Solid solution nanoparticles, method for producing the same, dispersion of solid solution nanoparticles, and catalyst

Abstract

The solid solution nanoparticles according to the present invention have a compositional makeup represented by formula PtxM1yM21-x-y (0<x<1, 0<y<1, x+y<1). M1 represents Ru or Ir. When M1 represents Ru, M2 represents at least one selected from the group consisting of Ir, Rh, Ag, Cu, and Au. When M1 represents Ir, M2 represents at least one selected from the group consisting of Rh, Pd, Ag, Cu, and Au. Pt, M1, and M2 form a solid solution.

Description

Technical Field

[0001] The present invention relates to solid solution nanoparticles, a method for producing the same, a dispersion of the solid solution nanoparticles, and a catalyst.

Background Art

[0002] Noble metal nanoparticles such as Pt nanoparticles are known to exhibit optical properties, electrical properties, and chemical properties different from those of the bulk material, and are used in various fields as electronic materials, magnetic materials, catalyst materials, pharmaceutical materials, cosmetic materials, or food material.

[0003] As described in Patent Documents 1 to 4, the catalyst is one of the most well-known uses of noble metal nanoparticles.

[0004] Patent Document 5 describes the use of noble metal nanoparticles as an antioxidant.

[0005] Patent Document 6 describes the use of noble metal nanoparticles as a material for a contrast agent.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0007] Among noble metal nanoparticles, Pt nanoparticles are highly useful because they exhibit various effects. In order to improve the desired effects, alloying a noble metal other than Pt with Pt can be considered.

[0008] However, except for specific combinations such as PtRh, many combinations of noble metal elements do not show a complete complete solid solution type alloy phase diagram. For example, PtIr forms a complete solid solution at high temperatures but is immiscible in a wide composition range below 1370 °C. Similarly, PtRu and IrRu also have an immiscible composition range of about 15 atom% in the entire temperature range. Therefore, even if noble metal nanoparticles are synthesized by a conventional reduction method using a solution containing a plurality of noble metal salts, only a mixture of single noble metal nanoparticles can be obtained.

[0009] If it is possible to obtain solid solution type metal nanoparticles that cannot be predicted from the alloy phase diagram, it is possible to expect an improvement in the desired effects in the existing uses of metal nanoparticles. The manifestation of new physical properties and the expansion of the uses of metal nanoparticles based on the new physical properties can also be expected.

[0010] An object of the present invention is to provide novel solid solution nanoparticles, a method for producing the same, a dispersion of the solid solution nanoparticles, and a catalyst using the solid solution nanoparticles.

Means for Solving the Problems

[0011] The present invention is Formula Pt x M1 y M2 1-x-y (0 < x < 1, 0 < y < 1, x + y < 1), having a composition represented by M1 is Ru or Ir, When M1 is Ru, M2 is at least one selected from the group consisting of Ir, Rh, Ag, Cu, and Au, When M1 is Ir, M2 is at least one selected from the group consisting of Rh, Pd, Ag, Cu, and Au, Pt, M1, and M2 form a solid solution. Provided are solid solution nanoparticles.

[0012] In another aspect, the present invention provides a catalyst comprising the solid solution nanoparticles of the present invention.

[0013] In yet another aspect, the present invention provides a solvent, the solid solution nanoparticles of the present invention dispersed in the solvent, and provides a dispersion liquid of the solid solution nanoparticles comprising the same.

[0014] In yet another aspect, the present invention provides a method for producing solid solution nanoparticles having a composition represented by the formula Pt x M1 y M2 1-x-y (0 < x < 1, 0 < y < 1, x + y < 1), the method comprising adding a solution containing a Pt salt, a salt of M1, and a salt of M2 to a liquid reducing agent heated to a temperature in the range of not less than 150°C and not more than 250°C and reacting them, wherein the salt of M1 is a Ru salt or an Ir salt, when the salt of M1 is a Ru salt, the salt of M2 contains at least one selected from the group consisting of an Ir salt, a Rh salt, an Ag salt, a Cu salt, and an Au salt, when the salt of M1 is an Ir salt, the salt of M2 contains at least one selected from the group consisting of a Rh salt, a Pd salt, an Ag salt, a Cu salt, and an Au salt, and provides a method for producing solid solution nanoparticles.

Advantages of the Invention

[0015] According to the present invention, novel solid solution nanoparticles, a method for producing the same, a dispersion liquid of the solid solution nanoparticles, and a catalyst using the solid solution nanoparticles can be provided.

Brief Description of the Drawings

[0016]

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Mode for Carrying Out the Invention

[0017] Conventionally, as a means for adjusting the catalytic activity of Pt, it has been known to support Pt on a carrier. By this, there are advantages that the specific surface area of Pt can be increased, or the catalytic activity can be changed by the interaction between Pt and the carrier.

[0018] Patent Documents 1 to 4 disclose catalysts using a plurality of noble metals. These catalysts are produced by impregnating a carrier with an aqueous solution containing a plurality of noble metal salts and then firing the carrier in air under the conditions of 550 to 600 °C for 3 to 6 hours. This method is called the impregnation method. According to the impregnation method, it is presumed that the plurality of noble metals constituting the catalyst exist in the state of single noble metal particles without alloying, or form noble metal particles with a non-uniform composition with variations. Therefore, the catalysts produced by the impregnation method cannot fully exhibit the effects of alloying.

[0019] For example, when the main component of hydrocarbons in the exhaust gas is methane, such as in the combustion exhaust gas of natural gas, it is not easy to sufficiently progress the oxidative decomposition of hydrocarbons (removal of methane) because methane has high chemical stability. In addition, there is also a problem that the activity of the catalyst decreases over time due to the deposition of reaction inhibitors such as sulfur oxides (SOx) derived from sulfur compounds contained in the fuel on the surface of the catalyst.

[0020] The present inventors have found that it is possible to produce solid solution nanoparticles in which metals that do not normally form a solid solution are solid-solved, and that such solid solution nanoparticles can be used as a novel catalyst. The present invention is based on this new finding.

[0021] Specifically, the present invention reveals that by mixing and alloying various metals with Pt, it is possible to adjust the catalytic activity of Pt at the nanoscale, which has been difficult in the past. This indicates new industrial applicability of Pt as a catalyst. The following embodiments show one aspect of Pt in which such catalytic activity is adjusted at the nano level.

[0022] (Embodiment) Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments.

[0023] The solid solution nanoparticles of the present embodiment have a composition represented by the following formula (1).

[0024] Pt x M1 y M2 1-x-y ···(1)

[0025] In formula (1), x and y satisfy 0 < x < 1, 0 < y < 1, and x + y < 1. M1 is Ru or Ir. When M1 is Ru, M2 is at least one selected from the group consisting of Ir, Rh, Ag, Cu, and Au. When M1 is Ir, M2 is at least one selected from the group consisting of Rh, Pd, Ag, Cu, and Au. Pt, M1, and M2 form a solid solution. In other words, Pt, M1, and M2 are mutually dissolved at the atomic level. The solid solution nanoparticles contain regions where each element is uniformly distributed. Preferably, each element is uniformly distributed throughout the solid solution nanoparticles.

