JPS60212230A - Preparation of electrode catalyst - Google Patents

Abstract

PURPOSE:To prepare highly active non-platinium catalyst for a methanol fuel battery, by synthesizing carbide of W and/or Mo so as to allow carbide with low carbon concn. to be present in a mixed state, and thereafter, oxidizing and eluting mixed carbide to obtain a high surface area. CONSTITUTION:Carbide of W and/or Mo is synthesized so as to allow carbide with low carbon concn. to be present in a mixed state. When a CVD film formed by the gas phase thermal decomposition of W/Mo carbonyl compounds is carbonized at 820 deg.C by CO-gas for the purpose of obtaining W/Mo carbide, a mixed phase of (W1-xMox)C[gamma-phase] and (W1-xMox)2C[beta-phase] is obtained. When this catalyst is immersed in an aqueous sulfuric acid solution, the beta-phase present in the surface of W/Mo carbide is selectively oxidized to be eluted and, as a result, only the gamma-phase remains in the surface thereof. Next, by placing this catalyst under positive polarization, the elution of the beta-phase can be promoted. The surface of the catalyst after the beta-phase was perfectly removed has a large roughness factor and is increased in its surface area. By this method, W/Mo carbide having extremely high catalytic activity per wt. can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for producing a catalytic catalyst, and in particular to a method for producing a tungsten and/or molybdenum carbide m-ridge catalyst suitable for a methanol electrode catalyst in a fuel cell, particularly a methanol fuel cell. Regarding the law.

[Background of the invention]

In addition to expensive platinum-based noble metals, tungsten and molybdenum carbide are known as conventional methanol electrolyte catalysts. As a manufacturing method, a catalyst synthesis method is known in which a thin film of tungsten and molybdenum is impregnated onto an electrode substrate by gas-phase thermal decomposition of a carbonyl compound, and this is carbonized with carbon oxide (Japanese Patent Application Laid-Open No. 140975/1983). It is possible to obtain highly active catalysts. However, the electrocatalytic performance is still insufficient for use in practical fuel cells.

[Purpose of the invention]

An object of the present invention is to provide a method for synthesizing a highly active non-platinum catalyst for methanol fuel cells.

[Summary of the invention]

The tungsten-molybdenum carbide catalyst synthesized by the above CVD method can be used to produce M O2C, etc., depending on the synthesis conditions.
A lower carbide with a carbon to metal concentration ratio of 1 or less is used as a second
Although they may be contained as a phase, we have found that by placing the catalyst under anodic polarization in an aqueous sulfuric acid solution, these lower carbides can be oxidized and eluted. Considering that this principle can be used to increase the surface area of a catalyst and increase its activity, we invented the following catalyst synthesis method.

CV with W(CO)s, Mo(CO)a as source
A W-MO alloy film is formed on an electrode substrate made of carbon fiber by method D. CVD so that when the composition of the alloy door is expressed as WI + g M Ox, it becomes 0.05 x 0.8.
If this is carried out, a highly active catalyst (Wl-, MO,)C will be produced after carbonization.
You can get angry when you get it. However, MO becomes MOC after carbonization.
It is more likely to produce a lower carbide having a composition of Mo and c, that is, the concentration ratio of carbon to metal is less than 1, and the catalytic activity of this carbide is extremely low. Therefore, it has high activity (W+
-Mo-) In order to obtain a single phase with the composition C, 0
．． It is desirable that 05<;:x<:0.5.

CVD is performed at 450C for 2 to 7 hours, but the substrate is
In order to completely cover the MO alloy film, CVD for 5 hours or more is desirable. When performing the above CVD, the MO concentration X during CVD is changed as follows as a means of synthesizing a catalyst with a large surface area.

That is, for the first few hours, 0.05<-x<0.8 is set, and a CVD film that becomes a highly active catalyst after carbonization is synthesized. For the next several hours, c'VD is performed with the Mo concentration increased to form an alloy film having a composition of 0.5 x 1.0. After carbonization, such a CVD film tends to become a lower carbide having a composition of (W+-Mo-)*C, and this phase is later oxidized and eluted. In order to easily obtain lower carbides, it is preferable to use Mo.
It is desirable that the concentration be 0.8 <X < 1.0.

Finally, the Mo concentration is lowered and set to 0.05<:x<0.8 again, and CVD is performed.

The time for increasing the Mo concentration to 0.5 <X < 1.0 is preferably 5% to 50% of the total CVD time.

When the CVD film produced as described above is carbonized with 820C'''cCO gas, a mixed phase of (Wl-RMOx)C (r phase) and (Ws-Mo-)2CCβ phase is obtained.

When this catalyst is immersed in an aqueous sulfuric acid solution, the β phase present on the surface is selectively oxidized and eluted, and as a result, only the r phase remains on the surface. By placing the catalyst under anodic polarization, the elution of the β phase can be promoted. The catalyst surface from which the β phase has been completely removed has a large roughness factor and a significantly increased surface area.

By the above method, tungsten molybdenum carbide having extremely high catalytic activity can be obtained.

[Embodiments of the invention]

The present invention will be explained in detail below.

Example 1 Made of carbon fiber, thickness 0.5 tan, size 13×
A 30 m long electrode substrate 1 was suspended in a quartz reaction tube 2 as shown in FIG. A vacuum pump (not shown) inside the reaction tube 2
The substrate 1 is heated to 450 c using the infrared image furnace 3 while being evacuated using the infrared image furnace 3. Cooling air is flowed into the outer shell 4 of the reaction tube 2 to maintain the inner wall temperature of the reaction tube 2 at 150C or less.

W (temperature of COLa 7 was set to 5C using constant temperature baths 5 and 6.

The temperature of Mo(CO)s8 is maintained at IC, and the evaporated gas of these compounds is introduced into the reaction tube 2, and C is heated on the substrate 1.
VD is performed to attach a tungsten-molybdenum alloy film. While measuring the CO gas partial pressure generated at this time, the flow rate of the evaporated gas is adjusted using valves 9 and 10 to determine the molybdenum concentration X in the alloy film. For the first three periods, x=0.
25, that is, (Wo, ys MOo, *m), but for the next hour, valve 9 is closed and only Mo is CVDed. For the last hour, (We, ys MOo, *m) is made again.
+ M O9,18) metal film is made. By performing such CVD on a highly uneven substrate, the surface of the Si alloy film has a mixture of regions with a composition of (We, 7g, MOo, 2s) and regions where only MO exists. Become. After closing the valves 9 and 10 to stop the CVD, the temperature of the substrate 1 was raised to 8200°C, and the valve 11 was operated to introduce CO at 50° Torr into the reaction tube 2, and this was maintained for 2 hours to form a CVD alloy film. carbonize.

During this time, the molybdenum concentration x = 0.25 in the CVD film
The region becomes completely carbonized (Wo, 75 M O0,2
5) C [r phase] is formed, but the region where x; becomes. After exhausting the CO gas, the heating is stopped, and after cooling to room temperature, the catalyst-equipped substrate 1 is taken out of the reaction tube 2.

A gold wire is attached to the substrate 1 to serve as an electrode.
Immerse in 50 tl'1 molar sulfuric acid aqueous solution.

The electrode potential is set to 0.5 V with respect to the reversible hydrogen electrode (R) (g) K, and Mo@C is oxidized and eluted. This operation can roughen the catalyst surface and increase the roughness factor.

After complete elution of More, methanol (CHsOH
) is added to a concentration of 5 mol/l, and the oxidation current of methanol due to the reaction CHsOH+8g0−+COz+6H”+66− of the following formula is measured.

Table 1 shows the electrode catalyst (A) synthesized by the above method,
(W
o, 78 MGo4s)C electrode catalyst (B)
The figure shows the methanol oxidation current and the BET surface area of the catalyst.

These results show that the tungsten molybdenum carbide catalyst synthesized using the method of this example has an increased surface area and higher catalytic activity per weight than the catalyst obtained by the conventional method.

Example 2 Mo concentration X during CVD was set as follows, and three types of catalysts C, D, and E were synthesized. That is, for the first 3 hours, X=0.
An alloy film having a composition of (We, 78 M 0 OJS ) as 25 is prepared. For the next hour, the concentration of Mo is set to X=0.5
(C), 0.8 (D), 0.9 (E), and 0.03 types and perform CVD. For the last hour, x=
Set to 0.25.

After CVD was carried out under the above conditions, a tungsten molybdenum carbide catalyst was synthesized in exactly the same manner as in Example 1.

Methanol oxidation current and BET surface area of this catalyst Comparing these results with the results in Table 1 of Example 1 shows that M
It can be seen that when increasing the o concentration 0.8 <x <1. It is clear that o is desirable.

〔Effect of the invention〕

According to the present invention, since it is possible to synthesize tungsten-molybdenum carbide with a large surface area, it is possible to obtain an activity improvement effect of about 2.5 times compared to catalysts produced by conventional synthesis methods, and its industrial value is extremely large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a catalyst manufacturing apparatus used in the present invention. DESCRIPTION OF SYMBOLS 1... Electrode base, 2... Reaction tube, 3... Infrared image furnace, 4... Outer shell part, 5, 6... Constant temperature bath, 7
...W(Co)s, ■ 1 Art and crafts

Claims (1)

[Claims] 1. A method for producing an electrocatalytic catalyst, which comprises synthesizing tungsten and/or molybdenum carbide so that carbide with a low carbon concentration is mixed therein, and then oxidizing and bathing it to obtain a surface area.