JP2015196144A - Method for producing core shell catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a core shell catalyst high in specific activity.SOLUTION: Provided is a method for producing a core shell catalyst provided with a core including palladium and a shell including platinum and covering the core, including a step of preparing palladium-containing particles having a peak at 2θ=40°±1° as a result of X-ray diffraction measurement, and in which crystalline size obtained from the half value width of the peak at 2θ=40°±1° by a Scherrer method is 5 to 8.5 nm; a copper precipitation step of precipitating copper on the surface of each palladium-containing particle by applying a potential nobler than the oxidation-reduction potential of copper to each palladium-containing particle in copper ion-containing electrolyte; and a substitution step of contacting the copper precipitated on the surface of each palladium-containing particle after the copper precipitation step with a platinum ion-containing solution, thus substituting the copper precipitated on the surface of each palladium-containing particle into platinum to form a shell.

Description

  The present invention relates to a method for producing a core-shell 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. Therefore, since the fuel cell is not subject to the Carnot cycle, it exhibits high energy conversion efficiency. A fuel cell is usually configured 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, platinum and platinum alloy materials have been employed as electrode catalysts for the fuel electrode (anode electrode) and oxidant electrode (cathode electrode) 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. Therefore, studies have been made to reduce the amount of platinum contained in the fuel electrode and the oxidant electrode of the fuel cell by combining platinum with a cheaper metal (for example, Patent Document 1, Non-Patent Documents 1 to 3).
For example, Patent Document 1 describes a method for producing a core-shell catalyst by displacement plating using copper underpotential deposition (Cu-UPD).
Cu-UPD is a phenomenon in which a copper monoatomic layer in a metallic state is formed on a surface of a dissimilar metal having a strong bonding force with copper at a potential nobler than the redox potential of copper. A core with a copper atomic layer formed by Cu-UPD on the surface is immersed in a solution containing shell metal ions, and the core is covered with the shell by substituting the shell metal with copper using the difference in ionization tendency. The core-shell catalyst can be produced.

US Application Publication No. 2010/0177462

2007-2008 Results Report, New Energy and Industrial Technology Development Organization, (Certificate) Chiba University, June 2008 Junlian Zhang, et, al Angew. Chem. Int. Ed. 2005, 44, 2132-2135 Radoslav R. Adzic, Electrocatal (2012), 3: 163-169

However, the core-shell catalyst obtained by the conventional production method has a problem that the catalyst activity per unit surface area of platinum (hereinafter referred to as specific activity) is low.
The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a method for producing a core-shell catalyst having high specific activity.

A method for producing a core-shell catalyst of the present invention is a method for producing a core-shell catalyst comprising: a core containing palladium; and a shell containing platinum and covering the core,
The X-ray diffraction measurement result has a peak at 2θ = 40 ° ± 1 °, and the crystallite size determined by the Scherrer method from the half-value width of the peak at 2θ = 40 ° ± 1 ° is 5 to 8.5 nm. Preparing palladium-containing particles;
In the copper ion-containing electrolyte, by applying a potential nobler than the copper redox potential to the palladium-containing particles, a copper deposition step of depositing copper on the surface of the palladium-containing particles,
After the copper deposition step, the copper deposited on the surface of the palladium-containing particles is contacted with a platinum ion-containing solution, thereby replacing the copper deposited on the surface of the palladium-containing particles with platinum. And a substitution step to be formed.

  In the method for producing a core-shell catalyst of the present invention, the palladium-containing particles are preferably supported on a carrier.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of a core-shell catalyst with high specific activity can be provided.

(A) is a TEM photograph of the core-shell catalyst, and (B) is a diagram showing a Pt / Pd atomic concentration ratio obtained by observing the core-shell catalyst shown in (A) in the arrow direction. It is a flowchart which shows an example of the manufacturing method of the core-shell catalyst of this invention. It is the perspective view which showed typically an example of the synthesis apparatus used for the manufacturing method of the core-shell catalyst of this invention. FIG. 4 is a diagram showing an XRD measurement result of palladium particles used in Example 1. FIG. 4 is a diagram showing an XRD measurement result of palladium particles used in Example 2. It is a figure which shows the XRD measurement result of the palladium particle | grains used by the comparative example 1. It is a figure which shows the XRD measurement result of the palladium particle | grains used by the comparative example 2. It is a figure which shows the relationship between the crystallite size of the palladium particle used by the Example and the comparative example, and the specific activity of a core-shell catalyst.

A method for producing a core-shell catalyst of the present invention is a method for producing a core-shell catalyst comprising: a core containing palladium; and a shell containing platinum and covering the core,
The X-ray diffraction measurement result has a peak at 2θ = 40 ° ± 1 °, and the crystallite size determined by the Scherrer method from the half-value width of the peak at 2θ = 40 ° ± 1 ° is 5 to 8.5 nm. Preparing palladium-containing particles;
In the copper ion-containing electrolyte, by applying a potential nobler than the copper redox potential to the palladium-containing particles, a copper deposition step of depositing copper on the surface of the palladium-containing particles,
After the copper deposition step, the copper deposited on the surface of the palladium-containing particles is contacted with a platinum ion-containing solution, thereby replacing the copper deposited on the surface of the palladium-containing particles with platinum. And a substitution step to be formed.

It is estimated that the larger the crystallite size, the closer the particle is to a single crystal having a regular atomic arrangement, and a higher proportion of the (111) plane that is stable in terms of surface energy.
In Non-Patent Document 1, the activity of palladium particles (hereinafter sometimes referred to as Pt / Pd) in which a single-layer platinum shell is formed varies depending on the crystal plane, and the order of activity is Pt / Pd (100) < It is reported that Pt / Pd (111) to Pt / Pd (110).
Furthermore, Non-Patent Documents 2 to 3 report that Pt / Pd (111) is more active than Pt (111).
From the above findings, if palladium-containing particles with a high proportion of the (111) plane are used, the specific activity of the core-shell catalyst increases, that is, the larger the crystallite size of the palladium-containing particles, the more stable the surface energy (111) It is expected that the specific activity of the core-shell catalyst will increase because of the higher surface ratio.
However, contrary to the above expectation, the present inventors have found that the specific activity of the core-shell catalyst is reduced unless the crystallite size of the palladium-containing particles is within the range of 5 to 8.5 nm, and completed the present invention. .
When the crystallite size of the palladium-containing particles is smaller than 5 nm, the proportion of atoms with a small number of coordinations appearing on the surface of the palladium-containing particles is small, and the interaction with the platinum shell is difficult to express. Inferred. Further, it is presumed that the ratio of the terrace portion of the crystal plane appearing on the surface of the palladium-containing particles is reduced, platinum atoms cannot be stably present, and it is difficult to form a uniform platinum shell on the surface of the palladium-containing particles.
On the other hand, if the crystallite size of the palladium-containing particles is larger than 8.5 nm, the potential energy of the adsorption state of copper and platinum on the surface of the palladium particles becomes high at a negative value, so that three-dimensional growth of platinum on the surface of the palladium particles occurs. It is assumed that this is because it becomes easy and it is difficult to form a uniform platinum shell. In addition, the potential energy of the adsorption state of copper and platinum on the surface of palladium particles is a value obtained when the structure of palladium is assumed to be a cubic octahedron by the first principle calculation program Interface (manufactured by Toyota Central R & D Co., Ltd.). Adopted.

  In support of the above, an energy dispersive x-ray spectroscopy (EDS) using a transmission electron microscope (TEM) for a core-shell catalyst manufactured using palladium particles having a crystallite size of about 10 nm. Analysis was performed. The results are shown in FIG. FIG. 1A is a TEM photograph of the core-shell catalyst, and FIG. 1B is a diagram showing the Pt / Pd atomic concentration ratio obtained by observing the core-shell catalyst shown in FIG. 1A in the arrow direction. As shown in FIG. 1 (B), the produced core-shell catalyst has a high platinum concentration on the catalyst surface and a long distance, so it is confirmed that three-dimensional growth of platinum occurs on the surface of the palladium core. did it.

  In the present invention, the shell covers the core not only in a form in which the entire surface of the core is covered by the shell, but also a part of the surface of the core is covered by the shell, and a part of the surface of the core is exposed. The form which is included is also included. Furthermore, the shell may be a monoatomic layer or a polyatomic layer in which two or more atoms are stacked, but from the viewpoint of improving specific activity, the shell is preferably a monoatomic layer.

Hereafter, the manufacturing method of the core-shell catalyst of this invention is demonstrated in detail.
FIG. 2 is a flowchart showing an example of a method for producing the core-shell catalyst of the present invention.
The manufacturing method of the core-shell catalyst shown in FIG. 2 includes (1) a preparation step, (2) a copper deposition step, (3) a replacement step, (4) a washing step, and (5) a drying step.
The method for producing a core-shell catalyst of the present invention has at least (1) a preparation step, (2) a copper deposition step, and (3) a substitution step, and if necessary, after the substitution step, (4) a washing step, (5) It has a drying process.
Hereinafter, each process is demonstrated in order.

(1) Preparatory Step The preparatory step has a peak at 2θ = 40 ° ± 1 ° as a result of X-ray diffraction measurement, and the crystallite size determined by the Scherrer method from the half width of the peak at 2θ = 40 ° ± 1 °. Is a step of preparing palladium-containing particles having a diameter of 5 to 8.5 nm.

The palladium-containing particles used in the present invention are not particularly limited as long as the crystallite size is 5 to 8.5 nm, and preferably 5.7 to 7.5 nm.
The crystallite size in the present invention can be determined by X-ray diffraction (XRD). As a specific method for calculating the crystallite size by XRD, for example, the following method may be mentioned.
A plurality of palladium-containing particles are irradiated with X-rays, and the crystallite size can be obtained from the half width of the peak at 2θ = 40 ° ± 1 ° using the following Scherrer equation (1).
[Formula (1)]
D = (Kλ) / (βcosθ)
In the above formula (1), the meaning of each symbol is as follows.
D: Crystallite size (nm)
K: Scherrer constant λ: Measurement X-ray wavelength (nm)
β: Half width (rad)
θ: Bragg angle (rad) of diffraction line

As the palladium-containing particles, at least one particle selected from palladium particles and palladium alloy particles can be used.
Examples of the palladium alloy include an alloy of palladium and a metal material selected from the group consisting of iridium, ruthenium, rhodium, iron, cobalt, nickel, copper, silver, and gold. The metal other than palladium constituting the palladium alloy is 1 Two or more species may be used.
The palladium alloy preferably has a palladium content of 80% by mass or more when the mass of the entire alloy is 100% by mass. This is because a uniform platinum-containing shell can be formed when the palladium content is 80 mass% or more.
The average particle size of the palladium-containing particles is not particularly limited, but is preferably 8 nm or less. When the average particle diameter of the palladium-containing particles exceeds 8 nm, the surface area per mass of platinum becomes small, and a lot of platinum is required to obtain the necessary activity, which is costly. If the average particle size of the palladium-containing particles is too small, the palladium itself is easily dissolved and the durability of the catalyst is lowered. Therefore, the average particle size of the palladium-containing particles is preferably 3 nm or more.
The calculation method of the average particle diameter of the particles used in the present invention is as follows. That is, using a scanning electron microscope (TEM), take a TEM photograph at a magnification of 1,000,000, and calculate the diameter of a perfect circle having the same area as the projected area of the particle on the plane (equivalent particle diameter) of the particle. Considered as particle size. The measurement of the particle size by photographic observation is performed for 500 particles of the same type, and the average of the particle sizes of these particles is defined as the average particle size. Note that the broken particles observed at the edge of the photograph are excluded from the analysis.
Therefore, in the present invention, the crystallite size and the average particle size are calculated based on different definitions, and the average particle size cannot be used as an approximate value of the crystallite size.

The palladium-containing particles are preferably supported on a carrier. Although it does not specifically limit as a support | carrier, When using the core-shell catalyst of this invention for the electrode catalyst layer of a fuel cell, it is preferable to use an electroconductive support | carrier from a viewpoint of ensuring electroconductivity for an electrode catalyst layer.
Specific examples of materials that can be used as a carrier for supporting palladium-containing particles include Ketjen Black (trade name: manufactured by Ketjen Black International), Vulcan (trade name: manufactured by Cabot), and Nolit (trade name: Norit). ), Black pearl (trade name: manufactured by Cabot), acetylene black (trade name: manufactured by Chevron), etc., conductive carbon materials such as carbon particles and carbon fibers, and metal materials such as metal particles and metal fibers. It is done.
A commercially available product can be used as the palladium-containing particle carrier supported on the carrier, or it can be synthesized. As a method of supporting the palladium-containing particles on the carrier, a conventionally used method can be employed. For example, there may be mentioned a method in which palladium-containing particles are mixed with a carrier dispersion in which a carrier is dispersed, filtered, washed, redispersed in ethanol or the like and then dried with a vacuum pump or the like. From the viewpoint of removing the solvent, and from the viewpoint of reducing the palladium oxide on the surface of the palladium-containing particles to palladium, drying may be performed as necessary. The firing method can be the same as the firing method in the method for controlling the crystallite size described later.

Examples of the method for controlling the crystallite size include a method of firing palladium-containing particles, a method of subjecting palladium-containing particles to potential cycle treatment, and the like.
Examples of the method for firing the palladium-containing particles include a method of firing the palladium-containing particles at 300 to 500 ° C. for 1 to 6 hours in an inert gas atmosphere such as argon gas and nitrogen gas. If the firing temperature is less than 300 ° C, the crystallite size may not increase, and if the firing temperature exceeds 500 ° C, the crystallite size may become too large.

