EP0033363B1 - Process for coating a porous electrode - Google Patents

Description

The invention relates to a method for coating a porous electrode for electrochemical processes with an activation layer which at least partially covers the electrode surface and contains metals or compounds of metals of the platinum group.

To activate electrodes for electrochemical processes, e.g. B. of anodes for chlor-alkali electrotysis, from a resistant to the electrotysis products and a passivation layer forming material under the conditions of electrolysis, numerous methods have become known, the purpose of which essentially consists of a platinum metal or platinum metal-containing compounds in an electrochemically effective degree of dispersion anchor the electrode or the electrode core mechanically. For example, according to DE-B-1155762 it is known to galvanically coat degreased and etched titanium plates with a platinum metal and the plates in a first thermal cycle in an inert atmosphere and in a second cycle in an oxidizing atmosphere up to a temperature of approximately To heat 800 ° C. This treatment gives a firmly adhering activation layer and at the same time improved protection of the electrode material by converting the titanium core, which is exposed in the pores of the activation layer and has a thin barrier layer made of titanium oxide, into rutile. However, this and other known coating methods are less suitable for porous electrodes, for example for sintered electrodes according to DE-A-23 05 175 or electrodes made of titanium suboxide according to DE-B-24 05 010. The adhesive strength of the activation layers is particularly favorable for porous electrodes and for The large surface area of the electrode is advantageous for numerous electrochemical processes. When the electrode is coated with an activation layer, however, losses of the activating agent occur when using the known methods, since some of the platinum metal or platinum metal compounds are also deposited in pores of the electrode which are remote from the surface, the surfaces of which are not involved in the electrochemical reactions. The loss is particularly great when the activating agent is precipitated from solutions, and less when the activating layer is electrodeposited. On the other hand, electroplated layers are less suitable because of their dense structure. It has also been proposed (e.g. DE-B-1671 422) to apply the metals or metal compounds from finely dispersed suspensions to the electrode surface. Because of the great difficulty of achieving a uniform distribution and good adhesion of the activating agents applied, the coating processes described above are preferred on an industrial scale.

The invention has for its object to provide a coating method for porous electrodes that does not have the disadvantages described, especially the comparatively large need for activating agents, and results in highly effective activation layers with small amounts of platinum metals.

The object is achieved according to the invention with a method of the type mentioned at the outset by coating the electrode surface with a fine-grained compounds of metals of the platinum group and a suspension containing the compounds which dissolve the compounds at an elevated temperature, the fine-grained compounds of the metals having a particle diameter, that only the dispersing agent can get into the pores of the electrode, but not the fine-grained compounds, that the dispersed phase of the suspending agent is dissolved, deposited on the electrode surface by evaporation of the agent and decomposed by heating the electrode to a temperature between 250 and 350 ° C and that the cycle is repeated several times and the electrode is then heated to a temperature between 400 and 600 ° C. in an oxygen-containing atmosphere.

The invention is based on the knowledge that particles dispersed in a suspension cannot get into pores which are only accessible through narrow hoses or channels, whereas the dispersing agent fills these pores. The particle diameter must therefore correspond to the pore diameter. If the electrode is heated after coating, the dispersant emerges from the pores and, because of the increasing solubility of the platinum group compounds used according to the invention, dissolves the particles concentrated at the pore entrances. The solution, which is in a thin layer, has a relatively high toughness and is distributed evenly over the outer electrode surface and the surface of larger pores accessible from the outer surface without penetrating into narrow channel and tube pores. The compounds deposited by evaporation of the solvent in a uniform layer thickness are then decomposed by heating the electrode to a temperature between 250 and 350 ° C., whereby a fissured, metallic surface coating having a large specific surface is formed. The layer thickness of about 1 µm required for technical purposes is obtained by repeating the coating cycle several times. Finally, the invention provides for the heating of the coated electrode in an oxidizing atmosphere, preferably in air, to a temperature between 400 and 600 ° C. The main purpose of the thermal treatment is to passivate the electrode surface exposed in the pores of the activation layer and to anchor the activation layer on this surface. Partial oxidation of the platinum metals contained in the activation layer is not harmful, since the growth of the metal crystals is inhibited and finely dispersed layers have greater electrochemical activity. The treatment temperature should therefore not fall below or exceed the temperature range of 400 to 600 ° C. The heating time is expediently about 3 to 60 minutes and can be easily determined by simple experiments for each electrode material and each compound used as an activating agent.

