BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to an electrolytic electrode and a mixed metal oxide coating thereon for the generation of hypochlorite. 2. Description of the Related Art

The use of mixed metal oxide coatings for the generation of hypochlorite by electrolyzing brine solutions is widely known in the art. Conventionally, when hypochlorite is manufactured through the electrolysis of brine, however, the available chlorine concentration of the hypochlorite product can be as low as 1 weight percent (wt%) or less.

Additionally, current efficiency and electrode lifetimes diminish where brine feed solutions are less concentrated (i.e., 10-30 g/l) and the desired hypochlorite concentrations exceed 8 g/l.

Various solutions have been proposed to achieve high concentration sodium hypochlorite solutions without deleteriously effecting current efficiency and electrode lifetime. For example, in U.S. Patent

4,495,048, there is taught a filter press type electrolytic cell where in sodium hypochlorite is produced at a reduced cell voltage and improved current efficiency. The anode in the electrolytic cell consists of a titanium substrate having a coating of a ternary mixture of 3 to 42% by weight platinum oxide, 3 to 34% by weight palladium oxide, 42% by weight ruthenium dioxide and 20-40% by weight titanium oxide.

In U.S. Patent 4,517,068, an electrode, especially for chlorine and hypochlorite production, comprises an electrocatalyst consisting of 22-44 mol% ruthenium oxide, 0.2-22 mol% palladium oxide and 44-77.8 mol% titanium oxide. The electrocatalyst may form a coating on a valve metal substrate and may be topcoated with a porous layer of titanium or tantalum oxide.

A method for manufacturing hypochlorite efficiently using an anode having a coating of palladium oxide by 10 to 45 weight%, ruthenium oxide

by 15 to 45 weight %, titanium dioxide by 10 to 40 weight % and platinum by 10 to 20 weight %, as well as an oxide of at least one metal selected from cobalt, lanthanum, cerium or yttrium by 2 to 10 weight % is described in U.S. Patent 5,622, 613. It would be desirable to provide an electrode having an electrocatalytic coating thereon which is capable of providing improved electrode lifetimes and operating efficiencies in electrolyte environments used for the generation of hypochlorite from 15-30 grams per liter (g/l) NaCI or KCI feed solutions and where desired hypochlorite concentrations exceed 8 g/l. It would be further desirable to provide such an electrode at reduced costs as compared to platinum based formulations.

SUMMARY OF THE INVENTION

There has now been found an electrode coating which provides improved lifetimes while maintaining high efficiencies in electrolytic solutions for the generation of hypochlorite. The coating is a mixed metal oxide coating consisting of combinations of the oxides of palladium, iridium, ruthenium and titanium.

In one embodiment, the invention is directed to an electrode for use in the electrolysis of an aqueous solution for the production of hypochlorite, the electrode having an electrocatalytic coating thereon, with the electrode comprising a valve metal electrode base; a coating layer of an electrochemically active coating on the valve metal electrode base, the coating comprising a mixed metal oxide coating of platinum group metal oxides and a valve metal oxide, the mixed metal oxide coating consisting essentially of platinum group metal oxides of ruthenium, palladium, and iridium, and a valve metal oxide of titanium; wherein a) the molar ratio of the platinum group metal oxides to the valve metal oxide is from about 90:10 to about 40:60; b) the molar ratio of the ruthenium to the iridium is from about

90:10 to about 50:50; and

c) the molar ratio of the palladium oxide to ruthenium plus iridium oxides is from about 5:95 to about 40:60, basis 100 mole percent of the metals present in the coating; whereby the electrode operates at a high current efficiency to produce hypochlorite concentrations of at least 8 grams per liter.

In another embodiment, the invention is directed to a process for the electrolysis of an aqueous solution in an electrolytic cell having at least one anode therein, the anode having an electrocatalytic coating thereon, the process comprising the steps of providing an unseparated electrolytic cell, establishing in the cell an electrolyte containing chloride, providing the anode in the cell in contact with the electrolyte, the anode having the electrocatalytic coating comprising a mixed metal oxide coating of platinum group metal oxides and a valve metal oxide, the mixed metal oxide coating consisting essentially of platinum group metal oxides of ruthenium, palladium, and iridium, and a valve metal oxide of titanium, wherein

(a) the molar ratio of the platinum group metal oxides to the valve metal oxide is from about 90:10 to about 40:60;

(b) the molar ratio of the ruthenium to the iridium is from about 90:10 to about 50:50 and (c) the molar ratio of the palladium oxide to ruthenium plus iridium oxides is from about 5:95 to about 40:60, basis 100 mole percent of the metals present in the coating; impressing an electric current on the anode; and oxidizing chloride at the anode to produce hypochlorite at concentrations of at least 8 grams per liter.

DESCRIPTION OF THE INVENTION

According to the present invention, an electrode having an electrocatalytic coating having a high current efficiency at high hypochlorite concentrations, e.g., > 8 gpl (grams per liter) and having a low electrode potential and improved lifetimes is provided. In one embodiment, depending upon the hypochlorite concentration, the current efficiency will be from about 90% to about 100% over a hypochlorite concentration of

from 16 to 0 grams per liter (g/l). The electrode having the electrocatalytic coating described herein will virtually always find service as an anode. Thus, the word "anode" is often used herein when referring to the electrode, but this is simply for convenience and should not be construed as limiting the invention.

The electrode used in the present invention comprises an electrocatalytically active film on a conductive base. The conductive base may be a metal such as nickel or manganese or a sheet of any film-forming metal such as titanium, tantalum, zirconium, niobium, tungsten and silicon, and alloys containing one or more of these metals, with titanium being preferred for cost reasons. By "film-forming metal" it is meant a metal or alloy which has the property that when connected as an anode in the electrolyte in which the coated anode is subsequently to operate, there rapidly forms a passivating oxide film which protects the underlying metal from corrosion by electrolyte, i.e., those metals and alloys which are frequently referred to as "valve metals", as well as alloys containing valve metal (e.g., Ti-Ni, Ti-Co, Ti-Fe and Ti-Cu), but which in the same conditions form a non-passivating anodic surface oxide film. Plates, rods, tubes, wires or knitted wires and expanded meshes of titanium or other film-forming metals can be used as the electrode base. Titanium or other film-forming metal clad on a conducting core can also be used. It is also possible to surface treat porous sintered titanium with dilute paint solutions in the same manner.

Of particular interest for its ruggedness, corrosion resistance and availability is titanium. As well as the normally available elemental metals themselves, the suitable metals of the substrate include metal alloys and intermetallic mixtures, as well as ceramics and cermets such as contain one or more valve metals. For example, titanium may be alloyed with nickel, cobalt, iron, manganese or copper. More specifically, grade 5 titanium may include up to 6.75 weight percent aluminum and 4.5 weight percent vanadium, grade 6 up to 6 percent aluminum and 3 percent tin, grade 7 up to 0.25 weight percent palladium, grade 10, from 10 to 13 weight percent plus 4.5 to 7.5 weight percent zirconium and so on.

By use of elemental metals, it is most particularly meant the metals in their normally available condition, i.e., having minor amounts of impurities. Thus, for the metal of particular interest, i.e., titanium, various grades of the metal are available including those in which other constituents may be alloys or alloys plus impurities. Grades of titanium have been more specifically set forth in the standard specifications for titanium detailed in ASTM B 265-79. Because it is a metal of particular interest, titanium will often be referred to herein for convenience when referring to metal for the electrode base. Regardless of the metal selected and the form of the electrode base, before applying a coating composition thereto, the electrode base is advantageously a cleaned surface. This may be obtained by any of the treatments used to achieve a clean metal surface, including mechanical cleaning. The usual cleaning procedures of degreasing, either chemical or electrolytic, or other chemical cleaning operation may also be used to advantage. Where the base preparation includes annealing, and the metal is grade 1 titanium, the titanium can be annealed at a temperature of at least about 450°C for a time of at least about 15 minutes, but most often a more elevated annealing temperature, e.g., 600°C to 875°C is advantageous.

For most applications, it is advantageous to obtain a base with a surface roughness. This will be achieved by means which can include intergranular etching of the metal, plasma spray application, which spray application can be of particulate valve metal or of ceramic oxide particles, or both, etching and sharp grit blasting of the metal surface, optionally followed by surface treatment to remove embedded grit and/or clean the surface, or combinations thereof. In some instances the base can simply be cleaned, and this gives a very smooth substrate surface. Alternatively, the film-forming conductive base can have a pre-applied surface film of film-forming metal oxide which, during application of the active coating, can be attacked by an agent in the coating solution (e.g. HCI) and reconstituted as a part of the integral surface film.

Etching will be with a sufficiently active etch solution to develop a surface roughness and/or surface morphology, including possible aggressive grain boundary attack. Typical etch solutions are acid solutions. These can be provided by hydrochloric, sulfuric, perchloric, nitric, oxalic, tartaric, and phosphoric acids as well as mixtures thereof, e.g., aqua regia. Other etchants that may be utilized include caustic etchants such as a solution of potassium hydroxide/hydrogen peroxide, or a melt of potassium hydroxide with potassium nitrate. Following etching, the etched metal surface can then be subjected to rinsing and drying steps. The suitable preparation of the surface by etching has been more fully discussed in U.S. Pat. No. 5,167,788, which patent is incorporated herein by reference.

In plasma spraying for a suitably roughened metal surface, the material will be applied in particulate form such as droplets of molten metal. In this plasma spraying, such as it would apply to spraying of a metal, the metal is melted and sprayed in a plasma stream generated by heating with an electric arc to high temperatures in inert gas, such as argon or nitrogen, optionally containing a minor amount of hydrogen. It is to be understood by the use herein of the term "plasma spraying" that although plasma spraying is preferred the term is meant to include generally thermal spraying such as magnetohydrodynamic spraying, flame spraying and arc spraying, so that the spraying may simply be referred to as "melt spraying" or "thermal spraying".

The particulate material employed may be a valve metal or oxides thereof, e.g., titanium oxide, tantalum oxide and niobium oxide. It is also contemplated to melt spray titanates, spinels, magnetite, tin oxide, lead oxide, manganese oxide and perovskites. It is also contemplated that the oxide being sprayed can be doped with various additives including dopants in ion form such as of niobium or tin or indium. It is also contemplated that such plasma spray application may be used in combination with etching of the substrate metal surface. Or the electrode base may be first prepared by grit blasting, as discussed hereinabove, which may or may not be followed by etching.

It has also been found that a suitably roughened metal surface can be obtained by special grit blasting with sharp grit, optionally followed by removal of surface embedded grit. The grit, which will usually contain angular particles, will cut the metal surface as opposed to peening the surface. Serviceable grit for such purpose can include sand, aluminum oxide, steel and silicon carbide. Etching, or other treatment such as water blasting, following grit blasting can be used to remove embedded grit and/or clean the surface.

It will be understood from the foregoing that the surface may then proceed through various operations, providing a pretreatment before coating, e.g., the above-described plasma spraying of a valve metal oxide coating. Other pretreatments may also be useful. For example, it is contemplated that the surface be subjected to a hydriding or nitriding treatment. Prior to coating with an electrochemically active material, it has been proposed to provide an oxide layer by heating the substrate in air or by anodic oxidation of the substrate as described in U.S. Patent 3,234,1 10. Various proposals have also been made in which an outer layer of electrochemically active material is deposited on a sublayer, which primarily serves as a protective and conductive intermediate. Various tin oxide based underlayers are disclosed in U.S. Patent Nos. 4,272,354,

3,882,002 and 3,950,240. It is also contemplated that the surface may be prepared as with an antipassivation layer.

