Electrode And Process For Making Same

Abstract

Electrodes suitable for use for electrolytic purposes, for example for the electrolysis of aqueous alkali metal chloride solutions, are disclosed. The electrodes of this invention have an electroconductive substrate with an electro-conductive outer surface containing a spinel thereon, and an intermediate electroconductive layer between the electro-conductive spinel surface and the substrate which intermediate layer contains an oxycompound of a second transition series platinum group metal (ruthenium, rhodium, palladium).

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 86,062, now abandoned filed Nov. 2, 1970, by Aleksandrs Martinsons for "Anode for the Electrolysis of Brines and Process for Making Same".

Description

BACKGROUND

Chlorine and alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, are produced commercially by either of two electrolytic processes--electrolysis in a diaphragm cell or electrolysis in a mercury cell. Alkali metal chlorates are made in a cell similar in structure to a diaphragm cell but not having a diaphragm.

The design and operation of mercury cells, diaphragm cells, and chlorate cells, are more fully treated in Mantell, Electrochemical Engineering, McGraw-Hill, New York, N. Y. (1960), and Sconce, Chlorine, Reinhold, New York, N. Y. (1962).

Common to all three processes has been the use of carbon anodes. These carbon anodes are a constant source of difficulty. They are short lived and subject to uneven wear and erosion. In mercury cells, frequent adjustment is required in order to maintain a constant gap between the cathode and anode and thereby maintain a constant voltage drop across the electrolyte. In diaphragm cells and chlorate cells, no provision is made for varying the anode-cathode space, and, accordingly, the voltage increases with time. Additionally, organic solvents present in the graphite electrodes may plug the diaphragm, resulting in a further increase in voltage. Reaction of the anode products with the graphite anode result in halogenated hydrocarbons being present in the anode product.

Many attempts have been made to remedy these problems. It has long been recognized that a superior anode would be one made of a solid precious metal. However, this is neither economical nor practical. The art shows many attempts to obtain a lower cost electrode having a long life and low overvoltage of a solid precious metal electrode, as well as the low cost of a graphite electrode. These attempts have sought to provide a durable coating, usually of a platinum group metal or platinum group metal oxide, on an electroconductive base, which base usually is composed of a valve metal such as titanium.

These electrodes are noticeably longer in life and operate at lower voltages than carbon electrodes. However, platinum group metals either as such or as their oxides are expensive.

SUMMARY OF INVENTION

It has now been found that a particularly satisfactory anode for the electrolysis of brines may be provided by an electrode having a spinel outer surface on a suitably electroconductive substrate wherein a layer comprising an oxycompound of a second transition series platinum group metal is between the substrate and spinel outer surface. The platinum group metals of the second transition series include ruthenium, rhodium, and palladium. The oxycompound of the second transition series platinum group metals may be present in the substrate and, additionally, may also be dispersed through the spinel coating. When the oxycompound is also dispersed through the spinel coating, it may be dispersed uniformly or non-uniformly. The resulting anode appears to offer economies of construction and operation over an anode having a metallic platinum coating on an electroconductive substrate.

As used herein, "spinel" means an oxycompound of a metal or metals which oxycompound has the unique crystal structure and formula characteristic of spinels. The spinel may be applied with a suitable binding agent over a suitably-treated metal structural member or substrate. Such spinel itself is essentially the ions of the metal or metals and oxygen in chemical combination. However, amounts, usually minor amounts, of other materials such as other metal oxides, sulfides, fluorides, or even metals in the metallic state, may be entrapped in or associated with the spinel crystal structure without seriously impairing the desirable properties of the spinel surface.

A suitable electroconductive substrate is one that retains its electroconductive properties during the formation of the spinel surface thereon and in the course of using the resulting electrode for its intended purpose. Such as substrate will be resistant to oxidation during fabrication and electrolysis, and will not be subject to attack by the gases liberated during electrolysis. Preferably, the substrate should be substantially more electroconductive than the spinel itself.

The resulting electrode is long lived in the electrolytic cell environment and has satisfactory overvoltage characteristics.

DESCRIPTION

In the FIGURE is an X-ray diffraction pattern of an electrode produced according to the procedure of Example I.

Suitable electroconductive substrates having a spinel outer coating and an intermediate layer of an oxy-compound of a second transition series platinum group metal (i.e., ruthenium, rhodium, palladium) therebetween produce an anode that is dimensionally stable in an environment where chlorine is electrolytically produced. Spinels are oxycompounds of one or more metals characterized by a unique crystal structure, stoichiometric relationship, and X-ray diffraction pattern.

Oxycompounds having the spinel structure may be represented by the empirical formula

M.sup.II M.sup.III 2.sup.O 4

where M.sup.II represents a metal having a valence of plus 2, M.sup.III represents a metal having a valence of plus 3. The metals may be the same metal as in Cr.sup.II Cr.sup.III 2.sup.O 4 or different metals as in NiCr.sub.2 O.sub.4. Spinels are more precisely represented by the empirical formula

M.sup.II (M.sup.IIIa M.sup.IIIb )O.sub. 4

where M.sup.II represents a metal having a valence of plus 2, and M.sup.IIIa and M.sup.IIIb represent metals having a valence of plus 3. M.sup.IIIa and M.sup.IIIb may either be the same metal or different metals, and one or both of them may represent the trivalent state of the metal present in the bivalent state, as Fe.sup.II (Cu.sup.III Fe.sup.III)O.sub.4, or all three metal ions may be ions of different metals, as MgAlFeO.sub.4.

