KR101135887B1 - High efficiency hypochlorite anode coating - Google Patents

Abstract

The present invention relates to an electrocatalytic coating and an electrode having the coating thereon, wherein the coating is a mixed metal oxide coating, preferably platinum group metal oxides, with or without a valve metal oxide. The electrocatalytic coating can be used especially as an anode component of an electrolysis cell and in particular a cell for the electrolysis of aqueous hypochlorite solutions.

Description

High efficiency hypochlorite anode coating

The present invention relates to an electrolyte electrode for the production of hypochlorite and a mixed metal oxide coating coated thereon.

The use of mixed metal oxide coatings to electrolyze a brine solution to produce hypochlorite is well known in the art. However, conventionally, when hypochlorite is produced via electrolysis of brine, the available chlorine concentration of the hypochlorite product may be as low as 1% by weight or less. In addition, current efficiency and electrode lifetimes decrease when the brine feed solution is less concentrated (ie, 10-30 g / l) and the desired hypochlorite concentration exceeds 8 g / l.

Various solutions have been proposed to achieve high concentration sodium hypochlorite solutions without degrading current efficiency and electrode life. For example, US Pat. No. 4,495,048 teaches filter compressed electrolytic cells in which sodium hypochlorite is produced at reduced cell voltage and improved current efficiency. In an electrolytic cell, the anode consists of a titanium substrate having a coating of a ternary mixture of 3 to 42 weight percent platinum oxide, 3 to 34 weight percent palladium oxide, 42 weight percent ruthenium dioxide and 20 to 40 weight percent titanium oxide. .

In US Pat. No. 4,517,068, in particular the electrode for the production of chlorine and hypochlorite comprises an electrocatalyst consisting of 22 to 44 mole percent ruthenium oxide, 0.2 to 22 mole percent palladium oxide and 44 to 77.8 mole percent titanium oxide. The electrocatalyst may form a coating on the valve metal substrate and topcoat with a porous layer of titanium oxide or tantalum oxide.

2 to 10% by weight of oxides of at least one metal selected from cobalt, lanthanum, cerium or yttrium, as well as 10 to 45% by weight of palladium oxide, 15 to 45% by weight of ruthenium oxide, 10 to 40% by weight of titanium dioxide and 10 to 20% by weight of platinum A method for effectively preparing hypochlorite using an anode having a% coating is described in US Pat. No. 5,622,613.

Electrocatalyst coatings that can provide improved electrode life and operating efficiency in an electrolyte environment where the desired hypochlorite concentration exceeds 8 g / l and is used to produce hypochlorite from 15-30 g / l NaCl or KCl feed solution. It may be desirable to provide an electrode having the above. In addition, it may be desirable to provide such electrodes at reduced cost compared to platinum based formulations.

Summary of the Invention

It has now been found that electrode coatings which provide improved lifetimes while maintaining high efficiency in electrolyte solutions for the preparation of hypochlorite. The coating is a mixed metal oxide coating consisting of a combination of oxides of palladium, iridium, ruthenium and titanium.

In one aspect, the invention

Valve metal electrode base and

On the valve metal electrode base, a coating layer of an electrochemically active coating comprising a mixed metal oxide coating of a platinum group metal oxide and a valve metal oxide of titanium, consisting essentially of platinum group metal oxides of ruthenium, palladium and iridium, wherein the coating Based on 100 mole percent of the metal present in the molar ratio of titanium to (a) the platinum group metal oxide is from about 90:10 to about 40:60, and (b) the molar ratio of ruthenium to iridium is about 90: 10 to about 50:50, and (c) the molar ratio of palladium oxide to ruthenium oxide and iridium oxide is about 5:95 to about 40:60], having an electrocatalyst coating and operating at high current efficiency. To an electrode used in the electrolysis of an aqueous solution for hypochlorite production, producing a hypochlorite concentration of 8 g / l or more.

In another aspect, the present invention

Providing a non-separated electrolytic cell,

Constructing an electrolyte containing chloride in an electrolytic cell,

Electrocatalyst coatings comprising a mixed metal oxide coating of a platinum group metal oxide with a valve metal oxide of titanium, consisting essentially of platinum group metal oxides of ruthenium, palladium and iridium, wherein based on 100 mole percent of the metal present in the coating (A) the molar ratio of titanium to platinum group metal oxide is about 90:10 to about 40:60, (b) the molar ratio of ruthenium to iridium is about 90:10 to about 50:50, and (c A molar ratio of ruthenium oxide to palladium oxide to iridium oxide is from about 5:95 to about 40:60.

Applying a current to the anode, and

Oxidizing the chloride at the anode to produce hypochlorite at a concentration of at least 8 g / l,

A method of electrolysis of an aqueous solution in an electrolytic cell having at least one anode having an electrocatalyst coating.

According to the present invention, there is provided an electrode having an electrocatalyst coating with high current efficiency, low electrode potential difference and improved lifetime at high hypochlorite concentrations (e.g. above 8 g / l). In one embodiment, depending on the hypochlorite concentration, the current efficiency can be from about 90% to about 100% for a hypochlorite concentration of 16-0 g / L. Electrodes with the electrocatalyst coatings described herein can be provided substantially as anodes at all times. Thus, the term “anode” is often used herein to refer to an electrode, but this is merely for convenience and should not be understood as limiting the invention.

Electrodes used in the present invention comprise an electrocatalytic active film on a conductive base. The conductive base can be a metal, such as nickel or manganese, or a sheet of all film forming metals, such as titanium, tantalum, zirconium, niobium, tungsten and silicon, and an alloy comprising one or more of these metals, Titanium is preferred for cost. The term "film forming metal" quickly forms a passivated oxide film that protects the underlying metal from erosion by the electrolyte when the coated anode is connected as an anode in a subsequently operated electrolyte, but under the same conditions the non-passivating cathodic Metals or alloys having a property of forming a surface oxide film (ie, these metals and alloys are often referred to as "valve metals"), and alloys comprising valve metals (eg Ti-Ni, Ti-Co, Ti- Fe and Ti-Cu). 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 in the conductive core may be used. In the same way, the porous sintered titanium may be surface treated with a dilute paint solution.

Titanium is particularly important for its robustness, erosion resistance and availability. In addition to the generally available elemental metals per se, suitable metals of the substrate include, for example, ceramics and cements comprising one or more valve metals as well as metal alloys and intermetallic mixtures. For example, titanium may be alloyed with nickel, cobalt, iron, manganese or copper. More specifically, grade 5 titanium may comprise up to 6.75% by weight of aluminum and 4.5% by weight of vanadium, grade 6 titanium may comprise up to 6% of aluminum and 3% of tin, and grade 7 titanium may comprise 0.25 wt% or less of palladium, and grade 10 titanium may include 10-13 wt% (+ 4.5-7.5 wt%) zirconium and the like.

The use of the term elemental metals means most particularly metals in their generally available conditions, ie metals with small amounts of impurities. Thus, for particularly important metals, ie titanium, it is possible to use metals of various grades, including metals whose other components may be alloys or alloys + impurities. The grade of titanium is described in more detail in the standard specification for titanium described in ASTM B 265-79. Since titanium is a particularly important metal, titanium may often be mentioned herein for convenience when referring to the metal for the electrode base.

Regardless of the type of metal and electrode base selected, the electrode base is advantageously a clean surface prior to application of the coating composition thereto. This can be obtained by any treatment used to achieve a clean metal surface including mechanical cleaning. General cleaning procedures or other chemical cleaning processes that degrease chemicals or electrolytes may be advantageously used. When the base preparation comprises annealing and the metal is a grade 1 titanium, the titanium may be annealed for at least about 15 minutes at a temperature of at least about 450 ° C., but most often at higher elevated annealing temperatures, for example from 600 ° C. to 875 ° C. is advantageous.

For most uses, it is advantageous to obtain a rough surface base. This means includes intergranular etching of the metal, plasma spray application, where the spray application can be spray application of microsphere valve metal or ceramic oxide particles, or both etching and sharp grit blasting of the metal surface. And optionally followed by surface treatment for removal of the embedded grit and / or cleaning of the surface or a combination thereof. In some cases, the base can be simply cleaned, which provides a very smooth substrate surface. Alternatively, the film forming conductive base may have a pre-coated surface film of film forming metal oxide that can be attacked with a formulation in a coating solution (eg HCl) and reconstituted as part of an integrated surface film during application of the active coating. Can be.

Etching is possible with an etching solution that is sufficiently active to develop surface roughness and / or surface morphology, including possible aggressive grain boundary attack. Typical etching solutions are acid solutions. They may be provided with hydrochloric acid, sulfuric acid, perchloric acid, nitric acid, oxalic acid, tartaric acid and phosphoric acid and mixtures thereof, such as aqua regia. Other etchant that may be used include caustic etchant, for example a solution of potassium hydroxide / hydrogen peroxide or a melt of potassium hydroxide with potassium nitrate. Subsequently, after etching, the etched metal surface can be introduced into a cleaning and drying step. Suitable preparation of the surface by etching is described in more detail in US Pat. No. 5,167,788, which is hereby incorporated by reference.

Suitably in plasma spraying for rough metal surfaces, the material may be used in the form of microspheres, for example as droplets of molten metal. For example, in such plasma spraying which can be used for the spraying of metals, the metal is produced by heating to an elevated temperature with an electric arc, for example in an inert gas containing argon or nitrogen, optionally a small amount of hydrogen. Is melted and sprayed into the plasma vapor. The use of the term “plasma spray” herein is preferably plasma spray but the term is generally thermal spray, eg magnetohydrodynamic, such that the spray may simply be referred to as “melt spray” or “thermal spray”. It is to be understood as meaning spray, flame spray and arc spray.

The microsphere materials used can be valve metals or oxides thereof, such as titanium oxide, tantalum oxide and niobium oxide. In addition, melt spraying of titanate, spinel, magnetite, tin oxide, lead oxide, manganese oxide and perovskite is conceived. It is also contemplated that the sprayed oxide may be doped with various additives, including dopants, in the form of ions such as niobium or tin or indium.

It is also contemplated that this plasma spray application can be used in combination with etching of the substrate metal surface. Alternatively, the electrode base may be prepared by first grit blasting as described above, and subsequently etching may or may not be performed.

It has also been found that suitably rough metal surfaces can be obtained by specific grit blasting with sharp grit, and optionally surface embedded grit can be removed. In general, grit, which may include each particle, can cut metal surfaces as opposed to peening the surface. Available grit for each purpose may include sand, aluminum oxide, steel and silicon carbide. After etching or other processing, such as water blasting, grit blasting can be used to remove embedded grit and / or to clean the surface.

It is then understood that the surface may be processed through a variety of processes that provide a pretreatment prior to the plasma spraying of the coating, eg, valve metal oxide coating, described above. In addition, other pretreatments may be useful. For example, it is understood that the surface can be hydrogenated or nitrogenized. It has been proposed that an oxide layer can be provided prior to application with an electrochemically active material by heating the substrate in air or anionic oxidation of the substrate as described in US Pat. No. 3,234,110. In addition, various proposals have been made in which the outer layer of electrochemically active material is deposited in a sublayer which is provided primarily as a protective and conductive intermediate. Various tin oxide based underlayers are described in US Pat. No. 4,272,354, US Pat. No. 3,882,002 and US Pat. No. 3,950,240. It is also contemplated that the surface can be prepared as an anti-dynamic layer.

For example, after surface preparation, which may include the pretreatment layer described above, an electrochemically active coating layer is applied to the substrate member. Since the electrochemically active coatings which are usually applied often are representative, they are provided from active oxide coatings, for example platinum group metal oxides, magnetite, ferrite, cobalt spinel or mixed metal oxide coatings. These may be aqueous systems, for example aqueous solutions or solvent systems using, for example, alcohol solvents. However, for the electrodes of the present invention, it has been found that preferred coating composition solutions are typically solutions consisting of mixed metal oxide coatings of platinum group metal oxides and valve metal oxides.

The platinum group metal oxides of the invention preferably comprise RuCl ₃ , PdCl ₂ , IrCl ₃ and hydrochloric acid, all of which are combined with the valve metal oxide in an alcohol solution. It is understood that RuCl ₃ , PdCl ₂ , IrCl ₃ can be used, for example, in the form of RuCl ₃ .xH ₂ O, PdCl ₂ .xH ₂ O and IrCl ₃ .xH ₂ 0. For convenience, this form may generally be referred to herein simply as RuCl ₃ , PdCl ₂ and IrCl ₃ . In general, the metal salts can be dissolved in alcohols such as isopropanol or butanol, all of which are formulated in the absence or presence of addition of small amounts of hydrochloric acid (preferably n-butanol). The components are substantially present as their oxides in the processed coatings, and it is understood that the reference of the metal is for convenience, especially when referring to proportions.

The valve metal component may further be present in the coating composition to stabilize the coating and / or alter the anode efficiency. Various valve metals can be used, including titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum and tungsten, with titanium being preferred. The valve metal component may be formed from the valve metal alkoxide in alcohol solvent in the presence or absence of acid. Such valve metal alkoxides contemplated for use in the present invention include methoxide, ethoxide, isopropoxide and butoxide. 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 has a molar ratio of titanium to platinum group metal oxide of about 90:10 to about 40:60, a molar ratio of ruthenium to iridium of about 90:10 to about 50:50, and Pd And a molar ratio of: (Ru + Ir) from about 5:95 to about 40:60. Particularly preferred compositions of the mixed metal oxide coatings of the present invention may comprise a molar ratio of titanium to noble metal oxide in the metal base of about 70:30 and a molar ratio of Pd: (Ru + Ir) of about 20:80. have.

The mixed metal oxide coating layer used herein may be applied to any means useful for applying the liquid coating composition to a metal substrate. Such methods include deep spin and deep drain techniques, brush application, roller coating and spray application, such as electrostatic spraying. Furthermore, spray application and combination techniques, such as deep drains in combination with spray application, can be used. With the coating compositions mentioned above for providing electrochemically active coatings, a roller coating process may be most available.

Regardless of how the coating is applied, conventionally, the coating procedure is repeated to provide a more uniform coating weight than that achieved with a single coating alone. However, provided the amount of the applied coating per one surface of a metal electrode base on the basis of the content of ruthenium, from about 0.05g / m ² to about 6g / m ^2, preferably from about 1g / m ² to about 4g / m ² May be sufficient.

After application of the coating, the applied composition can be heated to prepare the resulting mixed oxide coating by thermal decomposition of the precursors present in the coating composition. This results in a mixed oxide coating comprising the mixed oxides in molar ratio based on the metal of the oxide as described above. Such heating for pyrolysis may be performed at about 450 ° C. to about 550 ° C. for about 3 to about 15 minutes per coat. More typically, the applied coating may be heated at an elevated temperature of less than about 490-525 ° C. for a time of up to about 20 minutes. Suitable conditions may include heating in air or oxygen. In general, the heating technique used can be any technique that can be used to cure the coating on the metal substrate. Thus, an oven coating comprising a conveyor oven can be used. Moreover, ultraviolet curing techniques can be used. After such heating and before further coating in which further application of the coating composition may be used, the heated and applied substrate may generally be allowed to cool at least substantially to ambient temperature. In particular, after all the application of the coating composition is complete, postbaking can be used. Typical postbaking conditions for the coating may include a temperature of at least about 450 ° C. to about 550 ° C. Baking times may vary from about 1 hour to about 6 hours.

Unless otherwise stated as comparative examples, the following examples describe the preparation of high concentration hypochlorite by means of an anode comprising a coating of the invention generally at increased efficiency.

Example 1

Flat titanium plates of unalloyed grade 1 titanium, measured approximately 0.15 cm thick and approximately 10 x 15 cm, are grit blasted using alumina to achieve a rough surface. The sample is then etched in a 90-95 ° C. solution of 18-20% hydrochloric acid for 25 minutes.

The coating composition shown in Table 1 is applied to a separate sample measured in grade 1 titanium 10 cm x 15 cm x 0.15 cm, prepared by grit blasting with No. 54 grit alumina. Coating solutions A to D were prepared by dissolving a sufficient amount of metal as the chloride salt to achieve the concentrations listed in the table in a solution of n-butanol and 4.2 vol% concentrated HCl. The compounds used are RuCl ₃ , IrCl ₃ and PdCl ₂ (all anhydrides) and titanium orthobutyl titanate. After mixing to dissolve all salts, the solution is applied to each sample of the prepared titanium plate. The coatings are brushed and applied in layers, each coat applied separately and dried at 110 ° C. for 3 minutes and then heated to 500 ° C. for 6 minutes in air. A total of five coats are applied to each sample. Samples A through D are in accordance with the present invention. Sample E is considered a comparative example.

Sample Solution concentration (g / l) jacket 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 (except Ti, which is titanium orthobutyl titanate).

Hypochlorite efficiency of the sample was measured in a beaker-cell by immersing an area of 26 cm ² in a solution of 28 g / L NaCl with 1 g / L Na ₂ Cr ₂ O ₇ and measuring an anionic current of 4.86 amps (0.186 A / cm ² ). Is authorized. A titanium cathode 3 mm away from the anode is used. Samples are drawn out every 8 minutes and titrated against hypochlorite. Current efficiencies for the preparation of hypochlorite as a function of hypochlorite concentration are shown in FIG. 1 and Table 2.

Ru / Ir / Pd / Ti Ir / Pd / Ti Ru / Ir / Pd Ru / Pd / Ti Ru / Ir / Ti
(Comparative) NaOCl
(g / ℓ) efficiency
(%) NaOCl
(g / ℓ) efficiency
(%) NaOCl
(g / ℓ) efficiency
(%) NaOCl
(g / ℓ) efficiency
(%) NaOCl
(g / ℓ) efficiency
(%) 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

The set of samples A to E is then operated as an anode in an accelerated test as an oxygen-developing anode at a current density of 10 kA / m ² in an electrochemical cell containing 150 g / l H ₂ SO ₄ at 65 ° C. Cell voltage vs. time data is collected every 30 minutes and life is taken as the point of change where the voltage begins to increase quickly. The results are normalized to the amount of platinum group metal and summarized in FIG. 2 and Table 2. This is done by measuring the X-ray fluorescence counts for metal peaks using a Jordan Valley EX-300 spectrometer with Rh tube and 0.15 mm Sn filter. The applied voltage is 40 kV and the current is 25 mA. Peaks measured are Ru K-alpha, Pd K-alpha and Ir L-beta. The total count of Ru, Pd and / or Ir is used to generalize the lifetime.

Thus, samples made according to the present invention have increased current efficiency for the comparative example, while improving or meeting lifetime as evidenced by the extended time before significant rise in voltage (> 1 volt) generation. It is clear from the results in Table 2.

Although the best mode and preferred embodiments have been presented in accordance with the invention, the scope of the invention is not limited thereto but rather by the scope of the appended claims.

Claims (15)

Valve metal electrode base and On the valve metal electrode base an electrocatalyst coating comprising a mixed metal oxide coating of a platinum group metal oxide consisting of oxides of ruthenium, palladium and iridium and a valve metal oxide of titanium, wherein 100 mol% of the metal present in the coating The molar ratio of titanium to (a) platinum group metal oxides is 90:10 to 40:60, the molar ratio of ruthenium to iridium is 90:10 to 50:50, and (c) ruthenium oxide. And a molar ratio of palladium oxide to iridium oxide is 5:95 to 40:60]. An electrode for use in electrolysis of an aqueous solution for hypochlorite production. The electrode of claim 1, wherein the valve metal electrode base is a valve metal mesh, seat, blade, tube, punched plate, or wire member. 3. The hypochlorite of claim 1, wherein the valve metal electrode base is one or more of titanium, tantalum, aluminum, hafnium, niobium, zirconium, molybdenum or tungsten, alloys thereof or intermetallic mixtures thereof. Electrode used for electrolysis of aqueous solution for preparation. The electrode used in the electrolysis of the aqueous solution for hypochlorite manufacture of Claim 1 or 2 whose surface of the said valve metal electrode base is a rough surface. The electrode of claim 4, wherein the rough surface is obtained by at least one step of intergranular etching, grit blasting or thermal spraying. The electrode according to claim 4, wherein the ceramic oxide barrier layer is configured as a pretreatment layer on the rough surface, for use in the electrolysis of an aqueous solution for hypochlorite production. The electrolysis of an aqueous solution for preparing hypochlorite according to claim 1 or 2, wherein the molar ratio of titanium to the platinum group metal oxide is 70:30 and the molar ratio of palladium to ruthenium oxide and iridium oxide is 20:80. Electrode used. The electrode according to claim 1 or 2, wherein the molar ratio of ruthenium oxide to iridium oxide is 1: 1 for the electrolysis of the aqueous solution for hypochlorite production. The electrode according to claim 1 or 2, wherein the electrode is used in the electrolysis of an aqueous solution for hypochlorite production, which is used as an anode in the electrolysis of seawater. The electrolysis of an aqueous solution for the preparation of hypochlorite according to claim 1 or 2, wherein the electrode is used as an anode in a process for producing hypochlorite at a concentration of at least 8 g / l and a current efficiency in the range of 90% to 100%. Electrode used at the time. Providing a non-separated electrolytic cell, Constructing an electrolyte containing chloride in the electrolytic cell, Providing an anode in contact with the electrolyte in the electrolytic cell, Applying a current to the anode, and A method of electrolysis of an aqueous solution in an electrolytic cell equipped with at least one electrode according to claim 1, comprising the step of oxidizing the chloride at the anode to produce hypochlorite at a concentration of at least 8 g / l. 12. The method of claim 11, wherein the chloride electrolyte in the electrolytic cell is at least one of sodium chloride or potassium chloride. 12. The method of claim 11, wherein the surface of the anode is a roughened surface prepared by one or more steps of intergranular etching, grit blasting or thermal spraying. The aqueous solution of claim 13 wherein the anode roughness comprises titanium and the electrocatalyst coating is provided on the titanium by a process selected from the group of electrostatic spray application, brush application, roller coating, dip application and combinations thereof. Method of electrolysis. delete

Images