RU2379380C2 - High-efficiency anode coating for producing hypochlorite - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: electrocatalytalytic coating is a mixed metal-oxide coating on an electrode base made from a valve metal containing platinum group metal oxides and an oxide of titanium valve metal, wherein the said platinum group metal oxides mainly consist of oxides of ruthenium, palladium and iridium, where (a) molar ratio of said oxide of titanium valve metal to said platinum group metal oxides in terms of metal ranges from approximately 90:10 to approximately 40:60; (b) molar ratio of said ruthenium to said iridium ranges from approximately 90:10 to approximately 50:50; and (c) molar ratio of said palladium oxide to oxides of ruthenium plus iridium in terms of metal ranges from approximately 5:95 to approximately 40:60, in terms of 100 molar percents of these metals present in the coating. ^ EFFECT: electrolytic coating has longer service life with retention of high efficiency of formation of hypochlorite. ^ 14 cl, 2 dwg, 2 tbl, 1 ex

Description

BACKGROUND OF THE INVENTION

1. The technical field to which the invention relates.

The invention is directed to an electrolyte electrode and a mixed metal oxide coating on it to form hypochlorite.

2. Description of the Related Art

The use of mixed metal oxide coatings for the formation of hypochlorite in the electrolysis of saline solutions is widely known in the art. However, usually in the production of hypochlorite by electrolysis of a saline solution (brine), the concentration of available chlorine in the hypochlorite product can be so low that it is 1 mass percent (wt.%) Or less. In addition, the current efficiency and electrode life are reduced when the supplied salt solutions are less concentrated (i.e. 10-30 g / l) and the required hypochlorite concentrations exceed 8 g / l.

Various solutions have been proposed to obtain highly concentrated solutions of sodium hypochlorite without adversely affecting current efficiency and electrode life. For example, US Pat. No. 4,495,048 discloses a filter-press type electrolytic cell in which sodium hypochlorite is produced at a reduced cell voltage and with increased current efficiency. The anode in such an electrolytic cell consists of a titanium substrate having a ternary mixture coating of 3 to 42% by weight of platinum oxide, 3 to 34% by weight of palladium oxide, 42% by weight of ruthenium dioxide and 20-40% by weight of titanium oxide .

According to US Pat. No. 4,571,068, an electrode specifically designed to produce chlorine and hypochlorite contains an electrocatalyst consisting of 22-44 mol% of ruthenium oxide, 0.2-22 mol% of palladium oxide and 44-77.8 mol% of titanium oxide. The electrocatalyst can form a coating on a valve metal substrate and can be coated on top with a porous layer of titanium oxide or tantalum.

A method for the efficient production of hypochlorite using an anode having a coating of palladium oxide in an amount of 10 to 45 wt.%, Ruthenium oxide in an amount of 15 to 45 wt.%, Titanium dioxide in an amount of 10 to 40 wt.% And platinum in an amount from 10 to 20 wt.%, as well as oxide of at least one metal selected from cobalt, lanthanum, cerium or yttrium, in an amount of from 2 to 10 wt.%, is described in US patent 5622613.

It would be desirable to create an electrode with an electrocatalytic coating deposited on it, which can provide increased electrode life and work efficiency in electrolytic media used to form hypochlorite from the supplied NaCl or KCl solutions at a concentration of 15-30 grams per liter (g / l), and in cases where the required concentration of hypochlorite exceeds 8 g / l. Moreover, it would be desirable to create such an electrode with reduced costs compared to platinum based compositions.

SUMMARY OF THE INVENTION

An electrode coating has been found that provides improved durability while maintaining high

efficiencies in electrolytic solutions for the formation of hypochlorite. This coating is a mixed metal oxide coating consisting of combinations of palladium, iridium, ruthenium and titanium oxides.

In one embodiment, the invention is directed to an electrode for use in the electrolysis of an aqueous electrolyte solution to produce hypochlorite, comprising an electrode base of a valve metal and an electrocatalytic (electrochemically active) coating of said electrode base of a valve metal, the coating comprising a mixed metal oxide coating of metal oxides a platinum group and a valve metal oxide, wherein the mixed metal oxide coating consists essentially of metal oxides a platinum group ruthenium, palladium, and iridium and titanium oxide of the valve metal, wherein (a) the molar ratio of said titanium oxide of the valve metal to said platinum group metal oxides based on the metals is from about 90:10 to about 40:60; (b) the molar ratio of said ruthenium to said iridium is from about 90:10 to about 50:50; and (c) the molar ratio of said palladium oxide to ruthenium oxides plus iridium based on metals is from about 5:95 to about 40:60, based on 100 molar percent of these metals present in this coating.

Moreover, such an electrode operates at a high current efficiency to obtain hypochlorite concentrations of at least 8 grams per liter (g / l).

In another embodiment, the invention is directed to a method for electrolyzing an aqueous solution of an electrolyte in an electrolytic cell equipped with at least one electrocatalytic coated anode according to the first embodiment of the invention, comprising the steps of: providing an undivided (diaphragmless) electrolytic cell, placing chloride containing in this cell electrolyte, providing the specified anode in this cell in contact with the electrolyte, applying an electric current to the anode and obtaining hypochlorite concentrations of at least 8 g / l by oxidising said chloride at said anode.

Description of the invention

According to the present invention, there is provided an electrode with an electrocatalytic coating having a high current efficiency at high concentrations of hypochlorite, for example> 8 g / l, and having a low electrode potential and improved service life. In one embodiment, depending on the concentration of hypochlorite, the current output will be from about 90% to about 100% within the concentration of hypochlorite from 16 to 0 g / L. An electrode having an electrocatalytic coating described herein will in fact always function as an anode. Thus, the word "anode" is often used in this description with reference to the electrode, but this is done simply for convenience of presentation and should not be construed as limiting the invention.

The electrode used in the present invention contains an electrocatalytically active film on a conductive basis. 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, as well as alloys containing one or more of these metals, with titanium being preferred for cost reasons. By “film-forming metal” is meant a metal or alloy which has such a property that when it is connected as an anode in an electrolyte in which this coated anode is to subsequently operate, a passivating oxide film is quickly formed on it, which protects the underlying metal from corrosion under the action of an electrolyte, i.e. such metals and alloys, which are often called "valve metals", as well as alloys containing a valve metal (for example, Ti-Ni, Ti-Co, Ti-Fe and Ti-Cu), but which under the same conditions form non-passivating anode surface oxide films. As the electrode base can be used plates, rods, tubes, wires or knitted wires and stretched mesh of titanium or other film-forming metals. A shell of titanium or another film-forming metal on a conductive core may also be used. It is also possible to treat the surface of porous sintered titanium with diluted coloring solutions in a similar manner.

Of particular interest due to its high strength, corrosion resistance and availability is titanium. In addition to the commonly available elemental metals as such, suitable substrate metals include metal alloys and intermetallic mixtures, as well as ceramics and cermets, such as those containing one or more valve metals. For example, titanium can 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 , grades 10 - from 10 to 13 weight percent, plus from 4.5 to 7.5 weight percent of zirconium and so on.

The use of elemental metals most often in practice means metals in their usually accessible state, i.e. having minor amounts of impurities. So, in the case of metal of particular interest, i.e. titanium, various grades of this metal are available, including those in which other components may be alloys or alloys plus impurities. Grades of titanium are more specifically described in standard titanium specifications detailed in ASTM B 265-79.

Since titanium is a metal of particular interest, for convenience, reference will often be made to titanium when reference is made to a metal of an electrode base.

Regardless of the metal chosen and the shape of the electrode base, before applying the coating composition to it, the electrode base preferably has a cleaned surface. It can be obtained by any of the treatments used to achieve a clean metal surface, including mechanical cleaning. Conventional degreasing cleaning procedures, either chemical or electrolytic, or another chemical cleaning operation, can also be successfully used. In the case 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 at least about 15 minutes, but a higher annealing temperature is most often preferred. e.g. 600 ° C to 875 ° C.

