sign in sign up
<<
Home , Ruthenium tetroxide

3,668,005 United States Patent Office Patented June 6, 1972

2 metal support in a molten bath of an of a metal 3,668,005 of the under pressurized . Finally, PROCESS FOR THE COATING OF ELECTRODES coating methods have also been described which are Guy Sluse, Rixensart, and Gustave Joanaes, Abolens, Belgium, assignors to Solvay & Cie, Brussels, Belgium carried out by electroStatic pulverization under vacuum No Drawing. Filed Jan. 11, 1971, Ser. No. 105,628 or in the presence of oxygen and even by means of a Claims priority, applicag xemburg, Jan. 9, 1970, plasma generator. 168 SUMMARY OF THE INVENTION Int, Cl, B44d 1/18 U.S. C. 117. 25 13 Claims One of the objects of the present invention is to rem 0 edy difficulties encountered with known metal electrodes coated with a metal oxide of the platinum group by pro ABSTRACT OF THE DISCLOSURE viding a low over-voltage stable electrode which is espe Electrodes coated with dioxide are manu cially suitable as an anode for the electrolysis of aqueous factured by applying an anchoring layer containing at solutions of alkali metal halides, and which also may be least one compound oxidizable by ruthenium tetroxide 5 used advantageously for other electrolytic and electro to the area of an etched metal support wherein the ru chemical techniques, such as the production of thenium dioxide is to be fixed, exposing the thus coated salts, for the protection of the cathode, for the oxidation support to ruthenium tetroxide in the gaseous state, which of organic compounds and for fuel batteries. is decomposed to ruthenium dioxide upon contact with Another object of the present invention is to furnish a the anchoring layer on which it is preferentially fixed 20 simple and relatively inexpensive process for the produc and then heating the thus treated support. Electrodes pro tion of such an electrode. duced in this manner have an adherent coating of ru A process for producing a coated electrode has been thenium dioxide which is resistant to electrolyte corrosion discovered which does not require the installation of and support high current . costly apparatus and which may be carried out very eco 25 nomically, particularly with respect to the labor required, compared with that of the processes commonly employed at present, especially those coating processes which in BACKGROUND OF THE INVENTION volve the application of Successive coats of material. Fur The present invention pertains to electrodes used in ther, by means of the present invention, electrodes are electrochemical processes composed of a metallic support 30 obtained with an adherent coating which is completely which conducts electricity and which is resistant to corro resistant to electrolytic corrosion, particularly to nascent sion under the conditions prevailing in an electrochemical , and which develops with the release of chlorine cell and a metal oxide coating fixed on the Support which a very low over-voltage which hardly varies with time is also resistant to electrochemical corrosion and which and, in addition, the electrodes support high current favors the exchange of electrons between the support and 35 densities. the ions of the electrolyte. More particularly, this inven According to the present invention, the coating process tion concerns a process for manufacturing a metal Sup is carried out by applying an anchoring layer containing port coated with ruthenium dioxide. at least one compound oxidizable with ruthenium tetrox Recently, various types of metal electrodes have been ide to those areas of a previously etched metal support developed having a coating which is comprised of at 40 wherein ruthenium dioxide is to be fixed, exposing the least one metal oxide of the platinum group. It has been thus coated support to an atmosphere containing ruthe observed that with usage, there is a relatively rapid de nium tetroxide in the gaseous state, the ruthenium tetrox terioration of the electrochemical characteristics of these ide being decomposed to ruthenium dioxide on contact electrodes, which causes, in addition to the necessity of with the anchoring layer on which it is preferentially their replacement, the contamination of the products of 45 fixed, and then heating the thus treated and coated the electrolysis and a decrease in the efficiency of the Support. electric current. Various methods have already been proposed for the DESCRIPTION OF THE PREFERRED production of metal oxide coatings of the platinum group, EMBODIMENTS which are obtained either directly as oxide or in the 50 The support is generally composed of a film-forming metallic state; in the latter case, the metal coating is con metal, such as , , , and verted to the oxide by baking in an oxidizing atmosphere, , or of an alloy consisting principally of at least by heating at high frequency under vacuum, by electroly one of the foregoing metals, i.e. containing at least 50% sis in pulsated current or by immersion in a molten bath by weight of one or more of the foregoing metals. of an oxygenated salt. 55 The etching of the support is effected by any known In most of the proposed coating methods, the coating means such as electrolysis, immersion in a molten bath is carried out in the liquid phase, generally by applying of alkali salts or their mixtures, or in an a solution or Suspension of a compound of the platinum of alkali or organic or inorganic acid; but, generally, im group to the metal support by repeated painting by im mersion in an aqueous solution of oxalic acid or hydro mersion or by spraying, after which the oxide is precipi 60 chloric acid is preferred. tated by chemical, thermal or electrical means. of The composition applied to form the anchoring or the platinum group of metals can also be deposited di adhesive type layer must have a syrupy, i.e. slightly to rectly on the metal support starting with these solutions moderately viscous, liquid consistency so as to form a or suspensions by electrolysis with an alternating cur continuous layer, i.e. a coating without gaps, on the sur rent, or by electrophoresis, and also by immersion of the 65 face of the metallic support which is to be coated eventual 8,668,005 3 4 ly with ruthenium dioxide. The adhesive may be com RuO2 in the final layer, an anchoring layer which is thin posed of any compound or mixture thereof which is oxi ner than specified generally does not provide a coating of dized by ruthenium tetroxide. Ruthenium tetroxide is a RuO of the preferred thickness and, when the anchoring strong and is known to oxidize a wide layer is too thick, an excessive deposit of RuO2 is often variety of organic and inorganic compounds, as indicated obtained which causes a decrease in the electrical polari by numerous publications, for example, Berkowitz et al., zation of the electrode. The thickness of the final coating, J. Am. Chem. Soc. 80, 6682 (1958); Benyon et al., Proc. after thermal treatment, is preferably about 0.1 to 6 g./m. Chem. Soc. 1964 (Oct.), 342; Caputo et al. Tetrahedron and more preferably about 0.5 to 4 g/m. when obtained Lett. 1967 (47), 4729; Benyon et al., Carbohyd. Res. in a single series of operations. 1968, 6(4), 431; Rylander, Engelhard Ind. Tech. Bull. O The sequence of steps according to the invention, i.e. 1969,9(4), 135 and U.S. Pat. No. 3,278,558. application of the anchoring layer, exposure to gaseous Thus, the anchoring layer may contain any compound ruthenium tetroxide, may be repeated several times so as or mixture thereof which is oxidized by ruthenium te to obtain a coating of the desired thickness. When the troxide such as aromatic compounds, olefins, , sequence of steps is repeated, the final thermal treatment , amides, ethers, sulphides, hydrides, plasticizers 5 can be carried out driectly after the final application of for polymers, oils such as coriander oil, paraffin oil, sili the anchoring layer, i.e. the step of exposing the Support cone oil, greases such as silicone grease, hydrocarbon provided with the anchoring layer to ruthenium tetroxide, greases, or mixtures of these substances. The oxidizability may be omitted from the last sequence of steps. of a variety of specific compounds in the aforementioned In a particularly preferred embodiment of the invention, classes by means of ruthenium tetroxide is readily avail 20 the metal support material is titanium or one of its alloys able in the literature, several examples of which have al having anodic polarization properties similar to those of ready been specified. titanium. Such an electrode is particularly suitable as the In order to facilitate their application onto the support, anode in the electrolysis of aqueous solutions of alkali when desirable, any of the foregoing substances may be metal halides. placed in solution or in dispersion in a such as 25 The examples which follow further illustrate the best titanium tetrachloride, tetrachloride, trichloroeth mode currently contemplated for carrying out the inven ylene, perchloroethylene, methylene chloride, benzene, tion but must not be construed as restricting the invention toluene, petroleum ether, gasoline or petroleum. The main in any manner. role of the solvent is to reduce the viscosity of the coating EXAMPLE 1. in order to provide a homogeneous and continuous coating 30 of the support. The solvent may likewise play a role with Small plates of titanium which have been etched by respect to the reduction of gaseous ruthenium tetroxide. immersion over a period of 5 hours at approximately The coating can be carried out by any adequate tech 100 C. in an aqueous solution of 10% oxalic acid are nique, such as painting, smearing, spraying or immersion. coated with PB IV paraffin oil (Pharmacopée Belge, Due to their commercial availability, silicone and par 35 TVéme edition) with the aid of a cloth or impregnated afin oil and grease are preferably used for the anchoring wiping paper and then suspended in a sealed enclosure, layer of the present process. Among other compositions, over an acid solution of ruthenium sulfate obtained by oils and greases, such as silicones, for example polydi heating a solution of ruthenium chloride in the presence methylsiloxane and paraffins, for example Vaseline, which of sulfuric acid until the chlorine ions have been eliminated are in general use as lubricants for valves, taps, bearings 40 completely from the solution. and so forth provide the desired adhesive and oxidative A solution of permanganate is added to the properties required of the anchoring coating of the in solution of ruthenium sulfate and then the temperature vention. Such compositions are sufficiently viscous to pro is slowly increased. The rapid formation of a black de vide continuous coatings on the metallic supports, which posit of ruthenium dioxide is observed on the small plates coatings are capable of catching and anchoring the ruthe 45 of titanium in the areas coated with paraffin oil, resulting nium dioxide thereon. from the reduction of gaseous ruthenium tetroxide released The exposure of the coated support to ruthenium te by the reaction. The exposure to gaseous RuOis continued troxide can be carried out in the presence of air as well until such time that the temperature reaches 90° C., which as in an inert atmosphere. The temperature and the pres takes approximately 2 hours. The small plates are then Sure must be such that the ruthenium tetroxide is in the 50 Subjected to a thermal treatment in the air for a period of gaseous state and the operation is generally carried out 15 hours at different temperatures, as indicated in Table 1. between 20 and 300° C. under a pressure close to the The electrodes produced as described above were tested atmospheric pressure. The period of exposure to gaseous as anodes in the electrolysis of a circulating brine saturated ruthenium tetroxide may vary from 10 minutes to 10 with chloride and chlorine at 60° C., in the pres hours depending on the particular conditions, i.e. tempera 55 ence of a cathode of platinum and their polarization volt ture, and the nature of the anchoring layer. age was measured during the course of electrolysis in Preferably, the exposure to ruthenium tetroxide is car comparison with a saturated calomel electrode using a ried out at a temperature above 40 C. and in an atmos Luggin siphon. The results have been set forth in Table 1. phere composed essentially of air, vapor and RuO. The active surface of each electrode is 1 cm2. The thermal treatment is carried out in an atmosphere 60 containing oxygen at a temperature of about 200 to 550 EXAMPLE 2 C. For convenience, the support which has been coated with an anchoring layer and then exposed to ruthenium The same Small plates of titanium, etched as in Exam tetroxide in the gaseous state is generally heated in the ple 1, were coated with SISS-SI silicone grease for valves presence of air under a pressure equal or inferior to at 65 (supplied by the Société Industrielle des Silicones, Paris, mospheric pressure. Preferably, the periods of heat treat France). This grease has the consistency of petroleum ment does not exceed 15 hours. The particular objects of jelly at room temperature and retains its consistency from the treatment with heat include the oxidation of the re -40 to 500 F. The grease was applied by using differ maining anchoring layer and the crystallization of the ent techniques, in dispersion and in solution in various deposited oxide. 70 , as indicated in Tables 2, 3 and 4. After being Preferably, the anchoring layer, prior to treatment with coated with grease, the small plates were coated with gaseous RuO is of a thickness of about 0.1 to 24 g/m. ruthenium dioxide according to the technique set forth in and more preferably about 0.5 to 12 g/m3. Although Example 1. The thermal treatment was carried out in air there is not a direct correspondence between the thickness for a period of 15 hours at a temperature of 400 to 450 or wt./unit area of the anchoring layer and amount of 75 C. 3,668,005 5 6 The electrodes thus obtained were tested as anodes circulated brine saturated with sodium chloride and chlo under the same conditions as set forth in Example 1 and rine at 80° C., in a cell having a moving cathode the results are set forth in Tables 2, 3 and 4. The active and the variation of the electrolysis current was measured surfaces are 1 cm.. over a period of time for a constant voltage of 7.9 V. at Five of these electrodes must be considered to be the terminals of the cell. The results are set forth in unsatisfactory since their electrical polarization is below Table 6. 50 ma. under 1.800 v. The low electrical polarization of It is noted that after 72 hours of electrolysis under a these electrodes must be attributed to an excessive thick voltage of 7.9 v., a high polarization current is main ness of the anchoring layer which, in the case of silicone tained. grease, should never exceed 23 g/m. and preferably be O EXAMPLE 6 under 15 g./m... An anchoring layer which is too thick often results in an excessive deposit of ruthenium oxide. After etching as indicated in Example 1, the small plates The final coating weight should not exceed 5 g/m. after of titanium were coated by immersion in the silicone the final thermal treatment during which the silicone grease or oil already mentioned containing various quan grease is volatilized, and particularly that which has not 5 tities of pulverulent titanium hydride in suspension. After reacted with gaseous RuO4. the elimination of the excess coating by wiping with paper, the small plates were then covered with ruthenium di EXAMPLE 3 oxide and subjected to a thermal treatment in accordance The same small plates of titanium, etched in the manner with the technique described in Example 3. set forth in Example 1, were coated by means of immer 20 The small plates thus coated were tested as anodes in sion in solutions or emulsions having various concentra the same conditions as indicated in Example 5 and showed tions of SISS-SI silicone grease for valves in methylene polarizations indicated in Table 7. Here too, the high chloride. After immersion in the coating baths which has polarizations are maintained over a period of time. a very high viscosity (emulsions having a high concentra COMPARATIVE EXAMPLE tion and above all pure grease at 100%), the small plates 25 were subjected to wiping with paper in order to eliminate A comparative test was carried out on titanium elec any possible discontinuities or gaps in the coating layer trodes coated with RuO by means of the reduction of and in order to eliminate the excess coating in particular gaseous RuO on silicone grease according to the present at the of the small plates. invention, titanium electrodes coated with RuO obtained As in the preceding examples, the small plates were 30 by painting a solution of ruthenium chloride on the elec then suspended over the acid solution of ruthenium sul trodes and titanium electrodes coated with a mixture of fate in a sealed enclosure, but the technique of exposure RuOTiO2 containing 31 mole percent of RuO likewise to the vapors of RuO was modified after adding the solu obtained by painting. These electrodes were tested as tion of potassium permanganate, the temperature was anodes for the electrolysis of a brine under the conditions rapidly increased while placing the enclosure in an oven 35 described in Example 1, under anodic current densities of at 90° C. where it was kept for a period of 4 hours. Final 4 ka./m. and 8 ka, /m.. The potentials as compared ly, the small plates were heated to a temperature of 400 with the saturated calomel electrode were measured in to 450° C. in the air for a period of 15 hours. motion, while the anodes effectively delivered a current The electrodes thus obtained, whose active surface was of 4 or 8 ka,/m.. The results are given in Table 8 in mv.: still 1 cm., were tested as anodes under the same condi 40 tions set forth in Example 1. The results are set forth in TABLE 8 Table 5. Coating of RuO2 by Coating of Coating of It is noted that the wiping reduces the thickness of the Anodic current , gaseous RuO2 by RO-TiO2 anchoring layer for the high concentration baths so that Kaira. 3S painting by painting in no case the final coating weight exceeds 5 g/m. All 4------1,370 1,450 i,735 of the electrodes in this example show extremely advan 45 8------1,650 1,800 2,000 tageous polarization characteristics which are maintained during the course of time. After 72 hours of electrolysis, the potential of the EXAMPLE 4 electrode in accordance with the present invention was 50 maintained at 1650 mV. under 8 ka./m., with variations In the manner set forth hereinabove, a small plate of of 10 mV. titanium, identical to the plates described in the preceding Under the same conditions, the potential of the RuO examples, is etched and coated with an anchoring layer coated electrode, obtained by painting remained at ap of 2.5 g/m. by means of immersion in undiluted “M proximately 1800 mV. for a period of 24 hours and then 1028' silicone oil (Union Chimique Belge), which has a it increased considerably to 2250 mV. after 46 hours. viscosity of 50 centistokes, a flash point of 342 C. and a The coating process of the present invention has indis self-ignition temperature of 485 C. The coated plates are putable advantages over the known processes. Using the then treated with gaseous RuO and subjected to a thermal process of the invention, it is possible to treat a large treatment in accordance with the techniques described in number of electrodes simultaneously and the process is Example 3. The final coating weight was 3.51 g/m.2. 60 easily adapted to integral automation of the sequence of Tested as an anode under the same conditions as those required steps. Further, and above all, the process of the Set forth in the preceding examples, the small plate thus invention provides electrodes having an adherent coating coated showed a polarization of 950 ma. under a voltage with advantageous and durable electrochemical charac of 1800 y. teristics, in particular with respect to anodes used for the EXAMPLE 5 65 electrolysis of aqueous solutions of alkali metal halides. Finally, the process of the invention to the forma The same small plates of titanium which are etched in tion of a deposit of isotropic oxide having maximum the same manner as set forth in Example 1 were coated by electric conductivity. means of immersion in solutions of 10% SISSSI silicone TABLE 2 grease or M 1028 silicone oil in titanium tetrachloride and 70 Technique for applying anchoring layer: Impregnated cloth then covered with ruthenium dioxide and subjected to Solvent for the anchoring composition.------CHCI thermal treatment in accordance with the technique de Concentration of the dispersion, percent 00 100 ------scribed in Example 3. Final coating (g.fm.2).------4.65 3.70 2.44 Polarization (nv.f300 ma).--- -- 2, 175 1,430 1,300 The electrodes thus obtained, whose active surface is Polarization (ma.11.800 v.).------15 700 1,000 still 1 cm., were tested as anodes for the electrolysis of 75 3,668,005

TABLE 3 Technique for applying anchoring layer: Brush Solvent for the anchoring composition...... CHCl2 CEIC CH2Cl2 CH2Cl2 CC CC CC14 CaC Concentration of the Solution, percent.------5 20 30 5 O 20 10 Anchoring layer (gfm.).----- 6.1 9.9 26.1 48.2 4.6 1.8 2. 4.6 Final coating (gln).--- 4.93 ... 64 6.07 7. 03 1.26 1.45 98 Polarization (nv.f300 m.a.) 2,100 1,600 >2,000 >2,000 1,710 1,820 1,500 1,900 Polarization (mafi.800 v.). 130 430 7 6 380 290 570 230

TABLE 4 Technique for applying anchoring layer: Immersion Solvent for anchoring layer------, CC CC4 CC Concentration of solution, percent--- 5 10 20 30 O 20 O Anchoring layer (gfm.)------1.4 4.6 24,8 46.5 1. 6.9 46.0 4.2 Final coating------10 1.60 5.18 1.64 29 179 26.3 1.37 Polarization (m.w.f300 m.a.).------>2,000 1,800 >2,000 >2,000 1,650 >2,000 >2,000 1,360 Polarization (mall.800 v.)------160 300 5 0. 400 70 ... 4 770

ABLE 5 Technique of applying anchoring layer Immersion without wiping With wiping Percent of silicone grease in the coating bath. 0.1 3 3 5 5 10 00 Weight of the anchoring layer (gfm).------... 2 0.5 0.5 6 ... 4 4.2 3.2 0.7 0.5 0, Weight of the final coating (gln)--...-- 2.0 O. 61 2.82 496 4.50 2.70 2.43 4.27 3.62 3.27 Pelarization (m.w.f300 ma).------1,370 1,345 1,345 ------1,385 1,410 1,345 1,320 1,380 Polarization (mafi.800 v.)------780 900 925 950 10,50 840 770 870 960 730 TABLE 6 5. Process according to claim 1, in which said metallic support consists essentially of titanium or an alloy thereo Silicone compound Grease Grease Oil havingpp anodic polarizationy properties substantiallyy thereof the Anchoring layer (g-in.)------4.42 3.08 3.35 Final coating (g fin.)------4,0 3.93 4.04 30 same as that of titanium. Initial polarization (a.I.9 v.).------3.8 3, 4 3.7 6. Process according to claim 1, in which said com Polarization after 72 hours (a.17.9 w.)------3. 8 2.2 3.3 pound is at least one member selected from the group consisting of an organosilicon compound and a paraffin TABLE and in which said anchoring layer is applied to said sup 35 port in admixture with a solvent. Silicone compound Grease Grease Oil 7. Process according to claim 1, in which said com Hydride content,(percent in weight).------50 O 60 Final coating (gfm.')------3.60 3.80 3.80 pound is paraffin. Initial polarization (af7.9 v.).------3, 6 4, 1 3. 8 8. Process according to claim 1, in which the sequence 2.9 4.0 3. of steps including application of the anchoring layer, ex 40 posure to gaseous ruthenium tetroxide and the thermal What we claim as new and desire to secure by Letters treatment is repeated a plurality of times. Patent is: 9. Process according to claim 8, in which in the final 1. Process for manufacturing an electrode for electro sequence of steps, after applying the anchoring layer to chemical processes wherein the electrode is composed of said support, the step of exposing the support to gaseous a metallic support which conducts electric current and ruthenium tetroxide is omitted and the coated support is resistant to corrosion under the conditions prevailing 45 is subjected directly to a thermal treatment. in the electrochemical cell, and a coating which is resistant 10. Process according to claim 1, in which said com to electrochemical corrosion consisting essentially of ru pound is an organosilicon compound. thenium dioxide fixed on at least part of the surface of the support which comprises applying an anchoring layer 11. Process according to claim 10, in which the organo containing at least one compound oxidizable by ruthenium compound is selected from the group consisting of tetroxide on the area of said support where said coating 50 a silicone oil and a silicone grease. is to be fixed, exposing said support provided with said 12. Process according to claim 10, in which the organo anchoring layer to ruthenium tetroxide in the gaseous state silicon compound is polydimethylsiloxane. whereupon the ruthenium tetroxide is preferentially fixed 13. Electrode obtained by the process of claim 1. on the area of said support provided with said anchoring layer, and then subjecting the support to a thermal treat 55 References Cited ment. UNITED STATES PATENTS 2. Process according to claim 1, in which the exposure to said gaseous ruthenium tetroxide is carried out at a 3,562,008 2/1971 Martinsons. temperature of about 20 to 300 C. and under a pressure 60 FOREIGN PATENTS of about atmospheric pressure. 1,147,442 4/1969 Great Britain. 3. Process according to claim 1, in which the thermal treatment of the coated support is carried out in the pres ALFRED L. LEAVITT, Primary Examiner ence of air at a temperature of about 200 to 550' C. 4. Process according to claim 1, in which said metallic 65 C. K. WEIFFENBACH, Assistant Examiner support consists essentially of titanium, zirconium, niobi um, tantalum, tungsten, or an alloy constituted principally U.S. C. X.R. of at least one of the foregoing metals. 117-106, 161 ZA, 218, 230; 204-290 F