JPH0322361A - Catalyst for fuel electrode of liquid fuel cell and manufacturing method thereof - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a catalyst for a liquid fuel cell that uses a reducing agent such as methanol, hydrazine, formalin, or formic acid as a liquid fuel and air or oxygen as an oxidizing agent, and a method for manufacturing the same. In particular, it relates to a catalyst for methanol fuel cells and a method for producing the same.

Conventional technology Room-temperature liquid fuel cells include an alkaline type that uses a caustic potassium aqueous solution as an electrolyte, and an acidic type that uses a sulfuric acid aqueous solution as an electrolyte.However, considering economic efficiency, it is common to use air as an oxidizing agent. Many studies have been conducted on acidic liquid fuel cells in which the electrolyte does not deteriorate even when air is used. Improving the characteristics of this type of fuel cell is largely influenced by the form of the precious metal catalyst used in the electrode and its manufacturing method, and in particular supporting precious metal particles on carbon particles in a highly dispersed state is considered to be an important technology. There is. Therefore, much research has been conducted on methods of supporting noble metal catalysts. For example, after adding a reducing agent to an aqueous solution of a platinum compound, adding a water-soluble ruthenium compound in the coexistence of hydrogen peroxide and simultaneously introducing hydrogen gas, or by performing hydrogen gas reduction in the final step, platinum- It has been proposed to form binary cluster catalysts of ruthenium. Also, at this time, it was proposed that the catalyst exhibits high activity when the atomic composition of the catalyst is R u /P t = 1 (Japanese Patent Laid-Open No. 6397232).
Publication No.). In all of these conventional methods, catalyst particles are reduced to a metal-state catalyst.

Problems to be Solved by the Invention In such conventional manufacturing methods, in order to produce a catalyst in a metallic state, a catalyst for an air electrode or an oxygen electrode that uses air or oxygen as an oxidizing agent has relatively good polarization characteristics. Although it is obtained,
It had the disadvantage that sufficient polarization characteristics were not obtained as a fuel electrode for methanol or the like. Furthermore, since hydrogen gas is used in the manufacturing process, there are also problems in terms of safety.

The present invention solves the above conventional problems, and provides a catalyst for liquid fuel cells that has a simple manufacturing process, is highly safe, and exhibits high catalytic activity against liquid fuels such as methanol, and a method for manufacturing the same. The purpose is to

Means for Solving the Problems In order to solve the problems, the present invention provides a platinum-ruthenium catalyst highly dispersed on carbon fine particles of an electrode for a liquid fuel cell, in which at least platinum and ruthenium form an oxide. and its atomic composition ratio is 1< R u / P t <
The structure of the liquid fuel cell catalyst falls within the range of 2.

The present invention also provides a step of adding a reducing agent and an anti-flocculant to an aqueous solution of a platinum compound, and further adding a water-soluble ruthenium compound in the presence of the anti-flocculant to form a colloidal dispersion of platinum and ruthenium. A step of mixing the colloidal dispersion liquid with highly dispersed carbon fine powder in a suspended state and supporting binary catalyst particles of platinum and ruthenium on the carbon fine powder; This manufacturing method consists of a step of heat-treating the powder in an oxidizing atmosphere or an inert atmosphere, and forms an oxide catalyst of platinum and ruthenium alone or as an alloy on fine carbon powder.

Function: By using such a catalyst and its manufacturing method, a colloidal dispersion system of platinum and ruthenium formed by a reducing agent and an anti-aggregation agent and fine carbon powder in suspension can be produced without using hydrogen gas in the manufacturing process. The catalyst can be supported on the fine carbon powder in a highly dispersed state by bringing the catalyst into contact with the carbon powder. In addition, the fine carbon powder supporting the platinum-ruthenium catalyst is then heat-treated in an oxidizing atmosphere or an inert atmosphere to form oxide catalysts of platinum and ruthenium alone and alloys. Catalytic activity and durability for fuel can be improved. Further, by setting the atomic composition of the catalyst in the range of 1 < Ru / P t < 2, the catalytic ability for the electrode oxidation reaction of liquid fuel such as methanol can be optimized.

Example Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example 1) FIG. 1 shows a method for producing a catalyst for liquid fuel cells of the present invention. The manufacturing process will be explained below. First, commercially available fine carbon powder (acetylene black, carbon black, activated carbon, etc.) was subjected to hydrophilic treatment using nitric acid treatment, steam treatment, etc., and then this fine carbon powder was used as a single fine carbon powder for supporting a catalyst. Then, chloroplatinic acid (H2P t C j'6) 1
10 g of sodium bisulfite (NaISO3) as a reducing agent is added to 300 ml of an aqueous solution of g to produce a soluble colorless reduced intermediate. Then, the pH was adjusted to 3 to 6, diluted to 11 with water, and then added to 3 as a colloidal aggregation inhibitor.
0% hydrogen peroxide solution (H20:!) 150 m
Add l. This lowers the pH, so readjust to pH = 3-6 with sodium hydroxide (NaOH). After that, a ruthenium chloride aqueous solution (RuCi':+) was prepared using an ultrasonic disperser. Add Oml dropwise. Then, the pH was adjusted to 3 to 6, and 5 g of suspended fine carbon powder (for example, BP-2000 carbon black manufactured by Cabot Corporation), which was highly dispersed using an ultrasonic disperser, was added to this aqueous solution containing platinum and ruthenium. Stir vigorously overnight.

This is filtered, washed with water, dried, and then pulverized into a powder.
A fine carbon powder supporting a ruthenium catalyst was obtained. Next, this catalyst-supported carbon fine powder was heated to 200'C~
A heat treatment was applied at a temperature of 450°C for 3 to 30 minutes.

The amount of ruthenium supported is 1 in atomic ratio to platinum.
/ P t < 2.

It is thought that the following reaction is progressing up to the process of introducing hydrogen peroxide.

H2P t CI! 6+ 3 N a3H S O3
+ 2 H20=H3P t (SO3)zOH+Na
2sO4+ N a C l + 5 H C l
- ・= fl) H3P t (S 03) 20
H + 3 H202= P t 02= 3 H2
0 + 2 H2S 04 (2) The reason why the pH is initially adjusted to 3 to 6 is to complete reaction formula +11. In the next process, hydrogen peroxide finally becomes 1
Add 0 times the amount. Sulfite and excess sodium bisulfite in the platinum complex are oxidized to sulfuric acid using hydrogen peroxide. Here, the pH is again adjusted to 3 to 6.

In the step of adding ruthenium chloride described above, the following reaction proceeds.

RuCJ'3+3/2H:0: =RuO2+3HCi'+1/20: -・----
(At this point, the reason why pH adjustment is required again is from reaction formula (3).
This is to complete the process. Most of the excess hydrogen peroxide is also decomposed by the catalytic action of ruthenium. In this process,
Ruthenium oxide exists in a colloidal state and acts as an adsorption nucleus for platinum oxide, promoting the reaction.

3-30℃ in the above air at a temperature of 200℃-450℃
A is a catalyst-supported fine carbon powder that has been heat-treated for several minutes to form a composite oxide of platinum and ruthenium. This A and fine carbon powder treated with water repellency using a fluororesin were mixed and pressure molded into conductive carbon paper to prepare an electrode substrate. The amount of catalyst was such that platinum was 2 .mu./d. The same effect can be obtained even if the heat treatment is performed after the electrode substrate is fabricated. A lead was attached to this electrode substrate, a methanol electrode was formed, and the monopolar potential of the methanol electrode was measured.

The unipolar potential was measured using a 60°C sulfuric acid aqueous solution (1.5 M
) and methanol (2M). The methanol electrode prepared using this catalyst A is designated as A°.

(Example 2) In Example 1, the atmosphere of the heat treatment process of the fuel cell catalyst and the evaluation electrode was set to be a process performed in nitrogen. After that, the process is exactly the same as in the first embodiment. The catalyst-supported carbon fine powder used in this example is referred to as B. The methanol electrode prepared using this catalyst B is designated as B'.

(Comparative Example 1) In Example 1, the same thing as Example 1 was prepared except that the heat treatment process of the fuel cell catalyst and the prepared electrode for evaluation was not performed. The catalyst-supported carbon fine powder according to this comparative example is designated as C. The methanol electrode prepared using this catalyst C is designated as C°.

(Comparative Example 2) The same fuel cell catalyst and manufacturing method as in Example 1 were used except for the manufacturing method of performing a reduction treatment using hydrogen gas as described in the section of the prior art. The catalyst-supported carbon fine powder used in this comparative example is referred to as D. The methanol electrode made using this catalyst D is designated as D°.

X-ray photoelectron spectroscopy (XPS analysis) was performed to analyze the surface state of platinum and ruthenium in the catalyst-supported carbon fine powder produced in this manner. The measurement results are shown in FIG. 2 and Table 1.

Table 1 <XPS analysis of P t Ru carbon fine powder catalyst> 2nd
The figure shows the Pt4f XPS spectra of catalyst A produced by the catalyst manufacturing method of the present invention and catalysts C and D produced by Comparative Example 1.2. In Catalyst C which was not subjected to heat treatment, it is thought that two or more bonding states consisting of PtO2 and a lower oxide of platinum exist. Catalyst A heat-treated in air is considered to have three or more bonding states consisting of PtOz, PLO, and a lower oxide of platinum. In addition, catalyst D subjected to reduction treatment in hydrogen gas,
It is thought to be in an almost single bond state, with oxygen adsorbed to metallic platinum. From this analysis result,
It was found that the platinum catalyst according to the present invention forms two or more types of composite oxides. Catalyst B heat treated in nitrogen
The analysis results are not shown, but the surface conditions are A and C.
It was an intermediate state.

Next, the methanol pole A゜B'C produced as above
In order to measure the performance of ', D', it was combined with a hydrogen standard electrode and the electrode potential of the methanol electrode relative to the hydrogen electrode potential (NHE) was measured. The measurement results are shown in FIG.

In FIG. 3, methanol electrodes A' and B' produced by the catalyst manufacturing method of the present invention are different from methanol electrodes C' and D of the comparative example.
' shows superior current-voltage performance compared to '. The potentials of methanol poles C' and D' are 0.52 V and 0.41 V (6
0mA/cj), the potentials of A' and B' are respectively 0.
84V, 0.3TV (60mA/a1r). That is, the potentials of A' and B'ltc',D' (7) m1st comparison L
It can be seen that the performance is about 0.04 to 0.18 V (60 mA/aIr).

The reason that the characteristics of electrodes A' and B' are higher than those of C' and D' is thought to be due to the surface condition of the catalyst as shown in Figure 2 and Table 1. It is thought that the methanol oxidation activity was improved by forming a complex oxide such as .

Next, FIG. 4 shows the results of investigating the influence of the atomic composition of the catalyst on the methanol oxidation potential. Catalysts A and B of the present invention and Comparative Example D each have an atomic composition ratio of Ru/Pt=0.5 to 2.
．． 0 range of electrodes, each with a current density of 60 mA/
The electrode potential of the methanol electrode with respect to the hydrogen electrode potential (NHE′) at ad was measured. As a result, the methanol electrodes A and B° according to the present invention showed a different tendency from the comparative electrode D°, and the atomic composition ratio Ru/Pt was 1.2 to 1.
When the value was 5, the oxidation potential showed a minimum value. In the conventional example, it has been proposed that the optimum composition of the platinum ruthenium catalyst is the atomic composition R u /P t = 1, but in the catalyst and production method of the present invention, the catalyst atomic composition a 1 < R u /P
It showed high activity when t<2. When forming a composite oxide as in the present invention, it is considered that the catalyst has an optimum composition for the methanol oxidation reaction within the range shown above.

In addition, when heat treating catalyst-supported carbon fine powder in air,
Below 200°C, there is a problem that the heat treatment effect is small and it takes a long time to obtain an effective treatment effect, and above 450°C, there is a problem that the carbon material of the carrier deteriorates. Therefore, heat treatment within the temperature range of 200 to 450°C is optimal for improving catalyst properties.

In this example, carbon black (BP-2000) manufactured by Cabot Corporation was used as an example of the carbon material, but the same effect can be expected by using at least one of acetylene black, carbon black, and activated carbon. Also,
In particular, it is desirable to use a carrier that has been subjected to hydrophilic treatment such as nitric acid treatment or steam treatment.

In this example, a methanol fuel electrode was used as an example of an electrode for a liquid fuel cell, but it is also possible to apply the present invention to a hydrazine or formalin fuel electrode.

Further, although chloroplatinic acid and ruthenium chloride were used as the noble metal catalyst, similar effects can be expected for materials that are easily oxidized and reduced even if other noble metal salts are used. Furthermore, although a sulfuric acid aqueous solution was used as the electrolyte in the embodiment, it is also effective to use phosphoric acid, trifluoromethanesulfonic acid, or the like.

Effects of the Invention As described above, according to the present invention, a platinum and ruthenium copolymer oxide catalyst can be supported on carbon fine particles in a highly dispersed state, making it possible to obtain a high-performance fuel electrode. At the same time, the manufacturing process is simplified, and it is possible to provide an excellent, highly safe catalyst for liquid fuel cells and a method for manufacturing the same.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the manufacturing method of the present invention in the order of steps, Figure 2
The figure shows catalyst A of the present invention and catalysts C and D according to comparative examples 1 and 2.
Figure 3 shows the spectra of Pt4f obtained by X-ray photoelectron spectroscopy (XPS analysis) of methanol electrodes A' and B' using the catalyst of the present invention and methanol electrode C' of a comparative example,
Figure 4 shows the polarization characteristics of D', and Figure 4 shows the atomic composition ratio of the catalysts of the methanol electrodes A' and B' of the present invention and the conventional methanol electrode D°, and the methanol electrode at a current density of 60 mA/aI. FIG. 3 is a diagram showing the relationship with electric potential.

Claims (8)

[Claims] (1) A catalyst for liquid fuel cells comprising platinum and ruthenium highly dispersed on carbon fine particles of an electrode for liquid fuel cells, characterized in that at least platinum and ruthenium form an oxide. . (2) The platinum catalyst is platinum oxide (PtO), platinum dioxide (PtO)
tO_2), 2 of platinum oxygen adsorbates (PtOads)
The catalyst for liquid fuel cells according to claim 1, which is a composite oxide of more than one species. (3) The liquid fuel cell catalyst according to claim 1, wherein the ruthenium catalyst is an oxide of one or more of ruthenium trioxide (RuO_3) and ruthenium dioxide (RuO_2). (4) The atomic composition ratio of platinum and ruthenium is 1<Ru/P
The liquid fuel cell catalyst according to claim 1, wherein t<2. (5) adding a reducing agent and an anti-aggregating agent to an aqueous solution of a platinum compound, and further adding a water-soluble ruthenium compound in the presence of the anti-aggregating agent to form a colloidal dispersion of platinum and ruthenium; A process of mixing a dispersion liquid and highly dispersed carbon fine powder in a suspended state and supporting binary catalyst particles of platinum and ruthenium on the carbon fine powder, and a process of placing the carbon fine powder supporting platinum and ruthenium in an oxidizing atmosphere. A method for producing a catalyst for a liquid fuel cell, comprising a step of heat treatment in an inert atmosphere, or a step of heat treatment in an inert atmosphere, and forming a composite oxide catalyst of platinum and ruthenium alone or an alloy on carbon fine particles. (6) In the step of forming a colloidal dispersion of platinum and ruthenium, the reducing agent and anti-aggregation agent are sodium bisulfite and hydrogen peroxide, respectively, of the liquid fuel cell catalyst according to claim 5. Production method. (7) The liquid fuel cell catalyst according to claim 5, wherein the heat treatment temperature is 200 to 450°C in the step of heat treating the carbon fine powder supporting the platinum-ruthenium catalyst in an oxidizing atmosphere or an inert atmosphere. manufacturing method. (8) In the step of supporting a platinum-ruthenium catalyst and heat-treating it, a binder is mixed with fine carbon powder supporting a platinum-ruthenium catalyst, the mixture is press-molded, and then heat-treated in an oxidizing atmosphere or an inert atmosphere. A method for producing a catalyst for liquid fuel cells according to claim 5.