For most applications, it is preferable to obtain a base with a surface roughness. This can be achieved by such methods, which may include intergranular etching of the metal, applying by plasma spraying, and such spraying may occur from valve metal particles or ceramic oxide particles, or both, etching and shot peening of the metal surface with a sharp shot, optionally followed by surface treatment for removing disseminated shots and / or surface cleaning, or combinations thereof. In some cases, the substrate can simply be cleaned and this will give a very smooth surface to the substrate. Alternatively, the film-forming conductive substrate may have a pre-deposited film-forming metal oxide surface film which, during the application of the active coating, may be aggressively exposed to any agent in the coating solution (e.g., HCl) and be transformed as part of a common surface film.

The etching will be carried out with a sufficiently active etching solution for the formation of surface roughness and / or surface morphology, including possible aggressive effects on grain boundaries. Typical etching solutions are acid solutions. They can be represented by hydrochloric, sulfuric, perchloric, nitric, oxalic, tartaric and phosphoric acid, as well as their mixtures, such as aqua regia. Other etchants that can be used include caustic etchants such as potassium hydroxide / hydrogen peroxide solutions, or molten potassium hydroxide with potassium nitrate. After etching, the etched metal surface can then be subjected to washing and drying steps. Suitable etching surface preparation has been more fully discussed in US Pat. No. 5,167,788, which patent is hereby incorporated by reference.

In plasma spraying, in order to give a sufficient roughness to the metal surface, the material will be applied in the form of particles, such as droplets of molten metal. With this plasma spraying, such as would be used to spray metal, the metal melts and sprays in a plasma stream generated by heating an electric arc to high temperatures in an inert gas such as argon or nitrogen, optionally containing a small amount of hydrogen. When used in this description, the term "plasma spraying" it should be understood that, although plasma spraying is preferred, it is understood that this term includes in general thermal spraying, such as magnetohydrodynamic spraying, gas flame spraying and electric arc spraying, so that the spraying can simply be called " spraying in the liquid phase "or" thermal spraying. "

The particulate material used may be a valve metal or its oxides, for example titanium oxide, tantalum oxide and niobium oxide. Titanates, spinels, magnetite, tin oxide, lead oxide, manganese oxide and perovskites are also considered for spraying in the liquid phase. It is also contemplated that the sprayed oxide may be doped with various additives, including dopants in ionic form, such as niobium or tin or indium.

It is also contemplated that such a plasma spray application can be used in combination with etching a metal surface of a substrate. Alternatively, the electrode substrate may be first prepared by shot peening, as discussed above, 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 shot blasting with a sharp shot, optionally accompanied by the removal of disseminated shot from the surface. The fraction, which will usually contain angular particles, will leave notches on the surface of the metal, as opposed to hardening the surface. Suitable shots for this purpose can be sand, alumina, steel and silicon carbide. Pickling or other treatment, such as water-jet treatment, after shot-blasting, can be used to remove disseminated shots and / or to clean the surface.

From the foregoing, it will be understood that the surface can then be subjected to various operations that provide pre-treatment before coating, for example, the above-described plasma spraying of a valve metal oxide coating. Other pretreatments may also be useful. For example, it is contemplated that a surface is subjected to a hydrogenation or nitriding treatment. Before coating with an electrochemically active material, it was proposed to create an oxide layer by heating the substrate in air or by anodic oxidation of the substrate, as described in US Pat. No. 3,234,110. Various proposals have also been made in which the outer layer of the electrochemically active material is deposited on a sublayer, which primarily serves as a protective and conductive intermediate. Various underlying tin oxide-based layers are disclosed in US Pat. Nos. 4,272,354, 3,882,002 and 3,950,240. It is also contemplated that a surface can be prepared, as is the case with the anti-passivation layer.

After preparing the surface, which could include creating a pre-treatment layer, as described above, an electrochemically active coating layer is applied to the substrate element. Typical representatives of electrochemically active coatings that are often used are those selected from active oxide coatings, such as platinum group metal oxides, magnetite, ferrite, cobalt spinel, or mixed metal oxide coatings. They can be water based, for example, in the form of aqueous solutions, or solvent based, for example, using an alcohol solvent. However, it has been found that coating solution compositions preferred for the electrode of the present invention are usually those consisting of a mixed metal oxide coating of platinum group metal oxides and valve metal oxide.

The platinum group metal oxides of the present invention preferably include RuCl ₃ , PdCl ₂ , IrCl _3, and hydrochloric acid, all in an alcohol solution, in combination with valve metal oxide. It should be understood that RuCl ₃ , PdCl ₂ , IrCl ₃ can be used in such a form as RuCl ₃ × H ₂ O, PdCl ₂ × H ₂ O and IrCl ₃ × H ₂ O. For convenience, such forms will be generally referred to below just like RuCl ₃ , PdCl ₂ and IrCl ₃ . Typically, these metal salts will be dissolved in an alcohol, such as either isopropanol or butanol, both with or without minor additions of hydrochloric acid, with n-butanol being preferred. It should be understood that these components are present in the final coating mainly in the form of their oxides, and reference to metals is given for convenience, especially when quantitative ratios are indicated.

The valve metal component will be present in the coating composition in order to further stabilize the coating and / or change the efficiency of the anode. Various valve metals can be used, including titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum and tungsten, with titanium being preferred. The valve metal component can be formed from valve metal alcoholate in an alcohol solvent, with or without the presence of acid. Such valve metal alcoholates that are contemplated for use in the present invention include methylates, ethylates, isopropylates and butylates. For example, titanium ethylate, titanium propylate, titanium butylate, tantalum ethylate, tantalum isopropylate or tantalum butylate can be used, with titanium butylate being preferred.

The mixed metal oxide coating of the present invention will have a molar ratio of titanium oxide to metal oxides of the platinum group based on metals from about 90:10 to about 40:60, a molar ratio of ruthenium to iridium from about 90:10 to about 50:50, and a molar ratio palladium oxide to ruthenium oxides plus iridium based on metals, i.e. Pd: (Ru + Ir), from about 5:95 to about 40:60. A particularly preferred composition of the mixed metal oxide coating of the present invention will have a molar ratio of titanium oxide to noble oxides of the platinum group of about 70:30 based on metals and said molar ratio of Pd: (Ru + Ir) of about 20:80.

The mixed metal oxide coating layers used herein will be applied by any of those agents that are suitable for applying a liquid coating composition to a metal substrate. Such methods include immersion and swelling techniques after immersion, brushing, roller coating and spraying, such as electrostatic spraying. In addition, spray coating and combined technologies can be used, such as dripping after immersion with spray coating. With the aforementioned coating compositions for providing an electrochemically active coating, a roller coating operation may be most convenient.

Regardless of the coating method, usually the coating procedure is repeated to provide a uniform coating having a higher coating weight than that achieved in 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 (grams per square meter) to about 6 g / m ² , and preferably from about 1 g / m ² to about 4 g / m ² , according to the content of ruthenium in the form of a metal on each side of the electrode base.

After coating, the applied composition will be heated to obtain the resulting mixed metal oxide coating by thermal decomposition of the precursors contained in the coating composition. This gives a mixed oxide coating containing mixed oxides in molar ratios, based on the metals in these oxides, as discussed above. Such thermal decomposition heating will be carried out at a temperature of from about 450 ° C to about 550 ° C for a time of from about 3 minutes to about 15 minutes per coating layer. More typically, the applied coating will be heated to a higher temperature, up to about 490-525 ° C. over a period of no more than about 20 minutes per coating layer. Suitable conditions may include heating in air or in oxygen. Typically, the heating technique used may be any of those that can be used to cure the coating on a metal substrate. Thus, a drying oven for coating treatment, including conveyor ovens, can be used. In addition, infrared curing techniques may be useful. After such heating and before applying the additional coating, where additional application of the coating composition will be applied, the heated coated substrate will usually be allowed to cool to at least substantially ambient temperature. In particular, after all application of the coating composition has been completed, subsequent heat treatment may be used. Typical conditions for subsequent heat treatment of coatings may include temperatures from about 450 ° C to about 550 ° C. Heat treatment time intervals can vary from about 1 hour to as long as about 6 hours.

The following examples, unless otherwise indicated to them as comparative examples, generally demonstrate the production of high concentration hypochlorite at increased efficiencies using an anode containing a coating of the present invention.

Example 1

A flat titanium plate of unalloyed grade 1 titanium with a measured thickness of approximately 0.15 cm and dimensions of approximately 10 × 15 cm was subjected to bead blasting using alumina to obtain a rough surface. Then the sample was etched in a solution of 18-20% hydrochloric acid for 25 minutes at 90-95 ° C.

The coating compositions shown in table 1 were applied to individual samples with measured dimensions of 10 cm × 15 cm × 0.15 cm of grade 1 titanium, which were prepared by shot blasting with the 54th alumina shot. To achieve the concentrations shown in the table, AD coating solutions were prepared by dissolving a sufficient amount of metals in the form of chloride salts in a solution of n-butanol and 4.2 vol.% Concentrated HCl. The compounds RuCl ₃ , IrCl ₃ and PdCl ₂ (all hydrated) and orthobutyl titanium titanate were used. After mixing to dissolve all of these salts, the solutions were applied to individual samples of prepared titanium plates. The coatings were applied layer by layer by brushing, each coating layer being applied separately and allowed to dry at 110 ° C for 3 minutes, followed by heating in air to 500 ° C for 6 minutes. A total of 5 coating layers were applied to each sample. Samples AD obtained in accordance with the present invention. Sample E is considered as a comparative example.

Table I Sample The concentration of the solution (g / l) Coating Ru Ir Pd Ti A Ru / Ir / Pd / Ti 20.9 20.9 10.5 43.9 B Ru / Pd / Ti 20.9 10.5 43.9 C Ru / Ir / Pd 20.9 20.9 10.5 D Ir / Pd / Ti 20.9 10.5 43.9 E Ru / Ir / Ti (Comparative) 20.9 20.9 43.9 * Salts are chlorides, with the exception of Ti, which is orthobutyl titanium titanate.

The efficiency of samples on the yield of hypochlorite was measured in a beaker by immersing a 26 cm ² pad in a solution of 28 g / l NaCl with 1 g / l Na ₂ Cr ₂ O ₇ and applying an anode current of 4.86 amperes (0.186 A / cm ² ). A titanium cathode located at a distance of 3 mm from the anode was used. A sample was taken every 8 minutes and titrated to determine hypochlorite. The current outputs upon receipt of hypochlorite, depending on the concentration of hypochlorite, are plotted in figure 1 and are shown in table II.

Table II Ru / Ir / Pd / Ti Ir / Pd / Ti Ru / Ir / Pd Ru / Pd / Ti Ru / Ir / Ti (Comparative) NaOCl (g / l) Exit (%) NaOCl (g / l) Exit (%) NaOCl (g / l) Exit (%) NaOCl (g / l) Exit (%) NaOCl (g / l) Exit (%) 2,36 88.5 2.71 103.6 2.49 95.7 2,51 96.6 2.26 86.5 4.78 88.6 5.12 97.1 4.92 93.9 5.11 97.4 4.43 83.9 7.09 86.9 7.74 96.8 7.39 93.0 7.69 96.8 6.43 80.5 9.30 84.8 10.27 95.6 9.71 90.9 10.21 95.6 8.31 77.3 11.49 83.0 12.54 92.4 11.49 85.3 12.61 93.6 9.80 72.3

A series of AE samples were then exploited as anodes in an accelerated test for an anode of oxygen evolution at a current density of 10 kA / m ² in an electrochemical cell containing 150 g / l H ₂ SO ₄ at 65 ° C. Data on the cell voltage, depending on the time, was taken every 30 minutes, and for the service life we took the inflection point, at which the voltage began to increase rapidly. The results are summarized in figure 2 and in table II, while the data are normalized by the amount of metal of the platinum group. Normalization was carried out by measuring the X-ray fluorescence peak intensity accounts for metals using a Jordan Valley EX-300 spectrometer with an Rh tube and a 0.15 mm Sn filter. The applied voltage was 40 kV (kilovolt), and the current was 25 μA. The measured peaks were Ru K-alpha, Pd K-alpha and Ir L-beta. Common scores for Ru, Pd, and / or Ir were used to normalize the service lives.

