EP0004236A2 - Electrode for electrolysis - Google Patents

Abstract

The invention relates to an electrode comprising a valve metal substrate and an active surface without precious metals. This active surface comprises cobalt oxide and an oxygenated or hydrogenated compound of titanium in which this metal has a valence of less than 3. The electrode of the invention is intended in particular to be used as an anode in the electrolysis of chloride sodium.

Description

The invention relates to electrodes for electrolysis, in particular of sodium chloride, the coating of which consists of cobalt and titanium compounds.

It is known that for the past twenty years, metal anodes have been used more and more commonly for the electrolysis of sodium chloride, both in mercury cells and in diaphragm or membrane cells and both for the production of chlorine and soda only for that of oxygenated derivatives such as chlorates. The anodes used industrially generally comprise a substrate, generally of titanium, covered with metals of the platinum group or their oxides in mixture optionally with oxides of other metals and in particular of titanium in rutile form; which oxide is also formed in situ during electrolysis. The use of precious metals leads to high investments when setting up factories; moreover, the consumption of these metals, although low in weight, reaches a non-negligible value compared to the value of the products of electrolysis. It is therefore understandable that much research has been done to substitute non-precious metal compounds for platinum group metals. Among these compounds, cobalt oxide has been mentioned, for example, in US Pat. Nos. 3,399,966 and 3,977,958. There is no publication known on the industrial development of such electrodes, which shows that they do not give full satisfaction in the industrial production of chlorine in particular.

It has now been found that it is possible to produce electrodes not comprising precious metals but only compounds of moderate price and easy to obtain in coatings having an excellent resistance, during the electrolysis of alkali chlorides.

The invention relates to an electrode for electrolysis, in particular sodium chloride, consisting of a valve metal substrate and a coating comprising cobalt oxide, characterized in that the cobalt oxide corresponds to the formula Co _3-x O ₄ in which x is between 10 ^-1 and 10 ^-2 and in that the coating also comprises a compound of titanium and at least one element of the group formed by oxygen and hydrogen , the valence of titanium in this compound being less than 3.

The titanium compound can be an oxide of formula Ti O in which formula x is between 0.45 and 1.2 and preferably between 0.9 and 1.1. It can also be a hydride of formula Ti H _x in which formula x is between 0.1 and 2 and preferably between 1.8 and 2. These compounds, in which the titanium has a valence lower than its maximum valence, reveal better conductivity. One can also use as titanium compounds, the body corresponding to the formula Ti 0 _x H y in which x is between 2 and 1 and y is between O and 0.1. These compounds are described in particular in "Transaction of the Metallurgical Society of AIME Vol. 224 October 1962 p.928-935.

Cobalt oxide is preferably obtained by thermal decomposition of cobalt nitrate, in particular (N0 ₃ ) ₂ Co, 6 H ₂ 0, with air sweeping at a temperature of 200 to 600 ° C but preferably between 300 and 400 ° C.

The substrate is a valve metal, that is to say titanium, tantalum, molybdenum, zirconium, niobium or tungsten. Titanium is the preferred substrate; it can be in the form of a smooth metal plate, of canvas or obtained by powder sintering.

The titanium oxide or hydride can be previously deposited on the substrate in direct contact with it. These titanium compounds can also be deposited on the substrate at the same time as the cobalt oxide; they are then dispersed in the layer of this oxide.

The amount of cobalt oxide deposited is preferably 15 to 40 milligrams per square centimeter. More may be used, but this has no beneficial effect. In the case where a prior deposition of titanium oxide or hydride has been made on the substrate, it is necessary, for satisfactory operation of the electrode, that at least one face of the substrate is covered on at least 5% of its surface by this compound; the thickness of the titanium compound layer is greater than 0.5 microns.

In the case of direct deposition on a valve metal substrate of a mixture of titanium and cobalt compounds, the atomic ratio Ti / Co must be between 0.6 and 20 and preferably between 6 and 11. These latter values are therefore not imperative but it turns out that for a ratio less than 6, there is a voltage drift after a few hundred hours of electrolysis of sodium chloride under the usual industrial conditions, using these electrodes, while that the adhesion of the coating is poor when the ratio exceeds 11.

The prior deposition of titanium sub-oxide on a titanium substrate is described in the published French patent application No. 2 259 921. The hydride can also be formed in situ by hydriding solid or porous titanium. The deposition of these compounds at the same time as that of the titanium oxide can advantageously be carried out by suspending the sub-oxide or the titanium hydride previously prepared, in a solution of cobalt salt. The substrate is coated with this suspension then dried and finally heated to a temperature allowing the decomposition of the salt into oxide. This operation is repeated several times until the desired weight of cobalt and titanium is obtained: the proportion of titanium compound in the cobalt salt solution is obviously chosen accordingly. The preferred cobalt salt is nitrate which is decomposed at a temperature of 200 to 600 ° C and preferably 300 to 400 ° C under air sweep, whether it is deposited alone or with the titanium compound.

Titanium oxide, the particle size of which is preferably 0.5 to 20 microns, can be prepared separately, or deposited directly on the substrate previously pickled and washed, using a plasma torch either from the dioxide with, as carrier gas and plasma-forming agent, a mixture of hydrogen and argon, or from titanium powder with a mixture of oxygen and argon, or from a mixture of titanium dioxide and / or hydride with an argon plasma. The method described in the French patent application cited above for forming the sub-oxide on the substrate can be used to form it separately.

Titanium hydride can be prepared and deposited by a variety of methods such as those described in US Patents 2,401,326 or 3,732,157. Hydride can also be formed chemically in an acid solution hydrochloric acid from 4 to 13 N or electrochemically. Titanium dioxide can also be reduced by elements such as magnesium or carbon in the presence of hydrogen or by a hydride such as calcium.

The analyzes carried out in particular by X-rays have shown that the coatings of the invention were in fact a mixture of cobalt oxide of composition close to C ₀₃ 0 ₄ and of sub-oxide or of titanium hydride; the significant presence of a mixed compound of these two metals has never been observed.

This presence of a simple mixture gives the invention a character all the more surprising since experience has shown that a valve metal substrate coated with cobalt oxide alone only lasted a few hundred hours while this same substrate coated with any titanium oxide or hydride did not have acceptable electrochemical properties for the electrolysis of sodium chloride. The remarkable resistance in such an electrolysis for several thousand hours and under high current densities, combined with the simplicity and the low cost of these electrodes bring significant progress in the field of electrolysis.

The results obtained are mentioned in the examples below given solely for the purpose of illustrating the invention.

Example 1:

A substrate consisting of a sintered titanium plate covered with titanium sub-oxide over a thickness of approximately 1 mm, as described in the French application published under the number 2 259 921, is coated on the oxide layer, using a brush, with a solution of cobalt nitrate obtained by dissolving 1 ^g of Co (NO ₃ ) ₂ 6 H ₂ 0 in 2 cm 3 of mixture with equal volume of water and isopropyl alcohol. This substrate thus coated is dried in an oven and then heated for 10 minutes at 350 ° C. in an oven with air sweeping. This sequence of operations is repeated until 37 mg of cobalt oxide is obtained per cm2. The plate thus coated is used as an anode in a diaphragm electrolysis cell containing a Na Cl brine at 300 g / 1, the pH is 4.5 and the temperature 85 ° C. The voltage relative to a calomel electrode saturated (DHW voltage) measured after 3000 hours of operation is 1077 mV. under a current density of 25 A / dm2. At a current density of 200 A / dm2 after 600 h of operation, no degradation of the electrode was observed.

Example 2:

A solution of 1 g of Co (N0 ₃ ) ₂ 6 H ₂ 0 in 1 cm3 of water and 1 cm3 of isopropyl alcohol is deposited on a titanium plate previously sandblasted and washed. Ti Ox (x = 1 approx.) With a particle size between 0.5 and 20 microns. The Ti / Co atomic ratio in this suspension is 6. The treatment of the coated plate is carried out according to the process described in Example 1 and this plate is used as an electrode also under the same conditions as those indicated in the previous example (current density 25 A / dm2). The amount of cobalt oxide deposited is 27 mg / cm2). The DHW voltage measured after 957 hours of operation is 1090 mV.

Example 3:

The procedure is as in the previous example, replacing the oxide with Ti Hx hydride (x = 1.9) with different Ti / Co ratios. The weight of cobalt oxide is 37 mg / cm2.

The results obtained in an electrode with the conditions of the preceding examples are given in Table 1.

Claims (8)

1. Electrode without precious metal for electrolysis, in particular of sodium chloride, consisting of a valve metal substrate and a coating containing cobalt oxide, characterized in that the cobalt oxide corresponds to the formula Co _3-x O ₄ in which formula x is between 0.1 and 0.01 and in that the coating essentially comprises cobalt oxide and a compound of titanium and of an element taken from the group formed by hydrogen and oxygen , the valence of titanium in this compound being less than 3. 2. An electrode according to 1 characterized in that the titanium compound is an oxide of formula Ti Ox in which formula x is between 0.45 and 1.2 and preferably between 0.9 and 1.1. 3. Electrode according to 1 characterized in that the titanium compound is a hydride of formula Ti Hx in which formula x is between O, 1 and 2 and preferably between 1.8 and 2. 4. Electrode according to 1, characterized in that the amount of cobalt oxide is from 15 to 40 mg / cm2. 5. Electrode according to any one of the preceding claims, characterized in that the cobalt oxide used is the product obtained by thermal decomposition of cobalt nitrate. 6. Electrode according to any one of the preceding claims, characterized in that the titanium compound constitutes a layer in direct contact with the valve metal substrate. 7. An electrode according to any one of claims from 1 to 4 characterized in that the titanium compound is dispersed in the layer of cobalt oxide. 8. The electrode according to claim 7, in which the ratio of the number of titanium atoms to that of cobalt in the coating is between 0.6 and 20 and preferably between 6 and 11.