The potential cycle treatment can be performed, for example, by applying a predetermined potential to the palladium-containing particles in an electrolytic solution containing palladium-containing particles.
The method of applying a potential to the palladium-containing particles and the potential control device can be the same as the method and device performed in the copper deposition step described later.
The electrolytic solution that can be used for the potential cycle treatment is not particularly limited, and examples thereof include an acid solution. As an acid that can be used for the potential cycle treatment, specifically, an acid similar to the acid that can be used for the copper ion-containing electrolyte described later can be used.
In addition, when performing a potential cycle process and the copper precipitation process mentioned later in the same reaction container, you may add a copper ion containing electrolyte solution to the electrolyte solution used for the potential cycle process. For example, when sulfuric acid is used as the electrolytic solution for potential cycle treatment, a copper precipitation step may be performed by adding an aqueous copper sulfate solution to the sulfuric acid after use. The counter anion in the electrolytic solution used in the potential cycle treatment and the counter anion in the copper ion-containing electrolytic solution used in the copper deposition step may be the same or different.
In addition to controlling the crystallite size of the palladium-containing particles, the potential cycle treatment removes oxygen in the electrolyte solution as much as possible from the viewpoint of being able to quickly remove oxides on the surface of the palladium-containing particles. Nitrogen is preferably bubbled into the electrolytic solution.
Examples of the potential application signal pattern in the potential cycle process include a rectangular wave, a triangular wave, and a trapezoidal wave.
The potential range is not particularly limited, but is preferably 0.05 to 1.2 V (vs. RHE).
The number of potential cycles is not particularly limited when the potential application signal pattern is a triangular wave, but is preferably 10 to 6000 cycles, and the potential sweep rate can be set to 5 to 100 mV / second, for example.
It is speculated that the crystallite size of the palladium-containing particles can be controlled by subjecting the palladium-containing particles to potential cycle treatment for the following reason.
During the potential cycle treatment, palladium ions are usually exchanged between the palladium-containing particles via the electrolytic solution. Therefore, dissolution and reprecipitation of palladium occur in the electrolytic solution. At this time, in the electrolytic solution, so-called Ostwald ripening occurs in which the palladium-containing particles having a relatively small particle size are dissolved, and the dissolved palladium ions are reprecipitated on the surface of the palladium-containing particles having a relatively large particle size. It is inferred that by repeating dissolution and reprecipitation of palladium, the irregular arrangement of atoms becomes regular and the crystallite size of the palladium-containing particles increases.

(2) Copper deposition step In the copper deposition step, copper is deposited on the surface of the palladium-containing particles by applying a potential higher than the redox potential of copper to the palladium-containing particles in the copper ion-containing electrolyte. It is a process.

FIG. 3 is a perspective view schematically showing an example of a synthesis apparatus that can be used in the method for producing a core-shell catalyst of the present invention.
A synthesis apparatus 20 shown in FIG. 3 includes a reaction vessel 1, a reference electrode 4, a counter electrode 5, a counter electrode compartment 6, and a stirrer 7.
The reaction vessel 1 is made of Ti and functions as a working electrode. The surface of the reaction vessel 1 is coated with Ru ₂ O to give corrosion resistance. In the reaction vessel 1, palladium-containing particles 2 having a crystallite size of 5 to 8.5 nm and an electrolytic solution 3 containing copper ions are accommodated. The palladium-containing particles 2 and the electrolytic solution 3 accommodated in the reaction vessel 1 can be stirred with the stirring bar 7.
The reference electrode 4 and the counter electrode 5 are suspended by a platinum wire 8 and are disposed so as to be sufficiently immersed in the electrolytic solution 3. The reaction vessel 1, the reference electrode 4 and the counter electrode 5 which are also working electrodes are electrically connected to a potential control device (for example, a potentiostat or the like, not shown) so that the potential of the working electrode can be controlled. The reference electrode 4 and the counter electrode 5 are connected to a potential control device via a platinum wire 8. In order to prevent the palladium-containing particles 2 in the electrolytic solution 3 from coming into contact with the counter electrode 5, the counter electrode 5 is immersed in the electrolytic solution 3 while being accommodated in a glass counter electrode compartment 6. The counter electrode compartment 6 is formed of a porous glass frit at the bottom, and the contact property between the counter electrode 5 and the electrolytic solution 3 is ensured.

