METHOD OF COATING AN ELECTRODE
United States Patent 3720590
A novel method of producing an anode for use in the electrolysis of an aqueous solution such as of an alkali metal chloride. The method includes electrodeposition of a layer of a platinum group metal from a solution containing an organic material onto an electroconductive substrate, typically of titanium. After the electrodeposition, the coated substrate is heat treated to decompose the organic material and form a coating comprising a platinum group metal and carbon.
US Patent References:
PROCESS FOR CODEPOSITING PLATINUM METAL AND A WET-PROOFING POLYMER
Lyons, Jr. et al. - August 1969 - 3461044
PROCESS FOR CODEPOSITING PLATINUM METAL AND A WET-PROOFING POLYMER
Lyons, Jr. et al. - August 1969 - 3461044
Inventors:
Dewitt, Bernard J. (Akron, OH)
Martinsons, Aleksandrs (Wadsworth, OH)
Martinsons, Aleksandrs (Wadsworth, OH)
Application Number:
05/155647
Publication Date:
03/13/1973
Filing Date:
06/22/1971
View Patent Images:
Export Citation:
Assignee:
PPG Industries, Inc. (Pittsburgh, PA)
Primary Class:
Other Classes:
205/227, 205/264, 204/290.150, 428/670, 204/290.120, 205/234, 204/290.140
International Classes:
C25B11/08; C25B11/10; C23B5/52
Field of Search:
204/14N,37R,181,29F
Primary Examiner:
Mack, John H.
Assistant Examiner:
Solomon I, W.
Parent Case Data:
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of our copending U.S. Application Ser. No. 850,258, filed Aug. 14, 1969 and now abandoned.
Claims:
1. A method of producing an anode for electrolysis of aqueous alkali metal chloride, said anode including an anodically resistant substrate and an electroconductive coating, said method comprising:
2. The method of claim 1 wherein the electroconductive substrate is a titanium group metal.
3. The method of claim 2 wherein said decomposible organic material comprises a solvent in which said platinum group metal compound and said electrolyte are soluble.
4. The method of claim 2 wherein said solution further includes an organic solvent in which said platinum group metal compound and said electrolyte are soluble.
5. The method of claim 4 wherein the organic solvent is a member selected from the group consisting of alcohols and ketones.
6. The method of claim 4 wherein the solvent is a member selected from the group consisting of ethyl alcohol, propyl alcohol, butyl alcohol, acetone, ethyl methyl ketone, dioxane, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, N-Methyl pyrrolidone, furfural, acetylacetone, polyethylene glycols and phenol.
7. The method of claim 4 wherein the organic material includes at least one member selected from the group consisting of toluene and tetraethyl orthosilicate.
8. The method of claim 4 wherein the platinum group metal compound comprises a member selected from the group consisting of the inorganic acids and inorganic salts of the platinum group metal.
9. The method of claim 4 wherein the solution is free of uncombined water.
10. The method of claim 4 wherein an electrical potential is applied to the electroconductive substrate sufficient to provide a current density of between 2 and 50 amperes per square foot of plating surface.
11. The method of claim 4 wherein said electroplating solution further includes a compound of a second platinum group metal.
12. A method of electroplating a platinum group metal onto a substrate comprising a titanium group metal, the method comprising etching the base member, immersing the base member in a bath including by weight between 5 and 30 parts of a decomposible organic material, a platinum group metal compound providing 0.4 and 2.0 parts of a platinum group metal, 5 and 30 parts of a highly ionizible material capable of providing the electrolyte with a conductivity of at least 0.01 mho per 100 parts of an organic solvent capable of dissolving said organic material, platinum group metal compound, and ionizible material, applying sufficient electrical potential to said base member as a cathode to provide a current density of between 2 and 50 amperes per square foot of cathodic surface, removing the base member from said bath and heat treating the base member to decompose the included organic material forming an electroconductive layer comprising the platinum group metal and carbon.
13. The method of claim 12 wherein the heat treatment is at between 200°C. and 700°C. for between 15 and 120 minutes.
14. The method of claim 12 wherein the electrolyte is a member selected from the group consisting of organic acids and inorganic acids.
15. The method of claim 14 wherein said solution has a conductivity of at least 0.01 mho.
16. The method of claim 12 wherein the electrolyte is a mixture of acetonitrile and tetra-n-butyl ammonium bromide.
17. An electrode prepared according to claim 1.
18. An electrode prepared according to claim 12.
2. The method of claim 1 wherein the electroconductive substrate is a titanium group metal.
3. The method of claim 2 wherein said decomposible organic material comprises a solvent in which said platinum group metal compound and said electrolyte are soluble.
4. The method of claim 2 wherein said solution further includes an organic solvent in which said platinum group metal compound and said electrolyte are soluble.
5. The method of claim 4 wherein the organic solvent is a member selected from the group consisting of alcohols and ketones.
6. The method of claim 4 wherein the solvent is a member selected from the group consisting of ethyl alcohol, propyl alcohol, butyl alcohol, acetone, ethyl methyl ketone, dioxane, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, N-Methyl pyrrolidone, furfural, acetylacetone, polyethylene glycols and phenol.
7. The method of claim 4 wherein the organic material includes at least one member selected from the group consisting of toluene and tetraethyl orthosilicate.
8. The method of claim 4 wherein the platinum group metal compound comprises a member selected from the group consisting of the inorganic acids and inorganic salts of the platinum group metal.
9. The method of claim 4 wherein the solution is free of uncombined water.
10. The method of claim 4 wherein an electrical potential is applied to the electroconductive substrate sufficient to provide a current density of between 2 and 50 amperes per square foot of plating surface.
11. The method of claim 4 wherein said electroplating solution further includes a compound of a second platinum group metal.
12. A method of electroplating a platinum group metal onto a substrate comprising a titanium group metal, the method comprising etching the base member, immersing the base member in a bath including by weight between 5 and 30 parts of a decomposible organic material, a platinum group metal compound providing 0.4 and 2.0 parts of a platinum group metal, 5 and 30 parts of a highly ionizible material capable of providing the electrolyte with a conductivity of at least 0.01 mho per 100 parts of an organic solvent capable of dissolving said organic material, platinum group metal compound, and ionizible material, applying sufficient electrical potential to said base member as a cathode to provide a current density of between 2 and 50 amperes per square foot of cathodic surface, removing the base member from said bath and heat treating the base member to decompose the included organic material forming an electroconductive layer comprising the platinum group metal and carbon.
13. The method of claim 12 wherein the heat treatment is at between 200°C. and 700°C. for between 15 and 120 minutes.
14. The method of claim 12 wherein the electrolyte is a member selected from the group consisting of organic acids and inorganic acids.