According to an advantageous embodiment of the method according to the invention, compounds of non-platinum metals are dispersed in the dispersant in addition to compounds of metals from the platinum group. Suitable non-platinum metals are tantalum, zirconium, niobium, aluminum and especially titanium. After the oxidizing treatment, the activation layer then contains a finely dispersed mixture of platinum metals, oxides of platinum metals and oxides of non-platinum metals. According to another advantageous embodiment of the invention, thermally decomposable complex compounds which contain free acid, in particular compounds from the group oxotato, formiato, tartrato and citrate complexes of metals from the group ruthenium, are used as compounds of metals of the platinum group and of non-platinum metals , Rhodium, palladium, iridium and platinum and analogous compounds of non-platinum metals. The dispersant used according to the invention dissolves the complex compounds at elevated temperature, preferably with the electrode surface, in particular passivating layers, being formed by caustic solutions. Water and optionally aqueous oxalic acid solutions are particularly suitable for this purpose. In this process, the primer is improved without the formation of corrosive and harmful vapors as in the known use of hydrochloric acid platinum metal chloride solutions.

In principle, all electrically conductive metals, alloys and compounds that are stable under the conditions of electrochemical processes are suitable as electrodes. Are preferably used, e.g. B. as an anode for chlor-alkali electrolysis, passivating layers forming metals such as titanium, tantalum, zirconium and niobium and according to the invention preferably electrodes which consist at least in part of titanium suboxide. The electrodes according to the invention have a porosity of about 10 to 50% and are generally produced by sintering moldings from a metal powder or an oxide powder.

• 1. es wird nur der Teil der Gesamtoberfläche beschichtet, der an den elektrochemischen Reaktionen beteiligt ist,
• 2. die verwendeten Substanzen sind nicht korrosiv und gesundheitsschädlich,
• 3. die erzeugte Aktivierungsschicht ist feinkörnig ausgebildet und weist,eine hohe elektrochemische Aktivität auf,
• 4. die Aktivierungsschicht ist fest in der porösen Elektrode verankert.

The advantages of the method according to the invention for producing an activation layer on a porous electrode are essentially as follows:

• 1. only that part of the total surface which is involved in the electrochemical reactions is coated,
• 2. the substances used are not corrosive and harmful to health,
• 3. the activation layer produced is fine-grained and has high electrochemical activity,
• 4. the activation layer is firmly anchored in the porous electrode.

This results in a better use of the expensive and limited availability of platinum metals.

The invention is explained below by way of example:

example 1

41.4 parts by weight of titanium powder, grain size <0.06 mm, 38.6 parts by weight of rutile powder, grain size <0.01 mm were mixed in a high-speed mixer after addition of 5 parts by weight of a two percent aqueous polyvinyl alcohol solution, and the mixture was pressed on a die press with a pressure of about 50 bar pressed to moldings. The moldings were dried, heated to 1,250 ° C. in an argon atmosphere and then comminuted with a jaw crusher and ground to a particle size of <0.06 mm using an oscillating mill. The brittle, gray cast iron powder had a composition of TiO _0.56 .

100 parts by weight of powder were then mixed with 5 parts by weight of a 10% strength solution of hard paraffin in toluene, mixed for 5 minutes in a vortex mixer and pressed on a die press with a pressure of 2 kbar to form plate-shaped electrodes which were heated to 1,250 in a piercing furnace in an argon atmosphere ° C were heated. The sintered electrode plates, the porosity of which was approximately 15%, were coated with a 10% aqueous slurry of H [Ru (C ₂ O ₄ ) ₂ ] .2.5 H ₂ O (manufactured by OE Zviagintsev and SM Starostin, Zh. Neorgan Khim. 2 (1957) 1281/8), first dried at room temperature and finally at 105 ° C. To decompose the salt, the temperature was raised to 300 ° C. for 10 min. The cycle was repeated four times and a total amount of noble metal of approximately 7 g Ru / m ^{2 was} deposited. The coated electrode was heated to 500 ° C., the residence time at this temperature being 5 minutes.

• Stromdichte 20 kA/m²
• Temperatur ca. 70 °C
• Sole ca. 300 g/l NaCI

The electrode plate was then tested as an anode in an amalgam test cell. The conditions were:

• Current density 20 kA / m ²
• Temperature approx. 70 ° C
• Brine approx. 300 g / l NaCI

After predetermined periods of time, the anode potential was checked with a Luggin capillary measured the saturated calomel electrode.

Example 2

In a die, 100 parts of titanium sponge with a grain size of <2 mm were overlaid with 20 parts of TiO _0.58 powder - the production of which is described in Example 1 - and pressed at a pressure of about 2 kbar to form composite plates which were sintered as in Example 1. The titanium suboxide side of the plates was slurried with 66 parts of H [Ru (C ₂ O ₄ ) ₂ ] · 2.5 H ₂ O and 100 parts. Ti ₂ (C ₂ O ₄ ) ₃ .10 H ₂ O (manufactured according to A. Stähler, Ber. 38 (1905) 2619/29) in 1,000 parts of water in which 25 parts of oxalic acid were dissolved. The thermal treatment corresponded to Example 1, except for the oxidizing annealing at 550 ° C. and a holding time of 15 min.

The following potentials were measured:

Example 3

Composite plates as in Example 2 were treated with a slurry of 66 parts H ₂ [Ru (C ₂ O ₄ ) ₂ ] · 2.5 H ₂ O, 70 parts H ₂ [Ir (C ₂ O ₄ ) ₃ ], 100 parts Ti ₂ (C ₂ O ₄ ) ₃ .10H ₂ O coated in 1,000 parts of water and 50 parts of oxalic acid, annealed and the potentials measured.

Claims (7)

1. Process for the coating of a porous electrode for electro chemical processes with an activation layer containing metals and compounds of metals of the platinum group at least partially covering the electrode surface, characterised in that the electrode surface is coated with a suspension containing finely particulate compounds of metals of the platinum group and with a dispersing means dissolving the compounds at an elevated temperature, the finely particulate compounds of the metals possessing such a particle diameter that only the dispersion means can extend into the pores of the electrode, not however the finely particulate compounds, that the disperse phase of the suspension is dissolved in the dispersion means by warming the coated electrode, is deposited on the electrode surface by evaporation of the means and decomposed by heating of the electrode to a temperature between 250 and 350 °C, that the cycle is repeated several times and the electrode is then heated in an oxygen containing atmosphere to a temperature between 400 and 600 °C.

2. Process according to claim 1, characterised in that compounds of metals of the platinum group and a compound of non-platinum metals are dispersed in the dispersion means.

3. Process according to claims 1 and 2, characterised in that there is used a dispersion means forming a solution etching the electrode surface at an elevated temperature with the compounds of metals of the platinum group.

4. Process according to claims 1 to 3, characterised in that there are used compounds from the group of oxalato-, formato-, tartrato- and citrato-complexes at least of a metal from the group of ruthenium, rhodium, palladium, iridium and platinum.

5. Process according to claims 1 to 4, characterised in that water is used as dispersion means.

6. Process according to claims 1 to 5, characterised in that an aqueous oxalic acid solution is used as dispersion means.

7. Process according to claims 1 to 6, characterised in that there is used an electrode consisting at least partly of titanium sub-oxide.