JP6573333B2 - Carrier-supported palladium fine particle colloid, carrier-supported palladium core platinum shell fine particle colloid, carrier-supported palladium core platinum shell fine particle catalyst, method for producing them, and battery. - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a carrier-supported palladium core platinum shell fine particle colloid, a carrier-supported palladium core platinum shell fine particle catalyst, a production method thereof, and a battery, and more specifically, a carrier-supported palladium core and carrier before forming a platinum shell. The present invention relates to core-shell type fine particles synthesized using an alkali metal hydroxide such as potassium hydroxide and microwave irradiation to form a platinum shell after synthesizing a supported palladium core. The fine particles referred to in the present invention are primary particle diameters. When referring to palladium core fine particles or palladium core platinum shell fine particles, the fine particles may be mainly referring to nanoparticles, and in the case of a carrier carrying palladium core platinum fine particles. Often refers to particles of several tens of μm or less.

  Furthermore, the present invention also provides a core shell synthesized using sodium hydroxide and microwave irradiation to form a platinum shell after synthesizing a carrier-supported palladium core using sodium hydroxide and microwave irradiation. Type fine particles.

  A polymer electrolyte fuel cell (hereinafter referred to as PEFC) using hydrogen as a fuel is considered to be an environmentally friendly energy device that can be used by an electric transport vehicle. As an electrochemical catalyst of this PEFC, a metal nanoparticle, for example, a bimetallic metal particle such as a palladium core platinum sur nanoparticle exhibits a higher oxygen reduction activity than a catalyst using only platinum, Because it reduces the amount of precious platinum used, it attracts a great deal of interest.

  Catalysts containing metal nanoparticles using binary metal particles are dispersed in porous carbon materials with high surface areas such as activated carbon, carbon nanotubes, and graphene to maximize the catalytic effect and ensure conductivity. It is common to be fixed.

  In recent years, it has become clear that metal nanoparticles composed of a plurality of types of metal elements exhibit different characteristics compared to nanoparticles of only one type of metal. For this reason, alloy nanoparticles obtained by alloying two kinds of metals, core-shell type nanoparticles obtained by coating a certain metal nanoparticle with another metal, and the like have been actively studied, and various proposals have been made.

  The core-shell type nanoparticles are expected to be imparted with catalytic activity different from that of the shell metal-only nanoparticles if the shell layer in contact with the outside can be formed uniformly. This is presumed to be due to the modification of the electronic properties of the shell layer and the geometrical arrangement of the crystal structure due to the core particles as the base, and to lower the activation energy of the target reaction more effectively. Has been.

  Further, when a metal having catalytic activity is very expensive, it is economical because an expensive metal can be effectively arranged in the shell layer in contact with the reactant by forming a core-shell structure. In addition, by uniformly coating chemically stable noble metals as shells, base metals that are easy to oxidize and dissolve can be indirectly involved in the reaction while shielding them from the outside, and new catalytic properties can be exhibited. Will also be possible.

  Further, when a metal having catalytic activity is very expensive, it is economical because an expensive metal can be effectively arranged in the shell layer in contact with the reactant by forming a core-shell structure. In addition, by uniformly coating chemically stable noble metals as shells, base metals that are easy to oxidize and dissolve can be indirectly involved in the reaction while shielding them from the outside, and new catalytic properties can be exhibited. Is also possible.

  In recent years, attempts have been made to effectively use a small amount of platinum and obtain as good catalytic ability as possible. In recent years, palladium core platinum shell metal nanoparticles have been used to reduce the cost of catalyst for fuel cells for homes and automobiles. Is starting to be considered effective.

  However, since it is very difficult to uniformly coat the shell layer in the synthesis of core-shell type metal nanoparticles, the synthesis conditions are often limited in the laboratory, and the particle size is reduced to a small value of a single nanometer. Then, since detailed observation with an electron microscope and analysis using an electron beam or X-ray increase the difficulty, the data proving the core-shell structure often remain uncertain. Therefore, a process for mass-producing core-shell type metal nanoparticles having a uniform shell layer while maintaining stable quality at a low cost has not yet been realized.

  Non-Patent Document 1 claims that palladium core platinum shell particles are promising mainly for fuel cell catalyst applications.

  In Patent Documents 1 and 2, there is an underpotential deposition method in which a monolayer of copper is formed on the surface of palladium particles by adjusting the potential on the electrode, and subsequently, copper and platinum are substituted by adding a platinum salt. Have been described. In principle, this method is superior in that it can form a shell of a monoatomic layer, but because of the fact that electrons are not uniformly transmitted to the material and the reactants are not supplied uniformly, platinum is used. The shell coating may not be sufficient. In addition, this method is suitable for the purpose of analyzing the characteristics by forming a small amount of core-shell particles in the laboratory, but still has difficulty when considering productivity and automation in mass production.

  On the other hand, a method of forming a shell by electroless plating without using an electrode has also been proposed. In Patent Document 3, a platinum shell layer is formed on palladium particles by examining the type of platinum salt and the coverage is measured. However, this is not satisfactory, and it is also determined whether the shell layer has a uniform thickness. Not right.

  When using core-shell type metal fine particles as a catalyst, it is important to select conductive fine particles having a high surface area or a combination of fine particles as a carrier and to distribute the fine particles on the carrier with a suitable density. It is desirable that the particle diameter is as small as possible, and it is desirable that the particle diameters are uniform in order to achieve uniform characteristics. The impregnation method currently used as a catalyst preparation method has problems that the supported particles are likely to aggregate and that it is difficult to uniformly form two or more types of metals into alloy fine particles and core-shell fine particles. It has become.

  As a means for heating the reaction solution, the reaction solution is irradiated with microwaves. In Patent Literature 4, a semiconductor oscillator and a microwave resonance cavity are used, and a continuous flow system reaction tube is arranged at the position where the standing wave of the electric field is the largest, so that chemical reaction can be performed without impairing rapid heating or uniformity. Attempts have been made to heat. This method combining a single mode heating method and a continuous flow system is extremely useful when the reaction is sufficiently accelerated by microwave heating and is completed in a short time.

By synthesizing the core particles using the liquid phase reduction method, subsequent modifications such as shell formation can be performed easily. However, when trying to increase productivity using the batch method, nucleation occurs due to uneven heating and stirring. However, there are problems such as non-uniformity and uneven particle size. For this reason, at present, most of the metal nanoparticles that are said to have a uniform particle size are at the laboratory level, and it has been difficult to increase productivity without sacrificing quality when manufacturing metal nanoparticles. .
In particular, although palladium core platinum shell nanoparticles are expected to be used as a catalyst, it is difficult to produce a uniform shell layer with stable quality and low cost.

  As a technique corresponding to this point, the method described in Patent Document 5 uses microwave heating to synthesize palladium core particles, adjusts the shell formation reaction by adding hydroxide ions, and forms a uniform platinum shell layer. A technology for forming and achieving both stable quality and productivity improvement is described.

  Various methods have been proposed as methods for producing core-shell fine particles for oxygen reduction catalysts for fuel cells. However, it is difficult to continuously produce shell layers with stable quality and low cost. is there. For example, the method described in Patent Document 5 aims to form a uniform platinum shell layer by adjusting the shell formation reaction by adding sodium hydroxide ions. Detailed conditions for achieving both productivity improvements are not described. In addition, Patent Document 5 describes the possibility that addition of potassium hydroxide ions instead of sodium hydroxide ions can also be used for the production of core-shell fine particles, but it is described as a possibility. Only the details are not described at all.

  The synthesis of carrier-supported palladium core platinum shell fine particles, especially the synthesis conditions in the case of a carrier that exhibits conductivity, is often unknown, and it is considered extremely difficult to synthesize efficient carrier-supported palladium core platinum shell fine particles. Yes.

JP 2011-218278 A JP 2012-16684 A JP2012-120949A JP 2011-137226 A Japanese Patent Laying-Open No. 2015-223535

NEDO achievement report: "Development of technology for promoting the commercialization of polymer electrolyte fuel cells / Basic technology development / Low platinum technology" (2010-2012) Interim report for FY2010.

  One of the objects of the present invention is a production method capable of providing core-shell type fine particles supported on a carrier having conductivity such as carbon, for example, carrier-supported palladium core platinum shell fine particles for catalyst at high performance and at low cost. Is to provide.

  One of the objects of the present invention is a method for forming a shell for obtaining core-shell type fine particles carried on a carrier having conductivity such as carbon, for example, a carrier-supported palladium core platinum fine particle for catalyst at high performance and at low cost. Is to provide.

  One of the objects of the present invention is to provide, for example, high-performance and inexpensive carbon-supported palladium core platinum shell fine particles.

  In order to solve the above problems and achieve the object, an embodiment of the invention according to the present invention has the following configuration, for example.

