Platinum group nanoparticles and method for producing same

Abstract

To provide a platinum group nano-particle having a high catalytic activity in a broad pH range of solution, and a method of producing the same.SOLUTION: A platinum group nano-particle in the present invention is obtainable by a method including the steps of: (a) preparing solution containing protectant and platinum group complex; (b) charging reducer into the solution and stirring it at a desired temperature; and (c) increasing the temperature of the solution to a desired temperature and maturing the solution without stirring, in this order.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to platinum group nanoparticles and a method for producing the same. [Background technology]

[0002] Platinum group metals are chemically stable and excellent catalysts for chemical reactions. Nanoparticles made of platinum group metals, in particular, have excellent catalytic activity due to their large surface area; for example, Patent Document 1 discloses platinum nanoparticles as singlet oxygen quenchers. Other potential applications of platinum group nanoparticles include electrode catalysts for fuel cells, catalysts for chemical reactions, co-catalysts for photocatalysts in water splitting reactions, and biomarkers when combined with biomolecules.

[0003] Known methods for producing platinum group metal nanoparticles include the top-down method, in which coarse particles are mechanically crushed, and the bottom-up method, in which nanoparticles are precipitated from metal ions through a reduction reaction in the liquid phase.However, the bottom-up method is more widely used because it can control the inclusion of impurities and synthesize uniform particles without requiring expensive equipment.

[0004] In bottom-up methods from a liquid phase, aqueous or organic solvents are used as the liquid phase, but from the perspective of reducing environmental impact, it is desirable to carry out synthesis in an aqueous system.

[0005] However, conventional platinum nanoparticles have problems such as unstable behavior in aqueous solutions, causing aggregation due to changes in pH, etc., resulting in a significant decrease in catalytic efficiency, limiting their applications. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2005 / 018598 Summary of the Invention [Problem to be solved by the invention]

[0007] Platinum group nanoparticles produced by conventional manufacturing methods have problems with dispersibility in aqueous solutions, and are known to aggregate and exhibit reduced catalytic activity, particularly under neutral conditions such as those in vivo.The present invention aims to provide platinum group nanoparticles that are less likely to aggregate in aqueous solutions over a wide pH range, have a small average secondary particle size, and exhibit high catalytic activity, as well as a method for producing the same. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors reviewed the production process for reducing platinum group complexes to produce platinum group nanoparticles, and also conducted extensive research into protective agents for nanoparticles, resulting in the present invention. Specifically, the present invention provides platinum group nanoparticles having a protective agent, which have high dispersibility in aqueous solutions, a small average secondary particle size in aqueous solutions over a wide pH range, and high catalytic activity, as well as a method for producing the same. [Effects of the Invention]

[0009] The method for producing platinum nanoparticles of the present invention is characterized by the separate steps of mixing a reducing agent and a platinum group complex, followed by reduction at low temperature with stirring, and aging at high temperature without stirring. The platinum nanoparticles produced by the method of the present invention have excellent dispersibility in aqueous solutions, are less likely to aggregate in solutions over a wide pH range, and have a small average secondary particle size. Furthermore, the platinum nanoparticles of the present invention exhibit high oxidation catalytic activity over a wide pH range. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a TEM image of platinum group nanoparticles according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] In one aspect, the present invention is as follows. [1] A method for producing platinum group nanoparticles, comprising the following steps (a) to (c): (a) preparing an aqueous solution containing a protective agent and a platinum group complex; (b) adding a reducing agent to the aqueous solution and stirring at a desired temperature; (c) raising the temperature of the aqueous solution to a desired temperature and aging the solution without stirring; A method for producing platinum group nanoparticles, comprising the steps of: [2] The temperature in the step (b) is 0 to 40°C; The temperature in the step (c) is 60 to 80°C. [1] A method for producing platinum group nanoparticles. [3] The method for producing platinum group nanoparticles according to [1] or [2], wherein the protective agent is one or more protective agents selected from the group consisting of polyvinyl alcohol, dextran, inulin, polyvinylpyrrolidone, polyacrylic acid, polyoxazoline, and derivatives thereof. [4] The method for producing platinum group nanoparticles according to any one of [1] to [3], wherein the protective agent is polyoxazoline. [5] The method for producing platinum group nanoparticles according to [4], wherein the polyoxazoline is poly(2-methyl-2-oxazoline). [6] The method for producing platinum group nanoparticles according to any one of [1] to [5], wherein the platinum group nanoparticles are platinum nanoparticles or ruthenium nanoparticles. [7] Platinum group nanoparticles coated with a protective agent, The platinum group nanoparticles, characterized in that the protective agent is polyoxazoline. [8] The platinum group nanoparticles according to [7], wherein the average primary particle size of the platinum group nanoparticles is 5 nm or less. [9] The platinum group nanoparticles according to [7] or [8], characterized in that the platinum group nanoparticles exhibit oxidation catalytic activity in an aqueous solution of pH 7.5.

[10] The platinum group nanoparticles are platinum nanoparticles or ruthenium nanoparticles. The platinum group nanoparticles according to any one of [7] to [9].

[11] A platinum group nanoparticle dispersion in which the platinum group nanoparticles according to any one of [7] to

[10] are dispersed in an aqueous solution.

[0012] In this specification, the term "average primary particle size" refers to the average particle size of nanoparticles that can be determined by observation with an electron microscope. In one embodiment, the average primary particle size is determined by randomly extracting 100 primary particles from an image (TEM image) taken with a transmission electron microscope (TEM) and calculating the arithmetic mean value of the particle sizes of these particles. As used herein, the term "average secondary particle size" refers to the average particle size of aggregated particles in a nanoparticle dispersion. In one embodiment, the average secondary particle size refers to D50 (the particle size of particles at which the cumulative particle size distribution under a sieve is 50%) in the number-based cumulative particle size distribution measured by dynamic light scattering. Measurement by dynamic light scattering is preferably performed using a particle size measuring system ELSZ-1000ZS manufactured by Otsuka Electronics Co., Ltd.

[0013] As used herein, the term "nanoparticles" refers to nano-sized particles, and refers to particles having an average primary particle size of about 20 nm or less.

[0014] In this specification, the term "protective agent" refers to a water-soluble polymer compound that stabilizes platinum group complexes and platinum group atoms and maintains the dispersion stability of nanoparticles in aqueous solution by adsorbing to the surface of precipitated nanoparticles. Examples of water-soluble polymer compounds include, but are not limited to, polyvinyl alcohol, dextran, inulin, polyvinylpyrrolidone, polyacrylic acid, and polyoxazoline. In this specification, compounds exemplified as polymer compounds include compounds having a substituent. Examples of the substituent on the polymer compound include an alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 3 to 8 carbon atoms, and an aryl group having 5 to 12 carbon atoms. For example, polyoxazoline in this specification includes poly(2-alkyl-2-oxazoline), poly(2-alkyl-2-oxazoline), poly(2-cycloalkyl-2-oxazoline), poly(2-aryl-2-oxazoline), and the like.

[0015] In this specification, the term "reducing agent" refers to an inorganic or organic compound that undergoes a reduction reaction with a platinum group complex to precipitate platinum group nanoparticles. In this specification, compounds and salts thereof that can be hydrated or solvated may be in the form of a hydrate or solvate, and both forms are within the scope of the present invention.

[0016] As used herein, the term "platinum group" refers to a group of metals consisting of platinum, palladium, rhodium, ruthenium, and iridium.

[0017] In this specification, "stirring" is a general term for the operation of mechanically stirring a reaction solution using a stirrer, stirring blades, or shaker. In this specification, "aging" is a general term for an operation in which mechanical kinetic energy, such as stirring, is not applied to the reaction solution. This includes the case in which the reaction solution is sealed in a reaction vessel and not moved, and flow synthesis without the use of a mixer.

[0018] One embodiment of the present invention is a method for producing platinum group nanoparticles, comprising the following steps (a) to (c): (a) preparing an aqueous solution containing a protective agent and a platinum group complex; (b) adding a reducing agent to the aqueous solution and stirring at a desired temperature; (c) raising the temperature of the aqueous solution to a desired temperature and aging the solution without stirring; Contains, in this order:

[0019] The "protective agent" in step (a) is preferably polyvinyl alcohol, dextran, inulin, polyvinylpyrrolidone, polyacrylic acid, or polyoxazoline, more preferably polyoxazoline. Examples of the polyoxazoline used as the protective agent include poly(2-alkyl-2-oxazoline), poly(2-alkyl-2-oxazoline), poly(2-cycloalkyl-2-oxazoline), and poly(2-aryl-2-oxazoline), with poly(2-alkyl-2-oxazoline) being preferred, and poly(2-methyl-2-oxazoline) being more preferred. The polyoxazoline of the present invention preferably has a molecular weight of 10,000 or less, and more preferably a molecular weight of 5,000 or less. In one embodiment, the polyoxazoline of the present invention is poly(2-methyl-2-oxazoline) azide-terminated (n=approximately 50) manufactured by Tokyo Chemical Industry Co., Ltd. The "platinum group complex" in step (a) is preferably chloroplatinic acid, potassium chloroplatinate, palladium chloride, rhodium chloride, or ruthenium chloride, and more preferably potassium chloroplatinate or ruthenium chloride.

[0020] The "reducing agent" in step (b) may be any compound capable of reducing a platinum group complex, but is preferably sodium borohydride, lithium borohydride, or tetrakis(hydroxymethyl)phosphonium chloride, and more preferably sodium borohydride. The amount of the reducing agent added in the step (b) is preferably 1 to 10 times, more preferably 2.5 to 7.5 times, in terms of the ratio of the amount of substance to the platinum group element. The temperature in step (b) is preferably 0° C. to 40° C., more preferably 15° C. to 30° C. Step (b) is a step in which reduction is carried out with stirring at a relatively low temperature, and an excessively high temperature in this step adversely affects the catalytic activity of the resulting platinum nanoparticles. The stirring time in the step (b) is preferably 5 to 60 minutes, and more preferably 10 to 30 minutes. The weight percentage of the protecting agent relative to the entire solution at the completion of the step (b) (hereinafter referred to as the final concentration of the protecting agent) is preferably 0.02 to 1%, more preferably 0.02 to 0.05%.

[0021] The temperature in the step (c) is preferably 60° C. to 80° C., more preferably 65° C. to 75° C. The step (c) is a step of aging at a relatively high temperature without stirring, and is characterized by aging at a temperature elevated from the step (b) without stirring. The aging time in the step (c) is preferably 1 hour to 12 hours, more preferably 3 hours to 9 hours.

[0022] One embodiment of the present invention relates to platinum group nanoparticles coated with a protective agent, wherein the protective agent is a polyoxazoline. The platinum group elements of the platinum group nanoparticles are preferably platinum and ruthenium. The average primary particle size of the platinum group nanoparticles is preferably 5 nm or less, and more preferably 3 nm or less. The polyoxazoline as the protective agent is preferably the polyoxazoline described above.

[0023] Another embodiment of the present invention relates to a platinum group nanoparticle dispersion in which platinum group nanoparticles are dispersed in an aqueous solution. The average secondary particle size of the platinum group nanoparticle dispersion is preferably 30 nm or less, more preferably 20 nm or less, and particularly preferably 30 nm or less or 20 nm or less even at pH 7.5.

[0024] The platinum group nanoparticle dispersion of the present invention has oxidation catalytic activity over a wide pH range, and preferably has oxidation catalytic activity at pH 7.5, pH 4.5, and / or pH 1.5. The oxidation catalytic activity of the platinum group nanoparticle dispersion of the present invention can be quantified by known methods, for example, by observing the oxidation reaction of tetramethylbenzidine, a specific example of which is the method of Test Example 1. The platinum group nanoparticle dispersion preferably exhibits catalytic activity at pH 7.5 that is 10% or more, 20% or more, 30% or more, 40% or more, or 50% or more of its catalytic activity at pH 4.5.

[0025] The platinum group nanoparticles of the present invention can be used in a variety of industrially useful applications, such as oxidation catalysts for removing active oxygen, electrode catalysts for fuel cells, chemical reaction catalysts, co-catalysts for photocatalysts in water splitting reactions, and can also be used as biomarkers when complexed with biomolecules. [Example]

[0026] The present invention will be described in more detail below with reference to examples, although the scope of the present invention is not limited to the embodiments shown in the following examples. In the examples and comparative examples of this specification, unless otherwise specified, temperatures refer to liquid temperatures, and % refers to weight % unless otherwise specified.

[0027] [Example 1] (Platinum nanoparticle dispersion synthesis process) A 13.5 mL glass vial was charged with 7.6 mL of deionized water and 0.4 mL of a 5% concentration of carboxylated polyvinyl alcohol (molecular weight 1,000, manufactured by Nippon Vaccination & Poval Co., Ltd., AP-10) as a protective agent. To this protective agent solution, 1.0 mL of an 8 mM aqueous solution of potassium tetrachloroplatinate (II) (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was added at 28° C. while stirring with a magnetic stirrer. Stirring was continued at 28°C for 1 hour. To the above solution, 1.0 mL of a 40 mM aqueous sodium borohydride solution as a reducing agent was added while stirring with a magnetic stirrer at 28° C. The concentration of the protecting agent at this time (final protecting agent concentration) was 0.2%. Stirring was continued at 28°C for 30 minutes to complete the reduction reaction. After removing the magnetic stirrer from the reaction solution, the glass vial was aged for 3 hours in a dryer (DV600, manufactured by Yamato Scientific Co., Ltd.) set at 70°C. After the heat treatment, the glass vial was left to stand at room temperature to dissipate heat. After cooling, the reaction solution was centrifuged at a centrifugal acceleration of 20,000 G for 30 minutes in a centrifuge (Model 3500, manufactured by Kubota Shoji Co., Ltd.), and the supernatant was separated to obtain a platinum nanoparticle dispersion.

[0028] [Example 2] A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the protective agent was changed to polyvinylpyrrolidone (molecular weight 40,000, manufactured by Sigma-Aldrich).

[0029] [Example 3] The type of protective agent was polyvinylpyrrolidone (molecular weight 40,000, manufactured by Sigma-Aldrich), and A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% and the final concentration of the protective agent was set to 0.02%.

[0030] [Example 4] The procedure was the same as in Example 1, except that the entire process from preparation of the protective agent dispersion to the completion of the reduction reaction was carried out in an ice bath at 4°C, and the protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), to obtain a platinum nanoparticle dispersion.

[0031] [Example 5] The entire process from preparation of the protective agent dispersion to the completion of the reduction reaction was carried out in a cool incubator (CN-25B manufactured by Mitsubishi Engineering Co., Ltd.) at 16°C. The protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), and A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% and the final concentration of the protective agent was set to 0.02%.

[0032] [Example 6] The protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), and A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% and the final concentration of the protective agent was set to 0.02%.

[0033] [Example 7] The entire process from preparation of the protective agent dispersion to completion of the reduction reaction was carried out in a water bath at 40°C. The protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), and A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% and the final concentration of the protective agent was set to 0.02%.

[0034] [Examples 8 to 17] A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the reducing agent to be added, the type of protective agent, and the concentration of the protective agent were as shown in Table 1 below.

[0035] [Example 18] Instead of an aqueous solution of potassium tetrachloroplatinate (II) with a concentration of 8 mM, an aqueous solution of ruthenium (III) chloride (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) with a concentration of 8 mM was used. The protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), and A ruthenium nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% so that the final concentration of the protective agent was 0.02%.

[0036] [Comparative Example 1] The temperature of the reaction solution when the aqueous reducing agent solution was added was 70°C, and Instead of the process of stirring for 30 minutes at 28°C after adding the reducing agent solution and then aging for 3 hours at 70°C, the reduction reaction and heat treatment were carried out at 70°C for 3 hours with stirring using a magnetic stirrer. Other than that, the same preparation as in Example 1 was carried out to obtain a platinum nanoparticle dispersion.

[0037] Comparative Example 2 The type of protective agent was polyvinylpyrrolidone (molecular weight 40,000, manufactured by Sigma-Aldrich Co.), The temperature of the reaction solution when the aqueous reducing agent solution was added was 70°C, and Instead of the process of stirring for 30 minutes at 28°C after adding the reducing agent solution and then aging for 3 hours at 70°C, the reduction reaction and heat treatment were carried out at 70°C for 3 hours with stirring using a magnetic stirrer. Other than that, the same preparation as in Example 1 was carried out to obtain a platinum nanoparticle dispersion.

