US3849282A - Metal electrodes and coatings therefor - Google Patents

Abstract

Conductive coating for metal electrodes, consisting essentially of a compound ABO4 with a structure of the rutile-type, where A represents an element in the trivalent state selected from the group consisting of Rh, Al, Ga, La, and the rare earths and B represents an element in the pentavalent state selected from the group consisting of Sb, Nb and Ta, and an oxide of the type MO2 where M is selected from the group consisting of Ru and Ir.

Description

United States Patent [191 Degueldre et al.

METAL ELECTRODES AND COATINGS THEREFOR Inventors: Louis Degueldre; Charles Killens;

Louis Bourgeois, all of Brussels, Belgium Assignee: Solvay & Cie, Brussels, Belgium Filed: July 3, 1972 Appl. No.: 268,552

Foreign Application Priority Data July 9, 1971 Luxembourg 63506 US. Cl. 204/290 F, 204/290 R, 204/291 Int. Cl B0lk 3/06 Field of Search 204/290F, 290 R, 291

References Cited UNITED STATES PATENTS 1/1970 Bianchi et a1 204/242 Nov. 19, 1974 3,562,008 2/1971 Martinsons 117/221 3,632,498 l/1972 Beer 204/290 F 3,701,724 10/1972 Entwisle 204/290 F Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm-Spencer & Kaye [5 7] ABSTRACT 5 Claims, No Drawings METAL ELECTRODES AND COATINGS THEREFOR BACKGROUND OF THE INVENTION The present invention relates to a new type of coating intended for constituting the active surface of a metal electrode made at least superficially of titanium or a titanium alloy and optionally containing a core of a better conducting material than titanium, for example of copper, aluminum, iron or alloys of these metals.

Electrodes provided with a coating having catalytic properties according to the present invention may be used in various electrochemical processes such as cathodic protection, desalination or purification of water, electrolysis of water or hydrochloric acid, production of current in a fuel cell, reduction or oxidation of organic compounds of the electrolytic manufacture of per salts, but they are particularly useful as anodes in the electrolysis of aqueous solutions of alkali metal halides, particularly sodium chloride, both in diaphragm cells and mercury cells where they catalyze the discharge of chloride ions which is brought about at a re markably low and practically constant overpotential throughout the whole life of the electrode. Under the conditions ruling in these cells, wear of the coating is insignificant, and this assures a practically unlimited life for the anodes and avoids the burdensome operations of opening the cells and renewing the coatings.

SUMMARY OF THE INVENTION An object of the present invention, therefore, is to provide an electrocatalytically active coating which is particularly well adherent to its metallic support and is resistant to electrochemical corrosion.

This as well as other objects which will be apparent are achieved by a new type of coating for metal electrodes, which coating comprises a compound ABO, having a rutile-type structure, where A is an element in the trivalent state selected from the group rhodium, aluminum, gallium, lanthanum and the rare earths, while B is an element in the pentavalent state selected from the group antimony, niobium and tantalum, the compound ABO, being associated with an oxide of the type M where M is ruthenium and/or iridium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The compounds A80 and M0 must be present together in the coating. It may be possible to maintain the ABO /MO mol ratio in a wide range for severalelectrochemical processes. 7

However, it hasben found advantageous to maintain this ratio between well defined limitswhen the conductive coating is intended for the manufacture of metallic anodes for the electrolysis of aqueous solutions of alkali metal halides, especially sodium chloride. If the. proportion of M0 is too low, the overvolt age is higher than 300 mV (measured according to test of Example 1) and if the proportion of M0 is too high, the consumption of noble metal/ton of produced chlorine is prohibitive, i.e., higher than 40 mg/t C1 For these reasons, the ABO /MO mol ratio is maintained between l/l and U6.

Among all the elements for A in the trivalent state, Rh and Al have always given better results, so they are preferred.

Further illustrative of the present invention are the following Examples:

- Example 1 ABO .2MO where A Rh, B Sb and M v A solution A containing 0.5 g-atom ruthenium/liter was prepared by dissolving ruthenium trichloride (RuCl .xI-I O) in n-hexanol, a solution B containing 0.5 g-atom rhodium/liter was prepared by dissolving rhodium trichloride (RhCl .xI-l O) in 'n-hexanol and a solution C containing 1 g-atom antimony/liter was prepared by dissolving antimony pentachloride (SbCl in nhexanol.

By mixing 6.6 ml of solution A, 3.3 ml of solution B, 1.6 ml of solution C and 38.5 ml of n-hexanol, there was obtained a composition which was applied in 7 layers onto plates of titanium that had previously been hot-degreased in trichloroethylene and etched for. 5 hours at approximately 90C in an aqueous solution of oxalic acid containing 100 g/liter. For application of this composition, the titanium plates were placed in air on a hotplate at a temperature of about 75C.

