JP5660603B2 - Method for producing platinum core-shell catalyst - Google Patents

Description

  The present invention relates to a method for producing a platinum core-shell catalyst suitable for use as a catalyst for an oxygen reduction reaction in a fuel cell, and a fuel cell using the catalyst.

A polymer electrolyte fuel cell (PEFC) is known as a clean energy device that produces only water by causing an oxidation reaction of hydrogen on the anode side and a reduction reaction of oxygen on the cathode side. As this catalyst, a platinum-supported carbon catalyst (Pt / C catalyst) in which platinum fine particles are supported in a highly dispersed manner on a carbon black carrier is generally used. Pt / C catalysts have the advantages of high catalytic activity and high electrical conductivity, and because they are noble metals, they are less susceptible to corrosion and poisoning due to the state of the surrounding environment and substances present in the surrounding environment. Have
However, there is a problem that platinum is expensive and has a small amount of resources, and a reduction in the amount of platinum is required.

  In order to solve this problem, a platinum core-shell catalyst in which platinum is coated on a dissimilar metal at an atomic level has attracted attention. The platinum core-shell catalyst has a configuration in which different metal fine particles (core metal) coated with a platinum atomic layer (shell) are supported in a highly dispersed manner on a carrier (carbon black or the like). With such a configuration, the surface area can be increased while reducing the amount of platinum, so that the activity per mass of platinum can be improved and the amount of platinum can be reduced.

Gold is a noble metal and is as expensive as platinum, but it is less prone to ionization than platinum, is stable to oxidation, and is much more resource-rich than platinum, so it is expected as one of the core metals Has been. When trying to produce a platinum core-shell catalyst (Pt / Au / C catalyst) by reducing platinum ions in solution using a reducing agent such as hydrogen or sodium borohydride, and depositing platinum on gold (core metal) Since a thick platinum (shell) layer is deposited on gold (core metal) or in a solution, it is difficult to efficiently produce a platinum core-shell catalyst. Therefore, in order to efficiently produce a platinum core-shell catalyst using gold as a core, an underpotential deposition method (UPD method) is used (Non-Patent Document 1). A method for producing a platinum core-shell catalyst using the UPD method is schematically shown in FIG. According to the UPD method, by using specific conditions, the gold surface can be coated with a copper monoatomic layer, and then immersed in a hydrochloric acid solution containing chloroplatinate ions, For substitution, a monoatomic layer of platinum can be formed.
In the method using the UPD method, gold (core metal) can be theoretically coated with a monolayer, but it is synthesized because it requires a metal to be replaced such as copper or undergoes an electrochemical treatment. There was a problem that the method was complicated and mass synthesis was difficult.

Abstracts of 16th Fuel Cell Symposium, Tokyo (2009) pp. 92-95. "Development of Highly Active and Durable Platinum Core-Shell Catalysts for Polymer Electrolyte Fuel Cells" Satoshi Inaba, et al.

  Accordingly, an object of the present invention is to provide a method that can produce a platinum core-shell catalyst in a large amount at a low cost without using a reducing agent and having a simpler process than a method using an underpotential deposition method. .

  In the course of repeating the study to solve the above problems, the present inventors simply immersed the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions without using the underpotential precipitation method. The platinum core-shell catalyst produced by this method is comparable to the platinum core-shell catalyst produced by the underpotential deposition method, while it is found that a platinum monoatomic layer (shell) can be accurately and easily formed on the gold core particle. It was confirmed that the catalyst had catalytic activity, and the present invention was completed.

  That is, the present invention is characterized in that platinum is directly deposited on the gold core particles by immersing the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions in the absence of a reducing agent. And a method for producing a platinum core-shell catalyst.

  The gold core particles are preferably supported on the surface of an appropriate carrier (preferably carbon powder such as carbon black, or conductive oxide powder such as tin oxide or titanium oxide).