[0038] Comparative Example 3 A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that stirring with a magnetic stirrer was carried out at 70°C for 3 hours instead of the step of aging at 70°C for 3 hours.

[0039] Comparative Example 4 The temperature of the reaction solution when the aqueous reducing agent solution was added was 70°C, and After adding the reducing agent solution, instead of stirring for 30 minutes at 28°C, stirring was performed with a magnetic stirrer for 1 minute. Other than that, the same preparation as in Example 1 was carried out to obtain a platinum nanoparticle dispersion.

[0040] Comparative Example 5 The process from preparation of the protective agent dispersion to the completion of the reduction reaction was carried out in a dryer at 70°C. The protective agent was poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) (manufactured by Tokyo Chemical Industry Co., Ltd.), and A platinum nanoparticle dispersion was obtained in the same manner as in Example 1, except that the concentration of the protective agent added was set to 0.50% and the final concentration of the protective agent was set to 0.02%.

[0041] Comparative Example 6 As Comparative Example 5, a platinum nanoparticle dispersion was prepared by reproducing the method described in the examples of Patent Document 1 (WO 2005 / 018598). Specific details are as follows. 0.147 g of polyvinylpyrrolidone (molecular weight 40,000, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was placed as a protective agent in a 100 mL three-neck flask, and 23 mL of deionized water was placed in the flask. The mixture was stirred at room temperature with a magnetic stirrer for 10 minutes to dissolve the protective agent. To the above-mentioned protective agent solution, 2 mL of a 16.6 mM aqueous solution of hexachloroplatinic (IV) acid (Sigma-Aldrich) was added, and the mixture was stirred at room temperature for 30 minutes. The glass tube was connected to a flask, and 25 mL of 99.5% ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added while bubbling with dry nitrogen. The flask was placed in an oil bath at 100°C, and the mixture was heated to reflux for 2 hours while continuing to bubbling nitrogen and stirring with a magnetic stirrer. The flask was removed from the oil bath and allowed to cool to room temperature. After cooling, the ethanol was removed using an evaporator (N-1300 manufactured by Tokyo Rikakikai Co., Ltd.), and the solution was diluted with deionized water to a volume of 40 mL to obtain a platinum nanoparticle dispersion.

[0042] [Test Example 1] (Measuring catalytic activity) The platinum nanoparticle dispersion obtained in Example 1 was diluted 10 times and 20 times with deionized water. 10 μL of the 10-fold diluted solution and 10 μL of the 20-fold diluted solution were dropped into a 300 μL microwell. To 400 μL of tetramethylbenzidine solution (KPL), 50 μL of 1 M phosphate-acetate buffer solution adjusted to pH 7.5 with sodium hydroxide was added. 100 μL of the pH-adjusted tetramethylbenzidine solution was added to the 10-fold diluted solution and the 20-fold diluted solution in the microwells, and the mixture was shaken at 500 rpm for 10 seconds, and then allowed to stand at room temperature for 5 minutes. 100 μL of 4 M hydrochloric acid was added to each microwell, and the mixture was shaken at 500 rpm for 10 seconds to stop the catalytic reaction. The absorbance at 452 nm was measured using an absorption spectrophotometer (PerkinElmer, Inc., EnSpire), and the absorption due to the oxidized tetramethylbenzidinedimine dimer was confirmed. The absorbance at 650 nm was also measured as the background. The catalytic activity of the platinum nanoparticle dispersion was calculated using the following formula (1).