The electrolytic solution containing copper ions is not particularly limited as long as the electrolytic solution can deposit copper on the surface of the palladium-containing particles by Cu-UPD. The copper ion-containing electrolytic solution is usually composed of a predetermined amount of copper salt dissolved in a solvent, but is not particularly limited to this configuration, and some or all of the copper ions are dissociated and exist in the liquid. Any electrolyte solution may be used.
Examples of the solvent used for the copper ion-containing electrolytic solution include water and organic solvents, but water is preferable from the viewpoint of not preventing the precipitation of copper on the surface of the palladium-containing particles.
Specific examples of the copper salt used in the copper ion-containing electrolyte include copper sulfate, copper nitrate, copper chloride, copper chlorite, copper perchlorate, and copper oxalate.
In the electrolytic solution, the copper ion concentration is not particularly limited, but is preferably 10 to 1000 mM.
In addition to the solvent and copper salt, the copper ion-containing electrolytic solution may contain, for example, an acid or the like. Specific examples of the acid that can be added to the electrolytic solution containing copper ions include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid. The counter anion in the copper ion-containing electrolyte and the counter anion in the acid may be the same or different.
In addition, it is preferable that an inert gas is bubbled in advance in the electrolytic solution. This is because the oxidation of the palladium-containing particles is suppressed and uniform coating with the platinum-containing shell becomes possible. Nitrogen gas, argon gas, etc. can be used as the inert gas.

The palladium-containing particles may be immersed and dispersed in the electrolytic solution by adding to the electrolytic solution in a powder state. Alternatively, the palladium-containing particles are previously dispersed in a solvent to prepare a palladium-containing particle dispersion. You may immerse and disperse | distribute to electrolyte solution by adding to electrolyte solution. As the solvent used in the palladium-containing particle dispersion, the same solvents as those used in the above-described copper ion-containing electrolytic solution can be used. Moreover, the palladium-containing particle dispersion may contain the acid that can be added to the copper ion-containing electrolyte.
Alternatively, the palladium-containing particles may be fixed on the conductive base material or the working electrode, and the palladium-containing particle fixing surface of the conductive base material or the working electrode may be immersed in the electrolytic solution. As a method for fixing the palladium-containing particles, for example, a palladium-containing particle paste is prepared using an electrolyte resin (for example, Nafion (registered trademark)) and a solvent such as water or alcohol, and the conductive substrate or the action. The method of apply | coating to the surface of a pole is mentioned.

The method for applying a potential to the palladium-containing particles is not particularly limited, and a general method can be adopted. For example, a method in which a working electrode, a counter electrode, and a reference electrode are immersed in a copper ion-containing electrolytic solution, and a potential is applied to the working electrode.
As the working electrode, for example, a metal material such as titanium, platinum mesh, platinum plate, or gold plate, a conductive carbon material such as glassy carbon, carbon plate, or the like that can ensure conductivity can be used. Note that the reaction vessel may be formed of the above conductive material and function as a working electrode. When using a reaction vessel made of a metal material as a working electrode, it is preferable to coat the inner wall of the reaction vessel with RuO ₂ from the viewpoint of suppressing corrosion. When a carbon material reaction vessel is used as a working electrode, it can be used as it is without coating.
As the counter electrode, for example, a platinum mesh plated with platinum black and conductive carbon fiber can be used.
As the reference electrode, a reversible hydrogen electrode (RHE), a silver-silver chloride electrode, a silver-silver chloride-potassium chloride electrode, or the like can be used.
As the potential control device, a potentiostat, a potentiogalvanostat, or the like can be used.

The potential to be applied is not particularly limited as long as it is a potential at which copper can be deposited on the surface of the palladium-containing particles, that is, a potential nobler than the oxidation-reduction potential of copper. It is preferably within the range of (vs. RHE), particularly preferably 0.4 V (vs. RHE).
The time for applying the potential is not particularly limited, but is preferably 60 minutes or more, and more preferably until the reaction current becomes steady and approaches zero.
The application of the potential may be performed by sweeping the potential in a range including the potential range. Specifically, the potential range to be swept is preferably 0.3 to 0.8 V (vs. RHE).
The number of potential sweep cycles is not particularly limited, but is preferably 1 to 10,000 cycles. The potential sweep rate is, for example, 0.01 to 100 mV / sec.

The copper precipitation step is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere from the viewpoint of preventing oxidation of the surface of the palladium-containing particles and preventing oxidation of copper.
In the copper deposition step, it is preferable that the copper ion-containing electrolyte is appropriately stirred as necessary. For example, when a reaction vessel that also serves as a working electrode is used and palladium-containing particles are immersed and dispersed in the electrolytic solution in the reaction vessel, each of the palladium-containing particles is mixed with the reaction vessel that is the working electrode by stirring the electrolytic solution. The surface can be contacted and a potential can be uniformly applied to each palladium-containing particle. In this case, stirring may be performed continuously or intermittently during the copper deposition step.