15. The method of claim 14 wherein said solution has a conductivity of at least 0.01 mho.
16. The method of claim 12 wherein the electrolyte is a mixture of acetonitrile and tetra-n-butyl ammonium bromide.
17. An electrode prepared according to claim 1.
18. An electrode prepared according to claim 12.
Description:
BACKGROUND OF THE INVENTION
This invention relates to electrodes for electrolytic cells and, more particularly, to a corrosion-resistant, dimensionally stable anode such as for electrolysis of aqueous alkali metal chloride in the production of elemental chlorine or alkali metal chlorate.
The electrolysis of aqueous alkali metal chloride solutions such as solutions of sodium chloride or potassium chloride is conducted on a vast commercial scale. In the production of alkali metal chlorate, anodes and cathodes, or bipolar electrodes which wen arranged in a spaced electrical series in an electrolytic cell may serve as both anode and cathode, are immersed in an aqueous solution of the sodium chloride or the like and an electrical potential is established between the electrodes. In the past, graphite or carbon electrodes have been used as anodes or as the bi-polar electrodes in series. In consequence of the electrochemical reactions which occur, alkali metal chlorate is produced either directly in the cell or outside the cell after the solution is allowed to stand.
The electrolysis of alkali metal chloride to produce elemental chlorine and alkali metal hydroxide is conducted in two general types of cells--the diaphragm and the mercury cathode cell. In the diaphragm cell, the cell is divided into two compartments--the anode compartment and the cathode compartment--which are separated by a porous diaphragm usually of asbestos. The cathode is of perforate metal and the asbestos diaphragm is in contact with the cathode. The anode, usually of carbon or graphite, is disposed centrally in the anode compartment.
In the mercury cathode cell, the cathode is a flowing stream of mercury which flows along a solid metal base connected to the negative pole of a power source. The anode, again of carbon or graphite, is spaced from the mercury cathode and, as electric current flows, the sodium or like alkali metal is evolved and collected in the mercury as an amalgam which is removed from the cell. Outside the cell the mercury amalgam is contacted with water in a "denuder" to remove the sodium as sodium hydroxide solution and the mercury is then recycled.
In operating each of the above-described cells, one is confronted with a common problem; namely, that during the course of the electrolysis, the carbon or graphite electrode erodes and/or decomposes. Consequently, as the electrodes wear away or erode, the spacing between the electrodes increases with resulting increase in voltage between electrodes. This, together with the reactions which cause degradation of the anode, results in a loss of current efficiency for the production of the desired product. The graphite anodes ultimately must be replaced. In all these cells this erosion increases as the anode current density is increased. At the same time, the trend of operation is toward high current density to increase the amount of product produced per unit cell. Thus it has become necessary to resort to anodes or bipolar electrodes which remain dimensionally stable and do not erode appreciably over long periods of cell operation.
The present invention is directed to an improved stable electrode and method of producing a stable electrode. Electrodes herein contemplated normally should possess a certain degree of rigidity and, in any event, they must have surfaces which exhibit good electrolytic characteristics. Those characteristics, particularly in the case of anodes, include low oxygen and chlorine overvoltage, resistance to corrosion and decomposition in the course of use as anodes in the electrolytic cell, and minimum loss of coating during such use. It is well known that certain metals, metallic oxides, and alloys are stable during electrolysis and have other superior properties when used as anodes. Such metals typically include the members of the platinum group; namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum. These metals are not satisfactory for construction of the entire electrode since, for example, their cost is prohibitive. Therefore, these metals are commonly applied as a thin layer over a strength or support member such as a base member made of one of the titanium group metals. As used with respect to the present invention, the term titanium group metal is intended to include titanium, tantalum, zirconium, niobium, and alloys thereof. These support members or substrates have good chemical and electrochemical resistance to the alkali metal chloride electrolyte and the products of electrolysis, e.g., chlorine, hypochlorite, and/or chlorate, but may be lacking in good surface electroconductivity because of their tendency to form on their surface an oxide having poor electroconductivity.
In the past, such anodes generally have been produced by applying to the base member a layer of a thermally decomposable organic mixture containing a noble metal-organic compound such as platinum resinate. The temperature is slowly raised to a point, for example, between 400°F. and 700°F., in order to decompose and/or volatilize the organic matter and other components, leaving a deposit of the noble metal or noble metal oxide. This heat treatment usually takes several hours. In like manner, a plurality of layers of the organic mixture is applied, each layer being followed by the heat treatment. Although this previous method provides excellent anodes, it has certain inherent disadvantages. A commercial anode preferably should have a coating thickness of about 40 to 50 micro inches which requires application of about 25 coats of the organic mixture. The production of such a commercial anode requires several days. Thus, the method is costly in terms of the amount of handling during production, time required for production, and heat energy expended during production.
Electrodes have been produced by electroplating. Although the known methods for producing electrodes by electroplating are less expensive than the thermal-decomposition methods, the known methods of electroplating have not produced completely satisfactory anodes. For example, the resulting platinum metal coating lacks durability and has a high overvoltage as compared with anodes produced by the thermal-decomposition method. Furthermore, if the electroplated anode is heated to increase durability, the overvoltage is further increased.
The present invention provides a method for electrolytically depositing a platinum metal coating on a substrate that is anodically resistant. In other words, the substrate will withstand the conditions in the anodic zone of the cell and resist corrosion and/or decomposition, for example, by chlorine or oxygen. The titanium group metals including titanium, tantalum, zirconium, niobium, and alloys thereof are especially useful for this purpose. The present method provides for rapid production of high-quality electrodes. An anode having a platinum coating of between 40 and 50 micro inches can be produced by electroplating for about 1 hour. The resulting anode can be heated, thereby making a very durable electrode and yet the overvoltage remains very low. For example, anodes produced by this method have been found to have a life-time chlorine overvoltage of less than 0.05 volts and a consumption rate of 0.3 grams or less of platinum per ton of product when used in a chlorate cell and 0.1 gram or less per ton of chlorine when used in a diaphragm cell.
The present method of producing an anode for electrolysis off aqueous alkali metal halide preferably includes the steps of: (1) thoroughly cleaning the anodically resistant substrate, such as by chemical etching; (2) contacting the substrate with an organic-based solution or electroplating bath containing a platinum group metal compound; (3) applying a cathodic electrical potential to the substrate while it is in contact with the solution, thereby depositing a coating on the substrate including the platinum group metal and an organic material; and (4) treating the coating to decompose the organic material, leaving a coating including the platinum group metal and carbon.
The substrate may be cleaned by any method that will remove the foreign matter, such as oils, greases, and oxides, that is present on the surface to be plated. For example, such surface may be cleaned with a detergent and then etched with an acid such as hydrofluoric acid and/or hydrochloric acid.
The substrate may be contacted with the organic-based solution by any suitable method so long as the method permits application of a cathodic electrical potential during such contact. One method of contacting the substrate with the solution is by submerging the substrate in the solution. Another method is that of flowing the solution over the surface to be plated.
