Processes for the production of fine catalyst particles and processes for the production of carbon-supported catalysts

Abstract

A process for producing fine catalyst particles comprising a fine palladium-containing particle and a platinum-containing outermost layer covering at least a part of the fine palladium-containing particle, the process comprising: a potential application step of applying a potential to the fine palladium-containing particles in a first dispersion comprising fine palladium-containing particles dispersed in an acid solution and having an average particle diameter of 3.0 nm or more and 6.0 nm or less, until a peak indicating a Pd{111} surface in a reduction wave of a cyclic voltammogram and occurring in a range of 0.50 V to 0.55 V vs. RHE becomes larger than a peak indicating a Pd{110} or Pd{100} surface in the reduction wave of the cyclic voltammogram;a copper covering step of covering at least a portion of the fine palladium-containing particle with copper by preparing a second dispersion by mixing the first dispersion and a copper-containing solution after the potential application step and applying a potential more noble than the oxidation-reduction potential of copper to the fine palladium-containing particles in the second dispersion; and a platinum covering step of covering at least a portion of the fine palladium-containing particle with platinum by substituting the copper covering at least a portion of the fine palladium-containing particle with platinum by mixing the second dispersion and a platinum-containing solution after the copper covering step, wherein the potential in the potential application step is traversed in a range that includes at least 1.2 V vs. RHE.

Description

Technical field

[0001] The present invention relates to a process for producing fine catalyst particles with better catalytic performance than ever before and a process for producing a carbon-supported catalyst with better catalytic performance than ever before. Current state of the art

[0002] A technique involving fine catalyst particles is known as an electrode catalyst for the anode and cathode of a fuel cell. These particles have a structure comprising a core particle and an outermost layer covering the core particle (so-called "core-shell structure"). For these fine catalyst particles, using a relatively inexpensive material for the core particle reduces the cost of the particle's interior, which is hardly involved in a catalytic reaction.

[0003] A method for producing a core-shell catalyst, in which a copper shell layer is produced by the Cu-UPD process and the copper shell layer is then substituted by noble metal atoms, is disclosed in patent document 1. List of objections

[0004] Patent document 1: Unexamined US patent application publication no. 2010 / 0177462

[0005] Patent document 1 discloses a method for producing a monatomic copper layer on a core surface by Cu-UPD, wherein the method uses a mixed solution of 50 mM CuSO4 and 50 mM H2SO4. However, according to the research of the inventors of the present invention, it was found that a core-shell catalyst is obtained in which the shell exhibits numerous defects or flaws when Cu-UPD is simply carried out on the core surface, which – just like the invention of patent document 1 – is not subjected to any treatment.

[0006] The present invention was made in light of the above circumstances. One object of the present invention is to provide a process for producing fine catalyst particles with better catalytic performance than ever before and a process for producing a carbon-supported catalyst with better catalytic performance than ever before. Solution to the problem

[0007] The process for producing fine catalyst particles of the present invention is a process for producing fine catalyst particles comprising a fine palladium-containing particle and a platinum-containing outermost layer covering at least a part of the fine palladium-containing particle, wherein the process comprises: a potential application step of applying a potential to the fine palladium-containing particles in a first dispersion comprising fine palladium-containing particles dispersed in an acid solution and having an average particle diameter of 3.0 nm or more and 6.0 nm or less, until a peak indicating a Pd{111} surface in a reduction wave of the cyclic voltammogram becomes larger than a peak indicating a Pd{110} or Pd{100} surface in the reduction wave of a cyclic voltammogram;a copper covering step of covering at least a portion of the fine palladium-containing particle with copper by preparing a second dispersion by mixing the first dispersion and a copper-containing solution after the potential application step and applying a potential more noble than the oxidation-reduction potential of copper to the fine palladium-containing particles in the second dispersion; and a platinum covering step of covering at least a portion of the fine palladium-containing particle with platinum by substituting the copper covering at least a portion of the fine palladium-containing particle with platinum by mixing the second dispersion and a platinum-containing solution after the copper covering step.

[0008] In the potential application step of the present invention, it is preferred that the potential is traversed in a range that includes at least 1.2 V (vs. RHE).

[0009] The process for producing a carbon-supported catalyst of the present invention is a process for producing a carbon-supported catalyst in which the fine catalyst particles are supported on a carbon support, wherein fine palladium-containing particles, which are designed for use in the potential application step, are supported on a carbon support. Advantageous effects of the invention

[0010] According to the present invention, by allowing sufficient growth of an electrochemically more stable Pd{111} surface in the potential application step, the Pd{111} surface is covered with platinum in the platinum covering step, so that a Pt{111} surface is formed, and as a result, the Pt{111} surface, which has a higher catalytic activity, appears to a large extent on the surface of the outermost platinum layer; therefore, fine catalyst particles with better catalytic performance than conventional core-shell catalysts can be obtained. Brief description of the drawings

[0011] Fig. Figure 1 is a flowchart of an example of the process for producing a carbon-supported catalyst of the present invention.

[0012] Fig. Figure 2 schematically shows a perspective view of an example of an electrochemical device used for cyclic voltammetry.

[0013] Fig. Figure 3 schematically shows a perspective view of an example of an electrochemical device used to perform the potential application step.

[0014] Fig. Figure 4 is a TEM image of carbon-supported palladium used in Example 1.

[0015] Fig. Figure 5 is a histogram showing the particle size distribution of fine palladium particles in the carbon-supported palladium used in Example 1.

[0016] Fig. Figure 6 is a graph showing the palladium oxidation rates of samples taken during the preparation of carbon-supported palladium.

[0017] Fig.Figure 7 is a graph showing the cyclic voltammogram (thin curve) of fine palladium-containing particles before they were subjected to the potential application step in Example 1, and showing the cyclic voltammogram (thick curve) of the fine palladium-containing particles at the 2,500th cycle in Example 1, with the cyclic voltammograms overlapping.

[0018] Fig. Figure 8 is a graph showing the relationship between the diameter of fine catalyst particles and the platinum coverage for 40 samples that underwent the potential application step and for 40 samples that did not undergo the potential application step.

[0019] Fig. Figure 9 is a bar chart comparing the mass activities of Example 1 and Comparison Example 7.

[0020] Fig. Figure 10 is a histogram showing the particle size distribution of fine palladium particles in the carbon-supported palladium used in comparison example 4.

[0021] Fig. Figure 11 is a histogram showing the particle size distribution of fine palladium particles in the carbon-supported palladium used in comparison example 6.

[0022] Fig. Figure 12 is a graph showing the cyclic voltammogram (thin curve) of fine palladium-containing particles before they were subjected to the potential application step in Comparative Example 1, and showing the cyclic voltammogram (thick curve) of the fine palladium-containing particles at the 2,500th cycle in Comparative Example 1, with the cyclic voltammograms overlapping. Description of embodiments

[0023] The process for producing fine catalyst particles of the present invention is a process for producing fine catalyst particles comprising a fine palladium-containing particle and a platinum-containing outermost layer covering at least a part of the fine palladium-containing particle, wherein the process comprises: a potential application step of applying a potential to the fine palladium-containing particles in a first dispersion comprising fine palladium-containing particles dispersed in an acid solution and having an average particle diameter of 3.0 nm or more and 6.0 nm or less, until a peak indicating a Pd{111} surface in a reduction wave of a cyclic voltammogram becomes larger than a peak indicating a Pd{110} or Pd{100} surface in the reduction wave of the cyclic voltammogram;a copper covering step of covering at least a portion of the fine palladium-containing particle with copper by preparing a second dispersion by mixing the first dispersion and a copper-containing solution after the potential application step and applying a potential more noble than the oxidation-reduction potential of copper to the fine palladium-containing particles in the second dispersion; and a platinum covering step of covering at least a portion of the fine palladium-containing particle with platinum by substituting the copper covering at least a portion of the fine palladium-containing particle with platinum by mixing the second dispersion and a platinum-containing solution after the copper covering step.

[0024] As described above, Cu-UPD is used to form the shell of a core-shell catalyst. However, in conventional processes, a commercially available palladium-supported carbon (hereinafter also referred to as Pd / C) is primarily used as is for the Cu-UPD, and the surface area of ​​the fine core particle in front of the Cu-UPD is not taken into account at all.

[0025] However, the inventors of the present invention have determined through research studies that when commercially available Pd / C is used in its unmodified state for Cu-UPD, shell formation is not sufficiently promoted, resulting in a defective shell. This is because commercially available Pd / C contains palladium oxide and / or an organic substance on the surface of the fine palladium particle, or has many low-coordinate atoms on it, for the following reasons.