Following surface preparation, which might include providing a pretreatment layer such as described above, an electrochemically active coating layer is applied to the substrate member. As is typically representative of the electrochemically active coatings that are often applied, are those provided from active oxide coatings such as platinum group metal oxides, magnetite, ferrite, cobalt spinel or mixed metal oxide coatings. They may be water based, such as aqueous solutions, or solvent based, e.g., using alcohol solvent. However, it has been found that for the electrode of the present invention, the preferred coating composition solutions are typically those consisting of a mixed metal oxide coating of platinum group metal oxides and a valve metal oxide.

The platinum group metal oxides of the present invention preferably comprise, RuCb, PdCI ₂ , IrCb _, and hydrochloric acid, all in alcohol solution, in combination with a valve metal oxide. It will be understood that the RuCI _3, PdCI ₂ , IrCI ₃ may be utilized in a form such as RuCI ₃ xH ₂ O, PdCI ₂ xH ₂ O and IrCI ₃ XH ₂ O. For convenience, such forms will generally be referred to herein simply as RuCI ₃ , PdCI ₂ and IrCI ₃ - Generally, the metal salts will be dissolved in an alcohol such as either isopropanol or butanol, all combined with or with out small additions of hydrochloric acid, with n- butanol being preferred. It will be understood that the constituents are substantially present as their oxides in the finished coating, and the reference to the metals is for convenience, particularly when referring to proportions.

A valve metal component will be present in the coating composition in order to further stabilize the coating and/or alter the anode efficiency. Various valve metals can be utilized including titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum, and tungsten, with titanium being preferred. The valve metal component can be formed from a valve metal alchoxide in an alcohol solvent, with or without the presence of an acid. Such valve metal alchoxides which are contemplated for use in the present invention include methoxides, ethoxides, isopropoxides and butoxides. For example, titanium ethoxide, titanium propoxide, titanium butoxide, tantalum ethoxide, tantalum isopropoxide or tantalum butoxide may be useful, with titanium butoxide being preferred.

The mixed metal oxide coating of the present invention will contain a molar ratio of titanium to platinum group metal oxides of from about 90:10 to about 40:60, a molar ratio of ruthenium to iridium of about 90:10 to about 50:50 and a molar ratio of Pd:(Ru+lr) of about 5:95 to about 40:60. A particularly preferred composition of the mixed metal oxide coating of the present invention will contain a molar ratio of titanium to precious metal oxides of about 70:30 on a metals basis and a molar ratio of Pd:(Ru+lr) of about 20:80.

The mixed metal oxide coating layers utilized herein will be applied by any of those means which are useful for applying a liquid coating

composition to a metal substrate. Such methods include dip spin and dip drain techniques, brush application, roller coating and spray application such as electrostatic spray. Moreover, spray application and combination techniques, e.g., dip drain with spray application can be utilized. With the above-mentioned coating compositions for providing an electrochemically active coating, a roller coating operation can be most serviceable.

Regardless of the method of application of the coating, conventionally, a coating procedure is repeated to provide a uniform, more elevated coating weight than achieved by just one coating. However, the amount of coating applied will be sufficient to provide in the range of from about 0.05 g/m ² (gram per square meter) to about 6 g/m ² , and preferably, from about 1 g/m ² to about 4 g/m ² based on ruthenium content, as metal, per side of the electrode base.

Following application of the coating, the applied composition will be heated to prepare the resulting mixed oxide coating by thermal decomposition of the precursors present in the coating composition. This prepares the mixed oxide coating containing the mixed oxides in the molar proportions, basis the metals of the oxides, as above discussed. Such heating for the thermal decomposition will be conducted at a temperature of about 450°C to about 55O ⁰ C for a time of from about 3 minutes to about 15 minutes per coat. More typically, the applied coating will be heated at a more elevated temperature of up to about 490-525 ⁰ C for a time of not more than about 20 minutes per coat. Suitable conditions can include heating in air or oxygen. In general, the heating technique employed can be any of those that may be used for curing a coating on a metal substrate. Thus, oven coating, including conveyor ovens may be utilized. Moreover, infrared cure techniques can be useful. Following such heating, and before additional coating as where an additional application of the coating composition will be applied, the heated and coated substrate will usually be permitted to cool to at least substantially ambient temperature. Particularly after all applications of the coating composition are completed, postbaking can be employed. Typical postbake conditions for coatings can include

temperatures of from about 450°C up to about 55O ⁰ C. Baking times may vary from about 1 hour up to as long as about 6 hours.

The following examples, unless otherwise noted as comparative examples, generally demonstrate the production of a high concentration hypochlorite at increased efficiencies with an anode containing the coating of the present invention:

EXAMPLE 1

A flat, titanium plate of unalloyed grade 1 titanium, measuring approximately 0.15 cm thick and approximately 10 x 15 cm was grit blasted using alumina to achieve a roughened surface. The sample was then etched in a 90-95 ⁰ C solution of 18-20% hydrochloric acid for 25 minutes.

Coating compositions as set forth in Table 1 were applied to separate samples measuring 10 cm x 15 cm x 0.15 cm of Grade 1 titanium which was prepared by grit blasting with 54 grit alumina. The coating solutions A-D were prepared by dissolving sufficient amount of metals, as chloride salts, to achieve the concentrations listed in the table to a solution of n-butanol and 4.2 vol% concentrated HCI. The compounds used were RuCI ₃ , IrCb, and PdCI ₂ (all hydrated) and titanium orthobutyl titanate. After mixing to dissolve all of the salts, the solutions were applied to individual samples of prepared titanium plates. The coatings were applied in layers by brushing, with each coat being applied separately and allowed to dry at 1 1 O ⁰ C for 3 minutes, followed by heating in air to 500 ⁰ C for 6 minutes. A total of 5 coats was applied to each sample. Samples A-D are in accordance with the present invention. . Sample E is considered a comparative example.

Table I

' Salts are chlorides, except Ti, which is tan um orthobutyl titanate)

The hypochlorite efficiency of the samples was measured in a beaker-cell by immersing an area of 26 cm ² into a solution of 28 gpl NaCI with 1 gpl Na ₂ Cr ₂ O ₇ and applying an anodic current of 4.86 amps (0.186A/cm ² ). A titanium cathode was used, spaced 3 mm from the anode. A sample was pulled every 8 minutes and titrated for hypochlorite. The current efficiencies for the production of hypochlorite as a function of hypochlorite concentrations are plotted in Figure 1 and Table II.

Table Il

The set of samples, A-E, were then operated as anodes in an accelerated test as an oxygen-evolving anode at a current density of 10 kA/m ² in an electrochemical cell containing 150 g/l H ₂ SO ₄ at 65 ⁰ C. Cell voltage versus time data was collected every 30 minutes and the lifetime taken as the inflexion point at which the voltage began to increase rapidly. The results are summarized in Figure 2 and Table II, normalized for the amount of platinum group metal. Normalization was done by measuring the x-ray fluorescence count for the metal peaks using a Jordan Valley EX- 300 spectrometer with a Rh tube and a 0.15 mm Sn filter. The applied voltage was 4OkV (kilivolts) and current was 25 μA. The peaks measured

were the Ru K-alpha, Pd K-alpha and Ir L-beta. The total counts of the Ru, Pd and/or Ir were used to normalize the lifetimes.

It is, therefore, evident from the results of Table Il that samples prepared according to the present invention have substantially increased current efficiencies versus the comparison example while improving or meeting the lifetime as evidenced by the extended time before a significant rise in voltage (> 1 volt) occurs.

While in accordance with the patent statutes the best mode and preferred embodiment have been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims.