JP4560089B2 - Electrode used for electrolysis of aqueous solution to produce hypochlorite - 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

Background of the Invention
1. The present invention is directed to electrolyte electrodes and mixed metal oxides coated thereon for producing hypochlorite.

2. 2. Description of Related Art The use of mixed metal oxide coatings to produce hypochlorite by electrolyzing salt solutions is well known in the art. However, when hypochlorite is conventionally produced by electrolysis of brine, the available chlorine concentration of the hypochlorite product can be as low as 1 weight percent (wt%) or less. There is a possibility. In addition, if the solution containing salt water is not very high (ie, 10-30 g / l), the current efficiency and electrode life are reduced, and the hypochlorite concentration is desirably higher than 8 g / l.

  Various solutions have been proposed to achieve highly concentrated sodium hypochlorite solutions without detrimental effects on current efficiency and electrode life. For example, U.S. Pat. No. 4,495,048 teaches a filter press type electrolysis cell in which sodium hypochlorite is produced with low cell voltage and improved current efficiency. The anode in the electrolysis cell is a three-component coating film comprising 3 to 42 wt% platinum oxide, 3 to 34 wt% palladium oxide, 42 wt% ruthenium dioxide, and 20 to 40 wt% titanium oxide. It consists of a titanium substrate having

  In U.S. Pat. No. 4,517,068, electrodes for producing chlorine and hypochlorite in particular are 22 to 44 mol% ruthenium oxide, 0.2 to 22 mol% palladium oxide, and 44 Containing an electrocatalyst consisting of ˜77.8 mol% titanium oxide. The electrode catalyst can form a coating film on the valve metal substrate, and can further form an overcoat with a porous layer of titanium oxide or tantalum oxide.

  In U.S. Pat. No. 5,622,613, a coating comprising 10-45 wt% palladium oxide, 15-45 wt% ruthenium oxide, 10-40 wt% titanium dioxide, and 10-20 wt% platinum. An efficient process for the production of hypochlorite using an anode with a film and additionally 2 to 10% by weight of an oxide of at least one metal selected from cobalt, lanthanum, cerium or yttrium Has been.

  Electrolyte environment used to produce hypochlorite from solutions containing 15-30 grams / liter (g / l) NaCl or KCl, preferably having a hypochlorite concentration greater than 8 g / l It may be desirable to provide an electrode having an electrode catalyzing coating thereon that can provide improved electrode life and operating efficiency. Furthermore, it may be desirable to provide such an electrode at a lower cost compared to platinum-based formulations.

SUMMARY OF THE INVENTION In the present invention, electrode coatings have been found that provide improved lifetime and at the same time maintain high efficiency in electrolytes for producing hypochlorite. This coating film is a mixed metal oxide coating film comprising a combination of oxides of palladium, iridium, ruthenium and titanium.

In one embodiment, the present invention is directed to an electrode for use in electrolysis of an aqueous solution to produce hypochlorite, the electrode having an electrocatalyst coating thereon, the electrode comprising:
Valve metal electrode base;
A coating layer of an electrochemically active coating on the valve metal electrode base;
Wherein the coating comprises a coating of a mixed metal oxide of a platinum group metal oxide and a titanium valve metal oxide, the platinum group metal oxide being substantially ruthenium, palladium and Made of iridium oxide ;
Here, as a standard, the metal present in the coating film as 100 mole percent,
a) the molar ratio of titanium to platinum group metal oxide is from about 90:10 to about 40:60;
b) the molar ratio of ruthenium to iridium is from about 90:10 to about 50:50;
c) the molar ratio of palladium oxide to ruthenium oxide and iridium oxide is from about 5:95 to about 40:60;
The electrode thereby operates at a high current efficiency so as to produce a hypochlorite concentration of at least 8 grams / liter.

In another embodiment, the present invention is directed to a method for electrolyzing an aqueous solution in an electrolysis cell comprising at least one anode, the anode having an electrocatalyst coating thereon, the method comprising separating Providing an unelectrolyzed cell, placing a chloride-containing electrolyte in the cell, and providing the anode in contact with the electrolyte in the cell, wherein the anode is a platinum group An electrode catalyst coating comprising a mixed metal oxide coating of a metal oxide and a titanium valve metal oxide, wherein the platinum group metal oxide is substantially composed of an oxide of ruthenium, palladium and iridium; Become
Here, as a standard, the metal present in the coating film as 100 mole percent,
(A) the molar ratio of titanium to platinum group metal oxide is from about 90:10 to about 40:60;
(B) the molar ratio of ruthenium to iridium is from about 90:10 to about 50:50;
(C) the molar ratio of palladium oxide to ruthenium oxide and iridium oxide is from about 5:95 to about 40:60;
Applying current to the anode; and oxidizing chloride at the anode to produce hypochlorite at a concentration of at least 8 grams / liter.

DESCRIPTION OF THE INVENTION According to the present invention, it has high current efficiency at high hypochlorite concentration, for example, higher than 8 g / l (gram / liter), and exhibits lower electrode potential and improved lifetime, An electrode is provided having a coating that is electrocatalytic. In one embodiment, depending on the hypochlorite concentration, the current efficiency is expected to be about 90% to about 100% in the range of hypochlorite concentration of 16-0 grams / liter (g / l). The It is expected that an electrode having an electrocatalytic coating as described herein will always find use as an anode. Thus, when referring to electrodes herein, the term “anode” is often used, but this is merely for convenience and should not be construed as limiting the invention.

  The electrode used in the present invention includes a film having electrocatalytic activity on a conductive base. The conductive base forms a film such as a metal such as nickel or manganese, or an alloy including titanium, tantalum, zirconium, niobium, tungsten, and silicon and one or more of these metals. Any metal sheet is possible, preferably titanium for cost reasons. “Film-forming metal” means that when connected as an anode in an electrolyte, the coated anode operates and quickly forms a passive oxide film that protects the underlying metal from corrosion by the electrolyte. Which is often referred to as “valve metal” and also alloys containing valve metal (eg, Ti—Ni, Ti—Co, Ti—Fe, and Ti—Cu). Means metals and alloys that form a non-passive surface oxide film at the anode under the same conditions. Titanium or other film-forming metal plates, rods, tubes, wires or braided wires, and expanded meshes can be used as the electrode base. Also, titanium or other film-forming metal clad can be used on the conductive core. In the same way, it is also possible to surface-treat porous sintered titanium with a thin paint solution.

  Of particular interest for its durability, corrosion resistance and availability is titanium. In addition to the generally available elemental metals themselves, suitable metals for the substrate include metal alloys and intermetallic mixtures, plus ceramics and cermets that contain one or more valve metals. It is done. For example, titanium may be an alloy with nickel, cobalt, iron, manganese or copper. More specifically, grade 5 titanium may comprise 6.75 weight percent or less aluminum and 4.5 weight percent or less vanadium, and grade 6 titanium comprises 6 percent or less aluminum, And up to 3 percent tin, grade 7 titanium may contain up to 0.25 weight percent palladium, and grade 10 titanium is 10-13 weight percent and 4.5 May contain up to 7.5 weight percent zirconium and the like.

  When elemental metals are used, elemental metals most specifically mean those metals that are generally available, that is, those that contain only a small amount of impurities. Thus, in the case of a particularly interesting metal, i.e. titanium, various grades of metal are available, for example, where the other component is an alloy or an alloy containing impurities. Titanium grades are more specifically described in the standards for titanium detailed in ASTM B265-79. Because titanium is a particularly interesting metal, it is expected that titanium will often be described herein for convenience when describing the metal for the electrode base.

  Regardless of the metal selected and the electrode base configuration, the electrode base is advantageously a surface that has been cleaned prior to applying the coating composition to them. This may be achieved by any of the processes used to achieve a cleaned metal surface such as mechanical cleaning. It is also advantageous to use general cleaning methods for degreasing either chemically or electrolytically, or other chemical cleaning operations. If the base fabrication 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 in most cases A high annealing temperature, for example 600 ° C. to 875 ° C., is advantageous.

  For most applications it is advantageous to obtain a base having a surface roughness. This involves the application of metal grain boundary etching, plasma spraying (which allows the application of particulate valve metal and / or ceramic oxide particles spraying), metal surface etching, and pointed Grit blasting with grit, optionally followed by surface treatment to remove embedded grit and / or clean the surface, or expected to be achieved by means such as combinations thereof The In some cases, a very smooth substrate surface can be obtained by simply cleaning such a base. Alternatively, the conductive base forming the film may have a pre-coated film-forming metal oxide surface film, which is the material in the coating solution when applying the active coating film ( For example, it can be reconstituted as part of an integrated surface coating eroded by HCl).

  Etching will be due to an etching solution that is sufficiently active to develop surface roughness and / or surface morphology, including the possibility of erosive particle boundary corrosion. A typical etching solution is an acid solution. These can be provided by hydrochloric acid, sulfuric acid, perchloric acid, nitric acid, oxalic acid, tartaric acid and phosphoric acid, and mixtures thereof, such as aqua regia. Other etchants that may be used include corrosive etchants, such as a potassium hydroxide / hydrogen peroxide solution or a potassium hydroxide melt using potassium nitrate. After etching, the etched metal surface can be treated with a rinse and dry process. The production of suitable surfaces by etching is more fully discussed in US Pat. No. 5,167,788, which is included in the disclosure by this reference.

  In plasma spraying for a suitably roughened metal surface, the material is expected to be applied in granular form, for example in molten metal droplets. This plasma spraying is expected to be applied, for example, to spray metal, but in such cases it melts the metal and is inert such as argon or nitrogen (which may contain a small amount of hydrogen) Spray into a plasma stream generated by heating to high temperature with an electric arc in gas. Where the term “plasma spraying” is used herein, it is preferably plasma spraying, but this term generally refers to thermal spraying such as magnetohydrodynamic spraying, flame spraying and arc spraying. Therefore, the term “thermal spraying” may refer to “melt spraying” or “thermal spraying”.

  The particulate material used can be a valve metal or an oxide thereof, such as titanium oxide, tantalum oxide and niobium oxide. It is also conceivable to melt spray the titanate, spinel, magnetite, tin oxide, lead oxide, manganese oxide and perovskite. It is also conceivable to dope the sprayed oxide with various additives such as dopants in ionic form such as niobium or tin or indium.

  Further, such plasma spraying may be used in combination with etching of the metal surface of the substrate. Alternatively, the electrode base is first manufactured by grit blasting as described above, and may or may not be subsequently etched.

  It has also been found that a properly roughened metal surface can be obtained by special grit blasting with pointed grit, followed by removal of grit embedded in the surface. Good. Such grit is normally expected to contain angular particles, which are expected to cut the metal surface as opposed to surface peening. Practical grit for such purposes includes those containing sand, aluminum oxide, steel and silicon carbide. Etching or other treatments such as water blasting can be used after grit blasting to remove embedded grit and / or clean the surface.

  Of course, the surface may subsequently be pretreated by various operations prior to coating (eg, plasma spraying of the valve metal oxide coating described above). Other pretreatments can also be useful. For example, the surface may be hydrogenated or nitrided. Oxidize by heating the substrate in air or anodizing the substrate (as described in US Pat. No. 3,234,110) prior to coating with an electrochemically active material It has also been proposed to provide a physical layer. Various proposals have been made to deposit an outer layer of electrochemically active material in a layer below it, such a layer serving primarily as a protective and conductive intermediate. U.S. Pat. Nos. 4,272,354, 3,882,002, and 3,950,240 disclose various tin oxide based substrates. It is also conceivable to manufacture such a surface in the same manner as the non-passive layer.

  After surface preparation, which may include providing a pretreatment layer as described above, an electrochemically active coating layer is applied to the substrate member. Typical of the electrochemically active coatings often applied are typically active oxide coatings such as platinum group metal oxides, magnetite, ferrites, cobalt spinels, or And a coating film obtained from a coating film of a mixed metal oxide. Such a coating film may be water-based, for example, an aqueous solution, or may be solvent-based, for example, using an alcohol solvent. However, for the electrodes of the present invention, it has been discovered that the preferred coating composition solution is typically a mixed metal oxide coating solution of a platinum group metal oxide and a valve metal oxide. .

The platinum group metal oxide of the present invention preferably contains RuCl ₃ , PdCl ₂ , IrCl ₃ and hydrochloric acid in combination with the valve metal oxide, all of which are alcohol solutions. Of course, RuCl ₃ , PdCl ₂ and IrCl ₃ may be used in forms such as RuCl ₃ × H ₂ O, PdCl ₂ × H ₂ O, and IrCl ₃ × H ₂ O. For convenience, such forms shall be generally described herein simply as RuCl ₃ , PdCl ₂ and IrCl ₃ . In general, the metal salt is expected to be dissolved in an alcohol (eg, either isopropanol or butanol), in each case mixed with or without the addition of a small amount of hydrochloric acid, preferably n-butanol It is. Of course, the above components are present substantially as their oxides in the finished coating, and when referring to metals, the ratios are specifically mentioned for convenience.

  It is also conceivable that a valve metal component is 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 utilized, including titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum, and tungsten, with titanium being preferred. The valve metal component can be formed from the valve metal alkoxide in an alcohol solvent in the presence or absence of an 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, preferably titanium butoxide.

  In the mixed metal oxide coating of the present invention, the molar ratio of titanium to platinum group metal oxide is about 90:10 to about 40:60, and the molar ratio of ruthenium to iridium is about 90:10 to about 50: A molar ratio of 50 and Pd: (Ru + Ir) is expected to be included from about 5:95 to about 40:60. Particularly preferred compositions of the mixed metal oxide coatings of the present invention have a molar ratio of titanium to noble metal oxide of about 70:30 based on metal and a molar ratio of Pd: (Ru + Ir) of about 20 : Expected to be included at 80.

  The mixed metal oxide coating layer utilized herein is expected to be applied by any means useful for applying a liquid coating composition to a metal substrate. Such methods include dip spin and dip drain techniques, brushing, roller coating, and spray application (eg, electrostatic spraying). Furthermore, a combination technique with spray coating can be used, and for example, combined use of dip drain and spray coating can be mentioned. When using the coating composition to provide the electrochemically active coating described above, a roller coating operation may be most practical.

Regardless of how the coating is applied, it is customary to repeat the coating technique to provide a coating weight that is more uniform and higher than that achieved by a single coating. However, the amount of coating applied is from about 0.05 g / m ² (grams per square meter) to about 6 g / m ² , preferably from about 1 g / m ² based on the ruthenium content as metal per electrode base. An amount sufficient to provide a range of about 4 g / m ² is expected.

  After applying the coating, the applied composition is expected to be heated, thereby resulting in a mixed oxide coating obtained by pyrolyzing the precursor present in the coating composition. Is manufactured. Thereby, the coating film of the mixed oxide containing the mixed oxide is manufactured at the molar ratio (based on the metal of the oxide) as described above. Heating for pyrolysis in this manner is expected to occur at a temperature of about 450 ° C. to about 550 ° C. for a time of about 3 minutes to about 15 minutes per coating. More typically, the applied coating is expected to be heated at a higher temperature of about 490-525 ° C. or less for a time of about 20 minutes or less per coating. Suitable conditions include heating in air or oxygen. In general, the heating technique used can be any technique that can be used to cure a coating on a metal substrate. Accordingly, coating with an oven such as a conveyor oven is available. In addition, infrared curing techniques may be useful. After heating in this manner, the heated and coated substrate is typically cooled to at least substantially ambient temperature before additional coating in which additional application of the coating composition is expected to be applied. It is expected. In particular, post-baking can be used after the application of all coating compositions is complete. Typical coating post-baking conditions may include temperatures from about 450 ° C to about 550 ° C. Baking times may vary from about 1 hour to as long as about 6 hours.

  In the following examples, unless otherwise specified as in the comparative examples, generally, a high-concentration hypochlorite is produced with high efficiency by using an anode including the coating film of the present invention. Prove that:

Example 1
A flat titanium plate of unalloyed grade 1 titanium having a thickness of about 0.15 cm and dimensions of about 10 × 15 cm was grit blasted with alumina to obtain a rough surface. The sample was then etched for 25 minutes in an 18-20% hydrochloric acid solution at 90-95 ° C.

The coating composition described in Table 1 was applied to a separate sample consisting of grade 1 titanium of dimensions 10 cm × 15 cm × 0.15 cm produced by grit blasting with 54 grit aluminas. Coating solutions AD were prepared as chloride salts by dissolving a sufficient amount of metal in n-butanol and 4.2 vol% concentrated HCl solution to obtain the concentrations listed in the table. The compounds used were RuCl ₃ , IrCl ₃ , and PdCl ₂ (all hydrates) and titanium as orthobutyl titanate. After all the salts were mixed and dissolved, this solution was applied to individual samples of the manufactured titanium plates. Each paint film was applied by brushing and applied in layers, dried at 110 ° C. for 3 minutes, and then heated in air at 500 ° C. for 6 minutes. A total of five coatings were applied to each sample. Samples AD were in accordance with the present invention. Sample E was a comparative example.

A 26 cm ² area of the sample is immersed in a solution of 28 gpl NaCl containing 1 gpl (g / l) Na ₂ Cr ₂ O ₇ and an anode current of 4.86 amps (0.186 A / cm ² ) is applied. The efficiency of the hypochlorite of the sample was measured in a beaker cell. A titanium cathode was used at a distance of 3 mm from the anode. Samples were drawn every 8 minutes and titrated for hypochlorite. In FIG. 1 and Table II, the current efficiency for producing hypochlorite as a function of hypochlorite concentration is plotted.

Next, in the accelerated test, a group of samples A to E were operated as an anode for generating oxygen in an electrochemical cell containing 150 g / l H ₂ SO ₄ at a current density of 10 kA / m ² and 65 ° C. as an anode. It was. Cell voltage data over time was collected every 30 minutes and the lifetime was measured as the inflection point where the voltage began to increase rapidly. Figure 2 and Table II summarize the results (normalized to the amount of platinum group metals). Normalization was performed by measuring X-ray fluorescence counts on metal peaks using a Jordan Valley EX-300 spectrometer equipped with an Rh tube and a 0.15 mm Sn filter. The applied voltage was 40 kV (kilovolts) and the current was 25 μA. The measured peaks were RuK-alpha, PdK-alpha, and IrL-beta. Lifetime was normalized using the total count of Ru, Pd and / or Ir.

  Thus, from the results in Table II, it is clear that the samples produced according to the present invention have substantially higher current efficiency compared to the comparative examples and are improved at the same time, or the voltage increases significantly The lifetime is also sufficient, as evidenced by the longer time to cause (greater than 1 volt).

  Although the best mode and preferred embodiments have been described according to patent law, the scope of the invention is not limited thereto but is defined by the scope of the appended claims.

The current efficiency for producing hypochlorite as a function of hypochlorite concentration was plotted. It is a graph regarding the lifetime normalized to the quantity of the platinum group metal.

Claims (14)

An electrode for use in electrolysis of an aqueous solution to produce hypochlorite:
Valve metal electrode base;
An electrode catalyst coating comprising a mixed metal oxide coating of a platinum group metal oxide and a titanium valve metal oxide on the valve metal electrode base;
Wherein the platinum group metal oxide comprises an oxide of ruthenium, palladium and iridium;
Here, as a standard, the metal present in the coating film as 100 mole percent,
(A) the molar ratio of the titanium to the platinum group metal oxide is 90:10 to 40:60;
(B) the molar ratio of the ruthenium to the iridium is 90:10 to 50:50;
(C) The said electrode whose molar ratio of this palladium oxide with respect to ruthenium oxide and iridium oxide is 5: 95-40: 60.   The electrode according to claim 1, wherein the valve metal electrode base is a valve metal mesh, sheet, blade, tube, perforated plate, or wire member.   The valve metal electrode base is manufactured from any one of titanium, tantalum, aluminum, hafnium, niobium, zirconium, molybdenum, or tungsten, an alloy thereof, and an intermetallic mixture thereof. 2. The electrode according to 2.   The surface of the said valve metal electrode base is an electrode as described in any one of Claims 1-3 which is a rough surface.   The electrode according to claim 4, wherein the rough surface is obtained by one or more processes of grain boundary etching, grit blasting, or thermal spraying.   The electrode according to claim 4, wherein a barrier layer of ceramic oxide is established as a pretreatment layer on the rough surface.   The molar ratio of the titanium to the platinum group metal oxide is 70:30, and the molar ratio of the palladium oxide to ruthenium oxide and iridium oxide is 20:80. Electrode.   The electrode according to claim 1, wherein the molar ratio of ruthenium oxide to iridium oxide is 1: 1.   Use of an electrode according to any one of claims 1 to 8 as an anode in electrolysis of seawater.   9. Electrode according to any one of the preceding claims, as an anode in a process for producing hypochlorite at a concentration of at least 8 grams / liter and further with a current efficiency in the range of 90% to 100%. Use of. A method for electrolyzing an aqueous solution in an electrolytic cell comprising at least one anode according to claim 1, comprising:
Providing an unseparated electrolysis cell;
Placing an electrolyte containing chloride in the cell;
Providing the anode in contact with the electrolyte in the cell;
Applying an electric current to the anode; and
Oxidizing the chloride at the anode to produce hypochlorite at a concentration of at least 8 grams / liter;
Including the above method.   The method according to claim 11, wherein the chloride electrolyte in the cell is one or more of sodium chloride and potassium chloride.   The method according to claim 11 or 12, wherein the surface of the anode is a rough surface produced by one or more processes of grain boundary etching, grit blasting or thermal spraying.   The roughened anode surface contains titanium, and the electrode catalyst coating is selected from the group consisting of electrostatic spray application, brush application, roller application, dip application, and combinations thereof 14. The method of claim 13, wherein the method is provided on the titanium member.

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