The spinel crystal structure and the methods of identifying it by X-ray techniques are described in the literature. For example, the spinel structure is discussed in Wyckoff, Crystal Structure, Vol. 3 (2nd Edition), Wiley & Sons, New York (1963) at pages 75 to 86; in Wells, Structural Inorganic Chemistry, Oxford University Press, New York (1950) at pages 379 to 388; in Evans, An Introduction to Crystal Chemistry (2nd Edition), Cambridge University Press, New York (1966) at pages 171 to 175; and in Bragg, Claringbull and Taylor, The Crystalline State, Vol. 4; Crystal Structures of Minerals, G. Bell & Sons Ltd., London (1965) at pages 102 to 106.

According to those authorities, the spinel crystal structure may be characterized as comprising oxygen ions in an approximately cubic, close-packed relationship, with the metal ions lying in holes of the packing. Crystal structures having close-packed atoms or ions may be conveniently considered, for purposes of illustration, as being arranged in layers. In the spinel lattice having metal ions and layers of close-packed oxygen ions, the metal ions are smaller than the oxygen ions and are found between the layers of oxygen ions. The relationship within the lattice may be shown by imagining that the layers of oxygen ions are taken apart, leaving associated with each layer of oxygen ions the metal ions immediately in contact with the upper surface of the layer of oxygen ions. In this way the spinel structure may be regarded as built up of two kinds of alternate layers which layers are superposed one on top of the other in an alternating manner. The spinel structure may be further characterized in that one-third of the metal ions have 4 oxygen neighbors which oxygen neighbors are arranged tetrahedrally to the metal ion, and that two-thirds of the metal ions have 6 oxygen neighbors which oxygen neighbors are arranged octahedrally to the metal ion.

In each of the layers of close-packed oxygen ions are diagonal chains of metal ions having octahedral geometry. The octahedral metal ions are linked laterally above and below the layer of oxygen ions by the metal ions having tetrahedral geometry. The direction of the chains in any layer is normal to the direction of the chains in the adjacent layer. Four layers make up a unit cell.

The unit cell referred to above is an arbitrary parallelopiped which is the smallest repetitive unit identifiable as the crystal. The unit cell generally, as a matter of convenience, conforms to the symmetry of the system to which the crystal belongs. The unit cell is defined by the lengths of its edges, and the angles included between them. The edges of the unit cell are termed unit translations in the pattern. Starting from any point of origin in the lattice and going a distance equal to and parallel to any cell edge, or by any combination of such movements, we arrive at a point where the whole surrounding structure has the same form and orientation as at the point of origin. Because of the arbitrary nature of the definition of the unit cell, any one ion may be entirely within one cell or it may, alternatively, be divided between two, four, or eight unit cells. Additionally, the neighbor of any one ion may be the same unit cell or in an adjacent unit cell.

The spinel unit cell contains eight "formula weights," i.e., eight M.sup.II M.sub.2.sup.III 0.sub.4 units. More particularly, the crystallographic unit cell of the spinel structure contains 32 oxygen ions. There are equivalent positions in this cell for 8 metal ions surrounded tetrahedrally by 4 oxygen ions, and for 16 metal ions surrounded octahedrally by 6 oxygen ions.

Spinels may further be characterized on the basis of which metal ions occupy which positions in the crystal structure. Those spinels wherein the positions of tetrahedral coordination are occupied by the divalent metallic ions and the positions of octahedral coordination are occupied by the trivalent metal ions are "regular" spinels. Those spinels reported in the literature as being "regular" spinels, and their stoichiometric formulas, include: the magnesium-vanadium spinel (MgV.sub.2 O.sub.4); the zinc-vanadium spinel (ZnV.sub.2 O.sub.4); the magnesium-chromium spinel (MgCr.sub.2 O.sub.4); the manganese-chromium spinel (MnCr.sub.2 O.sub.4); the iron-chromium spinel (FeCr.sub.2 O.sub.4); the cobalt-chromium spinel (CoCr.sub.2 O.sub.4); the nickel-chromium spinel (NiCr.sub.2 O.sub.4); the copper-chromium spinel (CuCr.sub.2 O.sub.4); the zinc chromium spinel (ZnCr.sub.2 O.sub.4); the zinc-manganese spinel (ZnMn.sub.2 O.sub.4); the zinc-iron spinel (ZnFe.sub.2 O.sub.4); the cadmium-iron spinel (CdFe.sub.2 O.sub.4); the copper-cobalt spinel (CuCo.sub.2 O.sub.4); the zinc-cobalt spinel (ZnCo.sub. 2 O.sub.4); the magnesium-rhodium spinel (MgRh.sub.2 O.sub.4); the zinc-rhodium spinel (ZnRh.sub.2 O.sub.4); the magnesium-aluminum spinel (MgAl.sub.2 O.sub.4); the manganese-aluminum spinel (MnAl.sub.2 O.sub.4); the iron-aluminum spinel (FeAl.sub.2 O.sub.4); the cobalt-aluminum spinel (CoAl.sub.2 O.sub.4); the zinc-aluminum spinel (ZnAl.sub.2 O.sub.4); the nickel-aluminum spinel (NiAl.sub.2 O.sub.4); and the calcium-gallium spinel (CaGa.sub.2 O.sub.4).

Other spinels, wherein the tetrahedral positions are occupied by one-half of the trivalent metal ions and wherein the remaining one-half of the trivalent metal ions along with all of the divalent metal ions are in octahedral positions, are characterized as "inverse" spinels. In "inverse" spinels the arrangement of octahedral divalent and octahedral trivalent positions is substantially random. Such spinels, and their stoichiometric formulas, include: the titanium-magnesium spinel (TiMg.sub.2 O.sub.4); the vanadium-magnesium spinel (VMg.sub.2 O.sub.4); the magnesium-iron spinel (MgFe.sub.2 O.sub.4); the titanium-iron spinel (TiFe.sub.2 O.sub.4); the cobalt-iron spinel (CoFe.sub.2 O.sub.4); the nickel-iron spinel (NiFe.sub.2 O.sub.4); the copper-iron spinel (CuFe.sub.2 O.sub.4); the titanium-zinc spinel (TiZn.sub.2 O.sub.4); the tin-cobalt spinel (SnCo.sub.2 O.sub.4); the tin-zinc spinel (SnZn.sub.2 O.sub.4); the magnesium-gallium spinel (MgGa.sub.2 O.sub.4); the magnesium-indium spinel (MgIn.sub.2 O.sub.4); the manganese-indium spinel (MnIn.sub.2 O.sub.4); the iron-indium spinel (FeIn.sub.2 O.sub.4); the cobalt-indium spinel (CoIn.sub.2 O.sub.4); and the nickel-indium spinel (NiIn.sub.2 O.sub.4).