[0043] [Catalytic activity]=((I 1 / 10·452nm -I 1 / 10·650nm )-(I 1 / 20·452nm -I 1 / 20·650nm )) / 4...Equation (1)

[0044] I 1 / 10·452nm : Absorbance at 452 nm of 10-fold diluted solution I 1 / 10·650nm : Absorbance at 650 nm of 10-fold diluted solution I 1 / 20·452nm : Absorbance at 452 nm of 20-fold diluted solution I 1 / 20·650nm : Absorbance at 650 nm of 20-fold diluted solution 4: Constant for the absorbance change per unit volume of platinum nanoparticle dispersion Similarly, catalytic reactions were carried out in buffer solutions adjusted to pH 4.5 and pH 1.5, and the catalytic activity was calculated. Furthermore, in the same manner, the catalytic activity of the platinum nanoparticle dispersions obtained in Examples 2 to 18 and Comparative Examples 1 to 6 was measured. The results are shown in Table 1 below. In all test examples, (I 1 / 10·452nm -I 1 / 10·650nm ) and (I 1 / 20·452nm -I 1 / 20·650nm ) was approximately 2:1, confirming a proportional relationship between the absorbance and the concentration of the platinum nanoparticle dispersion.

[0045] [Test Example 2] (Measuring average secondary particle size using dynamic scattering method) The secondary particle diameters of the platinum nanoparticle dispersions obtained in Examples 1 to 18 and Comparative Examples 1 to 6 were measured using a particle size measuring system (ELSZ-1000ZS, manufactured by Otsuka Electronics Co., Ltd.). The results are shown in Table 1 below.

[0046] [Table 1]

[0047] The abbreviations in Table 1 have the following meanings: PVA-COOH: Carboxyl group-introduced polyvinyl alcohol (molecular weight 1,000) PVP: Polyvinylpyrrolidone (molecular weight 40,000) PMeOx: Poly(2-methyl-2-oxazoline) azide-terminated (n = approximately 50) D10: Particle size of particles where the cumulative particle size distribution under the sieve is 10% D50: Particle size at which the cumulative particle size distribution under the sieve is 50% (average secondary particle size) D90: Particle size of particles where the cumulative particle size distribution under the sieve is 90%

[0048] [Test Example 3] A TEM image of the platinum nanoparticle dispersion obtained in Example 6 was taken using the following equipment and conditions. Measurement device name: JEM-ARM200F, manufactured by JEOL Ltd. Measurement conditions: Magnification 2,000,000 (2 million times) Annular dark field image (ADF) The results are shown in Figure 1. The average primary particle size of the platinum nanoparticles obtained in Example 5 calculated from the TEM image in Figure 1 was 1.3 nm.

[0049] As shown in Table 1, the platinum group nanoparticles of the present invention exhibit higher catalytic activity than the platinum group nanoparticles of the prior art (Comparative Example 6). The platinum group nanoparticles of the present invention are particularly characterized by exhibiting high catalytic activity even under neutral conditions (pH 7.5). The method for producing platinum group nanoparticles of the present invention is characterized in that the reduction step and the heating step are separated, and that aging is carried out without stirring in the heating step. The platinum group nanoparticle dispersions of Comparative Examples 1 and 2, in which the reduction step and heating step were not separated, showed almost no catalytic activity under neutral conditions. The platinum group nanoparticle dispersion of Comparative Example 3, in which stirring was performed instead of aging in the heating step, also showed almost no catalytic activity under neutral conditions. Furthermore, the platinum group nanoparticle dispersions of Comparative Examples 4 and 5, in which heating and stirring were performed in the reduction step, also showed almost no catalytic activity under neutral conditions.

Claims

1. A method for producing platinum group nanoparticles, comprising the following steps (a) to (c): (a) preparing an aqueous solution containing a protective agent and a platinum group complex; (b) adding a reducing agent to the aqueous solution in an amount of 2.5 to 7.5 times the amount of the platinum group element in the platinum group complex in terms of the mass ratio, and stirring the mixture at a desired temperature; (c) raising the temperature of the aqueous solution to a desired temperature and aging the solution without stirring; in this order, wherein the protective agent is polyoxazoline, and the weight percentage of the protective agent relative to the total solution upon completion of step (b) is 0.02 to 0.05%; A method for producing platinum group nanoparticles.

2. The temperature in step (b) is 0 to 40°C, The temperature in step (c) is 60 to 80°C. The method for producing platinum group nanoparticles according to claim 1 .

3. 2. The method for producing platinum group nanoparticles according to claim 1, wherein the polyoxazoline is poly(2-methyl-2-oxazoline).

4. The method for producing platinum group nanoparticles according to any one of claims 1 to 3, wherein the platinum group nanoparticles are platinum nanoparticles or ruthenium nanoparticles.

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