JP2010198885A - Platinum ordered lattice catalyst for fuel cell, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a platinum catalyst of high performance for a fuel cell, and its manufacturing method. <P>SOLUTION: This is a platinum ordered lattice catalyst in which a platinum-cobalt ordered lattice alloy of L1<SB>0</SB>structure is made as a nucleus, and the platinum of the L1<SB>2</SB>structure is made as an outer layer. This is the manufacturing method of the platinum ordered lattice catalyst for the fuel cell including a process of carrying platinum-cobalt alloy particles consisting of A1 structure on a catalyst carrier, a process of changing this platinum-cobalt particles into a platinum-cobalt ordered lattice alloy particles of L1<SB>0</SB>structure, and a process of eluting cobalt from this platinum-cobalt ordered lattice alloy particles, and forming the outer layer composed of platinum of L1<SB>2</SB>structure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a platinum catalyst suitable as a catalyst for a cathode electrode of a fuel cell and a method for producing the same. More specifically, the present invention relates to a platinum ordered lattice having fine and highly dispersed platinum ordered lattice particles and high durability in an acidic atmosphere. The present invention relates to a method for producing a catalyst.

  Since the polymer electrolyte fuel cell is small and can take out a high current density, it is considered to be used as an automobile, a distributed power source for home use, and a portable power source.

  In this polymer electrolyte fuel cell, a catalyst in which metal particles mainly composed of platinum are supported on carbon is used. However, since platinum is expensive, the supported platinum is effectively used to increase the activity of the electrode catalyst. Need to increase. Thus, in order to increase the activity of the catalyst, a production method capable of carrying a finer and highly dispersed platinum particle has been proposed.

  In addition to the above-described methods for improving the fineness and high dispersion of platinum, a platinum alloy catalyst has been proposed in order to improve the activity of the catalyst. However, since the platinum alloy catalyst is unstable under the condition that oxygen is supplied in an acidic atmosphere in the fuel cell, various improvements have been made to stabilize the platinum alloy catalyst.

  For example, Patent Document 1 describes that durability and high activity can be obtained by removing a base metal element while maintaining a crystal form from a substituted solid solution alloy and producing a platinum skeleton alloy catalyst having a vacancy type defect structure. is there.

  Patent Document 2 proposes a fuel cell catalyst that can achieve high durability by covering metal particles that are less oxidized than platinum under acidic conditions with platinum.

  Patent Document 3 proposes a fuel cell catalyst that suppresses elution of additional elements by a core-shell structure having a noble metal-containing alloy layer as a core portion and a noble metal-containing layer having a composition different from the core portion as a shell portion, and has excellent life characteristics. .

Japanese Patent Laid-Open No. 10-69914 Japanese Patent Laid-Open No. 2002-289208 JP-A-2005-135900

  However, although the method of Patent Document 1 described above can be expected for the long-term stability of the catalyst, it cannot be said to be sufficient in terms of enhancing the catalytic activity, and fine platinum alloy particles can be formed on carbon in a highly dispersed manner. Therefore, even if the activity of individual particles is improved by alloying, there is also a problem that the decrease in activity due to the increase in particle diameter cancels out.

  Further, in the method of Patent Document 2, the metal particles are formed and then covered with platinum. However, platinum may not be completely covered with the metal particles, and the platinum particles may be supported on carbon. This is not sufficient in terms of long-term stability problems and catalyst activity due to the inability to perform the process.

  Furthermore, although the method of Patent Document 3 can be expected for the long-term stability of the catalyst, it cannot be said to be sufficient in terms of enhancing the catalytic activity, and the technique of performing heat treatment after the formation of the core-shell structure has a high noble metal content. There was also a problem that the crystal structure changed to the same FCC structure as platinum, resulting in a decrease in activity.

  In view of the above-described problems of the prior art, an object of the present invention is to provide a fuel cell excellent in durability in which platinum regular lattice particles are supported in a fine and highly dispersed state, highly active and capable of suppressing deterioration due to elution in an acidic atmosphere. The object is to provide a catalyst and a method for producing the same.

  As a result of various studies, the present inventors have found a platinum ordered lattice catalyst and a method for producing the same in order to achieve the above object, and have completed the present invention.