[0026] In the preparation of Pd / C, an organic dispersant or similar agent is generally used to control the particle diameter or dispersibility of fine palladium particles. The metallization of palladium in Pd / C is ultimately carried out by calcining a Pd / C precursor at a temperature of approximately 300 to 600°C. However, under such temperature conditions, it is likely that organic residues will remain in the Pd / C. Generally, in commercially available Pd / C, palladium oxide is present near the surface of fine palladium particles. However, the palladium oxide is not sufficiently reduced under the aforementioned calcination conditions. As just described, in commercially available Pd / C, sufficient copper adsorption to the surface of the fine palladium particle is not promoted during Cu-UPD because the surface of the fine palladium particle is covered with an oxide or organic substance.

[0027] In addition to crystal surfaces such as a Pd{111} surface, a Pd{110} surface, and a Pd{100} surface, calcined Pd / C contains many low-coordinate palladium atoms on the surface of the fine palladium particle. Low-coordinate palladium atoms are those that form a corner or edge. More precisely, they are palladium atoms located at a boundary between two surfaces of the fine palladium particle, or at a vertex of the surface. High-coordinate palladium atoms, on the other hand, are those that form the crystal surfaces of the fine palladium particle, as well as those located within the particle's interior.

[0028] As just described, a problem with prior art techniques is that the surface of the fine palladium particle is largely composed of low-coordinated palladium atoms, and therefore only a few highly active crystal surfaces are exposed, and many convexities and concavities occur on the surface, leading to low activity of the core-shell catalyst obtained by Cu-UPD.

[0029] In the present invention, the crystal surfaces of palladium are each represented by a combination of the chemical formula “Pd” with the crystal surface. In this description, equivalent surfaces are represented as a crystal surface enclosed in curly brackets. For example, the Pd(111) surface and the Pd(-1-1-1) surface are both represented as the Pd{111} surface.

[0030] Without increasing the surface activity of the fine palladium-containing particles, copper is not adsorbed in sufficient quantity onto the surface of the fine palladium-containing particles during Cu-UPD. Consequently, during the subsequent formation of the platinum-containing outer layer, the substitution of copper atoms by platinum atoms, etc., is less likely to occur, resulting in fine catalyst particles with a platinum-containing outer layer that is highly deficient or lacking in defects. The inventors of the present invention have focused on this issue and conducted research studies to increase the surface activity of the fine palladium-containing particles.As a result of their efforts, they discovered a method to increase the activity of the fine catalyst particles obtained by Cu-UPD more than ever before, by performing a step of applying a potential to the fine palladium-containing particles prior to the Cu-UPD to remove oxides and impurities from the surface of the fine palladium-containing particles, and by modifying the arrangement of atoms on the surface of the fine palladium-containing particles to expose Pd{111} with high catalytic activity. Consequently, they have realized the present invention.

[0031] Fig. Figure 1 is a flowchart of an example of the process for producing fine catalyst particles of the present invention.

[0032] The in Fig.1 The process shown for the production of fine catalyst particles includes (1) the potential application step, (2) the copper covering step, (3) the platinum covering step, (4) a washing step, (5) an acid treatment step and (6) a drying step.

[0033] The process for producing fine catalyst particles of the present invention comprises (1) the potential application step, (2) the copper covering step, and (3) the platinum covering step. If required, it includes (4) a washing step, (5) an acid treatment step, (6) a drying step, etc., after the platinum covering step.

[0034] These steps are described below in order. (1) Potential application step

[0035] The potential application step is a step of applying a potential to the fine palladium-containing particles in a first dispersion comprising fine palladium-containing particles dispersed in an acid solution and having an average particle diameter of 3.0 nm or more and 6.0 nm or less, until a peak indicating a Pd{111} surface area in a reduction wave of a cyclic voltammogram becomes larger than a peak indicating a Pd{110} or Pd{100} surface area in the reduction wave of the cyclic voltammogram.

[0036] In the present invention, “fine palladium-containing particle” is a generic term for fine palladium particle and fine palladium alloy particle.

[0037] As described below, the outermost layer covering the fine palladium-containing particles contains platinum. The catalytic activity, particularly the oxygen reduction reaction (ORR) activity of platinum, is excellent. While the lattice constant of platinum is 3.92 Å, the lattice constant of palladium is 3.89 Å, a value that lies within 5% of either side of the lattice constant of platinum. Accordingly, no lattice mismatch occurs between platinum and palladium, and the palladium is adequately covered by the platinum.

[0038] In the present invention, from the point of view of cost reduction, it is preferable that the fine palladium-containing particles contain a metallic material that is less expensive than the material described below, which is used for the platinum-containing outer layer. It is even more preferable that the fine palladium-containing particles contain a metallic material that can impart electrical conductivity.

[0039] In the present invention, the fine palladium-containing particles, from the above perspective, are preferably fine palladium particles or particles of an alloy of palladium with a metal such as cobalt, iridium, rhodium, or gold. In the case of the use of palladium alloy particles, the palladium alloy particles can contain palladium and only one type of metal, or they can contain palladium and two or more types of metal.

[0040] The average particle diameter of the fine palladium-containing particles used in the present invention is 3.0 nm or more and 6.0 nm or less. If the average particle diameter of the fine palladium-containing particles is less than 3.0 nm, the proportion of low-coordinated palladium atoms on the surface of the fine palladium-containing particles is high, so that the low-coordinated palladium atoms can be eluted in the potential application step. If, on the other hand, the average particle diameter of the fine palladium-containing particles is more than 6.0 nm, the surface energy of the fine palladium-containing particles is low and the fine palladium-containing particles themselves are stable, so that a rearrangement of the atoms on the particle surface does not occur sufficiently even after the potential application step, and the Pd{111} surface may not grow sufficiently.

[0041] The average particle diameter of the fine palladium-containing particles is preferably 3.5 nm or more and preferably 5.5 nm or less.

[0042] In the present invention, the average particle diameter of fine palladium-containing particles, fine catalyst particles, and carbon-supported catalyst is calculated using a conventional method. An example of the method for calculating the average particle diameter of fine palladium-containing particles, fine catalyst particles, and carbon-supported catalyst is as follows. First, for a particle shown in a TEM image at a magnification of 400,000 to 1,000,000×, the particle diameter is calculated, assuming the particle is spherical. Such a calculation of the particle diameter based on TEM observation is performed on 200 to 500 particles of the same type, and the average of the particles is taken as the average particle diameter.

[0043] In the case of the production of the carbon-supported catalyst, where the fine catalyst particles are supported on a carbon support, it is preferable to use fine palladium-containing particles supported on a carbon support in this step. The use of the carbon support allows the electrocatalyst layer of a fuel cell to be made electrically conductive by the carbon support.

[0044] Specific examples of carbon-containing materials that can be used as the carbon support include electrically conductive carbon-containing materials, including carbon particles and carbon fibers, such as: Ketjen Black (product name; manufactured by: Ketjen Black International Company), Vulcan (product name; manufactured by: Cabot), Norit (product name; manufactured by: Norit), Black Pearls (product name; manufactured by: Cabot), Acetylene Black (product name; manufactured by: Chevron) and OSAB (product name; manufactured by: Denka Co., Ltd.).

[0045] The acid solution is used in the potential application step. The acid solution is not subject to any particular restrictions, provided it is a solution capable of removing palladium oxide and impurities from the surface of the fine palladium-containing particles by means of a suitable potential sweep or pass in the acid solution, and of allowing the Pd{111} surface of the fine palladium-containing particle surface to grow sufficiently.

[0046] More precisely, the acid solution used in the potential application step can be an acid solution similar to one that can be used in the copper-containing solution described below.

[0047] If the potential application step and the copper covering step described below are performed in the same reaction vessel, the copper-containing solution can be added to the electrolyte used in the potential application step. For example, if sulfuric acid is used as the electrolyte in the potential application step, an aqueous copper sulfate solution can be added to the sulfuric acid and used in the copper covering step.

[0048] From the point of view of removing oxygen as much as possible from the acid solution and promoting rapid oxide removal, it is preferable to introduce nitrogen into the acid solution.

[0049] The first dispersion, comprising the fine palladium-containing particles dispersed in the acid solution and possessing the average particle diameter specified above, can be a pre-prepared dispersion or a commercially available product. In the case of preparing the first dispersion, the manufacturing process can be a well-established method.

[0050] A key feature of the present invention is the application of the potential to the fine palladium-containing particles in the first dispersion until the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram becomes larger than the peak indicating the Pd{110} or Pd{100} surface area in the reduction wave of the cyclic voltammogram.

