JP2012157833A - Method for producing catalytic fine particle, catalytic fine particle produced thereby, and catalytic fine particle-containing electrode catalyst for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently obtaining a catalytic fine particle having a high covering rate, the catalytic fine particle having such a structure that a center particle is covered with an outermost layer, and to provide the catalytic fine particle obtained by the method and a catalytic fine particle-containing electrode catalyst for fuel cells.SOLUTION: The method for producing the catalytic fine particle, which includes the center particle and the outermost layer for covering the center particle, includes a step of preparing the center particle; a step of applying an electrical potential to the center particle in a copper ion solution of the temperature lower than normal temperature to cover the surface of the center particle with a copper atom layer; and a step of substituting the outermost layer for the copper atom layer.

Description

  The present invention relates to a method for efficiently obtaining catalyst fine particles having a high coverage with respect to catalyst fine particles having a structure in which the outermost layer is coated on the center particle, the catalyst fine particles obtained by the method, and a fuel cell electrode containing the catalyst fine particles Relates to the catalyst.

  A fuel cell directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes and causing the fuel to be oxidized electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle, and thus exhibit high energy conversion efficiency. A fuel cell is usually formed by laminating a plurality of single cells having a basic structure of a membrane / electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes.

Conventionally, supported platinum and platinum alloy materials have been employed as electrode catalysts for anodes and cathodes of fuel cells. However, the amount of platinum required for current state-of-the-art electrocatalysts is still expensive to make mass production of fuel cells commercially feasible. The influence of the noble metal unit price on the catalyst price is great, and therefore further improvement in the activity per unit mass of the noble metal is desired.
In addition, platinum ions are eluted under a high potential environment, while platinum ions are precipitated under a low potential environment. Therefore, when high potential discharge and low potential discharge are alternately repeated, aggregation of platinum particles occurs. Such agglomeration of platinum particles causes a decrease in effective electrode area and contributes to a decrease in battery performance.

  One of the studies aimed at improving both the catalyst activity and the catalyst durability is a study of an electrocatalyst having a so-called core-shell structure. U.S. Patent No. 6,057,033 includes gold-coated metal particles bonded to a conductive support, the gold-coated metal particles being at least partially encapsulated by an atomically thin outer shell made of gold or a gold alloy. Also disclosed is an oxygen-reducing electrocatalyst characterized by comprising a noble metal-containing core.

Special table 2009-510705 gazette

Paragraph [0184] of the specification of Patent Document 1 describes a method for depositing a platinum layer on palladium particles by a natural redox substitution method. However, as a result of the study by the present inventors, as shown in the examples described later, among the natural redox substitution methods, in particular, when using a substitution plating method to which the copper underpotential deposition method is applied, copper serving as an electrolytic solution is used. It became clear that the temperature conditions of the ionic solution had a significant effect on the coating efficiency by subsequent displacement plating. However, although the patent document 1 has a description about covering a copper monoatomic layer on platinum in paragraph [0194] of the specification, the temperature condition of a copper ion solution used for copper monoatomic layer coating is described. There is no mention or suggestion.
The present invention has been achieved in view of the above-mentioned circumstances, and a catalyst fine particle having a structure in which the outermost layer is coated on the center particle is obtained by the method for efficiently obtaining catalyst fine particles having a high coverage rate. An object of the present invention is to provide catalyst fine particles and a fuel cell electrode catalyst containing the catalyst fine particles.

  The method for producing catalyst fine particles of the present invention is a method for producing catalyst fine particles comprising a center particle and an outermost layer covering the center particle, the step of preparing the center particle, in a copper ion solution at a temperature lower than room temperature, It is characterized by having a step of coating a copper atom layer on the surface of the center particle by applying a potential to the center particle, and a step of replacing the copper atom layer with the outermost layer.

  In this invention, it is preferable that the temperature of the said copper ion solution shall be 0-18 degreeC in the said copper atomic layer coating process.

  In the present invention, the center particle preferably contains at least one metal selected from the group consisting of palladium, iridium, rhodium and gold.

  In the present invention, it is preferable that the outermost layer contains at least one metal selected from the group consisting of platinum, iridium, ruthenium, rhodium and gold.

  In the present invention, in the copper atomic layer coating step, when the amount of copper coated on the central particles in the copper ion solution at normal temperature is assumed to be 100, the central particles in the copper ion solution at a temperature lower than normal temperature The ratio of the amount of copper coated on the surface may be 103 to 120.

  In the present invention, the amount of copper coated on the center particle is preferably an amount calculated based on the amount of charge adsorbed on the center particle.

  The catalyst fine particles of the present invention are produced by the above production method.

