JP2016128148A - Method for producing core-shell catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a core-shell catalyst capable of removing remaining Cu or uncoated Pd without destroying a core-shell structure.SOLUTION: There is provided a method for producing a core-shell catalyst including a palladium-containing metal as a core and a platinum-containing metal as a shell, which comprises: a copper coating step of coating at least a part of a palladium-containing metal particulate surface with copper by applying to palladium-containing metal particles a potential nobler than the oxidation-reduction potential of copper in a copper ion-containing electrolyte to obtain a copper-palladium composite; a substitution step of substituting copper with platinum in the copper-palladium composite in a solution containing platinum to obtain a platinum-palladium composite; and a post-treatment step of treating the platinum-palladium composite by introducing gas containing oxygen in a nitric acid solution at normal temperature.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a core-shell catalyst.

  As an electrode catalyst for a fuel cell, a core-shell catalyst intended to reduce the amount of noble metal used such as platinum is known. For example, Patent Documents 1 and 2 describe a method for producing a core-shell catalyst by displacement plating using a copper underpotential deposition method (Cu-UPD method).

When the core-shell catalyst produced using the Cu-UPD method is converted into an electrode and used as an MEA, there is a problem that the remaining Cu and uncoated Pd are eluted and the performance of the battery is lowered. FIG. 1 is a diagram showing the influence of the metal elution amount in MEA on proton resistance. As shown in FIG. 1, when metal elutes as ions in MEA, ionomer poisoning occurs and proton resistance increases. As a result of the increased proton resistance, the performance of the battery decreases.
Therefore, as a method for removing impurities such as remaining Cu and uncoated Pd before forming the core-shell catalyst into an electrode, Patent Document 2 discloses that after the core-shell catalyst is synthesized, post-treatment is performed using nitric acid. Have been described.

Japanese Patent Application Laid-Open No. 2014-117852 JP 2013-215701 A

In the case of performing the post-treatment of the core-shell catalyst using nitric acid, in the conventional method, it has been attempted to perform the treatment under nitric acid at 60 to 90 ° C. as in the case of catalysts such as Pt / C and PtCo / C. However, although the removal of Cu and Pd proceeds in this method, the heat load applied to the catalyst particles is large, and at the same time, the process proceeds to the dissolution of the Pt shell, thereby destroying the core-shell structure. As a result, the activity of the core-shell catalyst has been reduced.
On the other hand, when treatment is performed using nitric acid at room temperature, the core-shell structure is prevented from being destroyed, but there is a problem that the removal of the essential Cu and Pd does not proceed.

  Then, this invention makes it a subject to provide the manufacturing method of the core-shell catalyst which can remove remaining Cu and uncoated Pd, without destroying a core-shell structure.

  As a result of intensive studies, the present inventors synthesized a platinum / palladium complex using a Cu-UPD method, and treated the platinum / palladium complex in a nitric acid solution at room temperature by introducing a gas containing oxygen. By doing so, it was found that the remaining Cu and uncoated Pd can be reduced to a certain level without destroying the core-shell structure. The present invention has been completed based on this finding.

In order to solve the above problems, the present invention takes the following means. That is,
The present invention relates to a method for producing a core-shell catalyst having a palladium-containing metal as a core and a platinum-containing metal as a shell, wherein the palladium-containing metal fine particles have a potential nobler than the redox potential of copper in a copper ion-containing electrolyte. A copper coating step in which at least a part of the surface of the palladium-containing metal fine particles is coated with copper to obtain a copper / palladium composite, and the copper / platinum of the copper / palladium composite is replaced with platinum in a solution containing platinum. A core / shell catalyst comprising: a substitution step for obtaining a palladium / palladium complex; and a post-treatment step for treating the platinum / palladium complex by introducing a gas containing oxygen in a nitric acid solution at room temperature. Is the method.

  In the present invention, “normal temperature” means 15 ° C. or more and 40 ° C. or less.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the core-shell catalyst which can remove remaining Cu and uncoated Pd can be provided, without destroying a core-shell structure.

It is a figure which shows the influence which the metal elution amount of a core-shell catalyst has on proton resistance. It is a figure which shows the measurement result of Cu residual amount of the core-shell catalyst obtained in Example 1 and Comparative Examples 1-3. It is a figure which shows the measurement result of the amount of Pd components melt | dissolved in Example 1 and Comparative Examples 1-3. It is a figure which shows the measurement result of the electrochemical surface area (ECSA) and catalyst activity (MA) of the core-shell catalyst obtained in Example 1 and Comparative Examples 1-3.

The present invention will be described below. In addition, the form shown below is an illustration of this invention and this invention is not limited to the form shown below.

The present invention relates to a method for producing a core-shell catalyst having a palladium-containing metal as a core and a platinum-containing metal as a shell, wherein the palladium-containing metal fine particles have a potential nobler than the redox potential of copper in a copper ion-containing electrolyte. A copper coating step in which at least a part of the surface of the palladium-containing metal fine particles is coated with copper to obtain a copper / palladium composite, and the copper / platinum of the copper / palladium composite is replaced with platinum in a solution containing platinum. A substitution step for obtaining a palladium / palladium complex, and a post-treatment step for treating the platinum / palladium complex by introducing a gas containing oxygen in a nitric acid solution at room temperature.
Hereinafter, (1) the copper coating step, (2) the substitution step, and (3) the post-treatment step of the core-shell catalyst production method of the present invention will be described in order.

(1) Copper coating step In the copper coating step, a nobler potential than the oxidation-reduction potential of copper in the copper ion-containing electrolyte is applied to the palladium-containing metal fine particles, and at least a part of the palladium-containing metal fine particle surface is made of copper. This is a step of coating to obtain a copper / palladium composite.

As the palladium-containing metal fine particles, at least one fine particle selected from palladium fine particles and palladium alloy fine 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 diameter of the palladium-containing metal fine particles is not particularly limited, but is preferably 10 nm or less. When the average particle diameter of the palladium-containing metal fine particles exceeds 10 nm, the surface area per mass of platinum is reduced, and a large amount of platinum is required to obtain the required activity, which is costly. If the average particle diameter of the palladium-containing metal fine particles is too small, the palladium itself is easily dissolved and the durability of the catalyst is lowered. Therefore, the average particle diameter of the palladium-containing metal fine particles is preferably 3 nm or more.
The calculation method of the average particle diameter of the fine 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 fine particle on the plane (equivalent particle diameter) of the fine particle. Considered as particle size. The measurement of the particle size by photographic observation is performed for 500 fine particles of the same type, and the average of the particle sizes of these fine particles is defined as the average particle size. Note that broken particles observed at the edge of the photograph are excluded from the analysis.

The palladium-containing metal fine 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 fine metal particles include Ketjen Black (trade name: manufactured by Ketjen Black International), Vulcan (trade name: manufactured by Cabot), and Norrit (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, metal materials such as metal particles and metal fibers, and perylene. Non-conductive materials such as organic pigments such as red are included.

  The average particle size of the carrier is not particularly limited, but is preferably 0.01 to several hundred μm, more preferably 0.01 to 1 μm. When the average particle size of the support is less than the above range, the support may be deteriorated by corrosion, and the palladium-containing metal fine particles supported on the support may fall off with time. Moreover, when the average particle diameter of a support | carrier exceeds the said range, a specific surface area is small and there exists a possibility that the dispersibility of a palladium containing metal microparticle may fall.

The specific surface area of the support is not particularly limited, preferably 50~2000m ² / g, more preferably 100~1600m ² / g. If the specific surface area of the support is less than the above range, the dispersibility of the palladium-containing metal fine particles in the support may be reduced, and sufficient battery performance may not be exhibited. Moreover, when the specific surface area of a support | carrier exceeds the said range, there exists a possibility that the effective utilization factor of a palladium containing metal microparticle may fall and sufficient battery performance may not be expressed.

The support ratio of palladium-containing metal fine particles by the carrier [{(palladium-containing metal fine particle mass) / (palladium-containing metal fine particle mass + carrier mass)} × 100%] is not particularly limited, and generally ranges from 20 to 60%. It is preferable that If the amount of palladium-containing metal fine particles supported is too small, the catalytic function may not be sufficiently developed. On the other hand, if the amount of palladium-containing metal fine particles supported is too large, there may be no particular problem from the viewpoint of the catalytic function, but even if more palladium-containing metal fine particles are supported, it was commensurate with the increase in production cost. It becomes difficult to obtain the effect.
As a method for supporting palladium-containing metal fine particles on a carrier, a conventionally used method can be employed. For example, a method in which palladium-containing metal fine particles are mixed in 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. After drying, heat treatment may be performed as necessary. When palladium alloy particles are used, the synthesis of the alloy and the loading of the palladium alloy particles on the carrier may be performed simultaneously.

The electrolytic solution containing copper ions is not particularly limited as long as it is an electrolytic solution capable of obtaining a copper / palladium composite by coating copper on at least a part of the surface of the palladium-containing metal fine particles by the Cu-UPD method. 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, and water is preferable from the viewpoint of not preventing the precipitation of copper on the surface of the palladium-containing fine metal 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 0.01 to 1.0 mol / L.
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 metal fine 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 metal fine particles may be immersed and dispersed in the copper ion-containing electrolyte solution by adding it to the copper ion-containing electrolyte solution in a powder state, or may be previously dispersed in a solvent to prepare a palladium-containing metal particle dispersion. The palladium-containing fine metal particle dispersion may be immersed and dispersed in the copper ion-containing electrolytic solution by adding it to the copper ion-containing electrolytic solution. As the solvent used in the palladium-containing fine metal particle dispersion, the same solvent as that used in the above-described copper ion-containing electrolytic solution can be used. In addition, the palladium-containing fine metal particle dispersion may contain the acid that can be added to the copper ion-containing electrolyte.
Alternatively, the palladium-containing metal fine particles may be fixed on the conductive substrate or the working electrode, and the palladium-containing metal fine particle fixing surface of the conductive substrate or the working electrode may be immersed in the electrolytic solution. As a method for fixing the palladium-containing metal fine particles, for example, a palladium-containing metal fine particle paste is prepared using an electrolyte resin (for example, Nafion (registered trademark)) and a solvent such as water or alcohol, and a conductive substrate. And a method of applying to the surface of the working electrode.

A method for applying a potential to the palladium-containing metal fine 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 fine metal particles, that is, a potential noble than the oxidation-reduction potential of copper. It is preferably within the range of 7V (vs. RHE), particularly preferably 0.38V (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 copper coating 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 metal fine particles and preventing oxidation of copper.
In the copper coating 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 metal fine particles are immersed and dispersed in a copper ion-containing electrolyte in the reaction vessel, each of the palladium-containing metal fine particles is stirred by stirring the copper ion-containing electrolyte. Can be brought into contact with the surface of the reaction vessel, which is the working electrode, and a potential can be uniformly applied to each palladium-containing metal fine particle. In this case, stirring may be performed continuously or intermittently during the copper coating step.

(2) Substitution Step The substitution step is a step of obtaining a platinum / palladium complex by substituting copper and platinum of a copper / palladium complex in a solution containing platinum. The solution containing platinum is not particularly limited as long as it contains at least platinum ions. In the solution containing platinum, platinum may exist as ions or may exist as a platinum compound such as a platinum complex.
In the substitution step, by bringing a solution containing platinum into contact with the copper of the copper / palladium complex, copper and platinum can be substituted due to the difference in ionization tendency to obtain a platinum / palladium complex.

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 solution containing platinum, 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 solution containing platinum is not particularly limited, but is preferably 0.0005 to 0.1 mol / L.
The solvent that can be used for the solution containing platinum can be the same as the solvent used for the above-described copper ion-containing electrolytic solution. Moreover, the solution containing platinum may contain, for example, an acid or the like in addition to the solvent and the platinum salt. As an acid, it can be made to be the same as that of the acid used for the copper ion containing electrolyte solution mentioned above.
The solution containing platinum is sufficiently stirred in advance, and nitrogen is preferably bubbled into the solution in advance from the viewpoint of preventing oxidation of the surface of the palladium-containing metal fine particles and preventing oxidation of copper.
The temperature of the solution containing platinum is not particularly limited, but is preferably 3 to 10 ° C. from the viewpoint of improving the catalytic activity of the core-shell catalyst.
The replacement time (contact time between the platinum-containing solution and the copper / palladium complex) is not particularly limited, but it is preferable to ensure 10 minutes or more. As the platinum-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 coating process and a substitution process in the same reaction container, you may add the solution containing platinum to the copper ion containing electrolyte solution used for the copper coating process. For example, after the copper coating step, the potential control is stopped, and the solution containing platinum is brought into contact with the copper / palladium composite by adding a solution containing platinum to the copper ion-containing electrolyte used in the copper coating step. Also good.

(3) Post-treatment step The post-treatment step is a step of treating a platinum / palladium complex by introducing a gas containing oxygen in a nitric acid solution at room temperature. The post-processing step makes it possible to remove Cu remaining without being replaced with platinum or uncoated Pd in the replacement step.

It is important that the temperature of the nitric acid solution is room temperature (15 ° C. or more and 40 ° C. or less). According to the present invention, by using nitric acid at room temperature, the platinum / palladium composite is not subjected to a heat load as in the case of using the conventional method, so that the core-shell structure is not destroyed and the catalyst The activity does not decrease. On the other hand, by introducing a gas containing oxygen into the nitric acid solution, it is possible to reduce the amount of remaining Cu and uncoated Pd to a certain level.
Moreover, it is preferable that the density | concentration of a nitric acid solution is 0.05-1.0 mol / L.

The method for supplying the platinum / palladium complex to the nitric acid solution is not particularly limited, but the platinum / palladium complex may be added to the nitric acid solution in a powder state, or the platinum / palladium complex may be previously dispersed in a solvent. A body dispersion liquid may be prepared, and the platinum / palladium complex dispersion liquid may be added to the nitric acid solution. Examples of the solvent used in the platinum / palladium complex dispersion include water and organic solvents, and water is preferable.
The platinum / palladium complex is preferably immersed in a nitric acid solution from the viewpoint of facilitating dissolution of remaining Cu and uncoated Pd and performing the post-treatment step in a short time.

In the post-treatment process, it is important to introduce a gas containing oxygen into the nitric acid solution. By introducing a gas containing oxygen into the nitric acid solution, the internal potential can be raised to 1.0 V. Even when nitric acid at room temperature is used, the amount of remaining Cu or uncoated Pd is kept at a certain level. It becomes possible to reduce to. The system potential is preferably 0.9 to 1.1V.
The gas containing oxygen is not particularly limited, and examples thereof include air and pure oxygen, and pure oxygen is preferable.
The flow rate of the gas containing oxygen is not particularly limited, but is preferably 50 mL / min or more, and particularly preferably 50 to 100 mL / min from the viewpoint of keeping the internal potential constant.

It is preferable to stir the solution during the post-treatment process. A method of stirring is not particularly limited, and examples thereof include a method using an ultrasonic homogenizer, a magnetic stirrer, a motor with stirring blades, and the like.
The stirring speed is not particularly limited, but is preferably 200 to 1000 rpm when a magnetic stirrer is used.

  The time for performing the post-treatment can be appropriately adjusted according to the concentration, temperature and the like of the nitric acid solution, and is preferably 15 to 30 minutes.

  Hereinafter, although the manufacturing method of the core-shell catalyst which concerns on this invention is explained in full detail based on an Example, this invention is not limited to the following specific forms.