In still other spinels, the distribution of metal ions is less regular, the spinels exhibiting both normal and inverse arrangement, as is discussed, for example in Bragg, Claringbull, and Taylor, The Crystalline State, Vol 4; Crystal Structure of Minerals, G. Bell & Sons, Ltd., London (1965) at pages 105 and 106.

The spinel crystallographic unit gives a unique X-ray diffraction pattern corresponding to the spacings between the crystallographic planes. Typically, the observation of this X-ray diffraction pattern involves subjecting spinel samples to X-rays from a copper target. Methods of accomplishing this are more particularly described in chapter 5 of Klug and Alexander, X-ray Diffraction Procedures, John Wiley and Sons, Inc., New York (1954), at pages 235 to 318, and especially at pages 270 to 318, and in Newfield, X-ray Diffraction Methods, John Wiley and Sons, Inc., New York, N.Y., (1966), at pages 177 to 207. As described therein, these X-rays have a wave length of 1.5405 angstrom units. The X-rays diffracted by the sample are particularly intense at certain angles, .theta., resulting in peaks on diffractometer print-outs as in the FIGURE or in lines on photographic diffraction patterns. This high intensity is caused by the X-rays "reflected" from parallel planes in the crystal reinforcing each other. The wave length of the X-rays, the spacing of the planes in the crystal, and the angle, .theta., are related by Bragg's Law. Bragg's Law is

2d sin .theta. = n.lambda.

where d is the distance between the planes of the crystal, n is an integer, .lambda. is the wave length of the X-rays, and .theta. is the angle of incidence of the X-rays, and also the angle of reflection of the X-rays.

Typically, X-ray diffraction data is obtained from a diffractometer that is direct reading in 2.theta., wherein 180.degree. - 2.theta. is the angle between the incident ray and the reflected ray. One way of recording X-ray diffraction data is the form of a graph of the intensity of the reflected ray versus 2.theta.. X-ray diffraction data recorded in this way is shown in the FIGURE.

The FIGURE is a graph of the intensity of the reflective ray versus two theta for an electrode prepared according to Example I and consisting of a titanium member having thereon an outer surface of a cobalt aluminate bimetal spinel and silicon dioxide and an intermediate layer of palladium oxide between the cobalt aluminate, silicon dioxide outer surface and the titanium member. The peak at 33.92.degree. two theta is the characteristic PdO peak reported in the Literature. Also to be noted are the characteristic cobalt aluminate spinal peaks at (in numerical order) 31.28.degree. two theta, 36.78.degree. two theta, 59.18.degree. two theta, and 64.98.degree. two theta.

Good electrochemical results have been obtained with all spinels which have been tested. Spinels, including normal and inverse spinels, as well as those exhibiting both the normal and inverse arrangement are contemplated;

The spinels of magnesium, including titanium-magnesium spinel (TiMg.sub.2 O.sub.4), vanadium-magnesium spinel (VMg.sub.2 O.sub.4), and tin-magnesium spinel (SnMg.sub.2 O.sub.4).

The spinels of vanadium, including magnesium-vanadium spinel (MgV.sub.2 O.sub.4), iron-vanadium spinel (FeV.sub.2 O.sub.4), and zinc-vanadium spinel (ZnV.sub.2 O.sub.4).

The spinels of chromium including magnesium-chromium spinel (MgCr.sub.2 O.sub.4), manganese-chromium spinel (MnCr.sub.2 O.sub.4), iron-chromium spinel (FeCr.sub.2 O.sub.4), cobalt-chromium spinel (CoCr.sub.2 O.sub.4), nickel-chromium spinel (NiCr.sub.2 O.sub.4), copper-chromium spinel (CuCr.sub.2 O.sub.4), zinc chromium spinel (ZnCr.sub.2 O.sub.4), cadmium-chromium spinel (CdCr.sub.2 O.sub.4), and the chromium-chromium spinel (Cr.sub.3 O.sub.4).

The spinels of manganese, including titanium-manganese spinel (TiMn.sub.2 O.sub.4), and zinc-manganese spinel (ZnMn.sub.2 O.sub.4), and the manganese-manganese spinel (Mn.sub.3 O.sub.4).

The spinels of iron, including magnetite (Fe.sub.3 O.sub.4), magnesium-iron spinel (MgFe.sub.2 O.sub.4), titanium-iron spinel (TiFe.sub.2 O.sub.4), manganese-iron spinel (MnFe.sub.2 O.sub.4), cobalt-iron spinel (CoFe.sub.2 O.sub.4), nickel-iron spinel (NiFe.sub.2 O.sub.4), copper-iron spinel (CuFe.sub.2 O.sub.4), zinc-iron spinel (ZnFe.sub.2 O.sub.4), cadmium-iron spinel (CdFe.sub.2 O.sub.4), lead-iron spinel (PbFe.sub.2 O.sub.4), and aluminum-iron spinel (FeAlFeO.sub.4).

The spinels of cobalt including magnesium-cobalt spinel (MgCo.sub.2 O.sub.4), titanium-cobalt spinel (TiCo.sub.2 O.sub.4), copper-cobalt spinel (CuCo.sub.2 O.sub.4), zinc-cobalt spinel (ZnCo.sub.2 O.sub.4), and tin-cobalt spinel (SnCo.sub.2 O.sub.4).

The spinels of nickel including iron-nickel spinel (FeNi.sub.2 O.sub.4), and germanium-nickel spinel (GeNi.sub.2 O.sub.4).

The spinels of rhodium including magnesium-rhodium spinel (MgRh.sub.2 O.sub.4), cadmium-rhodium spinel (CdRh.sub.2 O.sub.4), cobalt-rhodium spinel (CoRh.sub.2 O.sub.4), copper-rhodium spinel (CuRh.sub.2 O.sub.4), manganese-rhodium spinel (MnRh.sub.2 O.sub.4), nickel-rhodium spinel (NiRh.sub.2 O.sub.4), and zinc-rhodium spinel (ZnRh.sub.2 O.sub.4).

The zinc spinels including titanium-zinc spinel (TiZn.sub.2 O.sub.4), and tin-zinc spinel (SnZn.sub.2 O.sub.4).

The aluminum spinels including magnesium-aluminum spinel (MgAl.sub.2 O.sub.4), strontium-aluminum spinel (SrAl.sub.2 O.sub.4), chromium-aluminum spinel (CrAl.sub.2 O.sub.4), molybdenum-aluminum spinel (MoAl.sub.2 O.sub.4), manganese-aluminum spinel (MnAl.sub.2 O.sub.4), iron-aluminum spinel (FeAl.sub.2 O.sub.4), cobalt-aluminum spinel (CoAl.sub.2 O.sub.4), nickel-aluminum spinel (NiAl.sub.2 O.sub.4), copper-aluminum spinel (CuAl.sub.2 O.sub.4), and zinc-aluminum spinel (ZnAl.sub.2 O.sub.4).

The gallium spinels including magnesium-gallium spinel (MgGa.sub.2 O.sub.4), zinc-gallium spinel (ZnGa.sub.2 O.sub.4), and calcium-gallium spinel (CaGa.sub.2 O.sub.4).

The indium spinels including magnesium-indium spinel (MgIn.sub.2 O.sub.4), calcium-indium spinel (CaIn.sub.2 O.sub.4), manganese-indium spinel (MnIn.sub.2 O.sub.4), iron-indium spinel (FeIn.sub.2 O.sub.4), and cobalt-indium spinel (CoIn.sub.2 O.sub.4), nickel-indium spinel (NiIn.sub.2 O.sub.4), cadmium-indium spinel (CdIn.sub.2 O.sub.4), and mercury-indium spinel (HgIn.sub.2 O.sub.4).

The spinels containing ions of three metals, such as magnesium-aluminum-iron spinel (MgFeAlO.sub.4), and nickel-aluminum-iron spinel (NiFeAlO.sub.4).

Particularly satisfactory results are obtained with the bimetal spinel. Bimetal spinels are those spinels having the formula M.sup.II M.sup.III.sub.2 O.sub.4 as described hereinabove where M.sup.II and M.sup.III are ions of two different metals.

Better electrolytic results are obtained with aluminate spinels, that is where one or both of the ions present in the plus 3 valence state is aluminum, as in CuAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, FeAlFeO.sub.4, and NiAl.sub.2 O.sub.4.

Best results are obtained with the heavy metal-aluminate spinels, that is where the metal ion present in the plus 2 valence state is iron, cobalt, or nickel, as in Fe.sup.II AlFe.sup.III O.sub.4, CoAl.sub.2 O.sub.4, and NiAl.sub.2 O.sub.4, and with the heavy metal ferrite spinels, that is, where iron is present in the plus 3 valence state, as in CoFe.sub.2 O.sub.4, MgFe.sub.2 O.sub.4, and NiFe.sub.2 O.sub.4.

Whenever FeAlFeO.sub.4 is referred to herein, it is understood that this material may actually be a mixture of Fe.sup.II Fe.sub.2.sup.III O.sub.4, FeAl.sub.2 O.sub.4, and Fe.sup.II AlFe.sup.III O.sub.4. This material may be characterized by the presence of iron in both the plus 2 and plus 3 valence states, as well as the presence of aluminum in the plus 3 valence state. Additionally, FeO, Fe.sub.2 O.sub.3, and Al.sub.2 O.sub.3 may also be present.

Preferably, the spinel itself should have some appreciable electroconductivity when measured in bulk. While good results have been obtained with a spinel having an electroconductivity as low as 10.sup.-.sup.5 (ohm-centimeters).sup.-.sup.1, generally the conductivity should be at least 10.sup.-.sup.1 (ohm-centimeters).sup.-.sup.1. Moreover, the thin spinel coatings appear to exhibit greater conductivity as the electrodes are used as anodes in the electrolysis of aqueous sodium chloride to produce chlorine and sodium hydroxide. Thus, some electrocatalytic effect may play a role in the electrolytic processes herein contemplated.

The temperatures which have been resorted to for the preparation of spinels, typically ranging from about 750.degree.C. to about 1,350.degree.C., are far above the temperatures which decompose and volatilize various compounds of the oxidation inhibitors and the binding agents and, in a normal atmosphere oxidize the surface of the substrate. For this reason when the spinel is formed in contact with the substrate, as for example, from mixed oxides of the metals, the substrate or support member may suffer some degree of oxidation and, in such a case, the electrode may show a much higher voltage than is desirable. But when the spinel is formed prior to being applied to the substrate or support member, the highest temperature that the member is exposed to is the higher of either the decomposition temperature of the oxidation inhibitor, the liquid solvent, or of the binding agent compound, and the degree of oxidation of the member is negligible.

For this reason, it is desirable that the spinel be formed prior to being applied to the support member. This can be accomplished by oxidation of the mixed metals, or by mixing and subsequent heating of the mixed oxides, or by coprecipitation from solutions of compounds of the metals followed by heating or by thermal decomposition of compounds of the mixed metals. The preferred compounds are those which decompose directly to the oxides on heating and do not leave a residue, as carbonates, formates, nitrates, and oxalates, e.g.:

CoCO.sub.3 + Al.sub.2 (CO.sub.3).sub.3.fwdarw.CoO + Al.sub.2 O.sub.3 + 4CO.sub.2 .uparw.

Co(NO.sub.3).sub.2 + 2Al(NO.sub.3).sub.3 .fwdarw.CoO + Al.sub.2 O.sub.3 + 8NO.sub.2 .uparw.+ 2O.sub.2 .uparw.

The resulting product is an intimate mixture of the two oxides which can be heated to form the spinel.

Various procedures may be resorted for the preparation of spinels to be used as electrode surfaces. For example, spinels may be prepared from the mixed oxides. Cobalt-aluminate, copper-aluminate, and nickel-aluminate spinels were prepared from the mixed oxides. The general procedure was to grind stoichiometric amounts of the oxides to minus 200 mesh, mix the ground oxides together, place the mixed, ground oxides in a crucible, and heat the mixed, ground oxides, thereby forming the spinel.

Another mixed oxide procedure may be utilized for the iron-aluminum spinel having the stoichiometric formula Fe.sup.II (Fe.sup.III Al)O.sub.4. By this method the iron-aluminum spinel is prepared from the mixed oxides Fe.sub.2 O.sub.3, FeO, and Al.sub.2 O.sub.3 according to the following procedure described in Blue and Claassen, Journal of the American Chemical Society, 71, 3839 (1949) and Couglin, King and Bonnickson, Journal of the American Chemical Society, 73, 3891 (1951).

According to this procedure, FeO was first prepared by heating metallic iron and Fe.sub.3 O.sub.4 in the presence of water vapor. The reactions believed to be taking place are:

Fe + H.sub.2 O .fwdarw. FeO + H.sub.2

Fe.sub.3 O.sub.4 + H.sub.2 .fwdarw. 3FeO + H.sub.2 O

The FeO prepared as described above, Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3, all ground to minus 200 mesh were mixed and heated under a vacuum overnight at a temperature of about 110.degree.C. The material was then heated to 1,200.degree.C., under vacuum, for 24 hours. The resulting product was black, magnetic material exhibiting the X-ray diffraction pattern reported in the literature to be characteristic of the Fe.sup.II (Fe.sup.III Al)O.sub.4 spinel.

Spinels may be prepared by precipitation from nitrate solution, followed by oxidation of the precipitate to yield the spinel. Copper-aluminate, copper-chromite, copper-ferrite, cobalt-aluminate, and cobalt-chromate spinels, inter alia, were prepared by precipitation from the nitrate solutions. The general procedure was to prepare an aqueous solution 0.5 molar in the nitrate salt of the divalent metal and 1.0 molar in the nitrate salt of the trivalent metal. This solution was evaporated to dryness by heating the solution to a temperature between 125.degree.C. and 140.degree.C. The dried product was then heated in an air-aspired furnace to drive off the nitrogen compounds, yielding thereby a mixed oxide. The mixed oxide was then ground to a power which was then heated in a furnace to a temperature sufficiently high to form the spinel.

Spinels useful for electrode coatings may also be prepared by precipitation from oxalate solution, and subsequent oxidation. A solution of MgSO.sub.4 and FeSO.sub.4 in distilled water was prepared. The solution was then filtered. The filtered solution was heated to boiling and ammonium oxalate and oxalic acid were added to the solution with stirring. Boiling and stirring were continued until a precipitate appeared. The resulting precipitate was filtered and washed. The washed precipitate was then dried and the dried product was placed in a porcelain crucible and heated to a temperature of 500.degree.C., held at the temperature of 500.degree.C. for 10 minutes and then slowly cooled to room temperature. The resulting product was ground to minus 200 mesh and then heated to 950.degree.C. and held at 950.degree.C. for 7 1/2 hours, thereby yielding the spinel.

Spinels may also be prepared by ammonium carbonate precipitation from the chloride solution ammonium carbonate. Nickel chromate, nickel aluminate, cobalt aluminate, and copper chromate spinels inter alia, were prepared by precipitation from chloride solutions. The general procedure was to prepare a solution containing the chloride salts of the di- and tri-valent metals. To this solution was added ammonium carbonate. The solution was then stirred under a nitrogen blanket and the precipitate which formed was separated by centrifuging under a nitrogen atmosphere. The centrifugate was dried under a nitrogen atmosphere and the solid material ground and heated under vacuum for 72 hours, thereby yielding the spinel.

Spinels may also be prepared by ammonium hydroxide precipitation from the chloride solution. In this procedure, a solution was prepared from stoichiometric quantities of the two chlorides. To this solution was added a concentrated solution of ammonium hydroxide. The resulting mixture was stirred until a precipitate appeared. The resulting precipitate was separated and dried. The dried precipitate was ground to minus 100 mesh and placed in a covered crucible which was placed in a vacuum furnace under a vacuum of 10.sup.-.sup.5 millimeters of mercury and heated to a temperature of 900.degree.C. or greater and maintained thereat for 24 hours, thereby yielding the spinel.

Depending on the method of forming the mixed oxide and the degree of comminution thereof, it is possible that all of the mixed oxide will not necessarily be converted to a spinel but that some will remain as the original oxide. This has no deleterious effect on the anode. The less soluble oxides, as Al.sub.2 O.sub.3, will remain on the anode without deleterious effect, while the more soluble oxide, as CoO or NiO, may be dissolved by the anolyte when the finished electrode is employed as an anode.

Also Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3 reportedly have structures which permit significant quantities of either or both to be present in the spinel lattice without deleterious effects, and without being readily detectable by X-ray diffraction.

The preferred spinel usually is applied together with a binding agent. These agents include organometallic compounds which, on heating, decompose to the metal or metal oxide and volatiles as well as more permanent binders.

Typically, regardless of the substrate, the spinel must be made to adhere to the substrate. This may be accomplished by providing a lattice or network within the spinel, with a suitable permanent binding agent, whereby the adherence of the spinel to the substrate is enhanced.

A suitable permanent binding agent has to be impervious to the chlorine environment of the electrolytic cell as for instance a metal compound, such as an oxide, sulfide, nitride, boride, or carbide of titanium, tantalum, niobium, aluminum, bismuth, tungsten, zirconium, hafnium, vanadium, chromium, or silicon. It has been found that particularly good binding results are obtained by the formation, in situ with the spinel coating, of a metal oxide that is substantially non-reactive with the anolyte. The formation of this oxide, in situ, must, moreover, take place at a temperature below the temperature at which any appreciable oxidation of the structural member occurs or any adverse effect on the undercoat occurs. For this reason, the thermal decomposition of a readily decomposed compound having volatile decomposition products as, for instance, an oxylate, carbonate, hydroxide, hydrated oxide, or resinate of titanium, tantalum, silicon, molybdenum, aluminum, bismuth, zirconium, hafnium, tungsten, niobium, or vanadium may be used. Generally the more permanent binders are inorganic. Titanium compounds are preferred. Whenever titanium dioxide is described as a binding agent, it will be understood that other binding agents as herein described may be used in lieu of or in addition thereof.

Small concentrations of the permanent binding agent are effective. Satisfactory results in terms of activity of the anode and the durability thereof have been obtained at titanium dioxide concentrations of from about 3.5 percent by weight (calculated as titanium metal) of the spinel surface coating to about 30 percent by weight (calculated as titanium metal) of the surface coating. Although it is possible to prepare electrodes using less than about 3.5 weight percent of the binding agent in the surface coating, spinel losses will be significant. With too great a concentration of a binding agent such as titanium dioxide, i.e., amounts above 30 weight percent of the surface coating (calculated as the metal), the anode activity will be influenced materially by the titanium dioxide. Titanium dioxide concentrations of from about 7 weight percent (calculated as titanium metal) to about 15 weight percent (calculated as titanium metal) are preferred.

In order to effectuate intimate mixing of the spinel and the permanent binder, both are put into a liquid medium. Either water or an organic solvent may be used. It is particularly important that the binding agent be dispersed in the liquid medium and that the spinel be in a fine enough state of subdivision that it is also readily dispersed in the liquid medium. Saturated aliphatic and aromatic liquid hydrocarbons yield satisfactory results. Better results are obtained with saturated aliphatic and aromatic liquid hydrocarbons having from six to 10 carbon atoms, as benzene, toluene, cumene, hexane, and cyclohexane. Toluene is preferred.

In one exemplification the spinel surface coating containing titanium dioxide binder is provided by applying a slurry of the spinel ground to minus 325 mesh and containing titanium resinate. Specifically, such a slurry is prepared by adding 0.5 gram of ground spinel to 3.0 grams of toluene and 1 gram of titanium resinate solution (containing 4.2 weight percent of titanium calculated as metal). This is vigorously stirred, providing a suspension which will not settle out for a period of from about 30 seconds to about 1 minute. Within this period and while the suspension still exists, it is brushed on over the layer of the second transition series platinum group metal oxide, and the coupon is then heated to a temperature of about 500.degree.C. A plurality of such brushings and subsequent heatings are performed until the spinel content is built up to the desired thickness, usually the process of brushing and heating being repeated by about 7 to about 20 times. It is to be understood that satisfactory results may also be obtained without heating after every coat of spinel so long as the resinate is ultimately decomposed. The resulting surface, on the order of about 200 to about 200 micro-inches thick, has on the order of about 0.02 to about 0.04 gram of spinel per square inch of spinel coated anode surface. Thicker coatings, rarely in excess of about 500 micro-inches in thickness, may also be applied in this manner.

When the spinel is applied in this way, it is likely that the resulting surface will exhibit some degree of porosity. For example, after only 5 coats of the slurry above-described comprising minus 325 mesh spinel, titanium resinate, and toluene have been applied, it is possible to optically observe at a magnification of 150 times the layers of material (as the substrate or the intermediate oxide layer between the substrate and the spinel) between individual spinel fragments. The surface under such magnification exhibits considerable irregularities, as ridges, valleys, peaks, crystal boundaries, and breaks.

The heating of the spinel slurry coating to form the desired surface bonded to the substrate may take place in air. While it may also take place under an inert atmosphere, as helium, argon, neon, krypton, xenon, carbon dioxide, or nitrogen, or other relatively inert gases, care must be taken to avoid recourse to temperatures or other conditions which cause the spinel lattice to break down. Thus, the presence of some oxygen in the surrounding atmosphere will prevent or minimize such breakdown of the spinel lattice.

The heating may take place at atmospheric pressure, or at a total pressure below atmospheric pressure, or at a total pressure above atmospheric pressure. The heating may also take place at a standard partial pressure of oxygen (approximately 2.6 pounds per square inch partial pressure of oxygen), or at a lower or higher partial pressure of oxygen, typically at oxygen partial pressures of from about .sup.-.sup.6 millimeters of mercury to about 15 pounds per square inch. Satisfactory results are obtained at normal atmospheric total pressure and at normal atmospheric partial pressure of oxygen. Satisfactory results may also be obtained by heating under an atmosphere having a standard atmospheric total pressure but a reduced partial pressure of oxygen as, for example, an inert atmosphere. Satisfactory results may also be obtained by heating under a total pressure greater than 14.7 pounds per square inch absolute and a partial pressure of oxygen less than 2.6 pounds per square inch absolute, as, for example, under a relatively inert gas atmosphere at a total pressure in excess of 14.7 pounds per inch. When, however, the heating takes place under a partial pressure of oxygen below the normal atmospheric partial pressure of oxygen, care must be exercised to prevent breakdown of the spinel lattice.

In still another embodiment of this invention, the spinel is dispersed in a fluxing agent prior to being applied to the structural member, thereby providing a more durable coating.

The fluxing agent should have a normal melting point, of from about 700.degree.C. to about 800.degree.C. The fluxing agent should also be resistant to be anolyte environment of the alkali-chlorine electrolytic cell. Glass frits slurried in water solution may be used.

Frits having a melting point in the desired range are generally comprised of mixed oxides and silicates of lead, potassium, zinc, boron, calcium, aluminum, and barium. They typically have from about 70 weight percent to about 80 weight percent lead oxides, about 1 weight percent silica, 10 to 16 weight percent zinc oxide, and about 10 weight percent boron oxide. The silica is present in the form of silicates. Various other compounds, as bismuth oxide, tin oxide, selenium oxide, tellurium oxide, and titanium dioxide may also be present in the frit.

The fluxing agent is ground to minus 325 mesh and slurried; typically using water as the liquid. The ground frit constitutes from about 60 to about 80 weight percent of the slurry. The spinel, ground to minus 325 mesh, is added to the slurry such that there is from 1.5 to 7 parts of spinel to 1 part of fluxing agent by weight. Better results are obtained when there is from 2 to 4 parts of spinel to 1 part of fluxing agent by weight.

This slurry is applied to a substrate which typically has been etched and thereafter provided with a protective sublayer (as by being treated with an oxide of a second transition series platinum group metal). The member is heated to melting point of the flux and held at that temperature for a short time-typically from about 1 to about 10 minutes.

Alternatively, the second transition series platinum group metal oxide may be applied with a fluxing agent, and the spinel-flux coating may be applied above the platinum group metal oxide flux coating. In still another variation of this exemplification, the platinum group metal oxide is dispersed in the spinel-flux coating as well.

While the spinel coating may be applied with effective results according to the methods described previously, in a general manner, other methods may also be resorted to. Thus, the spinel, in the form of a fine powder, may be pressed onto the palladium, ruthenium or rhodium oxide coated substrate. Thereafter, the substrate with the powder coating is subjected to a compressive force. The compressive force in in excess of one ton per square inch, and, typically, on the order of about 10 to 20 tons per square inch. Such forces may be obtained conveniently by passing the structural member with the finely-powdered spinel thereon between rollers held in compression. By this procedure, a compressive point force in excess of two tons per square inch may be applied to the finely-powdered spinel and the substrate.

According to a further embodiment, the titanium or like metal base cleaned of natural oxide and having a platinum group metal oxide surface thereon may be disposed as a cathode in an aqueous or electroconductive non-aqueous suspension of spinel powder with or without titanium oxide, or hydroxide, or aluminum hydroxide, or titanium resinate, and globules of a binder which migrates upon imposition of an electromotive force between a pair of electrodes to the cathode substantially as described in Ranney, Electrodeposition and Radiation Curing of Coatings, Hayes Data Corp., Park Ridge, N. J. (1970), pp. 101-109, to form a coating. This process is effective to produce a spinel coating of lower porosity. Furthermore, thicker coatings may be applied in a single coating operation. The substrate may then be heated to volatilize or burn up the organics and bond the oxide to the base described above. By maintaining the titanium member cathodic during the entire period that the titanium is in contact with the electrolyte and until the coating is deposited, oxidation of the substrate may be reduced.

In another embodiment, an alloy of two or three metals of the spinel may be electrodeposited directly upon a second transition series platinum group metal oxide coating on the substrate. The alloy coating which preferably should contain the metals substantially in the proportion of the spinel may then be heated in oxygen to oxidize the surface and to thereby form the spinel.

Any suitable electroconductive material resistant to attack by the chlorine cell environment may be used as the substrate or support member of the electrode of this invention useful as an anode for brine electrolysis. Most commonly used as the "valve" metals; that is, those metals which form a passivating oxide film, conductive only in the cathodic direction. The valve metals include titanium, tantalum, tungsten, hafnium, zirconium, aluminum, and columbium and alloys thereof. Such valve metals typically have an electrical conductivity of about 10.sup.5 (ohm centimeters).sup.-.sup.1 to about 10.sup.6 (ohm centimeters).sup.-.sup.1, and have an oxide coating having an electrical conductivity of from about 10.sup.-.sup.13 (ohm centimeters).sup.-.sup.1 to about 10.sup.-.sup.11 (ohm centimeters).sup.-.sup.1. Titanium and tantalum are preferred. Titanium yields best results. Carbon and graphite may also be used. These materials have a conductivity considerably greater than that of the spinel, usually being 10.sup.3 (ohm centimeters).sup.-.sup.1 or higher. In accordance with a preferred embodiment, the metal substrates are such that they will not normally permit bulk permeation of gases through the metal itself. Electrodes made of such gas impermeable metal substrates include electrodes contemplated herein having a metal mesh substrate wherein the metal substrate itself is substantially gas impervious, although the gases may pass through the openings in the mesh of the electrode.

The support members may be in the form of a solid structural member or of a thin imperforate plate, for example, up to about 1/4 inch thick. Alternatively, the support may be perforate or foraminous or mesh. They can be of any shape appropriate for anodes to be used in electrolytic cells. When anodes having perforate or foraminous supports are used in mercury cells, they may be totally or only partially immersed in the electrolyte. When totally immersed in the electrolyte, only the surface of the anode facing the flowing cathode need be coated with the spinel anodic surface, or all of the surfaces of the anode may be coated with the spinel anodic surface. Likewise, when such anodes having perforate or foraminous supports are used in diaphragm cells, either one surface or both surfaces of the support may be coated with the spinel anodic surface.

