JP2016128151A - Method for producing core-shell catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a core-shell catalyst having high catalytic activity while suppressing agglomeration of carbon with sulfuric acid.SOLUTION: There is provided a method for producing a core-shell catalyst including palladium-containing particulates as a core and a platinum-containing metal as a shell, which comprises: a first step of coating at least a part of the surface of the palladium-containing particulates with copper by applying a potential to palladium-containing particulates supported by a carbon carrier in a mixed solution containing more than 0 vol.% and less than 30 vol.% of isopropanol and further containing sulfuric acid and copper to obtain a copper-palladium composite; and a second step of substituting at least a part of copper in the copper-palladium composite with platinum in platinum ion-containing solution.SELECTED DRAWING: Figure 1

Description

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

  As a method for producing a core-shell catalyst for a fuel cell electrode, a technique is known in which copper is coated on a core material using a copper underpotential deposition (Cu-UPD) method in a copper sulfate solution, and then copper and platinum are replaced. (Patent Document 1). In addition, although different from the manufacturing process of a core-shell catalyst, the technique of adding isopropanol at the time of catalyst ink preparation is known (patent document 2).

JP, 2014-213212, A JP 2011-070926 A

  In Patent Document 1, sulfuric acid is required for Cu-UPD. However, sulfuric acid is also adsorbed on the carbon that is the catalyst carrier, and causes the carbons to aggregate. Therefore, Cu-UPD and Pt substitution cannot be performed appropriately, and the catalyst activity may be reduced. Therefore, an object of the present invention is to provide a method for producing a core-shell catalyst having high catalytic activity while suppressing carbon aggregation due to sulfuric acid.

In order to solve the above problems, the present invention adopts the following configuration. That is,
The present invention relates to a method for producing a core-shell catalyst having a palladium-containing fine particle as a core and a platinum-containing metal as a shell. A potential is applied to the palladium-containing fine particles supported on the carrier to coat at least a part of the surface of the palladium-containing fine particles with copper to obtain a copper / palladium composite, in the first step, in the platinum ion-containing solution, It is a manufacturing method of a core-shell catalyst provided with the 2nd process of substituting at least some copper and platinum of a copper / palladium composite.

  According to the present invention, by adding a predetermined amount of isopropanol to a solution containing sulfuric acid and copper, it is possible to suppress agglomeration of carbon in a Cu-UPD reaction or a Pt substitution reaction, and a core-shell catalyst having high catalytic activity. Can be manufactured.

It is data which shows the relationship between an isopropanol (IPA) addition amount and a core shell catalytic activity. It is the data which shows the CV measurement result of the core-shell catalyst manufactured about each of the case where a polyoxyethylene alkylphenyl ether type-surfactant (dispersant A) is used as a dispersing agent, and the case where isopropanol (IPA) is used. . It is the schematic for demonstrating the reactor used in the Example.

  A method for producing a core-shell catalyst according to the present invention is a method for producing a core-shell catalyst having a palladium-containing fine particle as a core and a platinum-containing metal as a shell, containing isopropanol in an amount of more than 0% by volume and less than 30% by volume, and sulfuric acid and copper. A first step of applying a potential to the palladium-containing fine particles supported on the carbon carrier in a mixed solution to cover at least a part of the surface of the palladium-containing fine particles with copper to obtain a copper / palladium composite; A second step of substituting at least part of the copper / platinum of the copper / palladium complex with platinum in the platinum ion-containing solution.

1. First step The first step is to apply a potential to the palladium-containing fine particles supported on the carbon support in a mixed solution containing isopropanol in an amount of more than 0% by volume and less than 30% by volume and containing sulfuric acid and copper. In this step, at least a part of the surface is coated with copper to obtain a copper / palladium composite.

1.1. Mixed solution The mixed solution contains sulfuric acid, copper and isopropanol. The mixed solution used for this invention can be set as the structure similar to the conventional solution (electrolyte solution) used for Cu-UPD method except that isopropanol is contained in a predetermined amount. That is, as a solvent constituting the mixed solution, either water or an organic solvent may be used, but water is particularly preferable. The sulfuric acid concentration in the mixed solution can be appropriately adjusted in consideration of the amount of palladium-containing fine particles to be treated. For example, the sulfuric acid concentration is preferably 0.001 mol / L or more, and particularly preferably 0.001 to 1.0 mol / L. The copper concentration in the mixed solution may be a concentration that allows the surface of the palladium-containing fine particles to be coated with Cu-UPD, and can be appropriately adjusted in consideration of the amount of palladium-containing fine particles to be treated. . In the mixed solution, copper is contained at least as copper ions. The anion which becomes a copper ion pair is not limited to sulfate ion. That is, copper (copper ions) may be included in the mixed solution by adding a copper salt other than copper sulfate separately from the above sulfuric acid. However, in order for the Cu-UPD method to proceed appropriately, it is necessary to co-adsorb sulfonic acid groups and copper on the surface of the palladium-containing fine particles, so that it is necessary to include sulfuric acid and copper in the mixed solution. Although the temperature of a mixed solution is not specifically limited, It is preferable that it is 15-30 degreeC.

The present invention is characterized in that isopropanol, which is a lower alcohol, is included as a dispersant in the mixed solution. When isopropanol is not present in the mixed solution, agglomeration of carbon that is a carrier of palladium-containing fine particles occurs. When the carbon aggregates, a sufficient amount of Cu ²⁺ / Pt ²⁺ cannot be supplied to the palladium-containing fine particles embedded in the carbon, and the Cu-UPD reaction and the Pt substitution reaction cannot be performed appropriately. As a result, for example, Pt segregation occurs at the time of Pt substitution, and the catalytic activity decreases. On the other hand, when there is too much content of isopropanol, precipitation of the copper ion in a mixed solution will arise by the reducing action by isopropanol. As a result, the Cu-UPD reaction does not proceed, and the catalytic activity decreases. As a result of intensive studies by the present inventors, it is possible to produce a core-shell catalyst having high catalytic activity while suppressing carbon aggregation due to sulfuric acid only when the content of isopropanol in the mixed solution is more than 0 volume% and less than 30 volume%. It was found out (see FIG. 1). The lower limit of the content of isopropanol is preferably 5% by mass or more, and the upper limit is preferably 20% by volume or less.

  In addition, even if it is a case where it replaces with isopropanol as a dispersing agent and another lower alcohol is included in a mixed solution, it is thought that a desired effect is show | played. Such lower alcohols include methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, cyclohexanol and the like. The content of these lower alcohols may be the same as in the case of isopropanol described above.

  The present inventors have also obtained the following knowledge through intensive studies. That is, when a known polymer dispersant such as a polyoxyethylene alkylphenyl ether surfactant is used without using a lower alcohol as a dispersant, not only the carbon surface but also the surface of the palladium-containing fine particles are coated with the dispersant. End up. As a result, the Pt substitution reaction does not proceed and the platinum shell cannot be formed properly (see FIG. 2). On the other hand, when a lower alcohol is used as a dispersant, such a situation does not occur, and the Cu-UPD reaction and the Pt substitution reaction can proceed appropriately. In this respect, there is an advantageous effect by using a predetermined amount of lower alcohol as a dispersant.

1.2. Palladium-containing fine particles As long as the palladium-containing fine particles can be used as the core of the core-shell catalyst for fuel cells, the form thereof is not particularly limited. For example, 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 at least one metal material selected from the group consisting of iridium, ruthenium, rhodium, iron, cobalt, nickel, copper, silver, and gold. The palladium alloy preferably contains 80% by mass or more of palladium based on the whole alloy (100% by mass). Thereby, a more uniform core-shell structure can be formed.

  The average particle diameter of the palladium fine particles is not particularly limited, but is preferably 10 nm or less. When the average particle diameter of the palladium-containing fine particles exceeds 10 nm, the surface area per mass of platinum becomes small, and a lot of platinum is required to obtain the necessary activity, which is expensive. On the other hand, if the average particle size of the palladium-containing fine 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 fine 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.

1.3. Carbon support In the present invention, it is assumed that the palladium-containing fine particles are supported on a carbon support. As the carbon carrier, Ketjen Black (trade name, manufactured by Ketjen Black International), Vulcan (trade name, manufactured by Cabot), Norrit (trade name, manufactured by Norit), Black Pearl (trade name, manufactured by Cabot) ), Carbon particles such as acetylene black (trade name, manufactured by Chevron) and conductive carbon materials such as carbon fibers can be used.

The average particle size of the carbon support 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 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 palladium containing microparticles | fine-particles 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 carrier is less than the above range, the dispersibility of the palladium-containing fine particles in the carrier 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 palladium containing fine particles may fall and sufficient battery performance may not be expressed.

  The palladium-containing fine particle supporting rate [{(palladium-containing fine particle mass) / (palladium-containing fine particle mass + carrier mass)} × 100%] by the carbon carrier is not particularly limited, and is generally in the range of 20 to 60%. It is preferable. If the supported amount of the palladium-containing fine particles is too small, the catalyst function may not be sufficiently exhibited. On the other hand, if the amount of palladium-containing fine particles supported is too large, there may be no particular problem from the viewpoint of the catalyst function, but even if more palladium-containing fine particles are supported, there is an effect commensurate with the increase in production cost. It becomes difficult to obtain.

  As a method for supporting the palladium-containing fine particles on the carbon carrier, a conventionally used method can be employed. For example, a method in which palladium-containing 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 palladium-containing fine particles are preferably subjected to a cleaning treatment by immersing them in an acid solution (referring to an acid-containing solution, which may be the above-mentioned mixed solution) before applying a potential. The oxide on the surface of the palladium-containing fine particles can be removed by the cleaning treatment. In this case, the acid solution is preferably stirred using an ultrasonic homogenizer, a magnetic stirrer, a motor with stirring blades, or the like.

In the first step, a potential is applied to the palladium-containing fine particles supported on the carbon support in the mixed solution. The method for applying the potential is not particularly limited, and a general method can be adopted. For example, a method of immersing a working electrode, a counter electrode, and a reference electrode in a mixed solution and applying a potential to the working electrode can be mentioned. 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 applied potential in this case 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 particles, that is, a potential nobler 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 first 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 fine particles and preventing oxidation of copper. In the first step, the mixed solution is preferably stirred as necessary. For example, when a reaction vessel also serving as a working electrode is used, and palladium-containing fine particles are immersed and dispersed in a mixed solution in the reaction vessel, the reaction containing palladium-containing fine particles as a working electrode is performed by stirring the copper ion-containing electrolyte. A potential can be uniformly applied to each palladium-containing fine particle by contacting the surface of the container. In this case, stirring may be performed continuously or intermittently in the first step.

  Through the first step, at least a part of the surface of the palladium-containing fine particles can be coated with copper by the Cu-UPD reaction, and a copper / palladium composite is obtained. In the present invention, since a predetermined amount of isopropanol is contained as a dispersant in the mixed solution, the Cu-UPD reaction can be advanced while suppressing aggregation of the carbon support, and segregation of copper and the like can be suppressed. .

2. Second Step The second step is a step of substituting at least part of copper and platinum of the copper / palladium complex in the platinum ion-containing solution.

2.1. Platinum ion-containing solution The shell of the core-shell catalyst produced according to the present invention contains platinum and / or 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 contains 90% by mass or more of platinum based on the whole alloy (100% by mass). Thereby, more excellent catalytic activity and durability can 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 0.0005 to 0.1 mol / L.

  The platinum ion-containing solution can be easily prepared, for example, by adding the above-described platinum salt to the acid solution. Examples of the acid constituting the acid solution include sulfuric acid, nitric acid, perchloric acid, hydrochloric acid, and hypochlorous acid. Moreover, water is preferable as the solvent constituting the acid solution. The platinum ion-containing solution is preferably bubbled in advance from the viewpoint of preventing oxidation of the surface of the palladium-containing fine particles. Although the temperature of a platinum ion containing solution is not specifically limited, It is preferable that it is 3-10 degreeC from a viewpoint of the catalytic activity improvement of a core-shell catalyst.

  In a 2nd process, copper and platinum can be substituted by the difference in ionization tendency by making a platinum ion containing solution contact the copper deposited on the surface of palladium containing fine particles. The substitution time (contact time between the platinum ion-containing solution and the palladium-containing fine particles) is not particularly limited, but it is preferable to ensure 10 minutes or more. Moreover, since the potential of the reaction solution increases as the platinum ion-containing solution is added, it is more preferable to leave the solution until the monitor potential does not change (until the OCV becomes steady).

  Through the above-described second step, the copper on the surface of the palladium-containing fine particles is replaced with platinum by the Pt substitution reaction, and a core-shell catalyst having the palladium-containing fine particles as the core and the metal containing platinum as the shell can be manufactured. In the present invention, since a predetermined amount of isopropanol is included as a dispersant in the mixed solution, the Pt substitution reaction can be promoted while suppressing the aggregation of the carbon support, and segregation of platinum or the like can be suppressed.

<Example 1>
1. First step (preparation of mixed solution and dispersion of Pd / C)
As shown in FIG. 3, in a reactor in which WE, CE, and RE were set in the P / G wiring, 500 ml of 0.1 M sulfuric acid was charged and nitrogen bubbling was performed for 30 minutes to degas oxygen. On the other hand, 2 g of carbon (Pd / C: palladium loading 30%) on which palladium fine particles are supported in a beaker is weighed, 0.1 M copper sulfate solution is added, and a Pd / C suspension is prepared using a homogenizer. Prepared. The Pd / C suspension was charged into the reactor to prepare a 0.05 M aqueous copper sulfate solution containing Pd / C. A predetermined amount of isopropanol was added here as a dispersing agent, and Pd / C was dispersed in the mixed solution by stirring and mixing for 30 minutes or more.

(Cu-UPD)
A potential of 0.38 V was applied to the WE electrode by P / G, and Cu-UPD was performed until the reduction current became zero. Thereby, at least a part of the surface of the palladium fine particles was coated with copper to obtain a copper / palladium composite.

2. Second step (Pt substitution)
The inside of the reactor was cooled from room temperature to 5 ° C. On the other hand, 0.85 g of potassium chloroplatinate was dissolved in 200 ml of 0.05 M sulfuric acid, and oxygen deaeration was performed by nitrogen bubbling to prepare a platinum ion-containing solution. The platinum ion-containing solution was charged into the reactor by a pump. The OCV was monitored and Pt substitution was performed until the potential converged in the range of 0.6 to 0.9V.

(Post-processing)
After convergence of the potential, vacuum filtration was performed. Thereafter, the solid content was washed with 4 L of ultrapure water and vacuum dried at 60 ° C. to obtain a core-shell catalyst having palladium-containing fine particles as a core and platinum-containing metal as a shell.

(Evaluation of core-shell catalyst)
The core-shell catalyst was evaluated by performing the following measurements. The results are shown in FIG.

Using a rotating disk electrode (electrode area 0.196 cm ² ) as a measuring device, the measurement was performed according to the following procedure.
(I) The glassy carbon (GC) electrode surface was mirror finished by buffing.
(Ii) The electrode was ultrasonically cleaned using ultrapure water.
(Iii) After ultrasonically dispersing an ink having the following composition, only 10 μL was applied to the electrode.
(Ink composition) Core-shell catalyst: 30 mg, ultrapure water: 30 ml, concentrated sulfuric acid: 7.5 μL, 5% Nafion: 131 μL
(Iv) A 0.1 mol / L HClO ₄ solution was placed in a glass cell, and an electrode was set. Under oxygen bubbling, the potential was run at a potential running range of 1.05 V to 0.1 V and a potential running speed of 10 mV / s (2 cycles). When running from 0.1 V to 1.05 V in the second cycle, the current value (I) at 0.9 V and the current value (Id) at 0.3 V were measured. The active dominant current (Ik) was calculated based on the following formula.
Ik = (Id × I) / (Id−I)
The metal composition of the core-shell catalyst obtained in advance was measured by ICP-AES, and the amount of platinum on the GC electrode surface was calculated from the applied catalyst ink. A value obtained by dividing Ik by the amount of platinum (g) was defined as mass active MA (A / g-Pt).

As shown in FIG. 1, the catalytic activity of the core-shell catalyst was dramatically improved only when the amount of isopropanol added was more than 0% by volume and less than 30% by volume. In particular, in the case of 5 volume% or more and 20 volume% or less, a more remarkable effect was confirmed.
When no isopropanol is added, agglomeration of the carbon carrier occurs, and a sufficient amount of Cu ²⁺ / Pt ²⁺ cannot be supplied to the palladium-containing fine particles embedded in the carbon, so that the Cu-UPD reaction and the Pt substitution reaction can be appropriately performed. As a result, it was considered that, for example, Pt segregation occurred at the time of Pt substitution, and the catalytic activity was lowered.
On the other hand, when there is too much content of isopropanol, precipitation of the copper ion in a mixed solution will arise by the reduction | restoration effect | action by isopropanol, and as a result, Cu-UPD reaction does not advance and it is thought that catalyst activity fell.

(Comparison with the use of polymer dispersant)
When a core-shell catalyst is produced by the above procedure using a mixed solution to which 20% by volume of isopropanol is added, and a polyoxyethylene alkylphenyl ether surfactant (dispersant A, trade name, which is a general-purpose polymer dispersant) : CV measurement was performed on the core-shell catalyst for each of the case where the core-shell catalyst was produced by the above-described procedure using a mixed solution to which 1% by volume of Triton X) was added. The results are shown in FIG.

  As shown in FIG. 2, when the polymer dispersant is applied, in the CV measurement after Pt substitution, since the hydrogen adsorption / desorption derived from Pt has disappeared, the Pt substitution reaction may not proceed. I understand. On the other hand, when isopropanol is applied, hydrogen adsorption can be clearly confirmed, and it can be seen that a platinum-containing shell is appropriately formed on the surface of the core without inhibiting the Cu-UPD reaction and the Pt substitution reaction.

  The core-shell catalyst produced according to the present invention can be widely used as a catalyst for fuel cell electrodes.

Claims (1)

A method for producing a core-shell catalyst having a palladium-containing fine particle as a core and a metal containing platinum as a shell,
In a mixed solution containing more than 0 volume% and less than 30 volume% of isopropanol and further containing sulfuric acid and copper, a potential is applied to the palladium-containing fine particles supported on the carbon support so that at least a part of the surface of the palladium-containing fine particles is formed. A first step of coating with copper to obtain a copper / palladium composite;
A second step of substituting at least a portion of copper and platinum of the copper / palladium composite in a platinum ion-containing solution;
A method for producing a core-shell catalyst.

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