<Example 1>
The platinum / palladium composite 2g prepared by the copper coating step and the substitution step (Cu-UPD method) is post-treated by the following methods (1) to (6), and the methods (7) and (8) The analysis was performed.
(1) 2 g of platinum / palladium complex is put into a nitric acid solution at 25 ° C. prepared to 0.1 mol / L and directly suspended using a homogenizer.
(2) O ₂ bubbling (100 mL / min) is performed while stirring the suspension with a stirrer (300 rpm). At this time, the temperature of the suspension is kept constant at 25 ° C.
(3) Leave in the state of (2) for 30 minutes.
(4) Filter the nitric acid solution with a vacuum filter.
(5) Wash the filtered powder with 2 L of ultrapure water.
(6) Vacuum dry at 60 ° C. to recover the core-shell catalyst.
(7) The Cu and Pd components dissolved in the filtrate obtained in (4) are analyzed and quantified by ICP-MS. The measurement result of the Cu residual amount calculated from the dissolved Cu component amount is shown in FIG. 2, and the measurement result of the dissolved Pd component amount is shown in FIG.
(8) As post-treatment performance, the electrochemical surface area (ECSA) and catalytic activity (MA) of the core-shell catalyst are measured by RDE. The results are shown in FIG.

<Comparative Example 1>
With respect to 2 g of the platinum / palladium complex used in Example 1, the residual amount of Cu and the measurement of (8) above were performed without post-treatment. The measurement result of Cu residual amount is shown in FIG. 2, and the measurement result of ECSA and MA is shown in FIG. In addition, since measurement of melt | dissolution Pd was not performed, it is set to 0 in FIG.

<Comparative example 2>
In Example 1, the temperature of the nitric acid solution and suspension was changed from 25 ° C. to 80 ° C., and post-treatment and performance evaluation were performed in the same manner as in Example 1 except that O ₂ bubbling was not performed. . The results are shown in FIGS.

<Comparative Example 3>
In Example 1, post-treatment and performance evaluation were performed in the same manner as in Example 1 except that O ₂ bubbling was not performed. The results are shown in FIGS.

[result]
2 and 3, in Example 1 using the manufacturing method of the present invention, the amount of Cu and Pd dissolved was equal to or greater than that of Comparative Example 2 using the conventional method. Further, it was confirmed from the comparison with Comparative Example 1 in FIG. 4 that ECSA and MA did not decrease in Example 1 even after the post-treatment. That is, according to the manufacturing method of the present invention, it can be said that remaining Cu and uncoated Pd can be reduced to a certain level without destroying the core-shell structure.
On the other hand, in Comparative Example 2 using the conventional method, ECSA and MA after post-treatment were lower than those in Example 1 and Comparative Examples 1 and 3. This is presumably because the core-shell structure was destroyed by the influence of heat load because high-temperature nitric acid was used. Therefore, it is considered that the Pd dissolved in Comparative Example 2 contains Pd dissolved by the collapse of the core shell. Further, in Comparative Example 3 in which O ₂ bubbling was not performed under normal temperature nitric acid, ECSA and MA after the post-treatment did not decrease, but the amount of Cu and Pd dissolved was significantly less than in Example 1 and Comparative Example 2, It was confirmed that the removal of Cu and Pd did not proceed sufficiently.

Claims (1)

A method for producing a core-shell catalyst having a palladium-containing metal as a core and a platinum-containing metal as a shell,
Copper that obtains a copper / palladium composite by applying a potential higher than the redox potential of copper in a copper ion-containing electrolytic solution to the palladium-containing metal fine particles and coating at least a part of the surface of the palladium-containing metal fine particles with copper A coating process;
Replacing the copper and the platinum in the copper / palladium complex in a solution containing platinum to obtain a platinum / palladium complex;
And a post-treatment step of treating the platinum / palladium composite by introducing a gas containing oxygen in a nitric acid solution at room temperature.