The electroplating solution of the present invention is an organic-based solution including a platinum group metal compound. The term organic-based solution, as used with respect to the present invention, means a solution in which the principal solvent is of an organic nature, such as ethyl alcohol. The solution is preferably substantially free of uncombined water, although in certain instances some free water may be present, for example, 5 percent or less by weight and, rarely if ever, in excess of 10 percent. If free water is present, it may be placed in a combined state by addition of absolute ethyl alcohol or concentrated sulfuric acid to the solution. The solution includes an organic material which will be deposited in the coating. The organic material is a substance that can be decomposed to provide carbon particles in the electroconductive coating, for example, by thermal or chemical means. The organic material may be the solvent for the solution or may be soluble or suspendible in the organic solvent. The solution further includes a highly ionizible electrolyte which makes the solution electroconductive.
An excellent solvent for use in the present invention is absolute ethyl alcohol. Other suitable solvents would include higher alcohols, such as propyl alcohol and butyl alcohol. Other solvents that may be used are acetone and higher ketones such as ethyl methyl ketone. Still other solvents would include, for example, dioxane, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, N-Methyl pyrrolidone, furfural, acetylacetone, polyethylene glycols, phenol, or a mixture of glycol and glycerine.
The highly-ionizible electrolyte is used for increasing the conductivity of the plating solution and may be any electrolyte that would be compatible with the other materials present and provide the plating solution with the necessary conductivity, for example, an inorganic acid such as sulfuric acid or hydrochloric acid. The ionizible electrolyte, alternatively, may be any ionizible organic acid, for example, acetic acid. Moreover, the electrolyte may be any electroconductive organic compound or mixture of compounds, for example, a mixture of acetonitrile and tetra-n-butyl ammonium bromide. The ionizible electrolyte should give the solution a conductivity of at least 0.01 mho and, preferably, about 0.04, although a higher conductivity would not be detrimental.
The organic material provides carbon particles for inclusion in the platinum group metal electroconductive coating and, preferably, is a compound having between 4 and 100 carbon atoms. The organic material may be a hydrocarbon or a surfactant, typically silicon compounds such as tetraethyl orthosilicate. In certain instances, the organic material may serve as the solvent for the electrolyte and platinum group metal compound in which case other solvents would be unnecessary. For example, butyl alcohol would serve this purpose. Although some polymerization of the organic material during electroplating is permissible and possibly even desirable, the organic material should not coagulate or precipitate during electroplating. The preferred organic material is toluene. Other aromatic solvents which would keep the anodic products in solution are also satisfactory.
The compound containing the platinum group metal is a compound that will permit deposition of a platinum metal coating during electrolysis, typically, acids or salts of the metal such as chloroplatinic acid or platinum nitrate. The term platinum group metal, as used with respect to the present invention, includes ruthenium, rhodium, palladium, osmium, iridium, and platinum. The solution preferably contains a small amount of a compound of another platinum group metal. The electroplating bath may also include other materials that may be electrolytically deposited on the electrode; for example, the bath may contain titanium compounds.
The solution or electroplating bath preferably contains on a basis of 100 parts by weight of solvent: 0.4 to 2.0 parts platinum group metal, 5 to 30 parts organic material, and 5 to 30 parts electrolyte. One highly-preferred electroplating bath contained 525 milliliters absolute ethyl alcohol, 125 milliliters concentrated sulfuric acid, 70 milliliters toluene, 17.8 grams chloroplatinic acid crystals, and 2 grams rhodium trichloride crystals.
The electrode is coated by placing a cleaned base member or substrate, typically titanium, as a cathode in an electrolytic plating cell containing the plating solution. An electrical potential is then applied to the cell, thereby depositing the platinum group metal. The electroplating cell may be operated at normal room temperature. Temperatures of between 25°C. and 35°C. have been found highly satisfactory. However, in any event, the temperature will rarely be greater than 70°C. or lower than 10°C.
The current density should be sufficient to deposit a layer of the platinum group metal of satisfactory thickness with a reasonable time. However, the within density should not be so great as to deposit a soft, non-durable layer. Current densities ranging from 2 to 50 amperes per square foot of plating surface have been satisfactorily used in the present invention. Generally, the current density will be within the range of 10 to 40 amperes per square foot. Preferably, the current density is about 25 to 30 amperes per square foot of plating surface. Reversal of the electrical potential for a short time periodically during the electroplating has been found to improve the durability and uniformity of the coating. For example, in one instance where the plating time was 1 hour, the current was reversed for 3 seconds every 15 minutes. The required plating time will vary depending on such factors as current density, solution concentrations, and desired plating thickness. However, the plating time will generally be between 15 and 120 minutes.
Following electrolytic deposition, the electrode is heat treated in order to drive off volatile matter and decompose the organic material producing a coating which comprises the platinum group metal and included carbon particles. The heat treatment will generally be at between 200°C. and 700°C. for between 15 and 120 minutes. However, the temperature may be lower in the case of easily decomposed organic material or higher if heated in a vacuum or inert atmosphere or if the heat treatment is for a shorter period of time.
The electrodes in the following examples were produced using for the substrate a titanium plate which was thoroughly cleaned and etched prior to electroplating. In the cleaning process, the titanium plate was washed with the detergent Comet (Trademark of Procter and Gamble) and rinsed in distilled water. The titanium plate was immersed for 1 minute in a solution containing 1 percent hydrofluoric acid and 37 percent hydrochloric acid by weight and rinsed in distilled water. The plate then was etched in 37 percent hydrochloric acid at a temperature of between 40°C. and 50°C. for from 1 to 4 hours and rinsed with distilled water, then acetone, and then dried in air. The portions of the titanium plate which were to be left uncoated were masked using a Teflon material, thus permitting deposition of the platinum metal only on the desired area.
As described in Examples I - VI, the electrodes of the present invention were tested in the following chlorate cell, chlorine diaphragm cell, and overvoltage cell.
The chlorate test cell container was constructed of rubber-lined steel and was 6 inches by 18 inches by 2 inches. The cathode was a flat steel plate and was 23/4 inches by 14 inches. Five test anodes were spaced 1 inch apart along the cathode. The test anodes were spaced one-fourth inch from the cathode. The feed solution was forced in at the bottom of the cell and the effluent was removed at the top of the cell. The chlorate cell was operated under the following conditions: The cell temperature was 105°C.; the solution feed rate was 5 milliliters per minute. The feed solution was maintained at about pH 6.8 and contained 120 grams sodium chloride, 688 grams sodium chlorate, and 2.0 grams potassium dichromate per liter of aqueous solution.
The chlorine diaphragm cell was constructed of Pyrex*(Trademark) pipe 16 inches in length and 6 inches in diameter. The cathode consisted of 10 mesh iron screens in the form of an enclosed tube having an outer diameter of 2 inches and a length of 6 inches. The diaphragm consisted of two layers of 0.016 inch asbestos paper which covered the screen. The asbestos paper was held in place with asbestos tape. The cathode was centrally located in the Pyrex pipe. Four test anodes were equally spaced around the cathode. The anodes were spaced three-fourths inch from the cathode. The chlorine cell was operated at 90°C. The cell feed solution was an aqueous brine containing 313 grams per liter sodium chloride. The feed solution was maintained at pH 10 and was fed to the cell at the rate of 350 milliliters per hour.
