JP7827641B2 - Exhaust gas purification catalyst for straddle-type vehicles - Google Patents

Description

The present invention relates to an exhaust gas purification catalyst for saddle-type vehicles such as motorcycles and a method for manufacturing the same, and in particular to an exhaust gas purification catalyst for saddle-type vehicles that consists of only one catalyst layer.

Exhaust gas from gasoline-fueled automobiles and motorcycles contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Purifying these components requires the use of a catalyst to purify each of these harmful components, such as by oxidizing hydrocarbons (HC) to water and carbon dioxide, oxidizing carbon monoxide (CO) to carbon dioxide, and reducing nitrogen oxides (NOx) to nitrogen. A three-way catalyst (TWC), capable of oxidizing and reducing CO, HC, and NOx, is used as a catalyst for treating such exhaust gases (hereinafter referred to as "exhaust gas purification catalyst"). As such a three-way catalyst, for example, a catalyst in which a platinum group metal (PGM) such as ruthenium, rhodium, palladium, osmium, iridium, platinum, or the like is supported as a catalytically active component on alumina base particles having a high specific surface area, and this is supported on a substrate, for example, a monolithic substrate having a refractory ceramic or metallic honeycomb structure, is widely known.

In the internal combustion engines of automobiles and other vehicles, the fuel-to-air ratio (A/F) varies significantly depending on engine operating conditions, such as acceleration, deceleration, low-speed driving, and high-speed driving. With the theoretical exhaust gas air-fuel ratio of 14.7 as the boundary, an oxygen-rich atmosphere (a lean atmosphere) favoring oxidation reactions and a fuel-rich atmosphere (a rich atmosphere) favoring reduction reactions alternate depending on driving conditions. Therefore, exhaust gas catalysts must be able to perform at a certain level under both lean and rich-fuel conditions. In particular, motorcycles tend to operate at higher engine speeds in a fuel-rich atmosphere to increase power output, so excellent catalytic performance in a fuel-rich atmosphere is required. For this reason, air-fuel ratio (A/F) control has traditionally been used. However, controlling the A/F ratio alone does not allow catalysts to fully demonstrate their catalytic performance. Therefore, the catalyst layer itself must also be able to control the A/F ratio.

Therefore, in order to prevent the deterioration of catalyst purification performance caused by changes in the air-fuel ratio through the chemical action of the catalyst itself, catalysts that combine a platinum group element, which is a catalytically active component, with a promoter are used. Known promoters of this type have oxygen storage capacity (OSC), which release oxygen in a reducing atmosphere and absorb oxygen in an oxidizing atmosphere (hereinafter, sometimes referred to as "OSC materials"). Specifically, ceria (cerium oxide, CeO ₂ ) and ceria-zirconia composite oxides are known as OSC materials with oxygen storage capacity. These OSC materials function as buffers that reduce the change between the oxidizing and reducing properties of exhaust gases and maintain the purification performance of the catalyst. Furthermore, ceria-zirconia composite oxides, which are ceria solid-solved with zirconia, have even better oxygen storage capacity (OSC), and are therefore added to many catalysts as OSC materials.

However, exhaust gas purification catalysts for motorcycles face unique challenges that differ from those for automobiles. For example, compared to automobiles, the space available for installing a catalyst is limited, so motorcycle catalysts must be small in volume yet offer high purification capabilities. Furthermore, motorcycles tend to have relatively short exhaust gas flow paths and use large amounts of fuel to prioritize power output, exposing catalysts to high temperatures. Furthermore, the oxygen concentration in the exhaust gas decreases accordingly, often resulting in an exhaust gas air-fuel ratio (A/F) below the stoichiometric air-fuel ratio of 14.7. Therefore, catalysts must have excellent heat resistance and be highly efficient at purifying exhaust gases, even in fuel-rich environments with an A/F ratio below 14.7.

For example, Patent Document 1 discloses a catalytic material for purifying exhaust gas _{from an internal combustion engine, characterized by having a carrier made of a cerium-zirconium composite oxide in which the amount of CeO2 is 45 to 70 mass%, the amount of ZrO2 is 20 to 45 mass%, the amount of Nd2O3 is 2 to 20 mass} %, and the amount of La2O3 is 1 to 10 mass%, and a catalytic component made of metal Pd or Pd oxide supported on the carrier.

Patent Document 2 also describes a catalyst having a first catalytic layer formed on the surface of a carrier made of a ceramic or metal material and a second catalytic layer formed on the first catalytic layer, the first catalytic layer having a carrier made of a cerium-zirconium composite oxide in which the amount of CeO ₂ is 45 to 70 mass %, the amount of ZrO ₂ is 20 to 45 mass %, the amount of Nd ₂ O ₃ is 2 to 20 mass %, and the amount of La ₂ O ₃ is 1 to 10 mass %, and a catalytic component made of metal Pd or Pd oxide supported on the carrier, and the second catalytic layer having a ZrO ₂ amount of 50 to 95 mass %, the amount of CeO ₂ is 0 to 40 mass %, the amount of Nd ₂ O ₃ is 2 to 20 mass %, and the amount of La ₂ O 3 is 1 to 10 mass %. The present invention discloses an exhaust gas purification catalyst for straddle-type vehicles, characterized by having a carrier made of a zirconium-based composite oxide in which the amount of ₃ is 1 to 10 mass % and a catalytic component made of metallic Rh or Rh oxide supported on the carrier, or having a catalytic component made of metallic Rh or Rh oxide and a catalytic component made of metallic Pt or Pt oxide supported on the carrier.

On the other hand, Patent Document 3 discloses an exhaust gas purification catalyst for saddle-type vehicles that is provided in the exhaust passage of an internal combustion engine and that aims to reduce costs by not using Pt or Rh, which are relatively expensive among precious metals, and that includes a substrate, palladium as a catalytically active component, an inorganic porous material such as porous γ-alumina as a catalyst support, and a catalyst layer as a single layer containing ceria (CeO ₂ ) particles and barium as promoter components.

JP 2010-227739 A WO2010/109734 Japanese Patent Application Laid-Open No. 2013-208578

Patent Document 1 uses a catalyst material (Pd-supported cerium-zirconium composite oxide) in which Pd is supported on the surface of a cerium-zirconium composite oxide of a specific composition as the base particle. However, this catalyst material alone still has insufficient purification performance (see Examples 1 and 2).

In addition, exhaust gases emitted from internal combustion engines of automobiles and other vehicles emit large amounts of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) when the catalyst temperature is low, such as during startup. To improve purification performance, it is important to quickly increase the catalyst's temperature and improve the catalyst's low-temperature purification performance. However, the catalyst material described in Patent Document 1 alone still has insufficient low-temperature catalytic activity (see Examples 1 and 2).

Meanwhile, Ce is an important rare earth element used in applications other than catalytic materials, such as phosphors and glass substrate polishing agents. Due to recent overseas circumstances, the supply of Ce has been in crisis, with supply severely restricted and international prices skyrocketing. Therefore, there is a demand for new catalytic materials that use less Ce. However, the catalytic material described in Patent Document 1 still uses a relatively large amount of Ce, and from this perspective, it does not meet recent demands.

The exhaust gas purification catalyst described in Patent Document 2 has a laminated structure in which a first catalyst layer containing the Pd-supported cerium-zirconium composite oxide described in Patent Document 1 as its catalytic component is provided on the surface of a substrate, and a second catalyst layer containing Rh- or Pt-supported cerium-zirconium composite oxide as its catalytic component is further provided on this first catalyst layer. Providing two or more catalyst layers in this way requires a series of processes for each catalyst layer: coating the catalyst slurry, calcining, supporting the PGM, drying, and calcining. This increases manufacturing costs, resulting in expensive exhaust gas purification catalysts. Furthermore, the exhaust gas purification catalyst described in Patent Document 2 has a laminated catalyst layer structure, which results in poor gas diffusion, and insufficient purification performance and low-temperature catalytic activity (see Tables 3 and 5).

On the other hand, Patent Document 3 describes an exhaust gas purification catalyst that uses a single-layer catalyst with a relatively small amount of Ce, but its purification performance is extremely low (see Tables 2 and 3).

The present invention was made in consideration of the above-mentioned problems. Specifically, the object of the present invention is to provide an exhaust gas purification catalyst for straddle-type vehicles that has a simple, low-cost configuration with only one catalyst layer, yet has excellent CO, HC, and NOx purification performance and low-temperature purification performance.

The inventors conducted extensive research to resolve the above-mentioned issues. As a result, they discovered a single-layer catalyst that combines specific Rh-supported composite catalyst particles and specific Pd-supported composite catalyst particles, and found that the above-mentioned issues can be resolved by using such a single-layer catalyst, which led to the completion of the present invention.

That is, the present invention provides various specific embodiments as shown below.
[1] An exhaust gas purification catalyst for a saddle-ride type vehicle that is provided in an exhaust gas passage of an internal combustion engine, the catalyst comprising: a metal substrate; and a single-layer catalyst layer provided on the metal substrate, the catalyst layer containing first composite catalyst particles having at least zirconia-based matrix particles and Rh and _CeO2 particles co-supported on the surfaces of the zirconia-based matrix particles; second composite catalyst particles including alumina matrix particles and Pd supported on the surfaces of the alumina matrix particles; and/or third composite catalyst particles having at least ceria-alumina matrix particles and Pd supported on the surfaces of the ceria-alumina matrix particles.

[2] An exhaust gas purification catalyst for saddle-type vehicles as described in [1], wherein the zirconia-based base particles are rare earth solid-doped zirconia base particles in which at least one rare earth element selected from the group consisting of Ce, Nd, and La is solid-dissolved.

[3] The exhaust gas purification catalyst for straddle-type vehicles according to [1] or [2], wherein the zirconia-based base particles contain, in oxide equivalent, 65 to 85 mass% of ZrO ₂ and 15 to 35 mass% of an oxide of a rare earth element.

[4] An exhaust gas purification catalyst for a saddle-type vehicle described in any one of [1] to [3], wherein the second composite catalyst particle comprises the alumina matrix particle and Pd and Ba supported on the surface of the alumina matrix particle.

[5] The exhaust gas purification catalyst for a saddle-type vehicle according to any one of [1] to [4], wherein the ceria _- alumina base particles of the third composite catalyst particles contain, in oxide equivalent, 70 to 90 mass% of Al 2 O ₃ and 10 to 30 mass% of CeO ₂ .

[6] An exhaust gas purification catalyst for a saddle-type vehicle described in any one of [1] to [5], wherein the catalyst layer further contains a fourth composite catalyst particle comprising ceria-alumina base particles and Rh supported on the surface of the ceria-alumina base particles.

[7] An exhaust gas purification catalyst for saddle-type vehicles described in any one of [1] to [6], wherein the coating amount of the catalyst layer is 1 to 200 g/L per 1 L of the metal substrate.

According to the present invention, it is possible to realize a high-performance exhaust gas purification catalyst for straddle-type vehicles that has excellent CO, HC, and NOx purification performance and low-temperature purification performance, and is highly productive and economical, even though it has a simple and low-cost configuration with only one catalyst layer. Furthermore, based on its composition and structure, the exhaust gas purification catalyst for straddle-type vehicles of the present invention is particularly suitable for use as a three-way catalyst (TWC) that reduces NOx, CO, HC, and other exhaust gases. In particular, the exhaust gas purification catalyst for straddle-type vehicles of the present invention is suitable for use in fields such as motorcycles, which require high heat resistance in a small volume and high exhaust gas purification efficiency even in a fuel-rich atmosphere.

1 is a schematic cross-sectional view showing an exhaust gas purification catalyst 100 for a straddle-type vehicle according to an embodiment. FIG. 2 is a schematic cross-sectional view showing the general configuration of a first composite catalyst particle 31. FIG. 2 is a schematic cross-sectional view showing the general configuration of a second composite catalyst particle 41. FIG. 2 is a schematic cross-sectional view showing a schematic configuration of a third composite catalyst particle 51. FIG. 2 is a schematic cross-sectional view showing a schematic configuration of a fourth composite catalyst particle 61. FIG. 1 is a diagram showing the purification performance (C450 purification rate) of an example and a comparative example. FIG. 1 is a diagram showing low-temperature purification performance (T50) of Examples and Comparative Examples.

Embodiments of the present invention will now be described in detail with reference to the drawings. However, the following embodiments are merely examples for explaining the present invention, and the present invention is not limited to these. In other words, the present invention can be implemented with any modifications within the scope of its gist. Note that in this specification, a numerical range such as "1 to 100" includes both the lower limit "1" and the upper limit "100". The same applies to other numerical ranges. Furthermore, in this specification, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings, unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown.

Figure 1 is a schematic cross-sectional view showing an exhaust gas purification catalyst 100 for a saddle-type vehicle according to this embodiment. Figures 2 to 5 are schematic cross-sectional views showing the general configurations of a first composite catalyst particle 31, a second composite catalyst particle 41, a third composite catalyst particle 51, and a fourth composite catalyst particle 61.

The exhaust gas purification catalyst 100 for a saddle-type vehicle of this embodiment comprises a metal substrate 11 and a single-layer catalyst layer 21 provided on the metal substrate 11, and the catalyst layer 21 contains first composite catalyst particles 31, second composite catalyst particles 41, and/or third composite catalyst particles 51. The exhaust gas purification catalyst 100 for a saddle-type vehicle of this embodiment is provided in the exhaust gas passage of an internal combustion engine, and purifies carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), etc. passing through the system.

The metal substrate 11 is a support that supports the single-layer catalyst layer 21 described above. The metal substrate 11 can be selected from those known in the industry as appropriate, and its type is not particularly limited. Examples of the metal substrate 11 include, but are not limited to, metal honeycomb substrates made of stainless steel or the like, wire mesh substrates made of stainless steel or the like, and steel wool-like knitted wire substrates. The shape of the metal substrate 11 is also not particularly limited, and any shape can be selected, such as a prismatic, cylindrical, spherical, honeycomb, sheet, or pellet shape. These may be used alone or in combination of two or more types.

The catalyst layer 21 is a single catalyst layer provided on the metal substrate 11. The catalyst layer 21 contains first composite catalyst particles 31, second composite catalyst particles 41 and/or third composite catalyst particles 51, and, if necessary, fourth composite catalyst particles 61.

The first composite catalyst particle 31 is a composite particle having at least zirconia-based base particles 32 and Rh and _CeO2 particles 33 co-supported on the surfaces 32a of the zirconia-based base particles 32. In the first composite catalyst particle 31, by co-supporting the Rh and _CeO2 particles 33 on the surfaces 32a of the zirconia-based base particles 32, a decrease in catalytic activity due to Rh particle growth is suppressed. Furthermore, the support of the _CeO2 particles 33 provides high purification performance over a wide A/F window for exhaust gas from saddle-ride vehicles, which exhibits significant changes in the concentrations of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), and also improves low-temperature purification performance. Note that the first composite catalyst particle 31 can be used alone or in appropriate combination of two or more types.

The zirconia-based base particles 32 are zirconia (ZrO ₂ ), a composite oxide in which zirconia (ZrO ₂ ) is doped with other elements, or a solid solution thereof. Specific examples of the zirconia-based base particles 32 include zirconium (IV) oxide, zirconium-rare earth element composite oxide, zirconium-transition element composite oxide, and zirconium-rare earth element-transition element composite oxide. Examples of rare earth elements include, but are not limited to, cerium, neodymium, praseodymium, lanthanum, and yttrium. The rare earth elements can be used singly or in appropriate combination of two or more. When a rare earth element is contained, its content is not particularly limited, but is preferably 0.1 mass % _or more, more preferably 5 mass % or more, even more preferably 10 mass % or _more , and preferably ₅₀ mass % or _less , more preferably 45 mass % or less, and even more preferably 40 mass % or less, in terms of the oxides of the rare earth elements (e.g., the sum of _La2O3 , Nd2O3, _Pr5O11 , etc.) relative to the total amount of the zirconia-based base particles 32. Transition elements are not particularly limited, but include chromium, cobalt, iron, nickel, titanium, manganese, copper, etc. The transition elements can be used alone or in appropriate combinations of two or more. When a transition element is contained, its content is not particularly limited, but is preferably 0.01 mass % _or more, more preferably 0.1 mass % or more, even more preferably 0.5 mass % or _more , and preferably ₁₀ mass % or less, more preferably 5 mass % or less, and even more preferably 3 mass % or less, in terms of the total amount of the transition element oxide (e.g., the sum of Fe2O3, TiO2, etc.) relative to the total amount of the zirconia-based base particles 32. The zirconia-based base particles 32 may be used singly or in appropriate combination of two or more types.

The zirconia-based base particles 32 are preferably zirconia-based _composite oxides containing, in oxide equivalents, 65 to 85 mass % of ZrO2 _and 15 to 35 mass % of an oxide of a rare earth element, and more preferably zirconia-based composite oxides containing, in oxide equivalents, 70 to 80 mass % of ZrO2 and 20 to 30 mass % of an oxide of a rare earth element.

In the zirconia-based base material particles 32, a portion of the zirconium may be substituted with an alkali metal element such as lithium, sodium, or potassium, or an alkaline earth metal element such as beryllium, magnesium, calcium, strontium, or barium. The alkali metal element and alkaline earth metal element may each be used alone or in any combination and ratio of two or more. The zirconia-based base material particles 32 may also contain hafnium (Hf) as an inevitable impurity, which is typically contained in zirconia ore at approximately 1 to 2% by mass.

The average particle diameter _D50 of the zirconia-based base particles 32 can be appropriately set depending on the desired performance and is not particularly limited. From the viewpoint of maintaining a large specific surface area, improving heat resistance, and increasing the number of catalytically active sites, the average particle diameter _D50 of the zirconia-based base particles 32 is preferably 1 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less. In this specification, the average particle diameter _D50 of the particles means the median diameter measured with a laser diffraction particle size analyzer (e.g., the laser diffraction particle size analyzer SALD-3100 manufactured by Shimadzu Corporation).

Rh and _CeO2 particles 33 are co-supported in a highly dispersed manner on the surfaces 32a of the zirconia-based base particles 32. Here, Rh functions as a catalytically active component, and the _CeO2 particles 33 function as a catalytically active component or a promoter having oxygen storage capacity (OSC). By co-supporting Rh and _CeO2 particles 33 on the surfaces 32a of the zirconia-based base particles 32, heat resistance is improved and a decrease in catalytic performance when exposed to high temperatures can be suppressed, thereby improving purification performance and low-temperature purification performance. Furthermore, since the _CeO2 particles 33 are supported on the surfaces 32a of the zirconia-based base particles 32, the interaction between Rh and Ce is promoted, which also improves purification performance and low-temperature purification performance.

The Rh content is not particularly limited and can be set appropriately according to the desired performance, taking into account the material and pore size of the zirconia-based base particles 32. However, from the standpoint of catalytic activity, etc., it is preferable that the total amount of Rh, in metal equivalent, be 0.001 to 5 mass%, more preferably 0.01 to 3 mass%, and even more preferably 0.1 to 1 mass%, relative to the total amount of the first composite catalyst particles 31.

The content ratio of the _CeO2 particles 33 may be appropriately set according to the desired performance, taking into consideration the material and pore diameter of the zirconia-based base particles 32, and is not particularly limited. However, from the viewpoint of catalytic activity and the like, the content ratio of the CeO2 particles 33 is preferably 5 to 20 mass %, more preferably 7 to 18 mass %, and even more preferably 10 to 15 mass %, in terms of oxide, relative to the total amount of the first composite catalyst particles 31 (excluding the amount of precious metals).

The second composite catalyst particles 41 are composite particles having at least an alumina matrix particle 42 and Pd supported on the surface 42a of the alumina matrix particle 42. The second composite catalyst particles 41 can be used alone or in appropriate combination of two or more types.

The alumina matrix particles 42 are matrix particles containing alumina (Al ₂ O ₃ ) as a main component. Here, "containing alumina as a main component" means that the alumina content is more than 90 mass % and not more than 100 mass % of the total amount of the alumina matrix particles 42. Specific examples of the alumina matrix particles 42 include, but are not limited to, alumina, silica-alumina, aluminosilicates, alumina-zirconia, alumina-chromia, alumina-ceria, alumina-magnesium oxide, alumina-barium oxide, and alumina-lanthanum oxide. The alumina matrix particles 42 may be used singly or in appropriate combination of two or more types.

The average particle diameter _D50 of the alumina base particles 42 is not particularly limited and can be appropriately set depending on the desired performance. From the viewpoint of maintaining a large specific surface area and increasing the number of catalytic active sites, the average particle diameter _D50 of the alumina base particles 42 is preferably 0.1 μm or more, more preferably 1 μm or more, and even more preferably 3 μm or more, and is preferably 50 μm or less, more preferably 45 μm or less, and even more preferably 40 μm or less.

Pd is highly dispersed and co-loaded on the surfaces 42a of the alumina matrix particles 42. Here, Pd functions as a catalytically active component. By loading Pd on the surfaces 42a of the alumina matrix particles 42, purification performance and low-temperature purification performance are improved.

The Pd content is not particularly limited and can be set appropriately according to the desired performance, taking into account the material and pore size of the alumina base particles 42. However, from the standpoint of catalytic activity, etc., it is preferable that the total amount of Pd, in metal equivalent, be 0.001 to 5 mass%, more preferably 0.01 to 3 mass%, and even more preferably 0.1 to 1 mass%, relative to the total amount of the second composite catalyst particles 41.

The second composite catalyst particle 41 may further support a Ba component, an alkaline earth metal, on the surface 42a of the alumina matrix particle 42. These components are known as NOx storage components. The Ba component can be supported on the surface 42a of the alumina matrix particle 42, for example, as barium carbonate or barium sulfate. The Ba component stores NOx in a lean, oxygen-rich environment and releases the stored NOx in a rich, oxygen-poor environment. The Ba component content is not particularly limited and can be set appropriately based on the desired performance, taking into account the material and pore size of the alumina matrix particle 42. However, from the perspective of catalytic activity, the Ba component content is preferably 0.1 to 10 mass% (oxide equivalent) of the total amount of the second composite catalyst particle 41 (excluding the amount of precious metals), more preferably 0.5 to 8 mass%, and even more preferably 1 to 5 mass%.

The third composite catalyst particle 51 is a composite particle having at least a ceria-alumina base particle 52 and Pd supported on the surface 52a of the ceria-alumina base particle 52. The third composite catalyst particle 51 can be used alone or in an appropriate combination of two or more types.

The ceria-alumina base particles 52 are a composite oxide of ceria (CeO ₂ ) and alumina (Al ₂ O ₃ ). The content ratio of ceria and alumina can be appropriately set depending on the desired performance and is not particularly limited. However, the content ratio of ceria:alumina, calculated as oxides, is preferably 10-30% by mass:70-90% by mass, and more preferably 15-25% by mass:75-85% by mass. In addition to ceria and alumina, the ceria-alumina base particles 52 may contain the above-mentioned rare earth elements, transition elements, alkali metal elements, alkaline earth metal elements, etc. Furthermore, the ceria-alumina base particles 52 can be used singly or in appropriate combination of two or more types.

The average particle diameter _D50 of the ceria-alumina base particles 52 is not particularly limited and can be appropriately set depending on the desired performance. From the viewpoint of maintaining a large specific surface area and increasing the number of catalytically active sites, the average particle diameter _D50 of the ceria-alumina base particles 52 is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more, and is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less.

Pd is highly dispersed and co-loaded on the surfaces 52a of the ceria-alumina base particles 52. Here, Pd functions as a catalytically active component. By loading Pd on the surfaces 52a of the ceria-alumina base particles 52, high heat resistance and OSC performance can be maintained, thereby improving purification performance and low-temperature purification performance.

The Pd content is not particularly limited and can be set appropriately according to the desired performance, taking into account the material and pore size of the ceria-alumina base particles 52. However, from the standpoint of catalytic activity, etc., it is preferable that the total amount of Pd, in metal equivalent, be 0.001 to 15 mass%, more preferably 0.1 to 5 mass%, and even more preferably 0.3 to 3 mass%, relative to the total amount of the third composite catalyst particles 51.

The fourth composite catalyst particle 61 is a composite particle having at least a ceria-alumina base particle 62 and Rh supported on the surface 62a of the ceria-alumina base particle 62. The fourth composite catalyst particle 61 can be used alone or in an appropriate combination of two or more types.

The ceria-alumina base particles 62 are a composite oxide of ceria (CeO ₂ ) and alumina (Al ₂ O ₃ ). The content ratio of ceria and alumina can be appropriately set depending on the desired performance and is not particularly limited. However, the content ratio of ceria:alumina, calculated as oxides, is preferably 10-30% by mass:70-90% by mass, and more preferably 15-25% by mass:75-85% by mass. In addition to ceria and alumina, the ceria-alumina base particles 62 may contain the above-mentioned rare earth elements, transition elements, alkali metal elements, alkaline earth metal elements, etc. Furthermore, the ceria-alumina base particles 62 can be used singly or in appropriate combination of two or more types.

The average particle diameter _D50 of the ceria-alumina base particles 62 is not particularly limited and can be appropriately set depending on the desired performance. From the viewpoint of maintaining a large specific surface area and increasing the number of catalytic active sites, the average particle diameter _D50 of the ceria-alumina base particles 62 is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more, and is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less.

Rh is highly dispersed and co-loaded on the surface 62a of the ceria-alumina base particles 62. Here, Rh functions as a catalytically active component. By loading Rh on the surface 62a of the ceria-alumina base particles 62, high heat resistance and OSC performance can be maintained, and degradation of catalytic performance when exposed to high temperatures can be suppressed, thereby improving purification performance and low-temperature purification performance.

The Rh content is not particularly limited and can be set appropriately according to the desired performance, taking into account the material and pore size of the ceria-alumina base particles 62. However, from the standpoint of catalytic activity, etc., it is preferable that the total amount of Rh, in metal equivalent, be 0.001 to 15 mass%, more preferably 0.1 to 5 mass%, and even more preferably 0.3 to 3 mass%, relative to the total amount of the fourth composite catalyst particles 61.

In the catalyst layer 21 of this embodiment, the Rh and Pd supported on each composite catalyst particle 31, 41, 51, and 61 can change into elemental metals or metal oxides depending on the external environment. Therefore, it is sufficient for the Rh and Pd supported on each composite catalyst particle 31, 41, 51, and 61 to be confirmed at least in a reducing atmosphere; the properties of Rh and Pd in an oxidizing atmosphere or stoichiometric atmosphere are not particularly limited to elemental metals. In this specification, a reducing atmosphere refers to a state in which the particles are left stationary in a hydrogen gas atmosphere at 400°C for 0.5 hours or more. The presence of Rh and Pd can be confirmed by various measurement methods such as observation with a scanning transmission electron microscope (STEM), powder X-ray diffraction (XRD), electron probe micro analyzer (EPMA), X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA).

The method for producing the composite catalyst particles 31, 41, 51, and 61 is not particularly limited as long as it can produce the Rh, Pd, or _CeO2 particles 33 and the barium component supported on the surfaces 32a, 42a, 52a, and 62a of the base particles 32, 42, 52, and 62, respectively. From the viewpoint of producing the composite particles simply and at low cost, an evaporation-to-dryness method (impregnation method) or the like is preferred.

The base particles 32, 42, 52, and 62 used as raw materials can be commercially available in various grades or can be manufactured by methods known in the art. While there are no particular limitations on the manufacturing methods, for example, the coprecipitation method or alkoxide method is preferred for the zirconia-based base particles 32. A preferred coprecipitation method involves adding an alkaline substance to an aqueous solution containing a cerium salt and/or zirconium salt, optionally mixed with other rare earth metal elements or transition elements, in a predetermined stoichiometric ratio, to hydrolyze or coprecipitate a precursor, and then calcining the hydrolysis product or coprecipitate. The types of salts used are not particularly limited. Generally, hydrochlorides, oxyhydrochlorides, nitrates, oxynitrates, carbonates, phosphates, acetates, oxalates, citrates, etc. are preferred. The type of alkaline substance is also not particularly limited. Generally, an ammonia solution is preferred. A preferred example of the alkoxide method involves hydrolyzing a mixture of cerium alkoxide and/or zirconium alkoxide with, optionally, other rare earth metal elements and transition elements in a predetermined stoichiometric ratio, followed by calcination. The type of alkoxide used is not particularly limited. Generally, methoxide, ethoxide, propoxide, isopropoxide, butoxide, or their ethylene oxide adducts are preferred. The rare earth metal elements may be compounded as metal alkoxides or as the various salts described above. Calcination conditions are not particularly limited and may follow conventional methods. The calcination atmosphere may be any of an oxidizing atmosphere, a reducing atmosphere, or air. The calcination temperature and treatment time vary depending on the desired composition and its stoichiometric ratio. However, from the standpoint of productivity, a temperature of 150°C to 1300°C for 1 to 12 hours is generally preferred, and a temperature of 350°C to 800°C for 2 to 4 hours is more preferred. Prior to the high-temperature firing, it is preferable to carry out reduced-pressure drying using a vacuum dryer or the like at a temperature of 50° C. to 200° C. for about 1 to 48 hours.

A preferred evaporation-to-dryness method involves impregnating each of the base particles 32, 42, 52, and 62 with an aqueous solution containing the Rh or Pd ions, Ce ions, Ba ions, and other ions to be supported, followed by heat treatment or chemical treatment. This impregnation process results in highly dispersed Rh or Pd ions, Ce ions, Ba ions, and other ions adsorbed (attached) to the surfaces 32a, 42a, 52a, and 62a of each base particle 32, 42, 52, and 62. The Rh and Pd ions can be incorporated into the aqueous solution as various Rh and Pd salts. The types of salts used are not particularly limited. Generally, sulfates, hydrochlorides, oxyhydrochlorides, nitrates, oxynitrates, carbonates, oxycarbonates, phosphates, acetates, oxalates, citrates, chlorides, oxides, composite oxides, complex salts, and the like are preferred. Furthermore, the content ratio of Rh and Pd ions in the aqueous solution is not particularly limited and can be appropriately adjusted so that the desired content ratio of Rh and Pd is achieved in each of the obtained composite catalyst particles 31, 41, 51, 61 or catalyst layer 21. Furthermore, it goes without saying that the aqueous solution used here may contain the above-mentioned optional components, for example, other rare earth elements and transition elements, and further unavoidable impurities.

After the impregnation treatment, if necessary, solid-liquid separation, water washing, and drying treatment to remove moisture, for example, in air at a temperature of approximately 50°C to 200°C for approximately 1 to 48 hours, can be performed according to conventional methods. The drying treatment can be natural drying, or a drying device such as a drum dryer, a vacuum dryer, or a spray dryer can be used. The drying atmosphere can be air, a vacuum, or an inert gas atmosphere such as nitrogen gas. Before or after drying, further grinding or classification treatment can be performed as needed. Chemical treatment can also be performed. For example, after the impregnation treatment in the evaporation-to-dryness method, a basic component can be used to hydrolyze Rh or Pd ions on each base material particle 32, 42, 52, 62. The basic component used here is preferably ammonia, amines such as ethanolamine, caustic soda, alkali metal hydroxides such as strontium hydroxide, or alkaline earth metal hydroxides such as barium hydroxide. By these heat treatments and chemical treatments, Rh, Pd, etc. highly dispersed to nano-order size, and further CeO ₂ particles 33 and barium components as required, are produced on each of the base particles 32 , 42 , 52 , 62 .

The firing conditions may be conventional and are not particularly limited. The heating means is not particularly limited, and known equipment such as an electric furnace or gas furnace can be used. The firing atmosphere may be an oxidizing atmosphere, air atmosphere, or reducing atmosphere, with an oxidizing atmosphere or air atmosphere being preferred. The firing temperature and treatment time vary depending on the desired performance. From the perspective of Rh and Pd production and productivity, however, firing at a temperature of 300°C to 1100°C for 0.1 to 12 hours is generally preferred, and firing at a temperature of 400°C to 800°C for 0.5 to 6 hours is more preferred.

In addition to the components described above, the catalyst layer 21 can be used in combination with catalysts, co-catalysts, and base particles known in the industry. Examples of known catalysts, co-catalysts, and base particles that can be used in combination include, but are not limited to, metal oxides or metal composite oxides such as silica, alumina, lanthanum oxide, neodymium oxide, and praseodymium oxide; perovskite-type oxides; composite oxides containing alumina such as silica-alumina, silica-alumina-zirconia, and silica-alumina-boria; barium compounds; and zeolites. The proportion of the catalysts, co-catalysts, and base particles used in combination can be appropriately determined depending on the required performance, but is not particularly limited. While the proportion is not particularly limited, a total of 0.01% by mass to 20% by mass is preferred, a total of 0.05% by mass to 10% by mass is more preferred, and a total of 0.1% by mass to 8% by mass is even more preferred.

In addition to the above-mentioned components, the catalyst layer 21 can be used in combination with additives known in the art. Examples of additives that can be used include, but are not limited to, various binders, dispersion stabilizers such as nonionic surfactants and anionic surfactants, pH adjusters, and viscosity adjusters. Examples of binders include, but are not limited to, various sols such as alumina sol, titania sol, silica sol, and zirconia sol. Soluble salts such as aluminum nitrate, aluminum acetate, titanium nitrate, titanium acetate, zirconium nitrate, and zirconium acetate can also be used as binders. Acids such as acetic acid, nitric acid, hydrochloric acid, and sulfuric acid can also be used as binders. The amount of binder used is not particularly limited, and can be any amount necessary to maintain the molded body. The proportion of the above-mentioned additives can be appropriately determined depending on the required performance, but is not particularly limited. A total of 0.01 to 20% by mass of the total amount is preferred, a total of 0.05 to 10% by mass is more preferred, and a total of 0.1 to 8% by mass is even more preferred.

In the present embodiment, the exhaust gas purification catalyst 100 for straddle-type vehicles has the above-described catalyst layer 21 provided on at least one side of the metal substrate 11. By adopting such a configuration, a low-cost catalyst configuration with a low manufacturing load can be realized, while also facilitating incorporation into manufacturing equipment, thereby increasing the applicability to a variety of applications. For example, by using a metal honeycomb structure carrier as the metal substrate 11, and placing this exhaust gas purification catalyst 100 for straddle-type vehicles in the flow path through which the gas flow passes, and passing the gas flow within the cells of the honeycomb structure carrier, highly efficient exhaust gas purification can be achieved.

In this specification, the term "single-layer catalyst" refers to a catalyst having only one catalyst layer. Furthermore, the term "provided on at least one surface of the metal substrate 11" encompasses an embodiment in which any layer other than the catalyst layer is interposed between one surface of the metal substrate 11 and the catalyst layer.

Such an exhaust gas purification catalyst 100 for a saddle-type vehicle can be obtained, for example, by providing a catalyst layer 21 on the metal substrate 11 described above. The method for forming the catalyst layer 21 may be performed according to a conventional method and is not particularly limited. For example, the exhaust gas purification catalyst 100 for a saddle-type vehicle of this embodiment can be obtained by applying a slurry mixture containing each of the components of the catalyst layer 21 described above to the metal substrate 11 using various known coating methods, washcoating methods, etc., and then drying and firing the mixture as necessary.

As a specific example, each of the above-mentioned composite catalyst particles 31, 41, 51, 61 is mixed with an aqueous medium and, if necessary, binders known in the industry, other catalysts, co-catalysts, OSC materials, various base material particles, additives, etc. in the desired proportions to prepare a slurry mixture, and the resulting slurry mixture is applied to the surface of a metal substrate 11, dried, and fired to obtain an exhaust gas purification catalyst 100 for saddle-type vehicles having a catalyst layer 21 provided on the metal substrate 11.

The aqueous medium used in preparing the slurry mixture should be in an amount that allows each main component to be uniformly dispersed in the slurry. If necessary, an acid or base can be added to adjust the pH, or a surfactant or dispersion resin can be added to adjust the viscosity or improve the dispersibility of the slurry. From the perspective of firmly adhering or bonding the resulting catalyst layer 21 to the metal substrate 11, it is preferable to use the binders described above. Furthermore, known grinding or mixing methods, such as grinding and mixing using a ball mill, can be used to mix the slurry.

After the slurry mixture is applied to the metal substrate 11, it can be dried and fired according to conventional methods. The drying temperature is not particularly limited, but is preferably 70 to 200°C, and more preferably 80 to 150°C. The firing temperature is also not particularly limited, but is preferably 300 to 650°C, and more preferably 400 to 600°C. Known heating methods, such as an electric furnace or gas furnace, can be used.

The total coating amount of the above-mentioned catalyst layer 21 is not particularly limited, but from the viewpoint of the balance between catalytic performance and pressure loss, it is preferable that it be 1 to 200 g/L (excluding the amount of precious metal) per 1 L of capacity of the metal substrate 11, and more preferably 50 to 180 g/L (excluding the amount of precious metal).

The total coating amount of Rh and Pd is not particularly limited, but is preferably 0.05 to 1.5 g/L, more preferably 0.1 to 1.0 g/L, in terms of metal equivalent, from the viewpoint of catalytic performance, balance of pressure drop, etc. The total coating amount of Rh is not particularly limited, but is preferably 0.02 to 0.6 g/L, more preferably 0.04 to 0.4 g/L, in terms of metal equivalent, from the viewpoint of catalytic performance, balance of pressure drop, etc. The total coating amount of Pd is not particularly limited, but is preferably 0.03 to 0.9 g/L, more preferably 0.06 to 0.6 g/L, in terms of metal equivalent, from the viewpoint of catalytic performance, balance of pressure drop, etc. The mass ratio of the coating amounts of Pd and Rh (Pd/Rh) is preferably 10/1 to 10/9, more preferably 10/2 to 10/8, in terms of metal equivalent.

The exhaust gas purification catalyst 100 for straddle-type vehicles of this embodiment described above can be placed in the exhaust system of various engines of straddle-type vehicles, and the number and locations of the catalysts can be designed as desired in accordance with exhaust gas regulations. Furthermore, the exhaust gas purification catalyst 100 for straddle-type vehicles of this embodiment has a simple, low-cost configuration with only one catalyst layer, yet is capable of demonstrating excellent CO, HC, and NOx purification performance and low-temperature purification performance.

The following examples and comparative examples further illustrate the features of the present invention, but the present invention is not limited by them in any way. In other words, the materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Furthermore, the values of various manufacturing conditions and evaluation results in the following examples represent preferred upper or lower limits for embodiments of the present invention, and preferred numerical ranges may be defined by combining the above-mentioned upper or lower limits with the values in the following examples or with each other.

Example 1
Zirconia composite oxide base particles containing 75.0 mass % of _ZrO2 and 25.0 mass % of a rare earth element oxide, calculated as oxides, were charged into a Nauta mixer, and predetermined amounts of cerium nitrate and pure water for dilution were further added. The mixture was stirred for 15 minutes to prepare composite particles in which 12.5 mass % of _CeO2 particles, calculated as oxides, were supported on the surfaces of the zirconia composite oxide base particles.

Next, a predetermined amount of rhodium nitrate and pure water were added to the stirrer and stirred for 5 minutes, and then the composite particles were added and stirred for an additional 15 minutes, thereby obtaining a Rh catalyst slurry containing first composite catalyst particles in which Rh and _CeO2 particles were co-supported on the surfaces of zirconia composite oxide base particles.

Next, predetermined amounts of palladium nitrate and pure water were added to the stirrer and stirred for 5 minutes, after which ceria- _alumina base particles containing 80.7 mass % _Al2O3 and 19.3 mass % _CeO2 , calculated as oxides, for the third composite catalyst particles were further added and stirred for 15 minutes, thereby obtaining a Pd catalyst slurry containing third composite catalyst particles in which Pd was supported on the surfaces of the ceria-alumina base particles.

The resulting Rh catalyst slurry and Pd catalyst slurry were each weighed out in predetermined amounts, placed in a stirrer, and stirred for 15 minutes. They were then pulverized in a ball mill until the average particle size D90 reached 11 μm. An alumina sol binder and distilled water were added to the resulting mixture, which was then stirred for an additional 10 minutes to obtain a catalyst slurry for washcoating.

A stainless steel metal honeycomb carrier (300 cpsi/50 μm, 40 x 90 mmL) was then immersed in the washcoat catalyst slurry, after which excess washcoat liquid was removed from the cells using an air blower. The carrier was then dried and calcined at 500°C for 1 hour to obtain an exhaust gas purification catalyst for straddle-type vehicles. The washcoat amount of this catalyst was 120 g/L (excluding precious metals) per 1 L of stainless steel metal honeycomb carrier, with an Rh content of 0.2 g/L and a Pd content of 0.3 g/L.

Example 2
A predetermined amount of rhodium nitrate and pure water were added to a stirrer and stirred for 5 minutes, and then the _CeO2 particle-supported composite particles obtained in Example 1 were added and stirred for an additional 15 minutes. Ceria-alumina base particles containing 80.7 mass% _Al2O3 and 19.3 mass% _CeO2 , calculated as oxides _, for the fourth composite catalyst particles were further added and stirred for 15 minutes. This resulted in a Rh catalyst slurry containing first composite catalyst particles in which Rh and _CeO2 particles were co-supported on the surfaces of zirconia composite oxide base particles and fourth composite catalyst particles in which Rh was supported on the surfaces of ceria-alumina base particles.

Next, a predetermined amount of palladium nitrate and pure water were added to the stirrer and stirred for 5 minutes, after which _Al2O3 powder _with Ba supported on its surface was added and stirred for an additional 15 minutes. 1N nitric acid was then added until the pH reached 4-5, and the mixture was stirred for 15 minutes. This yielded a Pd catalyst slurry containing second composite catalyst particles with Pd and Ba supported on the surfaces of alumina particles.

The resulting Rh catalyst slurry and Pd catalyst slurry were each weighed in a predetermined amount and placed in a stirrer. After stirring for 15 minutes, Ba-supported _{Al2O3 powder} was added and stirred for 15 minutes. The mixture was then pulverized in a ball mill until the average particle size D90 reached 11 μm. An alumina sol binder and distilled water were added to the resulting mixture, which was then stirred for another 10 minutes to obtain a catalyst slurry for washcoating.

A stainless steel metal honeycomb carrier (300 cpsi/50 μm, 40 x 90 mmL) was then immersed in the washcoat catalyst slurry, after which excess washcoat liquid was removed from the cells using an air blower. The carrier was then dried and calcined at 500°C for 1 hour to obtain an exhaust gas purification catalyst for straddle-type vehicles. The washcoat amount of this catalyst was 120 g/L (excluding precious metals) per 1 L of stainless steel metal honeycomb carrier, with an Rh content of 0.2 g/L and a Pd content of 0.3 g/L.

(Comparative Example 1)
A predetermined amount of rhodium nitrate and pure water were added to a stirrer and stirred for 5 minutes, after which ceria-alumina base particles were added and stirred for 15 minutes. This resulted in a mixed slurry containing composite catalyst particles in which Rh was supported on the surfaces of ceria-alumina base particles. This mixed slurry was then pulverized in a ball mill until the average particle diameter D90 reached 11 μm. An alumina sol binder and distilled water were added to the resulting mixture, and the mixture was stirred for an additional 10 minutes to obtain a Rh catalyst slurry containing composite catalyst particles in which Rh was supported on the surfaces of ceria-alumina base particles.

Next, a predetermined amount of palladium nitrate and pure water were added to the stirrer and stirred for 5 minutes, after which ceria-alumina base particles were added and stirred for 15 minutes. This resulted in a mixed slurry containing composite catalyst particles in which Pd was supported on the surface of ceria-alumina base particles. This mixed slurry was then pulverized in a ball mill until the average particle diameter D90 reached 11 μm. An alumina sol binder and distilled water were added to the resulting mixture, which was then stirred for an additional 10 minutes to yield a Pd catalyst slurry.

A stainless steel metal honeycomb carrier (300 cpsi/50 μm, 40 x 90 mmL) was then immersed in the Pd catalyst slurry, followed by air-blowing to remove excess washcoat liquid from the cells, drying, and firing at 500°C for 1 hour. The Pd catalyst slurry-coated stainless steel metal honeycomb carrier was then immersed in Rh catalyst slurry, followed by air-blowing to remove excess washcoat liquid from the cells, drying, and firing at 500°C for 1 hour to obtain a multilayer catalyst for saddle-type vehicles. The washcoat amount of this catalyst was 120 g/L (excluding precious metals) per 1 L of stainless steel metal honeycomb carrier, with an Rh content of 0.2 g/L and a Pd content of 0.3 g/L.

(Comparative Example 2)
A predetermined amount of the Rh catalyst slurry obtained in Comparative Example 1 and a predetermined amount of the Pd catalyst slurry obtained in Comparative Example 1 were weighed and placed in a stirrer, and an alumina sol binder material and distilled water were added thereto, followed by stirring for 10 minutes to obtain a catalyst slurry for wash coating.

A stainless steel metal honeycomb carrier (300 cpsi/50 μm, 40 x 90 mmL) was then immersed in the washcoat catalyst slurry, after which excess washcoat liquid was removed from the cells using an air blower. The carrier was then dried and calcined at 500°C for 1 hour to obtain an exhaust gas purification catalyst for straddle-type vehicles. The washcoat amount of this catalyst was 120 g/L (excluding precious metals) per 1 L of stainless steel metal honeycomb carrier, with an Rh content of 0.2 g/L and a Pd content of 0.3 g/L.

<Performance evaluation>
The purification characteristics of each catalyst were evaluated using a model gas evaluation device manufactured by Horiba, Ltd. Here, the ratio of HC, CO, and NO (nitrogen oxide) in the model gas reduced at 450°C (C450 purification rate) and the temperature at which 50% purification rate is reached [T50 (°C)] were measured to evaluate the three-way purification performance of each catalyst. In this evaluation, test pieces (25.4 x 50 mmL) were punched out from the exhaust gas purification catalysts for straddle-type vehicles obtained in Examples 1 and 2 and Comparative Examples 1 and 2, and these test pieces were placed in a rich atmosphere of 2% CO + 10% _H2O + balance _N2 and a rich atmosphere of 5% _O2 + 10% _H2O + balance N2. The catalysts were subjected to durability testing at 1050°C for 5 hours under conditions where the lean atmosphere of _{2 and 3} were varied at 5-minute intervals. The evaluation samples were used. The evaluation device used was a flow-type reactor consisting of stainless steel piping. A model gas with the following composition was introduced from the inlet, circulated through the exhaust gas purification reaction section, and discharged to the outlet. The model gas was heated by an external heater and sent to the exhaust gas purification reaction section, heating the purification reaction section. The gas composition on the outlet side (after passing through the catalyst section) was analyzed over a temperature range of 100°C to 450°C, and the rate of change in CO, HC, and NO concentrations was determined. The results are shown in Figures 6 and 7.

Model gas composition: CO: 0.3%, C ₃ H ₆ : 3000 ppm, NO: 3000 ppm, O ₂ : 0.15%, CO ₂ : 10%, H ₂ O: 10%, N ₂ : balance A/F = 14.5
Space velocity (SV): 72,000/h
Evaluation temperature: 100 to 500°C
Temperature rise rate: 10°C/min

The exhaust gas purification catalyst for straddle-type vehicles of the present invention has a simple, low-cost configuration with only one catalyst layer, yet it exhibits excellent CO, HC, and NOx purification performance and low-temperature purification performance, and is highly productive and economical. Based on its composition and structure, it can be widely and effectively used as a three-way catalyst (TWC) that reduces NOx, CO, HC, and other exhaust gases. In particular, the exhaust gas purification catalyst for straddle-type vehicles of the present invention can be effectively used in fields such as motorcycles, which require high heat resistance in a small volume and high exhaust gas purification efficiency even in a fuel-rich atmosphere.

DESCRIPTION OF SYMBOLS 100 Exhaust gas purification catalyst for straddle-type vehicle 11 Metal substrate 21 Catalyst layer 31 First composite catalyst particle 32 Zirconia-based matrix particle 32a Surface 33 _CeO2 particle 41 Second composite catalyst particle 42 Alumina matrix particle 42a Surface 51 Third composite catalyst particle 52 Ceria-alumina matrix particle 52a Surface 61 Fourth composite catalyst particle 62 Ceria-alumina matrix particle 62a Surface

Claims (5)

An exhaust gas purification catalyst for a straddle-type vehicle that is provided in an exhaust gas passage of an internal combustion engine,
A three-way catalyst including a metal substrate and a single catalyst layer provided on the metal substrate,
The catalyst layer is
First composite catalyst particles having at least zirconia-based base particles and Rh and _CeO2 particles co-supported on the surfaces of the zirconia-based base particles; and
The catalyst comprises second composite catalyst particles containing alumina matrix particles and Pd supported on the surfaces of the alumina matrix particles, and/or third composite catalyst particles having at least ceria-alumina matrix particles and Pd supported on the surfaces of the ceria-alumina matrix particles ,
The zirconia-based base particles contain, in oxide equivalent, 65 to 85 mass % of ZrO ₂ and 15 to 35 mass % of an oxide of a rare earth element,
The ceria-alumina matrix particles of the third composite catalyst particles contain, in oxide equivalent, 70 to 90 mass % of Al ₂ O ₃ and 10 to 30 mass % of CeO ₂ ;
Exhaust gas purification catalyst for saddle-type vehicles. The zirconia-based base particles are rare earth-doped zirconia base particles in which at least one rare earth element selected from the group consisting of Ce, Nd, and La is dissolved.
The exhaust gas purifying catalyst for a straddle-type vehicle according to claim 1. The second composite catalyst particles include the alumina matrix particles and Pd and Ba supported on the surfaces of the alumina matrix particles.
The exhaust gas purifying catalyst for a straddle-type vehicle according to claim 1 or 2 . the catalyst layer further contains fourth composite catalyst particles containing ceria-alumina matrix particles and Rh supported on the surfaces of the ceria-alumina matrix particles;
The exhaust gas purifying catalyst for a straddle-type vehicle according to any one of claims 1 to 3 . the coating amount of the catalyst layer is 1 to 200 g/L per 1 L of the metal substrate;
The exhaust gas purifying catalyst for a straddle-type vehicle according to any one of claims 1 to 4 .