US3293167A - Platinum bismuth alloy coated electrodes - Google Patents

Description

Dec. 20, 1966 L GREENSPAN 3,293,167

PLATINUM BISMUTH ALLOY COATED ELECTRODES LAWRENCE GREENSPAN Vii/ 4 ATTORNEY Dec. 20, 1966 GREENSPAN 3,293,167

PLATINUM BISMUTH ALLOY COATED ELECTRODES Original Filed Oct. 8, 1962 5 Sheets-Sheet 3 moo I lO-y (l3 fiawowvo srxoA "IVLLNELLOd INV'ENTOR. LAWRENCE GREENSPAN ATTORNEY Uited States Patent 3,293,167 PLATINUM BiSMUTH ALLOY COATED ELECTRODES Lawrence Greenspan, New York, N.Y., assignor to Engelhard Industries, Inc, Newark, N.J., a corporation of Delaware Original application Oct. 8, 1962, Ser. No. 228,935.

Divided and this application Feb. 15, 1966, Ser. No.

3 Claims. (Cl. 204290) This application is a division of my application Serial No. 228,935, filed October 8, 1962, entitled Method for Electroplating.

This invention relates to the electrolytic deposition of platinum from aqueous solutions, and is particularly concerned with the preparation of platinum-coated base metal articles which find particular utility as nonsacrificial anodes in electrochemical processes.

In electrochemical processes, the proper selection of material which may be employed in the fabrication of electrodes is very important. In these processes, it is conventional to use electrodes of iron, steel and the like in the cathode position, but there are only a few materials which may be used in the fabrication of insoluble anodes, because most materials, when made anodic, are susceptible to rapid corrosion. Metals of the platinum group have desirable characteristics when utilized as insoluble anodes, but because of the high cost of these metals, it is desirable to use substitute materials which are less costly, and graphite has often been employed heretofore as an anode material.

Graphite has a number of disadvantages, however, in that it undergoes continual disintegration and must be replaced frequently, thereby causing interruption of the electrochemical process. The selection of a suitable anode is particularly critical in highly corrosive electrochemical baths, such as the brine solutions used in the manufacture of chlorine, and the use of graphite anodes requires the product to be purified for removal of traces of carbon dioxide which result from oxidation of the graphite anodes. Additionally, disintegration of the graphite anode causes deposition of fine grains of graphite in the diaphragms which surround the electrodes necessitating replacement of the diaphragms along with the electrodes.

The diificulties encountered with graphite electrodes, particularly in brine electrolysis, are eliminated when platinum metal anodes are employed. Such anodes can be of pure platinum, or, as is known in the art, can comprise a corrosion-resistant core metal such as titanium, silver, and the like, clad with a coherent, impervious platinum metal sheath or covering.

In addition to the requirement of substantial noncorrosivity, anodes which are employed in electrolytic processes are required to have certain favorable currentcarrying characteristics for economic operation. Thus, the anode should be highly conductive and capable of carrying a high current load, i.e. operable at the highest possible current density, without undue polarization. In practice, polarization of the anode with resultant high overvoltage characteristics, reduces the efiiciency of the electrolysis and adversely affects the economics of the electrolytic process.

The overvoltage characteristics for chlorine discharge using platinum metal anodes, whether fabricated from sheets of precious metal or comprised of platinum-coated base metals such as titanium, are more favorable than when graphite anodes are used. It has been found, however, that at high current densities, e.g. in excess of 200 amperes per square foot and up to about 1000 amperes per square foot, neither platinum sheet nor conventional "ice platinum-coated base metals are entirely satisfactory with respect to chlorine overvoltage characteristics.

In the course of my research work directed to development of a method for plating base metals with platinum which would provide a suitable platinum-coated nonsacrificial anode, particularly for brine electrolysis, I have noted that electrodeposits of platinum from conventional plating baths, such as hexahydroxy platinate baths, are smooth and highly adherent. Such deposits, however, have poor overvoltage characteristics, roughly comparable to those of platinum sheet. I have further examined the overvoltage characteristics of platinum black deposits, i.e., platinum which has been deposited from a bath containing minor amounts of soluble lead compounds, and have found that such platinum black deposits provide excellent overvoltage characteristics Unfortunately, platinum black deposits have the disadvantage of extremely poor adherence, and are unsuitable for fabrication of corrosion-resistant anodes since even ordinary Wiping removes the highly corrosive-resistant coating of platinum, and exposes the base metal.

From an examination of the coatings formed in the foregoing manner, it appeared that the excellent overvoltage characteristics of platinum black, as compared to ordinary electrodeposited platinum, might in some way be related to the much greater active platinum surface of the more matte and relatively rough platinum black deposit. These results suggested that certain advantages would be gained by providing a platinum deposit of high adherence but having the matte appearance and unevenness of a platinum black deposit.

The present invention has for its principal object the provision of a stable bath from which platinum deposits can be obtained which have high adherence and are characterised by favorable overvoltage characteristics for chlorine discharge.

A further object is the provision of a platinum-coated base metal suitable for use as a non-sacrificial anode.

Another object is the provision of a method for depositing platinum from an electroplating bath to provide platinum-coated base metals suitable as anodes in electrochemical processes.

In accordance With this invention, adherent platinum deposits having desirable overvoltage characteristics are provided by depositing platinum from an aqueous solution of chloroplatinic acid containing an excess of hydrochloric acid and a small amount of a soluble bismuth compound. Suitable solutions for the practice of the invention contain from about 0.2 to about 2.5 weight percent bismuth based on platinum in the solution. Typical solutions contain from about 5 to about 15 gm./liter platinum (e.g. as H PtCl -6H O), from about 0.02 to about 0.08 gm./liter of bismuth (e.g. as BiCl and from about 1 to about 5 weight percent hydrochloric acid. The amount of hydrochloric acid employed is not critical, but should be at least sufficient to provide a homogeneous solution without formation of deposits during the plating process. The bismuth may be introduced as the chloride BiCl or in the form of the hydroxide, oxide, acetate or the like, sufficient HCl being present to maintain the bismuth in solution as the chloride.

I have found that in the absence of bismuth, firm platinum deposits can be obtained from solution but that these deposits do not have desirable overvoltage characteristics in electrolysis of brine solutions. I have further found that the concentration of bismuth employed is critical, too little providing poor overvoltage characteristics, and too much causing formation of non-adherent, spongy deposits which are unsuitable for the fabrication of platinum plated anodes.

The characteristics of the bismuth-containing solutions of my invention are unique, and cannot be duplicated even by substitution of such closely related material as antimony in the above formulation. Thus, I have tried antimony additions to solutions of chloroplatinic acid. Addition of 0.2 grn./liter of antimony chloride (SbCl to an aqueous solution of chloroplatinic acid containing HCl initially resulted in a matte grey-black deposit which was adherent. However, the effect appeared to be transient, since subsequent samples plated from the same bath, although dark-grey in color, were smooth. Increasing additions of SbCl up to 1 gram per liter, al-

though yielding, after the addition, matte deposits, after further electrolyzing resulted in smoothdeposits having poor overvoltage characteristics.

Contrariwise, the bismuth-containing solutions of the invention gave strongly adherent, uniform matte darkgrey deposits which were highly adherent and exhibited excellent overvoltage characteristics. While the desirable concentration of platinum and bismuth in the plating baths of the invention is indicated above, it will be recognized that the actual deposits which are obtained with such solutions do not necessarily have the same platinumbismuth ratio as present in solution. Thus, at a current density of 15 a.s.f. (amperes per square foot), using a solution containing 0.06 gm./liter BiCl and 8 gm./liter Pt as H PtCl -6H O, the bismuth content of the deposit was found to be about 0.1%. In general, the bismuth content of an effective deposit will 'be between about 0.01 and about 0.1% based on platinum.

The baths of the present invention are operated at a temperature between about 20 C. and about 75 C., preferably between about 20 and 60 C., and at a current density in the range of about 10 to about 30 amperes per square foot. In the plating process, it is desirable to provide mild agitation, e.g., by gently moving the cathode, .and plating is elfected for sufiicient time to provide a deposit of from about 25 to about 250, preferably about 50-100 microinches. The exact time required for obtaining deposits of the desired thickness will vary depending upon the concentration of the solution and the current density employed, for example, from about minutes to one hour or more.

I have noted that during the plating operation chlorine is liberated at the platinum anodes so that, in order to employ the solution of the invention on a production basis, adequate ventilation would have to be provided. On continued electrolysis of the solution, the deposit changes from a matte dark-grey to a smoother, greyer appearance. Since the matte finish is preferred, I have found it advantageous to bubble air or other inert gas, e.g. N argon, etc. continuously through the solution to reduce the free chlorine content thereof, and have thus obtained more consistent results from the standpoint of appearance of the deposit.

To prepare metals for plating in this solution, any standard accepted process may be employed, such as degreasing, electrocleaning or other operations required to prepare a clean receptive metal surface.

Among the metals which may have platinum electrodeposited thereon, in accordance with the present invention, are silver, nickel, gold, tantalum, tungsten, molybdenum, titanium and rhodium. Platinum-coated titanium is, of course, of particular value in electrolysis of brine because of those characteristics of titanium including light Weight, strength, corrosion-resistance and conductivity which make it of value as a base metal for platinized anodes in brine electrolysis. For certain applications, it may be desirable to employ the baths of this invention to deposit platinum on a platinum or palladium base.

In order to determine the overvoltage characteristics of platinized anodes prepared in accordance with this invention, apparatus was employed consisting basically of a Luggin capillary probe, calomel electrode and a vacuum tube voltmeter having a megohm resistance. The accuracy of the measurement with this instrument is about 10-20 millivolts.

4 OVERVOLTAGE MEASUREMENTS The Luggin probe consists of a tube one end of which is bent at right angles and is drawn out into a fine capillary which makes light contact with the surface of the electrode. The other end of the tube is connected with an open top cylindrical separatory funnel in which is immersed a reference electrode (calomel electrode). The funnel and tube are filled with a solution of the electrolyte, in this case a 22% brine solution. This simple device effectively extend the reference electrode electrolyte (in which no current flows) up to the surface of the electrode being studied. The leads from a vacuum tube voltmeter are connected to this electrode and the reference electrode. With this arrangement the electrode potential is measured with practically negligible solution IR drop.

The electrolyte used was a 22% solution of sodium chloride contained in a lucite cell. A porous clay cup contained a nickel cathode and prevented mixing of solutions in anolyte and catholyte compartments. Samples for testing were prepared by plating strips of silver and titanium, 3" x 1 wide, with a deposit of microinches of platinum from various solutions. The samples were masked with tape so as to expose a circular area approximately ,4; square inchto the action of the current.

The current density range used was from 20 to 1000 amperes per square foot. The sodium chloride electrolyte was employed at temperatures of 25 C. and 70 C.

The invention will be further illustrated by reference to the following specific examples:

Example I A solution was made up of the following composition:

Gm./liter Platinum (as H PtCI -H O) 8 Bismuth (as BiC1 .05 Hydrochloric acid 34 A titanium strip (3" x 1"), etched for 40 hours in concentrated hydrochloric acid, was used as the cathode. At a current density of 15 amperes per sq. ft., room temperature, and providing mild agitation by moving the cathode, the weight of deposit obtained in 15 minutes was 0.23 gram, representing an average plate thickness of 100 microinches. The deposit was uniform matte, dark grey, and strongly adherent. When wiped with filter paper, none of the deposit rubbed ofi. Chlorine overvoltage masurements made in brine solution (22% NaCl) showed no polarization occurring up to current densities of 1000 amperes per square foot. A titanium strip similarly prepared but plated with a 100 microinch deposit in a hexahydroxy platinate bath showed polarization occurring at 200 amperes per square foot accompanied by rapid increase in overvoltage.

Example I] A solution was made up of the following composition:

Gm./liter Platinum (as H PtOl -H O) 8 Bismuth (as BiCl .08 Hydrochloric acid 34 A strip of titanium metal (3" x l"), etched for 48 hours in concentrated hydrochloric acid, was used as the cathode. Using a current density of 15 amperes per sq. ft., room temperature, and providing mild agitation by moving the cathode, the weight of deposit in 15 minutes was 0.225 gram, representing an average thickness of 100 microinches. The deposit was a uniform black, 'but when given a swipe with filter paper, a small amount of deposit rubbed olf. It appeared to be adherent for the most part. Chlorine overvoltage measurements made in a 22% sodium chloride solution showed no tendency to polarize up 'to current densities of 1000 amperes per sq. ft.

COMPARATIVE TESTS Reference is here made to FIGURES 1, 2 and 3, attached hereto and forming a part hereof, wherein overvoltage measurements for various platinized anodes are plotted as a function of current density (amps/ft?) versus potential (volts). The curves plotted in the figures were obtained by testing the following sample structures used as anode in the test heretofore described:

Sample A-Platinum black on silver: The conventional solution for producing platinum black was used, containing 30 gm./liter chloroplatinic acid, 0.2 gm./ liter lead acetate, 83 ml./liter con. HCl (normal with respect to HCl). A current density of a.s.f. was used; time of plating, 15 minutes; deposited platinum equivalent to 100 rnicroinches deposit.

Sample B-Platinum-bismuth on silver: The solution contained gm./liter chlorplatinic acid, 83 m'l./liter HCl, and 0.06 gm./liter Bi(OH) dissolved in HCl. A current density of 15 a.s.f. was used; time of plating, 15 minutes; deposited thickness, 100 rnicroinches.

Sample C. Platinum-bismuth on titanium as described in Example I; deposited thickness 100 microinches.

Sample D Platinum-bismuth on titanium as described in Example I; deposit thickness, 50 rnicroinches.

Sample EPlatinum on titanium from platinate solution.

An etched titanium strip was plated with platinum from a solution containing 20 gm./liter sodium hexahydroxy platinate and 10 gin/liter sodium hydroxide at a temperature of 75 C. and current density of 7.5 a.s.f.; time of plating, minutes; deposit thickness, 1-00 rnicroinches.

Sample F .Platinum on titanium from chloroplatinic acid: An etched titanium strip was plated with platinum from a solution of 20 gm./ liter chloroplatinic acid and 83 ml./ liter hydrochloric acid. A current density of 15 a.s.f. was used; time of plating, 15 minutes; deposit thickness, 100 rnicroinches.

Sample G.Pure platinum strip.

Sample H. Platinum deposited on silver from platinate solution: A polished strip of silver was plated in the bath described under Sample E at a current density of 7.5 a.s.f. for 25 minutes; thickness of deposit, 100 microinches.

DISCUSSION Referring to FIGURE 1, wherein the overvoltage characteristics of certain of the sample anodes prepared as described above are shown as a plot of potential increase versus current density, (determined at 25 C.) it will be seen from Curve 1 that platinum black (Sample A) shows extremely low overvoltage values throughout the range of current densities tested, i.e. up to about 500 a.s.f. However, as stated heretofore, platinum black is nonadherent and therefore inadequate for fabrication of corrosion resistant anodes.

Curves 3, 4 and 5 of FIGURE 1 show that platinum electrodeposited on titanium (Sample E) or on silver (Sample H) and pure platinum sheet (Sample G) all show an undesirable rise in potential at applied current density in the range of 100 to 200 a.s.f.

In marked contrast, the overvoltage characteristics of Sample B, prepared in accordance with the invention, closely approach those of platinum black, as shown by Curve 2 of FIGURE 1, and are far superior to those of any of the other anodes tested.

FIGURE 2 shows the results obtained by testing anodes prepared in accordance with the invention on a titanium base in comparison with an anode prepared by depositing platinum from chloroplatinic acid in the absence of any added bismuth compound. It will be noted that the hismuth-containing deposits (Curves 6 and 7) show substantially constant potential for current density as high as 1000 a.s.f., and that varying the thickness of the platinum deposit from 50 to 100 rnicroinches has little effect on the anode characteristics. As shown by Curve 8, the anode prepared by depositing platinum on titanium from chloroplatinic acid in the absence of added bismuth showed a rapid increase in potential at a current density of 400 a.s.f., and would thus be unsuitable for 'brine electrolysis at high current density.

Referring to FIGURE 3, comparative tests are shown for over-voltage measurements determined at C. It will be noted that Sample C prepared in accordance with the invention showed substantially no potential increase up to a current density of 1000 a.s.f. (Curve 9). While anodes of pure platinum (Sample G) and of platinum deposited from sodium platinate solution (Sample E) showed improved overvoltage characteristics when tested at 70 C. (Curves 10 and 11 as compared to Curves 3 and 4 of FIGURE 1), neither of these samples exhibited the outstanding flat response of the anode of this invention.

While the method of the invention has been particularly described in connection with preparation and testing of platinum-coated anodes in the electrolysis of brine solutions used for production of chlorine and caustic soda, it will be apparent to those skilled in the art that the anodes of the invention can be used in diverse electroplating processes. Thus, platinized titanium anodes can be employed in electroplating processes using solutions of rhodium, acid gold, nickel and the like. Other major uses for the novel anode of the invention are in the conversion of sea water and brackish water by electrolysis, removable of valuable or harmful particles from industrial process water, etc.

It should be understood that this invention is not restricted to the use of any particular plating solution, apparatus or shape or form of platinum bismuth-coated anodes, and that the illustrative and descriptive matter herein is presented for purposes of exemplifying the invention and, thus, should not be construed as limitative in nature.

What is claimed is:

1. An anode for electrolyzing aqueous salt solutions comprising a metal core having a continuous adherent deposit of platinum containing from 0.01% to about 0.1 weight percent bismuth, the metal core having a composition difi'erent from the adherent deposit.

2. An anode as defined by claim 1 wherein the metal The American Journal of Science, vol. 36, 1888, pp. 437 and 438.

JOHN H. MACK, Primary Examiner. D. R. JORDAN, Assistant Examiner.

Claims (1)

1. AN ANODE FOR ELECTROLYZING AQUEOUS SALT SOLUTIONS COMPRISING A METAL CORE HAVING A CONTIUOUS ADHERENT DEPOSIT OF PLATINUM CONTANING FROM 0.01% TO ABOUT 0.1 WEIGHT PERCENT BISMUTH, THE METAL CORE HAVING A COMPOSITION DIFFERENT FROM THE ADHERENT DEPOSIT.

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