  A first invention of the present invention (hereinafter referred to as Invention 1) made to solve the problem is a solution containing platinum raw material ions in a colloid in which carrier-supported palladium fine particles are dispersed in a dispersion medium containing a glycol solvent. In the method of continuously forming a platinum shell on the support-supported palladium core particles by microwave heating to the reaction raw material solution obtained by adding the alkali metal hydroxide solution and the alkali metal hydroxide solution, the platinum raw material solution and the alkali metal hydroxide solution were added. Thereafter, the concentration of free, coordinated, adsorbed hydroxide ions is 2 to 6 times the amount of the platinum raw material, and this is a method for producing a carrier-supported palladium core platinum shell fine particle colloid.

  A second invention of the present invention (hereinafter referred to as Invention 2) made by developing Invention 1 is the method for producing a colloid according to Invention 1, wherein the carrier-supported palladium fine particles as one of the raw material components are glycol solvents. Carrier-supported palladium, characterized in that it is synthesized by microwave heating a reaction raw material liquid containing an alkali metal hydroxide having a substance amount 4 to 7 times the amount of palladium raw material atoms This is a method for producing a core platinum shell fine particle colloid.

  A third invention of the present invention (hereinafter referred to as Invention 3), which is developed from Inventions 1 and 2, is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to Inventions 1 and 2, wherein the carrier is carbon. Production of carbon-supported palladium core platinum shell particle colloid characterized by being synthesized by a continuous microwave irradiation method in which the formation process of carbon-supported palladium particles and the platinum shell formation process are connected by a continuous flow path Is the method.

  A fourth invention of the present invention (hereinafter referred to as invention 4) developed by developing the inventions 1-3 is a method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1-3. A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the metal hydroxide is potassium hydroxide.

  A fifth invention of the present invention (hereinafter referred to as Invention 5) made to solve the problem is a palladium raw material and a potassium hydroxide solution in a dispersion in which a carrier is dispersed in a dispersion medium containing a glycol solvent. In the method of continuously synthesizing carrier-supported palladium nanoparticles by microwave heating, adding 4 to 7 times the amount of potassium hydroxide as the palladium raw material atoms of the palladium raw material, This is a method for producing a carrier-supported palladium fine particle colloid.

  A sixth invention of the present invention (hereinafter referred to as Invention 6), which has been made to solve the problems, uses a colloid in which the fine particles described in Invention 5 are dispersed in a dispersion medium containing a glycol solvent as a raw material, and uses hexachloro In a method of forming a platinum shell on the carrier-supported palladium core particles by adding a platinum (4) acid ion-containing solution and a potassium hydroxide solution to form a platinum shell on the carrier-supported palladium core particles, after adding the platinum raw material solution and the potassium hydroxide solution, A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the concentration of free, coordinated and adsorbed hydroxide ions is 2 to 6 times the amount of the platinum raw material.

  The seventh invention of the present invention (hereinafter referred to as Invention 7) developed by developing Inventions 5 and 6 is the method for producing a colloid according to Inventions 5 and 6, wherein the carrier is a carbon fine particle, The method for producing a carbon-supported palladium core platinum shell fine particle colloid is characterized in that the particle colloid and the subsequent platinum shell forming method are connected by a continuous flow path and are carried out by a continuous method.

  The eighth invention of the present invention (hereinafter referred to as invention 8) made by developing the invention 1 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to the invention 1, wherein 2-aminoethanol is used as an additive. It is a method for producing a carrier-supported palladium core platinum shell fine particle colloid characterized in that it is contained.

  The ninth invention of the present invention (hereinafter referred to as invention 9) made by developing the inventions 1-3 and 8 is the production of the carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1-3 and 8. In the method, the carrier-supported palladium core platinum shell fine particle colloid is characterized in that the alkali metal hydroxide is sodium hydroxide.

  The tenth invention of the present invention (hereinafter referred to as Invention 10) made by developing Inventions 1-3 and 8 is the production of the carrier-supported palladium core platinum shell fine particle colloid according to any one of Inventions 1-3 and 8. In the method, the carrier-supported palladium core platinum shell fine particle colloid is characterized in that the alkali metal hydroxide is cesium hydroxide.

  The eleventh invention of the present invention (hereinafter referred to as invention 11) made by developing the inventions 1-3 and 8 is the production of the carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1-3 and 8. In the method, a carrier-supported palladium core platinum shell fine particle colloid is characterized in that the alkali metal hydroxide is lithium hydroxide.

  A twelfth invention of the present invention (hereinafter referred to as invention 12) made by developing the inventions 1 to 8 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 8. A method for producing a carrier-supported palladium core platinum shell fine particle colloid characterized by adding tetramethylguanidine together with a metal hydroxide.

  A thirteenth invention of the present invention (hereinafter referred to as invention 13) made by developing the inventions 1 to 8 is an alkali in the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 8. A method for producing a carrier-supported palladium core platinum shell fine particle colloid characterized by adding hexamethylenetetramine together with a metal hydroxide.

  A fourteenth invention of the present invention (hereinafter referred to as invention 14) made by developing the inventions 1 to 8 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 8. A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein triethanolamine is added together with a metal hydroxide.

  A fifteenth invention of the present invention (hereinafter referred to as invention 15) made by developing the inventions 1 to 8 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 8, wherein A method for producing a carrier-supported palladium core platinum shell fine particle colloid characterized by adding choline together with a metal hydroxide.

  The sixteenth invention of the present invention (hereinafter referred to as invention 16) made by developing the inventions 1 to 15 is used in the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any of the inventions 1 to 15. A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the glycol solvent to be used is ethylene glycol.

  The seventeenth invention of the present invention (hereinafter referred to as invention 17) developed by developing the inventions 1 to 15 is a process for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 15; A method for producing a carrier-supported palladium core platinum shell fine particle colloid, characterized in that glycerin is mixed with a system solvent.

An eighteenth invention of the present invention (hereinafter referred to as invention 18) made by developing inventions 2 to 17 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of inventions 2 to 17, wherein The palladium raw materials are palladium chloride (2) (PdCl ₂ ), sodium tetrachloropalladium (2) (Na ₂ [PdCl ₄ ]), potassium tetrachloropalladium (2) (K ₂ [PdCl ₄ ]), tetrachloro Ammonium palladium (2) acid ((NH ₄ ) [PdCl ₄ ]), palladium acetate (2) (Pd (CH ₃ COOH) ₂ ), palladium oxalate (2) (PdC ₂ O ₄ ), palladium nitrate (2) (Pd (NO ₃ ) ₂ ), palladium sulfate (2) (Pd (SO ₄ )), palladium acetylacetonate, tetraan A method for producing a carrier-supported palladium core platinum shell fine particle colloid comprising a salt raw material selected from a min palladium (2) salt and a dinitrodiammine palladium (2).

  A nineteenth invention of the present invention (hereinafter referred to as invention 19) made by developing the inventions 2 to 18 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 2 to 18, wherein A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the palladium raw material is potassium tetrachloropalladium (2).

  The twentieth invention of the present invention (hereinafter referred to as invention 20) made by developing the inventions 2 to 18 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of inventions 2 to 18, wherein A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the palladium raw material is sodium tetrachloropalladium (2).

  A twenty-first invention of the present invention (hereinafter referred to as invention 21) made by developing inventions 2 to 18 is the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of inventions 2 to 18, wherein A method for producing a carrier-supported palladium core platinum shell fine particle colloid, wherein the palladium raw material is palladium acetate (2).

  According to a twenty-second invention of the present invention (hereinafter referred to as invention 22) made by developing the inventions 1-21, in the method for producing a carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1-21, a carrier It is a method for producing a carrier-supported palladium core platinum shell fine particle colloid characterized in that the process proceeds to the next platinum shell forming step under the condition that the surface of the supported palladium core particle synthesis product is not contaminated.

  A twenty-third invention of the present invention (hereinafter referred to as invention 23) made by developing the inventions 1-22 is the carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1-22, wherein the majority of the shells It is a carrier-supported palladium core platinum shell fine particle colloid characterized in that the number of layers is 5 or less.

  The twenty-fourth invention of the present invention (hereinafter referred to as invention 24) made by developing the invention 23 is the carrier-supported palladium core platinum shell fine particle colloid according to the invention 23, wherein 90% or more of the shell layer is 5 or less. It is a support-supported palladium core platinum shell fine particle colloid.

  A twenty-fifth invention of the present invention (hereinafter referred to as invention 25) made by developing the inventions 23 and 24 is the carrier-supported palladium core platinum shell fine particle colloid according to the inventions 23 and 24, wherein a majority of the palladium core platinum shell fine particles are present. The carrier-supported palladium core platinum shell fine particle colloid is characterized in that the shell layer covers 50% or more of the core layer.

  The twenty-sixth invention of the present invention (hereinafter referred to as invention 26) made by developing the inventions 23 and 24 is the carrier-supported palladium core platinum shell fine particle colloid according to the inventions 23 and 24, wherein the majority of the palladium core platinum shell nanoparticle. The support-supported palladium core platinum shell fine particle colloid is characterized in that the shell layer covers 90% or more of the core layer.

  According to a twenty-seventh aspect of the present invention (hereinafter referred to as "invention 27") made by developing the inventions 1 to 26, the carrier-supported palladium core platinum shell fine particle colloid according to any one of the inventions 1 to 26 is formed into a dry powder. A method for producing a carrier-supported palladium core platinum shell fine particle colloid, characterized in that alkali metal ions remain in the dry powder.