  The fuel cell electrode catalyst of the present invention is characterized in that the catalyst fine particles are supported on a carbon support.

  According to the present invention, in the step of covering the surface of the center particle with the copper atomic layer, by using a copper ion solution at a temperature lower than room temperature, the potential range in which precipitation of the copper monoatomic layer occurs, and the copper bulk precipitation The overlapping portion of the potential range to be generated can be eliminated or made extremely small, and the ratio of the center particle covered with the copper monoatomic layer can be made higher than before.

It is the schematic diagram which showed the reduction wave of the cyclic voltammogram at the time of coat | covering a copper atomic layer to center particle | grains. It is the schematic diagram which showed the cross-sectional structure of the catalyst fine particle in the catalyst layer on a working electrode. It is the figure which accumulated and showed CV of Example 1, Example 2, and Comparative Example 1. FIG. 2 is a graph showing a copper adsorption peak area in CV of Example 1. FIG. 3 is a graph summarizing the results of Table 1. It is the isometric view schematic diagram which showed the apparatus which performs Cu-UPD.

  The method for producing catalyst fine particles of the present invention is a method for producing catalyst fine particles comprising a center particle and an outermost layer covering the center particle, the step of preparing the center particle, in a copper ion solution at a temperature lower than room temperature, It is characterized by having a step of coating a copper atom layer on the surface of the center particle by applying a potential to the center particle, and a step of replacing the copper atom layer with the outermost layer.

  In the present invention, applying a potential to the center particle means applying a potential to the center particle. The potential here includes not only a constant potential but also a potential that changes over time. Therefore, the application of a potential in the present invention includes sweeping a predetermined range of potential.

  As a method for producing an electrocatalyst having a so-called core-shell structure, a method using displacement plating applying a copper under potential deposition method (hereinafter, sometimes referred to as Cu-UPD) has been known. ing. Cu-UPD can make the amount of copper deposited on each center particle uniform in the step of coating the center particles with copper.

In the prior art using a copper deposition method typified by Cu-UPD, the ratio of the area of the copper monoatomic layer to the total surface area of the center particles (hereinafter, sometimes referred to as copper coverage by Cu-UPD) is low. For this reason, the surface of the center particle may be exposed, or an edge portion may be formed on the outermost layer formed by subsequent substitution plating. For this reason, both the center particles and the outermost layer are likely to elute, and the durability of the catalyst itself may be reduced. Moreover, since the area of the outermost layer formed mainly by the electrode catalyst metal is small, the catalytic activity is also low.
The present inventors have found that the cause of such a problem is that in the step of coating the center particles with copper, if the copper coverage by Cu-UPD is made close to 100%, one of the potential ranges in which Cu-UPD is generated. We found that the part overlaps part of the potential range where copper bulk deposition occurs. Here, the bulk deposition of copper means deposition in which the amount of copper deposited on each central particle is larger than that of the monoatomic layer and becomes non-uniform. For this reason, when it is going to prevent the bulk precipitation of copper, it is necessary to stop the application of the electric potential with respect to a center particle, before Cu-UPD coverage becomes 100%, As a result, the subject that Cu-UPD coverage falls was there.

FIG. 1 is a schematic diagram showing a reduction wave of a cyclic voltammogram (hereinafter sometimes referred to as CV) when a central particle is coated with a copper atomic layer. An arrow 2 in FIG. 1 indicates a direction in which the potential is swept.
FIG. 1A is a schematic diagram showing an ideal CV reduction wave when a central particle is coated with a copper monoatomic layer. Curve 1 shows a reduction wave of CV derived from Cu-UPD. The copper adsorption peak area indicated by oblique lines corresponds to the copper adsorption amount with respect to the center particle. Theoretically, all of the copper adsorption amount is used for forming the copper monoatomic layer, and the copper monoatomic layer can be uniformly deposited on the center particle.
FIG. 1B is a schematic diagram in which a curve 3a is superimposed on the curve 1 shown in FIG. Curve 3a shows a CV reduction wave derived from the bulk deposition of copper that occurs when a copper monoatomic layer is coated on the center particle using a room temperature copper ion solution. In addition, normal temperature means 20-25 degreeC.
As shown in FIG. 1B, there is a potential range 4 in which the deposition of a copper monoatomic layer by Cu-UPD and the bulk deposition of copper occur simultaneously. Therefore, the actual reduction wave of CV is a curve obtained by adding the curve 1 and the curve 3a. Therefore, in the conventional copper deposition method, in addition to copper derived from Cu-UPD, copper derived from bulk deposition also precipitated, and a copper monoatomic layer could not be deposited uniformly on the center particle. Further, as will be described later, there is a problem in that the variation in the amount of deposited copper in the catalyst layer becomes large.

Therefore, the present inventors have separated the potential range where the copper monoatomic layer is deposited by Cu-UPD from the potential range where the bulk deposition of copper occurs by controlling the temperature of the copper ion solution. investigated.
FIG.1 (c) is the schematic diagram which overlapped and showed the curve 3b on FIG.1 (b). Curve 3b shows the reduction wave of CV derived from the bulk deposition of copper that occurs when the central particle is coated with a copper monoatomic layer using a low-temperature copper ion solution. In addition, the low temperature here means the temperature below normal temperature.
As shown in FIG. 1 (c), by lowering the temperature of the copper ion solution than before, it is possible to lower the potential at which copper bulk deposition starts than before. As a result, the Cu-UPD The potential range 4 in which the deposition of the copper monoatomic layer and the bulk deposition of copper occur simultaneously becomes markedly narrow at low temperatures. Thereby, the ratio of the copper deposited by Cu-UPD can be increased, and the subsequent outermost layer can be uniformly coated.

  Due to the bulk deposition of copper, two or more atomic layers of copper are deposited on the surface of the center particle. For this reason, when the outermost layer is formed by the subsequent substitution plating, the outermost layer becomes a non-uniform layer, and there is a problem that the utilization rate of the electrode catalyst used for the substitution plating and the activity per mass are lowered. Further, since the outermost layer is a non-uniform layer, the structure of each catalyst fine particle becomes non-uniform, resulting in a variation in structure between the catalyst fine particles.

Such a problem is particularly noticeable when a catalyst paste containing central particles is applied to the working electrode to form a catalyst layer, and copper is coated using a method in which a potential is applied to the catalyst layer in a copper ion solution. It becomes. This is because a concentration gradient of copper ions occurs in the thickness direction of the catalyst layer.
FIG. 2 is a schematic diagram showing a cross-sectional structure of catalyst fine particles in the catalyst layer on the working electrode. In FIG. 2, catalyst fine particles 10 formed under different conditions are shown in 3 rows and 3 columns. The catalyst fine particles 10 are each composed of a center particle 11 and an outermost layer 12. The vertical axis in FIG. 2 indicates the thickness direction of the catalyst layer. The upper row of catalyst fine particles is formed on the surface side of the catalyst layer, and the lower row of catalyst fine particles is formed inside the catalyst layer. Also, the horizontal axis in FIG. 2 indicates the temperature of the copper ion solution, the catalyst fine particles in the right column are formed using a higher temperature copper ion solution, and the catalyst fine particles in the left column are the lower temperature copper ion solution. It shows that it was formed using.
As prominently shown by the schematic cross-sectional view in the right column in FIG. 2, the bulk deposition of copper preferentially occurs from the interface between the surface of the catalyst layer and the copper ion solution. On the other hand, copper bulk deposition hardly occurs inside the catalyst layer. This is because the bulk deposition of copper occurs in order from the highest copper ion concentration. As a result, the structure variation between the catalyst fine particles occurs, and the amount of the electrocatalyst metal used in the subsequent formation of the outermost layer increases, so that the utilization rate of the electrocatalyst metal and the activity per mass of the catalyst fine particles are increased. A decrease occurs.
However, as described above, the proportion of copper consumed for bulk deposition can be suppressed by using a copper ion solution having a temperature lower than that of the prior art. Therefore, as shown in the schematic cross-sectional view of the left column in FIG. 2, the variation in the structure between the catalyst fine particles can be suppressed.

  As described above, the inventors of the present invention can obtain catalyst fine particles having a uniform outermost layer by lowering the temperature of the copper ion solution used for copper coating, and the structure between the catalyst fine particles. The present inventors have found that variation can be suppressed and completed the present invention.

Conventionally, Cu-UPD does not perform any temperature control of the copper ion solution. It is usually considered that there is no advantage in controlling the temperature of the copper ion solution because a device for temperature control, labor and cost for monitoring the temperature are also required.
However, as a result of the study by the present inventors, as shown in the examples described later, by performing Cu-UPD by controlling the copper ion solution below room temperature, the amount of copper adsorbed is far beyond the initial expectation. It has been found that the ratio of the area of the copper monoatomic layer to the total surface area of the center particle is significantly improved as compared with the conventional case.

The present invention includes (1) a step of preparing a center particle, (2) a step of coating a copper monoatomic layer on the center particle in a cooled copper ion solution, and (3) replacing the copper monoatomic layer with an outermost layer. The process of carrying out. The present invention is not necessarily limited to only the above three steps, and may include, for example, a drying / washing step as described later, in addition to the above three steps.
Hereinafter, the steps (1) to (3) and other steps will be described in order.

1. Step of Preparing Center Particles The material constituting the center particles used in the present invention is preferably a metal material that does not cause lattice mismatch with the material used for the outermost layer described later. Further, from the viewpoint of cost reduction, it is preferable that the material constituting the central particle used in the present invention is a metal material that is cheaper than the material used for the outermost layer described later. Furthermore, the material constituting the central particles used in the present invention is preferably a metal material that can be electrically connected and can fix copper.
From such a viewpoint, it is preferable that the material contained in the central particle used in the present invention contains a metal such as palladium, iridium, rhodium or gold, or an alloy made of two or more of the metals. Of these metal materials, it is more preferable to use palladium or a palladium alloy containing the metal material as the central particle.

The average particle diameter of the center particles is not particularly limited as long as it is equal to or less than the average particle diameter of catalyst fine particles described later. In addition, from the viewpoint that the ratio of the surface area of the center particle to the cost per center particle is high, the average particle size of the center particle is preferably 4 to 40 nm, particularly preferably 5 to 10 nm.
In addition, the average particle diameter of the particle | grains used for this invention is computed by a conventional method. An example of a method for calculating the average particle size of the particles is as follows. First, in a TEM (transmission electron microscope) image having a magnification of 400,000 times or 1,000,000 times, a particle size is calculated for a certain particle when the particle is considered to be spherical. Calculation of the average particle diameter by such TEM observation is performed for 200 to 300 particles of the same type, and the average of these particles is taken as the average particle diameter.

The central particle may be supported on a carrier. In particular, when the catalyst fine particles according to the present invention are used in an electrode catalyst layer of a fuel cell, the support is preferably a conductive material from the viewpoint of imparting conductivity to the electrode catalyst layer.
Specific examples of the conductive material that can be used as a carrier include Ketjen black (trade name: manufactured by Ketjen Black International Co., Ltd.), Vulcan (product name: manufactured by Cabot), Norit (trade name: manufactured by Norit), Examples thereof include carbon particles such as black pearl (trade name: manufactured by Cabot), acetylene black (trade name: manufactured by Chevron), conductive carbon materials such as carbon fibers, and metal materials such as metal particles and metal fibers.

  Prior to the step of preparing the center particles, the center particles may be supported on the carrier. Conventionally used methods can be adopted as a method for supporting the center particles on the carrier. When an alloy is used for the center particles, the synthesis of the alloy and the loading of the center particles on the support may be performed simultaneously.

2. The process of coating the copper monoatomic layer on the center particle in a cooled copper ion solution This step covers the surface of the center particle by applying a potential to the center particle in a copper ion solution at room temperature or lower. It is a process to do.

The copper ion solution that can be used in this step is not particularly limited as long as it is a solution that can be used for Cu-UPD. The copper ion solution is usually composed of a predetermined amount of a copper salt dissolved in a solvent, but is not particularly limited to this configuration, and a solution in which a part or all of the copper ions are dissociated in the liquid. If it is.
Examples of the solvent used for the copper ion solution include water and organic solvents, but water is preferable from the viewpoint of not preventing the progress of Cu-UPD.
Specific examples of the copper salt used in the copper ion solution include copper sulfate, copper nitrate, copper chloride, copper chlorite, copper perchlorate, and copper oxalate.
The copper ion solution may contain, for example, an acid or the like in addition to the solvent and the copper salt. Specific examples of the acid that can be added to the copper ion solution include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid.

  One of the most important features of the present invention is that the temperature of the copper ion solution is less than room temperature in this step. As described in the description of FIG. 1, the CV reduction wave is generally a curve obtained by adding the CV reduction wave derived from Cu-UPD and the CV reduction wave derived from copper bulk precipitation. Moreover, the reduction wave of CV derived from the bulk deposition of copper shifts to a lower potential as the temperature of the copper ion solution is lower. By using a copper ion solution of less than room temperature, the ratio of copper deposited by Cu-UPD can be increased with respect to the amount of copper deposited in bulk.

  In this step, the temperature of the copper ion solution is not particularly limited as long as it is lower than room temperature, but is preferably 0 to 18 ° C, more preferably 6 to 13 ° C. When the copper ion solution is lower than 0 ° C., Cu-UPD may not proceed due to freezing of the solution. Moreover, when a copper ion solution exceeds 18 degreeC, as shown in the Example mentioned later, sufficient copper coating by Cu-UPD cannot be performed and there exists a possibility that the effect of this invention cannot fully be enjoyed.

  The amount of copper coated on the center particle can be calculated based on, for example, the amount of charge adsorbed on the center particle. The amount of charge adsorbed on copper is calculated from CV obtained by sweeping the potential of the central particles in the copper ion solution. Specifically, the copper adsorption charge amount can be calculated by determining the copper adsorption peak area in the reduction wave of CV. A specific example of the copper adsorption peak area is shown in FIG.

The copper adsorption peak area can be obtained by integrating a predetermined region of a CV reduction wave with a potential. Here, the potentials that determine the integration interval are a start potential E ₀ and a stop potential E ₁ .
The starting potential E ₀ is preferably a potential at which copper deposition derived from Cu-UPD starts. For example, the start potential E ₀ can be determined as follows. In the reduction wave of CV, the slope of the tangent of the reduction wave is 1.0 × 10 ^{−5 to} 0 (A / V), that is, almost 0 (A / V) at the initial stage of the sweep, that is, at a relatively high potential portion. There is a range of potentials that can be considered. This potential range is considered to be a region where the electrochemical reaction on the catalytic metal surface has not yet occurred and charging / discharging occurs in a carrier such as carbon. Immediately after this sweep the early part, the potential to be a predetermined inclination with the tangent of the slope of the reduction wave, it is possible to determine a potential E _0. Here, the predetermined inclination is, for example, an inclination in a range of 5.0 × 10 ^{−4 to} 1.0 × 10 ⁻⁴ (A / V).
In this step, the start potential E ₀ may be a potential within the range of 0.6 to 0.7 V (vs RHE).

The stop potential E ₁ is preferably a potential at which the amount of copper adsorbed charge is the largest and the ratio of the amount of copper adsorbed charge derived from Cu-UPD is the highest in the copper adsorbed charge amount. If you stop the sweeping at lower than the stop voltage E ₁ potential, the copper adsorption charge amount, the proportion of copper adsorption amount of charge derived from the Cu-UPD is lowered. On the other hand, when stopping the sweep at higher than the stop voltage E ₁ potential, copper adsorption charge amount itself is reduced.
Stop potential E ₁ is, for example, can be determined as follows. In the reduction wave of CV at the time of copper deposition, the potential range in which the slope of the tangent line of the reduction wave exceeds −1.0 × 10 ⁻³ (A / V) in the later stage of the sweep, that is, in the relatively low potential portion. Exists. This potential range is a range in which the amount of copper adsorbed charge derived from copper bulk deposition is larger than the amount of copper adsorbed charge by Cu-UPD. The potential having a predetermined slope with the slope of the tangent of the reduction wave can be determined as the potential E ₁ immediately before the potential range in the latter period of the sweep, that is, immediately before the bulk precipitation of copper becomes dominant. Here, the predetermined inclination is, for example, an inclination in a range of −1.0 × 10 ^{−3 to} 0 (A / V).
As shown in Examples described later, as the temperature of the copper ion solution is low, stopping potentials E ₁ is lower. This is because the lower the temperature of the copper ion solution, the lower the potential at which the bulk deposition of copper begins.
When the potential sweep rate is low, a maximum point or an inflection point may appear in the reduction wave immediately before copper bulk deposition becomes dominant. The potential corresponding to the maximum point or inflection point may stop potential E _1.
In this step, it may be a potential within the range stop potential _{E 1 0.34~0.4V} of (vsRHE). In the present process, to stop the sweep potential at the stop potential E _1, the time the potential of a predetermined, it preferably secures 30-60 minutes.

In this step, the potential sweep rate is preferably 0.01 to 10 mV / sec. If the potential sweep rate is too slow, the synthesis takes a long time and the high potential holding time becomes long, which may lead to elution of the material constituting the center particle.
Also, when the sweep rate of the potential is too fast, can not be sufficiently separate the CV waveforms derived from CV waveform and copper bulk precipitation derived from Cu-UPD, it is difficult to determine the stopping potential E ₁ There is a fear.

  In this step, it is preferable that the amount of adsorbed copper is larger than when a room temperature copper ion solution is used. Specifically, when the amount of copper coated on the center particles in the copper ion solution at room temperature is assumed to be 100, the ratio of the amount of copper coated on the center particles in the copper ion solution below room temperature is 103 to 120. It is preferable that

Hereinafter, a specific example of a method for subjecting the palladium alloy fine particles to the Cu-UPD method will be described.
First, a palladium alloy (hereinafter collectively referred to as Pd / C) powder supported on a conductive carbon material is dispersed in water, and a Pd / C paste obtained by filtration is applied to the working electrode of an electrochemical cell. The Pd / C paste may be adhered to the working electrode using an electrolyte such as Nafion (trade name) as a binder. A solvent such as water or alcohol may be appropriately added to the Pd / C paste. As the working electrode, platinum mesh or glassy carbon can be used.
Next, a copper ion solution is added to the electrochemical cell, the working electrode, the reference electrode and the counter electrode are immersed in the copper ion solution, and a copper monoatomic layer is deposited on the surface of the palladium alloy fine particles by the Cu-UPD method. . An example of specific conditions for the Cu-UPD method is shown below.
Copper ion solution: mixed solution of 0.05 mol / L CuSO ₄ and 0.05 mol / L H ₂ SO ₄ (nitrogen is bubbled)
-Temperature of the copper ion solution: 0 to 18 ° C
-Atmosphere: under nitrogen atmosphere-Sweep speed: 0.2-0.01 mV / sec-Potential: After sweeping from start potential E ₀ = 0.64 V (vsRHE) to stop potential E ₁ = 0.35 V (vsRHE), The potential is fixed at E ₁ = 0.35 V (vs RHE).
-Potential fixing time: 30-60 minutes

3. Step of Substituting Copper Monoatomic Layer with Outermost Layer The material constituting the outermost layer formed in this step preferably has high catalytic activity. The catalytic activity here refers to the activity when used as a fuel cell catalyst.
From such a viewpoint, the material included in the outermost layer is preferably at least one metal material selected from the group consisting of platinum, iridium, ruthenium, rhodium and gold.
Among these metal materials, the outermost layer particularly preferably contains platinum. Platinum is excellent in catalytic activity, particularly oxygen reduction reaction (ORR) activity. The lattice constant of platinum is 3.92３, whereas the lattice constant of palladium is 3.89Å, and the lattice constant of palladium is a value within a range of ± 5% of the lattice constant of platinum. Therefore, by using palladium or a palladium alloy for the center particle and platinum for the outermost layer, lattice mismatch does not occur between the center particle and the outermost layer, and the center particle is sufficiently covered with platinum.

From the standpoint that elution of the center particles can be further suppressed, the coverage of the outermost layer with respect to the center particles is preferably 0.8 to 1.
If the coverage of the outermost layer with respect to the center particles is less than 0.8, the center particles are eluted in the electrochemical reaction, and as a result, the catalyst fine particles may be deteriorated.

  The “covering ratio of the outermost layer to the center particle” as used herein refers to the ratio of the area of the center particle covered by the outermost layer when the total surface area of the center particle is 1. As an example of the method for calculating the coverage, several places on the surface of the catalyst fine particles were observed by TEM, and it was confirmed by observation that the center particles were covered with the outermost layer with respect to the entire area observed. The method of calculating the ratio of an area is mentioned.

In the catalyst fine particles obtained by this step, it is preferable that the proportion of the monoatomic layer in the outermost layer is large. Such catalyst fine particles have the advantage that the catalyst performance in the outermost layer is extremely high and the material cost is low because the coating amount of the outermost layer is small compared to the catalyst fine particles having the outermost layer of two atomic layers or more. There is.
The average particle size of the catalyst fine particles obtained by this step is 4 to 40 nm, preferably 5 to 10 nm.

Hereinafter, a specific example of a method for subjecting palladium alloy fine particles coated with copper to substitution plating with platinum will be described.
After the copper coating with Cu-UPD is completed, the working electrode is immediately immersed in a platinum solution, and copper and platinum are replaced by plating using the difference in ionization tendency. The displacement plating is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere, and it is more preferable to use a glove box or the like replaced with an inert gas atmosphere. In addition, the oxidation of copper after coating can be prevented by quickly moving the working electrode from the copper ion solution to the platinum solution under an inert gas atmosphere.
The platinum solution is not particularly limited. For example, a 0.005M K ₂ PtCl ₄ solution can be used. It is preferable that the platinum solution is sufficiently stirred and nitrogen is bubbled in advance in the solution. The displacement plating time is preferably secured for 90 minutes or more.
By the displacement plating, catalyst fine particles in which a platinum monoatomic layer is deposited on the surface of the palladium alloy fine particles are obtained.

4). Other Steps After the step of replacing the copper atomic layer with the outermost layer, the catalyst fine particles may be filtered, washed, and dried.
The filtering and washing of the catalyst fine particles is not particularly limited as long as it is a method that can remove impurities without impairing the coating structure of the produced catalyst fine particles. Examples of the filtration / washing include suction filtration using water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like.
The drying of the catalyst fine particles is not particularly limited as long as the method can remove the solvent and the like. As an example of the drying, a method of performing vacuum drying at room temperature for 0.5 to 2 hours and then drying under an inert gas atmosphere at a temperature of 60 to 80 ° C. for 1 to 4 hours can be given.

According to the present invention, in the copper atomic layer coating step for the center particle, by using a copper ion solution having a temperature lower than room temperature, a potential range in which precipitation of a copper monoatomic layer occurs and a potential range in which copper bulk deposition occurs. The overlapping portion can be eliminated or made extremely small, and the ratio of the central particles covered with the copper monoatomic layer can be made higher than before. In addition, according to the present invention, the coverage of the outermost layer with respect to the center particles can be increased, and the area where the center particles are exposed on the surface can be reduced as compared with the conventional electrode catalyst having a core-shell structure. In addition, according to the present invention, by using an electrode catalyst metal such as platinum for the outermost layer, when the outermost layer is a monoatomic layer, the utilization factor of the electrode catalyst metal is also improved.
As described above, according to the present invention, the amount of the electrocatalyst metal that is not used for the catalytic reaction can be reduced as compared with the prior art, and the elution of the center particles can be suppressed, so that high durability and activity can be realized. Furthermore, according to the present invention, variation among the catalyst fine particles can be suppressed, and uniform catalyst fine particles can be synthesized.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited only to these examples.
In this example, Cu-UPD was performed using copper ion solutions at different temperatures, and the copper adsorption amount calculated from the CV was evaluated.

1. Cu-UPD
[Example 1]
First, carbon-supported palladium particle powder (manufactured by Basf, 20% Pd / C) was prepared.
Next, 0.5 g of carbon-supported palladium particle powder and 0.2 g of Nafion (trade name) were dispersed in water, and a mixture paste obtained by filtration was applied to a glassy carbon electrode.

FIG. 6 is a schematic perspective view showing an apparatus for performing Cu-UPD. A copper ion solution 22 was added to the glass cell 21, and an electrode 24 coated with a mixture paste 23 of carbon-supported catalyst fine particles was set. In the glass cell 21, in addition to the electrode 24, a counter electrode 26 and a reference electrode 27 are disposed so as to be sufficiently immersed in the copper ion solution 22. These three electrodes are electrically connected to the dual electrochemical analyzer. It is connected. Further, the nitrogen introduction tube 28 is disposed so as to be immersed in the copper ion solution 22, and nitrogen is bubbled into the copper ion solution 22 at room temperature from a nitrogen supply source (not shown) installed outside the cell for a certain period of time, The copper ion solution 22 was saturated with nitrogen. A circle 29 indicates a nitrogen bubble.
Furthermore, the whole cell was put into the thermostat 30 which used water for temperature control, and the thermostat 30 was controlled so that the temperature of the copper ion solution 22 was maintained at 6 degreeC.
The details of the apparatus are as follows.
Copper ion solution: mixed solution of 0.05 mol / L CuSO ₄ and 0.05 mol / L H ₂ SO ₄ Electrode: electrode with glassy carbon Counter electrode: platinum electrode (manufactured by Hokuto Denko)
Reference electrode: Silver-silver chloride electrode (manufactured by Cypress)
Dual electrochemical analyzer: ALS700C manufactured by BAS
Cyclic voltammetry was performed at a sweep rate of 5 mV / sec to obtain CV.

[Example 2]
Except that the temperature of the copper ion solution 22 was 13 ° C., Cu-UPD was performed in the same manner as in Example 1 to obtain CV.

[Example 3]
Except that the temperature of the copper ion solution 22 was 18 ° C., Cu-UPD was performed in the same manner as in Example 1 to obtain CV.

[Comparative Example 1]
Cu-UPD was performed in the same manner as in Example 1 except that the temperature of the copper ion solution 22 was set to room temperature (20 to 25 ° C.) to obtain CV.

[Comparative Example 2]
Except that the temperature of the copper ion solution 22 was 40 ° C., Cu-UPD was performed in the same manner as in Example 1 to obtain CV.

[Comparative Example 3]
Except that the temperature of the copper ion solution 22 was 50 ° C., Cu-UPD was performed in the same manner as in Example 1 to obtain CV.

[Comparative Example 4]
Except that the temperature of the copper ion solution 22 was 60 ° C., Cu-UPD was performed in the same manner as in Example 1 to obtain CV.

2. Evaluation of CV FIG. 3A shows Example 1 (temperature of the copper ion solution: 6 ° C.), Example 2 (temperature of the copper ion solution: 13 ° C.), and Comparative Example 1 (temperature of the copper ion solution: 20). It is the figure which piled up and showed CV of -25 degreeC. FIG.3 (b) is the figure which expanded the part of 0.3-0.4V (vsRHE) in the reduction wave of Fig.3 (a). Incidentally, black circle in FIG. 3 (b) shows a maximum point in the reduction wave corresponding to the stop potential E _1.

As can be seen from FIG. 3B, the stop potential E ₁ of Comparative Example 1 (20 to 25 ° C.) is 0.371 V (vs RHE). On the other hand, the stop potential E ₁ of Example 1 (6 ° C.) is 0.342 V (vs RHE), and the stop potential E ₁ of Example 2 (13 ° C.) is 0.356 V (vs RHE). These results, as the temperature of the copper ion solution is low, indicating that the lower stop potential E _1. These results also support that the lower the temperature of the copper ion solution, the lower the potential at which copper bulk deposition begins.

3. Evaluation of Copper Adsorption Amount FIG. 4 is a graph showing the copper adsorption peak area in the CV of Example 1. In FIG. 4, white circles indicate points corresponding to the start potential E ₀ , and black circles indicate points corresponding to the stop potential E ₁ . The copper adsorption amount of Example 1 is calculated based on the copper adsorption peak area indicated by the oblique lines in FIG. The copper adsorption amount was similarly calculated for the CVs of Examples 2 to 3 and Comparative Examples 1 to 4.
Table 1 below is a table summarizing the copper adsorption amounts of Examples 1 to 3 and Comparative Examples 1 to 4. In Table 1, the ratio of each copper adsorption amount when the copper adsorption amount in Comparative Example 1 is set to 100 is shown.
FIG. 5 is a graph summarizing the results of Table 1. FIG. 5 is a graph in which the vertical axis represents the copper adsorption amount ratio and the horizontal axis represents the temperature (° C.) of the copper ion solution. The data of Comparative Example 1 in FIG. 5 was plotted assuming that the temperature of the copper ion solution was 25 ° C.

As can be seen from Table 1 above, in Comparative Examples 2 to 4 in which a copper ion solution of 40 ° C. or higher was used for Cu-UPD, the amount of adsorbed copper could be reduced as compared with the case of using a room temperature copper ion solution. I understand. On the other hand, as can be seen from Table 1 above, in Examples 1 to 3 in which a copper ion solution of 18 ° C. or lower is used for Cu-UPD, the amount of copper adsorption is increased as compared with the case of using a room temperature copper ion solution. I understand that. In particular, in Example 1 using a 6 ° C. copper ion solution, it can be seen that the increment of the amount of adsorbed copper exceeds 10%.
Further, as can be seen from FIG. 5, it can be seen that the lower the temperature of the copper ion solution, the greater the amount of copper adsorption.
From the above results, when the copper atom layer is coated on the surface of the palladium fine particles, the amount of copper adsorbed on the palladium fine particles can be increased more than before by using a copper ion solution at a temperature lower than room temperature. It can be seen that the catalyst fine particles with a high rate can be obtained efficiently.

1 Curve 2 showing a CV reduction wave derived from Cu-UPD 2 Arrow indicating the direction of potential sweep 3a Curve 3b showing a CV reduction wave derived from copper bulk precipitation using a copper ion solution at room temperature Low temperature copper Curve 4 showing a reduction wave of CV derived from bulk deposition of copper using an ionic solution Range of potential where copper deposition by Cu-UPD and bulk deposition of copper occur simultaneously 10 Catalyst fine particles 11 Center particle 12 Outermost layer 21 Glass Cell 22 Copper ion solution 23 Mixture paste of carbon supported catalyst fine particles 24 Electrode 26 Counter electrode 27 Reference electrode 28 Nitrogen introduction tube 29 Nitrogen bubble 30 Constant temperature bath

Claims (8)

A method for producing catalyst fine particles comprising a center particle and an outermost layer covering the center particle,
Preparing the central particle;
A step of coating a copper atom layer on the surface of the central particle by applying a potential to the central particle in a copper ion solution at a room temperature, and
A method for producing catalyst fine particles, comprising a step of replacing the copper atomic layer with the outermost layer.   The method for producing catalyst fine particles according to claim 1, wherein in the copper atomic layer coating step, the temperature of the copper ion solution is set to 0 to 18 ° C.   The method for producing catalyst fine particles according to claim 1 or 2, wherein the central particle contains at least one metal selected from the group consisting of palladium, iridium, rhodium and gold.   The method for producing catalyst fine particles according to any one of claims 1 to 3, wherein the outermost layer contains at least one metal selected from the group consisting of platinum, iridium, ruthenium, rhodium and gold.   In the copper atomic layer coating step, when the amount of copper coated on the central particles in the copper ion solution at normal temperature is assumed to be 100, the copper particles coated on the central particles in the copper ion solution below normal temperature The method for producing catalyst fine particles according to any one of claims 1 to 4, wherein the ratio of the amount is 103 to 120.   The method for producing catalyst fine particles according to claim 5, wherein the amount of copper coated on the center particle is an amount calculated based on the amount of charge adsorbed to the center particle.   Catalyst fine particles produced by the production method according to any one of claims 1 to 6.   8. An electrode catalyst for fuel cells, wherein the catalyst fine particles according to claim 7 are supported on a carbon support.

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