Titanium Or Tantalum Base Electrodes With Applied Titanium Or Tantalum Oxide Face Activated With Noble Metals Or Noble Metal Oxides

Abstract

Describes a titanium or tantalum base electrode having a protective and electrocatalytic layer applied to the faces exposed to the electrolyte, said protective and electrocatalytic layer consisting of mixtures of solid solutions of valve metal oxides and platinum group and noble metals as such or in the form of oxides and/or oxyhalides.

Description

This invention relates to an improvement in electrodes for use in anodic and cathodic reactions in cells used for example in the manufacture of chlorine and caustic alkali by the electrolysis of aqueous solution of alkali metal chlorides, for use in other processes in which an electrolysis current is passed through an electrolyte for the purpose of decomposing the electrolyte, for carrying out organic oxidations and reductions and for other purposes.

The invention is particularly valuable in flowing mercury cathode cells in which the cathode consists of mercury flowing over the cell base and in which the anodes are suspended above the flowing mercury cathode and the electrolysis of the brine takes place in the space between the anodes and the cathode. This, however, is only one illustration of the application of the invention and it may be used in diaphragm cells and in all other types of electrolysis cells and for other electrolysis and oxidation purposes.

With the advent of dimensionally stable anodes, based upon the use of valve metals, such as titanium, tantalum, zirconium, molybdenum and tungsten, which in service develop an oxide or barrier layer which prevents the further flow of anodic current through the anode, except at substantially higher voltage, it was considered necessary to cover the entire face of the titanium or tantalum anode with a conductive layer of noble metal from the platinum group (i.e., platinum, palladium, iridium, osmium, rhodium, ruthenium or alloys thereof), by electroplating or otherwise, which completely covered the titanium or tantalum base, except for inevitable pores through the coating metal, which pores were, however, sealed by the development of the barrier layer above referred to on the titanium or tantalum base.

The platinum group metals are, however, expensive, good adherence and good conduction between the titanium base and the platinum group metal coatings is difficult to secure, the platinum group metal coatings are consumed or lost in the electrolysis process, frequent cell shut downs are necessary to replace defective anodes, and the cost of the coating, per ton of chlorine produced, is high. Although many platinum group metal coated titanium base anodes have been produced and tested, no platinum group metal coated titanium base anodes have heretofore been produced either by the use of coatings of platinum group metals themselves or by the use of oxides of such metals, which gave reliable and reproducible results and which could be produced in commercial quantities and used satisfactorily in commercial electrolysis cells.

We have found that it is not necessary to cover the metal base of anodes, such as titanium or tantalum, with a conductive layer of a platinum group metal over the entire surface, but that a coating made of a valve metal oxide or oxyhalide, such as, for instance, titanium oxide, titanium oxychloride, tantalum oxide, tantalum oxychloride, zirconium oxide or oxyfluoride, may also develop electrocatalytic properties. if its structure is modified by the addition of platinum group and noble metals, either as such or in the form of oxides or oxyhalide. Furthermore, we have found that the coatings so applied will give equal or greater conduction than an entire anode surface coated with platinum or a platinum group metal or oxide and that the coating so applied is more resistant to the cell-operating conditions than platinum plated anodes and will cost far less per ton of chlorine produced than the platinum-plated titanium base anodes heretofore produced. The anodes of our invention will resist current reversals and amalgam dips, and resist the conditions of commercial operation in an electrolytic cell better than anodes which have their entire active surface coated with a platinum layer, and will produce a higher chlorine discharge with less overvoltage and have less wear per ton of chlorine produced than the previous anodes, completely covered with platinum group metals.

It is, therefore, an object of our invention to produce an anode for use in electrolysis processes which will have a long active life before developing an overvoltage for chlorine discharge and which will be resistive of the conditions of practical operation encountered in an electrolysis cell.

Another object of our invention is to produce a titanium or tantalum base anode provided with modified valve metal oxide or valve metal oxyhalide coatings which will continue to conduct electric current from the anode base to the electrolyte over long periods of time, without spalling or peeling of the coating from the anode base under practical cell conditions, and without material loss of the platinum group metal conductor, in the electrolysis process.

Another object of our invention is to provide a conductive coating on a titanium anode base which will be economical in the use of the platinum group and noble metals used for the coating and which will produce a high yield of chlorine per gram of the platinum group and noble metals used in the anode coating, or consumed in the electrolysis process.

Another object of our invention is to produce a titanium or tantalum base electrolysis anode provided with platinum group and noble metals dissolved as such, or in the form of oxides or oxyhalide, within the valve metal oxyhalide so that the platinum group and noble metals or oxides are protected from operative cell conditions by the presence of the valve metal oxide or oxyhalide which firmly adheres to the titanium or tantalum base and protects the platinum group and noble metals from destruction by current reversals, amalgam dips, short circuits and corrosive action of chlorine containing electrolyte encountered during operation of an electrolysis cell.

Another object of our invention is to provide a titanium or tantalum base anode having a coating comprising a mixture of titanium or tantalum oxides or oxychlorides of ruthenium oxide or oxychloride and of iridium metal, in which the weight ratio of noble metals to valve metal is not lower than 20/100 20/100 and not higher than 85/800.

Various other objects and advantages of our invention will appear as this description proceeds.

While any platinum group metals or alloy or oxides thereof may be used, we have secured our best results with iridium metal dissolved mixtures of ruthenium oxide or oxychloride embedded in and surrounded by a titanium or tantalum oxide or oxychloride coating applied on a titanium or tantalum base or core, and our invention will be described with reference to this preferred embodiment, without, however, intending to limit the invention to these embodiments.

Unlike prior platinum group metal coated titanium base anodes, which depend upon the development of a barrier layer of titanium oxide between the pores of the platinum group metal coating, our invention provides a valve metal oxide or oxyhalide coating on the titanium base which coating surrounds and protects the platinum group metal or oxide from destruction and wear under operative electrolysis cell conditions.

The electrodes of our invention are particularly useful as anodes for the electrolysis of sodium chloride brines in horizontal mercury cells and diaphragm cells as they have the ability to liberate chlorine at low-anode voltages essentially throughout the life of the platinum group metal or oxide conductors and conductive points and have a low wear rate (loss of noble metal per ton of chlorine produced).

Passivity of platinum or platinum group coated anodes of the prior art in electrolysis of brines has been a problem. Passivity refers to the rapid rise in potential in the said anodes after being used for some time at sufficiently high-current density under chlorine discharge. This rise in potential indicates that the anodic oxidation of the dissolved chloride ion to molecular chloride gas will proceed only at a higher overvoltage because of the diminished catalytic activity of the electrode surface.

The anodes of our invention have a lower anode potential and operate for a much longer time before reaching passivation than platinum group metal coated anodes, which demonstrate the high activity and long life of the anodes of our invention. This is particularly advantageous in electrolytic cells for the electrolysis of brines such as sodium chloride since the said cells can be operated for much longer periods of time before the electrodes have to be replaced, and shut down time of the cells is greatly reduced. Our invention also reduces the amount of the platinum group metal initially used, and eventually consumed, per ton of chlorine, making the process more economical.

In addition to the platinum group metals or oxides our anodes may be further improved by incorporating into the platinum group metal or oxides a minor amount of an activating element, such as antimony, as described in the copending application U.S. Ser. No. 506,852, filed Nov. 8, 1965, now U.S. Pat. No. 3,428,544.

The conductive coating of our invention may be applied in various ways, and to various forms of titanium or tantalum base anodes, such as solid rolled massive titanium plates, perforated plates, slitted, reticulated, titanium plates, titanium mesh and rolled titanium mesh, woven titanium wire, or screen or similar tantalum plates, all of which will be referred to as "mesh form." Our preferred method of application is by chemi-deposition in the form of painted on, dipped, sprayed or curtain coatings baked on the titanium anode base, but other methods of application, including electrophoretic deposition or electrodeposition, may be used.

In all applications the titanium or tantalum base must be cleaned and free of oxide or other scale. This cleaning can be done in any way, by mechanical or chemical cleaning, such as by sand blasting, etching, pickling or the like.

EXAMPLE I

An expanded titanium anode plate, with a surface of 50 cm..sup.2 projected area, was cleaned by boiling at reflux temperature of 110.degree. C. in a 20 percent solution of hydrochloric acid for 40 minutes. It was then given a liquid coating containing the following materials:

Ruthenium as Ru Cl.sub.3 .sup.. H.sub.2 0-- 10 mg. (metal)

Iridium as (NH.sub.4 ).sub.2 Ir Cl.sub.6 10 mg. (metal)

Titanium as TiCl.sub.3 56 mg. (metal)

Formamide (HCONH.sub.2)-- 10 to 12 drops

Hydrogen peroxide (H.sub.2 O.sub.2 30 %)--3 to 4 drops

The coating was prepared by first blending or mixing the ruthenium and iridium salts containing the required amount of Ru and Ir in a 2 molar solution of hydrochloric acid (5 ml. are sufficient for the above amounts) and allowing the mixture to dry at a temperature not higher than 50.degree.C. until a dry precipitate is formed. FOrmamide is then added to the dry salt mixture at about 40.degree.C. to dissolve the mixture. The titanium chloride, TiCl.sub.3, dissolved in hydrochloric acid (15 percent strength commercial solution), is added to the dissolved Ru-Ir salt mixture and a few drops of hydrogen peroxide (30% H.sub.2 O.sub.2) are added, sufficient to make the solution turn from the blue color of the commercial solution of TiCl.sub.3, to a brown-reddish color.

The coating mixture, thus prepared was applied to both sides of the cleaned titanium anode base, by brush, in eight subsequent layers. After applying each layer, the anode was heated in an oven under forced air circulation at a temperature between 300.degree.and 350.degree.C. for 10 to 15 minutes, followed by fast natural cooling in air between each of the first seven layers, and after the eighth layer was applied the anode was heated at 450.degree.C. for 1 hour under forced air circulation and then cooled.

The amounts of the three metals in the coating correspond to the weight ratios of 13.sup.. 15% Ir, 13.sup.. 15 % Ru and 73.sup.. 7% Ti and the amount of noble metal in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode area. It is believed that the improved qualities of our anode are due to the fact that although the three metals in the coating mixture are originally present as chlorides they are codeposited on the titanium base in other forms. Stoichiometric determinations indicate that in the final coating the iridium chloride is reduced to metallic Ir, whereas ruthenium chloride an titanium chloride are converted into ruthenium oxide RuO.sub.2 and titanium oxide or oxychloride TiOCl. In accelerated testing, the anode of this example showed a weight loss of zero after three current reversals, a loss of 0.152 mg./cm..sup.2 after three amalgam dips as against a weight loss of 0.93 mg./cm..sup.2 of a similar titanium base anode covered with ruthenium oxide. After 2,000 hours of operation this anode showed a weight increase of 0.7 mg./cm..sup. 2, whereas similar anodes covered with a layer of platinum or ruthenium oxide showed substantial weight losses. The weight increase had apparently become stabilized.

EXAMPLE II

The coating mixture was applied to a cleaned titanium anode base of the same dimensions as in example I according to the same procedure. The applied mixture consisted of the following amounts:

Ru as RuCl.sub. 3 .sup.. H.sub.2 O--20 mg. (metal)

Ir as (NH.sub.4).sub.2 IrCl.sub. 6 --20 mg. (metal)

Ti as TiCl.sub.3 --48 mg. (metal)

Hconh.sub.2 --10 to 12 drops

H.sub.2 o.sub.2 30 % --3 to 4 drops

The procedure for compounding the coating and applying it to the titanium base was the same as in example I. The quantities of the three metals in this mixture corresponded to the weight ratios of 22.6 % Ir, 22.6 % Ru and 54.8 % Ti and the amount of noble metal oxide in the active coating corresponded to 0.4 mg. Ir, and 0.4 mg. Ru per square centimeter of the active electrode area. After 2300 hours of operation this anode showed a weight increase of 0.9 mg./cm..sup.2 which had apparently become stabilized.

EXAMPLE III

Before being coated, the titanium anode after pre-etching, as described in example I, was immersed in a solution composed of 1 molar solution of H.sub.2 0 .sub.2 plus a 1 molar solution of NaOH at 20 to 30.degree. C. for 2 days. The surface of the titanium was thus converted to a thin layer of black titanium oxide.

The coating mixture of the same composition as given in example I was used, except that isopropyl alcohol was used as the solvent in place of formamide. The use of isopropyl alcohol resulted in a more uniform distribution of the coating films on the black titanium oxide substrate then when formamide was used as the solvent.

EXAMPLE IV

An expanded titanium anode plate of same size as in the former examples was submitted to the cleaning and etching procedure as described above and then given a liquid coating containing the following materials:

Ru as RuCl.sub.3 .sup.. H.sub.2 O-- 10 mg. (metal)

In as IrCl.sub.4 --10 mg. (metal)

Ta as TaCl.sub. 5 --80 mg. (metal)

Isopropyl alcohol--5 drops

20 % HC1--5 ml.

The coating was prepared by first blending or mixing the ruthenium and iridium salts in 5 ml. of 20 % HC1. The volume of this solution was then reduced to about one-fifth by heating to a temperature of 85.degree. C. The required amount of TaCl.sub.5 was dissolved in boiling 20 % HC1 so as to form a solution containing about 8 % TaC1.sub.5 by weight. The two solutions were mixed together and the overall volume reduced to about one-half by heating at 60.degree. C. The specified quantity of isopropyl alcohol was then added.

The coating mixture was applied to both sides of the cleaned titanium anode base in eight subsequent layers and following the same heating and cooling procedure as described in example 1.

The amounts of the three metals in the coating correspond to the weight ratios of 10 % Ru, 10 % Ir and 80 % Ta and the amount of noble metal in the coating corresponds to 0.2 mg. Ir and 0.02 mg. Ru. per square centimeter of projected electrode area. In accelerated testing, this anode showed a weight loss of 0.0207 mg./cm..sup. 2 after three current reversals and a loss of 0.0138 after two amalgam dips. After 514 hours of operation this anode showed a weight decrease of 0.097 mg./cm..sup.2.

EXAMPLE V

An expanded titanium anode plate of same size as in the former examples, after cleaning and etching, was given a liquid coating containing the following materials:

Ru as RuCl.sub.3 .sup. . H.sub.2 0-- 11.25 mg. (metal)

Au as HAuCl.sub.4 .sup. . nH.sub.2 O-- 3.75 mg. (metal)

Ti as TiCl.sub.3 --60 mg. (metal)

Isopropyl alcohol --5-10 drops

H.sub.2 o.sub.2 30 %--2-3 drops

The coating was prepared by first blending the ruthenium and gold salts in the required amount in a 2 molar solution of hydrochloric acid (5 ml.) and allowing the mixture to dry at a temperature of 50.degree. C. The commercial solution of TiCl.sub.3 was then added to the Ru-Au salt mixture and a few drops of hydrogen peroxide were stirred into the solution, sufficient to make the solution turn from blue to brown reddish. Isopropyl alcohol was finally added in the required amount. The coating mixture thus prepared was applied to both sides of the cleaned titanium anode base in eight subsequent layers, following the same heating and cooling procedure as described in example I.

The amounts of the three metals in the coating correspond to the weight ratios of 15 % Ru, 5 % Au, 80 % Ti and the amount of noble metal in the coating corresponds to 0.225 mg. Ru and 0.075 mg. Au per square centimeter of projected electrode area. In accelerated testing this anode showed a weight loss of 0.030 mg./cm..sup.2 after three current reversals and a loss of 0.043 mg./cm..sup.2 after two amalgam dips. After 514 hours of operation this anode showed a weight change of +0.2 mg./cm..sup.2.

Stoichiometric determinations indicate that the final deposit contains the three metals in the following form: gold is reduced by thermal decomposition to the metallic state Au, whereas ruthenium chloride and titanium chloride are converted into ruthenium oxide RuO.sub.2 and titanium oxychloride TiOCl, respectively.

Our tests so far have shown that when using the formulations and deposition methods described above the presence of titanium or tantalum oxide or oxychloride and iridium alone, i.e., without ruthenium oxide, gives a deposit which does not withstand the reducing action of current reversal (cathodic current) and amalgam dips, whereas the presence of titanium or tantalum oxide, oxychloride and ruthenium oxide, without iridium or iridium oxide, gives a deposit of low activity with a high-chlorine discharge voltage.

The anodes produced according to these examples showed the following advantages when compared to titanium base anodes covered with platinum group metals by electroplating or chemi-deposition. ##SPC1##

Weight losses on samples prepared according to the present invention were determined under simulated operating conditions and compared with weight losses determined under same conditions on titanium base samples coated with a Pt-Ir alloy. The tests were conducted in NaCl saturated solution at 65.degree. C. and under an anodic current density of 1 A./cm..sup.2. Anode potentials were measured by means of a Luggin tip against a saturated calomel electrode and converted to the normal hydrogen electrode scale. The relevant results are summarized in table II. The integrated weight change, as shown in the next to last column, was positive, that is increased, for most of the samples prepared according to the present invention; this is an indication that the coating, instead of gradually wearing off and thus decreasing its precious metal content, tends to build up an additional amount of protective valve metal oxide which reaches stability after a short period of operation as shown by sample B.

On the contrary, the results summarized in table I show that even the best noble metal alloy coatings suffer a greater wear rate, during operation; while such wear rate is not necessarily to be imputed exclusively to the spalling off of noble metals, it certainly involves also a substantial decrease of the noble metal content in the coating. The amount of noble metals in such noble metal alloy coatings, which is the amount necessary to obtain a satisfactory anode activity and a sufficiently long operating life, is from five to ten times greater than in the coatings prepared according to the present invention. ##SPC2##

Stoichiometric determinations indicate that the final coating contains the three metals in different form than the starting compounds. The iridium chloride apparently is reduced by thermal decomposition to metallic Ir, whereas ruthenium chloride and titanium chloride are converted into ruthenium oxide Ru0.sub.2 and titanium oxychloride TiOCl, respectively, in the anodes of examples I to IV, and in example V the gold is converted into metallic Au.

The average thickness of the final coating is 1.45 microns or 57 micro inches and the ratio of precious metals to nonprecious metals in the coatings may be between 20 to 100 and 85 to 100.

While we have set forth theories as to the final composition of our improved electrodes we do not intend to be bound by these theories but base our claim to invention on the procedures described to produce these electrodes and the results obtained in their use.

The word oxide in the following claims is intended to cover oxides of titanium and tantalum whether in the form of TiO.sub.2 and Ta.sub.2 O.sub.5 or TiOCl and TaO.sub.2 C1, or other oxides of these metals and the words noble metals is intended to include the platinum group metals and gold and silver.

Various modifications and changes may be made in the steps described without departing from the spirit of our invention or the scope of the following claims.

Claims

We claim:

1. An electrode comprised of a metal base from the group consisting of titanium and tantalum having a coating thereon, the major portion of which is an oxide of a valve metal from the group consisting of titanium and tantalum and the minor portion is an oxide of ruthenium, said minor portion also containing a material from the group consisting of iridium and gold, said coating being formed by applying onto said base a solution containing a metal compound from the group consisting of a titanium compound and a tantalum compound, a ruthenium compound and a metal compound from the group consisting of an iridium compound and a gold compound and heating the coating in air to bake said coating on said base.

2. The electrode of claim 1 in which the minor portion contains iridium and the iridium is in substantially equal amounts with the metal in the ruthenium oxide.

3. As a product of manufacture, a titanium base electrode in mesh form having a titanium oxide coating with a mixture of ruthenium oxide and one metal from the group consisting of iridium and gold extending through the coating and capable of conducting current from the titanium base through said coating to the exposed surface of said coating, said coating being formed on said base from a solution containing a titanium and a ruthenium compound and a compound from the group consisting of an iridium compound and a gold compound and baked on said base.

4. The product of claim 3 in which the metal in the ruthenium oxide and the metal compound from the group consisting of iridium and gold constitute from about 20 to about 85 percent of the weight of the metal in the titanium oxide.

5. The product of claim 3 in which the metal is iridium, said metal and the metal in the ruthenium oxide are in approximately equal amounts and constitute from about 20 to about 85 percent of the weight of the metal in the titanium oxide coating.

6. The product of claim 3 in which the coating is on all sides of the titanium base electrode.

7. The product of claim 3 in which the coating is applied in a plurality of layers and the electrode is heated in an oxidizing atmosphere after each layer is applied.

8. As a product of manufacture, an electrode consisting of a core of solid massive substantially nonporous valve metal from the group consisting of titanium and tantalum in mesh form, a substantially nonporous oxide layer of the same metal on said core in mixture with ruthenium oxide and a metal from the group consisting of iridium and gold, said layer being formed by applying a solution containing a compound from the group consisting of a titanium compound and a tantalum compound, a ruthenium compound and a compound selected from the group consisting of an iridium compound and a gold compound onto said core and baking on said core.

9. The product of claim 8 in which the oxide from the group consisting of titanium and tantalum oxides, the ruthenium oxide and the metal from the group consisting of iridium and gold are mixed together as chloride and applied to the core and baked on the core under air circulation.

10. As a product of manufacture, an electrode comprising a core of titanium and a conducting cover coating over said core consisting of (a) titanium oxide, (b) ruthenium oxide and (c) a metal from the group consisting of iridium and gold, said coating being formed by applying a solution containing a titanium compound, a ruthenium compound and a compound from the group consisting of an iridium compound and a gold compound onto said core and baking thereon, said coating being in multiple layers on said core and the metal in the titanium compound constituting more than 50 percent of the weight of the metals in said coating.