[0026] A solid solution is a form included in the concept of an alloy and means a state in which the constituent elements are mixed at the atomic level. Generally, the term "alloy" means a broad-sense alloy that includes not only solid solutions but also non-solid solution alloys. The local alloy composition is uniform in a solid solution, while it is not uniform in a non-solid solution system. Depending on whether multiple types of metal elements are mixed at the atomic level as a solid solution alloy or simply form a non-solid solution alloy, the physical properties of the alloy are generally different.

[0027] The solid solution nanoparticles of this embodiment can be noble metal nanoparticles. Generally, Cu is not classified as a noble metal, but in this specification, it is treated as a noble metal.

[0028] As a method for confirming that noble metal nanoparticles are a solid solution, there are element mapping by energy-dispersive X-ray analysis (EDX) using a scanning transmission electron microscope (STEM), EDX line analysis, structural analysis by X-ray diffraction (XRD), and the like.

[0029] The average particle size of the solid solution nanoparticles of the present embodiment may be in the range of 0.5 nm or more and 100 nm or less, or may be in the range of 1 nm or more and 10 nm or less. When the average particle size is sufficiently small, the solid solution nanoparticles can exhibit high activity. The average particle size can be calculated from the electron microscope image of the solid solution nanoparticles. In the electron microscope image, the particle sizes (major axes) of a plurality of solid solution nanoparticles (for example, 100) are measured. The average value of the measured particle sizes represents the average particle size of the solid solution nanoparticles.

[0030] In formula (1), x representing the content ratio of Pt satisfies, for example, 0.01 ≦ x ≦ 0.98. In formula (1), y representing the content ratio of M1 satisfies, for example, 0.01 ≦ y ≦ 0.98. In formula (1), (1 - x - y) representing the content ratio of M2 satisfies 0.01 ≦ (1 - x - y) ≦ 0.98. In other words, the respective content ratios of Pt, M1, and M2 are in the range of, for example, 1 mol% or more and 98 mol% or less based on the solid solution nanoparticles (100 mol%). The respective content ratios of Pt, M1, and M2 may be in the range of 5 mol% or more and 90 mol% or less, or may be in the range of 10 mol% or more and 80 mol% or less.

[0031] The solid solution nanoparticles of the present embodiment contain Pt as an essential component. Pt nanoparticles can be used in various applications such as electronic materials, magnetic materials, catalyst materials, pharmaceutical materials, cosmetic materials, and food material. However, Pt is expensive. Therefore, if it is possible to achieve functions and activities equal to or better than those of Pt nanoparticles while reducing the content ratio of Pt, solid solution nanoparticles with excellent economic efficiency can be provided. As will be described later, the solid solution nanoparticles of the present embodiment exhibit catalytic activity exceeding that of Pt nanoparticles. However, the catalytic activity is only one of the functions of the solid solution nanoparticles.

[0032] In formula (1), M1 can be Ru. Ru is less expensive than Pt and is suitable as a substitute element for Pt in solid solution nanoparticles.

[0033] Incidentally, the activity (catalytic activity) of single Ru is usually lower than that of Pt. Therefore, Ru nanoparticles themselves are not likely to be substitutes for Pt nanoparticles.

[0034] In formula (1), M1 may be Ir. Although Ir is an expensive noble metal like Pt, it often shows higher activity than Ru and is suitable as a material for the solid solution nanoparticles of the present embodiment. By including Ir, the solid solution nanoparticles can exhibit higher activity.

[0035] In formula (1), M1 may be Ru and M2 may be Ir. In this case, both the advantages of the combination of Pt and Ir and the advantages of the combination of Pt and Ru can be obtained.

[0036] M2 may consist of one, two, three, four, or five metal elements. When M2 consists of one metal element, the solid solution nanoparticles of the present embodiment are ternary solid solution nanoparticles. That is, in formula (1), when M1 is Ru, M2 is Ir, Rh, Ag, Cu, or Au. When M1 is Ir, M2 is Rh, Pd, Ag, Cu, or Au. Compared with quaternary or quinary systems, the production of ternary solid solution nanoparticles is easier.

[0037] When M1 is Ru and M2 consists of two metal elements, M2 = M a p2 M b q2 (wherein M a and M b are different from each other and are selected from the group consisting of Ir, Rh, Ag, Cu, and Au, and are represented by p2 = 0.01 to 0.99, q2 = 0.99 to 0.01, p2 + q2 = 1).

[0038] When M1 is Ir and M2 consists of two metal elements, M2 = M a p2 M b q2 (wherein M a and M bThey are different from each other and are selected from the group consisting of Rh, Pd, Ag, Cu and Au, and are represented by p2 = 0.01 to 0.99, q2 = 0.99 to 0.01, p2 + q2 = 1).

[0039] When M1 is Ru and M2 consists of three metal elements, M2 = M a p3 M b q3 M c r3 (In the formula, M a , M b and M c are different from each other and are selected from the group consisting of Ir, Rh, Ag, Cu and Au, and are represented by p3 = 0.01 to 0.98, q3 = 0.01 to 0.98, r3 = 0.01 to 0.98, p3 + q3 + r3 = 1).

[0040] When M1 is Ir and M2 consists of three metal elements, M2 = M a p3 M b q3 M c r3 (In the formula, M a , M b and M c are different from each other and are selected from the group consisting of Rh, Pd, Ag, Cu and Au, and are represented by p3 = 0.01 to 0.98, q3 = 0.01 to 0.98, r3 = 0.01 to 0.98, p3 + q3 + r3 = 1).

[0041] When M1 is Ru and M2 consists of four metal elements, M2 = M a p4 M b q4 M c r4 M d s4 (In the formula, M a , M b , M c and M d are different from each other and are selected from the group consisting of Ir, Rh, Ag, Cu and Au, and are represented by p4 = 0.01 to 0.97, q4 = 0.01 to 0.97, r4 = 0.01 to 0.97, s4 = 0.01 to 0.97, p4 + q4 + r4 + s4 = 1).

[0042] When M1 is Ir and M2 consists of four metal elements, M2 = M a p4 M b q4 M c r4 M d s4 (In the formula, M a , M b , M c and M d are different from each other and selected from the group consisting of Rh, Pd, Ag, Cu, and Au, and are expressed as p4=0.01 to 0.97, q4=0.01 to 0.97, r4=0.01 to 0.97, s4=0.01 to 0.97, and p4+q4+r4+s4=1).

[0043] When M1 is Ru and M2 consists of five metals, M2 = M a p5 M b q5 M c r5 M d s5 M e t5 (In the formula, M a , M b , M c , M d and M e are different from each other and selected from the group consisting of Ir, Rh, Ag, Cu, and Au, and are expressed as p5=0.01 to 0.96, q5=0.01 to 0.96, r5=0.01 to 0.96, s5=0.01 to 0.96, t5=0.01 to 0.96, p5+q5+r5+s5+t5=1).

[0044] When M1 is Ir and M2 consists of five metals, M2 = M a p5 M b q5 M c r5 M d s5 M e t5 (In the formula, M a , M b , M c , Md and M e are different from each other and are selected from the group consisting of Rh, Pd, Ag, Cu, and Au, and are represented by p5 = 0.01 to 0.96, q5 = 0.01 to 0.96, r5 = 0.01 to 0.96, s5 = 0.01 to 0.96, t5 = 0.01 to 0.96, p5 + q5 + r5 + s5 + t5 = 1).

[0045] In the solid solution nanoparticles of the present embodiment, the content ratio of Pt can be in the range of 10 mol% or more and 30 mol% or less.

[0046] For example, when using the solid solution nanoparticles as a catalyst, Pt is considered to be the main component that affects the catalytic activity. Therefore, it is predicted that when the content ratio of Pt is decreased, the catalytic activity will also decrease. However, surprisingly, when the content ratio of Pt is suppressed, the catalyst containing the solid solution nanoparticles exhibits the highest methane oxidative decomposition activity. Specifically, as the content ratio of Pt decreases to 50 mol%, 40 mol%, and 30 mol%, the catalytic activity tends to increase. Regarding the lower limit value, as the content ratio of Pt increases to 5 mol% and 10 mol%, the catalytic activity tends to increase. This will become clear from the examples described later.

[0047] The solid solution nanoparticles of the present embodiment can be suitably used as various catalysts.

[0048] Examples of catalytic reactions include chemical reactions such as reduction reactions, oxidation reactions, dehydrogenation reactions, and coupling reactions. The catalyst of this embodiment can be used in processes or apparatuses involving these catalytic reactions. Specific uses of the catalyst include environmental uses including exhaust gas purification, electrode uses, and chemical process uses. In environmental uses, the catalyst is used in at least one reaction selected from the group consisting of reduction reactions of nitrogen oxides, oxidation reactions of carbon monoxide, oxidation reactions of hydrocarbons, and oxidation reactions of volatile organic compounds (VOCs). In electrode uses, the catalyst is used in at least one reaction selected from the group consisting of hydrogen oxidation reactions, oxygen reduction reactions, and water electrolysis reactions. In chemical process uses, the catalyst is used in at least one reaction selected from the group consisting of hydrogenation reactions of unsaturated hydrocarbons and dehydrogenation reactions of saturated or unsaturated hydrocarbons. In particular, the catalyst of this embodiment can be preferably used for purification of exhaust gas discharged from heat engines, production of hydrogen in fuel cells, and removal of volatile organic compounds.

[0049] Among exhaust gases, the main component of hydrocarbons in the exhaust gas may be methane, such as combustion exhaust gas of natural gas. Methane has a greenhouse effect about 25 times that of carbon dioxide. Therefore, it is recommended from the perspective of global environmental protection to reduce the release of methane into the atmosphere as much as possible. The catalyst of this embodiment is suitable for oxidative decomposition of hydrocarbons, particularly oxidative decomposition of methane. Although methane has high chemical stability, the catalyst of this embodiment exhibits high activity, so that the oxidative decomposition of methane can proceed sufficiently at a relatively low temperature. In addition, the catalyst of this embodiment is also excellent in durability against reaction inhibitors such as sulfur oxides (SO x ). Examples of heat engines using natural gas as fuel include gas turbines. The catalyst of this embodiment is suitable for a purification device for combustion exhaust gas of gas turbines.

[0050] The catalyst of this embodiment may further include a carrier supporting solid solution nanoparticles. By supporting the solid solution nanoparticles on the carrier, aggregation of the solid solution nanoparticles can be suppressed. In addition, due to the electronic interaction from the carrier, adsorption and activation of reaction molecules on the surface of the solid solution nanoparticles can be promoted. Depending on the carrier, it is also possible to interact electronically with the solid solution nanoparticles and further improve the catalytic activity of the solid solution nanoparticles.

[0051] The structure of the carrier is not particularly limited. Typically, the carrier is a particle. The shape of the particle is also not particularly limited, and particles of various shapes such as spherical, ellipsoidal, and flaky can be used.

[0052] The material of the carrier is not particularly limited. Examples of the carrier material include oxides, nitrides, carbides, carbon materials, and metal materials. Examples of oxides include silica, alumina, ceria, titania, zirconia, niobia, silica-alumina, titania-zirconia, ceria-zirconia, tin oxide, tungsten trioxide, molybdenum trioxide, tantalum pentoxide, and strontium titanate. The oxide may be a metal oxide. Examples of nitrides include boron nitride, silicon nitride, gallium nitride, indium nitride, aluminum nitride, zirconium nitride, vanadium nitride, tungsten nitride, molybdenum nitride, titanium nitride, and niobium nitride. The nitride may be a metal nitride. Examples of carbides include silicon carbide, gallium carbide, indium carbide, aluminum carbide, zirconium carbide, vanadium carbide, tungsten carbide, molybdenum carbide, titanium carbide, niobium carbide, and boron carbide. The carbide may be a metal carbide. Examples of carbon materials include activated carbon, carbon black, graphite, carbon nanotubes, and activated carbon fibers. Examples of metal materials include pure metals such as iron, copper, and aluminum, and alloys such as stainless steel. One or a combination of two or more selected from these carriers can be used.

[0053] Among these, particles of metal oxides such as zirconia particles can be suitably used as the carrier. The metal oxide has a large specific surface area and is excellent in heat resistance, chemical stability, mechanical strength, and dispersibility.

[0054] The carrier may contain at least one selected from the group consisting of SnO2, WO3, MoO3, Ta2O5, and Nb2O5. These materials are excellent in durability against SO x and thus may have the effect of maintaining the activity of the catalyst over a long period. The carrier may contain any of these materials as the main component or may consist essentially of these materials. "Main component" means the component contained in the largest amount by mass ratio. "Consisting essentially of... " means that, excluding inevitable impurities, no materials other than the specific material are intentionally added.

[0055] The solid solution nanoparticles may be used as a catalyst without being supported on a carrier. When the solid solution nanoparticles are used as a catalyst in a solution, the solid solution nanoparticles may be protected with a protective agent.

[0056] Next, a method for producing the solid solution nanoparticles will be described. The method described below enables mixing at the atomic level even if the combination of metals is immiscible in the phase diagram.

[0057] The solid solution nanoparticles having the composition represented by formula (1) are produced through a step of adding a solution containing a Pt salt, a salt of M1, and a salt of M2 to a liquid reducing agent heated to a predetermined temperature T and reacting them. The composition of the solid solution nanoparticles can be controlled by adjusting the ratio of the metal salts as raw materials.

[0058] First, a solution containing a Pt salt, a salt of M1, and a salt of M2 is prepared. The solution is typically an aqueous solution. The salt of M1 is a Ru salt or an Ir salt. When the salt of M1 is a Ru salt, the salt of M2 contains at least one selected from the group consisting of an Ir salt, a Rh salt, an Ag salt, a Cu salt, and an Au salt. When the salt of M1 is an Ir salt, the salt of M2 contains at least one selected from the group consisting of a Rh salt, a Pd salt, an Ag salt, a Cu salt, and an Au salt. The Pt salt, the salt of M1, and the salt of M2 can each be water-soluble.

[0059] Examples of the Pt salt, Ru salt, Ir salt, Rh salt, Pd salt, Ag salt, Cu salt, and Au salt include the following salts. Pt: K2PtCl4, (NH4)2K2PtCl4, (NH4)2PtCl6, Na2PtCl6, [Pt(NO2)2(NH3)2] Ru: Ruthenium halides such as RuCl3 and RuBr3, ruthenium nitrate Ir: Iridium chloride, iridium acetylacetonate, potassium iridium cyanate, potassium iridate Rh: Rhodium acetate, rhodium nitrate, rhodium chloride Pd: K2PdCl4, Na2PdCl4, K2PdBr4, Na2PdBr4, palladium nitrate Ag: Silver nitrate, silver acetate Cu: Copper sulfate, cuprous chloride, cupric chloride, copper acetate, copper nitrate Au: Chloroauric acid, bromoauric acid, gold acetate

[0060] First, the Pt salt, the salt of M1, and the salt of M2 are weighed and added to water to prepare a solution. An acid or an alkali may be added to the water to adjust the pH of the solution. The temperature of the solution is, for example, room temperature (20 °C ± 15 °C).

[0061] Next, a carrier is added to the solution as necessary. The timing of adding the carrier to the solution is not particularly limited. When the reaction for forming the solid solution nanoparticles is allowed to proceed while the carrier is present in the solution, the solid solution nanoparticles can be directly supported on the carrier without using a protective agent such as a polymer.

[0062] Next, a liquid reducing agent and a solution are mixed to obtain a reaction solution. In one example, the liquid reducing agent and the solution are mixed by spraying the solution onto the liquid reducing agent heated to a predetermined temperature T. The reaction is allowed to proceed over a predetermined time t while maintaining the reaction solution at the predetermined temperature T. Instead of spraying, the solution may be dropped into the liquid reducing agent. Thereafter, the reaction solution is allowed to cool and solid-liquid separation is performed to obtain solid solution nanoparticles having a desired composition.

[0063] The predetermined temperature T is, for example, in the range of 150°C or higher and 250°C or lower. The predetermined time t is, for example, in the range of 1 minute or longer and 12 hours or shorter. Examples of the liquid reducing agent include polyhydric alcohols such as ethylene glycol, glycerin, diethylene glycol, and triethylene glycol. One or both of the liquid reducing agent and the solution may be preheated and then mixed.

[0064] The reaction solution may contain a protective agent. The protective agent serves to suppress aggregation of the solid solution nanoparticles. Examples of the protective agent include polymers, amines, and carboxylic acids. Examples of the polymer include poly(N-vinyl-2-pyrrolidone) (PVP) and polyethylene glycol (PEG). An example of the amine is oleylamine. An example of the carboxylic acid is oleic acid.

[0065] Note that it is also possible to post-carry the solid solution nanoparticles on the carrier by mixing the solid solution nanoparticles and the carrier particles. The mixing may be performed using a solvent. When a solvent is used, operations such as filtration, drying, and shaping may be performed as necessary.

[0066] The solid-liquid separation step may be omitted. That is, the solid solution nanoparticles may be provided in the form of a dispersion. The dispersion contains a solvent and the solid solution nanoparticles dispersed in the solvent. Depending on the application, it may be desirable to provide the solid solution nanoparticles in the form of a dispersion.

Example

[0067] Hereinafter, the present invention will be described in more detail by way of examples.

[0068] [[Example 1: PtRuIr Solid Solution Nanoparticles]] 0.117 ml of hydrochloric acid was added to 40 ml of water to prepare dilute hydrochloric acid. The pH of the dilute hydrochloric acid was 1.64 and the temperature was 24.7 °C. 0.1025 mmol of K2PtCl4 was dissolved in 8 ml of the dilute hydrochloric acid to obtain an aqueous solution of Pt salt. 0.205 mmol of RuCl3·nH2O was dissolved in 8 ml of the dilute hydrochloric acid to obtain an aqueous solution of Ru salt. 0.205 mmol of IrCl4·nH2O was dissolved in 8 ml of the dilute hydrochloric acid to obtain an aqueous solution of Ir salt. The aqueous solutions of Pt salt, Ru salt and Ir salt were mixed to obtain a mixed solution of noble metal salts. Using an ultrasonic homogenizer, 1920 mg of ZrO2 powder (manufactured by Daiichi Rare Element Co., Ltd., RC-100) was dispersed in 16 ml of the dilute hydrochloric acid to obtain a ZrO2 dispersion. While stirring the ZrO2 dispersion, the mixed solution of noble metal salts was added to the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 15 minutes.

[0069] On the other hand, 1.3 mmol of NaOH was dissolved in 2 ml of water to prepare an aqueous solution of NaOH. The aqueous solution of NaOH was slowly added to 400 ml of triethylene glycol to adjust the pH of the triethylene glycol to 7. Thereafter, the triethylene glycol was heated to 232 °C.

[0070] The raw material solution was sprayed into the heated triethylene glycol over 19 minutes. The temperature of the triethylene glycol during spraying was 229 - 232 °C. After completion of spraying, while maintaining the temperature at 232 °C, the reaction solution containing the triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation and washed with water. Thereafter, the separated solid was dried under vacuum. Thereby, PtRuIr solid solution nanoparticles supported on ZrO2 were obtained.

[0071] [Composition Analysis, TEM Observation] The composition of the PtRuIr solid solution nanoparticles was identified using a fluorescent X-ray analyzer. The results are shown in Table 1. The PtRuIr solid solution nanoparticles supported on ZrO2 were observed by a transmission electron microscope. The obtained TEM image is shown in Fig. 1.

[0072]

Table 1

[0073] The supported amount of Pt was 1.08 wt%, which was approximately consistent with the target value (1 wt%). The composition ratio (atom%) in the PtRuIr solid solution nanoparticles was Pt:Ru:Ir = 22:40:38, which was approximately consistent with the target of 1:2:2. HfO2 is an impurity inevitably contained in the zirconia particles.

[0074] In Fig. 1, the large particles are ZrO2 particles. The small particles attached to the surface of the ZrO2 particles are PtRuIr solid solution nanoparticles. The PtRuIr solid solution nanoparticles were uniformly attached to the surface of the ZrO2 particles. The average particle size of the PtRuIr solid solution nanoparticles was 2.2 ± 0.4 nm. In the notation of "A ± B nm", A represents the average particle size and B represents the standard deviation.

[0075] [Element Mapping and Line Analysis] For the PtRuIr solid solution nanoparticles supported on ZnO2 in Example 1, element mapping by energy dispersive X-ray analysis (EDX) and line analysis were performed. Images obtained by HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy) and the results of element mapping are shown in Figs. 2A to 2D. Fig. 2A shows the HAADF-STEM image. Figs. 2B, 2C, and 2D show the element mapping data of Pt, Ir, and Ru, respectively. The results of line analysis are shown in Figs. 3A and 3B. Fig. 3B shows the results of line analysis of the PtRuIr solid solution nanoparticles appearing in the STEM image of Fig. 3A.

[0076] The dark white portions shown in FIGS. 2B to 2D correspond to the portions of the small particles in FIG. 2A. These results indicate that PtRuIr solid solution nanoparticles were uniformly formed on the ZnO2 particles. The results of the line analysis in FIGS. 3A and 3B indicate that Pt, Ir, and Ru are not present separately from each other, but are uniformly distributed throughout the particles. That is, the data in FIGS. 2A to 2D, FIGS. 3A and 3B indicate that Pt, Ir, and Ru are solid-solved at the atomic level in the PtRuIr nanoparticles.

[0077] <<Examples 2 to 6: PtRuIr Solid Solution Nanoparticles>> PtRuIr solid solution nanoparticles of Examples 2 to 6 having different composition ratios were prepared in the same manner as in Example 1, except that the charging ratios of the Pt salt, Ru salt, and Ir salt were changed. The target compositions in each example were as follows. The target value of the Pt loading amount in each example was 1 wt%.

[0078] Example 2: Pt 0.2 Ru 0.6 Ir 0.2 Example 3: Pt 0.2 Ru 0.2 Ir 0.6 Example 4: Pt 0.25 Ru 0.25 Ir 0.5 Example 5: Pt 0.3 Ru 0.3 Ir 0.3 Example 6: Pt 0.6 Ru 0.2 Ir 0.2

[0079] <<Reference Example 1: Pt Nanoparticles>> 0.205 mmol of K2PtCl4 was dissolved in 20 ml of water to obtain an aqueous Pt salt solution. 3960 mg of ZrO2 powder was dispersed in 30 ml of water to obtain a ZrO2 dispersion. While stirring the ZrO2 dispersion, the aqueous Pt salt solution was added to the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 15 minutes.

[0080] The raw material solution was sprayed into 300 ml of triethylene glycol heated to 232 °C over 19 minutes. The temperature of the triethylene glycol during spraying was 228 - 233 °C. After completion of spraying, while maintaining at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation and washed with water. Thereafter, the separated solid was dried under vacuum. Thereby, Pt nanoparticles supported on ZrO2 were obtained. The supported amount of Pt was 1.02 wt%, which was approximately in agreement with the target value (1 wt%).

[0081] Figure 4 is a TEM image of the Pt nanoparticles supported on ZrO2 of Reference Example 1. In Figure 4, the large particles are ZrO2 particles. The small particles adhering to the surface of the ZrO2 particles are Pt nanoparticles. The average particle diameter of the Pt nanoparticles was 2.9 ± 0.8 nm.

[0082] <<Reference Example 2: Ru Nanoparticles>> 0.088 ml of hydrochloric acid was added to 30 ml of water to prepare dilute hydrochloric acid. 0.07689 mmol of RuCl3·nH2O was dissolved in 8 ml of the dilute hydrochloric acid to obtain an aqueous Ru salt solution. Using an ultrasonic homogenizer, 1462.45 mg of ZrO2 powder was dispersed in 12 ml of the dilute hydrochloric acid to obtain a ZrO2 dispersion. While stirring the ZrO2 dispersion, the aqueous Ru salt solution was added to the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 15 minutes.

[0083] On the other hand, 1 mmol of NaOH was dissolved in 2 ml of water to prepare an aqueous NaOH solution. The aqueous NaOH solution was slowly added to 300 ml of triethylene glycol to adjust the pH of the triethylene glycol to 7. Thereafter, the triethylene glycol was heated to 232 °C.

[0084] The raw material solution was sprayed into the heated triethylene glycol over 13 minutes. The temperature of the triethylene glycol during spraying was 229 - 232 °C. After spraying, while maintaining the temperature at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation and washed with water. Thereafter, the separated solid was dried under vacuum. Thereby, Ru nanoparticles supported on ZrO2 were obtained. The supported amount of Ru was 0.43 wt%, which was generally consistent with the target value (0.5 wt%).

[0085] Figure 5 is a TEM image of the Ru nanoparticles supported on ZrO2 of Reference Example 2. In Figure 5, the large particles are ZrO2 particles. The small particles attached to the surface of the ZrO2 particles are Ru nanoparticles. The average particle size of the Ru nanoparticles was 4.2 ± 0.8 nm.

[0086] <<Reference Example 3: Ir Nanoparticles>> 0.07689 mmol of IrCl4·nH2O was dissolved in 18 ml of water to obtain an Ir salt aqueous solution. 1462.45 mg of ZrO2 powder was dispersed in 12 ml of water to obtain a ZrO2 dispersion. While stirring the ZrO2 dispersion, the Ir salt aqueous solution was added to the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 15 minutes.

[0087] The raw material solution was sprayed into 300 ml of triethylene glycol heated to 232 °C over 12 minutes. The temperature of the triethylene glycol during spraying was 228 - 232 °C. After spraying, while maintaining the temperature at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation and washed with water. Thereafter, the separated solid was dried under vacuum. Thereby, Ir nanoparticles supported on ZrO2 were obtained. The supported amount of Ir was 1.1 wt%, which was generally consistent with the target value (1 wt%).

[0088] Figure 6 is a TEM image of Ir nanoparticles supported on ZrO2 in Reference Example 3. In Figure 6, the large particles are ZrO2 particles. The small particles attached to the surface of the ZrO2 particles are Ir nanoparticles. The average particle size of the Ir nanoparticles was 1.3 ± 0.3 nm.

[0089] <<Reference Example 4: Preparation of Pd Nanoparticles>> 0.3759 mmol of K2PdCl4 was dissolved in 20 ml of water to obtain an aqueous Pd salt solution. 3960 mg of ZrO2 powder was dispersed in 30 ml of water to obtain a ZrO2 dispersion. While stirring the ZrO2 dispersion, the aqueous Pd salt solution was added to the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 15 minutes.

[0090] The raw material solution was sprayed over 300 ml of triethylene glycol heated to 232 °C over 22 minutes. The temperature of the triethylene glycol during spraying was 228 - 233 °C. After completion of spraying, while maintaining at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred over 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation and washed with water. Thereafter, the separated solid was dried under vacuum. Thereby, Pd nanoparticles supported on ZrO2 were obtained. The loading amount of Pd was 1.06 wt%, which was approximately in agreement with the target value (1 wt%).

[0091] Figure 7 is a TEM image of Pd nanoparticles supported on ZrO2 in Reference Example 4. In Figure 7, the large particles are ZrO2 particles. The small particles attached to the surface of the ZrO2 particles are Pd nanoparticles. The average particle size of the Pd nanoparticles was 3.9 ± 0.8 nm.

[0092] <<Comparative Example 1: Preparation of Nanoparticles by Impregnation Method>> 2.94 g of ZrO2 powder was dispersed in 50 ml of water to obtain a ZrO2 dispersion. The ZrO2 dispersion was stirred at room temperature for 30 minutes. 0.666 g of Pt(NO2)2(NH3)2 solution (Pt content: 4.595 wt%), 1.043 g of Ru(NO)(NO3)3 solution (Ru content: 1.50 wt%), and 0.375 g of Ir(NO3)4 solution (Ir content: 7.91 wt%) were mixed to obtain a mixed solution. While stirring the ZrO2 dispersion, the mixed solution was slowly dropped into the ZrO2 dispersion to obtain a raw material solution. Stirring of the raw material solution was continued for 1 hour.

[0093] Using an evaporator, the solvent was removed from the raw material solution in a water bath at 50 °C. The remaining powder was collected, placed in a dryer, and dried in air under the conditions of 110 °C for 12 hours. The dried powder was pulverized in a mortar and then placed in an electric furnace and calcined in air under the conditions of 600 °C for 5 hours. Thereby, the PtRuIr / ZrO2 particles of Comparative Example 1 were obtained.

[0094] [Evaluation of Methane Oxidation Activity] The methane oxidation activity when the nanoparticles of Example 5, Reference Examples 1 to 4, and Comparative Example 1 were used as catalysts was examined. Methane oxidation activity means the catalytic ability to oxidatively decompose methane. Hereinafter, the catalyst using the nanoparticles of the example is referred to as the "catalyst of the example". The catalyst using the nanoparticles of the reference example is referred to as the "catalyst of the reference example". The catalyst using the nanoparticles of the comparative example is referred to as the "catalyst of the comparative example".

[0095] The methane oxidation activity of the catalysts of Example 5, Reference Examples 1 to 4, and Comparative Example 1 was evaluated using a fixed-bed flow-type reactor. First, 50 mg of the catalyst formed into pellets was filled into a quartz reaction tube with an inner diameter of 7 mm using quartz wool. The reaction tube was connected to a gas supply device, and a reaction gas (CH4: 0.1%, O2: 10%, SO2: 5 volppm, H2O: 3%, He: balance gas) simulating the combustion exhaust gas of natural gas was supplied toward the catalyst.

[0096] As a pretreatment before measurement, the catalyst was heated to 600 °C in the above reaction gas and held for 1 hour. Then, the temperature of the catalyst was lowered to 200 °C, and the reaction gas was supplied at a flow rate of 100 ml / min. The temperature of the catalyst was increased by 50 °C from 200 °C to 600 °C. The temperature of the catalyst was maintained at each temperature for 20 minutes, and the concentration of methane in the reaction gas passing through the catalyst in the steady state was measured. The methane conversion rate (%) was calculated from the measured concentration. The results are shown in Figure 8. The "methane conversion rate" on the vertical axis indicates the ratio of methane that has been oxidized and decomposed. The larger the methane conversion rate, the higher the methane oxidation activity of the catalyst.

[0097] The methane conversion rate of the catalyst of Example 5 at 400 °C was 41% (Figure 8). The methane conversion rate of the catalyst of Comparative Example 1 at 400 °C was 29%. When the rate constant k was calculated based on the following formula, the rate constant of the catalyst of Example 5 was 4.71×10 -5 mol / min / g-cat. The rate constant of the catalyst of Comparative Example 1 was 3.06×10 -5 mol / min / g-cat. The rate constant of the catalyst of Example 5 was about 1.5 times that of the catalyst of Comparative Example 1.

[0098] k = -(F / W)ln(1 - x) F: Molar flow rate of methane (= (0.1 / 100)×100 / 22400) W: Weight of the catalyst (50 mg) x: Conversion rate

[0099] The PtRuIr catalyst of Example 5 (Pt 0.3 Ru 0.3 Ir 0.3 / (ZnO2) showed higher activity than or equivalent to the Pd catalyst of Reference Example 4. In the low temperature range of 350 to 500 °C, the activity of the PtRuIr catalyst of Example 5 exceeded the activity of the catalyst of Comparative Example 1. The activity of the PtRuIr catalyst of Example 5 significantly exceeded the activities of the Pt, Ru, and Ir catalysts of Reference Examples 1 to 3. For example, at 400 °C, the methane conversion rates of the Pt, Ru, and Ir catalysts of Reference Examples 1 to 3 were less than 10%. In contrast, the methane conversion rate of the catalyst of Example 5 at 400 °C was approximately 40%.

[0100] "400 °C" is, for example, a temperature sufficiently lower than the exhaust gas temperature of a general gas turbine. Therefore, it can be said that a catalyst capable of exhibiting sufficient activity at 400 °C is suitable for use in removing methane from the exhaust gas of a gas turbine.

[0101] By the same method, the methane oxidation activities of the PtRuIr catalysts of Examples 1 to 4 and 6 were also examined. The results are shown in Fig. 9.

[0102] In Examples 1 to 6, the total supported amount (wt%) of the noble metals differed from each other. In Examples 1 to 6, the supported amount of Pt was equal at 1 wt%. For example, when comparing the activities (methane conversion rates) at 400 °C, the activity of the PtRuIr catalyst of Example 1 (Pt 0.2 Ru 0.4 Ir 0.4 ) was the highest. The activity of the PtRuIr catalyst of Example 6 (Pt 0.6 Ru 0.2 Ir 0.2 ) was the lowest. The supported amount of Pt in Example 1 and the supported amount of Pt in Example 6 were generally equal at approximately 1 wt%. When the Pt content ratio was relatively low, the PtRuIr catalyst showed high activity.

[0103] The methane oxidation activity of Ru nanoparticles alone is very low (Fig. 8). Nevertheless, the PtRuIr catalyst of Example 1 (Pt 0.2 Ru 0.4 Ir 0.4 ) and the PtRuIr catalyst of Example 2 (Pt 0.2 Ru 0.6 Ir0.2 ) showed extremely high activity, which is worthy of surprise.

[0104] In addition, even when the total loading amount of the noble metal is unified to a predetermined ratio (for example, 4 wt%), and the composition of the PtRuIr solid solution nanoparticles is changed, it is presumed that PtRuIr catalysts with various compositions show the same tendency as in the above examples.

[0105] Figure 10 is a triangular graph showing the relationship between the methane conversion rate at 400 °C and the composition of the solid solution nanoparticles. When the composition ratio of Pt-Ru-Ir was 0.2:0.4:0.4 (Example 1), the PtRuIr catalyst showed the highest activity (77.3%). As can be understood from FIGS. 9 and 10, when the content ratio of Pt in the PtRuIr solid solution nanoparticles was in an appropriate range, the catalyst using the PtRuIr solid solution nanoparticles showed high activity. From FIGS. 9 and 10, as the content ratio of Pt decreased to 50 mol%, 40 mol%, and 30 mol%, the catalytic activity tended to increase. Regarding the lower limit value, as the content ratio of Pt increased to 5 mol% and 10 mol%, the catalytic activity tended to increase. Therefore, the upper limit value of the content ratio of Pt in the PtRuIr solid solution nanoparticles is, for example, 50 mol%, 40 mol% or 30 mol%. The lower limit value of the content ratio of Pt in the PtRuIr solid solution nanoparticles is, for example, 5 mol% or 10 mol%. The content ratio of Pt in the PtRuIr solid solution nanoparticles may be in the range of 10 mol% or more and 30 mol% or less.

[0106] [Durability Test] Next, the durability of the catalysts of Example 1, Example 5, Reference Examples 1 to 4, and Comparative Example 1 was examined. Specifically, using the reaction apparatus and reaction gas described above, the change over time in the methane oxidation activity at 400 °C was examined until 30 hours had elapsed. The amount of the catalyst was changed from 50 mg to 200 mg. Pretreatment was performed in the same manner as the method described above. The results are shown in FIGS. 11A and 11B.

[0107] As shown in Fig. 11A, the activity of the catalyst (Pd nanoparticles) of Reference Example 4 decreased significantly over time. This result indicates that Pd was poisoned by SO2.

[0108] The activities of the catalysts of Example 1, Example 5, and Comparative Example 1 were generally constant over a 30-hour test period. This result indicates that the PtRuIr catalyst is less susceptible to sulfur poisoning. Also, the activities of the catalysts of Example 1 and Example 5 were much higher than the activity of the catalyst of Comparative Example 1.

[0109] As shown in Fig. 11B, the activities of the catalysts of Reference Examples 1 to 3 decreased slightly over time. However, the activities of the catalysts of Reference Examples 1 to 3 were low from the beginning. When comparing the rates of decrease based on the initial activity, the rates of decrease in the activities of the catalysts of Reference Examples 1 to 3 were large. This indicates that although not as much as Pd, Pt, Ru, and Ir are also susceptible to sulfur poisoning. Since the activity of the catalyst of Example 1 hardly decreased, it is considered that the PtRuIr catalyst newly acquired excellent durability against sulfur poisoning by forming a solid solution of Pt, Ru, and Ir.

[0110] <<Example 7: PtRuIr Solid Solution Nanoparticles / SnO2>> PtRuIr solid solution nanoparticles supported on SnO2 were obtained in the same manner as in Example 5, except that 1438.5 mg of SnO2 powder was used instead of ZrO2 powder, and the amount of metal salt charged was adjusted so that the total metal amount after loading was 4.1 wt%. That is, the composition of the PtRuIr solid solution nanoparticles in Example 7 is Pt 0.3 [[ID=..]]Ru 0.3 Ir 0.3 as follows.

[0111] [Durability Test] The durability of the following four types of catalysts was investigated. Specifically, using the reactor and reaction gas described above, the change in methane oxidation activity over time at 400 °C was investigated until 100 hours had elapsed. The results are shown in Fig. 12.

[0112] Catalyst of Example 3 (Pt 0.2 Ru 0.2 Ir 0.6 / ZrO2) 200 mg Catalyst of Example 5 (Pt 0.3 Ru 0.3 Ir 0.3 / ZrO2) 50 mg Catalyst of Example 7 (Pt 0.3 Ru 0.3 Ir 0.3 / SnO2) 200 mg Catalyst of Example 7 (Pt 0.3 Ru 0.3 Ir 0.3 / SnO2) 50 mg

[0113] Figure 12 is a graph showing the change over time in the methane oxidation activity at 400 °C of the catalysts of Examples 3, 5, and 7. The initial activity (0 - 50 hours) of 200 mg of the catalyst of Example 3 using ZrO2 as the carrier was superior to the initial activity of 200 mg of the catalyst of Example 7 using SnO2 as the carrier. However, after 100 hours, the activity of the catalyst of Example 7 exceeded the activity of the catalyst of Sample 1. That is, the catalyst using SnO2 as the carrier was excellent in durability.

[0114] The composition of the PtRuIr solid solution nanoparticles in the catalyst of Example 5 is the same as the composition of the PtRuIr solid solution nanoparticles in the catalyst of Example 7. The activity of 50 mg of the catalyst of Example 5 decreased immediately after the start of the test and then remained at around 40%. In contrast, 50 mg of the catalyst of Example 7 maintained its initial activity (60%) even after 100 hours. That is, the catalyst using SnO2 as the carrier was excellent in durability.

[0115] [Acid and base points of the carrier] The acidic and basic properties of the surfaces of ZrO2 particles and SnO2 particles as carriers were measured by temperature-programmed desorption (TPD). In the measurement of acid sites, ammonia, which is a basic probe molecule, was adsorbed onto the carrier, and the amount of ammonia desorbed when the temperature was continuously increased was measured (NH3-TPD). In the measurement of basic sites, carbon dioxide, which is an acid probe molecule, was used for the measurement (CO2-TPD). Specifically, pretreatment and measurement were carried out under the following conditions.

[0116] (Pretreatment) 100 mg of ZrO2 particles or SnO2 particles were filled into a reaction tube as samples, heated to 600 °C while flowing Ar, and Ar treatment was carried out for 30 minutes. Then, it was switched to 100% O2 and treated for 30 minutes, and then switched to Ar and treated for 30 minutes. Thereafter, the sample was cooled to 100 °C (NH3-TPD) or 50 °C (CO2-TPD). In NH3-TPD, an NH3 / Ar mixed gas containing NH3 at a concentration of 0.5 vol% was passed through the sample at 100 °C for 1 hour to adsorb NH3 onto the sample. In CO2-TPD, a CO2 / Ar mixed gas containing CO2 at a concentration of 0.5 vol% was passed through the sample at 50 °C for 1 hour to adsorb CO2 onto the sample. Then, it was switched to Ar, and the physically adsorbed species were desorbed from the sample over 30 minutes.

[0117] (Measurement) While flowing Ar at a flow rate of 40 ml / min, the sample was heated from 100 °C (NH3) or 50 °C (CO2) to 600 °C at a rate of 10 °C / min. In NH3-TPD, the MS signal with a mass number of 16, which is a fragment of NH3, was measured. In CO2-TPD, the MS signal with a mass number of 44 corresponding to CO2 was measured. The results are shown in Figures 13A and 13B.

[0118] Figure 13A is a graph showing the measurement results of NH3-TPD. Figure 13B is a graph showing the measurement results of CO2-TPD. The horizontal axis represents the temperature of the sample. The vertical axis represents the MS signal intensity. As shown in Figures 13A and 13B, ZrO2 showed the desorption of NH3 and CO2 over a wide temperature range. That is, both acid sites and base sites were present in ZrO2. On the other hand, SnO2 showed the desorption of NH3 over a wide temperature range but showed almost no desorption of CO2. No CO2 signal was detected at temperatures above 200 °C. That is, almost no base sites were present in SnO2. From these results, the reason why the decrease in methane conversion rate is slow when SnO2 is used as a carrier is considered to be that SO4 2- is difficult to adsorb on SnO2. By using a carrier on which no CO2 signal is detected at temperatures above 200 °C when performing CO2-TPD measurement, the effect of the catalyst can be maintained over a long period.

[0119] <<Example 8: PtIrPd Solid Solution Nanoparticles>> Using an ultrasonic homogenizer, 0.3 mmol of K2PtCl4, 0.3 mmol of IrCl4·nH2O, and 0.3 mmol of K2PdCl4 were dissolved in 40 ml of water. Thereby, a raw material solution containing noble metal salts was obtained.

[0120] 9 mmol of PVP was added to 300 ml of triethylene glycol and heated. The raw material solution was sprayed into the heated triethylene glycol over 15 minutes. The temperature of the triethylene glycol during spraying was 228 - 233 °C. After completion of spraying, while maintaining the temperature at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation. The supernatant was colorless and transparent. Thereby, 736.89 mg of PtIrPd solid solution nanoparticles were obtained.

[0121] [Composition Analysis, TEM Observation] The composition of PtIrPd solid solution nanoparticles was identified using a fluorescent X-ray analyzer. The results are shown in Table 2. The PtIrPd solid solution nanoparticles were observed by a transmission electron microscope. The obtained TEM image is shown in Fig. 14.

[0122]

Table 2

[0123] The composition ratio (atom%) in the PtIrPd solid solution nanoparticles was Pt:Ir:Pd = 32:35:32, which was generally consistent with the target 1:1:1. The quantitative results (wt%) were calculated assuming that the remainder other than the noble metals was PVP.

[0124] Fig. 14 is a TEM image of the PtIrPd solid solution nanoparticles of Example 8. As shown in Fig. 14, nano-sized PtIrPd solid solution nanoparticles were obtained. The average particle size of the PtIrPd solid solution nanoparticles was 4.4 ± 0.9 nm. The average particle size was calculated by measuring the particle sizes (major axes) of 100 particles in the TEM image and calculating their average. In the notation A ± B nm, A represents the average particle size and B represents the standard deviation. The "major axis" means the longest distance between two points on the outer edge of the particle.

[0125] [X-ray Diffraction Measurement] Powder X-ray diffraction measurement of the PtIrPd solid solution nanoparticles of Example 8 was carried out. Fig. 15 is the X-ray diffraction pattern of the PtIrPd solid solution nanoparticles of Example 8. The X-ray diffraction measurement was performed at room temperature using CuKα radiation. The X-ray diffraction pattern showed a single fcc pattern. This indicates that the sample is a solid solution rather than a mixture of Pt, Ir, and Pd. When multiple types of single noble metal nanoparticles are simply physically mixed, or when multiple types of single noble metals are phase-separated within a single particle, the lattice constants of each single noble metal are different, so multiple fcc patterns with different peak positions are observed. However, in the case of a solid solution in which all elements are uniformly mixed at the atomic level, its lattice constant is determined to be a single value by the composition ratio and atomic radius of each element, so only a single fcc pattern is observed.

[0126] <<Example 9: PtIrPdRh Solid Solution Nanoparticles>> Using an ultrasonic homogenizer, 0.25 mmol of K2PtCl4, 0.25 mmol of IrCl4·nH2O, 0.25 mmol of K2PdCl4, and 0.25 mmol of RhCl3·3H2O were dissolved in 40 ml of water. Thereby, a raw material solution containing noble metal salts was obtained.

[0127] 2 mmol of PVP was added to 300 ml of triethylene glycol and heated. The raw material solution was sprayed into the heated triethylene glycol over 16 minutes. The temperature of the triethylene glycol during spraying was 229 - 232 °C. After completion of spraying, while maintaining at 230 °C, the reaction solution containing triethylene glycol and the raw material solution was stirred for 10 minutes. After allowing the reaction solution to cool, the precipitate was separated by centrifugation. The supernatant had a pale brown color. Thereby, 105.19 mg of PtIrPdRh solid solution nanoparticles were obtained.

[0128] [Composition Analysis, TEM Observation] The composition of the PtIrPdRh solid solution nanoparticles was identified using a fluorescent X-ray analyzer. The results are shown in Table 3. The PtIrPdRh solid solution nanoparticles were observed by a transmission electron microscope. The obtained TEM image is shown in Fig. 16.

[0129] [Table 3]

[0130] The composition ratio (atom %) in the PtIrPdRh solid solution nanoparticles was Pt:Ir:Pd:Rh = 21:24:28:27, which was approximately consistent with the target 1:1:1:1. The quantitative results (wt%) were calculated assuming that the remainder other than the noble metals was PVP.

[0131] Figure 16 is a TEM image of the PtIrPdRh solid solution nanoparticles of Example 9. As shown in Figure 16, nano-sized PtIrPdRh solid solution nanoparticles were obtained. The average particle size of the PtIrPdRh solid solution nanoparticles was 3.9 ± 1.2 nm.

[0132] [X-ray diffraction measurement] Powder X-ray diffraction measurement of the PtIrPdRh solid solution nanoparticles of Example 9 was carried out. Figure 17 is the X-ray diffraction pattern of the PtIrPdRh solid solution nanoparticles of Example 9. The X-ray diffraction pattern showed a single fcc pattern. This indicates that the sample is a solid solution rather than a mixture of Pt, Ir, Pd, and Rh. When multiple types of single noble metal nanoparticles are simply physically mixed, or when multiple types of single noble metals are phase-separated within a single particle, the lattice constants of each single noble metal are different, so multiple fcc patterns with different peak positions are observed. However, in the case of a solid solution in which all elements are uniformly mixed at the atomic level, its lattice constant is determined by a single value based on the composition ratio and atomic radius of each element, so only a single fcc pattern is observed. [Industrial applicability]

[0133] The solid solution nanoparticles of the present invention are useful as electronic materials, magnetic materials, catalyst materials, pharmaceutical materials, cosmetic materials, or food material.

Claims

1. Formula Pt x M1 y M2 1-x-y Solid solution nanoparticles having a composition represented by (0<x<1, 0<y<1, x+y<1), (i)M1 is Ru and M2 is Ir, (ii)M1 is Ir and M2 is Pd, or (iii)M1 is Ir and M2 is Pd and Rh, In the solid solution nanoparticles, the content ratios of Pt, M1, and M2 are each 10 mol% or more and 80 mol% or less, When satisfying the above (i), in the solid solution nanoparticles, the content ratio of Pt is 10 mol% or more and 20 mol% or less, Solid solution nanoparticles.

2. A catalyst comprising the solid solution nanoparticles according to Claim 1.

3. Further comprising a carrier supporting the solid solution nanoparticles, The catalyst according to Claim 2.

4. The carrier is particles of a metal oxide, The catalyst according to Claim 3.

5. When the CO - TPD measurement of the carrier was carried out at a heating rate of 10 °C / min, no signal of CO was detected at 200 °C or higher. 2 When the CO - TPD measurement of the carrier was carried out at a heating rate of 10 °C / min, no signal of CO was detected at 200 °C or higher. 2 When the CO - TPD measurement of the carrier was carried out at a heating rate of 10 °C / min, no signal of CO was detected at 200 °C or higher. The catalyst according to Claim 3 or 4.

6. The carrier is SnO 2 , WO 3 , MoO 3 , Ta 2 O 5 , and Nb 2 O 5 and includes at least one selected from the group consisting of The catalyst according to any one of Claims 3 to 5.

7. A catalyst for oxidative decomposition of hydrocarbons, The catalyst according to any one of Claims 2 to 6.

8. The hydrocarbon contains methane, The catalyst according to Claim 7.

9. When satisfying the above (ii) or (iii), in the solid solution nanoparticles, the content ratio of Pt is 10 mol% or more and 30 mol% or less, The catalyst according to any one of Claims 2 to 8.

10. The catalyst is a catalyst for oxidative decomposition of the hydrocarbon contained in the combustion exhaust gas of natural gas, and the exhaust gas further contains sulfur dioxide and water, The catalyst according to Claim 7 or 8.

11. Solid solution nanoparticles having a composition represented by the formula Pt x M1 y M2 1-x-y (0 < x < 1, 0 < y < 1, x + y < 1), A carrier supporting the solid solution nanoparticles, Comprising, M1 is Ru, M2 is Ir, The carrier is SnO 2 particles, and In the solid solution nanoparticles, the content ratio of Pt is 10 mol% or more and 30 mol% or less, In the solid solution nanoparticles, the content ratios of M1 and M2 are each 10 mol% or more and 80 mol% or less, A catalyst for oxidative decomposition of methane, Catalyst.

12. Formula Pt x M1 y M2 1-x-y A method for producing solid solution nanoparticles having a composition represented by (0 < x < 1, 0 < y < 1, x + y < 1). Including adding a solution containing a Pt salt, a salt of M1, and a salt of M2 to a liquid reducing agent heated to a temperature in the range of 150°C or more and 250°C or less and reacting them, (i)The salt of M1 is a Ru salt and the salt of M2 is an Ir salt, (ii)The salt of M1 is an Ir salt and the salt of M2 is a Pd salt, or (iii)The salt of M1 is an Ir salt and the salt of M2 is a Pd salt and a Rh salt, In the solid solution nanoparticles, the content ratios of Pt, M1, and M2 are each 10 mol% or more and 80 mol% or less, When the above (i) is satisfied, in the solid solution nanoparticles, the content ratio of Pt is 10 mol% or more and 20 mol% or less. A method for producing solid solution nanoparticles.

13. The solution further contains a carrier. The method for producing solid solution nanoparticles according to claim 12.

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