(3) Substitution step The substitution step is a step of bringing the copper deposited on the surface of the palladium-containing particles into contact with the copper deposited on the surface of the palladium-containing particles by bringing a platinum ion-containing solution into contact therewith. The step of forming the shell by substituting platinum.
In the substitution step, copper and platinum can be substituted due to the difference in ionization tendency by bringing the platinum ion-containing solution into contact with copper deposited on the surface of the palladium-containing particles.

The shell in the present invention includes platinum and a platinum alloy.
Examples of the platinum alloy include an alloy with a metal material selected from the group consisting of iridium, ruthenium, rhodium, nickel and gold. The metal other than platinum constituting the platinum alloy may be one kind or two or more kinds.
The platinum alloy preferably has a platinum content of 90% by mass or more when the mass of the entire alloy is 100% by mass. This is because if the platinum content is less than 90% by mass, sufficient catalytic activity and durability cannot be obtained.
As the platinum salt used in the platinum ion-containing solution, for example, K ₂ PtCl ₄ , K ₂ PtCl ₆ or the like can be used, and an ammonia complex such as ([PtCl ₄ ] [Pt (NH ₃ ) ₄ ]) can be used. It can also be used.
The platinum ion concentration in the platinum ion-containing solution is not particularly limited, but is preferably 0.01 to 100 mM.
The solvent that can be used in the platinum ion-containing solution can be the same as the solvent used in the electrolytic solution containing copper ions described above. Further, the platinum ion-containing solution may contain, for example, an acid or the like in addition to the solvent and the platinum salt. Specific examples of the acid include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid.
The platinum ion-containing solution is sufficiently stirred in advance, and nitrogen is preferably bubbled into the solution in advance from the viewpoint of preventing oxidation of the palladium-containing particles and preventing oxidation of copper.
The replacement time (contact time between the platinum ion-containing solution and the palladium-containing particles) is not particularly limited, but it is preferable to ensure 10 minutes or more. As the platinum ion-containing solution is added, the potential of the reaction solution increases. Therefore, it is more preferable to substitute until the monitor potential does not change.
In addition, when performing a copper precipitation process and a substitution process within the same reaction container, you may add a platinum ion containing solution to the electrolyte solution used for the copper precipitation process. For example, after the copper deposition step, the potential control is stopped, and the platinum-containing solution is added to the copper ion-containing electrolyte used in the copper deposition step, so that the palladium-containing particles on which the copper is deposited are brought into contact with the platinum ion-containing solution. You may let them.

(4) Washing process The washing process is a process of washing the core-shell catalyst after the replacement process.
The washing of the core-shell catalyst is not particularly limited as long as it can remove impurities without impairing the core-shell structure of the produced core-shell catalyst. As an example of the cleaning, a method of suction filtration using water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like can be mentioned.

(5) Drying step The drying step is a step of drying the obtained core-shell catalyst after the substitution step.
The drying of the core-shell catalyst is not particularly limited as long as it is a method capable of removing a solvent and the like, and examples thereof include a method of maintaining a temperature of 50 to 100 ° C. for 6 to 12 hours in an inert gas atmosphere.
The core-shell catalyst may be pulverized as necessary. The pulverization method is not particularly limited as long as it is a method capable of pulverizing solids. Examples of the pulverization include pulverization using an inert gas atmosphere or mortar in the atmosphere, and mechanical milling such as a ball mill, a turbo mill, and a jet mill.

(Example 1)
[Preparation process]
A Ti cylindrical container having a diameter of 15 cm and having a surface coated with RuO ₂ was used as a reaction container. This reaction vessel was also functioned as a working electrode.
As a counter electrode, a platinum mesh plated with platinum black was prepared. The counter electrode was placed in a compartment with a frit glass on the bottom and placed in a reaction vessel with a polyethylene float.
An Ag / AgCl / KCl (3M) electrode (manufactured by Cypress Systems) was used as a reference electrode.
0.055 L of 0.05 M sulfuric acid was placed in a reaction vessel serving as a working electrode. The sulfuric acid was previously bubbled with nitrogen.
The reaction vessel, reference electrode, and counter electrode, which are working electrodes, were connected to a potentiostat as a potential control device from the outside so that the potential could be controlled.
While bubbling sulfuric acid in the reaction vessel with nitrogen, palladium-supported carbon (hereinafter sometimes referred to as Pd / C) was placed in the reaction vessel and dispersed. Cyclic voltammetry (CV) was performed at a potential range of 0.05 to 1.2 V (vs. RHE), a triangular wave, a potential cycle number of 1200 cycles, and a potential sweep rate of 100 mV / sec to control the crystallite size of the palladium particles. . The potential of the Ag / AgCl / KCl (3M) electrode is described in terms of RHE.
After the CV treatment, a part of Pd / C was collected, 10% by mass of silicon powder (NIST640b) was added, mixed in an agate mortar, and XRD measurement was performed on palladium particles. The XRD measurement results are shown in FIG.
The XRD measurement conditions are as follows.
・ Device: Rigaku Corporation RINT-TTRIII type wide angle X-ray diffractometer ・ X-ray source: CuKα ray ・ Tube voltage-Tube current: 50 kV-300 mA
-Step width: 0.01 deg
・ Measurement speed: 1 second / step
・ Slit system: 0.5 deg-0.15 mm-0.5 deg
Monochromator: Diffraction line curved crystal monochromator The crystallite size was calculated using the Scherrer formula from the half-value width of the (111) plane peak near 2θ = 40 ° shown in FIG. The peak near 2θ = 47 ° shown in FIG. 4 is a (200) plane peak. The crystallite size of the obtained palladium particles was 5.7 nm.
Moreover, the average particle diameter of the obtained palladium particle | grains was calculated | required from the TEM photograph. The average particle diameter of the obtained palladium particles was 3.1 nm.

[Copper deposition process]
Following the preparation step, copper sulfate pentahydrate was added to the reaction solution while bubbling sulfuric acid in the reaction vessel with nitrogen, and the copper ion concentration was adjusted to 50 mM.
Then, the potential of the reaction vessel as the working electrode was fixed to 0.4 V (vs. RHE), and copper was deposited on the palladium particles of Pd / C. Occasionally, the reaction vessel was stirred with a stir bar. Application of 0.4 V was performed until the reaction current became steady and approached zero.

[Replacement process]
Following the copper precipitation step, the potential control of 0.4 V (vs. RHE) was stopped, and while stirring the solution in the reaction vessel with a stirrer, a sulfuric acid solution of K ₂ PtCl ₄ was slowly added to the palladium particles. Copper was replaced with platinum. _{Incidentally,} sulfuric acid solution of K 2 PtCl ₄ is a platinum ion concentration was prepared by the _K 2 PtCl ₄ dissolved in 0.05M sulfuric acid solution 500mL such that 2 mM. Further, the sulfuric acid solution of K ₂ PtCl ₄ was previously bubbled with nitrogen before being put into the reaction vessel.

[Washing process, drying process]
After the replacement step, the solution in the reaction vessel was filtered to recover the powder. The collected powder was added with 4 L of room temperature pure water in 10 portions, and filtered and washed each time.
Then, it dried at 60 degreeC for 12 hours, and grind | pulverized using the agate mortar and the pestle, and obtained the core-shell catalyst.

(Example 2)
A core-shell catalyst was produced in the same manner as in Example 1 except that CV was carried out at a potential cycle number of 2500 in the preparation step, and palladium particles having an average particle size of 5.4 nm and a crystallite size of 7.5 nm were prepared. The XRD measurement result of the palladium particles in Example 2 is shown in FIG.

(Comparative Example 1)
A core-shell catalyst was produced in the same manner as in Example 1 except that CV was not performed in the preparation step and palladium particles having an average particle size of 2.8 nm and a crystallite size of 4.7 nm were prepared. The XRD measurement result of the palladium particles in Comparative Example 1 is shown in FIG.

(Comparative Example 2)
The same as in Example 1 except that CV was not performed in the preparation step, and calcined in an inert atmosphere at 600 ° C. for 1 hour to prepare palladium particles having an average particle size of 8.1 nm and a crystallite size of 9.9 nm. Thus, a core-shell catalyst was produced. The XRD measurement result of the palladium particles in Comparative Example 2 is shown in FIG.

