Pd and the second catalytic layer containing Rh, 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). [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a partial end view showing a state in which an exhaust gas purifying catalyst according to one embodiment of the present invention is disposed in an exhaust path of an internal combustion engine. [Diagram 2] FIG. 2 is an end view taken along line AA in FIG. [Diagram 3] FIG. 3 is an enlarged view of the region indicated by the symbol R in FIG. [Figure 4] FIG. 4 is an end view taken along line BB in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

[0012] As shown in Fig. 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 discharged from the internal combustion engine flows through an 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 drawings, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side of the exhaust gas flow direction X may be referred to as the "exhaust gas 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, upstream and / or downstream of the catalyst 1, other exhaust gas purifying catalysts may be arranged.

[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] <Base material> The substrate 10 will now be described.

[0016] 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 the ceramic material 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 the metal material 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] 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 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 cells 13 extend in the exhaust gas flow direction X, and have 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 of the cell 13 on the exhaust gas inlet side and the end of the cell 13 on the exhaust gas outlet side 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 inflow 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 outflow side also has a similar shape in plan view.

[0023] The cell density per square inch of the substrate 10 is, for example, from 100 cells to 1000 cells. 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 catalyst layer> The first catalyst layer 20 will be described below.

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

[0027] The first catalytic layer 20 may be composed of a portion (hereinafter referred to as a "protruding portion") protruding from the cell 13 side surface of the partition section 12 toward the cell 13 side, or may be composed of a portion (hereinafter referred to as an "internal portion") present inside the partition 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 catalyst layer 20 extends along the exhaust gas flow direction X from an end of the partition section 12 on the exhaust gas inlet side to an end of the partition section 12 on the exhaust gas outlet side. The first catalyst layer 20 may extend along the exhaust gas flow direction X from an end of the partition section 12 on the exhaust gas inlet side so as not to reach an end of the partition section 12 on the exhaust gas outlet side, or may extend from an end of the partition section 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X so as not to reach an end of the partition section 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 (mass after calcination) of the first catalyst layer 20 per unit volume of the portion of the substrate on which the first catalyst layer 20 is formed 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 catalyst layer 20 per unit volume of the portion of the substrate 10 on which the first catalyst layer 20 is formed is calculated by 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 of substrate 10 L 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 masses of precious metal elements in metal equivalent and the masses of metal elements other than precious metal elements in oxide equivalent of all 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 masses of precious metal elements contained in the first catalytic layer 20 in metal equivalent and the masses of metal elements other than precious metal elements contained in the first catalytic layer 20 in oxide equivalent. The "metal elements" also include metalloid elements such as Si and B.

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

[0032] In this specification, oxides of rare earth elements other than Ce, Pr, and Tb are referred to as sesquioxides (M2O3, where M represents a rare earth element other than Ce, Pr, and Tb), oxides of Ce are referred to as CeO2, and oxides of Pr are referred to as Pr6O 11 , oxide of Tb is Tb4O7, oxide of Al is Al2O3, oxide of Zr is ZrO2, oxide of Si is SiO2, oxide of B is B2O3, oxide of Cr is Cr2O3, oxide of Mg is MgO, oxide of Ca is CaO, oxide of Sr is SrO, oxide of Ba is BaO, oxide of Fe is Fe3O4, oxide of Mn is Mn3O4, oxide of Ni is NiO, oxide of Ti is TiO2, oxide of Zn is ZnO, and oxide of Sn is SnO2.

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

[0034] From the catalyst 1, the substrate 10 extends in the axial direction and has a 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 necessary. 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 by a plane perpendicular to the axial direction of the substrate 10, and the first cut piece, the second cut piece, ..., the nth cut piece are obtained in order from the end of the sample on the exhaust gas 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 by a plane perpendicular to the axial direction of the substrate 10, and the first cut piece, the second cut piece, ..., the nth cut piece are obtained in order from the end of the sample on the exhaust gas outlet side. In either case, the length of the cut piece is 5 mm. The composition of the cut pieces is analyzed using an X-ray fluorescence analyzer (XRF) (e.g., an energy dispersive X-ray analyzer (EDX), a wavelength dispersive X-ray analyzer (WDX), etc.), an inductively coupled plasma atomic emission spectrometer (ICP-AES), a scanning electron microscope-energy dispersive X-ray analysis method (SEM-EDX), etc., and based on the composition of the cut pieces, it is confirmed whether or not the cut pieces contain a part of the first catalyst layer 20.

[0035] It is not necessary to perform a composition analysis on a cut piece that clearly contains a part of the first catalytic layer 20. For example, a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA), or the like can be used to observe the cut surface to confirm whether or not the cut piece contains a part 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 piece contains a part of the first catalyst layer 20, the length of the first catalyst layer 20 contained in the sample is calculated based on the following formula. Length of the first catalytic layer 20 included in the sample = 5 mm x (number of cut pieces including a part of the first catalytic layer 20)

[0037] For example, if the first through kth cut pieces include a portion of the first catalytic layer 20, but the (k+1)th through 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 catalyst layer 20 included in a sample is as follows. The k-th cut piece (a cut piece obtained from the exhaust gas outlet side of the sample among the cut pieces including a part of the first catalyst layer 20 when the first catalyst layer 20 extends from the end of the exhaust gas inlet side of the partition section 12 along the exhaust gas flow direction X, and a cut piece obtained from the exhaust gas inlet side of the sample among the cut pieces including a part of the first catalyst layer 20 when the first catalyst layer 20 extends from the end of the exhaust gas outlet side of the partition section 12 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 catalyst layer 20 present on the cut surface is observed using SEM, EPMA, or the like, to measure the length of the part of the first catalyst layer 20 in the k-th cut piece. Then, the length of the first catalyst layer 20 included in the sample is calculated based on the following formula. Length of the first catalyst layer 20 included in the sample=(5 mm×(k−1))+(length of the part of the first catalyst layer 20 included in the kth cut 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 of the lengths was taken as the average length L of the first catalyst layer 20. 20 Let us assume that.

[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 capable of functioning as a catalytically active component, for example, in the form of a catalytically active component containing Pd, such as metallic Pd, an alloy containing Pd, or a compound containing Pd (for example, 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, based on the mass of 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.

[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 capable of functioning as a catalytically active component, for example, in the form of a catalytically active component containing a precious metal element other than Pd, such as a metal, an alloy containing a precious metal element, or a compound containing a precious metal element (for example, an oxide of a precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Pd is preferably in a particulate form.

[0043] When the first catalytic layer 20 contains Pd and a precious metal element other than Pd, there is a risk that the Pd and the precious metal element other than Pd form an alloy, and the active sites of Pd involved in the exhaust gas purification performance are reduced. Therefore, it is preferable that the first catalytic layer 20 does not substantially contain a precious metal element other than Pd.

[0044] "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, more preferably 0.010 mass% or less, based on the mass of the first catalytic layer 20. The lower limit is zero. "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 is 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 is calculated from the formula: (metal-equivalent mass of Pd in ​​the first catalytic layer 20) / (mass of the first catalytic layer 20)×100.

[0046] When 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 material 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 by a common method such as scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX), etc. Specifically, it is as follows.

[0048] The sample obtained from the first catalytic layer 20 is subjected to elemental analysis 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 metal element. The content (mass%) of each metal element is determined for each of 10 SEM visual fields, and the average value of the content (mass%) of each metal element in the 10 visual fields 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 supported on one or more types of carriers.

[0050] "At least a part of the catalytically active component is supported on the carrier" means that at least a part of the catalytically active component is physically or chemically adsorbed or held on the outer surface and / or the inner surface of the pores of the carrier. The fact that at least a part of the catalytically active component is supported on the carrier can be confirmed, for example, by using SEM-EDX. Specifically, when at least a part of the catalytically active component and the carrier are present in the same region in element mapping obtained by analyzing the cross section of the first catalyst layer 20 with SEM-EDX, it can be determined that at least a part of the catalytically active component is supported on the carrier.

[0051] The support can be selected from inorganic oxides, for example. 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 (for example, 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, zirconia (ZrO2), silica (SiO2), titania (TiO2), zeolite (aluminosilicate), and oxides based on MgO, ZnO, SnO2, etc.

[0053] The Al-based oxide refers to an oxide containing Al, in which Al is the element with the largest content by mass among the elements constituting the oxide other than O. However, those that fall under the category of Ce-Zr based composite oxides are not considered to be Al-based oxides.

[0054] Ce-based oxides refer to oxides that contain Ce and in which Ce is the element with the highest content by mass among the elements constituting the oxide other than O. However, Ce-Zr-based composite oxides are not considered to be Ce-based oxides.

[0055] A Ce-Zr-based composite oxide refers to a composite oxide containing Ce and Zr, in which the CeO2-converted content of Ce in the composite oxide is 5.0 mass% or more and 95.0 mass% or less, based on the mass of the composite oxide, the ZrO2-converted content of Zr 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 the Al2O3-converted content of Al in the composite oxide 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 catalyst layer> The second catalyst 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 second catalytic layer 30 is provided on the upper side of the first catalytic layer 20" means that a part or the whole of the second catalytic layer 30 is present on one of the two main surfaces of the first catalytic layer 20 opposite to the main surface on the partition wall portion 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 may be provided 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 a part of the main surface of the first catalytic layer 20 or may be provided 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 catalyst layer 30 extends along the exhaust gas flow direction X from the end of the partition section 12 on the exhaust gas inlet side to the end of the partition section 12 on the exhaust gas outlet side. The second catalyst layer 30 may extend along the exhaust gas flow direction X from the end of the partition section 12 on the exhaust gas inlet side so as not to reach the end of the partition section 12 on the exhaust gas outlet side, or may extend from the end of the partition section 12 on the exhaust gas outlet side along the direction opposite to the exhaust gas flow direction X so as not to reach the end of the partition section 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 (mass after calcination) of the second catalyst layer 30 per unit volume of the portion of the substrate 10 on which the second catalyst layer 30 is formed 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 on which the second catalyst layer 30 is formed is calculated by 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 of substrate 10 L 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 replaced with 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 In this case, the "first catalyst layer 20" is the "second catalyst layer 30" and the "average length L 20 " is the 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 capable of functioning as a catalytically active component, for example, in the form of a catalytically active component containing Rh, such as metallic Rh, an alloy containing Rh, or a compound containing Rh (for example, 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 capable of functioning as a catalytically active component, for example, in the form of a catalytically active component containing a precious metal element other than Rh, such as a metal, an alloy containing a precious metal element, or a compound containing a precious metal element (for example, an oxide of a precious metal element). From the viewpoint of improving exhaust gas purification performance, the catalytically active component containing a precious metal element other than Rh is preferably in a particulate form.

[0068] When 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 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, more preferably 0.010 mass% or less, based on the mass of the second catalytic layer 30. The lower limit is zero. "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, and 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 is 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 is 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 converted content of each metal element in the second catalytic layer 30 can be determined in the same manner as the metal converted 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, the second catalytic layer 30 is observed by SEM-EDX, and when Ce, Zr, and Al are present at 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 that the air / fuel ratio (air-fuel ratio) supplied to an internal combustion engine is controlled to be close to the theoretical air-fuel ratio (stoichiometric). However, the actual air-fuel ratio fluctuates from the stoichiometric ratio to the rich (excess fuel atmosphere) or lean (lean fuel atmosphere) side, so the air-fuel ratio of the exhaust gas also fluctuates to the rich or lean side. Since Ce-Zr-Al composite oxides have oxygen storage capacity, the use of Ce-Zr-Al composite oxides can alleviate the fluctuation of the oxygen concentration in the exhaust gas and expand the operating window of the catalyst. In addition, since Ce-Zr-Al composite oxides have heat resistance, they can prevent the burial of Rh and sintering of Rh that are caused by the aggregation of Ce-Zr-Al composite oxides and the disappearance of the pores of Ce-Zr-Al composite oxides (i.e., the decrease in the specific surface area) in high-temperature environments. Therefore, since the second catalytic layer 30 contains a Ce-Zr-Al based composite oxide, 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 the start of an internal combustion engine).

[0075] A Ce-Zr-Al based composite oxide refers to a composite oxide containing Ce, Zr, and Al, in which the CeO2 converted content of Ce in the composite oxide is from 1.0 mass% to 94.0 mass% based on the mass of the composite oxide, the ZrO2 converted content of Zr in the composite oxide is from 1.0 mass% to 94.0 mass% based on the mass of the composite oxide, and the Al2O3 converted content of Al in the composite oxide is from 5.0 mass% to 98.0 mass% based on the mass of the composite oxide.

[0076] The Ce-Zr-Al based composite oxide is, for example, particulate. 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] In the second catalytic layer 30, at least a part of the catalytically active component is preferably supported on a Ce-Zr-Al based composite oxide. 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 the oxygen storage capacity and heat resistance, the CeO2-equivalent content of Ce in the Ce-Zr-Al-based composite oxide 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 the oxygen storage capacity and heat resistance, the ZrO2-equivalent content of Zr in the Ce-Zr-Al-based composite oxide 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 the oxygen storage capacity and heat resistance, the Al content in the Ce-Zr-Al-based composite oxide, calculated as Al2O3, based on the mass of the Ce-Zr-Al-based composite oxide, 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.

[0081] From the viewpoint of improving the oxygen storage capacity and heat resistance, the total of the Ce content in the Ce-Zr-Al based composite oxide in terms of CeO2, the Zr content in terms of ZrO2, and the Al content in terms of Al2O3 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 the oxygen storage capacity and heat resistance, the ratio of the sum of the CeO2-converted content of Ce and the ZrO2-converted content of Zr in the Ce-Zr-Al-based composite oxide to the Al2O3-converted content of Al in the Ce-Zr-Al-based composite oxide 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 based 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, Lu, etc.

[0084] 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 in consideration of the CeO2-equivalent content of Ce, the ZrO2-equivalent content of Zr, and the Al2O3-equivalent content of Al in the Ce-Zr-Al based composite oxide. From the viewpoint of improving the 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. The "oxide-equivalent content of metal elements other than Ce, Zr, and Al in a Ce-Zr-Al-based composite oxide" means, when the Ce-Zr-Al-based composite oxide contains one metal element other than Ce, Zr, and Al, the oxide-equivalent content of that 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 that two or more metal elements is 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 composite oxide preferably contains La. From the viewpoint of improving heat resistance, the La2O3-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. The upper limit of the La2O3-converted content of La in the Ce-Zr-Al composite oxide can be appropriately adjusted in consideration of 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 La2O3-converted content of La in the Ce-Zr-Al 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.

[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 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 rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide can be appropriately adjusted in consideration of the CeO2-equivalent content of Ce, the ZrO2-equivalent content of Zr, and the Al2O3-equivalent content of Al in the Ce-Zr-Al composite oxide. The oxide-equivalent content of rare earth elements other than Ce and La in the Ce-Zr-Al 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 lower limits described above. "The oxide-equivalent content of rare earth elements other than Ce and La in the Ce-Zr-Al composite oxide" means the oxide-equivalent content of the one rare earth element when the Ce-Zr-Al composite oxide contains one rare earth element other than Ce and La, and means the sum of the oxide-equivalent contents of the two or more rare earth elements when the Ce-Zr-Al 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 calculated as an oxide can be determined from the composition of the Ce-Zr-Al based composite oxide.

[0088] 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 by analyzing a sample obtained from the second catalyst layer 30 by energy dispersive X-ray spectroscopy (EDX) and performing elemental mapping and EDX elemental analysis of the specified particles. Specifically, the Ce-Zr-Al 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 based composite oxide, Ce may form a solid solution phase (e.g., 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 may form a single phase which is a crystalline phase or an amorphous phase (e.g., a CeO2 single phase), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of Ce forms a solid solution phase.

[0090] In the Ce-Zr-Al based composite oxide, Zr may form a solid solution phase (e.g., 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 may form a single phase which is a crystalline phase or an amorphous phase (e.g., a ZrO2 single phase), or may form both a solid solution phase and a single phase, but it is preferable that at least a part of Zr forms a solid solution phase.

[0091] In the Ce-Zr-Al based composite oxide, Al may form a solid solution phase (e.g., a solid solution phase of CeO2 and Al2O3, a solid solution phase of ZrO2 and Al2O3, a solid solution phase of CeO2, ZrO2 and Al2O3, etc.) or may form a single phase (single Al2O3 phase) which is a crystalline phase or an amorphous 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 may form a solid solution phase (e.g., a solid solution phase of CeO2 and an oxide of a metal element other than Ce, Zr, and Al, a solid solution phase of ZrO2 and an oxide of a metal element other than Ce, Zr, and Al, a solid solution phase of CeO2, ZrO2, and an oxide of a metal element other than Ce, Zr, and Al, etc.) or may form a single phase which is a crystalline phase or an amorphous phase (a single phase of an oxide of a metal element other than Ce, Zr, and Al). However, 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 the deterioration of 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 in consideration of the content 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 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 material 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, the method is as follows.

[0096] (1) For the sample obtained from the second catalyst layer 30, elemental analysis is performed 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) For the sample obtained from the second catalytic layer 30, elemental mapping is performed using a standard method such as SEM-EDX to identify the types of particles contained in the sample (e.g., Ce-Zr-Al based composite oxide particles and possibly other particles). (3) For each type of particle, randomly selected multiple particles (e.g., 50 particles) are subjected to elemental analysis 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 calculated for each type of particle. (4) An equation expressing 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 is created and solved to calculate the content (mass%) of each type of particle in the sample, and this is 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 based 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 part of the catalytically active component is supported on the other carriers. The explanation of the carriers (including the explanation of the inorganic oxides) and the meaning and confirmation method of the support are the same as those in <First catalytic layer 20>.

[0098] <Conditions A and B> Catalyst 1 meets the following conditions A and B: (A) the first catalytic layer 20 contains Sr, and the content of Sr in the first catalytic layer 20 in terms of metal 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. At least one of the following is satisfied.

[0099] Since Pd in ​​the first catalyst layer 20 is easily poisoned by phosphorus (e.g., phosphorus in engine oil), the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 (particularly, the exhaust gas purification performance of Pd in ​​the first catalyst 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 catalyst layer 20 and / or the second catalyst layer 30 captures phosphorus, so that 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 (particularly, the exhaust gas purification performance of Pd in ​​the first catalyst 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 catalyst layer 20 after exposure to phosphorus). Incidentally, the present inventors have examined various alkaline earth metal elements for suppressing phosphorus poisoning, and have found that Mg and Ca are more likely to combine with Al2O3 that may be present in the first catalytic layer 20 or the second catalytic layer 30 than Sr, thereby more likely to reduce heat resistance, that Ba is more likely to combine with Al2O3 that may be present in the first catalytic layer 20 or the second catalytic layer 30 than Sr, thereby more likely to reduce heat resistance and more likely to cause a decrease in the exhaust gas purification performance of Rh, and that Sr is less likely to combine with Al2O3 that may be present in the first catalytic layer 20 or the second catalytic layer 30 than Mg, Ca, and Ba, thereby less likely to reduce heat resistance and less likely to cause a decrease in the exhaust gas purification performance of Rh and Pd, and based on these findings, Sr was selected.

[0100] The Ce-Zr-Al composite oxide has a large 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 a low physical blocking property against the intrusion of phosphorus. Therefore, as the content of the Ce-Zr-Al composite oxide in the second catalytic layer 30 increases, phosphorus passes through the second catalytic layer 30 and reaches the first catalytic layer 20 more easily, and Pd in ​​the first catalytic layer 20 is more likely to be poisoned by phosphorus. Therefore, when the content of the Ce-Zr-Al composite oxide in the second catalytic layer 30 is 80.0 mass% or more, the effect of the catalyst 1 satisfying at least one of the conditions A and B is remarkable.

[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 that the air / fuel ratio (air-fuel ratio) supplied to the internal combustion engine is controlled to be close to the theoretical air-fuel ratio (stoichiometric). However, since the actual air-fuel ratio fluctuates from the stoichiometric ratio to the rich side (excess fuel atmosphere) or lean side (lean fuel atmosphere) side, the air-fuel ratio of the exhaust gas also fluctuates to the rich side or lean side. In addition to the property of collecting 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 is released from Sr at once, and the released NOx may not be purified by the catalyst 1. In addition, the 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 in which the catalyst 1 can purify it.

[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 the start 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 the start 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" includes an embodiment in which the second catalyst layer 30 does not contain Sr, and an embodiment 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 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 mass % or less, more preferably 0.03 mass % or less, and even more preferably 0.01 mass % or less, based on the mass of the second catalyst 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 catalyst layer 20 does not contain Sr, and an embodiment in which first catalyst layer 20 contains Sr and the metal-equivalent content of Sr in first catalyst layer 20 is less than 0.1 mass % based on the mass of first catalyst layer 20. In the latter embodiment, the metal-equivalent content of Sr in first catalyst 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 catalyst 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 does not satisfy condition B, and (c) embodiments in which catalyst 1 satisfies condition B but does not satisfy condition A.

[0111] When the catalyst 1 satisfies condition B, the phosphorus contained in the exhaust gas can be captured by Sr in the second catalyst layer 30 before it reaches the first catalyst layer 20, and therefore, compared to when the first catalyst layer 20 satisfies condition A, there is an advantage in that the poisoning of Pd in ​​the first catalyst layer 20 by phosphorus can be more effectively prevented. On the other hand, when the catalyst 1 satisfies condition B, there is a disadvantage in that Sr in the second catalyst layer 30 reduces the exhaust gas purification performance of Rh in the second catalyst layer 30. In contrast, when the catalyst 1 satisfies condition A, compared to when the catalyst 1 satisfies condition B, the effect of preventing the poisoning of Pd in ​​the first catalyst layer 20 by phosphorus is inferior, but there is no disadvantage in that Sr in the first catalyst layer 20 reduces the exhaust gas purification performance of Pd in ​​the first catalyst layer 20. For this reason, 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 converted content of each metal element in the first catalytic layer 20.

[0113] When the catalyst 1 satisfies condition A, the first catalyst layer 20 contains one or more Sr sources, and when the catalyst 1 satisfies condition B, the 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 oxide, carbonate, and sulfate, for example. From the viewpoint of more effectively exerting the effect of Sr, it is preferable that Sr exists in one or two forms selected from SrO and SrCO3.

[0115] If catalyst 1 satisfies condition A, then catalyst 1 must also satisfy 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. It is preferable that the condition C is satisfied. The phenomenon in which Pd in ​​the first catalytic layer 20 becomes susceptible to phosphorus poisoning due to the large 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 first catalytic layer 20 that corresponds to the amount of Ce-Zr-Al-based composite oxide in the second catalytic layer 30, that is, by catalyst 1 satisfying condition C, it is possible to more effectively prevent phosphorus poisoning of 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 converted 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] If catalyst 1 satisfies condition B, catalyst 1 also satisfies 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. It is preferable that the condition D is satisfied. The phenomenon in which Pd in ​​the first catalytic layer 20 becomes susceptible to phosphorus poisoning due to the large 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 catalyst 1 satisfying condition D, it is possible to more effectively prevent phosphorus poisoning of 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 converted 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 on the upper side of the first catalytic layer 20. Therefore, the exhaust gas flowing in from the end (opening) of the exhaust gas inlet side of the cell 13 comes into contact with the second catalytic layer 30, and then comes into contact with the first catalytic layer 20. The exhaust gas that comes into contact with the second catalytic layer 30 is purified by the catalytic 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 catalytic active component (e.g., Pd) in the first catalytic layer 20. The second catalytic layer 30 contains a Ce-Zr-Al composite oxide, which 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 the catalyst 1 satisfying at least one of the 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, among the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalyst 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 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 catalyst layer 20 after exposure to phosphorus (e.g., phosphorus in engine oil) (particularly, among the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 after exposure to phosphorus, the exhaust gas purification performance of Pd in ​​the first catalyst layer 20 in a low-temperature environment immediately after the start of an internal combustion engine).

[0120] <Catalyst Production> 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 source of Rh (e.g., an Rh salt), a Ce-Zr-Al based composite oxide, optionally a source of Sr (e.g., an Sr salt), and optionally other components (e.g., an inorganic oxide other than the Ce-Zr-Al based 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 the Pd supply source include Pd salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of the Rh supply source include Rh salts, such as nitrates, ammine complex salts, acetates, and chlorides. Examples of the Sr supply source include Sr salts, such as nitrates, acetates, carbonates, and chlorides. Examples of the binder include alumina sol, zirconia sol, titania sol, silica sol, and ceria sol. Examples of the solvent 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. EXAMPLES

[0125] Example 1 (1) Preparation of slurry for forming lower layer An OSC material (Ce-Zr composite oxide, hereinafter sometimes referred to as "CZ") having the following composition was prepared. CeO2 content: 40.0 mass%, ZrO2 content: 50.0 mass%, oxide content of rare earth elements other than Ce: 10.0 mass%

[0126] Palladium nitrate solution, Ce-Zr composite oxide, La2O3 modified alumina (La2O3 modified amount: 1 mass%), strontium nitrate, alumina sol and water were added to a mixing vessel, mixed and stirred to prepare a slurry for forming a lower layer. The amount of each component in the slurry for forming a lower layer was adjusted so that Pd was 4.0 mass% in metal equivalent, Ce-Zr composite oxide was 40.0 mass%, La2O3 modified alumina was 45.4 mass%, Sr was 0.5 mass% in metal equivalent (0.6 mass% in SrO equivalent), and the solid content of alumina sol was 10.0 mass%, based on the mass of the lower layer after firing (100 mass%). Strontium nitrate is converted to strontium oxide by firing.

[0127] (2) Formation of the lower layer The flow-through substrate is made of cells extending in the axial direction, separated by partitions with a thickness of 50 to 70 μm, and has a density of 600 cells / inch on the plane perpendicular to the axial direction. 2 A flow-through type substrate having a density of 1.0 g / L and a volume of 1.0 L was prepared.

[0128] A flow-through type substrate was immersed in the underlayer forming slurry, and the flow-through type substrate coated with the underlayer forming slurry was dried at 150° C. for 0.5 hours, and then fired at 500° C. for 1 hour to form a underlayer. The mass of the underlayer per unit volume of the portion of the flow-through type substrate on which the underlayer 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. Ce content in terms of CeO2: 10.0 mass%, Zr content in terms of ZrO2: 35.0 mass%, Al content in terms of Al2O3: 45.0 mass%, La content in terms of La2O3: 5.0 mass%, Oxide content of rare earth elements other than Ce and La: 5.0 mass%

[0130] A rhodium nitrate solution, Ce-Zr-Al composite oxide, alumina sol, and water were added to a mixing vessel, and mixed and stirred to prepare a slurry for forming an upper layer. The amounts of each component 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, Ce-Zr-Al composite oxide was 89.0 mass%, and the solid content of alumina sol was 10.0 mass%.

[0131] (4) Formation of the upper layer The flow-through type substrate on which the lower layer was formed was immersed in the upper layer forming slurry, and the flow-through type substrate on which the upper layer forming slurry was applied 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 type substrate on which the upper layer was 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] In the lower layer forming slurry, the amount of strontium nitrate added was adjusted so that Sr was 1.0 mass% (Example 2), 2.5 mass% (Example 3), or 5.0 mass% (Example 4) in metal equivalent, based on the mass of the lower layer after firing (100 mass%), and the exhaust gas purification catalysts of Examples 2 to 4 were produced in the same manner as Example 1, except that the amount of strontium nitrate added was increased and the amount of La2O3-modified alumina added was reduced.