That is, the object is active and of high durability L1 ₀ structure platinum - cobalt superlattice alloy as a core, a platinum L1 ₂ structure is achieved by a platinum ordered lattice catalysts outer layer.

The above object, A1 structure platinum in a highly dispersed fine - cobalt alloy particles supported, platinum - platinum cobalt alloy particles L1 ₀ structure - is changed to cobalt ordered lattice alloy particles, by eluting the cobalt L1 This is achieved by a method for producing a platinum ordered lattice catalyst characterized by covering platinum-cobalt ordered lattice alloy particles with _two- structure platinum.
In order to obtain a catalyst having high activity and high durability according to the present invention, the A1 structure platinum-cobalt alloy particles are required to be formed at a low temperature of 400 ° C. or lower.

Incidentally, A1 structures described in this application FCC structure, L1 ₀ structure AuCu structure among FCT structure, L1 ₂ structure are what are referred to as Au ₃ Cu structure among FCT structure. It is considered that the outer layer L1 ₂ structure platinum ordered lattice on the platinum-cobalt ordered lattice alloy of L1 ₀ structure is suitable for the four-electron reduction reaction of oxygen.

The catalyst according to the present invention significantly improves the catalytic activity due to the effect of the outer layer L1 ₂ structure platinum and the core L1 ₀ structure platinum-cobalt ordered lattice alloy, and the fine platinum ordered lattice particles are uniformly distributed on the support. The catalytic activity can be further improved by the synergistic effect by producing the catalyst supported in high dispersion.
Also, platinum superlattice catalyst of the present invention, conventional platinum - the superior cobalt alloy catalyst performance is obtained by platinum L1 ₂ structure of the outer layer, remarkable improvement in durability while maintaining high catalytic performance Because it is made.

The oxygen reduction reaction occurring at the cathode electrode in the reaction in the fuel cell is rate-determining because it is the slowest of all the catalytic reactions. In order to improve the oxygen reduction activity of the catalyst used in the cathode electrode, a noble metal alloy catalyst has been proposed, and a platinum-cobalt alloy catalyst or the like is well known, but the catalyst activity is not sufficient and durability is also high. It is insufficient.
The environment in which the electrode catalyst is used is in the vicinity of a strongly acidic electrolyte membrane. Under such circumstances, there is also a problem that the alloyed metal is eluted.
Therefore, in the present invention, platinum L1 ₀ structure of platinum superlattice consisting of L1 ₂ structure - adopts a structure that covers the cobalt superlattice alloy particles. L1 ₂ structure platinum and L1 ₀ structure platinum - it is possible to improve the activity and durability due to cobalt alloy. And according to such a particle structure, since elution of a metal is suppressed, the problem of elution can also be solved.
Hereinafter, embodiments of the present invention will be specifically described.

The present invention is a fuel cell catalyst in which platinum regular lattice particles are supported on a catalyst carrier.
By "platinum superlattice particles" in the present application, L1 ₀ structure of the platinum - cobalt superlattice alloy as the core, the platinum L1 ₂ structure and the outer layer means a metal particle, characterized in that. The - "platinum L1 ₀ structure nucleus consisting of cobalt ordered lattice alloy", platinum consisting L1 ₀ structure - aspects that cobalt core - and manner of a cobalt core, platinum consisting L1 ₀ structure and L1 ₂ structure Including both. In the latter embodiment, in order to improve the oxygen reduction activity is preferably larger the ratio of L1 ₀ structure.
The fuel cell catalyst in which the platinum ordered lattice particles having the above structure are supported on a catalyst carrier is referred to as a “platinum ordered lattice catalyst” in the present application.

  The platinum regular lattice particles have an average particle diameter of 1 to 10 nm, more preferably 2 to 5 nm. When the particle diameter exceeds 10 nm, the active sites of the platinum particles decrease, and the activity improvement due to the crystal structure is negated. On the other hand, if the thickness is less than 1 nm, it enters into the pores of the catalyst carrier and does not contribute to the oxygen reduction reaction in the fuel cell, leading to a decrease in activity, or a problem that durability is reduced due to elution of particles.