[Mass activity evaluation]
30 mg each of the core-shell catalysts obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were collected, and 131.25 μL of 5% Nafion (registered trademark) dispersion, 30 mL of ultrapure water, and isopropanol were collected. A catalyst ink was prepared by dispersing in 7.5 mL of the mixed solution. The catalyst ink was applied onto a glassy carbon electrode of a rotating disk electrode (RDE) and allowed to dry naturally.
And the oxygen reduction reaction (ORR) measurement was performed about each core-shell catalyst.
The ORR measurement conditions are shown below.
・ Electrolyte: 0.1M perchloric acid aqueous solution (Oxygen saturated in advance by bubbling oxygen gas at 30mL / min for 30 minutes or more)
-Atmosphere: Under oxygen atmosphere-Sweep speed: 10 mV / sec-Potential sweep range: 0.1-1.05 V (vs. RHE)
・ Rotation speed: 1600 rpm
From the oxygen reduction wave obtained by ORR measurement, the catalytic activity (MA) per unit mass of platinum in each core-shell catalyst was measured.
In addition, the catalytic activity per unit mass of platinum in the core-shell catalyst is determined by using an oxygen reduction wave obtained by ORR measurement with a current value of 0.9 V (vs. RHE) as an oxygen reduction current (I _0.9 ), 0. The current value of 3 V (vs. RHE) is defined as the diffusion limit current (I _lim ), the activation dominant current (Ik) is obtained from the following formula (2), and the amount of platinum contained in the core-shell catalyst coated on the glassy carbon electrode ( The catalyst activity per unit mass of platinum (A / g-Pt) was calculated by dividing Ik (A) by g).
[Formula (2)]
Ik = (I _lim × I _0.9 ) / (I _lim −I _0.9 )
In the above formula (2), the meaning of each symbol is as follows.
Ik: Activation dominant current (A)
I _lim : diffusion limit current (A)
I _0.9 : Oxygen reduction current (A)

[Electrochemical surface area]
Cyclic voltammetry (CV) measurement was implemented about the core-shell catalyst obtained in Examples 1-2 and Comparative Examples 1-2, and the electrochemical surface area (ECSA) of the core-shell catalyst was computed.
A catalyst ink was prepared by the same method as the mass activity evaluation, and the catalyst ink was applied to a glassy carbon electrode (RDE) and dried, and CV measurement was performed.
CV measurement conditions are shown below.
Electrolyte: 0.1M perchloric acid aqueous solution (Ar gas was bubbled at 30 mL / min for 30 minutes or more in advance and saturated with Ar)
-Atmosphere: Under Ar atmosphere-Sweep speed: 50 mV / sec-Potential sweep range: 0.05 to 1.2 V (vs. RHE)
From the obtained cyclic voltammogram, the charge amount (C) of the hydrogen desorption peak was integrated.
Further, the mass (g) of platinum was calculated from the concentration and coating amount of the catalyst ink applied to the glassy carbon electrode.
From the value obtained by dividing the charge amount (C) of the hydrogen desorption peak by the charge amount per unit active surface area of platinum (C / m ² ) and the mass of platinum (g), the electrochemical surface area of platinum (m ² / g-Pt ) Was calculated.

[Specific activity]
For the core-shell catalysts obtained in Examples 1 and 2 and Comparative Examples 1 and 2, the catalytic activity per unit mass of platinum (A / g-Pt) is divided by the electrochemical surface area of platinum (m ² / g-Pt). The catalytic activity per platinum surface area (A / m ² ) was calculated. The results are shown in FIG.
As shown in FIG. 8, it can be seen that the core-shell catalysts of Examples 1 and 2 using palladium particles having a crystallite size of 5 to 8.5 nm have a high specific activity exceeding 4 A / m ² . On the other hand, the core-shell catalyst of Comparative Example 1 using palladium particles having a crystallite size of less than 5 nm and Comparative Example 2 using palladium particles having a crystallite size exceeding 8.5 nm has a specific activity lower than 4 A / m ^2. I understand that.

1 reaction vessel 2 palladium-containing particles 3 electrolyte 4 reference electrode 5 counter electrode 6 counter electrode compartment 7 stirrer (stirrer)
8 Platinum wire 20 Synthesizer

Claims (2)

A core-shell catalyst manufacturing method comprising: a core containing palladium; and a shell containing platinum and covering the core,
The X-ray diffraction measurement result has a peak at 2θ = 40 ° ± 1 °, and the crystallite size determined by the Scherrer method from the half-value width of the peak at 2θ = 40 ° ± 1 ° is 5 to 8.5 nm. Preparing palladium-containing particles;
In the copper ion-containing electrolyte, by applying a potential nobler than the copper redox potential to the palladium-containing particles, a copper deposition step of depositing copper on the surface of the palladium-containing particles,
After the copper deposition step, the copper deposited on the surface of the palladium-containing particles is contacted with a platinum ion-containing solution, thereby replacing the copper deposited on the surface of the palladium-containing particles with platinum. A core shell catalyst production method comprising: a substitution step to be formed.   The manufacturing method of the core-shell catalyst of Claim 1 with which the said palladium containing particle | grain is carry | supported by the support | carrier.