After each application the plates were dried and then heated for 15 minutes at 500C and, after the 7th application, heated for 1 hour at 500C, in the presence of air.

The amount of material thus deposited was approximately 4.5 g/m.

The coating, which contained 2 g-atom Ru to l gatom Rh and l g-atom Sb, showed excellent adherence to the substrate, as was shown in stripping tests with adhesive tape applied by pressure.

The plates of titanium thus coated were submitted to two types of evaluation.

The first relates to the over-potential for liberation of chlorine. It is measured at an anodic current density of 10 kA/m The second relates to the consumption of noble metal as a function of the amount of chlorine liberated.

The measurement of over-potential is carried out by immersing the plates in brine containing 250 g NaCl/kg, saturated with chlorine at C and at a pH of approximately 2. Under these conditions, the plates referred to in this example show an initial overpotential of approximately 165 mV and then an overpotential of 210 mV and 248 mV after a production of 67 and 210 tons of chlorine/m respectively, while positively polarized under a current density of 10 kA/m In another test the consumption of noble metal from these plates was evaluated by using them as anodes in a cell with a flowing mercury cathode containing brine saturated with chlorine and sodium chloride at a temperature between and C with a constant anodecathode potential. Under these conditions the plates tested have already produced more than 264 tons of chlorine/m at this time the limit of the useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 10 mg/ton of chlorine produced.

It was 'noted that the same plates showed an overpotential of approximately 210 mV at 10 kA/m after producing approximately 67 tons of chlorine/m Example 2: ABO.2MO where A Rh, B Nb and M Ru To 6 ml of solution A of Example 1 were added 3 ml of solution B of Example 1, 1.5 ml ofa solution D containing l g-atom niobium/liter obtained by dissolving niobium pentachloride (NbCl in n-hexanol, and 39.5 ml of n-hexanol.

Ten layers of this composition were applied to plates of titanium etched as in Example 1. The various heat treatments were carried out in the same manner.

The average weight of the coating thus obtained was approximately 3.7 g/m it contained 2 g-atom Ru to 1 g-atom Rh and l g-atom Nb. The adherence to the substrate was very good. When used as anodes under the conditions specified in Example 1, the plates thus coated showed over-potentials of approximately 245 mV at an anodic current density of kA/m During the consumption test carried out as in Example 1, the amount of chlorine already produced was 81 tons/m which corresponds to a consumption of noble metal equal to 26 mg/ton of chlorine.

Example 3: ABO.,.2MO where A A1, B Sb and M Ru A solution E containing 1 g-atom aluminum/liter was prepared by dissolving aluminum trichloride (AlCl in n-butanol.

To 10 ml of solution A of Example 1 were added 2.5 ml of solution C of Example 1, 2.5 ml of solution E and 35.0 ml of n-hexanol.

The application of this composition, in 8 coats, to 4 plates of etched titanium, and the heat treatments were carried out as in Example 1. The average weight of the coatings thus obtained was approximately 6 g/m they contained 2 g-atom Ru to 1 g-atom Al and l g-atom Sb. The adherence to the substrate was excellent.

When used as anodes under the conditions defined in Example 1, the plates thus coated showed an initial over-potential of 156 mV at an anodic current density of 10 kA/m During the consumption test carried out as in Example l, the amount of chlorine already produced was 125 tons/m which corresponds to a consumption of noble metal equal to mg/ton of chlorine.

Example 4: ABO .2MO where A Rh, B Sb and M I To 2.8 ml ofa solution F containing 1 g-atom of iridium/liter obtained by dissolving chloroiridic acid (H lrCl .xH O) in n-hexanol were added 2.8 ml of solution B of Example 1, 1.4 ml of solution C of Example I and 43.0 ml of n-hexanoln The application of this composition, in 10 coats, to plates of etched titanium and the heat treatments were carried out as in Example 1. The weight ofthe coatings thus obtained was approximately 6 glm they contained 2 g-atomlr to l g-atom Rh and 1 g-atom Sb. The adherence to the substrate was excellent.

When used as anodes under the conditions defined in Example 1, the plates thus coated showed initial overpotentials of approximately 200 mV at an anodic current density of 10 kA/m When submitted to the consumption test under the same conditions as in Example 1, the plates had produced more than 172 tons of chlorine/m At this time, the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 23 mg/ton of chlorine produced.

Example 5: ABO .4MO where A Rh, B Sb and M lr To 1.6 ml of the solution F of Example 4 there were added 0.8 ml of solution B of Example 1, 0.4 ml of solution C of Example 1 and 47.2 ml of n-hexanol.

The application of this composition, in 10 coats, on plates of etched titanium and the thermal treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 3 g/m they contained 4 g-atom lr to l g-atom Rh and l g-atom Sb. The adherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1, the plates thus coated showed overpotentials of approximately 230 mV at an anodic cur rent density of 10 kA/m When submitted to the consumption test under the same conditions as in Example 1, the plates had produced more than 198 tons of chlorine/m At this time, the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 11 mg/ton of chlorine produced.

Example 6: ABO .2MO where A Rh, B Sb and M Ru lr To 5.5 ml of the solution A of Example 1 there were added 1.4 ml of the solution F of Example 4, 4.2 ml of the solution B of Example 1, 2.1 ml of the solution C of Example 1 and 36.8 ml of n-hexanol.

The application of this composition, in 8 coats, to plates of etched titanium and the thermal treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 7 g/m they contained 4 g-atom Ru to 2 g-atom Ir, 3 g-atom Rh and 3 g-atom Sb; the adherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1, they showed an initial over-potential of approximately mV and then at 202 mV after the liberation of 233 tons/m of chlorine, at an anodic current density of 10 kA/m At this time, the limit of useful life of the electrode had not been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 18 mg/ton of chlorine produced.

Example 7: ABO .2MO where A Rh, B Sb and M M; Ru lr To 2.1 ml of the solution A of Example 1 there were added 2.1 ml of the solution F of Example 4, 3.2 ml of the solution B of Example 1, 1.6 ml of the solution C of g-atom Ir 1 and 41.0 ml of n-hexanol.

The application of the composition, in 8 coats, to plates of etched titanium and the heat treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 6 glm they contained 2 g-atom Ru to 4 g-atom Ir, 3 g-atom Rh and 3 g-atom Sb; the adherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1, the plates thus coated showed an initial overpotential of approximately 135 mV and then of 172 mV after the liberation of 150 tons of chlorine/m After production of 225 tons of chlorine/m the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 17 mg/ton of chlorine produced. Example 8: ABO .2MO where A Rh, B Sb and M Ru To 15.2 ml of the solution A of Example 1, there were added 7.6 ml of the solution B of Example 1, 3.8 ml of the solution C of Example 1 and 23.4 ml of nhexanol.

The application of the composition, in 8 coats, to plates of etched titanium and the heat treatments were carried out as in Example 1.

The average weight of the coatings thus obtained was approximately 12.0 g/m they contained 2 g-atom Ru to 1 g-atom Rh and 1 g-atom Sb. The adherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1, the plates thus coated showed an initial overvoltage of approximately 1 12 mV and then ofl60 mV after the liberation of 225 tons of chlorine/m of anodic surface at an anodic current density of kA/m At this time, the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 29 mg/ton of chlorine produced.

Example 9: ABO .4MO where A Rh, B Ta and M Ru A solution G containing 0.5 g-atom/liter was prepared by dissolving ruthenium trichloride RuCl x11 0 in isopropanol, a solution H containing 0.5 g-atom/liter by dissolving rhodium trichloride RhCl xH O in isopropanol and a solution 1 containing 1 g-atom Ta/liter by dissolving tantalum pentachloride TaCl in methanol.

By mixing 8 ml of solution G, 2 ml of solution H, l ml of solution 1 and 39 ml of isopropanol, a coating composition was obtained, which was applied, in 10 coats, to plates of titanium etched under the same conditions as in Example 1. After each application, the plates were dried and heated for minutes at 500C, and after the tenth application, heated for 2 hours at 500C in the presence of air.

The average weight of the coatings thus obtained was approximately 9.9 g/m they contained 4 g-atom Ru to 1 g-atom Rh and l g-atom Ta. The adherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1 the plates thus coated showed an initial overvoltage of approximately 190 mV and then of 198 mV after the liberation of 55 tons of chlorine/m of anodic surface at an anodic current density of 10 kA/m After production of 138 tons of chlorine/m the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 41 mg/ton of chlorine produced. Example 10 ABO .4MO where A Al, B Ta and 5 M Ru To 10 ml of the solution A of Example 1, there were added 1.25 ml of solution E of Example 3, 1.25 ml of the solution I of Example 9 and 37.5 ml of n-hexanol. The application of the composition, in 10 coats, to plates of etched titanium and the various heat treatments were carried out as in Example 1.

The average weight of the coatings thus obtained was approximately 6.8 g/m they contained 4 g-atom Ru to l 'g-atom Al and l g-atom Ta. The adherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1 the plates thus coated showed an initial overvoltage of approximately 145 mV, at a current density of 10 kA/m When submitted to the consumption test under the same conditions as in Example 1, the plates had produced more than 131 tons of chlorine/m of anodic surface. At this time, the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 26 mg/ton of chlorine produced.