The overvoltage cell consisted of a pair of Pyrex compartments (an anode compartment and a cathode compartment) each containing about 100 milliliters. The compartments were connected by a 3 inch piece of glass tubing having an internal diameter of 11/2 inches. The cathode, disposed centrally in the cathode compartment, was a piece of platinum foil 1 inch square and 0.006 inch in thickness. The cathode was welded to a platinum wire for electrical connection. The anode to be tested was disposed centrally in the anode compartment and had an electroconductive surface that was three-fourths inches by one-half inches. The cell feed solution was an aqueous solution containing 300 grams reagent grade sodium chloride per liter. The solution in the cell was maintained at about pH 7. A constant temperature of 90°C. was maintained by immersing the cell in a heated bath of glycol. A course of direct current was supplied to the cell by a Kocour Model 15 Silicon Rectifier. The voltage was measured with a Leeds and Northrup Type K-3 Universal Potentiometer in conjunction with an Eppley Laboratory, Inc., Standard Cell Model 100 and a GM Laboratories, Inc., Galvanometer Model 570-402. The voltages were measured against a saturated calomel electrode having an EMF of 0.2415 volts using a Luggin Tip and a saturated sodium chloride solution bridge. The overvoltages were calculated by assuming the equilibrium chlorine voltage at 90°C. to be 1.28 volts.
EXAMPLE I
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The titanium plate was cleaned and etched as aforedescribed, the etching being for 1 hour at 40°C. A plating solution was prepared by mixing 0.78 grams chloroplatinic acid crystals (Reagent A.C.S.), 75 milliliters absolute ethyl alcohol (Reagent Quality), 15 milliliters sulfuric acid (Reagent A.C.S.), 20 milliliters toluene (Certified A.C.S.), and 20 milliliters acetone (Certified A.C.S.).
In Examples I, II, and V the plating cell consisted of a glass tube, sealed at the bottom end, having an inside diameter of 11/2 inches and sufficient length to permit submersion of the portion of electrode to be electroplated. The plating cell included a platinized titanium anode which was 6 inches by 3/8 inch by 1/16 inch. The titanium substrate to be plated was placed in the cell as a cathode. The electrode of Example I was produced by applying a current of 0.5 amperes. The total plating area was 2 square inches, thus the current density was 36 amperes per square foot of plating area. The plating was continued for 30 minutes. The electrode was removed from the cell and heat treated by starting at room temperature and increasing the temperature 50°C. every 5 minutes up to a temperature of 350°C. The electrode was maintained at 350°C. for 15 minutes and then cooled. The electrode was operated as an anode in the aforedescribed chlorate cell at 360 amperes per square foot for 18 hours. The voltage remained constant at 3.4 volts during the 18 hours. A piece 11/4 inches by 1/2 inch, having an electroconductive surface of 3/4 inch by 1/2 inch, was cut from the electrode and tested as an anode in the overvoltage cell. The electrode was operated at 670 amperes per square foot of anode surface for 49.5 hours. The overvoltage remained below 0.05 volts throughout the test. The platinum coating had a thickness of 40 micro inches.
EXAMPLE II
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The electrode was cleaned and then etched for 1 hour. A plating solution was prepared by mixing 0.5 grams chloroplatinic acid crystals (Reagent A.C.S.), 30 milliliters absolute ethyl alcohol (Reagent Quality), 10 milliliters toluene (Certified A.C.S.), and 5 milliliters concentrated sulfuric acid (Reagent A.C.S.). The electrode was plated at 0.12 amperes for 1 hour. The electrode was heat treated by starting at room temperature and increasing the temperature 50°C. every 5 minutes until reaching a temperature of 400°C. The electrode was then held at 400°C. for 15 minutes. The electrode was examined by X-ray and found to have a platinum thickness of 26.8 micro inches. The electrode which had a plated area of 2 square inches was used as an anode in the chlorate cell. The cell voltage at an anode current density of 6 amperes was 3.30 volts after 4 hours of operation and 3.55 volts after 23 hours of operation.
EXAMPLE III
An electrode was prepared from a titanium plate 2 inches by 23/4 inches by 1/16 inch. The titanium plate was cleaned and etched for 1.5 hours at between 45°C. to 51°C. The electroplating contained -b 280 milliliters absolute ethyl alcohol (Reagent Quality), 60 milliliters concentrated sulfuric acid (Reagent A.C.S.), 70 milliliters toluene (Certified A.C.S.), 34 milliliters acetone (Certified A.C.S.), 9.85 grams chloroplatinic acid crystals (Reagent A. C. S.), and 0.5 milliliters of a rhodium plating solution. The rhodium plating solution contained 100 grams of rhodium as rhodium trichloride per liter of ethyl alcohol. The rhodium plating solution was of the type produced by Englehard Industries, Inc., in 1969 and designated Rhodium Plating Solution No. 221.
In this Example III (and also Examples IV, V, and VI), the plating cell was provided by a 16 ounce, wide-mouth bottle. The electrode was plated in the solution at 0.5 amperes for 1 hour and then heat treated starting at room temperature and raising the temperature 50°C. every 5 minutes up to 400°C. The temperature of 400°C. was maintained for 15 minutes. The platinum thickness as measured by X-ray was about 50 micro inches. X-ray disclosed the presence of traces of rhodium. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell for 23 hours at a current density of 6 amperes and a cell temperature of 38°C. The cell voltage was 3.11 volts.
EXAMPLE IV
An electrode was prepared by cleaning the titanium plate and etching for 1 hour. The titanium plate was 2 inches by 23/4 inches by 1/16 inch. The electrode was then plated in a solution containing 150 milliliters absolute ethyl alcohol (Reagent Quality), 30 milliliters concentrated sulfuric acid (Reagent A.C.S.), 25 milliliters toluene (Certified A.C.S.), 5.3 grams chloroplatinic acid crystals (Reagent A.C.S.), and 1.0 gram rhodium trichloride trihydrate crystals (Purified). The electrode was plated at 0.5 amperes for 1 hour, rinsed in water, and dried. The electrode was heat treated by slowly raising the temperature to 450°C. and holding at that temperature for 15 minutes. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell for 3 hours and 20 minutes. The final voltage was 3.17 volts at 6 amperes.
EXAMPLE V
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The titanium plate was cleaned and etched for 1.5 hours at 55°C., rinsed, and dried. The electrode was plated in a solution containing 135 milliliters absolute ethyl alcohol (Reagent Quality), 10 milliliters concentrated sulfuric acid (Reagent A.C.S.), 5 milliliters toluene (Certified A.C.S.), 40 milliliters tetraethyl orthosilicate (Practical), 4.12 grams chloroplatinic acid crystals (Reagent A.C.S.), and the electrode was plated at 0.2 amperes for 40 minutes, reversing the current for 5 seconds every 5 minutes. The electrode was heat treated to 450°C. The electrode had a plated area of 2 square inches. When operated in the chlorate cell at 360 amperes per square foot of anode surface for 71/2 hours, the voltage was 3.31 volts.
EXAMPLE VI
An electrode was prepared using a titanium plate 2 inches by 23/4 inches by 1/16 inch which had been cleaned and then etched for 2 hours and 30 minutes at between 40°C. and 47°C. The titanium plate was rinsed and dried. The electrode was plated in a solution containing 525 milliliters absolute ethyl alcohol (Reagent Quality), 125 milliliters concentrated sulfuric acid (Reagent A.C.S.), 70 milliliters toluene (Certified A.C.S.), 2 grams rhodium trichloride trihydrate crystals (Purified), and 17.8 grams chloroplatinic acid crystals (Reagent A.C.S.). The electrode was plated at 0.1 amperes for 15 minutes and the current was reversed for 15 seconds. This procedure was repeated 8 times. The plating voltage was 2.4 volts, the voltage on reversal was maintained less than 0.6 volts. The electrode was heat treated by slowly starting at room temperature and heating to 450°C. The electrode was held at that temperature for 15 minutes. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell and after 41 hours the voltage at 6 amperes was 3.25 volts and at 10 amperes was 3.90 volts. The electrode VI-A was tested in the overvoltage cell and the chlorine overvoltage at 1,000 amperes per square foot was about 0.1 volts.
EXAMPLE VII
An electrode was prepared using the same solution used in Example VI. The titanium substrate was 6 inches by 3/8 inch by 1/16 inch. The substrate was cleaned and etched for 1 hour and 45 minutes. The electrode was plated for 5 minutes at 0.06 amperes, for 10 minutes at 0.035 amperes, current was reversed for 3 seconds at 0.035 amperes, and then further plated for 60 minutes at 0.35 amperes. The current was reversed 4 times at 0.35 amperes during the final 60 minutes; at 17.5 minutes current was reversed for 3 seconds; at 50 minutes reversal was for 3 seconds; and after the 60 minutes reversal was for 5 seconds. The electrode was rinsed, dried, and heat treated by raising the temperature from room temperature to 550°C. in 17.5 minutes. The electrode was held at 550°C. for 1 hour. The electrode had a platinum coating of about 57 micro inches. The electrode, which had a plated area of 2 square inches, was tested in a chlorate cell as an anode at 360 amperes per square foot of anode surface and 30°C. The voltage was 3.20 volts after 22 hours of operation. The electrode was further tested in a diaphragm cell at 500 amperes per square foot. The voltage after 24 hours was 3.589 volts. After 35 days of operation the voltage was 3.680 volts; after 90 days, 3.552 volts; and after 174 days, 3.430 volts. The voltage after 365 days of operation was 3.434 volts. The platinum loss during the 365 days was 3.7 micro inches or 0.035 grams per ton of chlorine.
Electrodes made according to the present invention are highly suitable for use as anodes in cells used for the electrolysis of aqueous alkali metal chloride solutions, typically diaphragm cells, mercury cathode cells, and chlorate cells such as those shown in U.S. Pat. Nos. 3,400,055; 3,337,443; 3,312,614; 3,287,250; 3,203,882; 3,119,664; 3,116,228; 2,897,463; and 2,719,117.
Although the present invention has been described with reference to the specific details of particular embodiments thereof, it is not intended thereby to limit the scope of the invention except insofar as the specific details are recited in the appended claims.
This invention relates to electrodes for electrolytic cells and, more particularly, to a corrosion-resistant, dimensionally stable anode such as for electrolysis of aqueous alkali metal chloride in the production of elemental chlorine or alkali metal chlorate.
The electrolysis of aqueous alkali metal chloride solutions such as solutions of sodium chloride or potassium chloride is conducted on a vast commercial scale. In the production of alkali metal chlorate, anodes and cathodes, or bipolar electrodes which wen arranged in a spaced electrical series in an electrolytic cell may serve as both anode and cathode, are immersed in an aqueous solution of the sodium chloride or the like and an electrical potential is established between the electrodes. In the past, graphite or carbon electrodes have been used as anodes or as the bi-polar electrodes in series. In consequence of the electrochemical reactions which occur, alkali metal chlorate is produced either directly in the cell or outside the cell after the solution is allowed to stand.
The electrolysis of alkali metal chloride to produce elemental chlorine and alkali metal hydroxide is conducted in two general types of cells--the diaphragm and the mercury cathode cell. In the diaphragm cell, the cell is divided into two compartments--the anode compartment and the cathode compartment--which are separated by a porous diaphragm usually of asbestos. The cathode is of perforate metal and the asbestos diaphragm is in contact with the cathode. The anode, usually of carbon or graphite, is disposed centrally in the anode compartment.
In the mercury cathode cell, the cathode is a flowing stream of mercury which flows along a solid metal base connected to the negative pole of a power source. The anode, again of carbon or graphite, is spaced from the mercury cathode and, as electric current flows, the sodium or like alkali metal is evolved and collected in the mercury as an amalgam which is removed from the cell. Outside the cell the mercury amalgam is contacted with water in a "denuder" to remove the sodium as sodium hydroxide solution and the mercury is then recycled.
In operating each of the above-described cells, one is confronted with a common problem; namely, that during the course of the electrolysis, the carbon or graphite electrode erodes and/or decomposes. Consequently, as the electrodes wear away or erode, the spacing between the electrodes increases with resulting increase in voltage between electrodes. This, together with the reactions which cause degradation of the anode, results in a loss of current efficiency for the production of the desired product. The graphite anodes ultimately must be replaced. In all these cells this erosion increases as the anode current density is increased. At the same time, the trend of operation is toward high current density to increase the amount of product produced per unit cell. Thus it has become necessary to resort to anodes or bipolar electrodes which remain dimensionally stable and do not erode appreciably over long periods of cell operation.
The present invention is directed to an improved stable electrode and method of producing a stable electrode. Electrodes herein contemplated normally should possess a certain degree of rigidity and, in any event, they must have surfaces which exhibit good electrolytic characteristics. Those characteristics, particularly in the case of anodes, include low oxygen and chlorine overvoltage, resistance to corrosion and decomposition in the course of use as anodes in the electrolytic cell, and minimum loss of coating during such use. It is well known that certain metals, metallic oxides, and alloys are stable during electrolysis and have other superior properties when used as anodes. Such metals typically include the members of the platinum group; namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum. These metals are not satisfactory for construction of the entire electrode since, for example, their cost is prohibitive. Therefore, these metals are commonly applied as a thin layer over a strength or support member such as a base member made of one of the titanium group metals. As used with respect to the present invention, the term titanium group metal is intended to include titanium, tantalum, zirconium, niobium, and alloys thereof. These support members or substrates have good chemical and electrochemical resistance to the alkali metal chloride electrolyte and the products of electrolysis, e.g., chlorine, hypochlorite, and/or chlorate, but may be lacking in good surface electroconductivity because of their tendency to form on their surface an oxide having poor electroconductivity.
In the past, such anodes generally have been produced by applying to the base member a layer of a thermally decomposable organic mixture containing a noble metal-organic compound such as platinum resinate. The temperature is slowly raised to a point, for example, between 400°F. and 700°F., in order to decompose and/or volatilize the organic matter and other components, leaving a deposit of the noble metal or noble metal oxide. This heat treatment usually takes several hours. In like manner, a plurality of layers of the organic mixture is applied, each layer being followed by the heat treatment. Although this previous method provides excellent anodes, it has certain inherent disadvantages. A commercial anode preferably should have a coating thickness of about 40 to 50 micro inches which requires application of about 25 coats of the organic mixture. The production of such a commercial anode requires several days. Thus, the method is costly in terms of the amount of handling during production, time required for production, and heat energy expended during production.
Electrodes have been produced by electroplating. Although the known methods for producing electrodes by electroplating are less expensive than the thermal-decomposition methods, the known methods of electroplating have not produced completely satisfactory anodes. For example, the resulting platinum metal coating lacks durability and has a high overvoltage as compared with anodes produced by the thermal-decomposition method. Furthermore, if the electroplated anode is heated to increase durability, the overvoltage is further increased.
The present invention provides a method for electrolytically depositing a platinum metal coating on a substrate that is anodically resistant. In other words, the substrate will withstand the conditions in the anodic zone of the cell and resist corrosion and/or decomposition, for example, by chlorine or oxygen. The titanium group metals including titanium, tantalum, zirconium, niobium, and alloys thereof are especially useful for this purpose. The present method provides for rapid production of high-quality electrodes. An anode having a platinum coating of between 40 and 50 micro inches can be produced by electroplating for about 1 hour. The resulting anode can be heated, thereby making a very durable electrode and yet the overvoltage remains very low. For example, anodes produced by this method have been found to have a life-time chlorine overvoltage of less than 0.05 volts and a consumption rate of 0.3 grams or less of platinum per ton of product when used in a chlorate cell and 0.1 gram or less per ton of chlorine when used in a diaphragm cell.
The present method of producing an anode for electrolysis off aqueous alkali metal halide preferably includes the steps of: (1) thoroughly cleaning the anodically resistant substrate, such as by chemical etching; (2) contacting the substrate with an organic-based solution or electroplating bath containing a platinum group metal compound; (3) applying a cathodic electrical potential to the substrate while it is in contact with the solution, thereby depositing a coating on the substrate including the platinum group metal and an organic material; and (4) treating the coating to decompose the organic material, leaving a coating including the platinum group metal and carbon.
The substrate may be cleaned by any method that will remove the foreign matter, such as oils, greases, and oxides, that is present on the surface to be plated. For example, such surface may be cleaned with a detergent and then etched with an acid such as hydrofluoric acid and/or hydrochloric acid.
The substrate may be contacted with the organic-based solution by any suitable method so long as the method permits application of a cathodic electrical potential during such contact. One method of contacting the substrate with the solution is by submerging the substrate in the solution. Another method is that of flowing the solution over the surface to be plated.
The electroplating solution of the present invention is an organic-based solution including a platinum group metal compound. The term organic-based solution, as used with respect to the present invention, means a solution in which the principal solvent is of an organic nature, such as ethyl alcohol. The solution is preferably substantially free of uncombined water, although in certain instances some free water may be present, for example, 5 percent or less by weight and, rarely if ever, in excess of 10 percent. If free water is present, it may be placed in a combined state by addition of absolute ethyl alcohol or concentrated sulfuric acid to the solution. The solution includes an organic material which will be deposited in the coating. The organic material is a substance that can be decomposed to provide carbon particles in the electroconductive coating, for example, by thermal or chemical means. The organic material may be the solvent for the solution or may be soluble or suspendible in the organic solvent. The solution further includes a highly ionizible electrolyte which makes the solution electroconductive.
An excellent solvent for use in the present invention is absolute ethyl alcohol. Other suitable solvents would include higher alcohols, such as propyl alcohol and butyl alcohol. Other solvents that may be used are acetone and higher ketones such as ethyl methyl ketone. Still other solvents would include, for example, dioxane, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, N-Methyl pyrrolidone, furfural, acetylacetone, polyethylene glycols, phenol, or a mixture of glycol and glycerine.
The highly-ionizible electrolyte is used for increasing the conductivity of the plating solution and may be any electrolyte that would be compatible with the other materials present and provide the plating solution with the necessary conductivity, for example, an inorganic acid such as sulfuric acid or hydrochloric acid. The ionizible electrolyte, alternatively, may be any ionizible organic acid, for example, acetic acid. Moreover, the electrolyte may be any electroconductive organic compound or mixture of compounds, for example, a mixture of acetonitrile and tetra-n-butyl ammonium bromide. The ionizible electrolyte should give the solution a conductivity of at least 0.01 mho and, preferably, about 0.04, although a higher conductivity would not be detrimental.
The organic material provides carbon particles for inclusion in the platinum group metal electroconductive coating and, preferably, is a compound having between 4 and 100 carbon atoms. The organic material may be a hydrocarbon or a surfactant, typically silicon compounds such as tetraethyl orthosilicate. In certain instances, the organic material may serve as the solvent for the electrolyte and platinum group metal compound in which case other solvents would be unnecessary. For example, butyl alcohol would serve this purpose. Although some polymerization of the organic material during electroplating is permissible and possibly even desirable, the organic material should not coagulate or precipitate during electroplating. The preferred organic material is toluene. Other aromatic solvents which would keep the anodic products in solution are also satisfactory.
The compound containing the platinum group metal is a compound that will permit deposition of a platinum metal coating during electrolysis, typically, acids or salts of the metal such as chloroplatinic acid or platinum nitrate. The term platinum group metal, as used with respect to the present invention, includes ruthenium, rhodium, palladium, osmium, iridium, and platinum. The solution preferably contains a small amount of a compound of another platinum group metal. The electroplating bath may also include other materials that may be electrolytically deposited on the electrode; for example, the bath may contain titanium compounds.
The solution or electroplating bath preferably contains on a basis of 100 parts by weight of solvent: 0.4 to 2.0 parts platinum group metal, 5 to 30 parts organic material, and 5 to 30 parts electrolyte. One highly-preferred electroplating bath contained 525 milliliters absolute ethyl alcohol, 125 milliliters concentrated sulfuric acid, 70 milliliters toluene, 17.8 grams chloroplatinic acid crystals, and 2 grams rhodium trichloride crystals.
The electrode is coated by placing a cleaned base member or substrate, typically titanium, as a cathode in an electrolytic plating cell containing the plating solution. An electrical potential is then applied to the cell, thereby depositing the platinum group metal. The electroplating cell may be operated at normal room temperature. Temperatures of between 25°C. and 35°C. have been found highly satisfactory. However, in any event, the temperature will rarely be greater than 70°C. or lower than 10°C.
The current density should be sufficient to deposit a layer of the platinum group metal of satisfactory thickness with a reasonable time. However, the within density should not be so great as to deposit a soft, non-durable layer. Current densities ranging from 2 to 50 amperes per square foot of plating surface have been satisfactorily used in the present invention. Generally, the current density will be within the range of 10 to 40 amperes per square foot. Preferably, the current density is about 25 to 30 amperes per square foot of plating surface. Reversal of the electrical potential for a short time periodically during the electroplating has been found to improve the durability and uniformity of the coating. For example, in one instance where the plating time was 1 hour, the current was reversed for 3 seconds every 15 minutes. The required plating time will vary depending on such factors as current density, solution concentrations, and desired plating thickness. However, the plating time will generally be between 15 and 120 minutes.
Following electrolytic deposition, the electrode is heat treated in order to drive off volatile matter and decompose the organic material producing a coating which comprises the platinum group metal and included carbon particles. The heat treatment will generally be at between 200°C. and 700°C. for between 15 and 120 minutes. However, the temperature may be lower in the case of easily decomposed organic material or higher if heated in a vacuum or inert atmosphere or if the heat treatment is for a shorter period of time.
The electrodes in the following examples were produced using for the substrate a titanium plate which was thoroughly cleaned and etched prior to electroplating. In the cleaning process, the titanium plate was washed with the detergent Comet (Trademark of Procter and Gamble) and rinsed in distilled water. The titanium plate was immersed for 1 minute in a solution containing 1 percent hydrofluoric acid and 37 percent hydrochloric acid by weight and rinsed in distilled water. The plate then was etched in 37 percent hydrochloric acid at a temperature of between 40°C. and 50°C. for from 1 to 4 hours and rinsed with distilled water, then acetone, and then dried in air. The portions of the titanium plate which were to be left uncoated were masked using a Teflon material, thus permitting deposition of the platinum metal only on the desired area.
As described in Examples I - VI, the electrodes of the present invention were tested in the following chlorate cell, chlorine diaphragm cell, and overvoltage cell.
The chlorate test cell container was constructed of rubber-lined steel and was 6 inches by 18 inches by 2 inches. The cathode was a flat steel plate and was 23/4 inches by 14 inches. Five test anodes were spaced 1 inch apart along the cathode. The test anodes were spaced one-fourth inch from the cathode. The feed solution was forced in at the bottom of the cell and the effluent was removed at the top of the cell. The chlorate cell was operated under the following conditions: The cell temperature was 105°C.; the solution feed rate was 5 milliliters per minute. The feed solution was maintained at about pH 6.8 and contained 120 grams sodium chloride, 688 grams sodium chlorate, and 2.0 grams potassium dichromate per liter of aqueous solution.
The chlorine diaphragm cell was constructed of Pyrex*(Trademark) pipe 16 inches in length and 6 inches in diameter. The cathode consisted of 10 mesh iron screens in the form of an enclosed tube having an outer diameter of 2 inches and a length of 6 inches. The diaphragm consisted of two layers of 0.016 inch asbestos paper which covered the screen. The asbestos paper was held in place with asbestos tape. The cathode was centrally located in the Pyrex pipe. Four test anodes were equally spaced around the cathode. The anodes were spaced three-fourths inch from the cathode. The chlorine cell was operated at 90°C. The cell feed solution was an aqueous brine containing 313 grams per liter sodium chloride. The feed solution was maintained at pH 10 and was fed to the cell at the rate of 350 milliliters per hour.
The overvoltage cell consisted of a pair of Pyrex compartments (an anode compartment and a cathode compartment) each containing about 100 milliliters. The compartments were connected by a 3 inch piece of glass tubing having an internal diameter of 11/2 inches. The cathode, disposed centrally in the cathode compartment, was a piece of platinum foil 1 inch square and 0.006 inch in thickness. The cathode was welded to a platinum wire for electrical connection. The anode to be tested was disposed centrally in the anode compartment and had an electroconductive surface that was three-fourths inches by one-half inches. The cell feed solution was an aqueous solution containing 300 grams reagent grade sodium chloride per liter. The solution in the cell was maintained at about pH 7. A constant temperature of 90°C. was maintained by immersing the cell in a heated bath of glycol. A course of direct current was supplied to the cell by a Kocour Model 15 Silicon Rectifier. The voltage was measured with a Leeds and Northrup Type K-3 Universal Potentiometer in conjunction with an Eppley Laboratory, Inc., Standard Cell Model 100 and a GM Laboratories, Inc., Galvanometer Model 570-402. The voltages were measured against a saturated calomel electrode having an EMF of 0.2415 volts using a Luggin Tip and a saturated sodium chloride solution bridge. The overvoltages were calculated by assuming the equilibrium chlorine voltage at 90°C. to be 1.28 volts.
EXAMPLE I
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The titanium plate was cleaned and etched as aforedescribed, the etching being for 1 hour at 40°C. A plating solution was prepared by mixing 0.78 grams chloroplatinic acid crystals (Reagent A.C.S.), 75 milliliters absolute ethyl alcohol (Reagent Quality), 15 milliliters sulfuric acid (Reagent A.C.S.), 20 milliliters toluene (Certified A.C.S.), and 20 milliliters acetone (Certified A.C.S.).
In Examples I, II, and V the plating cell consisted of a glass tube, sealed at the bottom end, having an inside diameter of 11/2 inches and sufficient length to permit submersion of the portion of electrode to be electroplated. The plating cell included a platinized titanium anode which was 6 inches by 3/8 inch by 1/16 inch. The titanium substrate to be plated was placed in the cell as a cathode. The electrode of Example I was produced by applying a current of 0.5 amperes. The total plating area was 2 square inches, thus the current density was 36 amperes per square foot of plating area. The plating was continued for 30 minutes. The electrode was removed from the cell and heat treated by starting at room temperature and increasing the temperature 50°C. every 5 minutes up to a temperature of 350°C. The electrode was maintained at 350°C. for 15 minutes and then cooled. The electrode was operated as an anode in the aforedescribed chlorate cell at 360 amperes per square foot for 18 hours. The voltage remained constant at 3.4 volts during the 18 hours. A piece 11/4 inches by 1/2 inch, having an electroconductive surface of 3/4 inch by 1/2 inch, was cut from the electrode and tested as an anode in the overvoltage cell. The electrode was operated at 670 amperes per square foot of anode surface for 49.5 hours. The overvoltage remained below 0.05 volts throughout the test. The platinum coating had a thickness of 40 micro inches.
EXAMPLE II
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The electrode was cleaned and then etched for 1 hour. A plating solution was prepared by mixing 0.5 grams chloroplatinic acid crystals (Reagent A.C.S.), 30 milliliters absolute ethyl alcohol (Reagent Quality), 10 milliliters toluene (Certified A.C.S.), and 5 milliliters concentrated sulfuric acid (Reagent A.C.S.). The electrode was plated at 0.12 amperes for 1 hour. The electrode was heat treated by starting at room temperature and increasing the temperature 50°C. every 5 minutes until reaching a temperature of 400°C. The electrode was then held at 400°C. for 15 minutes. The electrode was examined by X-ray and found to have a platinum thickness of 26.8 micro inches. The electrode which had a plated area of 2 square inches was used as an anode in the chlorate cell. The cell voltage at an anode current density of 6 amperes was 3.30 volts after 4 hours of operation and 3.55 volts after 23 hours of operation.
EXAMPLE III
An electrode was prepared from a titanium plate 2 inches by 23/4 inches by 1/16 inch. The titanium plate was cleaned and etched for 1.5 hours at between 45°C. to 51°C. The electroplating contained -b 280 milliliters absolute ethyl alcohol (Reagent Quality), 60 milliliters concentrated sulfuric acid (Reagent A.C.S.), 70 milliliters toluene (Certified A.C.S.), 34 milliliters acetone (Certified A.C.S.), 9.85 grams chloroplatinic acid crystals (Reagent A. C. S.), and 0.5 milliliters of a rhodium plating solution. The rhodium plating solution contained 100 grams of rhodium as rhodium trichloride per liter of ethyl alcohol. The rhodium plating solution was of the type produced by Englehard Industries, Inc., in 1969 and designated Rhodium Plating Solution No. 221.
In this Example III (and also Examples IV, V, and VI), the plating cell was provided by a 16 ounce, wide-mouth bottle. The electrode was plated in the solution at 0.5 amperes for 1 hour and then heat treated starting at room temperature and raising the temperature 50°C. every 5 minutes up to 400°C. The temperature of 400°C. was maintained for 15 minutes. The platinum thickness as measured by X-ray was about 50 micro inches. X-ray disclosed the presence of traces of rhodium. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell for 23 hours at a current density of 6 amperes and a cell temperature of 38°C. The cell voltage was 3.11 volts.
EXAMPLE IV
An electrode was prepared by cleaning the titanium plate and etching for 1 hour. The titanium plate was 2 inches by 23/4 inches by 1/16 inch. The electrode was then plated in a solution containing 150 milliliters absolute ethyl alcohol (Reagent Quality), 30 milliliters concentrated sulfuric acid (Reagent A.C.S.), 25 milliliters toluene (Certified A.C.S.), 5.3 grams chloroplatinic acid crystals (Reagent A.C.S.), and 1.0 gram rhodium trichloride trihydrate crystals (Purified). The electrode was plated at 0.5 amperes for 1 hour, rinsed in water, and dried. The electrode was heat treated by slowly raising the temperature to 450°C. and holding at that temperature for 15 minutes. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell for 3 hours and 20 minutes. The final voltage was 3.17 volts at 6 amperes.
EXAMPLE V
An electrode was prepared from a titanium plate 6 inches by 3/8 inch by 1/16 inch. The titanium plate was cleaned and etched for 1.5 hours at 55°C., rinsed, and dried. The electrode was plated in a solution containing 135 milliliters absolute ethyl alcohol (Reagent Quality), 10 milliliters concentrated sulfuric acid (Reagent A.C.S.), 5 milliliters toluene (Certified A.C.S.), 40 milliliters tetraethyl orthosilicate (Practical), 4.12 grams chloroplatinic acid crystals (Reagent A.C.S.), and the electrode was plated at 0.2 amperes for 40 minutes, reversing the current for 5 seconds every 5 minutes. The electrode was heat treated to 450°C. The electrode had a plated area of 2 square inches. When operated in the chlorate cell at 360 amperes per square foot of anode surface for 71/2 hours, the voltage was 3.31 volts.
EXAMPLE VI
An electrode was prepared using a titanium plate 2 inches by 23/4 inches by 1/16 inch which had been cleaned and then etched for 2 hours and 30 minutes at between 40°C. and 47°C. The titanium plate was rinsed and dried. The electrode was plated in a solution containing 525 milliliters absolute ethyl alcohol (Reagent Quality), 125 milliliters concentrated sulfuric acid (Reagent A.C.S.), 70 milliliters toluene (Certified A.C.S.), 2 grams rhodium trichloride trihydrate crystals (Purified), and 17.8 grams chloroplatinic acid crystals (Reagent A.C.S.). The electrode was plated at 0.1 amperes for 15 minutes and the current was reversed for 15 seconds. This procedure was repeated 8 times. The plating voltage was 2.4 volts, the voltage on reversal was maintained less than 0.6 volts. The electrode was heat treated by slowly starting at room temperature and heating to 450°C. The electrode was held at that temperature for 15 minutes. The electrode, which had a plated area of 4 square inches, was operated in the chlorate cell and after 41 hours the voltage at 6 amperes was 3.25 volts and at 10 amperes was 3.90 volts. The electrode VI-A was tested in the overvoltage cell and the chlorine overvoltage at 1,000 amperes per square foot was about 0.1 volts.
EXAMPLE VII
An electrode was prepared using the same solution used in Example VI. The titanium substrate was 6 inches by 3/8 inch by 1/16 inch. The substrate was cleaned and etched for 1 hour and 45 minutes. The electrode was plated for 5 minutes at 0.06 amperes, for 10 minutes at 0.035 amperes, current was reversed for 3 seconds at 0.035 amperes, and then further plated for 60 minutes at 0.35 amperes. The current was reversed 4 times at 0.35 amperes during the final 60 minutes; at 17.5 minutes current was reversed for 3 seconds; at 50 minutes reversal was for 3 seconds; and after the 60 minutes reversal was for 5 seconds. The electrode was rinsed, dried, and heat treated by raising the temperature from room temperature to 550°C. in 17.5 minutes. The electrode was held at 550°C. for 1 hour. The electrode had a platinum coating of about 57 micro inches. The electrode, which had a plated area of 2 square inches, was tested in a chlorate cell as an anode at 360 amperes per square foot of anode surface and 30°C. The voltage was 3.20 volts after 22 hours of operation. The electrode was further tested in a diaphragm cell at 500 amperes per square foot. The voltage after 24 hours was 3.589 volts. After 35 days of operation the voltage was 3.680 volts; after 90 days, 3.552 volts; and after 174 days, 3.430 volts. The voltage after 365 days of operation was 3.434 volts. The platinum loss during the 365 days was 3.7 micro inches or 0.035 grams per ton of chlorine.
Electrodes made according to the present invention are highly suitable for use as anodes in cells used for the electrolysis of aqueous alkali metal chloride solutions, typically diaphragm cells, mercury cathode cells, and chlorate cells such as those shown in U.S. Pat. Nos. 3,400,055; 3,337,443; 3,312,614; 3,287,250; 3,203,882; 3,119,664; 3,116,228; 2,897,463; and 2,719,117.
Although the present invention has been described with reference to the specific details of particular embodiments thereof, it is not intended thereby to limit the scope of the invention except insofar as the specific details are recited in the appended claims.