Exhaust gas purification catalyst, method for producing same, and method for purifying exhaust gas using said exhaust gas purification catalyst

Abstract

The present disclosure provides a means capable of improving exhaust gas purification performance (particularly HC purification performance) at low temperature in an exhaust gas purification catalyst. The aforementioned problem is solved by an exhaust gas purification catalyst comprising a lower catalyst layer and an upper catalyst layer having prescribed compositions, wherein the ratio of the palladium concentration on a gas-inflow-side region of the lower catalyst layer to the palladium concentration on a gas-outflow-side region of the lower catalyst layer is controlled so as to be within a prescribed range.

Description

Exhaust gas purification catalyst, its manufacturing method, and exhaust gas purification method using said exhaust gas purification catalyst

[0001] The present invention relates to an exhaust gas purifying catalyst, a method for producing the same, and a method for purifying exhaust gas using the exhaust gas purifying catalyst.

[0002] Many technologies have been proposed for treating exhaust gases emitted from internal combustion engines. For example, in treating exhaust gases from gasoline engines, carbon monoxide (CO) and hydrocarbons (HC) are converted into carbon dioxide (CO). ２ ) and water (H ２ Oxidation of nitrogen oxides (NOx) to nitrogen (N ２ Three-way catalysts have been proposed that simultaneously reduce CO₂ to CO₂ and CO₂ to CO₂. Three-way catalysts generally have a structure in which a catalyst layer containing a noble metal and a refractory inorganic oxide is supported on a refractory three-dimensional structure (e.g., a honeycomb carrier).

[0003] At low temperatures, such as during engine start-up, three-way catalysts are not very active and are unable to sufficiently purify exhaust gases, resulting in the emission of gases containing large amounts of pollutants (especially HC). For this reason, development is underway to develop three-way catalysts that can provide sufficient purification performance at lower temperatures (i.e., within a shorter period of time after engine start-up).

[0004] For example, Japanese Patent Application Laid-Open No. 2019-69402 (corresponding to the specification of U.S. Patent Application Publication No. 2019 / 0105637) discloses an exhaust gas purification catalyst having a catalyst coating layer in which an upper layer and a lower layer are sequentially formed on the surface of a substrate; the upper layer contains Rh and Pd and a support; the upper layer includes a Pd outermost layer on the surface extending from the upstream end to a downstream direction of 20 mm or more, the Pd concentration of which is relatively higher than in other parts of the upper layer; the lower layer contains Pd and / or Pt and a support; and 60 mass % or more of the Pd contained in the Pd outermost layer is present from the surface of the Pd outermost layer to 50% of the thickness of the upper layer. According to this document, such a configuration makes it possible to provide an exhaust gas purification catalyst with excellent HC purification performance and warm-up performance.

[0005] However, according to the investigations of the present inventors, it has become clear that even with the techniques described in the above documents, sufficient exhaust gas purification performance cannot be obtained at low temperatures in some cases.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide a means for improving the exhaust gas purification performance (particularly, HC purification performance) of an exhaust gas purification catalyst at low temperatures (for example, 400°C or lower).

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in an exhaust gas purification catalyst comprising a lower catalytic layer and an upper catalytic layer having a predetermined composition, the above-mentioned problems can be solved by controlling the ratio of the palladium concentration in the gas inlet region of the lower catalytic layer to the palladium concentration in the gas outlet region of the lower catalytic layer to be within a predetermined range (so that the palladium concentration in the gas inlet region of the lower catalytic layer is higher than the palladium concentration in the gas outlet region of the lower catalytic layer), thereby completing the present invention.

[0008] That is, an exhaust gas purification catalyst according to one embodiment of the present invention comprises a refractory three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face; a lower catalyst layer formed in contact with the partition walls and containing palladium, alumina, and a cerium-zirconium composite oxide; and an upper catalyst layer formed on at least a portion of the lower catalyst layer and constituting the outermost layer and containing rhodium, alumina, and a cerium-zirconium composite oxide. The lower catalyst layer in the catalyst is located on the gas inlet side and has a palladium concentration of C Ｌ１ a gas inlet side region L1 having a palladium concentration of C [g / g]; Ｌ２ a gas outlet side region L2 having a palladium concentration C [g / g]; a boundary X between the gas inlet side region L1 and the gas outlet side region L2 is located within a range of 8% to 80% from the gas inlet side end face with respect to the length of the partition wall; Ｌ２ The palladium concentration C in the gas inlet side region L1 Ｌ１The ratio (C Ｌ１ / C Ｌ２ ) is between 30 and 230.

[0009] Fig. 1 is a front cross-sectional view schematically showing a part of an exhaust gas purifying catalyst according to one embodiment of the present invention. Fig. 2 is a front cross-sectional view schematically showing a part of an exhaust gas purifying catalyst according to another embodiment of the present invention. Fig. 3 is a front cross-sectional view schematically showing a part of an exhaust gas purifying catalyst according to yet another embodiment of the present invention.

[0010] Hereinafter, embodiments of the present invention will be described, but the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. In this specification, the numerical range "A to B" means "greater than or equal to A and less than or equal to B." Furthermore, "A and / or B" means "either A or B" or "both A and B."

[0011] <Catalyst for Purifying Exhaust Gas> A catalyst for purifying exhaust gas (hereinafter also simply referred to as "catalyst") according to one embodiment of the present invention comprises a refractory three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face; a lower catalyst layer formed in contact with the partition walls and containing palladium, alumina, and a cerium-zirconium composite oxide; and an upper catalyst layer formed on at least a portion of the lower catalyst layer and constituting the outermost layer and containing rhodium, alumina, and a cerium-zirconium composite oxide. The lower catalyst layer in the catalyst is located on the gas inlet side and has a palladium concentration of C Ｌ１ a gas inlet side region L1 having a palladium concentration of C [g / g]; Ｌ２ a gas outlet side region L2 having a palladium concentration C [g / g]; a boundary X between the gas inlet side region L1 and the gas outlet side region L2 is located within a range of 8% to 80% from the gas inlet side end face with respect to the length of the partition wall; Ｌ２ The palladium concentration C in the gas inlet side region L1 Ｌ１ The ratio (C Ｌ１ / CＬ２ ) is 30 or more and 230 or less. According to the inventors' studies, it has been found that a catalyst having the above-described configuration improves exhaust gas purification performance (particularly HC purification performance) at low temperatures (e.g., 400°C or less). While the mechanism by which this effect is achieved is not completely clear, the following mechanism is presumed. That is, during cold start-up, the catalyst temperature is higher on the gas inlet side (catalyst inlet side) than on the gas outlet side (catalyst outlet side) (the catalyst reaches its activation temperature in a shorter time). By controlling the ratio of the palladium concentration in the gas inlet side region to the palladium concentration in the gas outlet side region in the lower catalyst layer in contact with the refractory three-dimensional structure to be within a predetermined range (so that the palladium concentration in the gas inlet side region is higher than the palladium concentration in the gas outlet side region), the amount of palladium that reaches the activation temperature is increased, and exhaust gas purification performance is thought to be improved. Note that the above mechanism is merely speculative, and whether it is correct or incorrect does not affect the technical scope of the present invention.

[0012] The overall structure of the exhaust gas purifying catalyst according to this embodiment will be described in more detail below with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the sake of explanation, and may differ from the actual proportions.

[0013] 1 is a front cross-sectional view schematically showing a part of an exhaust gas purifying catalyst according to one embodiment of the present invention. This embodiment corresponds to catalyst A in Example 1 described below. As shown in FIG. 1, the exhaust gas purifying catalyst 1 has a fire-resistant three-dimensional structure 10, a lower catalyst layer 20, and an upper catalyst layer 30.

[0014] The fire-resistant three-dimensional structure 10 has partition walls extending from a gas inlet end face 10 a along a gas outlet end face 10 b, and defining a plurality of gas flow paths that penetrate from the gas inlet end face 10 a to the gas outlet end face 10 b. In the exhaust gas purifying catalyst 1 shown in Figure 1, the length of the partition walls from the gas inlet end face 10 a to the gas outlet end face 10 b is 130 mm.

[0015] The lower catalyst layer 20 is formed so as to contact the partition walls of the fire-resistant three-dimensional structure 10. The lower catalyst layer 20 is composed of a gas inlet region L1 and a gas outlet region L2, and the gas inlet region L1 and the gas outlet region L2 contact each other at a boundary X. In the exhaust gas purifying catalyst 1 shown in FIG. 1, the boundary X is located at a position 30 mm from the gas inlet end face 10a (a position 23.1% of the length of the partition walls). The lower catalyst layer 20 (gas inlet region L1 and gas outlet region L2) is made of palladium (Pd) and alumina (Al ２ O ３ ) and cerium-zirconium composite oxide (CeO ２ -ZrO ２ ) and Lantana (La ２ O ３ ) and barium sulfate (BaSO ４ ) and the palladium concentration C in the gas inlet side region L1 Ｌ１ The palladium concentration in the gas inlet side region L1 is 0.08582 [g / g], and the palladium concentration is substantially uniform throughout the gas inlet side region L1. Ｌ２ is 0.0006197 [g / g], and C Ｌ２ C against Ｌ１ The ratio (C Ｌ１ / C Ｌ２ ) is 138.

[0016] The upper catalyst layer 30 is formed on the lower catalyst layer 20 and is located as the outermost layer. In the exhaust gas purifying catalyst 1 shown in FIG. 1, the upper catalyst layer 30 is formed adjacent to the gas outlet side region L2 of the lower catalyst layer 20 and is not located on the gas inlet side region L1. This configuration can further improve the exhaust gas purifying performance at low temperatures. That is, in the catalyst according to a preferred embodiment, the gas inlet side region L1 of the lower catalyst layer constitutes the outermost layer. The upper catalyst layer 30 is composed of rhodium (Rh) and alumina (Al ２ O ３ ) and cerium-zirconium composite oxide (CeO ２ -ZrO ２ ) and Lantana (La ２ O ３ ) and consists of.

[0017] Fig. 2 is a front cross-sectional view schematically showing a part of an exhaust gas purifying catalyst according to another embodiment of the present invention. This embodiment corresponds to catalyst B in Example 2, which will be described later. The exhaust gas purifying catalyst 2 shown in Fig. 2 differs from the exhaust gas purifying catalyst 1 shown in Fig. 1 in that the upper catalyst layer 30 is also formed on the gas inlet side region L1 (i.e., the upper catalyst layer 30 is formed from the gas inlet side end face 10a to the gas outlet side end face 10b).

[0018] FIG. 3 is a front cross-sectional view schematically illustrating a portion of an exhaust gas purification catalyst according to yet another embodiment of the present invention. This embodiment corresponds to catalyst C in Example 3, which will be described later. The exhaust gas purification catalyst 3 shown in FIG. 3 differs from the exhaust gas purification catalyst 2 shown in FIG. 2 in that a predetermined proportion of palladium is present in both the gas inlet side region L1 of the lower catalytic layer and the region of the upper catalytic layer that contacts the gas inlet side region L1 of the lower catalytic layer (gas inlet side region U1 of the upper catalytic layer). In the exhaust gas purification catalyst 3 shown in FIG. 3, palladium is absent in the region of the upper catalytic layer that contacts the gas outlet side region L2 of the lower catalytic layer (gas outlet side region U2 of the upper catalytic layer). The boundary Y between the gas inlet side region U1 and the gas outlet side region U2, like the boundary X, is located 30 mm from the gas inlet side end face 10a (at a position 23.1% of the length of the partition wall). The palladium concentration C in the gas inlet side region L1 Ｌ１ The palladium concentration in the gas inlet side region L1 is 0.06031 [g / g], and the palladium concentration is substantially uniform throughout the gas inlet side region L1. Ｕ１ is 0.05972 [g / g], and C Ｌ１ C against Ｕ１ The ratio (C Ｕ１ / C Ｌ１ ) is 0.990.

[0019] Next, each of the components included in the exhaust gas purifying catalyst of this embodiment will be described.

[0020] [Fire-resistant three-dimensional structure] The fire-resistant three-dimensional structure serves as a support for the catalyst layer. There are no particular limitations on the type or size of the fire-resistant three-dimensional structure, and any known structure in the field of exhaust gas purification catalysts can be appropriately adopted. As the fire-resistant three-dimensional structure, a honeycomb support is preferably used. Examples of honeycomb supports include monolith honeycomb support, metal honeycomb support, and plug honeycomb support for particulate filters. The material of the honeycomb support is preferably a heat-resistant metal such as cordierite, silicon carbide, silicon nitride, stainless steel, or an Fe—Cr—Al alloy.

[0021] The honeycomb carrier is manufactured by extrusion molding or by winding sheet-like elements. The shape of the gas passage openings (cell shape) may be any of hexagonal, rectangular, square, triangular, and corrugated. A cell density (number of cells / unit cross-sectional area) of 100 to 1200 cells / square inch (15.5 to 186 cells / square centimeter) is sufficient for use, and preferably 200 to 900 cells / square inch (31 to 139.5 cells / square centimeter).

[0022] The length of the refractory three-dimensional structure along the gas flow path (length of the partition wall) is preferably more than 15 mm and not more than 1000 mm, more preferably 30 mm or more and not more than 500 mm, and even more preferably 50 mm or more and not more than 300 mm.

[0023] [Lower Catalyst Layer] The lower catalyst layer is formed so as to be in contact with the partition walls of the three-dimensional structure (directly above the partition walls). The lower catalyst layer is composed of a gas inlet side region L1 (hereinafter also simply referred to as "region L1") located on the gas inlet side, and a gas outlet side region L2 (hereinafter also simply referred to as "region L2") located on the gas outlet side. Region L1 and region L2 are in contact with each other at a boundary X. The lower catalyst layer (regions L1 and L2) contains palladium, alumina, and a cerium-zirconium composite oxide.

[0024] Palladium functions as a catalyst for the oxidation reaction. Ｌ２Palladium concentration C in region L1 [g / g] Ｌ１ [g / g] ratio (C Ｌ１ / C Ｌ２ ) is 30 or more and 230 or less. If this ratio is less than 30, there is a risk that exhaust gas purification performance (particularly HC purification performance) at low temperatures will not be fully exhibited. If this ratio exceeds 230, there is a risk that purification performance commensurate with the amount of palladium will not be obtained because palladium will not be sufficiently dispersed in region L1. From the viewpoint of further improving exhaust gas purification performance at low temperatures, this ratio is preferably 40 or more and 220 or less, more preferably 90 or more and 210 or less, and even more preferably 100 or more and 200 or less. Note that, Ｌ１ is not particularly limited, but is preferably 0.01900 g / g or more and 0.15000 g / g or less, more preferably 0.02000 g / g or more and 0.14000 g / g or less, even more preferably 0.06000 g / g or more and 0.13000 g / g or less, and particularly preferably 0.06500 g / g or more and 0.12000 g / g or less. Ｌ２ Although there is no particular limitation, the molecular weight is preferably 0.0001000 g / g or more and 0.001000 g / g or less.

[0025] The palladium concentration in the gas inlet side region L1 is preferably substantially uniform throughout the region L1. This configuration allows palladium to be highly dispersed in the region L1, further improving exhaust gas purification performance at low temperatures. In this specification, "the palladium concentration is substantially uniform throughout the gas inlet side region L1" refers to the value of {(maximum concentration - minimum concentration) / minimum concentration} x 100 being within 10%, when the palladium concentration is measured at any 10 points in the region L1 and the maximum value of the 10 palladium concentrations is defined as the "maximum concentration" and the minimum value as the "minimum concentration." This value is preferably within 5%, more preferably within 3%, even more preferably within 1%, and particularly preferably 0%. The composition of the catalyst layer can be confirmed by inductively coupled plasma (ICP) emission spectroscopy or X-ray fluorescence (XRF) analysis.

[0026] The palladium content per 1 L of the fire-resistant three-dimensional structure in the lower catalyst layer (region L1 and region L2) is preferably 0.05 g / L or more and 5 g / L or less, more preferably 0.1 g / L or more and 4.5 g / L or less, and even more preferably 1 g / L or more and 4 g / L or less.

[0027] Examples of the palladium raw material (palladium source) include palladium nitrate, acetate, ammonium salt, amine salt, carbonate, tetraamminepalladium salt, etc. According to a preferred embodiment, the palladium source in region L1 is palladium nitrate and tetraamminepalladium salt, and the palladium source in region L2 is palladium nitrate.

[0028] Alumina functions as a carrier for the noble metal (palladium). Specific examples of alumina include γ-alumina, δ-alumina, and θ-alumina, with γ-alumina being preferred.

[0029] Furthermore, alumina may be contained in the form of a composite oxide of alumina and an oxide of another element. In this case, examples of the other element include phosphorus, zirconium, silicon, titanium, and lanthanum. Specific examples of the composite oxide include Al-P composite oxide, Al-Zr-P composite oxide, Al-Zr composite oxide, Al-Si composite oxide, Al-Si-Zr composite oxide, Al-Ti composite oxide, Al-Ti-Zr composite oxide, Al-Si-Ti-Zr composite oxide, and Al-La composite oxide. The aluminum content in the above composite oxide is expressed as Al ２ O ３ In terms of conversion, it is preferably more than 50% by mass and less than 100% by mass, more preferably 60% by mass or more and 99% by mass or less, even more preferably 70% by mass or more and 97% by mass or less, and particularly preferably 75% by mass or more and 95% by mass or less.

[0030] The alumina content per L of the refractory three-dimensional structure in the lower catalyst layer (regions L1 and L2) is preferably 10 g / L to 300 g / L, more preferably 20 g / L to 250 g / L, and even more preferably 50 g / L to 200 g / L. If the alumina content is within the above range, the precious metal (palladium) can be sufficiently dispersed and supported.

[0031] Examples of alumina raw materials include powder of alumina (gamma alumina, delta alumina, theta alumina, or a composite oxide of alumina and an oxide of another element), and aluminum chloride, aluminum nitrate (e.g., aluminum nitrate nonahydrate), aluminum sulfate, aluminum acetate, and aluminum hydroxide, which become alumina upon firing. Furthermore, when alumina is contained in the form of a composite oxide of alumina and an oxide of another element, examples of raw materials for the oxide of the other element include the following. Examples of raw materials for phosphorus oxide (phosphorus sources) include phosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. Among these, phosphoric acid is preferred. Examples of raw materials for zirconia (zirconium sources) include zirconium oxynitrate, zirconium oxychloride, zirconium nitrate, basic zirconium sulfate, zirconium carbonate, and zirconium hydroxide. Among these, zirconium nitrate is preferred. Examples of raw materials for silica (silicon sources) include silicon oxide, orthosilicate, metasilicic acid, and silica sol. Examples of raw materials for titania (titanium sources) include inorganic titanium compounds such as titanium tetrachloride and titanium sulfate, titanium oxalate, and tetraisopropyl titanate. Examples of raw materials for lanthana (lanthanum sources) include lanthanum oxide, lanthanum nitrate, lanthanum sulfate, lanthanum carbonate, and lanthanum acetate. Among these, the alumina raw material is preferably a powder of alumina (gamma alumina, delta alumina, theta alumina, or a composite oxide of alumina and an oxide of another element).

[0032] Cerium-zirconium composite oxide (CeO ２ -ZrO ２ ) mainly acts as an oxygen storage / release material. Here, the oxygen storage / release material has the function of stably progressing the oxidation / reduction reaction by storing oxygen in an oxidizing atmosphere (lean) and releasing oxygen in a reducing atmosphere (rich) in accordance with fluctuations in the air-fuel ratio (A / F), which changes depending on the driving conditions.

[0033] The content of cerium (Ce) contained in the cerium-zirconium composite oxide is ２), the content of zirconium (Zr) contained in the cerium-zirconium composite oxide is preferably 3 mass % or more and 65 mass % or less, more preferably 5 mass % or more and 50 mass % or less, and even more preferably 10 mass % or more and 47 mass % or less. ２ When the cerium (Ce) and zirconium (Zr) contents are within the above ranges, oxygen occlusion and desorption can proceed even at low temperatures.

[0034] The cerium-zirconium composite oxide may be contained in the form of a composite oxide of a cerium-zirconium composite oxide and an oxide of at least one metal selected from the group consisting of lanthanum (La), yttrium (Y), neodymium (Nd), and praseodymium (Pr). Specific examples of such composite oxides include cerium-zirconium-lanthanum composite oxide and cerium-zirconium-lanthanum-yttrium composite oxide.

[0035] The content of the cerium-zirconium composite oxide per 1 L of the refractory three-dimensional structure in the lower catalyst layer (regions L1 and L2) is preferably 5 g / L to 200 g / L, more preferably 10 g / L to 100 g / L, and even more preferably 20 g / L to 90 g / L. If the content of the cerium-zirconium composite oxide is within the above range, the exhaust gas purification performance (particularly HC purification performance) at low temperatures can be further improved.

[0036] The lower catalyst layer (regions L1 and L2) optionally contains at least one promoter selected from the group consisting of Group 1 elements, Group 2 elements, and rare earth elements. Specific examples of the promoter include potassium, magnesium, calcium, strontium, barium, and lanthanum. These elements are contained in the catalyst in the form of oxides, sulfates, or carbonates after firing. It is particularly preferred that the lower catalyst layer (regions L1 and L2) contain both a rare earth element and a Group 2 element, and lanthanum oxide (La ２ O３ ) and barium sulfate (BaSO ４ It is more preferable that the lower catalyst layer (regions L1 and L2) contains a promoter, which can improve the efficiency of exhaust gas purification performance (particularly, HC purification performance). ２ O ３ ) per 1 L of the fire-resistant three-dimensional structure when containing lanthanum oxide (La ２ O ３ The content of barium sulfate (BaSO ) in the lower catalyst layer (region L1 and region L2) is preferably 0.5 g / L or more and 20 g / L or less, more preferably 0.8 g / L or more and 10 g / L or less, and further preferably 1 g / L or more and 5 g / L or less. ４ Barium sulfate (BaSO ) per 1 L of the fire-resistant three-dimensional structure when ４ The content of ) is preferably 1 g / L or more and 30 g / L or less, more preferably 3 g / L or more and 25 g / L or less, and even more preferably 5 g / L or more and 20 g / L or less.

[0037] The lower catalyst layer (region L1 and / or region L2) may contain other precious metals besides palladium, as long as the effects of the present invention are not significantly impaired. Examples of other precious metals include platinum (Pt) and rhodium (Rh). However, the content of other precious metals in the lower catalyst layer per 1 L of the refractory three-dimensional structure is preferably 3 g / L or less, more preferably 1 g / L or less, and even more preferably 0 g / L (not included).

[0038] The lower catalyst layer may contain the above-mentioned palladium, alumina, and cerium-zirconium composite oxide, an optional promoter, an optional other precious metal, and other components other than those described above, as long as the effects of the present invention are not significantly impaired. Examples of other components include manganese, iron, cobalt, nickel, and copper. However, the content of other components per 1 L of the refractory three-dimensional structure in the lower catalyst layer is preferably 10 g / L or less, more preferably 5 g / L or less, even more preferably 3 g / L or less, particularly preferably 1 g / L or less, and most preferably 0 g / L (not included).

[0039] The total loading amount of the lower catalyst layer is not particularly limited, but is preferably 50 g / L to 200 g / L, more preferably 80 g / L to 190 g / L, and even more preferably 120 g / L to 180 g / L. If the total loading amount is within the above range, it is possible to achieve the desired exhaust gas purification performance while suppressing an excessive increase in pressure loss.

[0040] The catalyst according to this embodiment is also characterized in that the boundary X between the regions L1 and L2 is located within a range of 8% to 80% from the gas inlet end face relative to the length of the partition wall. If the boundary X is located within a range of less than 8% or more than 80%, sufficient exhaust gas purification performance (particularly HC purification performance) at low temperatures may not be obtained. From the viewpoint of further improving exhaust gas purification performance (particularly HC purification performance) at low temperatures, the boundary X is located preferably within a range of 8% to 70% from the gas inlet end face, more preferably within a range of 9% to 50%, and even more preferably within a range of 10% to 30%.

[0041] The composition of each catalyst layer and the position of its boundary can be confirmed by inductively coupled plasma (ICP) emission spectroscopy or X-ray fluorescence (XRF) analysis.

[0042] [Upper catalytic layer] The upper catalytic layer is formed on the lower catalytic layer and is located as the outermost layer. Although other layers may be disposed between the lower catalytic layer and the upper catalytic layer, it is preferable that the lower catalytic layer and the upper catalytic layer are adjacent to each other. The upper catalytic layer contains rhodium, alumina, and a cerium-zirconium composite oxide.

[0043] Rhodium functions as a catalyst for the reduction reaction. The rhodium content per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 0.01 g / L or more and 2 g / L or less, more preferably 0.05 g / L or more and 1.5 g / L or less, and even more preferably 0.1 g / L or more and 1 g / L or less.

[0044] Examples of the rhodium raw material (rhodium source) include inorganic salts of rhodium such as nitrate, sulfate, acetate, ammonium salt, amine salt, hexaammine salt, carbonate, bicarbonate, nitrite, and oxalate; carboxylates such as formate; and hydroxides, alkoxides, and oxides. According to a preferred embodiment, the rhodium source is at least one selected from the group consisting of nitrate, ammonium salt, amine salt, and carbonate of rhodium.

[0045] In addition to the rhodium, as shown in FIG. 3, the upper catalytic layer contains palladium at a concentration of C Ｌ１ That is, in a preferred embodiment of the catalyst, the upper catalyst layer is located on the gas inlet side, and the palladium concentration is C Ｕ１ and a gas outlet side region U2 located on the gas outlet side and containing substantially no palladium, the boundary Y between the gas inlet side region U1 and the gas outlet side region U2 being located at substantially the same position as the boundary X with respect to the length of the partition wall, and the palladium concentration C Ｌ１ Palladium concentration C in the gas inlet side region U1 of the upper catalyst layer Ｕ１ The ratio (C Ｕ１ / C Ｌ１) is 0.800 or more and less than 1.000. By adopting such a configuration, the amount of palladium in the gas inlet side region U1 of the upper catalyst layer, which is likely to come into contact with gas, is increased, and therefore, compared to when palladium is not contained in the gas inlet side region U1, exhaust gas purification performance (particularly HC purification performance) at low temperatures can be further improved. In this specification, "substantially free of palladium" refers to "free of palladium (0 g / L)" or "containing palladium, and the palladium content per 1 L of the refractory three-dimensional structure is greater than 0 g / L and not more than 0.01 g / L." "Substantially the same position" refers to the distance between the position of boundary X and the position of boundary Y being within 5 mm. The distance between the position of boundary X and the position of boundary Y is preferably within 3 mm, more preferably within 1 mm, and most preferably 0 mm (the boundary X and boundary Y are at the same position).

[0046] The above ratio (C Ｕ１ / C Ｌ１ From the viewpoint of further improving the exhaust gas purification performance (particularly, HC purification performance) at low temperatures, the closer the ratio (C Ｕ１ / C Ｌ１ ) is 0.800 or more and less than 1.000, preferably 0.900 or more and less than 1.000, more preferably 0.950 or more and less than 1.000, and even more preferably 0.970 or more and less than 1.000.

[0047] The upper catalyst layer (region U1 and / or region U2) may contain platinum (Pt) in addition to rhodium and palladium, as long as the effects of the present invention are not significantly impaired. However, the content of platinum (Pt) per 1 L of the refractory three-dimensional structure in the lower catalyst layer is preferably 0.01 g / L or less, and more preferably 0 g / L (not contained).

[0048] The alumina and its raw materials contained in the upper catalyst layer are the same as those described in the section on the lower catalyst layer, so detailed description will be omitted here. The alumina contained in the upper catalyst layer may be the same as or different from that contained in the lower catalyst layer. The alumina content per 1 L of the refractory three-dimensional structure in the upper catalyst layer (regions U1 and U2) is preferably 5 g / L or more and 70 g / L or less, more preferably 10 g / L or more and 60 g / L or less, and even more preferably 20 g / L or more and 50 g / L or less. If the alumina content is within the above range, the precious metal (rhodium, or rhodium and palladium) can be sufficiently dispersed and supported.

[0049] The cerium-zirconium composite oxide (CeO ２ -ZrO ２ ) and its raw materials are the same as those described in the section on the lower catalytic layer, and therefore detailed description thereof will be omitted here. The cerium-zirconium composite oxide contained in the upper catalytic layer may be the same as or different from that contained in the lower catalytic layer. The content of the cerium-zirconium composite oxide per 1 L of the refractory three-dimensional structure in the upper catalytic layer (regions U1 and U2) is preferably 5 g / L or more and 70 g / L or less, more preferably 10 g / L or more and 60 g / L or less, and even more preferably 20 g / L or more and 50 g / L or less. If the content of the cerium-zirconium composite oxide is within the above range, exhaust gas purification performance (particularly HC purification performance) at low temperatures can be improved.

[0050] The upper catalyst layer (regions U1 and U2) contains at least one promoter selected from the group consisting of Group 1 elements, Group 2 elements, and rare earth elements, as necessary. Details of the promoter are the same as those explained in the section on the lower catalyst layer, and therefore detailed explanations will be omitted here. From the viewpoint of improving the efficiency of exhaust gas purification performance (particularly HC purification performance), the upper catalyst layer (regions U1 and U2) contains lanthanum oxide (La ２ O ３ It is preferable that the upper catalyst layer contains lanthanum oxide (La ２ O ３The content of ) is preferably 0.1 g / L or more and 10 g / L or less, more preferably 0.3 g / L or more and 5 g / L or less, and even more preferably 0.5 g / L or more and 3 g / L or less.

[0051] The upper catalyst layer may contain the above-mentioned rhodium, alumina, and cerium-zirconium composite oxide, as well as optional components such as platinum and a promoter, and other components other than those described above, within a range that does not significantly impair the effects of the present invention. Details of the other components are the same as those described in the section on the lower catalyst layer, and therefore detailed description will be omitted here. The content of the other components per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 10 g / L or less, more preferably 5 g / L or less, even more preferably 3 g / L or less, particularly preferably 1 g / L or less, and most preferably 0 g / L (not included).

[0052] The total loading amount of the upper catalyst layer is not particularly limited, but is preferably 50 g / L to 200 g / L, more preferably 60 g / L to 150 g / L, and even more preferably 65 g / L to 120 g / L. If the total loading amount is within the above range, it is possible to achieve the desired exhaust gas purification performance while suppressing an excessive increase in pressure loss.

[0053] <Method for producing exhaust gas purifying catalyst> Next, a method for producing the above-described exhaust gas purifying catalyst will be described. The method for producing the catalyst is not particularly limited, but the following method is preferred. That is, a method for producing an exhaust gas purifying catalyst according to another embodiment of the present invention includes: applying a lower catalyst layer slurry containing a palladium source, an alumina raw material, and a cerium-zirconium composite oxide raw material onto the partition walls of the refractory three-dimensional structure, drying and firing the slurry to form a lower catalyst layer (hereinafter also referred to as a "lower catalyst layer forming step (1)" or "step (1)"); applying an upper catalyst layer slurry containing a rhodium source, an alumina raw material, and a cerium-zirconium composite oxide raw material onto the lower catalyst layer, drying and firing the slurry to form an upper catalyst layer (hereinafter also referred to as an "upper catalyst layer forming step (2)" or "step (2)"); and applying a palladium solution containing a palladium-containing complex salt and a thickener from the gas inlet side end face to a position within a range of 8% to 80% of the length of the partition walls from the gas inlet side end face, drying and firing the palladium solution (hereinafter also referred to as a "Pd supporting step (3)" or "step (3)"). Each step will be described in detail below.

[0054] [Lower catalyst layer formation step (1)] In step (1), a lower catalyst layer slurry containing a palladium source, an alumina raw material, and a cerium-zirconium composite oxide raw material is applied onto the partition walls of the refractory three-dimensional structure, followed by drying and firing, thereby forming a lower catalyst layer.

[0055] The slurry for the lower catalyst layer contains a palladium source, an alumina raw material, and a cerium-zirconium composite oxide raw material. The palladium source, the alumina raw material, and the cerium-zirconium composite oxide raw material are the same as those described in the section on the lower catalyst layer, and therefore detailed description thereof will be omitted here.

[0056] In this embodiment, the alumina raw material is preferably a powder of alumina (γ-alumina, δ-alumina, θ-alumina, or a composite oxide of alumina and an oxide of another element), and more preferably has a large pore volume (high-porous alumina) in order to improve the dispersibility of the precious metal (palladium). The pore volume of the alumina raw material (alumina powder) can be measured by nitrogen adsorption. The pore volume of the alumina raw material (pore volume per 1 g of alumina raw material) is preferably 0.4 cm ３ / g or more 3.0cm ３ / g or less, and more preferably 0.6 cm ３ / g or more 2.5cm ３ / g or less, and more preferably 0.7 cm ３ / g or more 1.5cm ３ / g or less. The pore volume of the alumina raw material is 0.7 cm ３ / g or more, the noble metal (palladium) supported on the alumina is highly dispersed, which is preferable. ３ When the alumina content is 1 / g or less, the alumina is not excessively bulky and an increase in pressure loss can be suppressed, which is preferable.

[0057] The specific surface area of ​​the alumina raw material (alumina powder) can be measured by the BET multipoint method in accordance with ISO 9277: 2010. When the alumina raw material is highly porous alumina, the BET specific surface area (BET surface area per 1 g of the alumina raw material) is preferably 50 m ２ / g or more 500m ２ / g or less, more preferably 60m ２ / g or more 300m ２ / g or less, and more preferably 80m ２ / g or more 200m ２ / g or less. The BET specific surface area of ​​the alumina raw material is 80 m ２ / g or more, the noble metal (palladium) supported on the alumina is highly dispersed, which is preferable. ２ When the alumina has a surface area of ​​0.1 to 1.0 μm / g or less, the surface area is less likely to decrease when the alumina is exposed to high-temperature exhaust gas, which is preferable.

[0058] When the alumina raw material is high-porous alumina, the cerium-zirconium composite oxide raw material preferably has a small specific surface area, from the viewpoint of preferentially supporting the noble metal (palladium) on the high-porous alumina and improving the dispersibility of the noble metal (palladium). The BET specific surface area of ​​the cerium-zirconium composite oxide raw material is preferably 1.0 m ２ / g or more 30m ２ / g or less, more preferably 1.0m ２ / g or more 20m ２ / g or less, and more preferably 1.0m ２ / g or more 10m ２ / g or less. The BET specific surface area of ​​the cerium-zirconium composite oxide raw material is 1.0 m ２ On the other hand, when the BET specific surface area of ​​the cerium-zirconium composite oxide raw material is 10 m / g or more, sufficient oxygen storage and release performance is exhibited, which is preferable. ２ / g or less, the noble metal (palladium) is supported on the highly porous alumina in a highly dispersed state, which is preferable. That is, in a production method according to a preferred embodiment, the pore volume of the alumina raw material contained in the slurry for the lower catalyst layer measured by a nitrogen adsorption method is 80 cm ３ / g or more 200cm ３ / g or less, and the BET specific surface area of ​​the cerium-zirconium composite oxide raw material contained in the slurry for the lower catalyst layer is 1.0 m ２ / g or more 10m ２ / g or less.

[0059] The slurry for the lower catalyst layer may further contain optional components (the promoter, "other precious metals," and "other components" described above) and / or raw materials thereof other than those contained in the lower catalyst layer, as necessary. The promoter, "other precious metals," and "other components" are the same as those described in the section on the lower catalyst layer, and therefore detailed description thereof will be omitted here.

[0060] The solvent contained in the slurry for the lower catalyst layer is not particularly limited, but examples thereof include water (pure water, ultrapure water, deionized water, distilled water, etc.), lower alcohols such as ethanol and 2-propanol, and organic alkaline aqueous solutions. Of these, water and lower alcohols are preferred, and water is more preferred. The amount of solvent in the slurry for the lower catalyst layer is not particularly limited, but is an amount such that the proportion of solids in the slurry (solids mass concentration) is preferably 5 to 60 mass%, more preferably 10 to 50 mass%.

[0061] The method for applying the slurry for the lower catalyst layer onto the partition walls of the fire-resistant three-dimensional structure can be appropriately selected from known methods such as wash coating. The amount of the slurry to be applied is such that the amount of solids in the slurry and the amount of each component in the lower catalyst layer fall within the aforementioned ranges.

[0062] Known methods can also be appropriately used for drying and baking the coating film formed on the partition walls by the above-mentioned coating. In this specification, "drying" refers to removing the solvent contained in the coating film, and "baking" refers to further treating the coating film from which the solvent has been removed at a high temperature to adhere each component to the partition walls. The drying conditions are not particularly limited, but are preferably performed in air at a temperature of 50°C or higher but lower than 300°C, more preferably 80°C or higher but 200°C, for preferably 5 minutes or longer but 10 hours or shorter, more preferably 30 minutes or longer but 8 hours. The baking conditions are also not particularly limited, but are preferably performed in air at a temperature of 300°C or higher but 1200°C, more preferably 400°C or higher but 700°C, for preferably 10 minutes or longer but 10 hours or shorter, more preferably 30 minutes or longer but 5 hours.

[0063] [Upper catalyst layer formation step (2)] In step (2), an upper catalyst layer is formed by applying an upper catalyst layer slurry containing a rhodium source, an alumina raw material, and a cerium-zirconium composite oxide raw material onto the lower catalyst layer, followed by drying and firing.

[0064] The slurry for the upper catalyst layer contains a rhodium source, an alumina raw material, and a cerium-zirconium composite oxide raw material. The rhodium source, the alumina raw material, and the cerium-zirconium composite oxide raw material are the same as those described in the section on the upper catalyst layer, and therefore detailed description thereof will be omitted here.

[0065] In this embodiment, the alumina raw material is preferably a powder of alumina (γ-alumina, δ-alumina, θ-alumina, or a composite oxide of alumina and an oxide of another element). The pore volume of the alumina raw material (pore volume per 1 g of the alumina raw material) is not particularly limited, but is preferably 0.2 cm ３ / g or more 1.5cm ３ / g or less, and more preferably 0.3 cm ３ / g or more 1.2cm ３ / g or less, and more preferably 0.4 cm ３ / g or more 0.7cm ３ / g or less. ３ / g or more, the dispersion degree of the noble metal (rhodium) can be ensured, which is preferable. ３ When the SiO2 content is 1 / g or less, the thickness of the catalyst layer is prevented from increasing excessively, which is preferable because it is possible to prevent gas diffusion to the lower catalyst layer from being hindered.

[0066] The specific surface area of ​​the alumina raw material (alumina powder) is not particularly limited, but is preferably 50 m ２ / g or more 500m ２ / g or less, more preferably 60m ２ / g or more 300m ２ / g or less, and more preferably 80m ２ / g or more 200m ２ / g or less. The BET specific surface area of ​​the alumina raw material is 80 m ２ On the other hand, when the BET specific surface area of ​​the alumina raw material is 200 m / g or more, the noble metal can be dispersed efficiently, which is preferable. ２ When the alumina has a surface area of ​​0.1 to 1.0 μm / g or less, the surface area is less likely to decrease when the alumina is exposed to high-temperature exhaust gas, which is preferable.

[0067] The BET specific surface area of ​​the cerium-zirconium composite oxide raw material is not particularly limited, but is preferably 20 m ２ / g or more 300m ２ / g or less, more preferably 30m ２ / g or more 200m ２ / g or less, and more preferably 40m ２ / g or more 150m ２ / g or less. The BET specific surface area of ​​the cerium-zirconium composite oxide raw material is 40 m ２ On the other hand, when the BET specific surface area of ​​the cerium-zirconium composite oxide raw material is 150 m / g or more, sufficient oxygen storage and release performance is exhibited, which is preferable. ２ / g or less is preferable because the surface area is less likely to decrease when the cerium-zirconium composite oxide is exposed to high-temperature exhaust gas.

[0068] The slurry for the upper catalyst layer may further contain optional components (the promoter, "other precious metals," and "other components" described above) and / or raw materials thereof other than those contained in the upper catalyst layer, as necessary. The promoter, "other precious metals," and "other components" are the same as those described in the section on the upper catalyst layer, and therefore detailed description thereof will be omitted here.

[0069] The solvent and its amount contained in the slurry for the upper catalyst layer are the same as those explained in the section on step (1), and therefore detailed explanations will be omitted here.

[0070] The method for applying the upper catalyst layer slurry to the lower catalyst layer can be appropriately selected from known methods such as wash coating. The amount of the slurry to be applied is determined so that the amount of solids in the slurry and the amount of each component in the upper catalyst layer fall within the aforementioned ranges. When a portion of the lower catalyst layer (the portion that will become the gas inlet side region L1) is to be exposed, as in the embodiment shown in FIG. 1 , the upper catalyst layer slurry may be applied from the gas outlet side end surface 10 b to a predetermined length.

[0071] The drying and firing methods are the same as those explained in the section on step (1), and therefore detailed explanations are omitted here.

[0072] [Pd loading step (3)] In step (3), a palladium solution containing a palladium-containing complex salt and a thickener is applied from the gas inlet side end face to a position within a range of 8% to 80% from the gas inlet side end face with respect to the length of the partition wall from the gas inlet side end face to the gas outlet side end face, followed by drying and firing.

[0073] The complex salt containing palladium is not particularly limited, but may be tetraamminepalladium ion ([Pd(NH ３ ) ４ ] ２＋ The anion of the complex salt is not particularly limited. The tetraaminepalladium salt may be tetraaminepalladium(II) acetate ([Pd(NH ３ ) ４ ](CH ３ COO) ２ ), tetraamminepalladium(II) chloride ([Pd(NH ３ ) ４ ]Cl ２ ), tetraamminepalladium(II) hydroxide ([Pd(NH ３ ) ４ ](OH) ２ ), tetraamminepalladium(II) nitrate ([Pd(NH ３ ) ４ ](NO ３ ) ２ ), tetraamminepalladium(II) bicarbonate ([Pd(NH ３ ) ４ ](HCO ３ ) ２ Among these, from the viewpoint of the stability of the palladium solution, tetraaminepalladium(II) acetate and tetraaminepalladium(II) nitrate are preferred, and tetraaminepalladium(II) acetate is more preferred.

[0074] The amount of tetraamminepalladium salt in the palladium solution is preferably 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, relative to 100% by mass of the palladium solution, from the viewpoint of supporting a desired amount of palladium.

[0075] The thickener is added to control the amount of palladium carried and the position of the boundary (X, or X and Y) to the desired position. Examples of thickeners include β-1,3 glucans such as scleroglucan, curdlan, paramylon, patiman, and laminaran; and polysaccharides such as xanthan gum and guar gum. Among these, scleroglucan, xanthan gum, and guar gum are preferred from the viewpoint of ease of control, with scleroglucan being more preferred.

[0076] The amount of thickener in the palladium solution is preferably 0.1 mass% or more and 10 mass% or less, more preferably 0.15 mass% or more and 5.0 mass% or less, relative to 100 mass% of the palladium solution, from the viewpoint of facilitating control of the amount of palladium supported and the position of the boundary.

[0077] The solvent contained in the palladium solution is not particularly limited, but examples thereof include water (pure water, ultrapure water, deionized water, distilled water, etc.), lower alcohols such as ethanol and 2-propanol, and organic alkaline aqueous solutions. Of these, water is preferred.

[0078] The method for applying the palladium solution is not particularly limited, and examples include a method in which the catalyst precursor that has been subjected to step (2) is immersed in a container containing the palladium solution from the gas inlet end face. In this case, the region to which the palladium solution is applied is controlled so that the boundary (X, or X and Y) is at the desired position. The region to which the palladium solution is applied, starting from the gas inlet end face, ends at a position within 8% to 80% of the length of the partition wall from the gas inlet end face to the gas outlet end face. From the viewpoint of further improving exhaust gas purification performance at low temperatures, the position of the end point is preferably within a range of 8% to 70%, more preferably within a range of 9% to 50%, and even more preferably within a range of 10% to 30%. The region to which the palladium solution is applied becomes the gas inlet side region (L1, or L1 and U1) after subsequent drying and calcination.

[0079] The drying and firing methods are the same as those explained in the section on step (1), and therefore detailed explanations are omitted here.

[0080] <Method for purifying exhaust gas> According to yet another aspect of the present invention, there is provided a method for purifying exhaust gas, which comprises contacting the above-mentioned exhaust gas purification catalyst with exhaust gas emitted from an internal combustion engine. Examples of internal combustion engines include gasoline engines, gasoline hybrid engines, diesel engines, diesel hybrid engines, and engines that use natural gas, ethanol, dimethyl ether, or the like as fuel. Among these, gasoline engines and gasoline hybrid engines are preferred, and gasoline engines are more preferred.

[0081] As a method for contacting the exhaust gas with the catalyst, for example, a method may be mentioned in which the exhaust gas purification catalyst is placed in the exhaust flow path of the exhaust port of the internal combustion engine and the exhaust gas is allowed to flow into the exhaust flow path.

[0082] According to the present invention, even after high-temperature endurance, it is possible to improve exhaust gas purification performance (particularly HC purification performance) at low temperatures. Here, high-temperature endurance is performed by exposing the catalyst to an atmosphere of preferably 650°C to 1200°C for 5 hours to 500 hours, more preferably 800°C to 1100°C for 10 hours to 100 hours.

[0083] The temperature of the exhaust gas is not particularly limited as long as it is within the temperature range of exhaust gas during operation of a normal internal combustion engine (for example, a gasoline engine), but is preferably from 0° C. to 800° C., and more preferably from 50° C. to 700° C. The air-fuel ratio (A / F) of the exhaust gas is usually from 10 to 30, and preferably from 11 to 14.7.

[0084] In this specification, the term "exhaust gas temperature" refers to the temperature of the exhaust gas at the catalyst inlet. Here, the term "catalyst inlet" refers to the portion of the exhaust pipe in which the exhaust gas purification catalyst is installed, extending from the catalyst end face on the exhaust gas inlet side toward the internal combustion engine by 10 cm, and also refers to the central portion of the exhaust pipe in the longitudinal (axial) direction.

[0085] In addition, in this specification, the term "catalyst bed" refers to the central portion of the exhaust pipe between the catalyst end face on the exhaust gas inlet side and the catalyst end face on the exhaust gas outlet side, and also refers to the central portion of the cross section of the exhaust pipe (if the cross section of the exhaust pipe is not circular, the center of gravity of the cross section of the exhaust pipe).

[0086] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified in various ways within the scope of the claims. Furthermore, the embodiments described in this specification can be combined in any manner to form other embodiments.

[0087] The following embodiments are also included within the scope of the present invention: the catalyst for purifying exhaust gas according to claim 1 having the features of claim 2; the catalyst for purifying exhaust gas according to claim 1 or 2 having the features of claim 3; the catalyst for purifying exhaust gas according to any one of claims 1 to 3 having the features of claim 4; a method for producing the catalyst for purifying exhaust gas according to any one of claims 1 to 4 having the features of claim 5; the production method according to claim 5 having the features of claim 6; and a method for purifying exhaust gas using the catalyst for purifying exhaust gas according to any one of claims 1 to 4 having the features of claim 7.

[0088] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, each operation was carried out under the conditions of room temperature (25°C) and relative humidity of 40% RH or more and 50% RH or less.

[0089] <Examples of producing catalysts for purifying exhaust gas> [Example 1] (Formation of lower catalyst layer precursor) Palladium nitrate (Pd(NO ３ ) ２ ), γ-alumina (Al ２ O ３ , pore volume 0.9 cm ３ / g, BET specific surface area 150m ２ / g, average particle size (D50) 30 μm), cerium-zirconium composite oxide (CeO ２ -ZrO ２ , CeO ２ Content 44% by mass, BET specific surface area 1.7m ２ / g), lanthanum acetate (La(CH３ COO) ３ ) and barium sulfate (BaSO ４ ) to the Pd:Al after firing. ２ O ３ : CeO ２ -ZrO ２ :La ２ O ３ :BaSO ４ Each was weighed so that the mass ratio was 0.1:95:50:3:15. The weighed raw materials were added to deionized water and wet-pulverized to prepare slurry a1. Slurry a1 was applied to a cordierite honeycomb carrier (diameter 103 mm, length 130 mm, cylindrical, 1.083 L, 600 cells per square inch (1 inch = 25.4 mm), cell wall thickness 2.5 mil (1 mil = 0.0254 mm), cell shape square, hereinafter the same) as a refractory three-dimensional structure from the gas inlet end face to the gas outlet end face so that the loading amount after firing was 163.1 g / L, and then dried at 150 ° C. for 15 minutes, and then fired at 550 ° C. for 30 minutes to obtain A1 having a lower catalyst layer precursor provided on a refractory three-dimensional structure.

[0090] (Formation of Lower Catalyst Layer) A tetraamminepalladium (II) acetate solution and scleroglucan were added to deionized water so that the Pd concentration (metal equivalent) was 7.0% by mass and the scleroglucan concentration was 0.5% by mass, and the mixture was stirred for 2 hours to prepare palladium solution P1. Palladium solution P1 was applied to a position 30 mm (23.1% of the length of the partition wall) from the inlet end face of A1 so that the palladium loading after firing was 3.51 g / L, dried at 150 ° C. for 15 minutes, and then fired at 550 ° C. for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ A2 was obtained which was provided with a gas outlet region L2 having a thermal conductivity of 0.0006197 [g / g].

[0091] (Formation of upper catalyst layer) Rhodium nitrate (Rh(NO ３ ) ２ ), γ-alumina (Al ２ O ３ , pore volume 0.5 cm ３ / g, BET specific surface area 130m ２ / g, average particle size (D50) 50 μm), cerium-zirconium composite oxide (CeO ２ -ZrO ２ , CeO ２ Content 25% by mass, BET specific surface area 75m ２ / g) and lanthanum oxide (La ２ O ３ ) to the Rh:Al after firing. ２ O ３ : CeO ２ -ZrO ２ :La ２ O ３ The raw materials were weighed out so that the mass ratio was 0.2:40:35:1. The weighed raw materials were added to deionized water and wet-pulverized to prepare slurry a3. Slurry a3 was applied to a position 100 mm from the outlet end face of A2 so that the loading amount after calcination would be 76.2 g / L, and the mixture was dried at 150 ° C. for 15 minutes and then calcined at 550 ° C. for 30 minutes to obtain catalyst A having a refractory three-dimensional structure provided with a lower catalyst layer and an upper catalyst layer.

[0092] Example 2 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 (Formation of Lower Catalyst Layer Precursor).

[0093] (Formation of Lower Catalyst Layer) A2 was obtained in the same manner as in Example 1 (formation of lower catalyst layer).

[0094] (Formation of upper catalyst layer) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied from the gas inlet end face to the gas outlet end face of A2 so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, followed by firing at 550°C for 30 minutes, thereby obtaining catalyst B in which a lower catalyst layer and an upper catalyst layer were provided on a refractory three-dimensional structure.

[0095] Example 3 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0096] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0097] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 30 mm (23.1% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium supported after firing would be 3.51 g / L, and after drying at 150°C for 15 minutes, the mixture was fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst C was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0098] Comparative Example 1 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0099] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0100] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 10 mm (7.7% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium supported after firing would be 3.51 g / L, and the solution was dried at 150°C for 15 minutes and then fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration CＬ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst D was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0101] Example 4 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0102] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0103] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 15 mm (11.5% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium supported after firing would be 3.51 g / L, and the applied solution was dried at 150°C for 15 minutes and then fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst E was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0104] Example 5 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0105] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0106] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 65 mm (50.0% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium supported after firing would be 3.51 g / L, and the applied solution was dried at 150°C for 15 minutes and then fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst F was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0107] Example 6 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0108] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0109] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 100 mm (76.9% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium carried after firing would be 3.51 g / L, and the solution was dried at 150°C for 15 minutes and then fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration CＬ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst G was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0110] Example 7 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0111] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0112] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to a position 30 mm (23.1% of the length of the partition wall) from the inlet end face of C2 so that the amount of palladium supported after firing would be 1.75 g / L, and after drying at 150°C for 15 minutes, the mixture was fired at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ A catalyst H was obtained, which was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0113] Example 8 (Formation of Lower Catalyst Layer Precursor) A1 was obtained in the same manner as in Example 1 above.

[0114] (Formation of Upper Catalyst Layer Precursor) Slurry a3 in Example 1 (formation of upper catalyst layer) above was applied to A1 from the gas inlet side end face to the gas outlet side end face so that the loading amount after firing would be 76.2 g / L, and the coating was dried at 150°C for 15 minutes, and then fired at 550°C for 30 minutes, thereby obtaining C2 in which a lower catalyst layer precursor and an upper catalyst layer precursor were provided on a refractory three-dimensional structure.

[0115] (Formation of Lower and Upper Catalyst Layers) The palladium solution P1 in Example 1 (formation of lower catalyst layer) was applied to C2 up to a position 30 mm (23.1% of the length of the partition wall) from the inlet end face so that the amount of palladium supported after firing would be 2.63 g / L, and the solution was dried at 150°C for 15 minutes, followed by firing at 550°C for 30 minutes to form a lower catalyst layer (palladium concentration C Ｌ１ The gas inlet side region L1 and the palladium concentration C Ｌ２ 0.0006197 [g / g]) and the upper catalyst layer (comprising a gas outlet side region L2 having a palladium concentration C Ｕ１ The gas inlet side region U1 and the palladium concentration C Ｕ２ Catalyst I was provided with a gas outlet region U2 having a carbon black content of 0 [g / g].

[0116] Comparative Example 2 (Formation of Lower Catalyst Layer) Palladium nitrate (Pd(NO ３ ) ２ ), γ-alumina (Al ２ O ３ , pore volume 0.9 cm ３ / g, BET specific surface area 150m ２ / g, average particle size (D50) 30 μm), cerium-zirconium composite oxide (CeO ２ -ZrO ２ , CeO ２ Content 44% by mass, BET specific surface area 1.7m ２ / g), lanthanum acetate (La(CH ３ COO) ３ ) and barium sulfate (BaSO ４ ) to the Pd:Al after firing. ２ O ３ : CeO ２ -ZrO ２ :La２ O ３ :BaSO ４ Each was weighed so that the mass ratio was 3.6:95:50:3:15. Each weighed raw material was added to deionized water and wet-pulverized to prepare slurry j1. Slurry j1 was applied to a cordierite carrier (diameter 103 mm, length 130 mm, cylindrical, 1.083 L, 600 cells per square inch (1 inch = 25.4 mm), cell wall thickness 2.5 mil (1 mil = 0.0254 mm), hereinafter the same) as a refractory three-dimensional structure so that the loading amount after calcination was 166.6 g / L from the gas inlet end face to the gas outlet end face, dried at 150 ° C. for 15 minutes, and then calcined at 550 ° C. for 30 minutes to obtain J1 having a lower catalyst layer precursor provided on the refractory three-dimensional structure.

[0117] (Formation of Upper Catalyst Layer) An upper catalyst layer was formed in the same manner as in Example 1, to obtain catalyst J in which a lower catalyst layer and an upper catalyst layer were provided on a fire-resistant three-dimensional structure.

[0118] Comparative Example 3 (Formation of Lower Catalyst Layer) A1 was obtained in the same manner as in Example 1 (Formation of Lower Catalyst Layer Precursor).

[0119] (Formation of upper catalyst layer) Palladium nitrate (Pd(NO ３ ) ２ ), rhodium nitrate (Rh(NO ３ ) ２ ), γ-alumina (Al ２ O ３ , pore volume 0.5 cm ３ / g, BET specific surface area 130m ２ / g, average particle size (D50) 50 μm), cerium-zirconium composite oxide (CeO ２ -ZrO ２ , CeO ２ Content 25% by mass, BET specific surface area 75m ２ / g) and lanthanum oxide (La ２ O ３ ) to the Pd:Rh:Al after firing ２ O ３ : CeO ２ -ZrO ２ :La ２ O ３The materials were weighed so that the mass ratio was 3.5: 0.2: 40: 35: 1. The weighed raw materials were added to deionized water and wet-pulverized to prepare slurry k1. Slurry k1 was applied to a position 100 mm from the outlet end face of A1 so that the loading amount after calcination was 79.7 g / L, dried at 150 ° C. for 15 minutes, and then calcined at 550 ° C. for 30 minutes to obtain catalyst K having a lower catalyst layer and an upper catalyst layer on a refractory three-dimensional structure.

[0120] <Durability Test> A V8, 5.6 liter engine was operated for 50 seconds with an A / F ratio at the catalyst inlet of 14.6 and a catalyst bed temperature of 1000°C, followed by operation for 5 seconds at an A / F ratio of 12.0, and then operation for 5 seconds with fuel supply stopped. This cycle was repeated for a total of 50 hours, and heat treatment was performed.

[0121] <Catalyst Performance Test> An in-line 4-cylinder, 2.0-liter engine was operated at an A / F of 14.6, and the temperature of the exhaust gas was raised from 70°C to 400°C at a rate of 1,300°C / min. Gas discharged from the catalyst outlet was sampled, and the THC (total hydrocarbons) purification rate was calculated. The temperature at which each purification rate reached 50% was defined as T50, and the catalyst performance was evaluated based on the time required to reach T50. The results are shown in Table 1 below. The shorter the time required to reach T50, the higher the exhaust gas purification performance.

[0122]

[0123] As shown in Table 1, the present invention can improve the exhaust gas purification performance of exhaust gas purification catalysts at low temperatures. Note that, since the total amounts of the components in the lower catalyst layer and the upper catalyst layer are the same for catalysts A to G, the difference in purification performance is mainly due to the palladium concentration C in the gas inlet side region L1 of the lower catalyst layer. Ｌ１ and the palladium concentration C in the gas outlet side region L2 of the lower catalyst layer Ｌ２ The ratio of (C Ｌ１ / C Ｌ２ ) is considered to be within a specified range.

[0124] Comparing catalysts A to C, it is clear that catalyst A, in which the gas inlet side region L1 of the lower catalyst layer constitutes the outermost layer (the gas inlet side region L1 of the lower catalyst layer is not covered by another catalyst layer), has further improved exhaust gas purification performance at low temperatures.

[0125] A comparison of catalysts C to G also reveals that the exhaust gas purification performance at low temperatures is further improved when the boundary X is located in the vicinity of 11.5% from the gas inlet end face.

[0126] This application is based on Japanese Patent Application No. 2024-110282, filed on July 9, 2024, the disclosure of which is hereby incorporated by reference in its entirety.

[0127] 1, 2, 3: exhaust gas purification catalyst; 10: fire-resistant three-dimensional structure; 10a: gas inlet end face; 10b: gas outlet end face; 20: lower catalyst layer; 30: upper catalyst layer; L1, U1: gas inlet region; L2, U2: gas outlet region; X, Y: boundary.

Claims

1. A fire-resistant three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face; a lower catalyst layer formed in contact with the partition walls and containing palladium, alumina, and a cerium-zirconium composite oxide; and an upper catalyst layer formed on at least a portion of the lower catalyst layer and constituting the outermost layer and containing rhodium, alumina, and a cerium-zirconium composite oxide, wherein the lower catalyst layer is located on the gas inlet side and has a palladium concentration of C Ｌ１ a gas inlet side region L1 having a palladium concentration of C [g / g]; Ｌ２ a gas outlet side region L2 having a palladium concentration C [g / g], wherein a boundary X between the gas inlet side region L1 and the gas outlet side region L2 is located within a range of 8% to 80% from the gas inlet side end face with respect to the length of the partition wall, and Ｌ２ The palladium concentration C in the gas inlet side region L1 Ｌ１ The ratio (C Ｌ１ / C Ｌ２ ) is 30 or more and 230 or less.

2. The exhaust gas purifying catalyst according to claim 1, wherein the gas inlet side region L1 of the lower catalyst layer constitutes the outermost layer.

3. The upper catalyst layer is located on the gas inlet side, and the palladium concentration is C Ｕ１ a gas inlet side region U1 having a palladium concentration C of [g / g] and a gas outlet side region U2 located on the gas outlet side and containing substantially no palladium, a boundary Y between the gas inlet side region U1 and the gas outlet side region U2 is located at substantially the same position as the position of the boundary X with respect to the length of the partition wall, and Ｌ１ The palladium concentration C in the gas inlet side region U1 of the upper catalyst layer Ｕ１ The ratio (C Ｕ１ / C Ｌ１ 2. The exhaust gas purifying catalyst according to claim 1, wherein the value of (A) is 0.800 or more and less than 1.

000.

4. The exhaust gas purifying catalyst according to claim 1, wherein the palladium concentration is substantially uniform throughout the gas inlet side region L1.

5. A method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 4, comprising: applying a lower catalyst layer slurry containing a palladium source, an alumina raw material, and a cerium-zirconium composite oxide raw material onto the partition walls of the refractory three-dimensional structure, drying and firing the slurry to form a lower catalyst layer; applying an upper catalyst layer slurry containing a rhodium source, an alumina raw material, and a cerium-zirconium composite oxide raw material onto the lower catalyst layer, drying and firing the slurry; and applying a palladium solution containing a palladium-containing complex salt and a thickener from the gas inlet side end face to a position within a range of 8% to 80% from the gas inlet side end face with respect to the length of the partition walls, drying and firing the slurry.

6. The pore volume of the alumina raw material contained in the slurry for the lower catalyst layer measured by a nitrogen adsorption method is 0.7 cm ３ / g or more 1.5cm ３ / g or less, and the BET specific surface area of ​​the cerium-zirconium composite oxide raw material contained in the slurry for the lower catalyst layer is 1.0 m ２ / g or more 10m ２ 6. The method for producing an exhaust gas purifying catalyst according to claim 5, wherein the total mass of the catalyst is 1000 mol / g or less.

7. A method for purifying exhaust gas, comprising contacting the exhaust gas purifying catalyst according to any one of claims 1 to 4 with exhaust gas emitted from an internal combustion engine.

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