JPH045493B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION Technical Field The present invention relates to electrocatalysts. More specifically, the present invention relates to an electrode catalyst for methanol electrolytic oxidation, which comprises a conductive base material and a coating layer having high methanol electrolytic oxidation catalytic activity and good durability.

Prior art and its problems Starting with the depletion of oil resources, the effective use of energy is becoming an urgent issue for human society.
The use of _C1 fuel as an energy source has attracted attention in recent years. Methanol is a typical example of this _C1 fuel.

This is because methanol is expected to be available in large quantities and at low cost from coal, natural gas, biomass, etc. in the future.

One way to utilize methanol as an energy source is to utilize the chemical energy of methanol.
There are proposals to convert it into electrical energy using an electrochemical reactor.

In this case, in order to convert the chemical energy of methanol into electrical energy, an electrochemical reactor (for example, a fuel cell) may be configured using an anode having catalytic activity and using the methanol electrolytic oxidation reaction as the anode reaction.

At this time, it can be used as high-quality electrical energy, and
It is possible to save power. In addition, if the chemical energy of methanol is efficiently converted to electrical energy, the available energy associated with the 6-electron oxidation reaction of methanol to carbon dioxide gas is, in principle, per ton of methanol in terms of electrical energy.
It will be more than 5000Kwh, making it possible to use energy effectively.

Furthermore, if chemical energy is converted to electrical energy via an electrochemical reactor, unlike current power generation methods using heat engines, it is essentially free from the limitations of the Carnot cycle. Therefore, high thermal efficiency can be expected.

In this way, an electrochemical reactor is configured,
One of the attempts to effectively utilize the chemical energy of methanol for industrial electrolysis is to apply it to electrowinning from sulfuric acid-zinc sulfate solution to reduce power consumption.

In other words, in the current electrolytic method, the anode reaction is
This is an oxygen evolution reaction that takes place at 40 to 60°C, and the standard electrode potential for this reaction is 1.200V at 60°C, based on the hydrogen electrode.

On the other hand, when methanol is dissolved in an electrolytic solution and an anode having catalytic activity is used, the anode reaction can be replaced from an oxygen generation reaction to an oxidation reaction of methanol to carbon dioxide gas.
The oxidation reaction to CO ₂ is 0.034V at 60°C based on the hydrogen electrode.

That is, methanol is dissolved in the electrolyte,
By using a predetermined anode and incorporating an electrochemical reactor into the electrolytic cell itself, the electrolytic cell voltage is
In principle, compared to the current law, the difference between the two
It is expected that it will be possible to devalue 1.166V. and,
At this time, as an electrolytic energy source, it is theoretically possible to save 5,852 KWh of electrical energy per ton of methanol.

Furthermore, when a methanol-oxygen fuel cell is configured using an anode with catalytic activity as an electrochemical reactor, it is useful as a power generation device, and can be used as a future power generation device along with other hydrogen-oxygen fuel cells. It's promising.

By the way, platinum group elements such as platinum, palladium, ruthenium, rhodium, and iridium are conventionally known as substances having electrode reaction catalytic activity, and electrodes in which these are provided as a coating layer on a base material have been used in various ways. Used as an electrode for electrolytic reactions.

However, as an anode for methanol electrooxidation as described above, only when platinum is used as a coating layer, significant methanol electrooxidation catalytic activity is exhibited.
Other palladium, rhodium, etc. do not exhibit methanol electrocatalytic ability.

However, even when using an anode with a platinum coating layer, although it shows a certain degree of electrolytic oxidation catalytic activity at the beginning of methanol electrolytic oxidation, it is still insufficient, and as the electrolysis time progresses, the catalytic activity decreases significantly. This will cause a decline. That is, the resulting electrolytic current density is still unsatisfactory, the life span is short, and practically satisfactory characteristics cannot be obtained.

This decrease in catalytic activity over time of electrolysis is due to the adsorption and deposition of oxidation intermediate products on the electrode surface, which are thought to be generated during the process in which methanol reaches carbon dioxide, the final product of the electrolytic oxidation reaction. It is assumed that this is due to the catalyst activity being poisoned.

Under these circumstances, in order to put into practical use the technology that converts the chemical energy of methanol into electrical energy, it is necessary to use a coating layer forming material that has high methanol electrolytic oxidation catalytic activity and that does not poison the catalytic ability during electrolysis. It is desired to develop a long-life electrode catalyst material that does not cause a decrease in catalytic activity due to

In view of these actual circumstances, the present inventors first
A coating layer made of a platinum-rhodium composition has been proposed (Japanese Patent Application Laid-open No. 158391/1983).

According to this proposal, the catalyst activity is improved and the life span is also significantly improved compared to when a platinum coating layer is used.

However, the peak current for methanol oxidation is still low, and the catalytic activity is not practically satisfactory.

In addition, the catalyst activity is still greatly reduced due to poisoning, and the life span for practical use cannot be achieved.

Thus, in order to put it into practical use, there is a need to develop an electrode catalyst with higher activity and longer life.

Purpose of the Invention The present invention has been made in view of the above-mentioned circumstances, and has a methanol electrolytic oxidation catalyst activity sufficiently high for practical use, and has extremely little decrease in catalyst activity during long-term electrolysis. The main object of the present invention is to provide an electrode catalyst suitable for methanol electrolysis and a method for producing the same, which has a life that is sufficiently satisfactory for practical use.

The present inventors have conducted intensive research for this purpose, measured the methanol electrolytic oxidation catalytic activity and poisoning characteristics of various materials, especially platinum and other metals or oxides, and reported the results. , which led to the present invention.

That is, the first invention is an electrode catalyst formed by forming a coating layer having methanol electrooxidation catalytic activity on a conductive base material, in which the coating layer has a platinum-iridium oxide composition containing 15 to 60 mol% of iridium oxide. It is an electrolytic catalyst characterized by being made of

A second invention provides a method for producing an electrode catalyst in which a coating layer having methanol electrooxidation catalytic activity is formed on a conductive base material, which comprises: a compound that becomes platinum metal by thermal decomposition; By applying a coating solution containing a compound that becomes iridium oxide onto a conductive substrate and performing heat treatment, a coating layer made of a platinum-iridium oxide composition containing 15 to 60 mol% of iridium oxide is formed. This is a method for producing an electrode catalyst, characterized by forming an electrode catalyst.

According to the present invention, by using a platinum-iridium oxide composition in a predetermined quantitative ratio, high catalytic activity and strong resistance to poisoning of the catalytic activity over time of electrolysis are exhibited. When the oxide is replaced with another platinum group element, such as a metal or oxide such as ruthenium or rhodium,
Such an effect will not be realized. This fact will become clear from the examples described later.

Specific Configuration of the Invention The specific configuration of the present invention will be described in detail below.

The coating layer in the present invention is formed from a platinum-iridium oxide composition containing 15 mol% to 60 mol%, more preferably 20 mol% to 55 mol% of iridium oxide.

In this case, if the amount of iridium oxide is less than 15 mol % and greater than 60 mol %, the peak current for methanol oxidation becomes small and the catalyst is not practical in terms of catalytic activity. In addition, the resistance to poisoning becomes weaker, and the electrolytic current density deteriorates more with time.

The coating layer may be formed essentially only from the platinum-iridium oxide composition.

The platinum-iridium oxide composition that forms this coating layer contains platinum as a metal and iridium as an oxide (IrO ₂ ) in the composition layered on the conductive substrate described below. It is something that

The thickness of the coating layer is not particularly limited, but is usually about 0.2 to 20 μm, particularly about 0.5 to 5 μm.

On the other hand, there is no particular restriction on the material of the conductive base material serving as the core material, and various metals can be used.

However, when used as an anode in an acidic electrolyte, as in the case of applying it as an anode to the electrowinning of zinc in a sulfuric acid bath mentioned above, in order to have sufficient corrosion resistance as an insoluble anode,
It is preferable to use a so-called valve metal as the conductive substrate, which is known to form a dense corrosion-resistant film on its surface under anodic conditions.

Typical examples of such valve metals include titanium and titanium alloys.
In addition, tantalum, zirconium, niobium, and alloys thereof can also be used.

In addition, the platinum-iridium oxide composition coating layer of the present invention has extremely high methanol electrooxidation catalytic activity even in an alkaline electrolyte such as a caustic potassium aqueous solution, as well as in an acidic electrolyte. It has strong resistance to the accompanying poisoning of catalyst activity.

For example, when used as an anode for methanol electrolysis in a methanol-air fuel cell that uses a concentrated aqueous caustic potassium solution as the electrolyte, superior discharge characteristics can be obtained compared to the case of using electrodes that have been studied in the past, and this is a promising candidate for practical application. This is expected to make a great contribution.

When used in such applications in an alkaline electrolyte, an oxide film of a valve metal such as titanium as a conductive base material is not suitable for practical use due to its high solubility in the electrolyte. It is preferable to use a metal that forms a film insoluble in the alkaline electrolyte, such as nickel.

As described above, the shape and dimensions of the electrode catalyst of the present invention, which has a coating layer made of a predetermined platinum-iridium oxide composition on a conductive substrate, are not limited and may be selected according to the intended use.

In producing the electrode catalyst for methanol electrolysis of the present invention, various known methods can be used to form a coating layer on the conductive base material.

Among these various methods, the coating layer is preferably formed by a so-called pyrolysis method.

That is, for example, first, butanol or the like is used as a solvent, and then a compound that becomes platinum metal through thermal decomposition, such as chloroplatinic acid (H ₂ PtCl ₃ .6H ₂ O) or platinum resin acid, is added, and then a platinum resin acid is added to it through thermal oxidation. Compounds that become iridium metal oxide, such as iridium chloride (IrCl ₄ .H ₂ O), iridium chloride (H ₂ IrCl ₆ .
A coating solution is prepared by dissolving predetermined amounts of each of 6H ₂ O) and iridium resinate.

Next, this is applied to the surface of the conductive substrate by brush coating, thermal spraying, etc., and after drying, heat treatment is performed in an oxygen-containing atmosphere to form a platinum-oxidized product as a thermal decomposition and oxidation reaction product of the coating liquid components. A coating layer of the iridium composition can be baked onto the substrate.

According to such a method, an electrode catalyst for methanol electrolytic oxidation having extremely excellent performance can be obtained, as shown in Examples below.

In addition, when manufacturing the electrode of the present invention by such a pyrolysis method, in order to make the electrode sufficiently robust for practical use and capable of maintaining stable performance over a long period of use, the above-mentioned coating and heating process is necessary. It is more preferable to repeat the treatment step multiple times, and in particular,
When this was repeated 5 to 10 times, sufficiently favorable results were obtained.

Moreover, baking can be performed by heating at an optimum temperature of 200 to 800° C. for 5 to 10 minutes in a gas atmosphere with an oxygen partial pressure in the range of 0.1 to 0.5 atm.

Furthermore, water, ethanol, butanol, etc. are preferable as a solvent for the coating liquid.

The concentration of the coating solution is 0.01 to 10 g/metal equivalent, taking into account viscosity, ease of application, thickness of the coating, etc.
ml, particularly preferably 0.02 to 2 g/ml in terms of total metals. Further, the coating liquid may contain a coating property improving agent such as lavender oil or turpentine oil.

Note that the conductive base material may be subjected to a pretreatment such as a surface purification treatment or a surface roughening treatment, and then a coating layer may be formed thereon as described above.

Specific Effects of the Invention The electrode catalyst for methanol electrolysis of the present invention is useful as an anode for electrolytic oxidation of methanol.

That is, whether the electrolytic solution is acidic or alkaline, it exhibits extremely high methanol electrooxidation catalytic activity and can obtain a high methanol oxidation peak current density.

Furthermore, poisoning of the catalyst activity over time of electrolysis is extremely small, and a high current density is exhibited over a long period of time. In this case, the service life is improved by more than 10 times compared to conventional coating layers made of platinum or platinum compositions.

For this reason, as mentioned above, in the electrowinning of zinc, several moles of methanol are added to the sulfuric acid-zinc sulfate electrolyte, for example, several A/dm ² at 40 to 60°C.
As an anode when performing electrolysis at a rate of
Alternatively, it exhibits extremely good characteristics as an anode for the aforementioned methanol-oxygen fuel cells, etc., and greatly contributes to the effective use of energy.

Furthermore, since the electrode catalyst of the present invention has a low oxygen overvoltage with respect to oxygen generation, it has the features of excellent power efficiency and excellent durability.

In general, for fuel electrodes, an activation method is known in which the potential is increased to generate oxygen every certain period of use, and the poisoned layer is removed using bubbles of the generated oxygen.

In this case, for example, when activation is performed with an aqueous sulfuric acid solution, that is, when oxygen is generated, durability is particularly required, and there are few electrode catalysts that satisfy this requirement.

The electrode catalyst of the present invention has extremely little damage to the coating layer even after the activation treatment, and the current value after the activation treatment remains constant, which is extremely advantageous compared to conventional platinum or platinum composition coating layers. It has the following characteristics.

In addition, the oxygen overvoltage is also extremely low.

Compared to the platinum coating layer, the electrode catalyst of the present invention has
Since the oxygen overvoltage is 0.3 to 0.4 V lower, when used as an anode in the current zinc smelting process, it can be operated at a lower voltage.

On the other hand, when the anode electrodes are combined and used as an anode for zinc refining, as mentioned above, extremely high efficiency and durability are exhibited.

Therefore, by using the electrode catalyst of the present invention, when refining zinc, it is possible to operate using the current method using cheap late-night electricity, and operate using the methanol sub-electrode method during the day, thereby reducing the total electricity cost. It is possible.

Furthermore, it is said that methanol oxidation ability and oxygen reduction ability are the same, and among the platinum group metals, only platinum is active, so the electrode catalyst of the present invention is expected to be excellent as an oxygen reduction catalyst.

Therefore, by taking advantage of its excellent properties as an anode for oxygen generation, H ₂ −O ₂ fuel cells and
There is also the possibility of application to water batteries, which are power storage systems using H ₂ O electrolyzers.

As described above, the electrode catalyst of the present invention has the feature of functioning as a bifunctional anode.

Specific Examples of the Invention Hereinafter, the present invention will be explained in more detail by presenting specific examples of the present invention.

Example Chloroplatinic acid (H ₂ PtCl ₆ 6H ₂ O) and chloroiridic acid (H ₂ IrCl ₆ 6H ₂ O) were dissolved in butanol, and the total metal content was 0.1 g/metal equivalent.
A coating solution was prepared (hereinafter referred to as Solution A) with a charging composition of 70 mol % of platinum and 30 mol % of iridium.

Separately, in the same manner as Solution A, the total metal content was 0.1 g/ml, and iridium was added to
A total of 10 types with 10, 15, 20, 25, 35, 40, 50, 70, and 80 mol% were produced.

Furthermore, for comparison, the chloroiridic acid was replaced with rhodium chloride (RhCl _{3.3H 2} O), and the total metal content was 0.1 g/ml, 85 mol% platinum and 15 mol% rhodium (hereinafter referred to as solution B). A liquid containing only platinum (hereinafter referred to as liquid C) had a metal content of 0.1 g/ml, and a liquid containing only iridium had a total metal content of 0.1 g/ml.

Next, each of these coating solutions was degreased with a commercially available Triclean degreasing solution, and then the surface was treated with a boiling 10% oxalic acid aqueous solution for 300 minutes on a titanium wire base material (2 mmφ).
It was applied with a brush, dried and then fired.

Coating and baking were repeated in the same manner 10 times, and heat treatment was carried out at 500° C. in air for 10 minutes each time. In addition, in the coating liquid B, a reducing agent was contained in the coating liquid, and the heating temperature was set to 400°C.

In this way, 13 types of electrodes were produced using a total of 14 types of coating liquids. The coating layer thickness of these 14 types of electrodes was 2 μm.

When X-ray diffraction was performed on the platinum composition coating layer applied to each electrode, peaks of Pt and IrO ₂ were observed. In this case, it is possible that a small amount of Ir metal is dissolved in Pt, but there is almost no shift in the peak of Pt, and even if it is dissolved in solid solution, it is estimated that the amount is extremely small.

On the other hand, in electrode B, no peak attributed to Rh alone was observed, and the peak attributed to Pt was shifted to the Rh side, indicating that Rh is dissolved in Pt. Estimated.

Further, when fluorescent X-ray analysis was performed on each coating layer, it was confirmed that each coating layer had a composition that matched the charged composition within experimental error.

Next, a Teflon heat-shrinkable tube was fitted onto each of these electrodes, and the same predetermined area of the coating layer was exposed to form each anode sample.

Next, using each of these anodes, the catalytic activity of each electrode and its poisoning resistance were evaluated.

That is, as an electrolytic solution, 1 mol/
An aqueous solution containing H ₂ SO ₄ and 1 mol/methanol was used, and a cathode consisting of an anode and a platinum wire electrode was arranged. The electrolytic solution was maintained at 50°C in a constant temperature bath while monitoring the liquid temperature with a thermometer. and,
N ₂ bubbles were introduced into the electrolytic solution to eliminate dissolved oxygen in the solution, and the solution was stirred using a stirrer.

On the other hand, the electrolyte was connected via a bridge to a hydrogen electrode (NHE) placed in a 1 mol/H ₂ SO ₄ aqueous solution at 25°C.

Using such a device, for each electrode,
The cyclic voltamgram was measured with anode scanning at 0.1 V/sec. The results for coating layers A, B, and C are shown in FIG.

Further, FIG. 2 shows the relationship between the peak current density ip (A/cm ² ) at the oxidation peak of methanol and the iridium oxide content in the platinum-iridium oxide composition.

Furthermore, the anode potential is 0.6V based on the hydrogen electrode.
The change in electrolytic current density over time after the start of electrolysis was measured. The results are shown in Figure 3. In the figure, symbols A, B, and C indicate the electrode numbers used.

From these results, Pt− _IrO2 with 15–60 mol% Ir
It can be seen that the electrocatalyst of the present invention made of the composition has an extremely high methanol oxidation peak current density and has a lifetime more than 10 times that of other coating layers.

In addition, in each of the above electrodes, the composition containing Ir in the form of metal and the composition containing Rh in the form of oxide had almost the same results as electrode B containing Rh in the form of metal. .

From these results, it can be seen that unless Ir is contained in the form of IrO ₂ , a highly active, long-life electrode catalyst cannot be achieved, an effect that could not have been predicted at the outset.

[Brief explanation of drawings]

Figures 1 to 3 are diagrams for explaining the effects of the present invention, of which Figure 1 is a cyclic voltamgram in anode scanning, and Figure 2 is a cyclic voltamgram in anode scanning. FIG. 3 is a graph showing the relationship between the oxidation peak current density ip of methanol in the Crick Voltangram and the iridium oxide content in the platinum-iridium oxide composition, and FIG. 3 is a graph showing the change over time in the electrolytic current density. .

Claims (1)

[Scope of Claims] 1. An electrode catalyst comprising a coating layer having methanol electrooxidation catalytic activity formed on a conductive base material, wherein the coating layer has a platinum-iridium oxide composition containing 15 to 60 mol% of iridium oxide. An electrode catalyst characterized by being made of a material. 2. The electrode catalyst according to claim 1, wherein the iridium oxide content is 20 to 55 mol%. 3. In a method for producing an electrode catalyst in which a coating layer having methanol electrooxidation catalyst activity is formed on a conductive base material, a compound that becomes platinum metal by thermal decomposition and a compound that becomes iridium oxide by heating are used. A coating layer consisting of a platinum-iridium oxide composition containing 15 to 60 mol % of iridium oxide is formed by applying a coating solution containing the above onto a conductive substrate and performing heat treatment. A method for producing an electrode catalyst. 4. The method for producing an electrode catalyst according to claim 3, wherein the iridium oxide content in the platinum-iridium oxide composition is 20 to 55 mol%.