JP2012240000A - Method for manufacturing catalyst - Google Patents

Abstract

The present invention provides a method for producing a catalyst capable of stably improving the durability against potential cycle of platinum particles whose surface is modified with gold.
A method for producing a catalyst including at least platinum particles made of platinum alloy or platinum, the step of coating a surface of the platinum particles with a copper layer, and the platinum particles coated with the copper layer being converted into gold ions. It includes at least a step of replacing copper in the copper layer with gold by contacting with an aqueous solution, and a step of heat-treating the surface of the platinum particles replaced with gold in a non-oxidizing gas atmosphere.
[Selection figure] None

Description

  The present invention relates to a method for producing a catalyst comprising platinum particles and a conductive carrier carrying the platinum particles, and more particularly to a method for producing a catalyst capable of suitably modifying gold on the surface of the platinum particles.

  Conventionally, surface treatments such as plating, physical vapor deposition (PVD), and chemical vapor deposition (CVD) are generally performed by coating the surface of a substrate such as particles with a different material, while maintaining the properties of the substrate. This may be done to modify only the properties of the substrate surface. On the other hand, this type of surface treatment is used for reducing the amount of expensive metal material used as a coating material, improving the utilization rate, or suppressing characteristic deterioration.

  For example, an electrode for a fuel cell is a catalyst in which platinum particles are supported on a conductive carrier such as granular carbon, and a platinum particle itself is used as a catalyst for an oxygen sensor or the like. Such platinum particles are particles of several nanometers made of platinum or a platinum alloy. In such a catalyst, the surface of the platinum particles and the vicinity thereof function as a catalyst. Therefore, improvement in the utilization rate (reduction in the amount of use) of expensive platinum or a platinum alloy is desired.

  Furthermore, it is known that when such a catalyst is used in an environment where a fuel cell is used, platinum particles may be oxidized and eluted, and may be coarsened by reprecipitation, resulting in a decrease in cell performance. Therefore, a technique has been proposed in which a thin gold layer is formed on a part of the surface of the platinum particles (gold is modified on the surface of the platinum particles) to suppress the oxidation and elution of the platinum particles.

For example, (1) a catalyst containing platinum particles is fixed on an electrode (working electrode), this electrode is immersed in a copper sulfate aqueous solution in a nitrogen atmosphere, and an appropriate reduction potential is applied to the catalyst, Depositing a monoatomic layer on the surface of platinum particles to produce copper-coated platinum particles; (2) rinsing a catalyst containing copper-coated platinum particles with purified water to remove copper ions present in the solution; (3) There has been proposed a technology for producing a catalyst in which a monoatomic layer of copper is replaced with gold by immersing a catalyst containing copper-coated platinum particles in an HAuCl ₄ aqueous solution (see, for example, Patent Document 1).

  Thus, according to the manufacturing method described in Patent Document 1, the surface of platinum particles is coated with a copper layer (copper monoatomic layer) using an underpotential deposition (UPD) method, and this copper layer is made of gold. By performing galvanic substitution, a catalyst in which gold is modified on the surface of platinum particles can be obtained.

Special table 2009-510705 gazette

  Here, the catalyst shown in Patent Document 1 is intended to improve the durability of the platinum particles by modifying the surface of the platinum particles with gold. However, according to experiments described later by the inventors, when a potential cycle such as cyclic voltammetry (CV) is applied to the catalyst, platinum particles are eluted regardless of gold modification. The durability of the platinum particles sometimes deteriorated. In addition, the durability of platinum particles varied greatly from lot to lot.

  The present invention has been made in view of such problems, and the object of the present invention is to provide a catalyst that can stably improve the durability against potential cycles of platinum particles whose surfaces are modified with gold. It is in providing the manufacturing method of.

  As a result of intensive investigations, the inventors have found that gold adhering (modifying) to platinum particles is not sufficiently compatible with platinum, and gold is an electrochemically weak portion of platinum particles (for example, platinum particles). It was thought that this was caused by not covering the highly-reactive portions of the platinum particles, such as convex portions or corner portions, on the surface of the surface. Therefore, the inventors can diffuse (move) gold gold atoms once attached to the platinum particles that are easily eluted by applying thermal energy to platinum modified with gold. I got new knowledge.

  The present invention is based on the inventors' new knowledge, and a method for producing a catalyst according to the present invention is a method for producing a catalyst containing at least platinum particles made of platinum alloy or platinum, wherein the platinum A step of coating a copper layer on the surface of the particles, a step of replacing the copper of the copper layer with gold by bringing the platinum particles coated with the copper layer into contact with a gold ion aqueous solution, and the gold was replaced with gold And a step of heat-treating the surface of the platinum particles in a non-oxidizing gas atmosphere.

  According to the present invention, the surface of the platinum particles substituted with gold is heat-treated in a non-oxidizing gas atmosphere, so that gold gold atoms to which the platinum particles adhere are eluted from the platinum particles on the surface of the platinum particles. It is thought that it can diffuse (move) to an easy part. Thereby, the durability with respect to the potential cycle of the platinum particles whose surface is modified with gold can be stably improved.

  Here, the non-oxidizing gas referred to in the present invention is a gas that does not oxidize with respect to gold and platinum during heat treatment, and a reducing gas such as hydrogen gas or an inert gas such as nitrogen gas or argon gas. Can be mentioned.

  Moreover, as a more preferable aspect, in the heat treatment step, the platinum particles are heat-treated within a heating temperature range of 200 ° C to 400 ° C. According to the present invention, by performing heat treatment within this heating temperature range, gold atoms are preferably diffused into a portion (for example, an edge portion) where platinum particles are easily eluted on the surface of the platinum particles. Can be coated with gold.

  That is, when the heating temperature is less than 200 ° C., gold atoms may not diffuse sufficiently, and when the heating temperature exceeds 400 ° C., gold (gold particles) melts, Platinum alloying may occur.

  Furthermore, the heating temperature of the platinum particles is more preferably 300 ° C. or lower. When the heating temperature exceeds 300 ° C., the gold particles adhering (modified) to the surface of the platinum particles may start to coarsen, and the durability of the catalyst may be reduced.

In the step of replacing copper with gold, the type of aqueous solution is not particularly limited as long as the gold ion aqueous solution is an aqueous solution that can replace copper with gold, such as a salt containing AuCl ₄ ^−. . However, as a more preferable aspect, in the step of replacing copper with gold, an aqueous solution in which a salt containing AuBr ₄ ^— or AuI ₄ ^— and a hydrogen halide are added as the aqueous solution of gold ions is used.

According to this aspect, the platinum particles coated with the copper layer are brought into contact with an aqueous solution to which a salt containing AuBr ₄ ^— or AuI ₄ ^— and a hydrogen halide are added, whereby the copper in the copper layer is converted into gold. Replace. At this time, conventionally, an aqueous solution to which a salt containing AuCl ₄ ⁻ (for example, chloroauric acid (HAuCl ₄ )) is added is used. Compared to this, an AuBr ₄ ⁻ or a potential gap between Cu and Au is small. By using an aqueous solution to which a salt containing AuI ₄ ⁻ is added, the reaction of substituting copper with gold can be moderated, thereby suppressing the formation of coarse gold particles. As a result, the gold atoms by the subsequent heat treatment of the platinum particles are likely to diffuse on the surface of the platinum particles.

Furthermore, since hydrogen halide is added to this aqueous solution, Au ions are stabilized in the state of AuBr ₄ ⁻ or AuI ₄ ⁻ gold complex ions. This is because the same kind of anion (Cl ⁻ , Br ⁻ , I ⁻ or the like) contained in the gold complex ion is added. As a result, the state of the AuBr ₄ ⁻ or AuI ₄ ⁻ gold complex ion having a small potential gap between Cu and Au is maintained, so that the substitution reaction from copper to gold can be made more gentle.

Furthermore, as a more preferable embodiment, HBr or HI is used for the hydrogen halide. According to this aspect, by using HBr or HI, it can be stabilized in the state of the AuBr ₄ ⁻ or AuI ₄ ⁻ gold complex ion described above compared with HCl.

  ADVANTAGE OF THE INVENTION According to this invention, durability with respect to the electrical potential cycle of the platinum particle by which the surface was modified | denatured with gold can be improved stably.

It is a schematic diagram for demonstrating the manufacturing method of the catalyst which concerns on this embodiment, (a) is a partial expanded sectional view of a catalyst, (b) demonstrates the state of the surface of the platinum particle in heat processing. Schematic diagram for It is the figure which showed the result of the potential cycle test of the catalyst concerning Example 1 and Comparative Examples 1-3, (a) is the potential cycle number of the catalyst concerning Example 1 and Comparative Example 1, and the platinum surface area maintenance rate. FIG. 4B is a diagram showing the relationship between the number of potential cycles of the catalysts according to Comparative Example 2 and Comparative Example 3 and the platinum surface area maintenance rate. Example 2, Example 3, and Comparative Example 4 A graph showing the relationship between the number of potential cycles and the platinum surface area retention rate of such catalysts. The reference figure for demonstrating the relationship between the particle size of gold particle | grains, and its melting | fusing point. The reference figure for demonstrating the particle size and count number of the gold particle accompanying a temperature change. The figure which showed the relationship between the potential cycle number of the catalyst concerning Example 4, Example 5, and the comparative example 5, and a platinum surface area maintenance factor.

Embodiments of the catalyst production method according to the present embodiment will be described below.
The method for producing a catalyst according to the present invention is a method for producing a catalyst containing platinum particles whose surface is modified with a platinum alloy or platinum, and includes the following copper layer coating step, gold replacement step, and heat treatment step. After that, the catalyst can be manufactured.

1. Copper layer coating step First, platinum particles or a conductive carrier carrying platinum particles is prepared. The platinum particles are made of a platinum alloy or platinum, and examples of the platinum alloy include a PtFe alloy, a PtMo alloy, a PtCu alloy, a PtRu alloy, a PtSn alloy, a PtW alloy, a PtCo alloy, a PtNi alloy, a PtIr alloy, and a PtAu alloy. The platinum alloy is not particularly limited as long as it is a platinum alloy that can act as a catalyst. Further, the size of the platinum particles is not particularly limited. For example, when used for an electrode catalyst for a fuel cell, 2 to 10 nm is preferable.

Examples of the conductive carrier material include carbon, titanium oxide (TiO ₂ , Ti ₄ O _{7 and the} like), molybdenum oxide, tantalum oxide, and the like. It may be in the form of a rod or net.

  A copper monoatomic layer (copper layer) is coated on the surface of such platinum particles (or platinum particles supported on a conductive carrier) by the UPD method. More specifically, a working electrode (WE) in which platinum particles (or a conductive support) are fixed (adhered) in a solution containing copper ions to be coated, a counter electrode (CE) to the working electrode (CE), and a reference electrode ( D) RE). Next, a copper layer is deposited on the surface of the platinum particles by holding the working electrode at a predetermined potential with respect to the reference electrode using an external power source. Such UPD method is a generally known method as described in Patent Document 1 described above.

As another method, in an inert gas atmosphere, platinum particles or a conductive support carrying the copper ions are poured into an acid aqueous solution containing copper ions in which a copper electrode is immersed, and the acid aqueous solution is stirred to obtain platinum particles or A copper monoatomic layer (copper layer) may be deposited on the surface of the platinum particles by bringing a conductive carrier carrying this into contact with a copper electrode. Examples of the acid aqueous solution containing copper ions include copper sulfate (CuSO ₄ ), copper chloride (CuCl ₂ ), copper acetate (Cu ₂ (CH ₃ COO) ₄ ), and copper nitrate (Cu (NO ₃ ) ₂ ). it can.

The mechanism of copper deposition in this embodiment is as follows. When a copper electrode made of copper or a copper alloy is immersed in the acid aqueous solution described above, the copper electrode becomes a standard electrode potential of copper. When the aqueous acid solution is stirred after adding the platinum particles, the potential of the platinum particles in contact with the copper electrode in this potential state or the conductive carrier carrying the same becomes equal to the standard electrode potential of copper. Here, since the platinum particles are present in an acid aqueous solution containing copper ions, the copper electrode acts as a power source, and after contact, the copper ions are deposited on the surface of the platinum particles almost instantaneously (Pt + Cu ²⁺ + 2e ⁻ → Along with (Cu—Pt), copper of the copper electrode elutes accordingly (Cu → Cu ²⁺ + 2e ⁻ ). According to this method, the surface of the platinum particles can be coated with the copper monoatomic layer (copper layer) without using an external power source.

2. Gold replacement step In the gold replacement step, the copper in the copper layer is replaced with gold by bringing platinum particles coated with the copper layer into contact with a gold ion aqueous solution. Here, as the gold ion aqueous solution, an aqueous solution capable of replacing copper with gold, such as a salt containing AuCl ₄ ^— , may be used, and HCl may be further added to this aqueous solution. However, in this embodiment, the copper particles covered with the copper layer are brought into contact with an aqueous solution containing a salt containing AuBr ₄ ^— or AuI ₄ ^— and a hydrogen halide, whereby the copper in the copper layer is brought into contact. Replace with gold. Thereby, as shown in FIG. 1A, a catalyst in which platinum particles 11 whose surfaces are modified (attached) with gold (particles) 12 are supported on a conductive carrier 10 can be obtained.

Here, examples of the salt containing AuBr ₄ ^— or AuI ₄ ^— include HAuBr ₄ , NaAuBr ₄ , KAuBr ₄ , HAuI ₄ , NaAuI ₄ , KAuI _{4 and the} like. In the solution, AuBr ₄ ^— or AuI ₄ ^{— The} salt is not particularly limited as long as it can be ionized as a gold complex ion. The aqueous solution may further contain an acid such as perchloric acid (HClO ₄ ).

Here, each standard electrode potential of AuCl ₄ ⁻ , AuBr ₄ ⁻ , and AuI ₄ ⁻ is
AuCl ₄ ⁻ + 3e ⁻ = Au + 4Cl ⁻ +1.00 V
AuBr ₄ ⁻ + 3e ⁻ = Au + 4Br ⁻ +0.87 V
AuI ₄ ⁻ + 3e ⁻ = Au + 4I ⁻ + 0.56V
It is.

Therefore, the standard electrode potential decreases as the atomic number of the halogen element increases. Conventionally, substitution from copper to gold was performed using a salt containing AuCl ₄ ⁻ . However, in order to make the potential gap between Cu and Au during the substitution reaction from copper to gold smaller than when using a salt containing AuCl ₄ ⁻ , a salt containing AuBr ₄ ⁻ or AuI ₄ ⁻ is used. It can be seen that it should be used.

Thus, conventionally, an aqueous solution to which a salt containing AuCl ₄ ⁻ (for example, chloroauric acid (HAuCl ₄ )) was added was used. Compared with this, AuBr ₄ has a smaller potential gap between Cu and Au. By using an aqueous solution to which a salt containing ^— or AuI ₄ ^— is added, the reaction of substituting copper with gold can be moderated, thereby suppressing the formation of coarse gold particles. Thereby, the durability of the platinum particles against the potential cycle can be increased.

Further, AuBr ₄ ⁻ and AuI ₄ ⁻ may be changed to Au ³⁺ in an aqueous solution. Here, the standard electrode potential of Au ³⁺ is
Au ³⁺ + 3e = Au + 1.49V
Thus, the standard electrode potential of Au ³⁺ is larger than the standard electrode potential of AuBr ₄ ⁻ and AuI ₄ ⁻ gold complex ions. Therefore, when replacing copper with gold, it is desirable that the aqueous solution be stably present in a gold complex ion state of AuBr ₄ ⁻ or AuI ₄ ⁻ .

Therefore, in the present embodiment, hydrogen halide is further added to an aqueous solution of a salt containing AuBr ₄ ^— or AuI ₄ ^— . Since hydrogen halide is further added to the aqueous solution described above, Au ions are stabilized in the state of AuBr ₄ ⁻ or AuI ₄ ⁻ gold complex ions. This is because the same kind of anion contained in the gold complex ion is added.

As a result, since the state of AuBr ₄ ⁻ or AuI ₄ ^{− with} a small potential gap between Cu and Au is maintained, the substitution reaction from copper to gold can be made more gentle. In particular, by using HBr or HI as the hydrogen halide, it can be stabilized in the state of complex ions of AuBr ₄ ⁻ or AuI ₄ ⁻ described above compared with HCl.

More preferably, it is preferable that hydrogen halide (HBr or HI) is added to the aqueous solution so as to be 200 μM or more. By adding a hydrogen halide having a concentration in this range, the state of AuBr ₄ ⁻ or AuI ₄ ⁻ gold complex ions can be stabilized. On the other hand, hydrogen halide (HBr or HI) is more preferably added to the aqueous solution so as to be 1000 μM or less. When the concentration of hydrogen halide exceeds this range, platinum may be partially poisoned.

3. Heat treatment step In the heat treatment step, the catalyst after the gold replacement step is put into a heating furnace, and the surface of the platinum particles replaced with gold is heat-treated in a non-oxidizing gas atmosphere. Specifically, a reducing gas such as hydrogen gas or an inert gas such as nitrogen gas or argon gas is introduced in the heating furnace, and the platinum particles are more preferably within a heating temperature range of 200 ° C. to 400 ° C. Is heat-treated within a heating temperature range of 200 ° C to 300 ° C. The non-oxidizing gas is not limited to the above-described gas as long as it is a gas that does not oxidize gold and platinum during the heat treatment.

Here, as shown in FIG. 1B, when a gold ion aqueous solution to which, for example, HAuCl ₄ , HAuBr ₄ or the like is added is used in the gold substitution step, a halogen element (for example, Cl atom or Br atom (in the drawing) In this case, it is considered that Cl atoms)) adhere to unstable parts of the platinum particles.

  However, in this embodiment, by performing the heat treatment within the above-described heating temperature range, the halogen element is desorbed from the unstable portion of the platinum particle, and the gold gold atom to which the platinum particle is attached is removed from the unstable state. It is thought that it can be diffused (moved) to a part (for example, an edge part). Thereby, gold | metal | money is modified to the unstable part of platinum particle, and durability with respect to the electrical potential cycle of platinum particle can be improved stably.

  When the heating temperature is less than 200 ° C., gold atoms may not be sufficiently diffused. When the heating temperature exceeds 400 ° C., gold (gold particles) melts, And alloying of platinum may occur. The heat treatment time in the above temperature range is preferably 1 to 4 hours. When the heating time is less than 1 hour, the diffusion of gold atoms hardly occurs, and when it exceeds 4 hours, the gold may be coarsened.

Hereinafter, the present invention will be described by way of examples.
Example 1
First, in the copper coating step, the surface of the platinum particles was coated with a copper monoatomic layer (copper layer) by the UPD method (method not using the above-described external power source). Specifically, to 1 g of platinum-supporting carbon (30 mass% Pt / C) supporting 30 mass% of platinum particles in 25 ml of an aqueous copper sulfate solution (50 mM CuSO ₄ /0.1 MH ₂ SO ₄ ) in which a copper material is immersed. On the other hand, platinum-supporting carbon was added and stirred so that the sulfuric acid solution was 200 ml. As a result, a suspension containing platinum-supported carbon having a copper layer coated on the surface of platinum particles was obtained.

Next, in the gold replacement step, an aqueous solution containing 50 μM HAuCl ₄ and 0.1 M HClO ₄ was prepared, and a suspension containing platinum-supported carbon coated with a copper layer was added and stirred for 30 minutes. Thereby, the copper of the copper layer was replaced with gold. The suspension after stirring was filtered and dried to obtain platinum-supported carbon modified with gold.

  Next, in the heat treatment step, the platinum-supporting carbon was heated to 200 ° C. for 1 hour under an argon gas atmosphere, kept at this temperature for 2 hours, and then allowed to cool. Thereby, gold-modified platinum-supported carbon subjected to heat treatment was obtained.

(Comparative Example 1)
The platinum-supported carbon before coating the copper layer prepared in Example 1 was prepared, and heat treatment was performed under the same heating conditions as in Example 1 without modifying the platinum particles with gold. That is, the comparative example 1 is different from the example 1 in that the platinum particles are not modified with gold.

(Comparative Example 2)
In the same manner as in Example 1, gold-modified platinum-supported carbon was produced. The difference from Example 1 is that the heat treatment step is not performed. That is, Comparative Example 2 is gold-modified platinum-supported carbon that has not been subjected to heat treatment, and corresponds to platinum-supported carbon before heat treatment in Example 1.

(Comparative Example 3)
A platinum-supporting carbon before coating the same copper layer as in Example 1 was prepared, and this was used as the platinum-supporting carbon of Comparative Example 3. That is, Comparative Example 3 is a platinum-supported carbon that is not subjected to heat treatment and in which the platinum particles are not modified with gold, and corresponds to the platinum-supported carbon before heat treatment of Comparative Example 1.

<Measurement of particle size of platinum particles>
The platinum particle size of the platinum particles supported on the platinum-supported carbon of Example 1 and Comparative Examples 1 to 3 was measured by XRD. The results are shown in Table 1.

<Potential cycle test>
A potential cycle test was performed on the platinum-supported carbons of Examples 1 and 2 and Comparative Examples 1 to 3. Specifically, first, platinum-supported carbon was applied onto a glassy carbon electrode. Next, 0.1 N perchloric acid solution was bubbled with nitrogen gas to remove dissolved oxygen, and the electrode was immersed in this solution.

  Next, using a potentiostat, the potential of the electrode was repeatedly changed 5000 times by cyclic voltammetry at a sweep rate of 200 mV / s and in the range of 400 to 1100 mV, and the current with respect to the potential of the electrode was measured. The amount of hydrogen adsorbed on platinum (hydrogen adsorption amount) is calculated using the waveform (cyclic voltammogram) of the voltage value and current value every 1000 cycles from the first cycle obtained at this time. The surface area of the platinum particles was calculated from the amount of adsorption. The ratio of the surface area of the platinum particles for each potential cycle (platinum surface area maintenance ratio) relative to the surface area of the initial platinum particles before the potential cycle test was calculated. The results are shown in FIGS. 2 (a) and 2 (b). 2A and 2B show the results of measuring the platinum surface area maintenance rate for each number of potential cycles.

(Result 1 and discussion)
As shown in Table 1, the particle diameters of the platinum particles of Example 1 and Comparative Example 1 were larger than those of Comparative Examples 2 and 3. The platinum particles of Example 1 and Comparative Example 1 are considered to have increased in particle size due to the heat treatment.

  As is clear from FIG. 2B, the platinum surface area retention rates of Comparative Example 3 and Comparative Example 4 are substantially the same regardless of the number of potential cycles before the heat treatment. That is, it is considered that platinum of the platinum particles is eluted as the number of potential cycles increases regardless of the gold modification to the platinum particles.

  However, as shown in FIG. 2A, after the heat treatment, the platinum surface area retention rate of Example 1 was higher than that of Comparative Example 1. From the above, by performing the above-mentioned heat treatment on platinum particles modified with gold, gold diffuses into the portion where the platinum particles are easily eluted (edge portion), and the durability of the platinum particles can be improved. It is thought.

(Example 2)
As in Example 1, gold-modified platinum-supported carbon subjected to heat treatment was produced. In this example, in the heat treatment step, the platinum-supporting carbon was heated to 200 ° C. for 1 hour under an argon gas atmosphere, this temperature was maintained for 2 hours, and then allowed to cool.

(Example 3)
In the same manner as in Example 2, gold-modified platinum-supported carbon subjected to heat treatment was produced. The difference from Example 2 is that in the heat treatment step, the platinum-supported carbon was heated to 300 ° C. for 1 hour under an argon gas atmosphere, held at this temperature for 2 hours, and then allowed to cool.

(Comparative Example 4)
As in Example 1, platinum-supported carbon before coating the copper layer prepared in Example 1 was prepared and used as platinum-supported carbon in Comparative Example 4.

  A potential cycle test was performed on the platinum-supported carbons of Example 2, Example 3, and Comparative Example 4 described above in the same manner as in Example 1. The result is shown in FIG. FIG. 3 shows the results of measuring the platinum surface area maintenance rate for each number of potential cycles.

(Result 2 and discussion)
As shown in FIG. 3, the platinum surface area retention rates of Example 2 and Example 3 were higher than those of Comparative Example 4. From this, it is considered that the durability of the gold-modified platinum particles is improved if the heat treatment is performed at least within a heating temperature range of 200 ° C. to 300 ° C.

  Table 2 below shows the results of measuring the particle size of the gold particles when the platinum particles were heated. From this result, it can be seen that there is almost no change in the particle size up to the heating temperature of 400 ° C.

  FIG. 4 is a reference diagram for explaining the relationship between the particle size of the gold particles and the melting point thereof, and also from this result, the particle size of the gold increases as the heating temperature rises. When the heating temperature starts to exceed 300 ° C., it is considered that the coarsening of the gold particle diameter starts.

  Further, FIG. 5 is a reference diagram for explaining the particle diameter of gold particles and the count number thereof according to the temperature change. Also from this result, the melting point of gold particles having a particle diameter of about 3 nm is about 400 ° C. It is. From this, it is considered that when a catalyst containing gold is heated at a heat treatment temperature exceeding 400 ° C., the gold is melted and coarsened, and gold and platinum may be alloyed. . From this, it can be said that the heat treatment temperature in the heat treatment step is preferably 400 ° C. or lower.

  Thus, by heating the heat treatment temperature within a range of 200 ° C. to 400 ° C., the gold on the surface of the platinum particles moves to the edge portion (reactive portion) of the platinum particles and covers this portion. Therefore, it is considered that the durability of the platinum-supported carbon that was heat-treated with respect to the potential cycle was improved.

Example 4
As in Example 1, gold-modified platinum-supported carbon subjected to heat treatment was produced.

(Example 5)
As in Example 1, gold-modified platinum-supported carbon subjected to heat treatment was produced. The difference from Example 1 is that the same amount of HAuBr ₄ was used instead of HAuCl ₄ in the gold replacement step.

(Comparative Example 5)
As in Example 1, platinum-supported carbon before coating the copper layer prepared in Example 1 was prepared and used as platinum-supported carbon in Comparative Example 5.

  For the platinum-supported carbons of Example 4, Example 5, and Comparative Example 5, the platinum particle size of the platinum particles supported on the platinum-supported carbon was measured by XRD as described above. The results are shown in Table 3. In addition, the potential cycle test was performed on the platinum-supported carbons of Example 4, Example 5, and Comparative Example 5 in the same manner as in Example 1. The result is shown in FIG. FIG. 6 shows the results of measuring the platinum surface area maintenance rate for each number of potential cycles.

(Result 3 and discussion)
As shown in FIG. 6, the platinum surface area retention rate increased with the increase in the number of potential cycles in the order of Example 5, Example 4, and Comparative Example 5. In Example 4, an aqueous solution to which HAuCl ₄ was added was used. However, in Example 5, an aqueous solution to which HAuBr ₄ having a small potential gap between Cu and Au was used was used. Since the reaction of substituting copper with gold became mild and the formation of coarse gold particles could be suppressed, the platinum surface area maintenance rate of Example 5 was considered to be higher than that of Example 4. It is done.

  As mentioned above, although it explained in full detail using embodiment of this invention, a concrete structure is not limited to this embodiment and an Example, There exists a design change in the range which does not deviate from the summary of this invention. They are also included in the present invention.

Claims (3)

A method for producing a catalyst comprising at least platinum particles comprising platinum alloy or platinum,
Coating the surface of the platinum particles with a copper layer;
A step of replacing the copper of the copper layer with gold by bringing platinum particles coated with the copper layer into contact with a gold ion aqueous solution;
And a step of heat-treating the surface of the platinum particles substituted with gold in a non-oxidizing gas atmosphere.   The method for producing a catalyst according to claim 1, wherein in the heat treatment step, the platinum particles are heat-treated within a heating temperature range of 200 ° C to 400 ° C. In the step of replacing the copper with gold,
3. The method for producing a catalyst according to claim 1, wherein an aqueous solution to which a salt containing AuBr ₄ ^— or AuI ₄ ^— and a hydrogen halide are added is used as the gold ion aqueous solution.

Images