  The twenty-eighth invention of the present invention (hereinafter referred to as invention 28) made by developing inventions 1 to 27 uses the carrier-supported palladium core platinum shell fine particles produced by the method according to any one of inventions 1 to 27 as a catalyst. This is a polymer electrolyte fuel cell used.

The twenty-ninth invention (hereinafter referred to as invention 29) of the present invention made to solve the problem is:
0.1M perchloric acid aqueous solution was added to a glass 5-neck flask for convective voltammetry measurement having a width of 14.5 cm, a height of 12.5 cm, and a volume of 300 ml or more,
3 L / min of nitrogen was bubbled for 30 minutes to expel oxygen,
To the glassy carbon rotating electrode with a diameter of 5 mm of the glassy carbon portion coated with the catalyst ink so that 0.96 μg of platinum is contained,
At a temperature of 20 ° C
When a durability test was performed in which a potential cycle was applied using a reversible hydrogen electrode as a reference electrode and a potential of 0.6 V for 3 seconds and 1.0 V for 3 seconds as a square wave,
Add 0.1 M aqueous perchloric acid to the flask,
Use 3 L / min oxygen bubbling for 30 minutes to expel nitrogen and use as electrolyte
Electrode rotation speed 1600 [rotation / min],
Temperature 25 ° C,
Scanning speed to the high potential side of 20 [mV / sec],
In the condition of
In the mass activity calculation method for calculating an oxygen reduction current value at a potential of 0.9 V using a reversible hydrogen electrode as a reference electrode from the Koutecky-Levich equation and compensating for solution resistance,
The platinum mass activity as an oxygen reduction catalyst after 37000 cycles, that is, the value obtained by dividing the current value in the mass activity calculation method by the applied amount of platinum of 0.96 μg can maintain 75% of the mass activity before the durability test. It is a palladium core platinum shell fine particle colloid characterized by this.

  A thirtieth invention of the present invention (referred to as invention 30) made by developing the invention 1-27 is characterized in that, in the mass activity calculation method in invention 29, the initial activity is 600 [A / g] or more. The palladium core platinum shell fine particle colloid according to any one of Inventions 1 to 27.

  A thirty-first invention of the present invention (referred to as invention 31) made by developing the invention 1-27 is a carrier-supported palladium core platinum shell fine particle produced by the method according to any one of the invention 1-27. It is a catalyst.

  The thirty-second invention of the present invention (referred to as invention 32) made by developing the inventions 1-27 is that the initial activity of the catalyst calculated by the mass activity calculation method in invention 29 is 600 [A / g] or more. It is a palladium core platinum shell fine particle catalyst characterized by these.

  If a suitable additive such as potassium hydroxide is appropriately adjusted to synthesize the carbon-supported palladium core particles in the core-shell type fine particles of the carbon-supported palladium core platinum shell by the method found by the present invention, the hydroxide ion concentration and the like are appropriately adjusted. When the fine particles are catalyst fine particles, the core-shell type fine particles of the carbon-supported palladium core platinum shell having high catalyst performance and excellent durability can be provided at low cost.

It is a figure explaining the carrier carrying | support palladium core platinum shell fine particle manufacturing method of this invention. It is a figure explaining the transmission electron microscope image of the carbon carrying | support palladium core platinum shell microparticles | fine-particles produced at the process of the Example. It is the graph which compared the mass activity of the oxygen reduction catalyst synthesize | combined by the Example and the comparative example. It is a figure explaining transition of the maintenance rate of mass activity to the number of potential cycles in the endurance test of the carbon carrying palladium core platinum shell fine particles synthesized by the method of the example.

1: Raw material salt of palladium core particles, dispersant, carrier, raw material solution in which potassium hydroxide is dissolved and dispersed in ethylene glycol and pump for feeding it 2: Microwave in the resonator structure that confines the microwave electric field To form a standing wave and irradiate the reaction liquid flowing in the reaction tube arranged there with microwaves to heat the reaction system 3: high temperature reaction liquid immediately after microwave heating 4: Cooling apparatus 4: Potassium hydroxide ethylene glycol solution for adjusting the hydroxide ion concentration at the time of forming the platinum shell layer and pump system 5 for feeding the solution 5: Dissolving the raw material salt for forming the platinum shell layer Pump system 6a, 6b that feeds the raw material liquid and process using the mixer 7: Step 7 using a mixer: Microwave irradiation reaction system 8 for forming a platinum shell 8: Recovery tank 9: Carbon support Palladium core platinum shell catalyst 9a: Carbon support 9b: Palladium core platinum shell catalyst 10a: Initial activity in the load response durability test of Example 1 expressed as 100% Point 10b: 5000 cycles in the load response durability test of Example 1 Point 10c representing the subsequent mass activity retention rate: Point 10d representing the mass activity retention rate after 37000 cycles in the load response durability test of Example 1: Initial activity in the load response durability test of Example 2 Point 10e expressed as 100%: Point 10f representing the mass activity maintenance rate after 5000 cycles in the load response durability test of Example 2 Mass activity after 37000 cycles in the load response durability test of Example 2 Of the maintenance rate

  Embodiments of the invention according to the present invention will be described in detail below with reference to the drawings. The drawings used for the description schematically show the dimensions, shapes, arrangement relationships, and the like of each component to the extent that an example of the present invention can be understood. In addition, for convenience of explanation of the present invention, there may be cases where the enlargement ratio is partially changed for illustration, and there is a case where it is not necessarily similar to the real thing such as an embodiment. Moreover, in each figure, the same number is attached | subjected to the same structure and duplication description is abbreviate | omitted. Furthermore, in the following description, the description of the method for producing metal fine particles may also serve as the explanation for the metal fine particle or metal fine particle production apparatus within the range where no misunderstanding occurs, and vice versa.

  Taking catalyst fine particles as an example, it is well known that palladium core platinum shell fine particles exhibit an excellent catalytic effect. Therefore, since platinum is expensive, many people skilled in the art are making great efforts to reduce the amount of platinum used as much as possible, to increase the catalytic effect as much as possible, and to make the catalyst high quality and inexpensive. Currently. Further, regarding the carrier, it is desired that the carrier support and the core can be synthesized simultaneously from the point of synthesis of the core fine particles.

  In the palladium fine particle synthesis process by the liquid phase method in the presence of a carrier, it is considered that nucleation of palladium fine particles is likely to occur due to functional groups and defects on the carrier such as carbon black. Therefore, by adjusting the type, number, density, etc. of the functional groups present on the support and appropriately adjusting the reaction conditions such as the hydrogen ion concentration, hydroxide ion concentration, palladium raw material concentration, temperature during the reaction, the support The fine palladium particles can be supported simultaneously with the synthesis with the desired particle size and density.

Palladium chloride (2) (PdCl ₂ ), sodium tetrachloropalladium (2) acid (Na ₂ [PdCl ₄ ]), potassium tetrachloropalladium (2) acid (K ₂ [PdCl ₄ ]) are preferable as the palladium raw material. ), Ammonium tetrachloropalladium (2) ((NH ₄ ) [PdCl ₄ ]), palladium acetate (2) (Pd (CH ₃ COOH) ₂ ), palladium oxalate (2) (PdC ₂ O ₄ ), nitric acid A raw material selected from palladium (2) (Pd (NO ₃ ) ₂ ), palladium sulfate (2) (Pd (SO ₄ )), palladium acetylacetonate, tetraammine palladium (2) salt, and dinitrodiammine palladium (2). Can be included.

  It is known that the palladium core platinum shell structure is highly active and highly durable. However, the size of the catalyst fine particles is several nanometers, and the number is enormous even within a lot, and almost all of the catalyst fine particles are required to have the above highly active and highly durable structure. The This will be referred to as catalyst quality for convenience.

  In displacement plating and electroless plating, it is more difficult to form a shell structure on core metal particles supported on a conductive carrier. Since the carrier is conductive, electrons may flow through the carrier and the oxidation reaction and the reduction reaction may take place at a distance, and there may also occur side reactions such as the formation of single particles of shell metal on the carrier. .

  The formation of core-shell metal fine particles having a uniform shell structure seems to require a process similar to plating, but existing plating techniques cannot be applied as they are. However, in order to grow the shell layer uniformly over the entire surface of the fine particles without side reactions within and between the particles, the supply and reactivity of the shell raw material must be as uniform as possible.

As far as possible, the reactivity of the shell layer formation can be controlled so that only the reaction in which there is no pinhole on the surface of the palladium core particles and the platinum shell layer is formed proceeds, and the reaction does not proceed after the shell layer formation is completed. desirable.
There is a view that forming the shell layer as uniformly as possible is an important factor in controlling the characteristics of the core-shell type fine particles.
Form the shell layer uniformly, make the shell layer 5 or less, cover 50% or more of the core surface with the shell layer, cover 70% or more with the shell layer, and cover 90% or more with the shell layer It is also important to select according to the application and purpose.
It is preferable in terms of characteristics that the shell layer is 5 layers or less, and that 50% or more of the core surface is covered with the shell layer, and it is more preferable that 90% or more is covered with the shell layer.

  For example, in order to suppress unnecessary side reaction pathways, it is essential that the platinum salt precipitates as a zero-valent metal only when it comes into contact with palladium core particles or the surface of a platinum shell layer having palladium core particles as a base. In other words, it is necessary to suppress the reaction that draws electrons from the surrounding chemical species without being in contact with the underlying core particle to become a metal nucleus, or forms an excessive shell layer on the existing platinum shell layer. Don't be. Also, palladium (2) ions released into the solvent by displacement plating must not be reduced again in any way.

  In order to realize such reaction control for all the palladium core particles and platinum shell raw material salt present in the reaction solution, the method of adding various kinds of additives tends to result in uneven reaction conditions. In particular, in PEFC using hydrogen as a fuel, the adhesion of organic molecules to the catalyst surface may reduce the activity, necessitating a careful cleaning step.

  Here, synthesis of the core particles using the liquid phase reduction method facilitates subsequent modifications such as shell formation, but if the batch method is used to increase productivity, heating and stirring are not uniform. As a result, nucleation is likely to be uneven and particle diameters are not uniform. Most of the fine metal particles known to have a uniform particle size are synthesized on a laboratory scale, and it is difficult to increase productivity without sacrificing quality when producing fine metal particles. Will be proved.

  As a means for heating the reaction liquid in the liquid phase reduction method, the reaction liquid is irradiated with microwaves as in the invention described in Patent Document 5. That is, here, using a semiconductor oscillator and a microwave resonant cavity, a continuous flow reaction tube is placed at the position where the standing wave of the electric field is the strongest, so that rapid heating does not impair the uniformity. Heating for reaction is performed. Although not limited to this, a single mode heating method, which is a method of generating a single electromagnetic field mode in a cavity in which dimensions and resonance frequencies are adapted, is particularly preferable, and the reaction is sufficiently accelerated by microwave heating for a short time. This is extremely useful when completed with In addition, heating and reaction non-uniformity, which are problems in the batch method, can be remarkably improved.

  The effect of microwave irradiation varies depending on the frequency of the microwave irradiated to the reaction solution, the material of the reaction solution, the inner diameter of the reaction tube through which the reaction solution flows, the material of the reaction tube, and the like. In the case of Japan, various microwave frequencies such as 5.8 GHz and 0.9 GHz can be used in addition to frequencies often used for microwave ovens (frequency band: 2.4 to 2.5 GHz). In the examination of the present invention, the frequency of 2.45 GHz is mainly used, but other frequency bands such as 5.8 GHz and 0.9 GHz are also used, although not limited to this.

  For example, when using a microwave device having a frequency band of 2.4 to 2.5 GHz, it is considered that any frequency in the 2.4 to 2.5 GHz frequency band can be within the standard. The inner diameter of the reaction tube is not less than 2.9 mm, which is the optimum dimension at 2.45 GHz, and the standing wave absorption intensity is within a predetermined range even at 2.5 GHz, which is the highest frequency in the standard range. Thus, it is preferable to set the inner diameter of the reaction tube at 2.5 GHz to an optimum value. From this point of view, the inner diameter of the reaction tube was set to 2.9 ÷ 2.5 × 2.45 = 2.8 mm or less when the microwave irradiation condition was controlled particularly strictly.

  In consideration of these points, experiments for selecting a carrier were repeatedly performed using many materials and additives that have conductivity and are considered to be suitable for a carrier for supporting palladium (Pd). As a result, by using carbon as the carrier, the core shell fine particles of palladium gore platinum shell were synthesized, and the possibility of realizing a catalyst of good quality was found.

  When a palladium core platinum shell is synthesized as catalyst fine particles, the method of carrying fine particles after synthesis has the difficulty that uniform loading is difficult and loss of noble metal fine particles is difficult, and that a dispersing agent removal step is necessary. . When catalyst fine particles are mixed and supported on the carrier, it is difficult to adsorb and support the catalyst fine particles on the surface of the carbon carrier. However, if excessive catalyst fine particles are added, agglomeration occurs on the carbon carrier, making effective use. A catalyst surface that cannot be produced or free catalyst particles that are not supported are produced, and the noble metal raw material does not function effectively as a catalyst. In PEFC, polymer dispersants and organic compounds that do not participate in the battery reaction are adsorbed on the catalyst surface to block the active sites, and carbon monoxide can cause catalyst poisoning, resulting in battery failure. Because it causes, it is necessary to remove such substances as much as possible. However, when carbon having a high surface area is used as a carrier, a compound having a hydrophobic group such as a polymer dispersant or a surfactant is easily adsorbed on the carbon carrier and is difficult to remove. Complete removal requires careful cleaning and firing in an inert atmosphere, which increases the number of steps and increases costs. From this point of view, if a method of supporting noble metal fine particles on a carbon support by microwave heating is used, the metal fine particle catalyst can be uniformly supported on the support, and if the conditions are adjusted, it is possible to convert almost all the noble metal raw material as a catalyst. Yes, and very useful.

  By using a solvent that absorbs microwaves well, heating can be performed more rapidly and uniformly than normal heating such as heaters and oil baths. Advantages such as completion of the reaction in a short time and effective use of energy can be achieved. Available.

  Glycol solvents have a high boiling point and viscosity, dissolve various substances, are relatively safe for the human body, easily absorb microwaves, and have a weak reducing ability, and are suitable for the technology of this patent. Solvent group. Specific examples include, but are not limited to, ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, and the like. Absent. In addition, polyols such as glycerin, glycol ethers such as ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and diethylene glycol dimethyl ether, glycol esters, and the like can be added to the glycol solvent as necessary.

  From these viewpoints, although not limited to this, one of the embodiments of the present invention is a typical solvent that absorbs microwaves well, and heats up to its boiling point (196 ° C.) in a short time. We decided to use ethylene glycol.

  When the ionomer for a fuel cell is used as a carbon dispersant, it is also important that the fuel cell ionomer need not be completely removed by purification, calcination or the like. Polymeric dispersants such as polyvinylpyrrolidone are widely used for the synthesis of metal nanoparticles, but these dispersants improve the dispersibility of metal nanoparticles and carbon, while being strongly adsorbed on the surface of fine particles. It has been found that it is difficult to remove the polymer dispersant, and it is also known that the polymer dispersant remaining on the catalyst surface significantly impairs the catalyst performance. Since the ionomer for fuel cells is one of the constituent components of the cell added in the cell preparation process for transferring protons to the catalyst surface, even if a small amount remains, the fuel cell performance is not significantly affected.

Furthermore, in the embodiment of the present invention, metal nanoparticles are synthesized by reducing the raw metal salt. This reaction is expressed in chemical formula:
M ⁺ (metal ion) + e ⁻ (electrons supplied from the reducing agent) → M ⁰ (metal) (1)
It becomes.
Several thousands of zero-valent metals on the right are collected to form metal fine particles.

  Various synthesis methods are conceivable depending on what is used as the raw material for the metal ions and the reducing agent for supplying electrons. In the embodiment of the present invention, sodium tetrachloropalladate and chloride are used as the metal ions. Platinum acid was used as the reducing agent and ethylene glycol.

When ethylene glycol is heated, it has a reducing power.
Ethylene glycol → e ⁻ + ethylene glycol oxidation product + hydrogen ion (2)
It is expressed.
By rapidly heating ethylene glycol containing metal raw material salt by microwave, a large amount of electrons are rapidly supplied from ethylene glycol, and fine metal particles with small particle diameter can be synthesized by rapidly reducing the raw metal salt. .

  Slow heating of ethylene glycol tends to moderate metal particle nucleation and crystal growth and produce nano or micron particles with large particle sizes.

  In the formula (2), hydrogen ions are released with the supply of electrons from ethylene glycol. However, when this is accumulated, the reaction rate decreases and it becomes difficult to form fine particles having a small particle diameter. However, by adding a compound containing hydroxide ions, it is possible to suppress an increase in hydrogen ion concentration and maintain a high reaction rate. As a result, nanoparticles with a small particle diameter are synthesized. be able to.

  In the embodiment of the present invention, a palladium core particle having a small particle diameter is synthesized by using microwave heating, and platinum shell is obtained by slowly reacting chloroplatinic acid with microwave heating or room temperature. The generation of single particles is suppressed, and a shell having a uniform thickness is formed. In these cases, potassium hydroxide is used as an accelerator for the reduction reaction. By adding potassium hydroxide, the reduction reaction at a high temperature becomes faster and the reaction proceeds slowly even at room temperature, so that the reactivity can be delicately controlled. For this reason, a shell having a uniform thickness can be formed. In particular, by using potassium hydroxide, the shell can be formed with higher quality.

Amines can also be considered as compounds that suppress the increase in hydrogen ion concentration, but the amines tend to form complexes with palladium raw materials and platinum raw materials, and are adsorbed on the surface of the generated fine particles. The state of particle growth changes in a complex manner. Metal hydroxides are also considered as a source of hydroxide ions, but must be sufficiently soluble in ethylene glycol. As described in detail in the present invention, as a result of many experiments, the inventor of the present invention uses potassium hydroxide as a hydroxide for the synthesis of palladium core particles in the core-shell type catalyst fine particles of the carbon-supported palladium core platinum shell. Was particularly preferred, and the conditions were found.
In the embodiment of the present invention, the above reaction is performed in the presence of a carbon support. In this case, the palladium nanoparticles are generated while adhering to the carbon, and the platinum shell grows on the surface of the palladium fine particles on the carbon, and a core-shell structure is formed on the carbon support.

  In Patent Document 5, synthesis of palladium core particles and platinum shell particles is performed in an environment in which no carbon carrier coexists. However, in the coexistence of the carbon carrier, carbon agglomeration, clogging in a carbon reaction tube, and on the palladium core particle carrier. Undesirable distribution paths to the catalyst and formation of platinum single particles on carbon may cause undesirable reaction paths for the catalyst. For this reason, the production method of the present invention is known in which the synthesis of core particles and their loading on carbon can be performed simultaneously in a continuous reaction system, and then a uniform shell layer can be formed on the core particles supported on carbon. There wasn't.

  The inventors have found that the concentration of hydroxide ions in the reaction and that the counter ions are potassium ions are extremely important in the formation of palladium core particles and subsequent platinum shells. . As a result of a great deal of trial and error, the carbon-supported palladium core platinum shell fine particles with high performance are suppressed by using a continuous synthesis method that suppresses the above-mentioned undesirable reaction path and irradiates the reaction liquid with microwaves while circulating the reaction liquid. The process of obtaining the catalyst could be completed.

  Particularly important problems in producing carrier-supported palladium core platinum shell fine particles, such as carbon-supported palladium core platinum shell fine particles for battery catalysts, are cost reduction, performance improvement, and quality improvement. A synthesis method is desired that has as high a catalytic performance as possible, has excellent durability, and can be manufactured at low cost.

  As a result of many experiments, in order to effectively advance the reaction for solving the problems of the present invention, it has been found that it is effective to use an alkali metal hydroxide as an additive for the reaction. We decided to investigate the relationship between oxide type and catalyst quality in detail.

  Specifically, lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, etc. can be used as the alkali metal hydroxide. The purpose of increasing the hydroxide ions involved in the reaction is to add basic substances such as amines and guanidines as long as they do not induce unwanted side reactions. Specifically, ammonia, 2-aminoethanol, diethanolamine, triethanolamine, choline, butylamine, taurine, glucosamine, trishydroxymethylaminomethane, pyridine, triethylamine, guanidine, tetramethylguanidine, hexamethylenetetramine, N, N- Examples include diisopropylethylamine, triisopropylamine, diazabicycloundecene, diazabicyclononene, and triethylenediamine.

  Although the mechanism is not clear, as a result of experiments, potassium hydroxide has an important effect on improving the catalyst quality if the concentration to be added is appropriately selected among alkali metal hydroxides as additives for promoting the reaction. I decided to find out what I was doing and look into the details.

  Since it is important to adjust the potassium hydroxide concentration in the reaction solution, it is considered that potassium ions and hydroxide ions in the reaction system play an important role. Substances that generate these two species, such as potassium acetate, potassium carbonate, potassium phosphate, etc., and mixtures of substances that generate these two species, such as sodium hydroxide and potassium chloride, but also potassium ions and hydroxides It is considered that the above-mentioned preferable effect is produced in the formation of the platinum shell because the product ions are generated. However, in the presence of a carrier, ions other than potassium ions and hydroxide ions may cause aggregation of the carrier, and potassium hydroxide was the most suitable additive in a reaction system using ethylene glycol as a solvent.

  Potassium ions are considered to have an effect of depositing [PtCl4] 2− ions and / or [PtCl 6] 2− ions in ethylene glycol, and may suppress the formation of free platinum particles. Also, hydroxide ions adsorb on the surface of the palladium core particles while interacting with alkali metal ions, and the above [PtCl4] 2- ions and / or [PtCl6] 2-ions are not on the support but on the palladium particles. It seems to have the effect of selectively depositing.

  Sodium ions are also strongly attracted to hydroxide ions and exist on the palladium surface, but their adsorption is so strong that they tend to inhibit uniform shell formation, and Na2 [PtCl6] has a solubility in ethylene glycol. It is considered difficult to form a high-quality platinum shell with only sodium ions because it is difficult to form a platinum raw material deposition structure. On the other hand, since potassium ions are not as strong as sodium ions in attracting hydroxide ions, the effect of inhibiting shell formation is not as strong as sodium ions. In the shell formation process, when the reaction is performed slowly at room temperature without using microwave heating, there is a significant difference in the activity of the catalyst between when sodium hydroxide and potassium hydroxide are used.

  Even when sodium hydroxide is used without using potassium hydroxide, if a platinum shell is formed using the microwave continuous heating method by selecting the coexisting hydroxide ion concentration and temperature as the preferred conditions, the shell slowly grows at room temperature. It has also been found that catalyst particles having a greater mass activity than that formed can be synthesized.

  By adjusting the potassium hydroxide concentration, in the platinum shell formation process, the deposit composed of chloroplatinate ions and potassium ions is adsorbed with moderate strength on the carrier-supported palladium core particles and heated rapidly by microwaves. As a result, the shell formation reaction proceeds only on the surface of the palladium core particles, and the present invention is not limited to this. However, there is a possibility that a platinum shell having a uniform thickness or as close as possible to it may be formed.

  The shell layer of fine particles such as nanoparticles preferably covers the core particle in a perfect layer, but the actual synthesized shell layer covers the core particle in a perfect layer and in the case of the core particle. There are many forms, such as when partially attached to the surface. Of course, it is no wonder that the shell layer is not disposed on the portion of the core particle supported by the carrier. In the production of the core-shell type fine particles, those in which at least a part of the performance intended for use is improved are referred to as the core-shell type fine particles in the present invention.

  By using the microwave continuous heating method, the present inventors have also solved the trade-off of maintaining quality within a lot, suppressing variation between lots, and simplifying the process. Rapid heating by microwave heating can be performed more rapidly than normal heating using an oil bath or steam, and is used in various organic synthesis and inorganic synthesis processes. In addition, if the volume of the object to be heated is reduced to some extent, the energy of the electric field can be concentrated to further increase the heating rate, and only the solvent can be heated uniformly, so the timing of nucleation and growth of nanoparticles etc. is important Suitable for reaction. In addition, it was possible to eliminate the lot-to-lot variation and productivity trade-off by configuring the microwave heating process by a continuous method using a flow path. Although microwave heating can also be performed by a batch method, if the volume of an object to be heated is increased in order to increase productivity, the uniformity is impaired and the quality in the lot varies. On the other hand, the microwave continuous irradiation method is always easy to automate because synthetic products can be obtained under the same conditions. Furthermore, even when trouble occurs, it is only necessary to switch the flow path to separate defective products, and there is no situation where the entire tank of valuable precious metal raw materials is wasted unlike the batch method.

  In addition, the advantage of connecting the palladium fine particle forming step and the subsequent platinum shell forming step with a continuous reaction tube to form a continuous method is also important. Immediately after synthesis, the surface of the palladium fine particles is thought to adsorb coexisting hydroxide ions and chloride ions, but by leaving it in the atmosphere, an oxide layer is formed on the surface, or sulfur from the outside It is thought that the surface is contaminated due to strong adsorption of impurity atoms such as. Such surface contamination is considered to hinder the formation of a uniform platinum-coated structure, and if the two processes are separated by a batch method or the like, it causes variation. Therefore, it is ideal that the palladium fine particles are coated with platinum atoms as quickly as possible, and the synthesis by the continuous method satisfies this requirement.

  In the field of plating, it is widely practiced to suppress unwanted side reactions and abnormal growth of deposited metal layers using additives. The inventors believe that the same method can be applied to the formation of a platinum shell on palladium core particles in the presence of a support. The inventors have repeated numerous experiments and the additives and reaction conditions that can form a high-quality platinum shell layer However, at least it was not possible to find an effective additive in the reaction at room temperature. Most of the cause seems to be because the additive strongly adsorbs on the surface of the palladium core particle and inhibits the formation of the platinum shell. Batch heating method because there is a problem that palladium core particles agglomerate on the support and the surface area of the catalyst decreases when heated for a long time in the presence of the support even if it is heated to advance the reaction. However, it was difficult to search for the optimum conditions when using a reaction-suppressing additive.

  However, it has been found that high-quality platinum shells may be formed even when coexisting with reaction-suppressing additives, by appropriately selecting reaction conditions and performing microwave heating rapidly in a short time. . In the microwave continuous heating method, in which the core synthesis process and the shell synthesis process are followed by microwave irradiation in both processes, the raw material solution can be heated uniformly only for the time required to form the platinum shell. It has become possible to form a shell that greatly suppresses aggregation of particles on the carrier. Additives are thought to play an important role in controlling the shape, surface irregularities, and crystal planes of fine metal catalyst, and may be an important control factor when forming a core-shell structure with binary metal species. There is. For this reason, the microwave continuous method is a very excellent technology in that only the efficacy of a reaction-inhibiting additive is realized, side reactions are suppressed, and a highly controlled metal fine particle catalyst can be produced. .

  When performing the two-stage reaction by the microwave continuous heating method, there is a problem that the residue of the first step does not interfere with the second step in the second step. For the residue of the palladium fine particle synthesis process, the raw materials and reaction conditions that do not adversely affect the platinum shell formation process were selected, and the residue was used for deposition, such as the use of potassium hydroxide.

  When performing the microwave continuous heating method in the presence of a carrier, a pump capable of transporting the slurry without pulsation is required. Appropriate use of a mono pump, precision diaphragm pump, etc. has been found useful in conductive carbon supports most commonly used as fuel cell catalyst supports. By connecting the raw material and the carrier slurry to a pump and heating them continuously with microwaves, an ideal process was achieved in which a supported high-quality catalyst slurry could be produced.

  By carrying out the reaction of forming and supporting palladium particles in the presence of a carrier, followed by the reaction of forming a platinum shell layer, it was possible to greatly reduce the amount of platinum used and improve productivity. As described above, the method of adsorbing the catalyst particles after the synthesis of the catalyst particles is not suitable for actual production due to the poor loading rate and non-uniform loading. In addition, in the case of a catalyst using a noble metal as in the present invention, it becomes an inefficient process in which a fine particle catalyst such as a core-shell structure, which is troublesome in steps, is lost at the final stage of synthesis. After extensive experimentation and examination, the present inventors do not use any polymers or surfactants that are not included in the final product, and form a palladium-platinum composite catalyst on the support without any purification process. I found a way. After the synthesis, only the process of removing water-soluble salts and solvent residues by washing with water. Repeated washing to remove the dispersant and baking to remove organic matter are not required.

  According to the observation with an electron microscope, platinum single particles may be slightly observed, but the number can be greatly reduced by appropriately selecting the synthesis conditions.

  In the microwave continuous heating method, the reaction tube can be thickened and scaled up in a range where microwaves can be uniformly and rapidly heated. In the batch method, there are various considerations such as heat transfer and stirring when reacting in a large tank with a scale-up, but in the microwave continuous method, production is relatively easy by controlling the input energy and residence time. The amount can be increased.

  The method as described above can be applied not only to the formation of a platinum shell structure with palladium fine particles as a base, but also to the uniform formation of a platinum thin film with a thickness of several nanometers or less on a palladium thin film as a base. In addition, the method of depositing platinum raw material salt is applied not only to carbon support, but also to forming platinum thin film on various bases including ceramics, and bases with a composite function of carbon, metal and ceramics. be able to. Therefore, this method is extremely useful in that it can be widely applied not only to the palladium core platinum shell structure but also to the formation of platinum composite fine particles having a platinum thin film structure and a platinum composite structure.

  As described above, various trade-offs related to production that have existed so far could be solved by an appropriate combination of the elemental technologies of the present invention, such as deposition, continuous microwave method, and synthesis in the presence of a carrier. According to the present invention, it is possible to produce palladium-platinum composite fine particles having both quality, cost, and productivity, and it can be said that this is a great progress toward wide-ranging practical application of solid polymer fuel cells.

  An embodiment of the present invention based on the above will be specifically described below.

  FIG. 1 is a diagram schematically showing the configuration of a carbon-supported palladium core platinum shell fine particle production apparatus as an embodiment of the present invention.

  In FIG. 1, reference numeral 1 is composed of a raw material salt of palladium core particles, an ionomer, a carbon carrier, a reaction raw material liquid in which a potassium hydroxide raw material is dissolved and dispersed in a solvent, and a pump for feeding the reaction raw material liquid. Specifically, a reaction raw material liquid in which potassium tetrachloropalladium (2) acid (K2PdCl4), Nafion (registered trademark), Vulcan (registered trademark) XC72, an aqueous potassium hydroxide solution and EG (ethylene glycol) are mixed with a liquid feed pump. The process of sending the liquid to the next process is shown.

  Reference numerals 2 and 7 are resonator structures for confining a microwave electric field and irradiating and heating the liquid in the reaction tube. Specifically, the reaction liquid is produced and sent in the reaction raw material production process. Is a resonator structure such as a cavity for irradiating the reaction raw material liquid flowing in the reaction tube through the microwave resonator structure with microwaves, and constitutes a microwave irradiation system. Reference numeral 3 denotes an apparatus for cooling and keeping the temperature of the reaction liquid heated by reference numeral 2, and a double tube or a Peltier element capable of circulating a refrigerant can be used.

  Reference numeral 4 is composed of a potassium hydroxide solution and a pump for feeding the potassium hydroxide solution, and reference numeral 6 a is a potassium hydroxide solution fed by the reference numeral 4 and the carbon-supported palladium nanocrystal generated by the reference numeral 2 and cooled by the reference numeral 3. The process of mixing particle dispersion liquid with a mixer is shown.

  Reference numeral 5 is composed of a solution in which a raw material salt for forming a platinum shell layer is dissolved and a pump for feeding the solution. Reference numeral 6b is a dispersion liquid mixed with the platinum raw material solution fed by the reference numeral 5 and the reference numeral 6a. The process of mixing with a mixer. The carbon-supported palladium particle dispersion mixed with the platinum raw material solution is heated by the microwave irradiation in the cavity denoted by reference numeral 7 to form a platinum shell. Reference numeral 8 denotes a tank for recovering the produced carbon-supported palladium core platinum shell particles. In the case where platinum shell formation is allowed to proceed slowly at room temperature, it can be said that the shell formation reaction is carried out in the tank indicated by reference numeral 8 without heating at reference numeral 7.

  Hereinafter, although the present invention is explained in detail, referring to an example and a comparative example, the present invention is not limited to this narrowly.

Example 1
When adding microwave hydroxide and using microwave heating for shell formation.
Vulcan® XC72 is 0.1 weight percent, Nafion® (DE520 CS type) is mixed to 0.03% with ethylene glycol, and sonicated to make Vulcan XC72 a uniform dispersion. To be. To this dispersion, potassium tetrachloropalladium (2) was added to a concentration of 2 mM and potassium hydroxide to 13 mM and completely dissolved, and this was used as a reaction raw material liquid. This reaction raw material liquid is fed by a pump capable of handling the carbon dispersion liquid.

  As a result of repeated detailed experiments conducted by the inventors, it is desirable to add potassium hydroxide 4 to 7 times the amount of potassium tetrachloropalladium (2) to be added to the reaction raw material liquid. Was revealed. When the hydroxide ion concentration is low, nucleation is delayed and the particle size is increased, and unreacted palladium ions may remain. When the hydroxide ion concentration is high, the reduction rate is too high and aggregates of free particles and palladium nanoparticles that are not supported are generated.

  The reaction raw material solution was fed at a flow rate of 6 ml / min through a PFA (Perfluoroalkoxy alcohol) reaction tube having an inner diameter of 2 mm and heated at 100 ° C. The reaction tube was arranged along the cylindrical central axis of a microwave cavity having a cylindrical space with an inner diameter of 90 mm and a height of 10 cm. Heating was performed by irradiating the cavity with a microwave having a frequency at which a standing wave of TM010 mode was formed, and the temperature of the reaction raw material liquid was measured at a position in the center of the reaction tube with a radiation thermometer. Further, the frequency at which the TM010 mode standing wave is formed varies depending on the temperature of the reaction raw material liquid, but the frequency of the microwave to be irradiated was adjusted so as to always match the frequency. The frequency range at this time was 2.45 GHz.

  To the carbon-supported palladium particle dispersion synthesized by the above method, the potassium hydroxide ethylene glycol solution required for the platinum shell formation reaction is added with fine adjustment of the concentration by the mixer 6a (mixer in the pipe joint flow path). It was done. In this example, 0.5 M potassium hydroxide ethylene glycol solution was added so that the potassium concentration increased by 3 mM.

  After adding a potassium hydroxide ethylene glycol solution, the carbon-supported palladium particle dispersion synthesized by the above method is mixed with a mixer 6b so that the molar ratio of palladium to platinum in the mixed solution becomes 100 mM. Chloroplatinic acid (4) mixed with ethylene glycol solution.

  In FIG. 1, the hydroxide ions added in the process indicated by reference numeral 4 react with the hydrogen ions generated from the platinum raw material added in the process indicated by reference numeral 5, so that the amount of added hydroxide ions is entirely a shell forming reaction. Not involved in. However, free hydroxide ions still remaining after the neutralization reaction, hydroxide ions complexed with platinum ions, and hydroxide ions adsorbed on the palladium particle surface are considered to be involved in the shell formation reaction. It is done. As a result of repeatedly examining detailed experiments, the inventors have found that these hydroxide ion concentrations are 2 to 6 times the amount of hexachloroplatinate (4) ion, which is a precursor of platinum shell formation as a raw material. Clarified that it is desirable If the hydroxide concentration in the shell formation reaction is low, the reaction rate becomes slow and the platinum raw material becomes unreacted. If the hydroxide concentration is too high, platinum is not formed as a shell but as particles on the carbon support or palladium core surface. Or platinum free particles are easily produced as a by-product. When the platinum raw material salt is a salt obtained by neutralizing chloroplatinic acid, when the palladium core particles are formed by another method, or when the palladium core particles are purified before the shell formation reaction, the hydroxide ions are Therefore, it is necessary to adjust the concentration of sodium hydroxide to be added so as to fall within the above range.

  The carbon-supported palladium particle dispersion mixed with the mixer 6b and the ethylene glycol solution of platinum chloride (4) acid are fed at a flow rate of 7 ml / min through a reaction tube disposed in the microwave cavity 7 to reach 80 ° C. So that it was heated. The synthesized carbon-supported palladium core platinum shell fine particle colloid was recovered in 8 tanks.

(Example 2)
When potassium hydroxide is added and microwave heating is not used to form the shell.
Carbon-supported palladium particles were synthesized in the same manner as in Example 1, mixed with potassium hydroxide ethylene glycol solution and platinum chloride (4) acid ethylene glycol solution, and then the mixture was collected in a collection container without heating at 7. In order to complete the reaction at room temperature, it was left at room temperature for 72 hours.

(Example 3)
When adding sodium hydroxide and using microwave heating for shell formation.
Palladium core particles were synthesized and platinum shells were formed using sodium hydroxide instead of potassium hydroxide in Example 1 and sodium tetrachloropalladium (2) instead of potassium tetrachloropalladium (2). . Specifically, for the production of one reaction raw material solution, a 5M sodium hydroxide aqueous solution was added to 13 mM. In No. 4, a 0.5 M aqueous sodium hydroxide solution was added so that the sodium concentration increased by 3 mM, and the mixture was mixed using the mixer 6a. Except for the above, the same raw material concentration and heat treatment as in Example 1 were performed to recover the product.

Example 4
Vulcan® XC72 is 0.1 weight percent, Nafion® (DE520CS) is mixed with 0.03% ethylene glycol and sonicated to make Vulcan XC72 a uniform dispersion. To. To this dispersion, 3 mM potassium tetrachloropalladium (2), 20 mM potassium hydroxide, and 6 mM monoethanolamine were added and completely dissolved, and this was used as a reaction raw material liquid. In the same manner as in Example 1, after flowing the reaction raw material liquid and performing microwave heating, the concentration after mixing the ethylene glycol solution of hexachloroplatinum (4) acid and monoethanolamine in reference numeral 5 is platinum chloride (4) acid. 2 mM and monoethanolamine were mixed at 24 mM so as to be 24 mM, and heated at 160 ° C. at 7.

(Comparative Example 1)
When carbon-supported platinum particles are synthesized by the microwave continuous method.
As Comparative Example 1, a carbon-supported platinum catalyst was synthesized by a microwave continuous method, and the catalytic activity of the oxygen reduction reaction was compared. The manufacturing method is described below. Vulcan (registered trademark) XC72 as a carbon carrier and Nafion (registered trademark) (DE520 CS type) as a dispersing agent were mixed in ethylene glycol, and sufficiently irradiated with ultrasonic waves to thoroughly disperse the carbon. A hexachloroplatinic acid (4) ethylene glycol solution and a potassium hydroxide ethylene glycol solution are added to the dispersion, and finally carbon is 0.1 g / L, Nafion (registered trademark) is 0.03 g / L, hexachloroplatinum (2 ) A raw material solution was prepared so that the concentration of acid was 2 mM and potassium hydroxide was 20 mM. In the same manner as in the example, the raw material liquid was passed through a PFA reaction tube having an inner diameter of 2 mm disposed on the central axis of the microwave cavity at a flow rate of 2 ml / min, and heated so that the temperature of the raw material liquid became 85 ° C. .

(Comparative Example 2)
When sodium hydroxide is added and microwave heating is not used to form the shell.
Carbon-supported palladium core particles were synthesized in the same manner as in Example 3 except that microwave heat treatment was not performed at 7 and mixed with an aqueous sodium hydroxide solution and a platinum chloride (4) ethylene glycol solution. The shell formation reaction was carried out by leaving it at room temperature for 72 hours in a collection container.

(Comparative Example 3)
When a highly active catalyst cannot be obtained even when sodium hydroxide is added and microwave irradiation is applied.
Vulcan® XC72 is 0.1 weight percent, Nafion® (DE520CS) is mixed with 0.03% ethylene glycol and sonicated to make Vulcan XC72 a uniform dispersion. To. To this dispersion was added sodium tetrachloropalladium (2) acid at 3 mM and sodium hydroxide at 20 mM so as to be completely dissolved, and this was used as a reaction raw material liquid. This reaction raw material liquid was fed by a pump capable of handling the carbon dispersion liquid, and heated at a flow rate of 6 ml / min and 100 ° C. in a microwave cavity denoted by reference numeral 2 as in Example 1. Subsequently, a 100 mM chloroplatinic acid (4) ethylene glycol solution was mixed at 5 with a mixer 6b so that the molar ratio of palladium to platinum in the mixed solution was 3: 2. Next, it heated at 130 degreeC and the flow rate of 7 ml / min in the microwave cavity of the code | symbol.

  The formed carbon-supported palladium core platinum shell particles and carbon-supported platinum particles are purified by centrifugation, and are subjected to TEM (transmission electron microscopy), STEM (scanning electron microscopy) using an electron microscope (JEM-2200FS manufactured by JEOL). Observation by microscopy and analysis by EDS (energy dispersive X-ray analysis).

Measurement and load of oxygen reduction catalytic activity with reference to the standard method proposed by FCCJ (Fuel Cell Practical Use Promotion Council) (the method described in "Proposal of Targets, R & D Issues and Evaluation Methods for Solid Divided Fuel Cells") A response potential cycle endurance test was performed, and the catalyst of the example and the comparative example were compared.
A detailed method is described below.

The catalyst particles obtained in the above-mentioned Examples and Comparative Examples were repeatedly purified by centrifugation to remove ethylene glycol and water-soluble salts and the like. Finally, the carbon concentration was 0.0123 wt%, Nafion (registered trademark) ) A catalyst ink having a concentration of 0.0125% by weight was prepared. As a solvent for the catalyst ink, a mixed solvent of water and IPA (isopropanol) (weight ratio 3: 1) was used. 20 mg of this catalyst ink was applied to a polished glassy carbon rotating electrode having a diameter of 5 mm, dried, and used for electrochemical measurement. The electrolyte used was 0.1M perchloric acid, and a reversible hydrogen electrode was used as the reference electrode. Cleaning of the catalyst surface by potential scanning in the range of 0.05 to 1.2 V (hereinafter, the potential value is expressed with reference to the reversible hydrogen electrode) in an electrolytic cell in which nitrogen was purged by bubbling 3 L / min of nitrogen. The electrochemically active surface area was measured from the shape of hydrogen adsorption / desorption peak. Subsequently, oxygen was dissolved in the electrolytic solution by bubbling using 3 L / min of oxygen, and the rotating electrode was rotated at 1600 rpm to perform convective voltammetry, and the oxygen reduction activity was measured. The scanning speed was 20 mV / sec from the low potential side to the high potential side, the value of the oxygen reduction current at 0.9 V was recorded, and the oxygen reduction activity of the catalyst was produced from the Koutecky-Levich equation. The temperature of the electrolyte during the activity measurement was 25 ° C. The oxygen reduction activity was calculated in the form of oxygen reduction current (mass activity) per weight of platinum used as a raw material.
The unit of mass activity is ampere per gram [A / g]. However, in Example 4, platinum quantified from the result of EDS analysis after purification by centrifugation was used for calculation of mass activity.
Nitrogen bubbling and oxygen bubbling nitrogen and oxygen gas flow rates and sample setting conditions can be changed as appropriate depending on the purpose of the sample test. , Showing an example of the performance comparison of the fine particles of the present invention under the above conditions, the initial activity of the catalyst calculated by the mass activity calculation method of the fine particles of the present invention is 500 [A / g] or more, It was confirmed that it was possible to achieve 600 [A / g] or more by setting more preferable conditions for manufacturing conditions, and 700 [A / g] or more by setting particularly preferable conditions.

  The potential cycle in the load response endurance test is as follows. Using the above electrolyte and rotating electrode, a square wave of 0.6 V for 3 seconds and 1.0 V for 3 seconds is 1 cycle, and nitrogen is bubbled through the electrolyte to saturate. Under these conditions, a potential of 5000 and 37000 cycles was applied at 20 ° C., and the surface area and oxygen reduction activity were measured by the same method as described above before and after the potential cycle.

  2 is an STEM observation image of the carbon-supported palladium core platinum shell catalyst synthesized by the method of Example 1. FIG. The carbon support 9a and the palladium core platinum shell catalyst fine particles 9b supported thereon constitute the carbon supported palladium core platinum shell catalyst synthesized in Example 1. As illustrated by reference numeral 9b, particles of 3 to 4 nm are almost uniformly supported on the carbon support. Further, according to the results of EDS, the distributions of palladium and platinum were uniform and consistent over a wide range, and the ratio was also substantially consistent with the ratio of charged amounts at the time of synthesis. From this fact, it can be said that the synthesis method proposed by the present inventors is used so as to greatly reduce the waste of precious metals, and also achieves a high surface area necessary for the oxygen reduction catalyst.

  FIG. 3 is a graph comparing the mass activities of the oxygen reduction catalysts before and after the durability test of the example and the comparative example. The mass activity values of Examples 1, 2, 3, and 4 are 696, 643, 548, and 637 [A / g], respectively, and the mass activity values of Comparative Examples 1, 2, and 3 are 241, 394, and 100 [A], respectively. / G]. The catalyst synthesized by the method of the example has several times the activity of the carbon-supported platinum catalyst synthesized by the microwave of Comparative Example 1, and potassium hydroxide and microwave heating were not used for shell formation. The activity was higher than that of the carbon-supported palladium core platinum shell catalyst of Comparative Example 2. From these facts, it can be seen that the catalyst synthesized by the method of the example can effectively use expensive and rare platinum as a catalyst.

  First, the effect of forming a platinum shell by heating with microwaves will be described. When a platinum shell is synthesized by microwave heating, the synthesis rate is clearly faster than when a shell is slowly formed at room temperature, which can contribute to an improvement in productivity. Furthermore, by combining with the continuous method, it is possible to achieve lot-to-lot variations and simplification of the process, which were the problems of the present invention. What about the quality of the catalyst that synthesized platinum shell by microwave heating? Example 1 and Example 2 are equivalent conditions except that microwave heating is used to form the platinum shell. Comparing the mass activity values of Example 1 and Example 2 in FIG. 3, Example 1 has a mass activity that is equal to or slightly above that of Example 2, and is gentle at room temperature even if the platinum shell formation time is short. It was found that the performance is equal to or better than the reaction. In addition, Example 3 and Comparative Example 2, which are reaction systems in which potassium ions do not exist, have the same conditions except for the microwave heat treatment in shell formation. Example 3 showed higher activity than Comparative Example 2, and the effectiveness of microwave heating in platinum shell formation became clear.

  However, as in Comparative Example 3, there is a case where a highly active catalyst cannot be obtained even when microwaves are used for shell formation. When the reaction temperature is high and the nucleation rate is high, platinum does not form a shell, but becomes free particles, which are lost in the purification process and become a low activity catalyst. In order to improve productivity by using the microwave continuous method for shell formation in the presence of a carrier, the hydroxide ions coexisting in the reaction process, the pairing cations, and the reaction temperature must be adjusted appropriately.

  Next, the effect of coexisting potassium ions will be described. In the process of Example 1, sodium ions are not contained in the reaction system, and the cations contained in the palladium raw material and the hydroxide are potassium ions. In contrast, potassium ions are not contained in the reaction system in the process of Example 3, and the cations contained in the palladium raw material and the hydroxide are sodium ions. Example 1 and Example 3 are equivalent in other conditions, and comparing these shows that Example 1 has higher mass activity. In addition, Example 2 and Comparative Example 2 are also in contrast. Example 2 contains only potassium ions, and Comparative Example 2 contains only sodium ions. Comparing Example 2 and Comparative Example 2, it can be seen that Example 2 has higher mass activity. From these results, it can be seen that when the reaction conditions are the same, a more active catalyst tends to be synthesized when the cation of the palladium raw material and the added hydroxide pair is potassium than sodium.

  Although the details of the action of potassium ions and sodium ions in the process of forming a palladium core and a platinum shell in the presence of a carrier are not clear, the phenomenon is as described. One of the clear differences is the solubility of the starting metal salt. Potassium tetrachloropalladium (2) and potassium hexachloroplatinum (4) are more likely Low solubility in ethylene glycol solvent. This difference in solubility may be related to the production of free palladium particles and platinum particles that are lost during the purification process.

  Another distinct difference is the ability to form ion pairs by hydroxide ions. Sodium ions have a smaller ionic radius than potassium ions, and strongly attract surrounding water, polar solvents, and hydroxide ions. In contrast, potassium ions are weaker than sodium ions than sodium ions. Since the surface of the palladium nanoparticle is negatively charged by adsorbing hydroxide ions in the presence of hydroxide ions, it is considered that cations are covered outside. Sodium ions are thought to interact more strongly than hydroxide ions and potassium ions on the surface of palladium nanoparticles, and the strong presence of sodium ions on the surface of palladium nanoparticles results in the growth of palladium core nanoparticles and the subsequent formation of a platinum shell. May have an impact.

  It is difficult to observe these processes in situ, and it is difficult to directly prove the difference between how sodium ions and potassium ions work in platinum shell formation. There is no doubt that the above differences are influencing, but there may be unknown effects, and it may be intricately affecting other factors such as reaction temperature. However, in spite of such difficulties, the inventors have found the influence of sodium ions and potassium ions, and have succeeded in improving the performance and productivity of palladium core platinum shell fine particle catalyst synthesis in the presence of a carrier. .

In Example 4, palladium core particles and a platinum shell are formed in the presence of monoethanolamine in the presence of a carrier. Since primary amines such as monoethanolamine and ammonia form stable complexes with palladium and platinum ions and complexes, they have the effect of suppressing the reduction reaction of these raw materials. For this reason, when monoethanolamine is added, the particle diameter of the palladium core particles is slightly increased, and the platinum shell hardly reacts at room temperature. It is possible to proceed the platinum shell forming reaction even in the presence of monoethanolamine by heating in a batch method with a normal heating means such as an electric heater, oil bath, steam or the like. However, in order to form a uniform shell, it is necessary to react for a long time at a low reaction rate. As described above, heat transfer and diffusion tend to be non-uniform when scaled up to increase productivity in a batch. There is a problem. Thus, the type of additive that controls the particle growth and shell formation by suppressing the reaction rate and the normal heating method are incompatible.
However, when microwave heating is used, uniform heating can be rapidly performed, and the reaction may be allowed to proceed in a short time even in the presence of a reaction-suppressing additive. In addition, in the case of reactions in which the crystallinity of nanoparticles and the diffusion of raw material atoms on the surface are important factors, it is necessary to react at a high temperature. The combination method is effective. In Example 4, it was possible to form a platinum shell even under the condition that the primary amino group was present several times as much as the platinum raw material. It was found that some of the platinum added as a raw material was not taken into the catalyst, but the mass activity based on platinum contained in the catalyst showed a high value. In addition, Example 4 maintains a mass activity of 98% or more of the initial activity even after a 5000 cycle load response durability test, and the additive and temperature in the shell formation process may have an important influence on the durability. Recognize. The synthesis of such a catalyst is difficult by the conventional batch method, and the effectiveness of the microwave continuous heating method has been clarified.

  FIG. 4 shows the transition of the maintenance ratio of mass activity with respect to the number of potential cycles in the durability test of the carbon-supported palladium core platinum shell catalyst of Example 1 and Example 2. Here, the maintenance rate represents the activity after the durability test with respect to the initial activity in 100 minutes, and is an index indicating the maintenance rate of the activity due to the increase in the number of potential cycles, with the initial activity as 100%. Assuming that the initial activity 10a in Example 1 is 100%, the activity 10b after 5000 cycles was 84%, and the activity 10c after 37000 cycles was 80%. In Example 2, when the initial activity code 10d is 100%, the code 10e, which is activity after 5000 cycles, is 93%, and the code 10f, which is activity after 37000 cycles, is 84%. This shows that the carbon-supported palladium core platinum shell catalyst of the present invention maintains an activity of 80% or more of the initial activity even after 37,000 cycles of potential cycle, which is an important characteristic in the practical application of fuel cells. It became clear that the durability was excellent.

  As described above, by using potassium hydroxide in the synthesis of the carbon-supported palladium core platinum shell catalyst fine particles of the present invention in the presence of the core and shell support, and effectively using microwave heating for the shell formation reaction, Carbon-supported palladium core platinum shell catalyst fine particles having high catalyst performance and small performance deterioration in the durability test can be obtained.

  While the present invention has been described with reference to the drawings and examples and comparative examples, the present invention is not limited to this, and many variations are possible based on the technical idea of the present invention. It is clear that it is. For example, as for microwaves, the present invention has been described with respect to single-mode microwave irradiation, but the present invention is not limited to this, and also includes the use of multi-mode microwave irradiation. The intensity distribution range of the microwave applied to the body and the method of applying the intensity are determined by the purpose applied to the irradiated object.

  In the present invention, the carbon-supported palladium core platinum shell fine particle catalyst synthesized by a method using potassium hydroxide and microwave heating for the synthesis of the core and shell in the presence of the carrier has high catalytic efficiency and excellent durability. It can be used for chemical reactions and fuel cell catalysts.

Claims (1)

  A method in which a palladium raw material and a potassium hydroxide solution are added to a dispersion in which a carrier is dispersed in a dispersion medium containing a glycol-based solvent to obtain a reaction raw material liquid, and carrier-supported palladium nanoparticles are continuously synthesized by microwave heating. In claim 1, a method for producing a carrier-supported palladium fine particle colloid comprising adding potassium hydroxide in an amount of 4 to 7 times the amount of palladium raw material atoms of the palladium raw material.