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

[0135] An OSC material (Ce-Zr based composite oxide, hereinafter sometimes referred to as "CZ") having the following composition was prepared. CeO2 content: 15.0 mass%, ZrO2 content: 75.0 mass%, oxide content of rare earth elements other than Ce: 10.0 mass%

[0136] A rhodium nitrate solution, Ce-Zr composite oxide, La2O3-modified alumina (La2O3 modification amount: 1 mass%), alumina sol and water were added to a mixing vessel, and mixed and stirred to prepare a slurry for forming an upper layer. The amount of each component in the slurry for forming an upper layer was adjusted so that, based on the mass of the upper layer after firing (100 mass%), Rh was 1.0 mass% in terms of metal, Ce-Zr composite oxide was 50.0 mass%, La2O3-modified alumina was 39.0 mass%, and the solid content of alumina sol was 10.0 mass%. Except for using this slurry for the upper layer, the upper layer was formed in the same manner as in Example 2.

[0137] Comparative Example 2 The exhaust gas purification catalyst of Comparative Example 2 was produced in the same manner as in Example 1, except that in the lower layer forming slurry, strontium nitrate was not added, and the amount of La2O3-modified alumina added was increased to compensate for the decrease in the amount of strontium nitrate added.

[0138] [Test Example] (1) Heat treatment of catalyst The exhaust gas purification catalyst cored to 30 mL was subjected to heat treatment in a quartz tube furnace at 1000°C for 20 hours in a 10% by volume H2O atmosphere in the FC mode described below. 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 rates: C3H 66 mL / min, O2 71 mL / min, N2 2923 mL / min (total 3 L / min). 10% by volume H2O was vaporized from a water tank and mixed into the model gas or air as water vapor. The saturated water vapor pressure was adjusted by temperature to obtain the above volume percent water vapor amount.

[0139] (2) Phosphorus poisoning treatment of catalyst In a quartz tubular furnace, while spraying a solution containing zinc dialkyldithiophosphate from the upstream side of the catalyst after the heat treatment in (1) above, phosphorus poisoning treatment was performed at 750°C for 20 hours in a 10% by volume HO atmosphere under the following gas flow conditions. The gas flow rates were: N2 11.9 L / min, CO2 3.0 L / min, O2 0.1 L / min (total 15 L / min). 10% by volume H2O was vaporized from a water tank and mixed into the model gas or air as water vapor. The saturated water vapor pressure was adjusted by temperature to obtain the above volume percent water vapor amount.

[0140] (3) Evaluation of exhaust gas purification performance (T50) The catalyst treated as described in (1) or (2) above was placed in the exhaust passage, and the exhaust model 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 passed through the exhaust passage at a space velocity of 100,000 / h. The temperature was raised to 500°C at a rate of 20°C / min, and the purification rates of total hydrocarbons (THC), nitrogen oxides (NOx), and carbon monoxide (CO) were continuously measured, and 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% was obtained. The light-off temperature T50 was obtained 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 atmosphere was switched in the following order: lean condition, N2, and rich condition, and the mixture was allowed to flow for 5 minutes each at a space velocity of 100,000 / h and 400°C. Lean conditions: NO 1000volppm, O25000volppm, N2 balance Rich condition: CO 5000volppm, N2 balance

[0142] The total amount of NO and NO2 emitted when switching to rich conditions was evaluated as transient NOx emissions (μmol).

[0143] [Table 1]

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

[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 is formed under the following condition A: (A) The lower layer contains Sr, and the metal converted 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 meet the following condition A: (A) The lower layer contains Sr, and the metal converted content of Sr in the lower layer is 0.1 mass% or more based on the mass of the lower layer. In addition, the exhaust gas purifying catalyst of Comparative Example 2 does not satisfy the following condition B: (B) The upper layer contains Sr, and the metal converted content of Sr in the upper layer is 0.1 mass% or more based on the mass of the upper layer. does not satisfy either.

[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 the start of 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). [Explanation of symbols]

[0150] P...Exhaust pipe of internal combustion engine 1. Exhaust gas purification catalyst 10...Base material 11... Cylindrical part 12...Bulkhead part 13 Cell 20...1st 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 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.

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