Platinum Group Metal Oxide Coated Electrodes

Abstract

A dimensionally stable electrode comprises an electrically-conductive substrate having bound to the surface thereof a coating which consists essentially of a solid solution of platinum group oxides. The electrodes are fabricated by applying a solution of a binder having dispersed therein a preformed particulate solid solution of the foregoing type.

Description

BACKGROUND OF THE INVENTION

Dimensionally stable electrodes are known to be useful in a variety of electrolytic applications such as chlor-alkali electrolysis, electrowinning, cathodic protection, electroplating, electro-organic synthesis and the like. Generally, these electrodes consist of an electrically conductive substrate, inert to the electrolytic environment, bearing on the surface thereof on electrically conductive electrocatalytically-active coating. Typical of such electrodes are valve metal (titanium, tantalum) substrates coated with a platinum group metal or a mixed oxide coating of valve and platinum metal oxides, although many others are known.

Generally, however, no one electrode has proven useful in a variety of electrochemical applications, the inherent and relatively uncontrollable and unchangeable properties of the coatings dictating limited uses. For example, while an electrode composed of a solid solution coating of ruthenium and titanium oxides on a titanium substrate is an excellent chlor-alkali anode, it fails rapidly in environments wherein oxygen is evolved, e.g., aqueous electrowinning.

STATEMENT OF THE INVENTION

Therefore, it is an object of the present invention to provide a dimensionally stable electrode useful to advantage in a variety of electrochemical reactions.

It is a further object of the present invention to provide a method for obtaining such an electrode, which method allows control of the electrochemical properties of the resulting electrode.

These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims which follow.

There has now been found an electrode comprising an electrically-conductive substrate, an electrocatalytically active material and a binder for adhering said material to at least a portion of the surface of said substrate, the material consisting essentially of a solid solution of at least two platinum group metal oxides. Such an electrode may be obtained by applying to an electrically-conductive substrate a dispersion of said solid solution in a solution of the binder and heating the thus-coated substrate in the presence of oxygen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

By the term "electrically conductive substrate," it is intended to refer to the metallic support for the electrode. It is essential that this substrate be resistant to the electrolytic environment in which it will be employed in order that, upon mechanical failure of the coating, excessive attack will not occur. A variety of materials are useful depending upon the intended application. Often, a valve metal such as titanium, tantalum, zirconium or an alloy thereof is employed. However, it is not essential that the entire substrate be of this material. Rather, a core of a more highly conductive metal, such as copper or aluminum, may be employed if clad or coated with the valve metal. Alternately, a valve metal may carry a surface coating designed to alter electrode properties, such as a platinum metal layer. The physical form of the substrate is independent of the invention and it may be flat or shaped, continuous or foraminous, as required and desired for the particular end use envisioned.

By the term "binder," it is intended to refer to any material, organic or inorganic, capable of adhering the particulate solid solution to the underlying substrate in a manner relatively permanent under the conditions obtaining during use. The binder itself need not be, and most often is not, electrically conductive, a sufficient amount of the solid solution being dispersed throughout the binder to provide a path for current from the underlying substrate to the electrolyte. Thus, a diverse group of materials is contemplated, such as the fluorocarbons, e.g., polyvinylidene fluoride; sodium silicate (subjected to a subsequent acid treatment); an inorganic glaze of refractory materials such as titania or silica glasses, and the like. Particularly preferred, however, in order to facilitate production and for the results obtained therewith, are the amorphous oxide binders which may be obtained by thermal decomposition of a solution of the salts thereof in the presence of oxygen, especially the amorphous valve metal oxides and particularly tantalum and niobium oxides. These latter two oxides have been noted to produce remarkably uniform and adherent coatings without detracting from the activity of the electrocatalytic material.

By the phrase "solid solution," it is intended to refer to those mixtures of platinum group metal oxides in which atoms of the different platinum metals are randomly distributed throughout the parent metal oxide crystal lattice, thus giving the resultant solid solution properties different from either platinum metal oxide alone or a mere physical combination of same. The platinum group metals contemplated are ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably ruthenium, palladium, rhodium and iridium. Particularly preferred are solid solutions of ruthenium and iridium oxides. Generally, and in order to achieve the desired beneficial results, at least one percent of the second platinum metal oxide must be present in the parent platinum metal oxide crystal lattice. That is to say, for example, at least one mole percent of ruthenium oxide must be present in an iridium-ruthenium system. It is by manipulation of the relative amounts of platinum metal oxides present that it is possible to control the properties of the electrodes, particularly with respect to the relative overvoltages at which different elements are discharged when employing the electrodes as anodes in aqueous solution. (Overvoltage referring to the potential in excess of that theoretically required to discharge the element in question at the electrode surface.) For example, where the electrode is to be employed as an anode for the electrolysis of aqueous sodium chloride, a low chlorine overvoltage may be coupled with a high oxygen overvoltage, especially when the solid solution is of 25% RuO.sub.2 /75% IrO.sub.2, on a mole basis, thereby reducing the likelihood of contamination of the chlorine with oxygen and the attendant current inefficiency. On the other hand, for use as an electrowinning anode, a low oxygen overvoltage electrode may be designed. Still further, an anode employing 92-93% RuO.sub.2 /7-8% IrO.sub.2 in an amorphous tantalum binder and on a titanium substrate is particularly suited to use in the electrolysis of dilute aqueous brine to form hypochlorite.

It has been noted that these solid solution materials are particularly effective when employed in a particle size of 0.1 micron or less, an advantage being noticed both with respect to current efficiency and operating life of the resultant electrode.

Generally, the amount of binder present will be that sufficient to adhere the particulate solid solution to the underlying substrate in a continuous fashion without being so much as to completely isolate the individual particles. This is particularly important where the binder is not itself electrically-conductive under conditions of use since, otherwise, disruption of electrical continuity will result. In most instances, this means that from 0.01-1.0 part by weight of binder will be used per part of particulate solid solution.

The electrodes of this invention depend for their versatility upon the formation of the particulate solid solution prior to its application to the electrode. While this preparation may be accomplished in a number of ways known to the art, it is preferred, in order to obtain the material in the desired particle size of less than 0.1 micron, that the solid solution be precipitated from an acidic solution of platinum metal salts. Thus, the desired concentrations of the appropriate metals are dissolved in acid solution, either jointly or separately followed by combination. The pH of the solution is then slowly raised, generally to a value in excess of 8, causing precipitation of the material as a hydrated oxide. This precipitate is now filtered, washed and dried at a temperature sufficient to remove the water, e.g., 110.degree. C. The solid solution of platinum metal oxides is then preferably heat-treated at a temperature within the range of 300.degree.-900.degree. C preferably 450.degree.-800.degree. C, still more preferably 600.degree.-700.degree. C, to crystallize and stabilize same.

Next, a dispersion of the aforesaid particulate solid solution is formed in a solution containing the desired binder. This solution may consist of an organic polymeric binder dissolved in an appropriate solvent, such as polyvinylidene fluoride in 1-methyl-2-pyrrolidinone, or an inorganic salt, such as sodium silicate, in water, in which case a subsequent acid treatment of the electrode may be required to "insolubilize" the binder. Preferably, however, the solid solution will be dispersed in a solution of a valve metal salt, such as TaCl.sub.5 or NbCl.sub.5, in an appropriate solvent, such as a combination of water, butyl alcohol and hydrochloric acid. The salt used will be one decomposable at elevated temperatures, e.g., within the range of 300.degree.-600.degree. C, and in the presence of oxygen to an amorphous valve metal oxide. Generally, the amount of solvent required to form the dispersion is that sufficient to dissolve the binder, suspend the dispersion and still result in a sufficient viscosity to facilitate application to the underlying substrate. Such amounts will vary depending upon the materials involved, the ratio of binder to solid solution and the like. Amounts within the range sufficient to form a dispersion containing from 5-50 percent by weight solids are exemplary.

The electrode is formed by applying the dispersion, e.g., by brushing, spraying or roller coating, to the surface of the electrically conductive substrate followed by heating the thus-coated substrate to a temperature within the range of from 300.degree.-600.degree. C in the presence of oxygen.

In order that those skilled in the art may more readily understand the invention and certain preferred embodiments by which it may be carried into effect, the following specific examples are afforded.

EXAMPLE 1

An electrocatalytic material is prepared by dissolving 34.5 grams IrCl.sub.3.sup.. xH.sub.2 O (52.9% Ir) in 1 liter of water containing 25 milliliters HCl and 10 ml HNO.sub.3. After boiling for 5 minutes and cooling, there is added a solution of 23.3 g RuCl.sub.3.sup.. xH.sub.2 O (40.7% Ru) in 1 liter of water containing 10 ml HCl. The pH of the resultant solution is adjusted to within the range of 1.5 to 2.5 with concentrated KOH and heated almost to boiling. While hot, a 10 percent aqueous KOH solution is slowly added, with stirring, to precipitate a hydrous oxide. When a pH of 10 is obtained, the solution is brought to a boil, cooled and acidified to a pH of 6.5 with dilute aqueous HCl. The precipitate is then removed by filtration and washed several times with water, following which it is dried at about 100.degree. C. Finally, the oxide is ground and heated in air to a temperature of about 660.degree. C for about 16 hours. The resultant material has a particle size of less than 0.1 micron and contains 50 mole percent each of iridium and ruthenium oxides. The X-ray diffraction pattern shows a single rutile phase with peaks intermediate those of the pure oxides, thus indicating the formation of a solid solution. This same procedure is also employed to prepare solid solutions of other platinum metal oxides by simply altering the identity of the metal salts used. Likewise, the relative proportions of platinum metal oxides in the electrocatalytic material are varied by employing differing amounts of the salts in question.

For testing purposes, electrodes are prepared by forming a dispersion of 1.0 g of electrocatalytic material, 0.5 g of anhydrous TaCl.sub.5 and 1 ml HCl (35%) in 10 ml butanol. A dispersion is then applied to a previously degreased and etched titanium sheet in two coats, with air drying between coats, before heating in air at a temperature of 500.degree. C for 5 minutes. This procedure is repeated three times to give the desired electrode for test purposes.

The electrodes thus prepared are tested as anodes versus a saturated calomel electrode in (1) a 300 g/l sodium chloride solution for chloride evolution and (2) a 1 M H.sub.2 SO.sub.4 solution for oxygen evolution, with the half-cell voltage measured at 70.degree. C and a current density of 3 amperes/square inch. The results appear in Table I.

TABLE I ______________________________________ Electrocatalytic Material Anode (mole %) Half-Cell Voltage ______________________________________ RuO.sub.2 IrO.sub.2 Cl.sub.2 O.sub.2 1 100 -- 1.08 1.23 2 75 25 1.14 1.38 3 50 50 1.13 1.36 4 25 75 1.15 1.55 5 -- 100 1.09 1.40 ______________________________________

From this table, it can be seen that the relative chlorine and oxygen overvoltages vary with the composition of the electrocatalytic material, the material containing 75% IrO.sub.2 showing the greatest displacement and therefore being most efficient with respect to the loss of current to oxygen generation in sodium chloride electrolysis.

EXAMPLE 2

For the purpose of comparison, Anode No. 4 of Example 1 is compared with an anode prepared in the same manner, with the exception that the electrocatalytic material, rather than being a solid solution of platinum metal oxides is a mixture, in the same proportions, of separately prepared IrO.sub.2 and RuO.sub.2. By this procedure, an oxygen half-cell voltage of 1.35, as compared to 1.55 volts for Anode No. 4, is obtained, thus dictating considerably greater oxygen evolution during alkali metal chloride electrolysis.

EXAMPLE 3

A further series of anodes is prepared by the procedure of Example 1 with varying amounts of IrO.sub.2 as indicated in Table II below. The substrate is an etched titanium mesh and four coatingheating cycles as described in Example 1 are employed. The anodes are used in the production of sodium hypochlorite by the electrolysis of a dilute (30 g/l) sodium chloride solution at a temperature of 25.degree. C, and an anode current density of 1.0 a.s.i., opposite a titanium cathode. Table II indicates the current efficiencies obtained.

TABLE II ______________________________________ Electrocatalytic Material Current Efficiency Anode (mole %) (%) ______________________________________ RuO.sub.2 IrO.sub.2 6 96.5 3.5 73.1 7 92.5 7.5 81.7 8 88.5 11.5 66.8 9 84.5 15.5 65.4 ______________________________________

From Table II, it is apparent that control of the solid solution composition dictates its efficiency in various electrochemical reactions. Further, in Table III, Anode No. 7 is compared with anodes of the same composition prepared in the same manner, with the exception that firing of the electrocatalytic material proceeds at temperatures above and below that previously employed.

TABLE III ______________________________________ Electrocatalytic Material Firing Current Material Temperature Efficiency Anode (mole %) (.degree.C) (%) ______________________________________ RuO.sub.2 IrO.sub.2 10 92.5 7.5 500 74.2 7 92.5 7.5 660 81.7 11 92.5 7.5 825 76.5 ______________________________________

It is apparent that temperature has an effect upon the efficiency of the resultant anode. Thus, the present invention, which allows preparation of the electrocatalytic material extra the electrode and hence a wider range of heat treatment, affords a substantial advantage.

EXAMPLE 4

A further example of the importance of an ability to control the temperature at which the electrocatalytic material (as opposed to the entire electrode) is fired, is provided by Table IV. Here Anode No. 9 from Table II is compared with anodes prepared in identical fashion with the exception of the indicated variance in firing temperature. Electrolysis is conducted employing the electrode as an anode, opposite a titanium sheet cathode (electrode gap 1.5 inches) in 1 M H.sub.2 SO.sub.4 at 35.degree. C and a current density of 3 a.s.i. The "lifetime" is determined by noting the time necessary for the cell voltage to rise from an original level of about 4.0 volts to a value of 8.0 volts, such a rise indicating passivation (inactivity) of the anode.

TABLE IV ______________________________________ Electrocatalytic Material Firing Material Temperature Lifetime Anode (mole %) (.degree.C) (hrs) ______________________________________ RuO.sub.2 IrO.sub.2 12 84.5 15.5 500 14 9 84.5 15.5 660 134 13 84.5 15.5 825 31 ______________________________________

A remarkable and unexpected advantage is observed at elevated temperatures, especially temperatures on the order of 660.degree. C. On the other hand, attempts to directly deposit an iridium-ruthenium oxide solid solution coating on a titanium substrate at a temperature in excess of 600.degree. C, lead to the formation of excessive amounts of nonconducting oxides with the result that the initial half-cell voltage is prohibitively high.

EXAMPLE 5

To illustrate the use of an electrode of the present invention in the area of cathodic protection, Anode No. 3 from Example 1 is employed opposite a titanium metal cathode in synthetic sea water (28 g/l NaCl) having a pH of 9.5 at a temperature of 21.degree.-26.degree. C and an anode current density of 3.0 a.s.i. After 6,000 hours continuous operation, there is no change in the initially favorable operating voltage. By comparison, an anode comprising a 2:1 mole ratio solid solution coating of TiO.sub.2 and RuO.sub.2 on a Ti substrate fails (passivates) after 600 hours. Comparable results obtain on cold (<6.degree. C) sea water electrolysis to hypochlorite.

EXAMPLE 6

A solid solution is formed by dissolving 34.5 g IrCl.sub.3.sup.. xH.sub.2 O (52.9% Ir) and 25.0 g RhCl.sub.3.sup.. xH.sub.2 O (39.1% Rh) in one liter of water containing 25 ml HCl. The pH of the resulting solution is adjusted to pH 1.5 - 2.5 with concentrated KOH. The hydrous oxides are coprecipitated by slowly adding a 10 percent aqueous KOH solution while stirring until a pH value of 10 is obtained. The solution is then warmed to 60.degree.-80.degree. C, while maintaining a pH of 10, cooled and filtered. After several washings with water, the precipitate is dried at about 100.degree. C. Finally, the oxide is ground and heated in air at 600.degree. C for 16 hours. The resultant material contains 50 mole percent each of iridium and rhodium oxides and its X-ray diffraction pattern shows a single phase corundum structure with peaks displaced from that of pure Rh.sub.2 O.sub.3. This catalyst, when bound to a titanium substrate as in Example 1, results in an anode exhibiting excellent behavior in neutral and basic brine.

EXAMPLE 7

A solid solution is prepared as in Example 1 using 23.3 g RuCl.sub.3.sup.. xH.sub.2 O (40.7% Ru) and 27.4 g RhCl.sub.3.sup.. xH.sub.2 O (39.1% Rh). The coprecipitated hydrous oxides are fired at 660.degree. C for 20 hours. This material, containing 10 mole percent rhodium and 90 mole percent ruthenium oxides, exhibits an X-ray diffraction pattern showing a single rutile phase with the peaks shifted from those of pure RuO.sub.2. An anode prepared from this material has shown excellent behavior in the electrolysis of brine and in the electrolysis of acid solutions producing oxygen.

EXAMPLE 8

Another catalyst material is prepared by dissolving 23.3 g RuCl.sub.3 and 18.5 g PdCl.sub.2 (anhydrous) in 500 ml water, precipitating the hydrous oxides as in Example 5 and firing at 550.degree. C for 16 hours. The resulting material contains 10 mole percent palladium and 90 mole percent ruthenium oxides. X-ray shows a single rutile phase with peaks shifted from pure RuO.sub.2. An electrode prepared from this electrocatalyst is useful in acidic, oxygen-evolving applications.

EXAMPLE 9

By the same procedures, 10% PdO.sub.2 - 90% IrO.sub.2 and 10% PdO.sub.2 - 90% RhO.sub.2 solid solutions are prepared. The Pd-Ir electrocatalyst, bound to a tantalum substrate with an amorphous tantalum oxide binder as in Example 1, shows use as a chlor-alkali anode, as does the Pd-Rh catalyst in amorphous niobium oxide on a Nb or Ta substrate.

EXAMPLE 10

To illustrate the use of an electrode of the invention in aqueous electrowinning, an experiment isi conducted in an electrolytic cell employing a titanium mesh cathode, a porous polyfluorocarbon spacer (NAFION, trademark of E. I. du Pont de Nemours and Co., Inc.) and an anode comprising a titanium mesh substrate bearing a coating of 1 part amorphous tantalum oxide and 2 parts of a 90% RuO.sub.2 - 10% IrO.sub.2 solid solution electrocatalyst. The anolyte is a 200 g/l aqueous slurry of chalcopyrite ore concentrate and ferric chloride in 10% HCl. Hydrochloric acid (10%) is also the medium for the catholyte which is a leach of cuprous, ferric and ferrous ions from the ore slurry. Electrolysis is conducted initially at a temperature of 30.degree. C and an anode current density of 0.3 a.s.i. (electrode gap, 0.75 inch), the operating cell voltage being 5.15. Copper metal (powder) and H.sub.2 are the cathode products while Cl.sub.2, CuCl, FeCl.sub.2, S and H.sub.3 O.sup.+ are all found in the anolyte (the important effect being the dissolution of additional copper values for later reduction on recycle to the catholyte). At 1.0 a.s.i., a voltage of 17.7 follows. Changing the operating temperature to 54.degree. C reduces voltages to 3.45 and 9.35 at 0.3 and 1.0 a.s.i., respectively.

While the invention has been described with reference to certain preferred embodiments thereof, it is not to be so limited since changes and alterations may be made therein, while still remaining within the intended scope of the appended claims.

Claims

We claim:

1. An electrode consisting essentially of an electrically conductive substrate, an electrocatalytically active material and an amorphous valve metal oxide binder for adhering said material to at least a portion of the surface of said substrate, the material being a particulate solid solution of iridium and ruthenium oxides having a particle size of less than 0.1 micron, iridium oxide being present within the range of 1.0-99 mole percent, the balance being ruthenium oxide.

2. An electrode as in claim 1 wherein the substrate is a metal selected from the group consisting of titanium and tantalum.

3. An electrode as in claim 1 wherein the binder is an amorphous oxide selected from the group consisting of tantalum and niobium oxides.

4. An electrode consisting essentially of an electrically-conductive substrate, an electrocatalytically-active material and an amorphous valve metal oxide binder for adhering said material to at least a portion of the surface of said substrate, the material consisting essentially of a particulate solid solution of at least two platinum group metal oxides having a particle size of less than 0.1 micron with one oxide being present with the range of 1 to 99 mole percent and the balance being the second oxide.

5. An electrode as in claim 4 wherein the substrate is a metal selected from the group consisting of titanium and tantalum.

6. An electrode as in claim 4 wherein the amorphous binder is an oxide selected from the group consisting of tantalum and niobium oxides.

7. An electrode as in claim 4 wherein the material is a solid solution of at least two oxides selected from the group consisting of ruthenium, rhodium, palladium and iridium.