Thus, from the results shown in table II, it is obvious that the samples prepared according to the present invention have significantly increased current outputs compared with the comparative example, while providing an increased or corresponding service life, as evidenced by the extended period of time until how a significant increase in voltage (> 1 V) occurs.

Although the best method and preferred embodiments of the invention have been set forth in accordance with patent law, the scope of the invention is not limited by them, but by the scope of the attached claims.

Claims (14)

1. An electrode for use in the electrolysis of an aqueous electrolyte solution to produce hypochlorite, comprising an electrode base of a valve metal, an electrocatalytic coating on said electrode base of a valve metal, comprising a mixed metal oxide coating of platinum group metal oxides and titanium valve metal oxide, said metal oxides the platinum group consist essentially of oxides of ruthenium, palladium and iridium, where (a) the molar ratio of the specified valve metal oxide t ethane to the indicated oxides of metals of the platinum group in the calculation of metals is from about 90:10 to about 40:60; (b) the molar ratio of said ruthenium to said iridium is from about 90:10 to about 50:50; and (c) the molar ratio of said palladium oxide to ruthenium oxides plus iridium based on metals is from about 5:95 to about 40:60, based on 100 mol% of these metals present in the coating. 2. The electrode according to claim 1, in which the specified electrode base of the valve metal is a mesh, sheet, spatula, tube, perforated plate or wire element of the valve metal. 3. The electrode according to claim 1, wherein said valve metal electrode base is made of one or more metals selected from the group of titanium, tantalum, aluminum, hafnium, niobium, zirconium, molybdenum or tungsten, their alloys and their intermetallic mixtures. 4. The electrode according to any one of the preceding paragraphs, in which the surface of the specified electrode base of the valve metal is a rough surface. 5. The electrode according to claim 4, in which the specified rough surface is obtained through one or more stages of intergranular etching, shot peening or thermal spraying. 6. The electrode according to claim 4, in which on the specified rough surface there is a formed ceramic oxide barrier layer as a pretreatment layer. 7. The electrode according to claim 1, in which the molar ratio of said titanium oxide to said platinum group metal oxides, based on metals, is about 70:30, and the molar ratio of said palladium oxide to ruthenium oxides plus iridium per metal is about 20: 80. 8. The electrode according to claim 1, in which the molar ratio of ruthenium oxide to iridium oxide based on metals is about 1: 1. 9. The use of the electrode according to any one of the preceding paragraphs as an anode in the electrolysis of sea water. 10. The use of an electrode according to any one of claims 1 to 8 as an anode in a method for producing hypochlorite in concentrations of at least 8 g / l and with a current output within the range of from about 90% to about 100%. 11. The method of electrolysis of an aqueous solution of an electrolyte in an electrolytic cell equipped with at least one anode according to any one of claims 1 to 8, which includes the following stages:
providing an undivided electrolytic cell;
placement in the specified cell containing chloride electrolyte;
providing said anode in said cell in contact with said electrolyte;
supplying electric current to said anode; and
obtaining hypochlorite in concentrations of at least 8 g / l by oxidizing said chloride at said anode. 12. The method according to claim 11, in which the specified chloride electrolyte in the specified cell is one or more of sodium chloride or potassium chloride. 13. The method according to claim 11 or 12, in which the surface of the specified anode is a rough surface prepared by one or more stages of intergranular etching, shot peening or thermal spraying. 14. The method according to item 13, wherein said rough surface of the anode contains titanium, and said electrocatalytic coating is provided on said titanium using a procedure selected from the electrostatic deposition group, brush application, roller coating, dipping, and combinations thereof.

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