  According to the method of the present invention, a highly active platinum core-shell catalyst can be produced in large quantities at a low cost. Since this catalyst can be used as a catalyst for a fuel cell, the cost of the fuel cell can be drastically reduced.

(a) is a figure explaining typically the manufacturing method of the conventional platinum core-shell catalyst using an underpotential method, (b) is a figure explaining the manufacturing method of the platinum core-shell catalyst of this invention typically. . (a) is a figure which shows typically the apparatus used for the method of this invention, (b) is a figure which shows typically the rotating ring disk electrode used for oxygen reduction activity evaluation. (a) is a figure which shows the cyclic voltammogram of the Pt / Au / C catalyst created in Example 1 and Comparative Example 2 (UPD method), (b) explains the peak of the voltammogram shown in (a). It is a figure to do. (a) is a view showing a cyclic voltammogram of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method) and a gold-supported carbon support (Au / C) not subjected to platinum coating treatment. (B) is a partially enlarged view for explaining the voltammogram peak shown in (a). The figure which shows the convective voltammogram of the Pt / Au / C catalyst created in Example 1 (1.0 mM), Example 2 (0.1 mM), and Comparative Example 2 (UPD method), and a gold | metal | money carrying | support carbon support (Au / C). is there. It is a figure which shows the UV spectrum of the platinum ion containing solution immediately after preparation, and the UV spectrum of the supernatant liquid of the platinum ion containing solution after the platinum core-shell catalyst preparation, (a) is a potassium tetrachloroplatinate (II) acid solution, b) shows the results for an aqueous solution of potassium hexachloroplatinate (IV). It is a transmission electron microscope (TEM) image, (a) is the gold | metal | money carrying | support carbon support (Au / C) used in Example 7, (b) is the image of the Pt / Au / C catalyst produced in Example 7. Indicates. It is a graph which shows a particle size distribution, Comprising: (a) is a gold | metal | money carrying | support carbon support | carrier (Au / C) used in Example 7, (b) shows the graph of the Pt / Au / C catalyst created in Example 7. FIG.

The gold core particles used in the present invention preferably have a particle size of about 1 nm to 30 nm. The gold core particle may be a particle whose surface is made of gold, and may contain a metal other than gold or platinum.
In addition, it is preferable to carry out the method of the present invention using a gold-supported carrier on which a large number of gold core particles are supported. At this time, the gold core particles are dispersed on the surface of the carrier and It is preferably supported in an amount that occupies 1% to 70% of the combined weight of the carrier and the carrier. In order to carry the gold core particles in a highly dispersed state, the specific surface area of the carrier is preferably 10 to 1000 m ² / g, and the particle size of the carrier is preferably in the range of 10 nm to 1 mm. By using the carrier having such properties, the gold core particles are supported on the surface of the carrier, whereby the nanometer-scale gold core particles can be held in a highly dispersed state with high density.
The gold core particle size means an average crystallite size measured by the XRD method, and the carrier particle size means an average particle size of primary particles observed by an electron microscope.
As an example of a preferable gold-supporting carrier, a gold-supporting carrier in which gold fine particles having a particle size of 1 nm to 30 nm are supported in a highly dispersed state on the surface of a carbon black carrier. As such a gold-supported carrier, the same gold-supported carrier used in the conventional method for producing a platinum core-shell catalyst (method using the UPD method) can be used.

Examples of the solution containing divalent platinum ions or tetravalent platinum ions used in the present invention include, for example, an aqueous solution of tetrachloroplatinum (II) acid, an aqueous solution of potassium tetrachloroplatinate (II), and diaminemineplatinum platinum (II). An aqueous solution, an aqueous solution of diamminedinitroplatinum (II), an aqueous solution of tetraammineplatinum (II) chloride (monohydrate), an aqueous solution of hexachloroplatinum (IV) acid, an aqueous solution of potassium hexachloroplatinum (IV), and the like. Further, these platinum ions may exist in the solution in any state of an anion complex, a cation complex, and a nonionic complex.
In the present invention, immersing the gold core particles in a solution containing platinum ions means that the gold core particles only need to be immersed in the platinum ion-containing solution as a result. That is, the gold core particles may be immersed in a solution that already contains platinum ions, or the platinum complex may be added after the gold core particles are immersed in a solvent.

  The amount of platinum ions contained in the solution may be appropriately adjusted according to the amount of gold core particles. That is, in order to cover the surface of the gold core particle with the shell of the platinum monoatomic layer, it is sufficient that sufficient platinum ions are included in the solution. Specifically, it is preferable to prepare the solution so that the amount of platinum ions is about 1 to 1000 times the amount covered by the platinum atoms of the monoatomic layer per surface area of the gold used. If the concentration of the solution is too thin, it takes time to coat, so the platinum ion concentration is preferably 0.05 mM or more. In general, an aqueous solution containing 0.1 mM to 100 mM of platinum ions may be used.

  In the present invention, “in the absence of a reducing agent” means that a reducing agent (such as hydrogen or borohydride) is not intentionally added to the solution. That is, even if a substance having a reducing action is present in a small amount in the solution, it is included in “in the absence of a reducing agent” as long as platinum ions are not reduced.

  The method according to the present invention is intended to coat the surface of the gold core particle with a platinum monoatomic layer by depositing platinum atoms on the gold core particle. Even if not, sufficient catalytic activity is obtained. It is preferable to cover 60% or more, particularly 70% or more, of the surface area of the gold core particles with platinum atoms.

  When performing the method concerning this invention, the temperature of a solution is not specifically limited, Reaction can be advanced also at room temperature.

The method according to the present invention can be carried out, for example, as follows. First, a carrier on which gold fine particles (gold core particles) are highly dispersed and supported is prepared, added to water, and subjected to ultrasonic treatment for about 10 to 60 minutes to disperse the carrier in water. A sufficient amount of Pt complex (divalent or tetravalent) for coating is added and stirred for about 1 to 24 hours under an inert atmosphere such as an Ar atmosphere. Thereafter, centrifugation and washing with ultrapure water are performed, and the obtained powder (platinum core-shell catalyst) is dried.
In the powder thus obtained, 60 to 70% or more of the surface of the gold core particle on the support is coated with a platinum monoatomic layer, and has high catalytic activity.
The platinum coverage of the gold core particles described above is the cyclic voltammogram obtained by performing cyclic voltammetry on the gold core particles (Au) before platinum coating and the gold core particles (Pt / Au) after platinum coating. It can obtain | require from the reduction | restoration peak of a gold oxide film using the following formula | equation (for the details of a measurement, refer Example 5).
Formula (1) Coverage (%) =
{[(Au peak area) − (Pt / Au peak area)] / (Au peak area)} × 100

In the conventional underpotential deposition method, carbon black in which gold fine particles are supported in a highly dispersed state is supported on an electrode, and this electrode is held in a state in which a potential is applied in a solution containing copper ions. First, it was necessary to deposit a copper monoatomic layer underpotentially and then immerse it in a chloroplatinic acid solution to replace the copper atoms with platinum atoms. For this reason, it is necessary to prepare an electrode and perform an electrochemical treatment, and it is difficult to directly produce a catalyst in a powder state. Furthermore, the gold core particles could not be directly covered with a platinum monoatomic layer, and copper atoms had to be replaced with platinum atoms (see FIG. 1a).
On the other hand, in the method of the present invention, the gold core particles can be directly coated with a monoatomic platinum shell (see FIG. 1b) and can be produced from powder with the simple equipment and steps described above. So mass production is possible. In platinum and gold, gold is a noble metal (less ionization tendency), so from the common knowledge in the art, even if a gold core is immersed in a solution of platinum ions, the gold core is covered with platinum atoms. It cannot be predicted. Therefore, the underpotential deposition method has been used so far, and the phenomenon in the method of the present invention is an unexpected phenomenon found in the research process for improving the platinum core-shell catalyst.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely using an Example, this invention is not limited to an Example.

[Example 1] Production of platinum core-shell catalyst
0.1 g of gold-supported carbon support (Au / C, gold average particle diameter: 5 nm, carbon support: 50 nm particle diameter, ketjen black with a specific surface area of 800 m ² / g, gold support density of 28.6% by weight), ultrapure for 30 minutes After ultrasonically dispersing in 1 liter of water, it was sufficiently deaerated with argon gas, and potassium tetrachloroplatinate (II) (K ₂ PtCl ₄ ) was added so as to be 1.0 mM. The mixture was stirred at 30 ° C. for 24 hours while flowing argon gas, to obtain a Pt / Au / C suspension (see FIG. 2a). This suspension was centrifuged and washed with ultrapure water (× 3 times), and then dried at room temperature to produce Pt / Au / C powder.

The Pt / Au / C powder obtained, rotating disk electrode composed of glassy carbon disk with a diameter of 6 mm (geometrical area: 0.283cm ²⁾ uniformly dispersed and supported so as to 14.1μg (Au) cm ^-2 on did. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast to a film thickness of 0.1 mm and dried to prepare a performance evaluation electrode.

[Example 2] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the concentration of potassium tetrachloroplatinate (II) (K ₂ PtCl ₄ ) was 0.1 mM. did.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 3] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the reaction temperature was 60 ° C.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 4] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the platinum source used was potassium hexachloroplatinate (IV) (K ₂ PtCl ₆ ).
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Comparative Example 1]
A commercially available platinum-supported carbon catalyst (Pt / C, platinum average particle size: 2.8 nm, carbon support: Ketjen black with a particle size of 50 nm, platinum support density: 50% by weight) is composed of a glassy carbon disk with a diameter of 6 mm. rotating disk electrode (geometric area: 0.283cm ²⁾ was uniformly dispersed and supported so as to 14.1μg (Pt) cm ^-2 on. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast so as to have a film thickness of 0.1 μm, and dried to produce a performance evaluation electrode.

[Comparative Example 2]
Gold-supported carbon support (Au / C, gold average particle diameter: 5 nm, carbon support: particle diameter 50 nm, specific surface area 800 m ² / g ketjen black, gold support density 28.6% by weight) from a glassy carbon disk with a diameter of 6 mm composed rotating disk electrode (geometric area: 0.283cm ²⁾ was uniformly dispersed and supported so as to 14.1μg (Au) cm ^-2 on. This electrode was degassed with argon gas in 0.5M sulfuric acid aqueous solution containing 2mM copper sulfate, held at 0.3V for 10 minutes against a reversible hydrogen electrode (RHE), thereby forming a copper monoatomic layer on the gold core particle. Underpotential deposition. This electrode was quickly washed with water and immersed in a 5 mM aqueous potassium tetrachloroplatinate (II) solution that had been degassed with argon gas for 10 minutes to replace the copper atoms with platinum atoms, resulting in a platinum monolayer core-shell catalyst (Pt / Au / C) was obtained. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast to a film thickness of 0.1 mm and dried to prepare a performance evaluation electrode.

[Example 5] Performance evaluation of catalyst The electrodes obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to electrochemical evaluation as follows, and compared with the electrodes obtained in Comparative Examples 1 and 2. Oxygen reduction activity was evaluated. The structure of the rotating ring disk electrode used in this example is shown in FIG. The catalyst was supported on the disk portion and the working electrode was rotated to generate a constant convection in the electrolyte solution to control mass transfer.
For each electrode, cyclic voltammetry at a scan rate of 50 mVs ^{-1 in} a potential range of 0.05-1.2 V against a reversible hydrogen electrode (RHE) in 0.1 M perchloric acid at 25 ° C saturated with argon gas Went.
FIG. 3A shows a cyclic voltammogram of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method). The horizontal axis represents the potential, and the vertical axis represents the current value. FIG. 3B is a diagram for explaining the peak of the voltammogram shown in FIG. Both Pt / Au / C prepared in Example 1 and Comparative Example 2 exhibited Pt-specific hydrogen adsorption / desorption and platinum oxide coating generation / reduction peaks. Pt / Au / C prepared in Examples 2 to 4 also showed the same peak.
From the area of the hydrogen desorption peak appearing at 0.05-0.4 V in the obtained voltammogram, the electrochemical surface areas of the Pt / Au / C catalyst (Examples 1 to 4 and Comparative Example 2) and the Pt / C catalyst (Comparative Example 1) (cm ² ) was calculated.

In addition, except for Comparative Example 1, with respect to Au / C not modified with platinum and Pt / Au / C after modification, cyclic voltammetry at a scanning speed of 50 mVs ⁻¹ at a potential of 0.05 to 1.7 V with respect to RHE. Went.
FIG. 4 (a) shows cyclic voltammograms of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method), and the gold-supported carbon support (Au / C) not subjected to the platinum coating treatment. . The horizontal axis represents the potential, and the vertical axis represents the current value. FIG. 4B is a partially enlarged view of the reduction peak portion of the voltammogram shown in FIG. As can be seen from FIG. 4, the reduction peak of the Au oxide film near 1.2 V seen with Au / C was reduced by the coating with Pt. Similarly, in the Pt / Au / C prepared in Examples 2 to 4, a reduction in the reduction peak of the Au oxide film was observed.
The platinum coverage (%) was calculated using the following formula (1) using the reduction peak of gold oxide appearing near 1.2 V.
Formula (1) Coverage (%) =
{[(Au / C peak area) − (Pt / Au / C peak area)] / (Au / C peak area)} × 100

The rotating ring disk electrode is then rotated at a rate of 1600 rpm (rev / min) in 0.1 M perchloric acid saturated with oxygen at 25 ° C. and 0.1 V to 1.0 V relative to RHE at a scan rate of 10 mVs ^−1. The oxygen reduction current flowing up to was measured. FIG. 5 shows convective voltammograms of the Pt / Au / C catalyst prepared in Example 1 (1.0 mM), Example 2 (0.1 mM) and Comparative Example 2 (UPD method), and a gold-supported carbon support (Au / C). Indicates. The horizontal axis represents the potential, and the vertical axis represents the current value. As shown in FIG. 5, the core-shell catalyst produced by the method of the present invention exhibited an oxygen reduction reaction behavior similar to that of the core-shell catalyst produced by the UPD method, with the oxygen reduction current rising from around 1.0 V. Moreover, the same behavior was seen also in Pt / Au / C created in Examples 3-4.
Than the current I in 0.9V, using a limiting current I _L in 0.4V, it was calculated activity dominant current I _K in 0.9V according the following equation (2).
Formula (2) I _K = (I × I _L ) / (I _L −I)

Surface area specific activity (μA cm ^-2 ) obtained by dividing the obtained _IK value by the electrochemical surface area of platinum, and mass activity (A mg (Pt) ^-1 ) divided by the mass of platinum in the catalyst for oxygen reduction activity It was calculated as an index. The mass of platinum in each catalyst was estimated by dissolving the catalyst powder in aqua regia (a mixture of 3 mL of concentrated hydrochloric acid and 1 mL of concentrated nitric acid) and analyzing by inductively coupled plasma emission spectrometry (ICP). Table 1 shows the electrochemical surface area, coverage, surface area specific activity, and mass activity of platinum obtained for the electrodes of Examples 1 to 4 and Comparative Examples 1 and 2.

  From the results shown in Table 1, the platinum core-shell catalyst (Pt / Au / C: Examples 1 to 4) prepared according to the method of the present invention was compared with a commercially available platinum carbon supported catalyst (Pt / C: Comparative Example 1). Thus, both have high surface area specific activity and mass activity. In addition, when compared with the platinum core-shell catalyst prepared using the conventional UPD method (Pt / Au / C-UPD method: Comparative Example 2), the platinum coverage is low, but the surface area specific activity and mass activity are It was confirmed that it would be higher. For this reason, it was found that the platinum core-shell catalyst produced by the method of the present invention exhibits a catalytic activity comparable to that of the platinum core-shell catalyst produced by the UPD method in the oxygen reduction reaction.

  Further, from the results of Example 1 and Example 2, it was found that even if the concentration of platinum ions was changed, no significant change was observed in the coverage ratio or the catalytic activity. Therefore, regarding the amount of platinum ions in the solution, the concentration is not particularly limited as long as sufficient platinum ions are present in the solution to cover the surfaces of the gold core particles with the platinum monoatomic layer.

  In addition, from the results of Example 1 and Example 3, it was found that even when the temperature of the solution was increased, no significant change was observed in the coverage and the catalytic activity. Therefore, in the method of the present invention, it was found that a platinum core-shell catalyst having excellent catalytic activity could be produced even if it was carried out at room temperature without the need to manipulate the temperature. In Example 1, Example 2 and Example 4, the experiment was carried out while keeping the temperature at 30 ° C. in order to make the production conditions completely the same. It has been confirmed that can be manufactured.

  Moreover, from the results of Example 1 and Example 4, it was found that when the immersion time was the same, the coverage decreased when the valence of the platinum complex was increased. Therefore, there is a possibility that the deposition rate of platinum is slower for tetravalent platinum ions than for divalent platinum ions. In addition, as described above, the cyclic voltammogram showed a peak peculiar to platinum regardless of whether it was divalent or tetravalent, and showed the same behavior of oxygen reduction current.

[Example 6] Measurement of UV spectrum of supernatant liquid Based on the same procedure as in Example 1, the gold-supported carbon support was stirred in an aqueous solution of potassium tetrachloroplatinate (II) (K ₂ PtCl ₄ ) for 24 hours. After standing, the supernatant was collected and the UV spectrum was measured. This UV spectrum was compared with the UV spectrum of an aqueous solution of potassium tetrachloroplatinate (II) immediately after preparation (no gold-supported carbon support).
The same experiment was also conducted on an aqueous solution of potassium hexachloroplatinate (IV) (K ₂ PtCl ₆ ). The measurement results are shown in FIG. (a) is a UV spectrum of an aqueous solution of potassium tetrachloroplatinate (II), and (b) is an aqueous solution of potassium hexachloroplatinate (IV).
As shown in the figure, the peak due to the Pt-Cl charge transfer transition near 215 nm and the peak due to zero-valent Pt near 300 nm were almost the same regardless of whether or not the catalyst was prepared. . Therefore, it is considered that a part of the platinum complex in the solution is selectively reduced on the gold core particle to form a platinum core-shell catalyst, and the remaining platinum complex is not reduced but remains in the supernatant.

  In contrast, when gold core particles were immersed in a platinum-containing solution in the presence of a reducing agent, platinum precipitated not only on the gold core particles but also in the solution. Observation of a TEM image of the Pt / Au / C catalyst prepared in the presence of a reducing agent revealed that the particles became larger as the concentration of platinum ions increased. This is presumably because the gold core particles were not covered by a single layer but were covered with a platinum atomic layer that overlapped many layers.

[Consideration of platinum deposition]
In the method of the present invention, if platinum is deposited on the carbon of the gold-supported carbon support, platinum that does not contribute to the reaction increases, so that the mass activity decreases, and the convective voltammograms of Examples 1 to 4 The behavior should not be consistent with the convective voltammogram of Comparative Example 2 (a platinum core-shell catalyst monolayer coated by the UPD method). However, since the catalyst produced by this example has the same mass activity and convective voltammogram behavior as the catalyst produced by the UPD method, the gold core particle is a platinum monolayer as in the catalyst produced by the UPD method. It is believed that a coated catalyst was produced.
Further, as observed in the cyclic voltammogram, the catalyst produced by the method of the present invention has a reduced reduction peak of the gold oxide film as compared with the untreated gold-supported support (Au / C). Furthermore, a hydrogen adsorption / desorption peak and a production and reduction peak of a platinum oxide film appear. The reduction in the reduction peak of the gold oxide film is based on the fact that the gold surface was coated with platinum, and the hydrogen adsorption / desorption peak and the formation and reduction peak of the platinum oxide film are considered to be based on platinum deposited on the gold surface. From the cyclic voltammogram, it can be seen that gold was coated with platinum. Furthermore, when the experiment was carried out by reducing the concentration of platinum ions, as the concentration of platinum ions decreased, that is, the amount of platinum decreased, the hydrogen adsorption / desorption peak and the formation and reduction peak of the platinum oxide film decreased. Along with this, it was observed that the reduction peak of the gold oxide film increased. This also shows that platinum was deposited on the gold surface.
From these, it is considered that the catalyst produced by the method according to the present invention has a core-shell structure in which gold particles are coated with platinum, like the catalyst produced by the UPD method.

  In the process of catalyst preparation, centrifugation and washing with ultrapure water were not performed, and the Pt / Au / C catalyst was allowed to settle, then the supernatant was removed and dried to produce a Pt / Au / C catalyst. In the cyclic voltammogram, changes were observed in the hydrogen adsorption / desorption peak and the platinum oxide film-forming reduction peak. This was because the platinum ion-containing solution was not washed and removed, so the platinum complex remained on the generated catalyst, and the platinum complex was reduced by the potential cycle, and platinum was deposited. It is thought that it precipitated. In order to improve the mass activity of the platinum-based catalyst, it is desirable to deposit platinum as a single atom. Therefore, after manufacturing a platinum core-shell catalyst in a platinum ion containing solution, it turned out that it is preferable to dry a catalyst, after removing an unreacted platinum complex from a catalyst by methods, such as centrifugation and washing | cleaning. In addition, it is not always necessary to use centrifugation for the removal of the platinum complex, and it is possible to sufficiently repeat normal filtration and washing.

[Example 7] Production of platinum core-shell catalyst A gold-supported carbon support (Au / C, gold average particle size: 5 nm, carbon support: particle size 50 nm, specific surface area 800 m ^{2) in} a different production lot from that used in Example 1 Pt / Au / C powder was produced in the same manner as in Example 1, using Ketjen black / g, gold loading density: 29.5 wt%.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 8] Observation with a transmission electron microscope and calculation of particle size distribution and average particle diameter The Au / C carrier used in Example 7 and the Pt / Au / C powder obtained in Example 7 were measured with a transmission electron microscope (TEM). ) And the diameters of 500 noble metal fine particles in the obtained TEM image were measured to obtain a particle size distribution. Further, the average particle size of the noble metal fine particles was calculated from the particle size distribution.
The obtained TEM image is shown in FIG. (a) shows an image of Au / C support, and (b) shows an image of Pt / Au / C powder. It can be seen that the noble metal fine particles on the Au / C support and the Pt / Au / C powder are both supported on the carbon support with good dispersion. FIG. 8 shows the particle size distribution of the noble metal fine particles obtained from the TEM image. (a) shows the particle size distribution of the Au / C support, and (b) shows the particle size distribution of the Pt / Au / C powder. After the platinum atom coating, the particle size distribution is shifted to the large particle size as a whole, but there is no significant change in the distribution state, especially the phenomenon that the small particle size is increasing, It can be seen that the overall particle size was increased by depositing on the gold fine particles without depositing platinum on the carbon. The average particle size calculated from the particle size distribution was 4.4 nm for gold particles on the Au / C support and 5.1 nm for noble metal particles on the Pt / Au / C powder. This average particle size difference of 0.7 nm is close to 0.54 nm, which is twice the atomic diameter of platinum of 0.27 nm, indicating that platinum is deposited in a monoatomic layer on the gold core particles.

[Example 9] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 1 hour.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 10] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 3 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 11] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 6 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 12] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 48 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

[Example 13] Measurement of platinum coverage The platinum coverage (%) of the electrodes obtained in Examples 7 and 9-12 was determined in the same manner as described in Example 5.

[Example 14] Measurement of platinum, gold content and platinum / gold atomic ratio A small amount of the Pt / Au / C powder obtained in Examples 7 and 9 to 12 was taken, and aqua regia (hydrochloric acid and nitric acid in a volume ratio of 3: 1). The concentration of platinum and gold in the solution was determined using inductively coupled high-frequency plasma emission (ICP) analysis. From the obtained results, the platinum and gold contents (% by weight) and the platinum / gold atomic ratio in the noble metal of the Pt / Au / C powder were calculated.

[Comparative Example 3] Monoatomic layer model particle It is assumed that a monoatomic Pt layer (thickness: 0.27 nm, equivalent to platinum atom diameter) is deposited on a spherical gold core particle having a particle size of 4.4 nm. Using the density (19.3 g / cm ³ ) and the density of platinum (21.5 g / cm ³ ), the platinum and gold content (wt%) of the fine gold particles covered with monoatomic platinum and the platinum / gold atom The theoretical value of the ratio was obtained.

  The platinum coverage of the electrodes obtained in Examples 7 and 9-12, and the Pt / Au / C catalyst obtained in Examples 7 and 9-12 and the platinum monoatomic layer model particles obtained in Comparative Example 3 Table 2 shows the platinum and gold content and platinum / gold atomic ratio.

  From the results shown in Table 2, although the platinum coverage and the platinum content increase with the reaction time up to 6 hours, the platinum coverage and the platinum content do not increase even if the reaction is continued thereafter, and become constant. Further, even when compared in terms of platinum / gold atomic ratio, it tends to increase up to 6 hours of reaction, but after that, it becomes constant at about 40%. This value of 40% is very close to the theoretical platinum / gold atomic ratio of 40.6% obtained for the gold core particle having a particle diameter of 4.4 nm coated with the platinum monoatomic layer obtained in Comparative Example 3. I know that there is. That is, in the platinum core-shell catalyst produced according to the method of the present invention, the platinum coating amount can be controlled by the reaction time, and the platinum atomic layer does not cover the monoatomic layer.

  From the above examples, according to the method of the present invention, it is possible to produce a highly active platinum core-shell catalyst by a very simple process compared to the method using the UPD method, and the method using a reducing agent. As compared with the above, it was proved that a platinum core-shell catalyst can be produced efficiently and contributes to a reduction in the amount of platinum.

Claims (6)

A platinum core-shell catalyst characterized in that platinum is directly deposited on the gold core particles by immersing the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions in the absence of a reducing agent. Manufacturing method. A method for producing a platinum core-shell catalyst, wherein a platinum monoatomic layer is formed on the gold core particle by the deposited platinum, and 60% or more of the gold core particle surface is covered with the layer. The method for producing a platinum core-shell catalyst according to claim 1 or 2, wherein the gold core particles are supported on the surface of a support. The method for producing a platinum core-shell catalyst according to claim 3, wherein the gold core particles are gold fine particles having a particle diameter of 1 nm to 30 nm, and the carrier is carbon black. The method for producing a platinum core-shell catalyst according to any one of claims 1 to 4, wherein the solution is an aqueous solution containing 0.1 to 100 mM of the platinum ions. a) a step of adding a carrier carrying gold core particles to water and ultrasonically dispersing the carrier;
The platinum core-shell catalyst according to any one of claims 3 to 5, comprising a step of b) adding a divalent or tetravalent platinum complex to the water and stirring in an inert atmosphere. Manufacturing method.