  The supported amount of the platinum ordered lattice particles on the catalyst carrier is 0.1 to 60 mass%, preferably 1 to 50 mass%. If the amount is less than 0.1 mass%, a sufficient amount of catalyst cannot be obtained because the amount of the catalyst is insufficient, and if it exceeds 60 mass%, there is a problem that aggregation of particles occurs and the utilization rate of platinum decreases.

  The catalyst carrier is not particularly limited, and those conventionally used as catalyst carriers can be used as well, and in particular, carbon black, acetylene black, carbon nanotubes, carbon nanohorns, fullerenes, and other conductive oxides. Carriers such as products are preferred.

The platinum ordered lattice catalyst having the above structure can be produced through the following steps a to c.
a) A step in which platinum-cobalt alloy particles having an A1 structure are supported on the surface of a carbon carrier by attaching a binuclear complex comprising platinum and cobalt to a catalyst carrier and alloying the binuclear complex at the same time as reduction.
In addition to heat treatment cobalt alloy particles, L1 ₀ structure of platinum - - b) The platinum step of changing the cobalt ordered lattice alloy particles.
c) The L1 ₀ structure platinum - an outer layer of cobalt superlattice alloy particles (surface layer portion), eluting the cobalt, to form an outer layer of platinum of L1 ₂ structure.

  Although the implementation method of each process is not specifically limited, It will be as follows when a desirable example is given and demonstrated.

The platinum-cobalt alloy particles having an A1 structure used in the present invention must have a composition ratio (molar ratio) of platinum to cobalt of 4: 6 to 6: 4.
However, even when platinum and cobalt having this composition ratio are used, platinum-cobalt alloy particles having an A1 structure described later cannot usually be obtained.

  In the production process of platinum-cobalt alloy particles having an A1 structure in the present invention, a binuclear complex composed of platinum and cobalt is attached to a catalyst carrier, and the binuclear complex can be alloyed simultaneously with reduction. For example, a platinum-cobalt binuclear complex with an oxalic acid group and a catalyst carrier are mixed and then dried, and the resulting carrier with the platinum alloy complex attached thereto is reduced gas such as hydrogen or carbon monoxide or Heat treatment is performed at 200 ° C. to 400 ° C., preferably 200 ° C. to 300 ° C. in an inert gas atmosphere such as nitrogen, helium, or argon. Thereby, it is possible to obtain platinum-cobalt alloy particles having an A1 structure by alloying simultaneously with the reduction. In addition, according to the above-described method using platinum oxalate and cobalt oxalate, it is possible to form an alloy having an A1 structure at a low temperature, so that the growth of particles during heat treatment is suppressed and fine alloy particles are supported. Can do.

L1 ₀ structure of platinum - The manufacturing process of the cobalt ordered lattice alloy particles, for example, the platinum produced in step - hydrogen cobalt alloy particles, inert gas atmosphere such as a reducing gas or nitrogen, helium or argon, such as carbon monoxide Then, heat treatment is performed at 500 ° C. to 1000 ° C., preferably 500 ° C. to 800 ° C. Thus, platinum consisting A1 structure - cobalt alloy particles, platinum L1 ₀ structure according to the invention - can be changed to a cobalt ordered lattice alloy particles.

Incidentally, platinum L1 ₀ structure according to the invention - cobalt superlattice alloy particles can be formed by obtaining the A1 structure. The A1 structure formed at high temperature platinum L1 ₀ structure suitable for the fuel cell - not possible to obtain a cobalt superlattice alloy.

In the step of eluting cobalt from the outer layer portion of the platinum-cobalt ordered lattice alloy particles obtained in the above step, for example, an inorganic acid such as nitric acid, sulfuric acid or hydrochloric acid or an organic acid such as oxalic acid or acetic acid is allowed to act on the alloy particles. Thus, cobalt on the surface can be eluted. Thus, the platinum L1 ₂ structure and the outer layer, L1 ₀ structure of the platinum - containing cobalt ordered lattice alloy core, can be obtained platinum superlattice catalyst.

  The present invention will be described in the following examples, but is not limited to the embodiments.

Example 1
Disperse 48g of carbon support (Ketjen Black EC300J) in a platinum and cobalt binuclear complex solution with oxalic acid groups containing 40g of platinum and 12g of cobalt, and mix and dry with a rotary evaporator. To attach a binuclear complex to the surface of the carbon support and reduce platinum and cobalt at 300 ° C in a nitrogen atmosphere to obtain 50 mol% platinum-50 mol% cobalt alloy supported carbon (platinum support rate = 40 mass%) with A1 structure. . Platinum of the A1 structural - to obtain a cobalt-ordered lattice alloy-supported carbon - cobalt alloy-supported carbon in a nitrogen atmosphere, platinum containing only L1 ₀ structure can be heat-treated at 800 ° C.. Platinum of the L1 ₀ structure - 4 hours cobalt ordered lattice alloy-supported carbon in 1 mol / L nitric acid aqueous solution of 25 ° C., eluting the cobalt by dispersing the platinum ordered lattice L1 ₂ structure and an outer layer, L1 ₀ platinum structure - cobalt superlattice alloy to obtain a conversion rate of 50% platinum superlattice catalysts -1 to L1 ₂ structure at the core.

The conversion rate mentioned above was calculated | required according to the following formula.
[(L1 ₂ abundance after elution with acid - abundance of L1 ₂ before elution with acid) / L1 ₀ abundance of prior to elution with acid] × 100

(Example 2)
Disperse 48g of carbon support (Ketjen Black EC300J) in a platinum and cobalt binuclear complex solution with oxalic acid groups containing 40g of platinum and 12g of cobalt, and mix and dry with a rotary evaporator. To attach a binuclear complex to the surface of the carbon support and reduce platinum and cobalt at 200 ° C in a nitrogen atmosphere to obtain 50 mol% platinum-50 mol% cobalt alloy supported carbon (platinum support rate = 40 mass%) of A1 structure. . Platinum this A1 structure - cobalt alloy-supported carbon in a nitrogen atmosphere, the molar ratio of the L1 ₀ structure and L1 ₂ structure can be heat-treated at 500 ° C. is 1: 0.5 platinum - obtain cobalt ordered lattice alloy-supported carbon. The platinum-cobalt ordered lattice alloy-supported carbon of L1 ₀ structure and L1 ₂ structure is dispersed in 1 mol / L nitric acid aqueous solution at 25 ° C for 4 hours to elute cobalt, and the platinum ordered lattice of L1 ₂ structure is outer layer. and then, L1 ₀ structure of platinum - to obtain cobalt ordered lattice alloy conversion rate of 40% platinum superlattice catalyst into L1 ₂ structure the core 2.

Mol ratio of L1 ₀ structure and the L1 ₂ structure platinum superlattice catalyst of the present invention, the particle size, oxygen reduction activity, the amount of elution of cobalt into sulfuric acid solution shown in Table 1.

Each measurement method is as follows.
Mol ratio of L1 ₀ structure and L1 ₂ structure: It depends on the peak area of XRD.
Particle size: Calculated from a TEM photograph.
Oxygen reduction activity: Measured under the conditions of electrolyte = 0.5 mol / L sulfuric acid aqueous solution (oxygen saturation), rotation speed = 1500 rpm, potential sweep speed = 5 mV / sec, and evaluated by current value per unit platinum weight at 0.85V.
Cobalt elution amount: Dispersed in a 1 mol / L sulfuric acid aqueous solution at 80 ° C. for 96 hours, and the cobalt elution amount was evaluated from the cobalt concentration in the sulfuric acid solution.

Claims (2)

L1 ₀ structure of the platinum - cobalt superlattice alloy as the core, platinum ordered lattice catalyst platinum L1 ₂ structure and the outer layer. In the method for producing a platinum ordered lattice catalyst,
Obtaining platinum-cobalt alloy particles having an A1 structure on a catalyst support;
Cobalt alloy particles, L1 ₀ structure of platinum - - platinum and step of changing the cobalt ordered lattice alloy particles,
The platinum - cobalt was eluted from the cobalt ordered lattice alloy particles, a step of forming an outer layer consisting of platinum L1 ₂ structure,
A method for producing a platinum ordered lattice catalyst for a fuel cell, comprising: