EXHAUST CATALYSATOR

Abstract

having an exhaust gas purification catalyst (10, 20): a substrate (11, 21); and a catalyst coating layer (14, 24) formed on the substrate (11, 21), wherein the catalyst coating layer (14, 24) has a two-layer structure, wherein the catalyst coating layer (14, 24) comprises an upstream section (12, 22) on an upstream side and a downstream section (13, 23) on a downstream side in an exhaust gas flow direction, and a part or all of the upstream section (12, 22) is formed on a part of the downstream section (13, 23), wherein the downstream section (13, 23) contains Rh fine particles, and where the Rh fine particles have a mean particle size, measured by observation with a transmission electron microscope, of 1.0 nm or more than 2.0 nm or less and has a standard deviation σ of particle size of 0.8 nm or less.

Description

BACKGROUND Technical Area

[0001] The present invention relates to an exhaust gas purification catalyst. State of the art

[0002] Exhaust gas emitted by an internal combustion engine in a car or similar vehicle contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). These harmful components are removed by an exhaust gas purification catalyst before being released into the atmosphere. Conventionally, a three-way catalytic converter is used, which simultaneously oxidizes CO and HC and reduces NOx. A catalyst using a precious metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) as the catalyst metal is a widely used three-way catalytic converter.

[0003] While emissions regulations have become stricter recently, the amount of precious metals used in the exhaust gas purification catalyst is to be reduced in light of resource risk. Among the precious metals, rhenium plays a role in NOx reduction activity, and highly activating rhenium suggests that a reduction in the amount of precious metals required while still complying with emissions regulations.

[0004] One known method for reducing the use of precious metals in exhaust gas purification catalysts involves using the precious metal as fine particles on a support. For example, JP 2016-147256 A discloses a method for producing a catalyst that includes a step of carrying precious metal particles on an oxide support to obtain a precious metal-supported catalyst, and a step of performing a heating process for the precious metal-supported catalyst in a reducing atmosphere to adjust the particle size of the precious metal to a predetermined range. JP 2016-147256 A discloses examples showing that the particle sizes of the precious metal particles on the oxide support could be controlled to a range from 2.8 nm or more to 3.8 nm or less.

[0005] JP 2007-038085 A discloses a process for producing a catalyst, which includes a step in which a reducing agent acts on a catalyst in which precious metal particles are carried on an oxide support, wherein the precious metal particles are enlarged from small particle sizes until the minimum particle size of the precious metal particles of 1 nm or more is achieved. JP 2007-038085 A discloses by way of examples that the particle sizes of the precious metal particles on the oxide support could be controlled to 3.0 nm or more down to 4.1 nm or less.

[0006] An addition position or location for a catalyst metal to maximize the catalytic effect was investigated. For example, JP 2009-273986 A discloses an exhaust gas purification catalyst comprising a catalyst coating layer with a two-layer structure consisting of a lower and an upper layer. The lower layer is formed on the surface of a support substrate and carries rhodium. The upper layer is formed on a surface of the lower layer and carries at least platinum. The platinum in the upper layer is carried at a concentration increased towards the upstream side of the exhaust gas, and the rhodium in the lower layer is carried at a concentration increased towards the downstream side of the exhaust gas.

[0007] In conventional catalysts that use rhrhodium particles with controlled particle size, the rhrhodium particles aggregate and decompose during the catalytic reaction, resulting in insufficient catalyst stability in some cases. Since improving catalyst stability ensures effective rhrhodium utilization, the amount of precious metal can be reduced. For conventional catalysts using controlled rhrhodium particles, the optimal addition position for maximizing their effectiveness has not been sufficiently investigated.

[0008] Furthermore, the published patent applications JP 2020-099840 A and WO 2020 / 175142 A1, which were published after the priority date of the present invention, disclose exhaust gas purification catalysts from the prior art. SUMMARY

[0009] As described above, for the conventional exhaust gas purification catalyst, which uses Rh fine particles whose particle sizes are controlled, there is scope for improving the catalyst's durability and the additive position in order to maximize its effectiveness. Accordingly, the present invention provides an exhaust gas purification catalyst with improved durability.

[0010] The inventors investigated various means of solving the problem and found that the use of Rh fine particles, where a mean particle size and a standard deviation σ of the particle size for a downstream section of a catalyst coating layer are controlled within specific ranges, ensures improved durability of an exhaust gas purification catalyst. This led to the present invention.

[0011] This means that the core of the present invention is as follows. (1) An exhaust gas purification catalyst comprising: a substrate; and a catalyst coating layer formed on the substrate, the catalyst coating layer having a two-layer structure, the catalyst coating layer comprising an upstream section on an upstream side and a downstream section on a downstream side in an exhaust gas flow direction, and part or all of the upstream section being formed on part of the downstream section, the downstream section containing Rh fine particles, and the Rh fine particles having a mean particle size, as measured by observation with a transmission electron microscope, of 1.0 nm or more to 2.0 nm or less and a standard deviation σ of the particle size of 0.8 nm or less. (2) The exhaust gas purification catalyst according to (1) wherein the content of Rh fine particles in the downstream section is 0.01 g / L or more up to 0.7 g / L or less, based on a volume of the substrate. (3) The exhaust gas purification catalyst according to (1) or (2), wherein the downstream section further contains an OSC material which has an oxygen storage capacity and wherein a mass fraction of the OSC material in the downstream section is 5% or more up to 55% or less. (4) The exhaust gas purification catalyst according to (1), (2) or (3), wherein the upstream section contains Pt as a catalyst metal.

[0012] The present invention can provide an exhaust gas purification catalyst with improved durability. BRIEF DESCRIPTION OF THE DRAWING Fig.Figure 1 is a schematic cross-sectional diagram representing a first embodiment of an exhaust gas purification catalyst of the present invention; Fig. Figure 2 is a schematic cross-sectional diagram representing a second embodiment of the exhaust gas purification catalyst of the present invention; Fig. Figure 3 is a diagram showing the NOx conversion rates in a rich atmosphere for catalysts of Example 1 and comparison examples 1 to 4; Fig. Figure 4 is a diagram showing a relationship between the amount of Rh fine particles added in a downstream section and the NOx conversion rate in the rich atmosphere; Fig. Figure 5 is a diagram showing the NOx conversion rates in the rich atmosphere for catalysts from Example 4 and comparison examples 7 and 8; and Fig.Figure 6 is a diagram showing a relationship between the mass fraction of an OSC material in the downstream section and the NOx conversion rate in the rich atmosphere. DETAILED DESCRIPTION

[0013] The embodiments of the present invention are described in detail below.

[0014] An exhaust gas purification catalyst of the present invention comprises a substrate and a catalyst coating layer formed on the substrate. The catalyst coating layer has a two-layer structure, comprising an upstream section on the upstream side and a downstream section on the downstream side in an exhaust gas flow direction, and part or all of the upstream section is formed on a portion of the downstream section. That is, the downstream section has a single-layer section that is not coated with the upstream section.

[0015] Fig. Figure 1 represents a first embodiment of the exhaust gas purification catalyst of the present invention. As in Fig.As shown in Figure 1, an exhaust gas purification catalyst 10 has a substrate 11 and a catalyst coating layer 14 of a two-layer structure formed on the substrate 11. The catalyst coating layer 14 includes an upstream section 12 and a downstream section 13, and part of the upstream section 12 is formed on part of the downstream section 13. Fig.Figure 1 shows the direction of exhaust gas flow. In the exhaust gas purification catalyst with this structure, the exhaust gas first flows through the upstream section 12, where some of the harmful components in the exhaust gas are removed. The exhaust gas then flows through the downstream section 13, which contains Rh fine particles. Therefore, the Rh can be effectively used for NOx removal, ensuring improved NOx removal capacity and reduced Rh utilization of the catalyst.

[0016] Here, for the catalyst coating layer, the upstream section is formed in a predetermined region from an upstream end surface in the exhaust gas flow direction. Since the upstream section in the exhaust gas purification catalyst of the present invention is formed on a portion of the downstream section, it does not extend all the way to the downstream end surface. That is, the thickness of the upstream section is a length in a region of less than 100% of the total length of the substrate from the upstream end surface. Meanwhile, the downstream section only needs to be formed at least from the downstream end surface and can extend over the entire length of the substrate.This means that a layer width of the downstream section is a length in the range of 100% or less of the total length of the substrate from the downstream end surface. If the downstream section is formed over the entire length of the substrate, then the entire upstream section is formed on a portion of the downstream section.

[0017] Fig. Figure 2 represents a second embodiment of the exhaust gas purification catalyst of the present invention. As in Fig.As shown in Figure 2, a catalyst coating layer 24 in an exhaust gas purification catalyst 20 comprises an upstream section 22 and a downstream section 23, and the entire upstream section 22 is formed on a portion of the downstream section 23. In the exhaust gas purification catalyst 20, the downstream section 23 is formed as a lower layer on a substrate 21, and the upstream section 22 is formed as an upper layer on a portion of the downstream section 23. Fig.Figure 2 shows an arrow indicating the direction of exhaust gas flow. In the exhaust gas purification catalyst with this structure, similar to the first embodiment, the exhaust gas first flows through the upstream section 22 as the upper layer, thereby purifying some of the harmful components in the exhaust gas in the upstream section 22. The exhaust gas then flows through the downstream section 23, which contains Rh fine particles. Therefore, the Rh can be effectively used for NOx purification, ensuring improved NOx purification capacity and reduced Rh utilization of the catalyst.

[0018] The thickness of the upstream portion of the catalyst coating layer is a length in the range of 80% or less of the total length of the substrate from the upstream end surface in some embodiments, or may be 70% or less, or 50% or less. For example, the thickness of the upstream portion may be a length in the range of 40% or less, or 30% or less.

[0019] The thickness of the downstream portion of the catalyst coating layer is a length in the range of 100% or less of the total length of the substrate from the downstream end surface, as described above, and the length in the range of 90% or less, or 80% or less in some embodiments. For example, the thickness of the downstream portion may be the length in the range of 60% or less, or 40% or less. The thickness of the downstream portion of the catalyst coating layer is the length in the range of 60% or more to 100% or less of the total length of the substrate from the downstream end surface in some embodiments.

[0020] In the catalyst coating layer of the exhaust gas purification catalyst of the present invention, a portion or all of the upstream section overlaps a portion of the downstream section. The width of the section in which the upstream section overlaps the downstream section corresponds to a length in the range of 10% or more to 60% or less of the total length of the substrate in some embodiments, or may be 10% or more to 40% or less.

[0021] The substrate used for the exhaust gas purification catalyst of the present invention is not particularly limited, and a commonly used multi-cell honeycomb material can be employed. The substrate material comprises a heat-resistant ceramic material, such as cordierite (2MgO·2Al₂O₃·5SiO₂), aluminum oxide, zirconium dioxide, and silicon carbide, and a metallic material formed from a metal foil, such as stainless steel. For cost reasons, cordierite is used in some embodiments.

[0022] The downstream section of the catalyst coating layer contains rhodium (Rh) fine particles (hereinafter also referred to as particle size-controlled Rh fine particles or Rh fine particles), whose mean particle size and standard deviation σ of the particle size are controlled or set to specific ranges. Due to their relatively small mean particle size, the particle size-controlled Rh fine particles have a significantly large specific surface area and thus high catalytic activity. Since the particle size-controlled Rh fine particles exhibit a narrow particle size distribution and low proportions of coarse and fine particles, they have high durability and high catalytic activity.By using particle size-controlled Rh fine particles for the downstream section of the catalyst coating layer, the Rh can be effectively used for NOx purification, thus ensuring improved NOx purification capability and reduced Rh use of the catalyst.

[0023] The Rh fine particles have a mean particle size of 1.0 nm or more to 2.0 nm or less. In the present invention, the mean particle size of the Rh fine particles is a number-averaged particle size obtained by directly measuring the equivalent diameters of the projected area based on an image or recording taken during an observation with a transmission electron microscope (TEM), and by analyzing particle groups with a cardinal number of 100 or more.

[0024] By controlling the mean particle size of the Rh fine particles to 1.0 nm or more, the proportion of fine particles with a particle size of less than 1.0 nm, which are considered to aggregate and cause coarsening during a catalytic reaction, can be reduced. Therefore, the deterioration of the Rh fine particles can be suppressed, ensuring improved catalyst durability. Meanwhile, by controlling the mean particle size of the Rh fine particles to 2.0 nm or less, the surface area of ​​the Rh fine particles can be increased, ensuring improved catalytic activity. The mean particle size of the Rh fine particles is 1.1 nm or more in some embodiments, but can also be 1.2 nm or more. The mean particle size of the Rh fine particles is 1.9 nm or less in some embodiments, or can be 1.8 nm or less, or 1.6 nm or less.The mean particle size of the Rh fine particles is 1.1 nm or more to 1.9 nm or less in some embodiments, or may be 1.2 nm or more to 1.8 nm or less.

[0025] The Rh particles have a standard deviation σ of particle size, measured by transmission electron microscopy, of 0.8 nm or less. Because the Rh particles have a standard deviation σ of particle size of 0.8 nm or less, the particle size distribution is sharp, and the proportions of fine and coarse particles are low. Due to the low number of fine particles, aggregation of the Rh particles during the catalytic reaction is suppressed, thus reducing Rh degradation and improving catalyst durability. Conversely, the low number of coarse particles increases the surface area of ​​the Rh particles, thereby enhancing catalytic activity.

[0026] The standard deviation σ of the particle size of the Rh fine particles is, in some embodiments, 0.7 nm or less, or can be 0.6 nm or less, or 0.5 nm or less. While the particle sizes of the Rh fine particles can be monodispersed, the effects of the invention can also be provided when the standard deviation σ is 0.2 nm or more, 0.3 nm or more, or 0.4 nm or more.

[0027] In the Rh fine particles, the proportion of fine particles with a particle size of less than 1.0 nm is particularly reduced. Due to the low proportion of fine particles with a particle size of less than 1.0 nm, the aggregation of Rh fine particles during the catalytic reaction is suppressed, thereby reducing Rh degradation and improving the catalyst's durability. In some embodiments, the mass fraction of Rh fine particles with a particle size of less than 1.0 nm is 5% or less, based on the total weight of the Rh fine particles. This value can be 4% or less, 3% or less, 2% or less, 1% or less, 0.5% or less, 0.3% or less, or 0.1% or less. Rh fine particles with a particle size of less than 1.0 nm need not be included at all.

[0028] In some embodiments, when measured with the transmission electron microscope, the Rh fine particles have a mean particle size of 1.2 nm or more to 1.8 nm or less, and the mass fraction of the Rh fine particles with a particle size of less than 1.0 nm is 5.0% or less, based on the total weight of the Rh fine particles.

[0029] In some embodiments, the rhr-fine particles are carried on carrier particles. The carrier particles are not particularly restricted, and oxide carrier particles, for example, can be used. A common carrier method, such as an impregnation carrier method, an adsorption carrier method, or a water absorption carrier method, can be used.

[0030] Particles of a metal oxide can be used as oxide carrier particles. The metal contained in the metal oxide includes, for example, one or more metals selected from Groups 3, 4, and 13 of the periodic table, as well as a lanthanide-based metal. If the oxide carrier particles contain two or more metal oxides, they can be a mixture of two or more metal oxides, a composite oxide containing two or more metals, or a mixture of one or more metal oxides and one or more composite oxides.

[0031] The metal oxide can be the oxide of one or more metals selected, for example, from scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), europium (Eu), lutetium (Lu), titanium (Ti), zirconium (Zr), and aluminum (Al), or, in some embodiments, the oxide of one or more metals selected from Y, La, Ce, Ti, Zr, and Al. In some embodiments, the metal oxide is a composite oxide of yttrium oxide (Y₂O₃), lanthanum oxide (La₂O₃), and zirconium dioxide (ZrO₂).

[0032] The particle size of the carrier particles can be adjusted by a specialist depending on the purpose.

[0033] When the Rh fine particles carried on the carrier particles are used, the carrier mass fraction of the Rh fine particles is, for example, 5% or less, 3% or less, 1% or less, 0.7% or less, 0.5% or less, 0.3% or less, or 0.2% or less, based on the mass of the carrier particles. The carrier mass fraction of the Rh fine particles is, for example, 0.01% or more, 0.02% or more, 0.05% or more, 0.07% or more, 0.1% or more, 0.2% or more, 0.5% or more, or 1% or more, based on the mass of the carrier particles.

[0034] When the Rh fine particles carried on the support particles are used, the Rh-carried catalyst particles can be produced by bringing the support particles into contact with a dispersion of Rh fine particle precursors containing Rh fine particle precursors that have been pre-controlled to a predetermined particle size distribution, and then performing calcination.

[0035] The dispersion of the Rh fine particle precursors can be produced, for example, by one of the following methods. (1) a process (Process 1) in which an acidic solution of an Rh compound is reacted with a basic solution in a reactor in which the distance of a reaction field is set to a predetermined range; and (2) a process (Process 2) in which the acidic solution of the Rh compound is mixed with the basic solution to be reacted and a stirring treatment is then carried out in a high-speed mixer.

[0036] In method 1, by using the reactor in which the distance of the reaction field is set to the predetermined range when the acidic solution of the Rh compound (e.g. an inorganic acid salt of Rh) is reacted with the basic solution (e.g. an aqueous solution of a nitrogenous organic compound), the particle size and particle size distribution of the Rh fine particle precursors (e.g. hydroxides of Rh) contained in the obtained dispersion can be controlled.

[0037] The components used for spacing control within the reactor can be two flat plates, a combination of a flat plate and a corrugated plate, narrow tubes, and the like. The spacing of the reaction field can be appropriately adjusted according to a desired particle size and particle size distribution. For example, a microreactor containing a suitable spacing adjustment component can be used as a reactor in which the spacing of the reaction field is set to a predetermined range.

[0038] In process 2, the acidic solution of the Rh compound (e.g., inorganic acid salt of Rh) is reacted with the basic solution (e.g., aqueous solution of a nitrogenous organic compound) to produce the Rh fine particle precursors as particles with large particle sizes, the stirring treatment of the Rh fine particle precursors is carried out in the high-speed mixer, and a strong shear force is applied to disperse the Rh fine particle precursors and thereby control the mean particle size and the particle size distribution of the dispersed Rh fine particle precursors.

[0039] The Rh-supported catalyst particles can be obtained by bringing the dispersion of the Rh fine particle precursors produced as described above into contact with the support particles and then carrying out the calcination.

[0040] The concentration of rhrhodium particles in the downstream section of the catalyst coating layer is 0.01 g / L or more to 0.7 g / L or less, based on the volume of the substrate in some embodiments, or can be 0.1 g / L or more to 0.5 g / L or less. If the concentration of rhrhodium particles in the downstream section is 0.01 g / L or more to 0.7 g / L or less, both improved catalyst durability and reduced rhrhodium consumption can be ensured.

[0041] When the Rh fine particles carried on the support particles are used as the Rh-carried catalyst particles, the content of the Rh-carried catalyst particles in the downstream section of the catalyst coating layer is 10 g / L or more to 80 g / L or less, based on the volume of the substrate in some embodiments, or may be 20 g / L or more to 60 g / L or less.

[0042] The downstream section of the catalyst coating layer can contain an OSC material, which has an oxygen storage capacity. The OSC material is an inorganic material that possesses this oxygen storage capacity. The OSC material stores oxygen when a lean exhaust gas is supplied and releases the stored oxygen when a rich exhaust gas is supplied. The OSC material is not particularly limited and includes cerium oxide (CeO₂), a cerium oxide-containing composite oxide (e.g., cerium oxide-zirconium dioxide (ZrO₂) composite oxide (CZ composite oxide or ZC composite oxide)), and similar materials. Among the OSC materials described above, the cerium oxide-zirconium dioxide composite oxide is used in some embodiments due to its high oxygen storage capacity and relatively low price. The cerium oxide-zirconium dioxide composite oxide can be used in the form of a composite oxide with lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃), and the like.The mixing ratio of cerium oxide to zirconium dioxide in the cerium oxide-zirconium dioxide composite oxide is CeO2 / ZrO2 = 0.2 or more to 9.0 or less in some embodiments.

[0043] If the downstream portion of the catalyst coating layer contains the OSC material, its mass fraction is 5% or more to 55% or less, based on the mass of the downstream portion in some embodiments, or may be 10% or more to 50% or less. If the mass fraction of OSC material in the downstream portion is 5% or more to 55% or less, the durability of the catalyst can be effectively improved.

[0044] If the downstream portion of the catalyst coating layer contains the OSC material, its content is 5 g / L or more to 75 g / L or less, based on the volume of the substrate in some embodiments, or can be 10 g / L or more to 60 g / L or less. If the OSC material content in the downstream portion is 5 g / L or more to 75 g / L or less, based on the volume of the substrate, the catalyst's durability can be effectively improved.

[0045] The downstream portion of the catalyst coating layer may contain any component other than the particle size-controlled Rh fine particles and the OSC material. This other component is not particularly restricted and includes a metal oxide and the like. If the downstream portion of the catalyst coating layer contains this other component, its concentration is 80 g / L or less, based on the volume of the substrate in some embodiments, or may be 60 g / L or less.

[0046] The metal oxide is not particularly restricted and can consist of the oxides of one or more metals, selected, for example, from scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), europium (Eu), lutetium (Lu), titanium (Ti), zirconium (Zr), and aluminum (Al). Aluminum oxide (Al₂O₃) or a composite oxide of Al₂O₃ and lanthanum oxide (La₂O₃) is used as the metal oxide in some embodiments.

[0047] The upstream section of the catalyst coating layer contains a platinum group noble metal as a catalyst metal in some embodiments. Platinum group noble metals include ruthenium (Ru), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), and Pt is used in some embodiments. When the upstream section of the catalyst coating layer contains Pt as the catalyst metal, hydrocarbons (HC) can be converted in the upstream section through which the exhaust gas initially flows, and NOx can be converted in the downstream section, which contains the particle size-controlled Rh fine particles, while suppressing the effects of HC poisoning. Accordingly, the Rh can be used effectively for NOx removal, ensuring improved NOx removal capability and reduced Rh utilization of the catalyst.

[0048] The catalyst metal can be carried on support particles. The support particles are not particularly limited; for example, oxide support particles can be used. A common support method, such as an impregnation support method, an adsorption support method, or a water absorption support method, can be employed.

[0049] The oxide carrier particles can be, for example, particles of a metal oxide. The metal contained in the metal oxide includes, for example, one or more metals selected from Groups 3, 4, and 13 of the periodic table of elements, as well as a lanthanide-based metal. If the oxide carrier particles contain two or more metal oxides, they can be a mixture of two or more metal oxides, a composite oxide containing two or more metals, or a mixture of one or more metal oxides and one or more composite oxides.

[0050] The metal oxide can be the oxides of one or more metals selected, for example, from scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), europium (Eu), lutetium (Lu), titanium (Ti), zirconium (Zr), and aluminum (Al), or, in some embodiments, the oxides of one or more metals selected from Y, La, Ce, Ti, Zr, and Al. Aluminum oxide (Al₂O₃) or a composite oxide of Al₂O₃ and lanthanum oxide (La₂O₃) is used as the metal oxide in some embodiments.

[0051] The catalyst metal content in the upstream section of the catalyst coating layer is 0.01 g / L or more to 1 g / L or less, based on the volume of the substrate in some embodiments, or may be 0.1 g / L or more to 0.6 g / L or less. If the catalyst metal content in the upstream section is 0.01 g / L or more to 1 g / L or less, HC is sufficiently converted in the upstream section, thus ensuring NOx purification while suppressing the effects of HC poisoning in the downstream section.

[0052] When the catalyst metal is carried on the support particles which are to be used as supported catalyst particles, the content of the supported catalyst particles in the upstream section is 1 g / L or more to 50 g / L or less, based on the volume of the substrate in some embodiments, or may be 10 g / L or more to 30 g / L or less.

[0053] The upstream portion of the catalyst coating layer may contain any component other than the catalyst metal. This other component is not particularly restricted and includes OSC material and the like. If the upstream portion of the catalyst coating layer contains the other component, its concentration is 80 g / L or less, based on the volume of the substrate in some embodiments, or may be 60 g / L or less, or 40 g / L or less.

[0054] The OSC material is not particularly limited and includes cerium oxide (CeO₂), a cerium oxide-containing composite oxide (e.g., cerium oxide-zirconium dioxide (ZrO₂) composite oxide (CZ composite oxide or ZC composite oxide)), and similar materials. Among the OSC materials described above, the cerium oxide-zirconium dioxide composite oxide is used in some embodiments due to its high oxygen storage capacity and relatively low cost. The cerium oxide-zirconium dioxide composite oxide can be used in the form of a composite oxide with lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃), and the like. The mixing ratio of cerium oxide to zirconium dioxide in the cerium oxide-zirconium dioxide composite oxide is CeO₂ / ZrO₂ = 0.2 or more, up to 9.0 or less in some embodiments.

[0055] The exhaust gas purification catalyst of the present invention can be produced by coating the substrate with a slurry containing the components of the catalyst coating layer according to a method known to those skilled in the art. In one embodiment, for example, a predetermined area is coated with a slurry containing particle size-controlled Rh fine particles, an OSC material, and a metal oxide, produced according to a known method from a downstream end surface of a substrate. Drying and calcination are carried out at a predetermined temperature for a predetermined time period, thereby forming a downstream section of a catalyst coating layer on the substrate. Subsequently, a predetermined area is coated with a slurry containing a catalyst metal, such as...Pt, and an OSC material, is coated from an upstream end surface of the substrate according to a known method, and drying and calcination are carried out at a predetermined temperature for a predetermined period of time, thereby forming an upstream section. Examples

[0056] The present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to examples. <Bestätigung der Wirkungen der partikelgrößengesteuerten Rh-Feinpartikel und Untersuchung der Zugabeposition> 1. Production of the catalysts: Raw materials used Material 1: Al2O3: 4% mass fraction - Al2O3 composite material Al2O3 Material 2: ZY: 84% by mass ZrO2, 6% by mass La2O3, 10% by mass Y2O3 composite oxide Material 3: Pt / Al2O3: Material in which Pt is worn on top of Material 1 Material 4: particle size-controlled Rh dispersion Material 5: Rh / ZY: Material where Rh is worn on material 2 Material 6: particle size-controlled Rh / ZY: material in which the Rh of material 4 is carried on material 2 Material 7: Beaker method Rh dispersion Material 8: Beaker method Rh / ZY: Material in which the Rh of material 7 is carried on material 2 Material 9: ZC (OSC material): 21% by mass CeO2, 72% by mass ZrO2, 1.7% by mass La2O3, 5.3% by mass Y2O3 composite oxide

[0057] Substrate: Cordierite honeycomb substrate of 875 cc (400 cells square, wall thickness 4 mil or 4 thousandths of an inch)

[0058] Materials 3 to 8 were produced as follows. Material 3: Pt / Al2O3

[0059] A Pt nitrate solution was brought into contact with material 1 and then calcined, resulting in material 3 in which Pt was carried on material 1 in a carrier mass fraction of 1%. Material 4: particle size-controlled Rh dispersion

[0060] 0.2 g of Rh (III) nitrate were added to 50 mL of ion-exchanged water and dissolved, producing an acidic solution (pH 1.0) of the Rh compound.

[0061] An aqueous tetraethylammonium hydroxide solution (concentration 175 g / L, pH 14) was prepared as an organic base solution.

[0062] Using a reactor (microreactor) incorporating two flat plates for spacing control, with a method for introducing the acidic solution of the Rh compound and the organic base solution into a reaction chamber where the spacing was set to 10 µm, both liquids were reacted under a molar ratio (TEAH / RN) of tetraethylammonium hydroxide (TEAH) to Rh nitrate (RN) of 18 to produce a dispersion of Rh fine-particle precursors. The resulting dispersion of Rh fine-particle precursors had a pH of 14. The median diameter (D50) of the Rh fine-particle precursors contained in the resulting dispersion was measured using a dynamic light scattering (DLS) method and was found to be 2.0 nm. Material 5: Rh / ZY

[0063] A Rh nitrate solution was brought into contact with material 2 and subsequently calcined, yielding material 5 in which Rh was carried on material 2 at a carrier mass fraction of 0.45%. The mean particle size of the Rh fine particles, measured using a transmission electron microscope, was 0.70 nm. Material 6: particle size-controlled Rh / ZY

[0064] Material 4 was brought into contact with material 2 and subsequently calcined, yielding material 6 in which Rh was carried on material 2 at a carrier mass fraction of 0.45%. The mean particle size of the Rh fine particles, measured by transmission electron microscopy, was 1.40 nm, and the standard deviation σ of the particle size was 0.48 nm. In the particle size distribution of the particle size-controlled Rh, the proportion of fine particles with a particle size of less than 1.0 nm was smaller than in the beaker method of Rh for material 8. Material 7: Beaker method Rh dispersion

[0065] Material 7 was produced similarly to material 4, except that the acidic solution of the Rh compound was reacted with the organic base solution in a beaker without using the reactor which contained the components for spacing control. Material 8: Beaker method Rh / ZY

[0066] Material 8, in which Rh from material 7 was carried on material 2, was produced similarly to the production of material 6. The mean particle size of the Rh fine particles, measured by transmission electron microscopy, was 1.42 nm, and the standard deviation σ of the particle size was 0.94 nm. Example 1

[0067] Material 6, Material 1, Material 9, and an Al₂O₃-based binder were stirred into distilled water to create a slurry 1 in which these materials were suspended. The prepared slurry 1 was then poured from a downstream end surface into a substrate, and any excess was blown off, coating a wall surface of the substrate with the materials. The coating width was set to 80% of the total length of the substrate from the downstream end surface. The coating rates were adjusted so that Material 6 was 40 g / L, Material 1 was 40 g / L, and Material 9 was 35 g / L, based on the volume of the substrate.Finally, the material was dried for two hours in a dryer at 120 °C and then calcined for two hours in an electric furnace at 500 °C, producing a downstream section of catalyst coating layer.

[0068] Similarly, material 3, material 9, and the Al₂O₃-based binder were stirred into distilled water to form a slurry 2 in which these materials were suspended. The slurry 2 was poured from an upstream end surface onto the substrate on which the downstream section was formed, and any excess was blown off by the blower, thus coating the substrate wall surface with the materials. The coating width was set to 40% of the total length of the substrate from the upstream end surface. The coating quantity was adjusted so that material 3 was 20 g / L and material 9 was 30 g / L, based on the volume of the substrate.Finally, the product was dried in a dryer at 120 °C for two hours and then calcined in an electric oven at 500 °C for two hours, producing an upstream section of the catalyst coating layer. Comparative example 1

[0069] The slurry 2 used in Example 1 was poured into the substrate from the downstream end surface, and any excess was blown off by the blower, thus coating a wall surface of the substrate with the materials. The coating width was set to 80%. The coating quantity was adjusted so that material 3 had a density of 40 g / L and material 960 g / L, based on the volume of the substrate. Finally, drying in a dryer at 120 °C for two hours and subsequent calcination in an electric furnace at 500 °C for two hours were carried out, thus preparing the downstream portion of the catalyst coating layer.

[0070] Similarly, the slurry 1 used in Example 1 was poured from the upstream end surface into the substrate, where the downstream section formed, and any excess was blown off by the blower, thus coating the wall surface of the substrate with the materials. The coating width was set to 40%. The coating quantity was adjusted so that material 6 was 20 g / L, material 1 was 20 g / L, and material 9 was 15 g / L, based on the volume of the substrate. Finally, the substrate was dried in a dryer at 120 °C for two hours and then calcined in an electric furnace at 500 °C for two hours, producing the upstream section of the catalyst coating layer. Comparative example 2

[0071] The catalyst of comparative example 2 was produced similarly to that of comparative example 1, except that material 6 was replaced by material 5. Comparative example 3

[0072] The catalyst of comparison example 3 was produced similarly to that of example 1, except that material 6 was replaced by material 5. Comparative example 4

[0073] The catalyst of comparative example 4 was produced similarly to that of example 1, except that material 6 was replaced by material 8.

[0074] Table 1 lists the compositions and amounts of precious metals in the upstream and downstream sections of the catalysts from Example 1 and Comparative Examples 1 to 4. The amount of precious metal is the amount of precious metal (g / substrate 1 L) relative to the volume of the substrate. [Table 1] Upstream section downstream section Materials used Material composition Amount of precious metal (g / L) Materials used Material composition Amount of precious metal (g / L) Example 1 Material 3 Pt / Al2O; Pt 0.4 Material 6 particle size controlled Rh 0.12 Material 9 ZC Rh / ZY Material 1 Al2O3 Material 9 ZC Example 2 Material 3 Pt / Al2O; Pt 0.4 Material 6 particle size controlled Rh 0.6 Material 9 ZC Rh / ZY Material 1 Al2O3 Material 9 ZC Example 3 Material 3 Pt / Al2O; Pt 0.4 Material 6 particle size controlled Rh 0.48 Material 9 ZC Rh / ZY Material 1 Al2O3 Material 9 ZC Comparison example 1 Material 6 particle size controlled Rh 0.12 Material 3 Pt / Al2O; Pt 0.4 Rh / ZY Material 9 ZC Material 1 Al2O3 Material 9 ZC Comparison example 2 Material 5 Rh / ZY Rh 0.12 Material 3 Pt / Al2O; Pt 0.4 Material 1 Al2O3 Material 9 ZC Material 9 ZC Comparison example 3 Material 3 Pt / Al2O3 Pt 0.4 Material 5 Rh / ZY Rh 0.12 Material 9 ZC Material 1 Al2O3 Material 9 ZC Comparison Example 4 Material 3 Pt / Al2O; Pt 0.4 Material 8 beaker method Rh 0.12 Material 9 ZC Material 1 Rh / ZY Material 9 Al2O3 ZC Comparison example 5 Material 3 Pt / Al2O; Pt 0.4 Material 5 Rh / ZY Rh 0.36 Material 9 ZC Material 1 Al2O3 Material 9 ZC Comparison example 6 Material 3 Pt / Al2O3 Pt 0.4 Material 5 Rh / ZY Rh 0.48 Material 9 ZC Material 1 Al2O3 Material 9 ZC 2. Shelf life test

[0075] Durability testing was performed for each of the manufactured catalysts using a real engine. Specifically, the durability test was carried out as follows: The catalysts were each installed in the exhaust system of an eight-cylinder V-type engine, and the exhaust gases in the respective stoichiometric and lean atmospheres were repeatedly passed through for a specific period (in a 3:1 ratio) at a catalyst bed temperature of 900 °C for 46 hours. 3. Performance evaluation

[0076] Exhaust gases with air-fuel ratios (L / K) of 14.1 and 15.1 were alternately fed at a catalyst bed temperature of 500 °C, switching every three minutes, and the NOx conversion rate at Ga = 30 g / s was evaluated. The NOx conversion rate in a rich atmosphere was calculated by averaging the NOx conversion rates over 15 seconds, starting two minutes and 45 seconds after the rich (L / K = 14.1) third switchover. Table 2 shows the details of the catalysts from Example 1 and comparison examples 1 to 4. Fig. Figure 3 shows the NOx conversion rates in the rich atmosphere for the catalysts of Example 1 and the comparison examples 1 to 4. [Table 2] Rh addition position Rh fine particles Rh fine particles mean particle size (nm) Rh fine particle standard deviation σ (nm) Example 1 downstream section Material 6 1,40 0,48 Comparative example 1 Upstream section Material 6 1,40 0,48 Comparative example 2 Upstream section Material 5 0,70 - Comparative example 3 downstream section Material 5 0,70 - Comparative example 4 downstream section Material 8 1,42 0,94

[0077] Out of Fig.Figure 3 shows that, in a comparison between Example 1 and Comparative Example 1, the catalyst from Example 1, in which the downstream section contains the particle size-controlled Rh fine particles, whose mean particle size and the standard deviation σ of the particle size were controlled to certain ranges of the present invention, exhibited a significantly higher NOx conversion rate compared to the catalyst of Comparative Example 1, in which the upstream section contained the particle size-controlled Rh fine particles. It is assumed that this is due to the fact that HC in the exhaust gas can be converted in the upstream section through which the exhaust gas initially passes, and NOx in the exhaust gas can be converted by Rh in a state of suppression of the influence of HC poisoning in the downstream section through which the exhaust gas passes after the upstream section. The increase range, orThe increase in the NOx conversion rate of Example 1 compared to Comparison Example 1 was greater than the increase in the NOx conversion rate of Comparison Example 3 compared to Comparison Example 2, in which the mean particle sizes of the Rh fine particles were outside the predetermined range of the present invention. Accordingly, it can be said that the improvement in NOx conversion capability is an effect specifically achieved by adding the particle size-controlled Rh fine particles to the downstream section. In the comparison between Example 1 and Comparison Examples 3 and 4, it is shown that the NOx conversion rate is significantly increased by controlling the mean particle size and the standard deviation σ of the particle size of the Rh fine particles within the specific range of the present invention.Here, the NOx conversion rate of the catalyst from Example 1, whose standard deviation σ of the Rh fine particle size was smaller, was significantly higher than that of the catalyst from Comparison Example 4, when the mean particle sizes of the Rh fine particles were equivalent between Example 1 and Comparison Example 4. This is assumed to be due to the fact that the Rh fine particles used for the catalyst in Example 1 have the smaller standard deviation σ and a more uniform particle size. <Einfluss der Zugabemenge der Rh-Feinpartikel des stromabwärtigen Abschnitts auf die NOx-Umwandlungsrate> Examples 2 and 3

[0078] The catalysts of Examples 2 and 3 were prepared similarly to those of Example 1, with the amount of carrier of the particle size-controlled Rh being changed as shown in Table 1. Comparative examples 5 and 6

[0079] The catalysts of comparative examples 5 and 6 were prepared in a similar manner to comparative example 3, with the amount of Rh carrier being changed as specified in Table 1.

[0080] For the catalysts of Examples 1 to 3 and the comparison examples 3, 5 and 6, the NOx conversion rates were measured similarly to the measurement described above. Table 1 gives the compositions and precious metal amounts in the upstream and downstream sections of the catalysts of Examples 1 to 3 and the comparison examples 3, 5 and 6. Fig. Equation 4 represents a relationship between the amount of Rh fine particles added in the downstream section and the NOx conversion rate in the rich atmosphere.

[0081] As in Fig.As shown in Figure 4, the NOx conversion rates of the catalysts of Examples 1 to 3, in which the particle size-controlled Rh fine particles were used, were significantly high compared to the catalysts of Comparative Examples 3, 5, and 6, whose mean particle sizes of the Rh fine particles were outside the specified range of the present invention. Increasing the amount of Rh fine particles added in the downstream section increases the amount of Rh fine particles as the catalyst metal and thus improves the activity of the catalyst. Therefore, the contribution of the particle size-controlled Rh fine particles to improving the NOx removal capacity was reduced.Accordingly, the catalyst that uses the particle size-controlled Rh fine particles has a intended range for its addition quantity from the point of view of improving NOx cleaning capability, and the addition quantity of the particle size-controlled Rh fine particles is in a range of 0.01 g / L or more to 0.7 g / L or less in some embodiments. <Einfluss des OSC-Materialanteils im stromabwärtiger Abschnitt auf die NOx-Umwandlungsrate> 1. Production of catalysts: Examples 4 to 6

[0082] The catalysts of Examples 4 to 6 were prepared similarly to those in Example 1, except that the coating amounts of material 1, material 6 and material 9 in the downstream section were changed as shown in Table 3. Comparative examples 7, 9 and 10

[0083] The catalysts of comparison examples 7, 9 and 10 were produced similarly to comparison example 3, except that the coating amounts of material 1, material 5 and material 9 in the downstream section were changed as shown in Table 3. Comparative example 8

[0084] The catalyst of Comparative Example 8 was produced in a similar manner to that in Comparative Example 7, except that material 5 was replaced by material 8. 2. Shelf life test

[0085] Durability testing was performed for each of the manufactured catalysts using a real engine. Specifically, the durability test was carried out as follows: The catalysts were each installed in the exhaust system of an eight-cylinder V-type engine, and the exhaust gases in the respective stoichiometric and lean atmospheres were repeatedly passed through for a specific period (in a 3:1 ratio) at a catalyst bed temperature of 900 °C for 50 hours. 3. Performance evaluation

[0086] Exhaust gases with air-fuel ratios (L / K) of 14.1 and 15.1 were alternately fed in at a catalyst bed temperature of 500 °C, switching every three minutes, and the NOx conversion rate at Ga = 30 g / s was evaluated. The NOx conversion rate in a rich atmosphere was calculated by averaging the NOx conversion rates over 10 seconds, starting from a point 2 minutes and 45 seconds after the rich (L / K = 14.1) fourth switchover.

[0087] Table 3 shows the details of the catalysts of Examples 4 to 6 and the comparison examples 7 to 10, and Table 4 shows the details of the Rh fine particles of the catalysts of Example 4 and the comparison examples 7, 8. Fig. Figure 5 represents the NOx conversion rate in the rich atmosphere for the catalysts from Example 4 and the comparative examples 7 and 8. [Table 3] Upstream section downstream section Materials used Material composition Coating quantity (g / L) Materials used Material composition Coating quantity (g / L) Al2O3 mass fraction (%) ZC mass fraction (%) Example 4 Material 3 Pt / Al2O3 20 Material 6 particle size controlled Material 9 ZC 30 Rh / ZY 50 Material 1 Al2O3 35 27 32 Material 9 ZC 40 Example 5 Material 3 Pt / Al2O3 20 Material 6 particle size controlled Material 9 ZC 30 Rh / ZY 50 46 12 Material 1 Al2O3 60 Material 9 ZC 15 Example 6 Material 3 Pt / Al2O3 20 Material 6 particle size controlled 15 44 Material 9 ZC 30 Rh / ZY 50 Material 1 Al2O3 20 Material 9 ZC 55 Comparative example 7 Material 3 Pt / Al2O3 20 Material 5 Rh / ZY 50 Material 9 ZC 30 Material 1 Al2O3 40 27 32 Material 9 ZC 35 Comparative example 8 Material 3 Pt / Al2O3 20 Material 8 beaker method 50 Rh / ZY Material 9 ZC 30 Material 1 40 27 32 Al2O3 Material 9 35 ZC Comparative example 9 Material 3 Pt / Al2O3 20 Material 5 Rh / ZY 50 Material 9 ZC 30 Material 1 Al2O3 60 46 12 Material 9 ZC 15 Comparative example 10 Material 3 Pt / Al2O3 20 Material 5 Rh / ZY 50 Material 9 ZC 30 Material 1 Al2O3 20 15 44 Material 9 ZC 55 [Table 4] Rh fine particles Rh fine particles mean particle size (nm) Rh fine particle standard deviation (J (nm) Example 4 Material 6 1,40 0,48 Comparative example 7 Material 5 0,70 - Comparative example 8 Material 8 1,42 0,94

[0088] As in Fig. Figure 5 shows that the NOx conversion rate was also significantly increased by controlling the mean particle size and the standard deviation σ of the particle size of the Rh fine particles to the predetermined ranges of the present invention when the weight fraction of the OSC material in the downstream section was constant.

[0089] Fig. Figure 6 represents a relationship between the weight fraction of OSC material in the downstream section and the NOx conversion rate in the rich atmosphere. As in Fig.As shown in Figure 6, the NOx conversion rates of the catalysts in the examples using particle size-controlled Rh fine particles were significantly higher compared to the catalysts in the comparison examples, whose mean Rh fine particle sizes were outside the specified range of the present invention, when the mass fraction of the OSC material in the downstream section was in the range of 5% or more to 55% or less. The influence of the mass fraction of the OSC material in the downstream section on the NOx conversion rate is taken into account as follows. Specifically, since the aggregation of the Rh fine particles is suppressed after the shelf-life test, the particle sizes of the particle size-controlled Rh fine particles become smaller than those of the comparison examples. Accordingly, the number of active Rh points increases, but conversely, the interaction with the OSC material increases, thereby slightly inactivating Rh.Therefore, if the mass fraction of the OSC material is high, e.g., exceeding 55%, the contribution of the particle-size-controlled Rh fine particles to improving NOx removal efficiency is expected to decrease. Conversely, if the mass fraction of the OSC material is low, e.g., less than 5%, the proportion of OSC material that strongly interacts with Rh decreases to reduce the likelihood of causing Rh inactivation, thereby improving the NOx conversion rate. However, since Rh is also less likely to be inactivated in the catalysts of the comparison examples in this case, the effects are considered reduced compared to the catalysts of the comparison examples. DESCRIPTION OF SYMBOLS 10 Exhaust gas purification catalyst 11 Substrat 12 upstream section 13 downstream section 14 Catalyst coating layer 20 Exhaust gas purification catalyst 21 Substrat 22 upstream section 23 downstream section 24 catalyst coating layer

Claims

[1] Exhaust gas purification catalyst (10, 20) comprising: a substrate (11, 21); and a catalyst coating layer (14, 24) formed on the substrate (11, 21), wherein the catalyst coating layer (14, 24) has a two-layer structure, wherein the catalyst coating layer (14, 24) includes an upstream portion (12, 22) on an upstream side and a downstream portion (13, 23) on a downstream side in an exhaust gas flow direction, and a part or the whole of the upstream portion (12, 22) is formed on a part of the downstream portion (13, 23), wherein the downstream portion (13, 23) contains Rh fine particles, and wherein the Rh fine particles have an average particle size, as measured by observation with a transmission electron microscope, of 1.0 nm or more to 2.0 nm or less and a standard deviation σ of the particle size of 0.8 nm or less. [2] The exhaust gas purifying catalyst (10, 20) according to claim 1, wherein a content of the Rh fine particles in the downstream portion (13, 23) is 0.01 g / L or more to 0.7 g / L or less based on a volume of the substrate (11, 21). [3] The exhaust gas purifying catalyst (10, 20) according to claim 1 or 2, wherein the downstream portion (13, 23) further contains an OSC material having an oxygen storage capacity, and wherein a mass fraction of the OSC material in the downstream portion (13, 23) is 5% or more to 55% or less. [4] An exhaust gas purifying catalyst (10, 20) according to any one of claims 1 to 3, wherein the upstream portion (12, 22) contains Pt as a catalyst metal.

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