[0051] A cyclic voltammogram is a current-potential curve that develops as a result of a potential sweep, with the current on the vertical axis and the potential on the horizontal axis. Generally, positive current is defined as oxidation current, and negative current as reduction current. Therefore, the reduction wave of the cyclic voltammogram corresponds to a negative current wave.

[0052] In the present invention, the cyclic voltammogram can be obtained simultaneously with the application of the potential, or it can be obtained separately from the application of the potential.

[0053] In the case where the cyclic voltammogram is obtained simultaneously with the application of the potential, examples include the case where the cyclic voltammetry is performed with a suitable potential sweep.

[0054] In the case where the cyclic voltammogram is obtained separately from the application of the potential, examples include a case where a suitable sample quantity is taken from the first dispersion in the potential application step, and the sample is treated by a Fig. 2 device shown, etc., a cyclic voltammetry is performed for monitoring.

[0055] Fig. Figure 2 schematically shows a perspective view of an example of an electrochemical device used for cyclic voltammetry. An electrochemical device 100 includes a glass cell 1 , an electrolyte added to the cell 2 , a dispersion 3 (the first dispersion) and a working electrode 4 with the first dispersion applied to it 3 (approximately 10 μl). In the glass cell 1 are the working electrode 4 , a reference electrode 5 and a counter electrode 6so that they are sufficiently immersed in the electrolytes 2 are immersed, and the three electrodes are electrically connected to a potentiostat / galvanostat. A gas inlet tube 7 is positioned so that it is in the electrolytes 2 is immersed. At room temperature, an inert gas from an inert gas supply source (not shown) located outside the cell is introduced into the electrolyte for a certain period of time. 2 encapsulated to retain the electrolyte 2 to saturate with the inert gas. Bubbles 8 The bubbles indicate the presence of an inert gas. Nitrogen, argon, or a mixture thereof can be used as the inert gas. Cyclic voltammetry is then performed.

[0056] A measuring electrode made of an electrically conductive material, such as a glassy carbon electrode, can be used as the working electrode. If the cyclic voltammetry of the carbon-supported catalyst is performed using a regenerative electrode (RDE) as the working electrode, the cyclic voltammetry is preferably carried out, from the perspective of potential stability, after immersing the RDE in an electrolyte, rotating the RDE in the electrolyte, and stopping the rotation a few minutes after immersion.

[0057] A reversible hydrogen electrode (hereinafter also referred to as RHE), created by injecting hydrogen into platinum, or a silver-silver chloride electrode is used as the reference electrode. In the case where a measurement taken with the silver-silver chloride electrode is converted to a measurement with the reversible hydrogen electrode, the potential difference between the RHE and the silver-silver chloride electrode is measured beforehand and subsequently corrected.

[0058] A platinum electrode or similar can be used as the counter electrode.

[0059] The conditions for cyclic voltammetry are preferably those that do not lead to deterioration of the fine palladium-containing particles or deterioration of the support (carbon). A specific example of cyclic voltammetry conditions for monitoring using RDE is as follows. Electrolyte: aqueous 0.1 M perchloric acid solution (into which inert gas is bubbled) Atmosphere: under an inert gas atmosphere Sweep Rate: 50 mV / s Potential sweep rate (potential window): 0.35 to 0.70 V (vs. RHE) Reference electrode: reversible hydrogen electrode (RHE)

[0060] The peak indicating the Pd{111} surface is a peak that appears in the reduction wave of the cyclic voltammogram in a range of 0.50 V to 0.55 V (vs. RHE), depending on the cyclic voltammetry conditions.

[0061] In the present invention, applying the potential "until the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram becomes larger than the peak indicating the Pd{110} or Pd{100} surface area in the reduction wave of the cyclic voltammogram" means applying the potential to the fine palladium-containing particles where the Pd{111} surface area does not appear at all on the surface of the fine palladium-containing particles before the potential application step, or to the fine palladium-containing particles where the proportion of the Pd{111} surface area in the surface area is small, until the proportion of the Pd{111} surface area in the surface area becomes larger than the proportion of the Pd{110} or Pd{100} surface area and the Pd{111} surface area grows sufficiently. The peak that appears in the reduction wave of the cyclic voltammogram is an indicator of the degree of growth of the Pd{111} surface.In the present invention, the potential application step is terminated when the peak indicating the Pd{111} surface becomes larger than the peak indicating the Pd{110} or Pd{100} surface in the reduction wave. That is, this step is a step of increasing the purity and crystallinity of the surface of the fine palladium-containing particles.

[0062] The size of the peak indicating the Pd{111} surface area and the size of the peak indicating the Pd{110} surface area and / or the Pd{100} surface area can be compared by calculating the areas of these peaks in the reduction wave of the cyclic voltammogram.

[0063] An example of applying the potential "until the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram becomes larger than the peak indicating the Pd{110} or Pd{100} surface area in the reduction wave of the cyclic voltammogram" is a method of repeatedly applying a potential over a broad potential range. A broad potential range is defined as a potential range such that palladium reaches a reduction state at at least at the lower limit of the range and palladium reaches an oxidation state at at least at the upper limit of the range.As just described, by alternately repeating the reduction and oxidation states of palladium through the potential sweep, oxygen and impurities can be removed from the surface of the fine palladium-containing particle, and the arrangement of palladium atoms on at least the surface of the fine palladium-containing particle can be modified. As a result, the most electrochemically stable Pd{111} surface can grow on the surface of the fine palladium-containing particle.

[0064] The lower limit of the potential range (hereinafter also referred to as the lower limit potential) is preferably 0.1 V (vs. RHE) or less, and preferably 0.05 V (vs. RHE) or less. Since the oxidation-reduction potential of palladium is 0.915 V (vs. RHE), the lower limit potential can be lower than the oxidation-reduction potential. However, the lower limit potential is preferably −0.5 V (vs. RHE) or more, because if the lower limit potential is less than −0.5 V (vs. RHE), hydrogen is drastically produced from the solution.

[0065] In contrast, the upper limit of the potential range (hereinafter also referred to as the upper limit potential) is preferably higher than 1.0 V (vs. RHE), more preferably 1.1 V (vs. RHE) or more, and even more preferably 1.2 V (vs. RHE) or more. Particularly in this step, it is preferred that the potential is traversed within a range that includes at least 1.2 V (vs. RHE). The upper limit potential is also preferably 2 V (vs. RHE) or less, since a significant amount of palladium can be dissolved if the upper limit potential is greater than 2 V (vs. RHE).

[0066] As just described, raising the upper limit potential to above 1.0 V requires the remodification of the palladium atoms on the surface of the fine palladium-containing particles, and as a result, the proportion of the Pd{111} surface area in the surface can be increased so that it is higher than that before the potential application step.

[0067] In this step, from the perspective of rapid oxide removal, it is preferable to repeatedly cycle the potential back and forth within a predetermined potential range. Examples of the signal pattern of the applied potential include a square wave, a triangular wave, and a trapezoidal wave. The signal pattern of the applied potential is not subject to any particular restrictions, as it depends on the repetition of a low potential and a high potential.

[0068] In the case that the signal pattern of the applied potential is a square wave, the number of potential cycles is not subject to any particular limitations. Holding 0.05 V (vs. RHE) for 15 to 60 seconds and then holding the aforementioned upper limit potential for 15 to 60 seconds is considered one cycle, and it is preferable to perform 1,000 to 3,000 cycles.

[0069] In the case where the signal pattern of the applied potential is a triangular wave, the number of potential cycles is not subject to any particular limitations. It is preferably 100 cycles or more, and more preferably 800 to 3,000 cycles. The potential sweep rate can be, for example, 5 to 100 mV / s.

[0070] In the potential application step, the temperature within a reaction system is not subject to any particular restrictions. If the potential application step, the copper covering step, and the platinum covering step are carried out in the same reaction vessel, it is preferable, from the perspective of rapidly adjusting the temperature within the reaction system to -3°C or more and 10°C or less during the platinum covering step, to maintain the temperature at -3°C or more and 10°C or less. In the present invention, "within the reaction system" is a term that includes areas used for reactions (such as reaction vessels and apparatus), as well as gases, liquids, and solids held in these areas.

[0071] The method for applying the potential to the fine palladium-containing particles and the potential control device can be the same as those used in the copper covering step described below.

[0072] The method for applying the potential to the fine palladium-containing particles in the first dispersion is not subject to any particular restrictions and can be a general method. For example, it can be a method of immersing the working electrode, the counter electrode, and the reference electrode in the first dispersion and then applying the potential to the working electrode.

[0073] The working electrode can be a material that ensures electrical conductivity, such as metallic materials including titanium, platinum lattice, platinum plate, and gold plate, and electrically conductive carbon-containing materials including glassy carbon and carbon plate. Alternatively, the reaction vessel itself can be made of the electrically conductive material and used as the working electrode. If the reaction vessel is made of a metallic material and used as the working electrode, it is preferable, from a corrosion prevention standpoint, for the inner wall of the reaction vessel to be coated with RuO₂. If the reaction vessel is made of a carbon-containing material and used as the working electrode, the vessel can be used as is, without any coating.

[0074] For example, a platinum grid plated with platinum moraine or electrically conductive carbon fibers can be used as the counter electrode.

[0075] For example, a reversible hydrogen electrode (RHE), a silver-silver chloride electrode, or a silver-silver chloride-potassium chloride electrode can be used as the reference electrode.

[0076] A potentiostat or potentio-galvanostat, for example, can be used as the potential control device.

[0077] Fig. Figure 3 schematically shows a perspective view of an example of the electrochemical device used to perform the potential application step.

[0078] One in Fig. 3 electrochemical device shown 200 includes a reaction vessel 11 , a reference electrode 14 , a counter electrode 15 , a room 16for the counter electrode and a stirring rod 17 .

[0079] The reaction vessel 11 It is made of graphite and also functions as a working electrode. An acid solution 13 is in the reaction vessel 11 containing a palladium-supported carbon atom. 12 (hereinafter also referred to as Pd / C) contains, in which palladium particles are supported on a carbon support.

[0080] In the reaction vessel 11 can the acid solution 13 , which the Pd / C 12 contains, with the stirring rod 17 be stirred.

[0081] The reference electrode 14 and the counter electrode 15 are positioned so that they are completely immersed in the acid solution 13 are immersed. The reaction vessel 11 , which acts as a working electrode, the reference electrode 14 and the counter electrode 15are electrically connected to the potentiostat / galvanostat so that they can control the potential of the working electrode. To establish contact between the Pd / C 12 in the acid solution 13 with the counter electrode 15 To prevent this, the counter electrode 15 in the state of storage in the room 16 for the counter electrode in the acid solution 13 immersed, with the room being made of glass. The floor of the room. 16 The counter electrode is made of a porous glass frit to ensure contact between the counter electrode and the other electrode. 15 and the acid solution 13 to ensure.

[0082] First, the Pd / C is dispersed. 12 by stirring the acid solution 13 with the stirring stick 17 nitrogen from a nitrogen supply source (not shown) located outside the reaction vessel is introduced into the acid solution for a certain period of time 13sprinkled to contain the acid solution 13 to saturate with nitrogen.

[0083] Then, hydrogen is introduced into the acid solution for a certain period of time from a hydrogen supply source located outside the reaction vessel (not shown). 13 sprinkled to contain the acid solution 13 to saturate with hydrogen.

[0084] Then, nitrogen is released from the nitrogen supply source back into the acid solution for a certain period of time. 13 sprinkled to contain the acid solution 13 to saturate with nitrogen.

[0085] Then the potential of the reaction vessel is set by the potentiostat / galvanostat. 11 (working electrode) passed through to establish a specific potential at the surface of the reaction vessel 11 Pd / C brought into contact 12to apply the potential, thereby removing oxides from the surface of the palladium particles. An example of the conditions for the potential application step is shown below. Electrolyte: 0.05 mol / l H2SO4aq (into which inert gas is bubbled) Atmosphere: under an inert gas atmosphere Sweep Rate: 50 mV / s Potential sweep rate: -0.05 V to 1.2 V (vs. RHE) Reference electrode: reversible hydrogen electrode (RHE)

[0086] This step removes impurities from the surface of the fine palladium-containing particles, and the Pd{111} surface area grows sufficiently. Therefore, in the subsequent copper covering step, a larger amount of copper than ever before adsorbs onto the surface of the fine palladium-containing particles, and a Pt{111} surface with high catalytic activity is formed more extensively on the surface of the fine catalyst particles. As a result, the catalytic activity of the fine catalyst particles can be increased. (2) Copper covering step

[0087] The copper covering step is a step of covering at least a part of the fine palladium-containing particle with copper by preparing a second dispersion by mixing the first dispersion and a copper-containing solution after the potential application step and applying a potential that is more noble than the oxidation-reduction potential of copper to the fine palladium-containing particles in the second dispersion.

[0088] The copper-containing solution is not subject to any particular restrictions, provided it is a solution in which copper can be deposited onto the surface of the fine palladium-containing particles by Cu-UPD. In the copper-containing solution, the copper may be present in the form of ions or a copper compound, such as a copper complex. The copper-containing solution generally consists of a solvent in which a predetermined amount of copper salt is dissolved. However, it is not limited to this composition and may be a solution in which copper, or all or some of its ions, are present separately.

[0089] Examples of solvents used for the copper-containing solution include water and organic solvents. Water is preferred because it does not inhibit the deposition of copper on the surface of the fine palladium-containing particles.

[0090] Examples of copper salts used for copper-containing solutions include copper sulfate, copper nitrate, copper chloride, copper chlorite, copper perchlorate, and copper oxalate.

[0091] The copper concentration of the copper-containing solution is not subject to any particular restrictions and is preferably 10 to 1000 mM.

[0092] In addition to the solvent and the copper salt, the copper-containing solution can, for example, contain an acid. Examples of acids that can be added to the copper-containing solution include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxoacid. Counter-anions in the copper-containing solution and counter-anions in the acid can be of the same type or different types.

[0093] It is also preferable to introduce an inert gas into the copper-containing solution beforehand. This prevents the fine palladium-containing particles from oxidizing and allows them to be uniformly coated with the platinum-containing outer layer. Nitrogen gas, argon gas, or the like can be used as the inert gas.

[0094] In this step, the second dispersion is prepared by mixing the first dispersion with the copper-containing solution after the potential application step.

[0095] The method for applying the potential to the fine palladium-containing particles in the second dispersion is not subject to any particular restrictions and can be a general procedure. Examples include the following procedure: the second dispersion is prepared by adding the copper-containing solution to the reaction vessel. 11 the in Fig. 3 electrochemical device shown200 while stirring the acid solution 13 through the stirring rod 17 ; then, by applying the potential to the reaction vessel 11 Copper on the surface of the palladium particles of the Pd / C 12 deposited, which is in contact with the surface of the reaction vessel 11 was put in contact.

[0096] The applied potential is not subject to any particular restrictions, provided it is a potential capable of depositing copper on the surface of the fine palladium-containing particles, i.e., a more noble potential than the oxidation-reduction potential of copper. For example, the applied potential is preferably in the range of 0.35 to 0.7 V (vs. RHE) and particularly preferably 0.4 V (vs. RHE).

[0097] The potential application time is not subject to any particular restrictions. It is preferable that the potential be applied for 60 minutes or more, and it is more preferable that the potential be applied until a reaction current becomes steady and close to zero.

[0098] The potential can be applied by a potential sweep within a range that includes the aforementioned potential range. More precisely, the potential sweep range is preferably 0.3 to 0.8 V (vs. RHE).

[0099] The number of potential sweep cycles is not subject to any particular restrictions and is preferably 1 to 10,000 cycles. The potential sweep rate is, for example, 0.01 to 100 mV / s.

[0100] From the point of view of preventing oxidation of the surface of the fine palladium-containing particles and preventing oxidation of the copper, it is preferable to carry out the copper covering step under an inert gas atmosphere such as a nitrogen atmosphere.

[0101] It is also preferable in the copper coating step to stir the second dispersion appropriately if necessary. For example, in the case that – as in the case of the Fig.In the apparatus shown in Figure 3, the reaction vessel is used as the working electrode, and the fine palladium-containing particles in the second dispersion are dispersed in the reaction vessel. Stirring the second dispersion brings the fine palladium-containing particles into contact with the surface of the reaction vessel (working electrode), thus applying a uniform potential to the fine palladium-containing particles. In this case, the stirring in the copper covering step can be continuous or intermittent.

[0102] In the copper covering step, the temperature within the reaction system is not subject to any particular restrictions. However, if the copper covering step and the platinum covering step described below are carried out in the same reaction vessel, it is preferable, from the perspective of rapidly adjusting the temperature within the reaction system to -3°C or more and 10°C or less in the platinum covering step, to maintain the temperature at -3°C or more and 10°C or less. (3) Platinum covering step

[0103] The platinum covering step is a step of covering at least a part of the fine palladium-containing particle with platinum by substituting the copper covering at least a part of the fine palladium-containing particle with platinum by mixing the second dispersion and a platinum-containing solution after the copper covering step.

[0104] In the platinum coating step, the temperature within the reaction system is preferably maintained at -3°C or higher and 10°C or lower. To achieve the formation of a uniform coating on the surface of the fine palladium-containing particles, the temperature is more preferably maintained at 3°C ​​or higher and 9°C or lower, and particularly preferably at 5°C or higher and 8°C or lower. If the temperature is lower than -3°C, the solution is frozen and no reaction can occur. If the temperature is higher than 10°C, sufficient mass activity of platinum may not be obtained.

[0105] The method for maintaining the temperature within the reaction system is not subject to any particular restrictions. For example, it can be a method that uses a circulating cooling device (cooling apparatus), a cooling tube, or the like.

[0106] The platinum-containing solution is not subject to any special restrictions, provided it contains at least platinum ions and a reaction inhibitor. In the platinum-containing solution, the platinum can be present in the form of ions or a platinum compound such as a platinum complex.

[0107] The reaction inhibitor is not subject to any special restrictions, provided it can inhibit a substitution reaction between copper and platinum. Examples of reaction inhibitors include a complexing agent that forms a complex in solution with the platinum, the copper deposited on the surface of the fine palladium-containing particles, and the palladium exposed on the surface of the fine palladium-containing particles.

[0108] Examples of complexing agents include citric acid, sodium citrate, potassium citrate, ethylenediaminetetraacetic acid (hereinafter also referred to as EDTA), sodium EDTA, and potassium EDTA. Citric acid is preferred. These complexing agents may be used alone or in a combination of two or more. In solution, these complexing agents form a complex with the platinum and the copper; therefore, the substitution reaction between the copper and the platinum is inhibited, and as a result, the surface of the fine palladium-containing particles can be uniformly coated with the platinum-containing shell.

[0109] The concentration of the reaction inhibitor in the platinum-containing solution is not subject to any particular restrictions and is preferably 1 to 10 times higher than the platinum ion concentration.

[0110] A platinum salt is used for the platinum-containing solution. Examples of platinum salts include K₂PtCl₄ and K₂PtCl₆. An ammonia complex such as ([PtCl₄][Pt(NH₃)₄]) can also be used.

[0111] The platinum ion concentration of the platinum-containing solution is not subject to any particular restrictions and is preferably 0.01 to 100 mM.

[0112] A solvent is used for the platinum-containing solution. This solvent can be the same as the solvent used for the copper-containing solution described above.

[0113] In addition to the solvent, the reaction inhibitor, and the platinum salt, the platinum-containing solution may also contain an acid, etc. The acid may be the same as the acid used for the copper-containing solution described above.

[0114] From the perspective of maintaining the temperature within the reaction system at -3°C or more and 10°C or less, it is preferable to pre-adjust the temperature of the platinum-containing solution to -3°C or more and 10°C or less. The platinum-containing solution is also stirred sufficiently, and from the perspective of preventing oxidation of the surface of the fine palladium-containing particles or preventing oxidation of the copper, it is preferable to pre-inject nitrogen into the solution.

[0115] The substitution time (contact time between the platinum-containing solution and the fine palladium-containing particles) is not subject to any particular limitations and is preferably 10 minutes or more. Since the potential of the reaction solution is increased by adding the platinum-containing solution, it is more preferable to continue the substitution until the monitored potential shows no further change.

[0116] The method for contacting the copper deposited on the surface of the fine palladium-containing particles with the platinum-containing solution is not subject to any particular restrictions. If the copper covering step and the platinum covering step are carried out in the same reaction vessel, the platinum-containing solution may be added to the copper-containing solution used in the copper covering step. For example, if the copper is used in the copper covering step, the platinum-containing solution may be added to the copper-containing solution used in the copper covering step. Fig. 3 electrochemical device shown 200 the potential control in the reaction vessel 11 The process is stopped after the copper covering step, and the platinum-containing solution is stirred into the acid solution. 13 with the stirring stick 17 stepwise into the reaction vessel 11 added, thereby bringing the fine palladium-containing particles, on which copper is deposited, into contact with the platinum-containing solution.

[0117] It is preferable that the platinum-containing outermost layer formed by this step possesses high catalytic activity. As used herein, “catalytic activity” refers to the activity required of a fuel cell catalyst, in particular oxygen reduction reaction (ORR) activity.

[0118] The platinum-containing outer layer can consist solely of platinum, or it can also contain iridium, ruthenium, rhodium, or gold. If a platinum alloy is used for the platinum-containing outer layer, the alloy can contain platinum and only one other metal, or it can contain platinum and two or more other metals.

[0119] In the potential application step described above, the Pd{111} surface area grows to a sufficiently larger size than both the Pd{110} and Pd{100} surfaces. Since it is likely that a Pt{111} crystal will grow on the Pd{111} surface, the proportion of Pd{111} surface area on the surface of the fine catalyst particles subjected to the platinum coating step is the highest. The catalytic activity of the fine catalyst particles thus obtained can be increased by enhancing the proportion of the Pt{111} surface area exhibiting high catalytic activity, making it larger than ever before. (4) Washing step

[0120] The washing step is a step involving washing with water the fine palladium-containing particles, which have undergone copper substitution with platinum, after the platinum coating step. From the perspective of eluting the reaction inhibitor, which is physically adsorbed onto the support surface, the washing step is preferably carried out before the acid treatment step.

[0121] In this washing step, the water can be cold or warm. Alternatively, a combination of cold and warm water can be used. More specifically, the fine palladium-containing particles can be washed with cold water at less than 30°C and then rinsed with warm water.

[0122] From the point of view of eluting the reaction inhibitor, which is physically adsorbed onto the support surface, the temperature of the warm water is preferably 30°C or more and 100°C or less.

[0123] The washing step preferably involves washing the fine palladium-containing particles by dispersing them in water, preferably warm water. The method for dispersing the fine palladium-containing particles in water is not subject to any particular restrictions. It can, for example, be an ultrasonic dispersion method, a method of pulverizing the particles with a ball mill and then adding them to water, or a method of dispersing the particles with a device using shear force, such as a nanometer. Of these, the ultrasonic dispersion method is preferred due to the relatively lower damage it causes to the structure of the fine palladium-containing particles.

[0124] It is preferable to repeat the washing step until the conductivity of the water used for washing (hereinafter also referred to as wash water) reaches 10 μS / cm or less. This is because the amount of reaction inhibitor that is physically adsorbed onto the support surface is still considered large if the conductivity of the wash water is high. Specifically, wash water refers to supernatant water obtained by adding the fine palladium-containing particles to water (10 g per liter of water) in a container and dispersing the mixture. (5) Acid treatment step

[0125] The acid treatment step involves contacting an acid solution with the fine palladium-containing particles, which have undergone copper substitution with platinum following the platinum coating step. The acid treatment selectively elutes the exposed fine palladium-containing particles, thus reducing their size. This repairs defects in the outermost platinum layer, thereby increasing the mass activity of platinum within the fine catalyst particles.

[0126] Examples of acid solutions include nitric acid, sulfuric acid, perchloric acid, hydrochloric acid, and hypochlorous acid. Of these, nitric acid is preferred because it is strongly acidic.

[0127] The concentration of the acid solution is as follows: for example, if nitric acid is used as the acid solution, the nitric acid concentration is preferably 1.0 × 10 –4up to 2 mol / l, more preferably 1.0 × 10 –3 up to 1 mol / l and even more preferably 1.0 × 10 –2 up to 1.0 × 10 –1 minor.

[0128] In the case of using sulfuric acid as the acid solution, the sulfuric acid concentration is preferably 1.0 × 10 –4 up to 2 mol / l, more preferably 1.0 × 10 –3 up to 1 mol / l and even more preferably 1.0 × 10 –2 up to 1.0 × 10 –1 minor.

[0129] The temperature of the acid solution is preferably 40°C or higher, particularly preferably 50°C or higher, as this allows for effective and efficient repair of defects in the platinum-containing outer layer. Furthermore, to prevent aggregation of the fine palladium-containing particles, etc., the temperature of the acid solution is preferably 90°C or lower, particularly preferably 80°C or lower.

[0130] The contact time for the fine palladium-containing particles with the acid solution can be adjusted depending on the type, concentration, temperature, etc. of the acid solution. For example, it can range from approximately 30 minutes to 2 hours.

[0131] The method for contacting the fine palladium-containing particles with the acid solution is not subject to any particular restrictions. From the perspective of ensuring that the acid treatment proceeds adequately, a method for immersing the fine palladium-containing particles in the acid solution is preferred. At the time of immersion, it is preferable to stir the acid solution and disperse the particles using an ultrasonic homogenizer, a magnetic stirrer, a motor with impeller blades, etc. (6) Drying step

[0132] The drying step is a step in drying the fine catalyst particles obtained after the platinum covering step.

[0133] The method for drying the fine catalyst particles is not subject to any particular restrictions, provided it is a method capable of removing the solvent, etc. For example, it could be a drying process involving maintaining a temperature of 50 to 100°C for 6 to 12 hours under an inert gas atmosphere.

[0134] If necessary, the fine catalyst particles can be pulverized. The pulverization process is not subject to any particular restrictions, provided it is a process capable of pulverizing solids. Examples of pulverization include pulverization using a mortar or the like under an inert gas atmosphere or in the atmosphere, and mechanical grinding such as with a ball mill, turbo mill, or the like.

[0135] The fine catalyst particles obtained by the present invention are preferably intended for use in fuel cells. From the perspective of excellent oxygen reduction activity, the fine catalyst particles obtained by the present invention are preferably used in electrodes for fuel cells, and more preferably in cathode electrodes for fuel cells.

[0136] From the perspective that the elution of the fine palladium-containing particles can be more strongly inhibited, the coverage of the fine palladium-containing particle with the platinum-containing outer layer is generally 0.5 to 2, preferably 0.8 to 1. If the coverage of the fine palladium-containing particle with the platinum-containing outer layer is less than 0.5, the palladium-containing particles are eluted in an electrochemical reaction and as a result, the fine catalyst particles can degrade.

[0137] As used herein, “coverage of the fine palladium-containing particle with the platinum-containing outer layer” means the fraction of the surface area of ​​the fine palladium-containing particle that is covered by the platinum-containing outer layer, assuming that the total surface area of ​​the fine palladium-containing particle is 1. An example of the procedure for calculating the coverage is as follows. First, the metal content (A) of the outer layer in the fine catalyst particle is measured by inductively coupled plasma mass spectrometry (ICP-MS), etc. Meanwhile, the average particle diameter of the fine catalyst particles is measured by a transmission electron microscope (TEM), etc.From the average particle diameter thus measured, the number of atoms on the surface of a particle with the same diameter is estimated, and the metal content (B) of the outermost layer is estimated if an atomic layer on the particle surface is substituted by the metal contained in the platinum-containing outermost layer. The value obtained by dividing the metal content (A) of the outermost layer by the metal content (B) of the outermost layer is the "coverage of the fine palladium-containing particle with the platinum-containing outermost layer".

[0138] The platinum-containing outer layer covering the fine palladium-containing particle is preferably a monoatomic layer. The fine catalyst particle with such a structure is advantageous in that, compared to a fine catalyst particle with a platinum-containing outer layer composed of two or more atomic layers, the catalytic performance of the platinum-containing outer layer is much higher, and because the amount of platinum-containing outer layer covering the fine palladium-containing particle is small, the material costs are lower.

[0139] The average particle diameter of the fine catalyst particles is preferably 3.0 nm or more, and more preferably 3.5 nm or more. The average particle diameter of the fine catalyst particles is preferably 6.0 nm or less, and more preferably 5.5 nm or less. Examples

[0140] The present invention is explained in more detail below with reference to examples and comparative examples. The scope of the present invention is not limited to these examples and comparative examples. 1. Preparation of a carbon-supported catalyst [Example 1] 1-1. Preparation of a first dispersion

[0141] OSAB (product name; manufactured by: Denki Kagaku Kogyo Kabushiki Kaisha) was used as a carbon support. The carbon support was dispersed in nitric acid. Chloropalladium acid was added to the resulting dispersion mixture. Under heating at a temperature of 100°C or less, sodium borohydride (NaBH4) was added to the mixture to reduce the palladium. After completion of the reaction, the resulting reaction mixture was filtered. A solid thus obtained was washed and then dried for 24 hours under an inert gas atmosphere, producing carbon-supported palladium. The average particle diameter of fine palladium particles in the carbon-supported palladium was 5.2 nm. Then, 5 g of the carbon-supported palladium thus prepared were added to 1 L of pure water and dispersed therein using an ultrasonic homogenizer, thus preparing the first dispersion.

[0142] Fig. Figure 4 is a TEM image of carbon-supported palladium used in Example 1. In the TEM image, white dots are fine palladium particles, and a bright white blurred area is a carbon support. For 500 or more fine palladium particles shown in this TEM image, their equivalent particle circle diameters were calculated to form a histogram of the particle size distribution.

[0143] Fig. Figure 5 is a histogram showing the particle size distribution of fine palladium particles in the carbon-supported palladium used in Example 1. As shown in the histogram, the most common equivalent circular diameter of the fine palladium particles is 4 nm. As described above, the average particle diameter is 5.2 nm. 1-2. Potential investment step

[0144] The first dispersion obtained in this way was placed in the reaction vessel (reaction vessel).11 ) one in Fig. 3 electrochemical device shown (electrochemical device) 200 ) given. Sulfuric acid was added so that the sulfuric acid concentration of 0.05 mol / l was reached. The electrochemical apparatus 200 The mixture was placed inside a glovebox. For deoxidation, a sufficient amount of inert gas (N2 gas) was introduced into the first dispersion. A triangular wave of the potential was applied to the working electrode (reaction vessel). 11 ) the electrochemical device 200 applied in a potential range of 0.05 to 1.2 V (vs. RHE) for 2,500 cycles, thereby sufficiently cleaning the surface of the fine palladium particles and the surface of the carbon support.

[0145] Fig.Figure 6 is a graph showing the palladium oxidation rates of samples taken during the preparation of the carbon-supported palladium used in Example 1. More specifically, it is a graph showing the palladium oxidation rate of a sample not subjected to the potential application step (number of cycles: 0), the palladium oxidation rate of a sample at the 800th cycle (number of cycles: 800), the palladium oxidation rate of a sample at the 1200th cycle (number of cycles: 1200), the palladium oxidation rate of a sample at the 1600th cycle (number of cycles: 1600), and the palladium oxidation rate of a sample at the 2000th cycle (number of cycles: 2000). The procedures for measuring and calculating palladium oxidation rates are as follows.

[0146] The carbon-supported palladium samples underwent constant voltage (CV) cleaning and were then analyzed by X-ray photoelectron spectroscopy to quantify the amount of oxygen present. The palladium oxidation rate was calculated assuming that the observed oxygen was present only on the palladium surface. Device: Multifunctional scanning X-ray photoelectron spectrometer (Product name: Versa Probe II; manufactured by: ULVAC-PHI, Inc.) Measurement conditions: X-ray source: A1Kα (monochrome, 25 W) Analysis area: 1.0 × 0.5 mm 2 (scanned with a 0.1 mmφ probe) Note: A charge neutralization mechanism (electron beam and ion beam) was used.

[0147] As from Fig.As can be clearly seen in Figure 6, the palladium oxidation rate for the sample with 0 cycles is over 55%. This indicates that over half of the palladium surface not subjected to the potential treatment is covered with palladium oxide. With increasing cycle count, the palladium oxidation rate gradually decreased. For the sample with 2,000 cycles, the palladium oxidation rate decreased to 10%.

[0148] The above result clearly shows that by performing the potential application step, palladium oxide is removed from most of the palladium surface of the carbon-supported palladium.

[0149] Fig.Figure 7 is a graph showing the cyclic voltammogram (thin curve) of fine palladium-containing particles before they were subjected to the potential application step in Example 1, and showing the cyclic voltammogram (thick curve) of the fine palladium-containing particles at the 2,500th cycle in Example 1, with the cyclic voltammograms overlapping. The cyclic voltammetry was determined by the Fig. The electrochemical apparatus shown in section 2 was used. The cyclic voltammetry conditions are as follows. Electrolyte: aqueous 0.1 M perchloric acid solution (into which an inert gas was bubbled) Atmosphere: under an inert gas atmosphere Sweep Rate: 50 mV / s Potential sweep range (potential window): 0.35 to 0.70 V (vs. RHE) Reference electrode: reversible hydrogen electrode (RHE)

[0150] As seen from the thin line in Fig.As becomes clear in Figure 7, in the initial cyclic voltammogram, the peak indicating the Pd{111} surface, which occurs around 0.52 V (vs. RHE), is still small, while the peak indicating Pd{110} or Pd{100}, which occurs in a region below 0.50 V (vs. RHE), is larger. However, as can be seen from the thick line in Fig. As can be clearly seen in Figure 7, the peak occurring at 0.50 V (vs. RHE) almost disappeared in the cyclic voltammogram at the 2,500th cycle, and the peak at 0.52 V (vs. RHE) is the largest. This is thought to be due to the fact that, as a result of the potential sweep across a broad potential range of 0.05 to 1.2 V (vs. RHE), the growth of the Pd{111} surface, which is electrochemically more stable, was promoted. 1-3. Copper covering step (Cu-UPD)

[0151] The in Fig. 3 electrochemical device shown 200was also used in this step. Nitrogen was introduced into the first dispersion in the reaction vessel. 11 A copper-containing solution of 14.6 g of copper sulfate pentahydrate dissolved in 66 ml of 0.05 M sulfuric acid was added to the reaction vessel to prepare a second dispersion. By setting the potential of the working electrode (reaction vessel) 11 ) at 0.4 V (vs. RHE) for two hours, the potential was applied to fine palladium-containing particles in the second dispersion to deposit copper onto the fine palladium particles. Steps 1-4: Platinum covering

[0152] The potential control was stopped at 0.4 V (vs. RHE). A platinum-containing solution of 161.3 mg K₂PtCl₄ and 4.5 g citric acid monohydrate, dissolved in 140 ml 0.05 M sulfuric acid, was added to the reaction vessel. 11The mixture containing the second dispersion was added for approximately 80 minutes. The resulting mixture was then stirred for one hour to substitute the copper with platinum. The amount of platinum added was 100 at%, if 100 at% was determined to be the minimum amount of platinum required to cover the fine palladium particles with a monatomic platinum layer. 1-5. Follow-up treatment

[0153] The resulting reaction solution was filtered to obtain a carbon-supported catalyst. The carbon-supported catalyst was washed, dried, and then pulverized using an agate mortar and pestle, thus preparing the carbon-supported catalyst of Example 1. [Example 2]

[0154] The carbon-supported catalyst of Example 2 was prepared in the same way as Example 1, except that in the potential application step the upper limit of the applied potential range was changed from 1.2 V (vs. RHE) to 1.4 V (vs. RHE). [Example 3]

[0155] The carbon-supported catalyst of Example 3 was prepared in the same way as Example 1, except that a carbon-supported palladium in which the fine palladium particles have an average particle diameter of 3.8 nm was prepared and used. [Comparison example 1]

[0156] The carbon-supported catalyst of Comparative Example 1 was prepared in the same way as Example 1, except that in the potential application step the upper limit of the applied potential range was changed from 1.2 V (vs. RHE) to 1.0 V (vs. RHE).

[0157] Fig.Figure 12 is a graph showing the cyclic voltammogram (thin curve) of fine palladium-containing particles before they were subjected to the potential application step in Comparative Example 1, and showing the cyclic voltammogram (thick curve) of the fine palladium-containing particles at the 2,500th cycle in Comparative Example 1, with the cyclic voltammograms overlapping.

[0158] How to determine the thin and thick curve in Fig.As is evident from Figure 12, there is no significant difference between the peak appearing in the cyclic voltammogram of the fine palladium-containing particles before the potential application step and the peak appearing in the cyclic voltammogram of the fine palladium-containing particles at the 2,500th cycle. More precisely, the peak appearing around 0.50 V (vs. RHE), indicating Pd{110} or Pd{100}, appears to be the largest in both cyclic voltammograms. This suggests the following: Due to the potential sweep in a relatively narrow range of 0.05 to 1.0 V (vs. RHE), the Pd{111} surface was not fully grown, and the Pd{110} or Pd{100} surface is still the largest crystal surface. [Comparative example 2]

[0159] The carbon-supported catalyst of comparative example 2 was prepared in the same way as example 1, except that in the potential application step the upper limit of the applied potential range was changed from 1.2 V (vs. RHE) to 0.8 V (vs. RHE). [Comparative example 3]

[0160] The carbon-supported catalyst of Comparative Example 3 was prepared in the same way as Example 1, except that in the potential application step the upper limit of the applied potential range was changed from 1.2 V (vs. RHE) to 0.45 V (vs. RHE). [Comparative example 4]

[0161] The carbon-supported catalyst of Comparative Example 4 was prepared in the same way as Example 1, except that a carbon-supported palladium in which the fine palladium particles have an average particle diameter of 1.6 nm was prepared and used.

[0162] Fig. Figure 10 is a histogram showing the particle size distribution of the fine palladium particles in the carbon-supported palladium used in Comparative Example 4. As shown in the histogram, the most common equivalent circular diameter of the fine palladium particles is 1.5 nm. As described above, the average particle diameter is 1.6 nm. [Comparative example 5]

[0163] The carbon-supported catalyst of Comparative Example 5 was prepared in the same way as Example 1, except that a carbon-supported palladium in which the fine palladium particles have an average particle diameter of 8.2 nm was prepared and used. [Comparative example 6]

[0164] The carbon-supported catalyst of Comparative Example 6 was prepared in the same way as Example 1, except that a carbon-supported palladium in which the fine palladium particles have an average particle diameter of 10.5 nm was prepared and used.

[0165] Fig. Figure 11 is a histogram showing the particle size distribution of the fine palladium particles in the carbon-supported palladium used in Comparative Example 6. As shown in the histogram, the most common equivalent particle circle diameter of the fine palladium particles is 11 nm. As described above, the average particle diameter is 10.5 nm. [Comparative example 7]

[0166] The carbon-supported catalyst of Comparative Example 7 was prepared in the same way as Example 1, except that the potential application step was not carried out. 2. Evaluation of the catalytic activity of carbon-supported catalysts

[0167] For the carbon-supported catalysts of Examples 1 to 3 and Comparative Examples 1 to 7, the mass activity was obtained by using an RDE. (a) RDE measurement

[0168] Each carbon-supported catalyst was dried to obtain a powder, which was then pulverized using a mortar and pestle. The powder was dispersed in a mixed solution of 6.0 ml of ultrapure water, 1.5 ml of isopropanol, and 35 μl of a 5% electrolyte dispersion based on a perfluorocarbon sulfonic acid polymer (trade name: Nafion; manufactured by: DuPont). This dispersion was applied to the RDE and allowed to dry naturally.

[0169] The prepared RDE was immersed in an aqueous 0.1 M perchloric acid solution. Linear sweep voltammetry (LSV) was performed while rotating the RDE at 1600 rpm. This time, an aqueous 0.1 M perchloric acid solution was used, into which oxygen gas had been bubbled for 30 minutes or longer at a gas flow rate of 30 ml / min.

[0170] The LSV procedure is as follows. First, a potential sweep was repeatedly performed in a range from 1.05 V to 0.05 V (vs. RHE) at a rate of 10 mV / s. The potential sweep was repeated until current values ​​at 0.9 V (vs. RHE) and 0.35 V (vs. RHE) became stable. Then, from the reduction wave of a linear sweep voltammogram thus obtained, the current value at 0.9 V (vs. RHE) was determined as the oxygen reduction current value (I). 0,9 ) determined, and the current value at 0.35 V (vs. RHE) was determined as the diffusion-limited current value (I lim). determined. From these current values, an activation-controlled current value (Ik) was obtained based on the following formula (1).

[0171] The catalytic activity per mass unit of platinum (A / g-Pt) was calculated by dividing the activation-controlled current value (Ik) by the amount (g) of platinum applied to the RDE. Formula 1) Ik = (I lim × I 0,9 ) / (I lim – I 0,9 )

[0172] (In formula (1) Ik is the activation-controlled current value (A); I lim is the diffusion-limited current value (A); and I 0,9 is the oxygen reduction current value (A).) (b) Evaluation of TEM-EDS coverage

[0173] Energy-dispersive X-ray spectroscopy with transmission electron microscopy (TEM-EDS) was performed on 40 samples that had undergone the above “1-2 potential application step” and on another 40 samples that had not undergone the above “1-2 potential application step” to measure the platinum coverage of the palladium surface. Details of the TEM-EDS measurement method are as follows. TEM: Field emission transmission electron microscope (Cs-corrected) (Product name: JEM-2100F; manufactured by: JEOL Ltd.) Acceleration voltage: 120 kV EDS: UTW Si (Li) Semiconductor Detector (manufactured by: JEOL Ltd.) Beam diameter: 0.2 nm Particle Analysis: Particle Analysis Ver. 3.5 (Product name; manufactured by: Sumitomo Metal Technology, Inc.)

[0174] Fig.Figure 8 is a graph showing the relationship between the diameter of fine catalyst particles and the platinum coverage for the aforementioned 80 samples. It is a graph with the platinum coverage (Pt / Pd(At-% / At-%)) on the vertical axis and the diameter of the fine catalyst particles (nm) on the horizontal axis. Fig. Figure 8 shows the data from those samples that underwent the above “1-2 potential application step” represented by white squares, and the data from those samples that did not undergo the above “1-2 potential application step” represented by black squares. Also, a Fig.Figure 8, a thick curve, is a graph showing the theoretical value of platinum coverage relative to the particle diameter in the ideal case, where the entire surface of the fine palladium particle is covered with the monatomic platinum layer (hereinafter also referred to as the 1 ML curve). The squares shown above the curve can be judged to have a sufficiently high platinum coverage. Conversely, the squares shown below the curve can be judged to be deficient with respect to the platinum-containing outermost layer.

[0175] As from Fig.As is clearly shown in Figure 8, many white squares are drawn and distributed above the 1 ML curve, but many black squares are densely packed below the 1 ML curve. This result indicates that the platinum coverage can be increased by performing the potential application step on the carbon-supported palladium, regardless of the particle diameter.

[0176] For the carbon-supported catalysts of Examples 1 to 3 and Comparative Examples 1 to 7, the result of the Pd{111} peak evaluation and the mass activity are shown in the following Table 1 together with the details of the manufacturing process. Table 1 Conditions of the potential investment step Average article diameter of (nm) fine palladium particles Potential upper limit (V vs. RHE) The Pd{111) peak is larger than the Pd{110} peak and the Pd{100} peak. Mass activity (A / g-Pt) Example 1 5,2 1,2 o 960 Example 2 5,2 1,4 o 870 Example 3 3,8 1,2 o 950 Comparative example 1 5,2 1,0 x 650 Comparative example 2 5,2 0,8 x 640 Comparative example 3 5,2 0,45 x 450 Comparative example 4 1,6 1,2 x 240 Comparative example 5 8,2 1,2 x 540 Comparative example 6 10,5 1,2 x 420 Comparative example 7 - - - 400 3. Catalyst evaluation

[0177] As shown in Table 1, the potential limit in the potential application step in comparison examples 1 to 3 was set to 1.0 V (vs. RHE), 0.8 V (vs. RHE), and 0.45 V (vs. RHE), respectively. For the fine palladium particles subjected to the potential application step in comparison examples 1 to 3, the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram could not be larger than the peak indicating the Pd{110} surface area and the peak indicating the Pd{100} surface area.

[0178] The mass activity of comparison examples 1 to 3 is 650 (A / g-Pt), 640 (A / g-Pt) and 450 (A / g-Pt) respectively. This is less than 70% of the mass activity of example 1, in which the fine palladium particles with the same average particle diameter were used.

[0179] The following is assumed to be the reason for such low mass activities in comparative examples 1 to 3: The Pd{111} surface did not appear to a sufficient extent on the surface of the fine palladium particles and consequently the proportion of the Pt{111} surface with high catalytic activity in the surface of the fine catalyst particles obtained by the platinum covering step was low.

[0180] As shown in Table 1, in comparative example 4, due to the use of fine palladium particles with an average particle diameter of 1.6 nm in the potential application step, the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram could not be larger than the peak indicating the Pd{110} surface area and the peak indicating the Pd{100} surface area.

[0181] The mass activity of comparison example 4 is 240 (A / g-Pt). This is the lowest mass activity found in the experimental results shown in Table 1.

[0182] The reason for such a low mass activity in Comparative Example 4, in which the fine palladium particles with an average particle diameter of less than 3.0 nm were used, is assumed to be as follows: Since the proportion of low-coordinate palladium atoms (palladium atoms located at edges and corners) on the surface of the fine palladium particles is high, the low-coordinate palladium atoms were eluted in the potential application step, and as a result, palladium was deposited on the surface of the fine catalyst particles obtained by the platinum covering step; therefore, the Pt{111} surface area with high catalytic activity could not be large.

[0183] As shown in Table 1, in comparative examples 5 and 6, due to the use of fine palladium particles with an average particle diameter of 8.2 nm and 10.5 nm respectively, the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram could not be larger than the peak indicating the Pd{110} surface area and the peak indicating the Pd{100} surface area in the potential application step.

[0184] The mass activity of comparison examples 5 and 6 is 540 (A / g-Pt) and 420 (A / g-Pt), respectively. This is less than 60% of the mass activity of example 1, in which the same potential range was used in the potential application step.

[0185] The following is assumed to be the reason for such low mass activities in comparative examples 5 and 6, in which the fine palladium particles with an average particle diameter of over 6.0 nm were used: Since the surface energy of the fine palladium particles is low and the palladium itself is stable, the rearrangement of the palladium atoms on the surface of the fine particles was not promoted even after the potential application step, and the Pd{111} surface area did not grow sufficiently; therefore, the proportion of the Pt{111} surface area with high catalytic activity in the surface area of ​​the fine catalyst particles obtained by the platinum covering step was low.

[0186] As shown in Table 1, in comparative example 7, where the potential application step was not performed, the mass activity is 400 (A / g-Pt). This is approximately 40% of the mass activity of example 1, where the potential application step was performed.

[0187] Fig. Figure 9 is a bar chart comparing the mass activities of Example 1 and Comparison Example 7. As shown from Fig. As is evident from Figure 9, the mass activity of Example 1 is more than twice that of Comparative Example 7. Therefore, it is clear that the potential application step of the present invention modifies the arrangement of the palladium atoms on the surface of the fine palladium particles and promotes the growth of the Pd{111} surface; therefore, the mass activity of the carbon-supported catalyst thus obtained is dramatically increased.

[0188] As shown in Table 1, in Examples 1 to 3, the fine palladium particles with an average particle diameter of 5.2 nm and 3.8 nm, respectively, were used, and the potential application step was performed by setting the potential limit to 1.2 V (vs. RHE) and 1.4 V (vs. RHE), respectively. Consequently, the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram was larger than the peak indicating the Pd{110} surface area and the peak indicating the Pd{100} surface area. The carbon-supported catalysts of Examples 1 to 3 obtained by the potential application step under such conditions resulted in a mass activity of 870 (A / g-Pt) or more, which is a higher mass activity than previously achieved.

[0189] As just described, the carbon-supported catalyst with better catalytic performance than ever before can be obtained by the manufacturing process of the present invention, in which the fine palladium particles with an average particle diameter of 3.0 nm or more and 6.0 nm or less are used and in the potential application step the potential is applied to the fine palladium-containing particles until the peak indicating the Pd{111} surface area in the reduction wave of the cyclic voltammogram becomes larger than the peak indicating the Pd{110} surface area and the peak indicating the Pd{100} surface area. Reference symbol list 1 glass cell 2 Electrolyte 3 Dispersion 4 Working electrode 5 Reference electrode 6 Counter electrode 7 Gas inlet pipe 8 bubbles 11 reaction vessels 12 Carbon-supported palladium (Pd / C) 13 Acid solution 14 Reference electrode 15 Counter electrode 16 Space for counter electrode 17 Stirring rod 100, 200 Electrochemical device

Claims

[1] Method for producing fine catalyst particles comprising a fine palladium-containing particle and a platinum-containing outermost layer covering at least part of the fine palladium-containing particle, the procedure includes: a potential application step of applying a potential to the fine palladium-containing particles in a first dispersion comprising fine palladium-containing particles dispersed in an acid solution and having an average particle diameter of 3.0 nm or more and 6.0 nm or less, until a peak indicating a Pd{111} surface in a reduction wave of a cyclic voltammogram becomes larger than a peak indicating a Pd{110} or Pd{100} surface in the reduction wave of the cyclic voltammogram; a copper coating step of coating at least a portion of the fine palladium-containing particle with copper by preparing a second dispersion by mixing the first dispersion and a copper-containing solution after the potential application step and applying a potential more noble than the oxidation-reduction potential of copper to the fine palladium-containing particles in the second dispersion; and a platinum covering step of covering at least a part of the fine palladium-containing particle with platinum by substituting the copper covering at least a part of the fine palladium-containing particle with platinum by mixing the second dispersion and a platinum-containing solution after the copper covering step. [2] Method for producing the fine catalyst particles according to claim 1, wherein the potential in the potential application step is traversed in a range which includes at least 1.2 V (vs. RHE). [3] Method for producing a carbon-supported catalyst, wherein the fine catalyst particles defined by claim 1 or 2 are supported on a carbon support, wherein fine palladium-containing particles designed for use in the potential application step are supported on a carbon support.

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