Example 11 :ABO .2MO where A 0.9 Rh 0.] Yb, B Sb and M Ru A solution .1 containing 1 g-atom Sb/liter was prepared by dissolving antimony pentachloride in isopropanol and a solution K containing 0.1 g-atom Yb/liter by attacking ytterbium oxide Yb O with hydrochloric acid and dissolving in isopropanol.

By mixing 10 ml of solution G of Example 9, 4.5 ml of solution H of Example 9, 2.5 ml of solution K, 2 ml of solution J and 30.5 ml ofisopropanol, a coating composition was obtained, which was applied, in 10 coats, to plates of titanium etched under the same conditions as in Example 1. The various heat treatments were carried out by the same manner.

The average weight of the coatings thus obtained was approximately 8.0 g/m they contained 2 g-atom Ru to 0.9 g-atom Rh, 0.1 g-atom Yb and 1 g-atom Sb. The adherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1 the plates thus coated showed an initial overvoltage of approximately 122 mV at an anodic current density of 10 kA/m When submitted to the consumption test under the same conditions as in Example 1, the plates had produced more than 103 tons of chlorine/m of anodic surface. At this time, the limit of useful life of the electrode had not yet been reached and the consumption test was continued, but it was already apparent that the consumption of noble metal would be less than 41 m-g/ton of chlorine produced.

The data of the illustrative Examples 1 to 11 are collected together in Table 1.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

TABLE 1.

COMPILATION OF RESULTS ACHIEVED IN THE EXAMPLES. Example Coating composition Wt. of Adherence of Initial ovcr' Consumption test of the coating No. coating the coating voltage in m\/ Cl produced noble metal con g/m at kA/m in t/m sumcd mg/t Cl produced l RhSbO..2RuO 4.5 excellent 165 264 10 2 Rl1NbO .2RuO 3.7 very good 245 8| 26 3 AlSbO .2RuO 6 excellent I56 125 4 RhSb()..2lrO 6 excellent 200 172 23 5 RhSbO l46 .4lr0 3 excellent 230 198 11 6 RhSbO .2(Ru,,;,1 )O 7 excellent 135 233 18 7 RhSbO .2(Ru ,;,l )0 6 excellent 135 225 17 8 RhSbO HZRuO 12 very good 112 225 29 9 RhTuO .4RuO- 9.) very good 190 138 4] l0 AlTaO .4RuO, 68 very good 145 131 26 ll (Rh Yb )SbO .2RuO 8 very good 122 103 41 We claim: 2. A metallic electrode as claimed in claim 1, where l. A metallic electrode for electrochemical processes A i an l t in th trivalent state selected from the comprising a metal support and on at least a portion of group consisting f Rh d All said support a conductive coating consisting essentially 20 of a compound ABO; with a structure of the rutile- 3. A metallic electrode 88 claimed in claim 1, where type, where A represents an element in the trivalent B iS pentavalen Sbstate selected from the group consisting of Rh, Al, and Ga and B represents an element in the pentavalent state selected from the group consisting of Sb, Nb and Ta, and an oxide of the type M02 where M is Selected from 5. A metallic electrode as claimed in claim 1 wherein the group consisting of Ru and Ir, and the mole ratio of the compound ABO4 is RhSbO.

4. A metallic electrode as claimed in claim 1 wherein the metal support is a valve metal.

Claims (5)

1. A METALLIC ELECTRODE FOR ELECTROCHEMICAL PROCESSES COMPRISING A METAL SUPPORT AND ON AT LEAST A PORTION OF SAID CUPPORT A CONDUCTIVE COATING CONSISTING ESSENTIALLY OF A COMPOUND ABO4 WITH A STRUCTURE OF THE RUTILE-TYPE, WHRE A REPRESENTS AN ELEMENT IN THE TRIVALENT STATE SELECTED FROM THE GROUP CONSISTING OF RH, AL AND GA AND B REPRESENTSS AN ELEMENT IN THE PENTAVALENT STATE SELECTED FROM THE GROUP CONSISTING OF SB, NB AND TA, AND AN OXIDE OF THE TYPE MO2 WHERE M IS SELECTED FROM THE GROUP CONSISTING OF RU AND IR, AND THE MOLE RATIO OF ABO4 TO MO2 IS FROM 1/1 TO 1/6.

2. A metallic electrode as claimed in claim 1, where A is an element in the trivalent state selected from the group consisting of Rh and Al.

3. A metallic electrode as claimed in claim 1, where B is pentavalent Sb.

4. A metallic electrode as claimed in claim 1 wherein the metal support is a valve metal.

5. A metallic electrode as claimed in claim 1 wherein the compound ABO4 is RhSbO4.