EXHAUST GAS CATALYST

Abstract

Exhaust gas purification catalyst (10, 20) comprising: a substrate (11, 21); and a catalyst coating layer (14, 24) formed on the substrate (11, 21), the catalyst coating layer (14, 24) having 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 upstream section (12, 22) contains Rh fine particles and Pt, wherein the Rh fine particles have an average particle size 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, wherein the downstream section (13, 23) contains Rh.

Description

BACKGROUNDTechnical FieldThe present invention relates to an exhaust gas purifying catalyst.Prior ArtExhaust gas discharged from an internal combustion engine of an automobile and the like contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), and the harmful components are purified by an exhaust gas purification catalyst before being discharged to the atmosphere. Conventionally, as the exhaust gas purifying catalyst, a three-way catalyst that simultaneously performs oxidation of CO and HC and reduction of NOx is used, and a catalyst that uses a noble metal such as platinum (Pt), palladium (Pd), and rhodium (Rh) as the catalyst metal is widely used as the three-way catalyst.Recently, as emission regulations have become more stringent, the amount of noble metals used for the exhaust gas purification catalyst is to be reduced from the aspect of resource risk. Among the noble metals, Rh plays a role in NOx reduction activity, and highly activating Rh makes it expected to reduce the amount of noble metal while complying with emission regulations.As a method for reducing the use of the noble metal in the exhaust gas purifying catalyst, a method is known in which the noble metal is used by supporting the noble metal as fine particles on a carrier. For example, JP 2016-147256 A discloses a method for producing a catalyst, which includes a step of supporting noble metal particles on an oxide carrier to obtain a noble metal supported catalyst, and a step of performing a heating process for the noble metal supported catalyst in a reducing atmosphere to control a particle size of the noble metal within a predetermined range. JP 2016-147256 A discloses in Examples that the particle sizes of the noble metal particles on the oxide carrier could be controlled to a range of 2.8 nm or more to 3.8 nm or less.JP 2007-38085 A discloses a method for producing a catalyst, which includes a step of applying a reducing agent to a catalyst in which noble metal particles are carried on an oxide carrier, enlarging the noble metal particles having small particle sizes, and reaching the minimum particle size of the noble metal particles of 1 nm or more. JP 2007-38085 A discloses in examples that the particle sizes of the noble metal particles on the oxide carrier could be controlled to 3.0 nm or more to 4.1 nm or less.A structure of a catalyst coating layer and an addition position of a catalyst metal for maximum providing a catalyst effect were examined. For example, JP 2014-151306 A discloses a catalyst including a substrate having a cell structure through which an exhaust gas flows, and a catalyst coating formed on a cell wall surface of the substrate. The catalyst layers include a first catalyst layer disposed on an upstream side in an exhaust flow direction and a second catalyst layer disposed on a downstream side in an exhaust flow direction. The first catalyst layer contains rhodium and the second catalyst layer contains palladium or platinum.However, in the conventional catalyst using Rh fine particles whose particle size is controlled, the Rh fine particles are aggregated during the catalytic reaction, so that they decompose during the catalytic reaction and thus do not ensure sufficient durability of the catalyst in some cases. Since the improvement in the durability of the catalyst ensures effective utilization of Rh, the use of Rh can be reduced. For the conventional catalyst using the Rh fine particles whose particle sizes are controlled, the addition position for further activation of Rh has not been sufficiently studied.Furthermore, published patent applications EP 2 045 010 A1 and EP 3 932 544 A1 disclose exhaust gas purification catalysts from the prior art.SUMMARYAs described above, for the conventional exhaust gas purifying catalyst using the Rh fine particles whose particle sizes are controlled, the catalyst durability is insufficient in some cases, and there is still room for improving the addition position for further activating Rh. Accordingly, the present invention provides an exhaust gas purifying catalyst having improved Rh activation.The inventors studied various ways to solve the problem and found that the use of Rh fine particles in which an average particle size and a standard deviation σ of the particle size are controlled in certain ranges on an upstream portion of a catalyst coating layer ensures further Rh activation. Thus, the present inventors achieved the present invention.That is, the gist 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, wherein the catalyst coating layer has a two-layer structure, wherein the catalyst coating layer includes an upstream portion on an upstream side and a downstream portion on a downstream side in an exhaust gas flow direction, and wherein a part or the whole of the upstream portion is formed on a part of the downstream portion, wherein the upstream portion contains Rh fine particles and Pt, wherein the Rh fine particles have an average particle size 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, and wherein the downstream portion contains Rh.(2) The exhaust gas purification catalyst according to (1), wherein a content of the Rh fine particles in the upstream portion is a mass ratio of 1.0% or more to 45% or less based on a total content of the Rh fine particles in the upstream portion and Rh in the downstream portion.(3) The exhaust gas purification catalyst according to (1) or (2), wherein the upstream portion further contains an OSC material having an oxygen storage capacity.The present invention can provide the exhaust gas purifying catalyst having the improved Rh activation.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a cross-sectional diagram illustrating a first embodiment of an exhaust gas purification catalyst of the present invention; FIG. 2 is a cross-sectional diagram illustrating a second embodiment of the exhaust gas purification catalyst of the present invention; FIG. 3 is a graph illustrating NOx conversion rates in the alternating atmosphere of rich and lean for the catalysts of Example 1 and Comparative Examples 1 to 4; and FIG. 4 is a graph illustrating a relationship between a ratio of the Rh content in an upstream portion and each of a NOx conversion rate and a NOx 50% conversion temperature in the rich and lean repeated atmosphere for catalysts.DETAILED DESCRIPTIONHereinafter, embodiments of the present invention will be described in detail.An exhaust gas purifying 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 including an upstream portion on the upstream side and a downstream portion on the downstream side in an exhaust gas flow direction, and a part or the entire upstream portion is formed on a part of the downstream portion. That is, the downstream portion has a single-layer portion that is not coated with the upstream portion.FIG. 1 illustrates a first embodiment of the exhaust gas purification catalyst of the present invention. As shown in FIG. 1, an exhaust gas purifying catalyst 10 includes a substrate 11 and a catalyst coating layer 14 having a two-layer structure formed on the substrate 11. The catalyst coating layer 14 includes an upstream portion 12 and a downstream portion 13, and a part of the upstream portion 12 is formed on a part of the downstream portion 13. In FIG. 1, an arrow shows the exhaust gas flow direction.Here, for the catalyst coating layer, the upstream portion is formed in a predetermined range from an upstream end surface in the exhaust flow direction. In the exhaust gas purifying catalyst of the present invention, since the upstream portion is formed on a part of the downstream portion, the upstream portion is not formed up to a downstream end surface. That is, a layer width of the upstream portion is a length in a range of less than 100% of the total length of the substrate from the upstream end surface. On the other hand, the downstream portion need only be formed at least from the downstream end surface, and may be formed over the entire length of the substrate. That is, a layer width of the downstream portion is a length in a range of 100% or less of the total length of the substrate from the downstream end surface. When the downstream portion is formed over the entire length of the substrate, the entire upstream portion is formed on a part of the downstream portion.FIG. 2 illustrates a second embodiment of the exhaust gas purification catalyst of the present invention. As illustrated in FIG. 2, in an exhaust gas purification catalyst 20, a catalyst coating layer 24 includes an upstream portion 22 and a downstream portion 23, and the entire upstream portion 22 is formed on a part of the downstream portion 23. In the exhaust gas purifying catalyst 20, the downstream portion 23 is formed as a lower layer on a substrate 21, and the upstream portion 22 is formed as an upper layer on a part of the downstream portion 23. In FIG. 2, an arrow shows the exhaust gas flow direction.The layer width of the upstream portion of the catalyst coating layer is a length in a 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 layer width of the upstream portion may be a length in a range of 40% or less, or 30% or less.The layer width of the downstream portion of the catalyst coating layer is a length in a range of 100% or less of the total length of the substrate from the downstream end surface as described above, and the length in a range of 90% or less and 80% or less, respectively, in some embodiments. For example, the layer width of the downstream portion may be the length in a range of 60% or less, or 40% or less. The layer width of the downstream portion of the catalyst coating layer is the length in a range of 60% or more to 100% or less of the total length of the substrate from the downstream end surface in some embodiments.In the catalyst coating layer of the exhaust gas purifying catalyst of the present invention, a part or the whole of the upstream portion overlaps a part of the downstream portion. A width of the portion where the upstream portion overlaps the downstream portion is a length in a 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.The substrate used for the exhaust gas purifying catalyst of the present invention is not particularly limited, and a generally used material in a honeycomb shape having a plurality of cells may be used. The material of the substrate includes a ceramic material having heat resistance such as cordierite (2MgO·2Al 2 O 3 ·5SiO 2), alumina, zirconia, and silicon carbide, and a metal material made of a metal foil such as stainless steel. From a cost standpoint, cordierite is used in some embodiments.The upstream portion of the catalyst coating layer contains rhodium (Rh) fine particles (hereinafter also referred to as particle size-controlled Rh fine particles) whose mean particle size and standard deviation σ of the particle size are controlled to specific ranges, as a catalyst metal. Due to the relatively small average particle size, the particle size controlled Rh fine particles have a significantly large specific surface area and thus a high catalytic activity. Since the particle size-controlled Rh fine particles have a narrow particle size distribution and small amounts of coarse particles and fine particles, the particle size-controlled Rh fine particles have high durability and high catalytic activity. By using the particle size controlled Rh fine particles to the upstream portion of the catalyst coating layer, the NOx purification ability in the upstream portion through which the exhaust gas first passes is improved, and Rh can be effectively utilized for the NOx purification even in the downstream portion through which the exhaust gas subsequently passes, thereby ensuring reduction in consumption of Rh in the exhaust gas purification catalyst.The particle size-controlled Rh fine particles have the average particle size of 1.0 nm or more to 2.0 nm or less. In the present invention, the average particle size of the particle size-controlled Rh fine particles is a number-average particle size obtained by directly measuring the equivalent diameters of the projected area on the basis of an image taken in observation with a transmission electron microscope and analyzing particle groups having a cardinal number of 100 or more.By controlling the average particle size of the particle size-controlled Rh fine particles to 1.0 nm or more, the proportion of fine particles having a particle size of less than 1.0 nm, which are considered to be aggregated to cause coarsening during a catalytic reaction, can be reduced. Thereby, the decomposition of the Rh fine particles can be suppressed, thereby ensuring the improved catalyst durability. Meanwhile, by controlling the average particle size of the particle size-controlled Rh fine particles to 2.0 nm or less, the surface areas of the Rh fine particles can be increased, thereby ensuring the improved catalytic activity. The average particle size of the particle size-controlled Rh fine particles is 1.1 nm or more in some embodiments, but may be 1.2 nm or more. The average particle size of the particle size-controlled Rh fine particles is 1.9 nm or less, or may be 1.8 nm or less or 1.6 nm or less, in some embodiments. The average particle size of the particle size-controlled 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.The particle size-controlled Rh fine particles have a standard deviation σ of the particle size measured by observation with a transmission electron microscope of 0.8 nm or less. Since the particle size-controlled Rh fine particles have a standard deviation σ of the particle size of 0.8 nm or less, the particle size distribution is sharp, and the proportions of the fine particles and the coarse particles are small. Due to the small number of the fine particles, aggregation of the Rh fine particles during the catalytic reaction is suppressed, thereby suppressing decomposition of Rh and improving catalyst durability. The small number of the coarse particles increases the surface area of the Rh fine particles, thereby improving the catalytic activity.The standard deviation σ of the particle size controlled particle size of the Rh fine particles is 0.7 nm or less, or may be 0.6 nm or less, or 0.5 nm or less, in some embodiments. While the particle sizes of the particle size-controlled Rh fine particles may be monodisperse, the effects of the invention may be exhibited even when the standard deviation σ is 0.2 nm or more, 0.3 nm or more, or 0.4 nm or more.In the particle size-controlled Rh fine particles, the proportion of the fine particles having the particle size of less than 1.0 nm is particularly reduced. Because of the small amount of the fine particles having the particle size of less than 1.0 nm, aggregation of the Rh fine particles during the catalytic reaction is suppressed, thereby suppressing decomposition of Rh and improving catalyst durability. In the particle size-controlled Rh fine particles, the proportion of the Rh fine particles having a particle size of less than 1.0 nm is 5% by mass or less based on the total weight of the Rh fine particles in some embodiments. This value may be 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less. The Rh fine particles having a particle size of less than 1.0 nm need not be contained at all.In some embodiments, the particle size-controlled Rh fine particles have the average particle size of 1.2 nm or more to 1.8 nm or less when measured by observation with a transmission electron microscope, and the proportion of the Rh fine particles having a particle size of less than 1.0 nm is 5.0% by mass or less based on the total weight of the Rh fine particles.The content of the particle size-controlled Rh fine particles in the upstream portion of the catalyst coating layer is 0.01 g / L or more to 0.5 g / L or less based on the volume of the substrate in some embodiments, or may be 0.05 g / L or more to 0.2 g / L or less. When the content of the particle size-controlled Rh fine particles in the upstream portion is 0.01 g / L or more to 0.5 g / L or less, the high NOxpurifying ability and the high low temperature activity of the catalyst can be ensured simultaneously.The content of the particle size-controlled Rh fine particles in the upstream portion of the catalyst coating layer is 1.0% by mass or more to 45% by mass or less based on the total content of the particle size-controlled Rh fine particles in the upstream portion and Rh in the downstream portion in some embodiments, or may be 1.0% by mass or more to 40% by mass or less. When the content of the particle size-controlled Rh fine particles in the upstream portion is 1.0% by mass or more to 45% by mass or less, the high NOxpurifying ability and the high low temperature activity of the catalyst can be ensured at the same time.The upstream portion of the catalyst coating layer contains platinum (Pt) as a catalyst metal in addition to the particle size-controlled Rh fine particles. The upstream portion of the catalyst coating layer containing Pt provides sufficient conversion of HC in the exhaust gas in the upstream portion and ensures the conversion of NOx in the state of suppressing HC poisoning in the downstream portion containing Rh.The content of Pt in the upstream portion of the catalyst coating layer is 0.01 g / L or more to 10 g / L or less based on the volume of the substrate in some embodiments, or may be 0.1 g / L or more to 5 g / L or less. When the content of Pt in the upstream portion is 0.01 g / L or more to 10 g / L or less, the HC in the exhaust gas can be sufficiently converted.The upstream portion of the catalyst coating layer may contain a catalyst metal other than Rh or Pt. As the catalyst metal, a platinum group noble metal such as ruthenium (Ru), palladium (Pd), osmium (Os), and iridium (Ir) can be used.The upstream portion of the catalyst coating layer contains an OSC material that has oxygen storage capacity in some embodiments. The OSC material is an inorganic material having the 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, thereby absorbing and reducing variations in the exhaust atmosphere to ensure that the exhaust atmosphere is maintained near a stoichiometric air-fuel ratio. A three-way catalyst most efficiently purify harmful components such as CO, HC and NOx in the vicinity of the stoichiometric air-fuel ratio. Accordingly, the upstream portion of the catalyst coating layer containing the OSC material provides for efficient cleaning of the noxious components in the upstream portion through which the exhaust gas first flows.The OSC material is not particularly limited, and includes cerium oxide (cerium oxide: CeO 2), a mixed oxide containing the cerium oxide (e.g., cerium oxide-zirconium dioxide (ZrO 2)- mixed oxide (CZ mixed oxide or ZC mixed oxide)), and the like. Among the above-described OSC materials, ceria-zirconia (CeO 2- ZrO 2)- composite oxide is used in some embodiments because of the high oxygen storage capacity and the relatively low price. The ceria-zirconia composite oxide may contain an oxide of a metallic element other than Ce or Zr. The metallic element other than Ce or Zr is a rare earth element (note that Ce is excluded) in some embodiments. The rare earth element may include yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), erbium (Er), ytterbium (Yb), lutetium (Lu), and the like. Among them, in some embodiments, the rare earth element is one or more selected from Y, La, Pr, Nd and Eu, or it may be Y and La. The ceria-zirconia mixed oxide is used in the form of a mixed oxide including lanthanum (La 2 O 3) and yttrium oxide (Y 2 O 3) in some embodiments. The mixing ratio of ceria to zirconia in the ceria-zirconia mixed oxide is CeO 2 / ZrO 2= 0,2 or more to 9.0 or less by weight, in some embodiments.The content of the OSC material in the upstream portion 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. When the content of the OSC material in the upstream portion is 10 g / L or more to 80 g / L or less, the high NOxpurifying ability is ensured in the upstream portion.The upstream portion of the catalyst coating layer may contain any other constituent besides the catalyst metal and the OSC material. The other component is not particularly limited, and includes a metal oxide and the like. When the upstream portion of the catalyst coating layer contains the other component, its content is 80 g / L or less based on the volume of the substrate, in some embodiments, or may be 60 g / L or less.A metal contained in the metal oxide includes, for example, one or more metals selected from Group 3, Group 4, and Group 13 of the periodic table, and a lanthanide-based metal. When the metal oxide contains the oxides of two or more metals, it may be a mixture of two or more metal oxides, a mixed oxide containing two or more metals, or a mixture of one or more metal oxides and one or more mixed oxides.The metal oxide may be the oxide of one or more metals selected 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 the oxide of one or more metals selected from Y, La, Ce, Ti, Zr and Al, in some embodiments. As the metal oxide, aluminum oxide (Al 2 O 3) or a mixed oxide of Al 2 O 3 and lanthanum oxide (La 2 O 3) is used in some embodiments.In the upstream portion of the catalyst coating layer, the particle size-controlled Rh fine particles and Pt are supported on support particles in some embodiments. The carrier particles are not particularly limited, and for example, the above-described OSC materials and the other metal oxides may be used. In some embodiments, the particle size controlled Rh fine particles are supported on the OSC materials. In other embodiments, Pt may be supported on the alumina (Al 2 O 3) or the mixed oxide of Al 2 O 3 and lanthanum (La 2 O 3). As the support method, a usual support method such as an impregnation support method, an adsorption support method and a water absorption support method can be used.When the particle size-controlled Rh fine particles supported on the support particles are used, the support mass ratio of the particle size-controlled 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 weight of the support particles. The content of the carrier mass of the particle size-controlled 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 weight of the carrier particles.When using the particle size-controlled Rh fine particles supported on the support particle, the particle size-controlled Rh fine particles can be supported on the support particle by bringing the support particles into contact with a Rh fine particle precursor dispersion containing Rh fine particle precursors preliminarily controlled to a predetermined particle size distribution and then carrying out calcination.The Rh fine particle precursor dispersion can be prepared, for example, by any of the following methods.(1) a method (Method 1) that reacts an acidic solution of an Rh compound with a basic solution in a reactor in which a distance of a reactive field (clearance of a reactive field) is set to a predetermined range; and(2) a method (Method 2) in which the acidic solution of the Rh compound is mixed with the basic solution for reaction, and then a stirring treatment is performed in a high-speed mixer.In Method 1, by using the reactor in which the reaction field distance is set to the predetermined range, when 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 nitrogen-containing organic compound), the particle size and the particle size distribution of the Rh fine particle precursors (e.g., hydroxides of Rh) contained in the obtained dispersion can be controlled.The clearance adjustment members (memories) contained in the reactor may be two flat plates, a combination of a flat plate and a corrugated plate, narrow tubes, and the like. The distance of the reaction field can be adjusted appropriately according to a desired particle size and a desired particle size distribution. As a reactor in which the reaction field distance is set to the predetermined range, for example, a microreactor having a corresponding distance regulating member can be used.In Method 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 nitrogen-containing organic compound) to produce the Rh fine particle precursors as large particle size particles, the stirring treatment of the Rh fine particle precursors is performed in the high-speed mixer, and a strong shear force is applied to disperse the Rh fine particle precursors, thereby controlling the average particle size and the particle size distribution of the dispersed Rh fine particle precursors.The particle size-controlled Rh fine particles can be supported on the support particle by bringing the Rh fine particle precursor dispersion prepared as described above into contact with the support particles and then carrying out the calcination.Meanwhile, Pt supported on the support particles is used, and the support mass ratio of Pt is, for example, 10% or less, 5% or less, 3% or less, or 2% or less based on the weight of the support particles. The content of Pt by mass 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, or 0.5 % or more, based on the weight of the carrier particles.The downstream portion of the catalyst coating layer contains rhodium (Rh) as a catalyst metal. In the exhaust gas purification catalyst of the present invention, since a part of the harmful component in the exhaust gas in the upstream portion containing the particle size-controlled Rh fine particles and Pt is efficiently purified, the use of Rh in the downstream portion can be reduced. In particular, since NOx in the exhaust gas having the small use amount of Rh in the upstream portion is efficiently purified by the use of the particle size-controlled Rh fine particles in the upstream portion, the consumption of Rh in the downstream portion can also be reduced.In the downstream portion of the catalyst coating layer, Rh in the form of Rh fine particles is used in some embodiments. The average particle size of the Rh fine particles is not particularly limited, and is typically 0.5 nm or more to 5.0 nm or less, or 1.0 nm or more to 2.0 nm or less in some embodiments.As Rh in the downstream portion of the catalyst coating layer, the above-described particle size-controlled Rh fine particles are used in some embodiments. The use of the particle size controlled Rh fine particles in the downstream portion of the catalyst coating layer ensures further improvement in the NOxpurifying ability of the catalyst. Accordingly, in some embodiments, the upstream portion of the catalyst coating layer contains the above-described particle size-controlled Rh fine particles and Pt, and the downstream portion of the catalyst coating layer contains the above-described particle size-controlled Rh fine particles.The content of Rh in the downstream portion of the catalyst coating layer is 0.01 g / L or more to 1.0 g / L or less based on the volume of the concentrate, in some embodiments, or may be 0.1 g / L or more to 0.4 g / L or less. When the Rh content in the downstream portion is 0.01 g / L or more to 1.0 g / L or less, the high NOxpurifying ability and the high low temperature activity of the catalyst can be ensured simultaneously.The total content of the particle size-controlled Rh fine particles in the upstream portion and Rh in the downstream portion of the catalyst coating layer is 0.01 g / L or more to 1.0 g / L or less based on the volume of the substrate in some embodiments, or may be 0.1 g / L or more to 0.5 g / L or less.The downstream portion of the catalyst coating layer may contain a catalyst metal other than Rh. As the catalyst metal, a platinum group noble metal such as ruthenium (Ru), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pr) can be used.The downstream portion of the catalyst coating layer contains an OSC material in some embodiments. The downstream portion including the OSC material ensures the high NOx purification capability in the downstream portion.The OSC material is not particularly limited, and includes cerium oxide (cerium oxide: CeO 2), a mixed oxide containing the cerium oxide (e.g., cerium oxide-zirconium dioxide (ZrO 2)- mixed oxide (CZ mixed oxide or ZC mixed oxide)), and the like. Among the above-described OSC materials, cerium oxide-zirconia (CeO 2- ZrO 2)- composite oxide is used in some embodiments because of the high oxygen storage capacity and the relatively low price. The ceria-zirconia composite oxide may contain an oxide of a metallic element other than Ce or Zr. The metallic element other than Ce or Zr is a rare earth element (note that Ce is excluded) in some embodiments. The rare earth element may include yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), erbium (Er), ytterbium (Yb), lutetium (Lu), and the like. Among these, in some embodiments, the rare earth element is one or more selected from Y, La, Pr, Nd and Eu, or may be La and Nd. The ceria-zirconia mixed oxide is used in the form of a mixed oxide including lanthanum oxide (La 2 O 3) and neodymium oxide (Nd 2 O 3) in some embodiments. The mixing ratio of ceria to zirconia in the ceria-zirconia mixed oxide is CeO 2 / ZrO 2= 0,2 or more to 9.0 or less by weight in some embodiments.The content of the OSC material in the downstream portion of the catalyst coating layer is 1 g / L or more to 80 g / L or less based on the volume of the substrate in some embodiments, or may be 5 g / L or more to 50 g / L or less. When the content of the OSC material in the downstream portion of the catalyst coating layer is 1 g / L or more to 80 g / L or less, the high NOxpurifying ability in the downstream portion is ensured.The downstream portion of the catalyst coating layer may contain any other ingredient besides the catalyst metal and the OSC material. The other component is not particularly limited and includes a metal oxide and the like. When the downstream portion of the catalyst coating layer contains the other component, its content is 100 g / L or less based on the volume of the substrate in some embodiments, or may be 80 g / L or less.A metal contained in the metal oxide includes, for example, one or more metals selected from Group 3, Group 4, and Group 13 of the periodic table, and a lanthanide-based metal. When the metal oxide contains the oxides of two or more metals, it may be a mixture of two or more metal oxides, a mixed oxide containing two or more metals, or a mixture of one or more metal oxides and one or more mixed oxides.The metal oxides may be the oxides of one or more metals selected from, for example, 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. As the metal oxide, aluminum oxide (Al 2 O 3), a mixed oxide of Al 2 O 3 and lanthanum oxide (La 2 O 3) or a mixed oxide of yttrium oxide (Y 2 O 3), lanthanum oxide (La 2 O 3) and zirconium dioxide (ZrO2) is used in some embodiments.In the downstream portion of the catalyst coating layer, Rh is supported on support particles in some embodiments. The carrier particles are not particularly limited, and for example, the above-described OSC materials and the other metal oxides may be used. In some embodiments, Rh is supported on the metal oxides other than the OSC materials in the downstream portion of the catalyst coating layer. As the support method, a usual support method such as an impregnation support method, an adsorption support method and a water absorption support method can be used.When Rh supported on the support particles is used, the support mass ratio of Rh 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 weight of the support particles. The content of Rh as a carrier mass 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 weight of the carrier particles.The exhaust gas purifying catalyst of the present invention can be prepared 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 region is coated with a slurry containing Rh, an OSC material, and a metal oxide from a downstream end surface according to a known method, and drying and calcining are performed at a predetermined temperature for a predetermined time period, thereby forming a downstream portion of a catalyst coating layer on a substrate. Subsequently, a predetermined region is coated with a slurry containing the particle size-controlled Rh fine particles, Pt, an OSC material, and a metal oxide from an upstream end surface of the substrate using a known method, and drying and calcining are performed at a predetermined temperature for a predetermined time period, thereby forming an upstream portion.ExamplesThe present invention will be described in more detail below by way of examples. However, the technical scope of the present invention is not limited to examples.<Production of Catalysts>Raw Material UsedMaterial 1: Al 2 O 3: 4 % mass fraction-La 2 O 3 composite or composite material Al 2 O 3Material 2: ZC (OSC material): 21% by mass of CeO 2, 72 % by mass of ZrO 2, 1,7 % by mass of La 2 O 3, 5,3 % by mass of Nd 2 O 3- mixed oxideMaterial 3: ZC (OSC material): 30% by mass of CeO 2, 60 % by mass of ZrO 2, 5 % by mass of La 2 O 3, 5 % by mass of Y 2 O 3 mixed oxideMaterial 4: 6% by mass fraction of La 2 O 3, 10% by mass fraction of Y 2 O 3 composite ZrO 2Material 5: Pt / Al 2 O 3: Material in which Pt is supported on the material 1Material 6: Particle size controlled Rh DispersionMaterial 7: Rh / ZY: Material in which Rh is supported on the material 4Material 8: Particle size controlled Rh / ZC: Material in which Rh of the material 6 is supported on the material 3Material 9: Rh Dispersion by Beaker MethodMaterial 10: Rh / ZC: Material in which Rh is supported on the material 3Material 11: Particle size controlled Rh / ZY: Material in which Rh of the material 6 is supported on the material 4Material 12: Rh / ZC by beaker method: Material in which the Rh of the material 9 is supported on the material 3Substrate: 875 cc (400 cells square, 4 mil and 4 thousandths of an inch wall thickness) cordierite honeycomb formed substrateMaterials 5 to 12 were prepared as follows.Material 5: Pt / Al2O3A Pt nitrate solution was contacted with the material 1 and then calcined to obtain the material 5 in which Pt was supported on the material 1 in the supporting mass ratio of 3%.Material 6: Particle size controlled Rh Dispersion110 g of Rh (III) nitrate was added to 110 mL of ion-exchanged water and dissolved, thereby preparing an acidic solution (pH 1.0) of the Rh compound.A tetraethylammonium hydroxide aqueous solution (concentration 175 g / L, pH 14) was prepared as an organic base solution.Using a reactor (microreactor) including two flat plates as spacer regulating members, with a method of introducing the acidic solution of the Rh compound and the organic base solution into a reaction field in which the distance was set to 10 μm, both liquids were reacted under a condition in which a molar ratio (TEAH / RN) of tetraethylammonium hydroxide (TEAH) to Rh nitrate (RN) was 18 to prepare a Rh fine particle precursor dispersion. The obtained Rh fine particle precursor dispersion had a pH of 14. the median diameter (D50) of the Rh fine particle precursors contained in the obtained Rh fine particle precursor dispersion was measured by a dynamic light scattering method (DLS), and the median diameter (D50) was 2.0 nm.Material 7: Rh / ZYA Rh nitrate solution was contacted with the material 4 and then calcined to obtain the material 7 in which Rh was supported on the material 4 in the supported mass ratio of 0.9%. The average particle size of the Rh fine particles measured by a transmission electron microscope was 0.70 nm.Material 8: Particle size controlled Rh / ZCThe material 6 was brought into contact with the material 3 and then calcined to obtain the material 8 in which Rh was supported on the material 3 in the supported mass ratio of 0.45%. The average particle size of the Rh fine particles measured by the transmission electron microscope 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 the fine particles having a particle size of less than 1.0 nm was smaller than that of the Rh of the material 12 according to the beaker method.Material 9: Rh Dispersion according to Beaker MethodMaterial 9 was prepared similarly to Material 6, except that the acidic solution of the Rh compound was reacted with the organic base solution in a beaker without using the reactor including the spacer elements.Material 10: Rh / ZCA Rh nitrate solution was contacted with the material 3 and then calcined to obtain the material 10 in which Rh was supported on the material 3 in the supported mass ratio of 0.45%. The average particle size of the Rh fine particles measured by a transmission electron microscope was 0.70 nm.Material 11: Particle size controlled Rh / ZYThe material 6 was brought into contact with the material 4 and then calcined to obtain the material 11 in which Rh was supported on the material 4 in the supported mass ratio of 0.9%. The average particle size of the Rh fine particles measured by the transmission electron microscope was 1.40 nm, and the standard deviation σ of the particle size was 0.48 nm.Material 12: Rh / ZC according to beaker methodMaterial 12 in which Rh of material 9 was supported on material 3 was prepared similarly to Material 8 preparation. the average particle size of the Rh fine particles measured by the transmission electron microscope was 1.42 nm, and the standard deviation σ of the particle size was 0.94 nm.Example 1The material 1, the material 2, the material 7, and an Al 2 O 3- based binder were placed in distilled water with stirring, and a slurry 1 in which these materials were suspended was prepared. Subsequently, the prepared slurry 1 was poured into a substrate from a downstream end surface, and an unnecessary portion was blown off with a blower, thereby coating a surface of the substrate with the materials. The layer width was set to 80% of the total length of the substrate. The coating amount was adjusted so that the material 1 was 25 g / L, the material 2 was 15 g / L, and the material 7 was 50 g / L, based on the volume of the substrate. Finally, it was dried at 120° C. for two hours, and then calcined at 500° C. with an electric furnace for two hours, thereby preparing a downstream portion of a catalyst coating layer.Similarly, the Al 2 O 3- based material 5, material 8 and binder were placed in distilled water with stirring, and a slurry 2 in which these materials were suspended was prepared. The slurry 2 was poured into the substrate on which the downstream portion was formed from an upstream end surface, and an unnecessary portion was blown off by the blower, thereby coating the materials on the surface of the substrate. The layer width was set to 40% of the total length of the substrate from the upstream end surface. The coating amount was adjusted so that the material 5 was 30 g / L and the material 8 was 40 g / L based on the volume of the substrate. Finally, drying was carried out at 120° C. for two hours in the dryer, and then calcining was carried out at 500° C. in the electric furnace for two hours, thereby preparing an upstream portion of the catalyst coating layer.Examples 2 and 3The catalysts of Examples 2 and 3 were prepared similarly to that of Example 1, except that the Rh amounts of Slurries 1 and 2 were changed, respectively, as illustrated in Table 1.Comparative Example 1The catalyst of Comparative Example 1 was prepared similarly to that of Example 1, except that the material 8 in the upstream portion was replaced with the material 3 and the amount of Rh on the slurry 2 was changed as illustrated in Table 1.Comparative Example 2The catalyst of Comparative Example 2 was prepared similarly to that of Comparative Example 1, except that the material 7 on the downstream portion was replaced with the material 11.Comparative Example 3The catalyst of Comparative Example 3 was prepared similarly to that of Example 1, except that the material 8 on the upstream portion was replaced with the material 10.Comparative Example 4The catalyst of Comparative Example 4 was prepared similarly to that of Example 1, except that the material 8 on the upstream portion was replaced with the material 12.Comparative Examples 5 and 6The catalysts of Comparative Examples 5 and 6 were prepared similarly to those of Comparative Example 3 except that the Rh amounts of Slurries 1 and 2 were changed, respectively, as illustrated in Table 1.Table 1 shows compositions and amounts of noble metals in the upstream portion and the downstream portion of the catalysts of Examples 1 to 3 and Comparative Examples 1 to 6, and the amount of noble metal is an amount of noble metal (g / substrate 1L) to the volume of the substrate. [Table 1] Table 1] [Table 1] Table 1]Example 1Material 5 Material 8Pt / Al 2 O 3 Particle size controlled Rh / ZCPt 1.0 Rh 0,09Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,36Example 2Material 5 Material 8Pt / Al 2 O 3 Particle size controlled Rh / ZCPt 1.0 Rh 0,15Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,30Example 3Material 5 Material 8Pt / Al 2 O 3 Particle size controlled Rh / ZCPt 1.0 Rh 0,19Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,26Comparative Example 1Material 5 Material 3Pt / Al 2 O 3 ZCPt 1, 1,0Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,45Comparative Example 2Material 5 Material 3Pt / Al 2 O 3 ZCPt 1, 1,0Material 1 Material 2 Material 11Al 2 O 3 ZC Particle Size Controlled Rh / ZYRh 0, Rh 0,45Comparative Example 3Material 5 Material 10Pt / Al 2 O 3 Rh / ZCPt 1.0 Rh 0,09Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,36Comparative Example 4Material 5 Material 12Pt / Al 2 O 3 Rh / ZC according to beaker methodsPt 1.0 Rh 0,09Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,36Comparative Example 5Material 5 Material 10Pt / Al 2 O 3 Rh / ZCPt 1.0 Rh 0,15Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,30Comparative Example 6Material 5 Material 10Pt / Al 2 O 3 Rh / ZCPt 1.0 Rh 0,19Material 1 Material 2 Material 7Al 2 O 3 ZC Rh / ZYRh 0, Rh 0,26< Test>The durability test was performed for each of the prepared catalysts using a real motor. Specifically, the durability test was performed as follows. The catalysts were each incorporated into an exhaust system of a V-type eight-cylinder engine, and the exhaust gases in the respective stoichiometric and lean atmospheres were repeatedly flown for a certain period of time (in the ratio of 3:1) at a catalyst bed temperature of 900° C. for 46 hours.< Rating>NOx conversion rateThe exhaust gases having the air-fuel ratios (A / F) 14.4 and 14.8 were alternately supplied at a catalyst inlet gas temperature of 350° C., and the NOx conversion rate at Ga=28 g / s was evaluated. The NOx conversion rate was calculated by averaging the NOx conversion rates in 60 seconds from a time point after the lapse of two minutes.NOx 50% conversion temperatureFor the catalysts of Examples 1 to 3 and Comparative Example 1, the exhaust gas purification catalysts on which the durability test was performed were each mounted on an exhaust system of an L-type four-cylinder engine, the exhaust gas was supplied with the air-fuel ratio (L / K)=14.4, and the temperature at the catalyst bed was raised from 200° C. to 600° C. (20° C. / minute) under the condition Ga=35 g / s. The temperature (NOx50% conversion temperature) at which the NOx conversion rate was 50% was measured, and the low temperature activity was evaluated. The lower this value, the higher the low temperature activity.Table 2 shows the Rh amounts in the upstream portion and the downstream portion, and information on the Rh fine particles of the catalysts of Example 1 and Comparative Examples 1 to 4. FIG. 3 illustrates the NOx conversion rate in the repeated atmosphere of rich and lean for the catalysts of Example 1 and Comparative Examples 1 to 4. [Table 2] [Table 2]Example 10,090,36Material 8Particle size controlled Rh1,400,48Upstream PortionComparative Example 100.45Material 7Rh nitrate0,70-Downstream PortionComparative Example 200,45Material 11Particle size controlled Rh1,400,48Downstream PortionComparative Example 30,090,36Material 10Rh nitrate0,70-Upstream PortionComparative Example 40,090,36Material 12Rh according to beaker method1,420,94Upstream PortionAs shown in FIG. 3, in all the catalysts in which the particle size-controlled Rh fine particles and the Rh fine particles whose average particle size was outside the specific range of the present invention, by adding a part of the Rh fine particles in the downstream portion to the upstream portion, the NOx conversion rate tended to increase compared to the catalysts in which the Rh fine particles were added only to the downstream portion (Example 1 and Comparative Examples 1 to 3). The range of increase in the NOx conversion rate was larger when the particle size-controlled Rh fine particles were used than when the Rh fine particles whose average particle size was outside the specific range of the present invention were used. Accordingly, it was shown that the effect of improving the NOxpurifying ability of the catalyst of the example was a specific effect obtained by controlling the particle size and the addition position of the Rh fine particles. Moreover, it was shown that the NOxpurifying ability was improved not only by controlling the average particle size of the Rh fine particles but also by controlling the standard deviation σ of the particle size to the predetermined range for reducing the fine particles (Example 1 and Comparative Example 4).FIG. 4 illustrates a relationship between a ratio of the Rh content in the upstream portion to the total content of the Rh in the upstream portion and the downstream portion, and the NOx conversion rate and the NOx 50% conversion temperature, respectively, in the rich and lean repeated atmosphere. In FIG. 4, the NOx conversion rate is indicated by the measurement results of Examples 1 to 3 and Comparative Examples 1, 3, 5, and 6, and the NOx 50% conversion temperature is indicated by the measurement results of Examples 1 to 3 and Comparative Example 1. As shown in FIG. 4, as the Rh content in the upstream portion increased, the NOx conversion rate in each of the catalysts of Examples and the catalysts of Comparative Examples increased. The NOx conversion rate of the catalysts of the examples in which the particle size controlled Rh fine particles were used was significantly higher than that of the catalysts of the comparative examples. However, when the proportion of the Rh content in the upstream portion increased to about 40%, the increase range of the NOx conversion rate tends to decrease in the catalysts of the examples compared with the catalysts of the comparative examples. As the proportion of the Rh content in the upstream portion increased and the proportion of the Rh content in the downstream portion decreased, the NOx 50% conversion temperature tends to increase to reduce the low temperature activity of the catalyst. Accordingly, the content of the particle size-controlled Rh fine particles in the upstream portion has an intended range from the viewpoint of simultaneously providing the high NOxpurifying ability and the high low temperature activity, and may be in a range of 1.0% by mass or more to 45% by mass or less.DESCRIPTION OF THE CHARACTERS10 Exhaust gas purification catalyst 11 Substrate 12 Upstream portion 13 Downstream portion 14 Catalyst coating layer 20 Exhaust gas purification catalyst 21 Substrate 22 Upstream portion 23 Downstream portion 24 Catalyst coating layer

Claims

An exhaust gas purification catalyst (10, 20) comprising: a substrate (11, 21); and a catalyst coating layer (14, 24) formed on the substrate (11, 21), the catalyst coating layer (14, 24) having a two-layer structure, the catalyst coating layer (14, 24) including an upstream portion (12, 22) on an upstream side and a downstream portion (13, 23) on a downstream side in an exhaust flow direction, and a part or the whole of the upstream portion (12, 22) being formed on a part of the downstream portion (13, 23), the upstream portion (12, 22) containing Rh fine particles and Pt, the Rh fine particles having an average particle size 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, the downstream portion (13, 23) containing Rh.The exhaust gas purification catalyst (10, 20) according to claim 1, wherein a content of the Rh fine particles in the upstream portion (12, 22) is a mass proportion of 1.0% or more to 45% or less based on a total content of the Rh fine particles in the upstream portion (12, 22) and Rh in the downstream portion (13, 23).The exhaust gas purification catalyst (10, 20) according to claim 1 or 2, wherein the upstream portion (12, 22) further contains an OSC material having an oxygen storage capacity.

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