Electrode coating

BACKGROUND OF THE INVENTION

This invention relates to electrodes for electrolytic cells and, more particularly, to a corrosion-resistant, dimensionally-stable anode such as for electrolysis of aqueous alkali metal chloride in the production of elemental chlorine or alkali metal chlorate.

The electrolysis of aqueous alkali metal chloride solutions such as solutions of sodium chloride or potassium chloride is conducted on a vast commercial scale. In the production of alkali metal chlorate, anodes and cathodes, or biopolar electrodes which when arranged in a spaced electrical series in an electrolytic cell may serve as both anode and cathode, are immersed in an aqueous solution of the sodium chloride or the like and an electrical potential is established between the electrodes. In the past, graphite or carbon electrodes have been used as anodes or as the bipolar electrodes in series. In consequence of the electrochemical reactions which occur, alkali metal chlorate is produced either directly in the cell or outside the cell after the solution is allowed to stand.

The electrolysis of alkali metal chloride to produce elemental chlorine and alkali metal hydroxide is conducted in two general types of cells—the diaphragm and the mercury cathode cell. In the diaphragm cell, the cell is divided into two compartments—the anode compartment and the cathode compartment—which are separated by a porous diaphragm usually of asbestos. The cathode is of perforate metal and the asbestos diaphragm is in contact with the cathode. The anode, usually of carbon or graphite, is disposed centrally in the anode compartment.

In the mercury cathode cell, the cathode is a flowing stream of mercury which flows along a solid metal base connected to the negative pole of a power source. The anode, again of carbon or graphite, is spaced from the mercury cathode and, as electric current flows, the sodium or like alkali metal is evolved and collected in the mercury as an amalgam which is removed from the cell. Outside the cell the mercury amalgam is contacted with water in a “denuder” to remove the sodium as sodium hydroxide solution and the mercury is then recycled.

In operating each of the above-described cells one is confronted with a common problem; namely, that during the course of the electrolysis, the carbon or graphite electrode erodes and/or decomposes. Consequently, as the electrodes wear away or erode, the spacing between the electrodes increases with resulting increase in voltage between electrodes. This, together with the reactions which cause degradation of the anode, results in a loss of current efficiency for the production of the desired product. The graphite anodes ultimately must be replaced. In all these cells this erosion increases as the anode current density is increased. At the same time, the trend of operation is toward high current density to increase the amount of product produced per unit cell. Thus it has become necessary to resort to anodes or bipolar electrodes which remain dimensionally stable and do not erode appreciably over long periods of cell operation.

The present invention is directed to the provision of an improved stable electrode and to electrolytic cells, particularly to cells of the type described above which contain such electrodes as the anode or anodic surface thereof.

Electrodes herein contemplated normally should possess have surfaces which exhibit good electrolytic characteristics. These characteristics, particularly in the case of anodes, include low oxygen and chlorine overvoltage, resistance to corrosion and decompoistion in the course of use as anodes in the electrolytic cell, and minimum loss of coating during such use. It is well known that certain metals, metallic oxides, and alloys are stable during electrolysis and have other superior properties when used as anodes. Such metals typically include the members of the platinum group; namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum. These metals are not satisfactory for construction of the entire electrode since, for example, their cost is prohibitive. Therefore, these metals, particularly as metallic oxides or mixtures of metallic oxides, are commonly applied as a thin layer over a strength or support member such as a base member made of one of the titanium group metals. As used with respect to the present invention, the term titanium group metal is intended to include titanium, tantalum, zirconium, niobium, and alloys thereof. These support members have good chemical and electrochemical resistance to the alkali metal chloride electrolyte and the products of electrolysis, e.g., chlorine, hypochlorite and/or chlorate, but may be lacking in good surface electroconductivity because of their tendency to form on their surface an oxide having poor electroconductivity.

In the past the electrodes have been made by depositing the platinum group metal on the titanium, such as by the method disclosed in the commonly-assigned application Ser. No. 767,281, filed Oct. 14, 1968, and heating the electrode in air or oxygen to form the platinum group metal oxide. The electrode coating is made more durable by more extensive heating; however, the degree of heating is limited due to the fact that the electroconductivity of the electrode is severely damaged once the electrode is heated beyond a certain point. The present invention enables more extensive heating of the electrode without damage to the electroconductivity of the electrode. The present invention thus provides a method for making a more durable electrode.

DESCRIPTION OF THE INVENTION

The present invention provides an electrode having excellent electrolytic characteristics and excellent coating durability. The electrode has a base member or substrate preferably constructed of titanium or a titanium group metal. Alternatively, the base member may be of a material other than titanium but having a titanium metal coating, for example, a titanium clad steel plate. The base member may be a solid plate or a foraminous plate. The electrode has a first layer of platinum or a platinum group metal which is deposited on the base member. The electrode further includes a second layer or exposed surface of a platinum group metal oxide or a platinum group metal oxide mixed with another oxide such as titanium dioxide or silicon dioxide.

The first layer, namely, the layer of a platinum group metal, should have a thickness of at least 2 micro-inches, generally at least 5 micro-inches, and preferably about micro-inches; however, a greater thickness is not detrimental. Moreover, the platinum group metal in the first

3,663,414 layer should be one that is not easily oxidized; therefore, platinum is preferred. However, any platinum group metal which is deposited as the metal may be used, for example, iridium or osmium. The second layer, namely, the layer of the platinum group metal oxide or mixture of a platinum group metal oxide and another oxide, should have a thickness of at least 1 micro-inch, generally at least 5 micro-inches, and preferably at least 25 micro-inches. The platinum group metal used in the second layer is one that is more easily oxidized than the platinum group metal in the first layer. Typically, the second layer is comprised of ruthenium, palladium or rhodium oxides. Thus, the platinum group metal in the first layer is more noble than the platinum group metal in the second layer.

The first layer and the second layer may be deposited on the base member by any of several methods; for example, the layers may be deposited by precipitation of the metal or metallic oxide by chemical, thermal, or electrolytic methods. The second layer is generally heated to form the oxide; however, the oxide may be formed by other methods, such as chemically. But, in any event, the electrode is heated following the application of the second layer in order to make the platinum group metal oxide more durable.

According to one method of producing the anode of the present invention, a thermally-decomposable platinum organic compound, such as platinum resinate, is applied to a conductive, chemically-resistant substrate of titanium metal. The adhesion of the platinum group metals to the substrate may be increased by reducing the concentration of the resinate. For example, the metallic resinates may be mixed with an organic solvent or diluent, such as terpenes and aromatics, typically oil of turpentine, xylene, and toluene, before being applied to the base member. The electrode is heated to decompose and/or to volatilize the organic matter and other components, leaving a deposit of an electroconductive layer of platinum metal Care should be taken at this stage in heating the electrode to avoid formation of platinum oxide, for example, by limiting the temperature of heating (such as to 400° C.) or by effecting the heating of the electrode in an oxygenfree atmosphere such as in a vacuum or under a nitrogen or argon blanket. Alternatively, the platinum can be electroplated onto the titanium base plate such as from an organic or inorganic solution.

A thermally-decomposable organic material including a thermally-decomposable platinum group metal organic compound such as palladium resinate or ruthenium resinate is then applied over the layer of platinum. The electrode is heated in the presence of oxygen and under conditions that will decompose and/or volatilize the organic matter and other components leaving a deposit of an electroconductive layer including a platinum group metal oxide. Alternatively, thermally-decomposable inorganic compounds of the respective platinum group metals may be used. Such inorganic compounds include the platinum group metal oxalates, nitrates, acetates, formates, and chlorides.

As a general rule, if the first and second layers are applied by thermo-decomposition of an organic or an inorganic compound or mixture of such compounds, then preferably each layer is applied as a series of thin coatings in order to promote maximum adhesion. The coatings are then heated between coating operations to volatilize or drive off the organic matter, solvent, decomposition products, etc.

When applying the first layer, care should be taken to avoid forming the oxide, such as by heating the thermallydecomposable compounds in air at a temperature below that at which the oxide is formed or by heating such compounds in a vacuum or inert atmosphere such as in nitrogen or argon. Care should be taken in the case of depositing the second layer to select temperatures and duration of heating that will provide the platinum group metal as an oxide. The temperature to which the electrode may be heated will vary dependent upon the particular platinum group metal oxide present. Typically, the temperature may be in the range of 300° C. to 1000° C., preferably 450° C. to 750° C., for between 10 minutes and 2 hours. When treating the second layer, the heating step is most advantageously conducted in an atmosphere containing elemental oxygen such as air or other oxygen-inert gas mixtures although an atmosphere of pure oxygen can be used. The platinum group metal oxide thus formed is crystalline or amorphous depending upon the temperature of heating; the higher the temperature and the longer the heating, the greater the crystallinity of the product. Both crystalline, particularly if such crystals are very small in size, and non-crystalline coatings have good electroconductivity. However, products of improved adhesion and conductivity are obtained when care is exerted to maintain the coatings in a state where crystallinity is low. As used herein, low crystallinity will mean an X-ray diffraction pattern of less than 700% above background when measured on a Philips Diffractometer under the following conditions: the detector is a sealed, proportional counter operated at 35 kv., 15 milliamperes on the X-ray tube and at 1000 counts per second full scale. Copper radiation is used and the Philips Diffractometer is adjusted as follows: 1° divergence slit, 0.006 inch receiving slit, and 1° scatter slit. The detector is rotated at 2° two theta per minute with a time constant of 2 seconds and the specimen is rotated at 1 ° per minute.

The organic platinum group metal compounds may, if desired, be applied by brushing a coating on the titanium base member or, alternatively, by any other method of application such as spraying or dipping. The electrode must then be heated to a temperature sufficient to drive off the organic and inorganic products and to form the metal and the metallic oxide described above.

Although the present invention is directed to an electrode having a titanium group metal base member, a platinum group metal first layer and a platinum group metal oxide second layer, it should be recognized that other materials could also be present. The platinum group metal oxide layer could also have a small amount of elemental platinum group metal, for example, about 10% platinum group metal. In one preferred embodiment, the layer of platinum group metal oxide further includes titanium dioxide in a ratio by weight of one part platinum group metal oxide to between 0.01 and 5 parts of titanium dioxide. Preferably, the second layer contains at least 2 parts platinum group metal oxide to 1 part titanium metal oxide and, more preferably, at least 4 parts platinum group metal oxide to 1 part titanium metal oxide. In another preferred embodiment, the layer of platinum group metal oxide further includes silicon and/or silicon dioxide in a ratio by weight of 1 part platinum group metal oxide to between 0.01 and 5 parts silicon. In one embodiment of the present invention, the platinum group metal inner layer is deleted and the electrode is comprised of a titanium group metal substrate and a single electroconductive layer comprising a mixture of a platinum group metal oxide (typically ruthenium oxide) and silicon and/or silicon dioxide. In still other preferred embodiments, the platinum group metal oxide layer includes other metal oxides such as aluminum and/or selenium oxides.

In each of the following examples, the cell used in testing the electrodes consisted of a 2 liter wide mouth bottle, an anode, according to the present invention, made from a titanium plate 2 inches by 2¾ inches by Vie inch and a cathode of an expanded sheet of platinized titanium 2 inches by 3 inches by inch. The anode had electroconductive layers of platinum metal and a platinum group metal oxide which covered an area 1½ inches by 1½ inches on the titanium plate. The spacing between the anode and the cathode was inch. An aqueous solution containing 300 grams per liter sodium chloride was used as the electrolyte in the cell and sodium chlorate was produced. The cell temperature was maintained between

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45° C. and 50° C. The pH of the electrolyte was maintained between 9 and 11. The cell current was maintained at 6 amperes giving an anode current density of 385 amperes per square foot.

In each of the following examples, the titanium base plate was cleaned and etched prior to application of the coatings. The etching process comprised submerging the plate for about 1 minute in a solution containing 1 percent HF, said solution being concentrated with respect to HC1. The plate was then washed in water and submerged in 37 percent HC1 at about 40° C. to 50° C. for 1½ hours. The plate was rinsed in water, then in acetone, and dried in air.

Example I

An Electrode I-A was prepared having a first layer of platinum metal deposited on the titanium plate and a second layer of a mixture including palladium oxide and titanium dioxide. The platinum metal was electrolytically deposited on the titanium plate by placing the plate in a sulphato-dinitro-platinous acid solution (H₂Pt(NO₂)₂SO₄) (produced by J. Bishop & Company and designated DNS solution). The solution contained 5 grams of platinum per liter of solution. The current density was 5 amperes per square foot of plating surface. The current was applied for 20 minutes. The pH of the solution was maintained between 0.5 and 2.0. The temperature of the solution during plating was 30° C. The electrode was rinsed in water, dried, and heated in air to 400° C. for 15 minutes. Under these conditions, the platinum is deposited as the metal and not as an oxide. The platinum metal layer was about 20 micro-inches thick. Five coatings of a mixture including 2.72 grams palladium resinate, 2.72 grams titanium resinate, and 2.72 grams toluene were applied over the platinum layer. The resinates used herein were of the type produced by Hanovia Division of Englehart Industries in 1969. The palladium resinate contained 9% palladium and the titanium resinate contained 4.2% titanium. Following each of the coatings 1, 2, and 3, the electrode was slowly heated in the presence of air to 400° C. and held at that temperature for 10 minutes. Following coating 4 the electrode was slowly heated in the presence of air to 450° C. and held at that temperature for 15 minutes. The final coaing was heated in the presence of air to 600° C. for 15 minutes. The outer layer containing palladium oxide (PdO) and titanium dioxide (TiO₂) was about 10 micro-inches in thickness. The electrode was operated in the cell for 96½ hours at 48.5° C. The cell voltage varied between 2.98 and 3.06 volts. The electrode was found to have excellent durability.

A similar Electrode I-B was prepared; however, Electrode I-B was a control electrode and did not have a platinum metal layer. Five coats of a mixture including 5 grams palladium resinate, 1 gram titanium resinate and 5 grams toluene were applied directly to the titanium plate. The electrode was slowly heated in air to 450° C. and held at that temperature for 15 minutes following each coating. The electrode was operated for 90 hours in the cell at 45° C. The cell voltage was 2.99 volts. The electrical current was discontinued and it was found that the electrode coating was dissolving into the cell solution. The Electrode I-B was removed from the cell, rinsed in water, dried, and slowly heated in air to 550° C. and held at that temperature for 15 minutes. The electrode was again operated in the cell and the cell voltage was 5 volts. The electrical current was discontinued and the electrode showed showed excellent activity; that is, a substantial amount of gas was being formed at the electrode surface. The reaction taking place was apparently the conversion of CIO^- to Cl^- and oxygen. This activity indicates that the heating to 550° C. did not adversely affect the surface characteristics of the electrode but, instead, apparently affected the interface between the coating and the titanium plate or substrate. This effect is believed to be due to interaction between palladium oxide and titanium, producing a non-conducting layer of titanium dioxide. The presence of a layer of platinum between the titanium and the palladium oxide apparently prevents or at least reduces the adverse effects of such heating. The Electrode I-A, for example, was heated to 600° C. and yet a cell voltage of 3.06 volts was obtained, whereas Electrode I-B was heated to 550° C. and the cell voltage was 5 volts. Apparently the platinum layer resulted in a voltage savings of nearly 2 volts and yet permitted heating of the electrode an additional 50° C.

Example II

An Electrode II-A was prepared having a first layer of platinum metal deposited on the titanium plate and a second layer of a mixture including palladium oxide and silicon and/or silicon dioxide. The platinum metal was electrolytically deposited on the titanium plate by placing the plate in an organic solution containing 0.78 gram chloroplatinic acid crystals, 75 milliliters absolute ethyl alcohol, 15 milliliters sulfuric acid, 20 milliliters toluene, and 20 milliliters acetone. Current was applied at a rate of 0.2 ampere for 15 minutes. The solution was at room temperature. The electrode was rinsed, dried, and heated in air to 450° C. for 15 minutes. Six coats of a resinate mixture were applied to the electrode. The resinate mixture included 5 grams palladium oxide, 1 gram of a toluene solution (containing 10 percent by weight vinyl methyl silicone rubber) and 5 grams toluene. The first fiive coats were slowly heated in air to 450° C. and maintained at that temperature for 15 minutes. The sixth coat was slowly heated in air to 575° C. and held there for 15 minutes. The electrode was operated as an anode in the afore-described cell for 51½ hours at about 46° C. The cell voltage was 3.20 volts. When the electrical current was discontinued, the activity at the surface of the electrode was good with a substantial amount of gas being formed, thus indicating a good electrode activity. The outer layer was about 12 micro-inches in thickness.

A similar Electrode Π—B was prepared except Electrode Π—B did not have a platinum layer. Also, following each of the six coatings the Electrode II-B was slowly heated to 450° C. and maintained at that temperature for 15 minutes. Electrode Π-B was operated in the cell and the cell voltage was 4.55 volts. The activity at the surface of the electrode was good when electrical current was discontinued, thus showing a good surface but poor voltage characteristics. Electrode Π-B was further heated to 700° C. and the cell voltage was 8.45 volts.

Example III

An Electrode III-A was prepared having a first layer of platinum metal deposited on the titanium plate and a second layer of a mixture including palladium oxide and silicon and/or silicon dioxide deposited on the platinum layer. The platinum layer was provided by applying 2 coats of platinum resinate (manufactured by Hanovia Division of Englehart Industries and labelled Liquid Bright Platinum 05—X). The platinum concentration of the resinate was 7 percent. The platinum resinate was diluted with toluene in a ratio of 1 part platinum resinate to 1 part toluene. The electrode was slowly heated to 400° C. following the first coat and held there for 10 minutes. The electrode was slowly heated to 500° C. following the second coating and held there for 15 minutes. The platinum layer was about 4 micro-inches in thickness. The second layer including palladium oxide and silicon and/or silicon dioxide was provided by applying five coats of a mixture including 5 grams palladium resinate and 1 gram of a toluene solution containing 10 percent vinyl methyl silicone rubber. The mixture was diluted with 5 grams toluene. The electrode was slowly heated in air to 400° C. after coats 1, 2, and 3 and held there for 10 minutes. Following coating 4, the electrode was heated in air to 550° C. and held there for 15 minutes. The Electrode

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ΠΙ-A was slowly heated in air to 600° C., following the final coating, and held at that temperature for 15 minutes. The outer layer was about 10 micro-inches in thickness. The electrode was operated as an anode in the test cell at 55° C. and the cell voltage was 3.75 volts at an anode current density of 385 amperes per square foot.

A similar Electrode III-B was prepared except that Electrode III-B did not have a platinum layer. In other words, the coatings of the palladium resinate and silicone rubber mixture were applied directly to the titanium plate. Electrode III-B received the same heat treatment as Electrode III-A except that the fifth coating was only heated to 550° C. The Electrode III-B was operated in the cell under the same conditions as Electrode III-A and cell voltage was 5.0 volts. Thus, the presence of the platinum layer apparently accounted for a voltage reduction in this instance of at least 1.25 volts.

Electrodes made according to the present invention are highly suitable for use as anodes in cells used for the electrolysis of aqueous alkali metal chloride solutions, typically diaphragm cells, mercury cathode cells, and chlorate cells such as those shown in U.S. Pats. Nos. 3,400,055; 3,337,443; 3,312,614; 3,287,250; 3,203,882; 3,119,664; 3,116,228; 2,897,463; and 2,719,117.

Although the present invention has been described with reference to the specific details of particular embodiments thereof, it is not intended thereby to limit the scope of the invention except insofar as the specific details are recited in the appended claims.