In most cases even when the spinel is applied following the above procedure directly to an untreated valve metal metallic substrate, as commercial grade titanium metal, in the presence of oxygen, the voltage drop across the cell with such an anodic surface is very high--on the order to about 10 volts. This increased voltage appears to be caused by the formation of oxides of the metal used in fabricating the substrate at the spinel-substrate interface. While not wishing to be bound by this explanation, it is believed that either some actual oxidation of the unprotected bulk titanium or like metal takes place at the spinel-substrate interface or that there is some migration or intermetallic diffusion of oxygen atoms into the bulk metal, or possibly that the spinel itself may tend to oxidize the substrate.

According to this invention especially good results are obtained by the application of an intervening layer of an oxide of a second transition series platinum group metal between the spinel and the titanium or like metal base and in electrical contact with the base to inhibit this oxidation and/or to prevent or minimize this undesirably high anode voltage drop.

The second transition series platinum metals include ruthenium, rhodium, and palladium. Oxides of all three members of the second transition series platinum group metals appear to have good oxidation-inhibiting effects.

Such oxides of the second transition series platinum group metals typically have electrical conductivities of from about 10.sup.-.sup.5 (ohm centimeters).sup.-.sup.1 to about 10.sup.-.sup.1 (ohm centimeters).sup.-.sup.1.

Good results are obtained when the intermediate layer comprises metal oxycompounds of the second transition series platinum metals with other metals such as the rhodium spinels, cadmium-rhodium spinel (CdRh.sub.2 O.sub.4), cobalt-rhodium spinel (CoRh.sub.2 O.sub.4), copper-rhodium spinel (CuRh.sub.2 O.sub.4), magnesium-rhodium spinel (MgRh.sub.2 O.sub.4), nickel-rhodium spinel (NiRh.sub.2 O.sub.4), and zinc-rhodium spinel (ZnRh.sub.2 O.sub.4), and the outer surface is a heavy metal aluminate spinel as CoAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, or FeAlFeO.sub.4. Good results are also obtained with other metallic oxide compounds of the second transition series platinum metals such as the alkaline earth ruthenite, as beryllium ruthenite (BeRuO.sub.3), magnesium ruthenite (MgRuO.sub.3), calcium ruthenite (CaRuO.sub.3), strontium ruthenite (SrRuO.sub.3), and barium ruthenite (BaRuO.sub.3).

Good results are also obtained with an intermediate coating comprising the alkaline earth ruthenates as beryllium ruthenate (BeRuO.sub.4), magnesium ruthenate (MgRuO.sub.4), calcium ruthenate (CaRuO.sub.4), strontium ruthenate (SrRuO.sub.4), and barium ruthenate (BaRuO.sub.4). Satisfactory results are also obtained with the analogous, rhodites, rhodates, palladates, and palladites. Also contemplated are the oxycompounds of the second transition series platinum group metals (ruthenium, rhodium, palladium) with the rare earths or lanthanides (cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, halmium, erbium, thulium, ytterbium and lutetium) and scandium.

Entirely satisfactory results are obtained with mixed oxides of the platinum metals of the second transition series, such as ruthenium oxide--palladium oxide, ruthenium oxide--rhodium oxide, palladium oxide--rhodium oxide, and ruthenium oxide--rhodium oxide--palladium oxide. Satisfactory results are also obtained when the coating contains one or more oxides of second transition series of platinum metals (as palladium oxide, rhodium oxide, ruthenium oxide) and one or more oxides of third transition series platinum metals (as osmium oxide, platinum oxide, and iridium oxide). Examples of such coatings include palladium oxide--platinum oxide, palladium oxide--iridium oxide, palladium oxide--osmium oxide, ruthenium oxide--platinum oxide, ruthenium oxide--osmium oxide, ruthenium oxide-- iridium oxide, rhodium oxide--platinum oxide, rhodium oxide-- osmium oxide, and rhodium oxide--iridium oxide. Particularly good results seem to be obtained with mixtures of platinum group metal oxides in the same column of the periodic chart, as ruthenium oxide--osmium oxide, rhodium oxide--iridium oxide, and palladium oxide--platinum oxide.

Additionally, satisfactory results may also be obtained with a coating comprising one or more oxides of platinum group metals of the second transition series with one or more platinum group metals of the third transition series being present in the metallic state. Such coatings comprise ruthenium oxide--osmium, ruthenium oxide--iridium, ruthenium oxide--platinum, rhodium oxide--osmium, rhodium oxide--iridium, rhodium oxide--platinum, palladium oxide--osmium, palladium oxide--iridium, and palladium oxide--platinum. Particularly good results also seem to be obtained with mixtures containing an oxide of a second transition series platinum group metal, and the corresponding third transition series metal, in the metallic state, as ruthenium oxide--osmium, rhodium oxide--iridium, and palladium oxide--platinum.

The oxide is deposited as a discrete layer between the substrate and the spinel coating. It is to be understood, however, that there may be some diffusion between layers. This may occur during the deposition of subsequent layers of the oxide and spinel; or it may occur during electrolysis. As a result of these effects, concentration gradients may exist with some oxide being present in the substrate and also in the spinel coating. Additionally, small amounts of the metal used in fabricating the substrate may be present in the second transition series platinum group metal oxide undercoating, either as the oxide or as the metal, but most likely as the oxide. Small amounts of the spinel or of the metallic binder or both may be present in the oxide undercoating.

Whenever a layer of a second transition series platinum group metal is referred to herein, it is understood that such layer may also contain some of the second transition series platinum group metal in the metallic state. Typically, less than thirty percent of the second transition series platinum group metal will be in the metallic state.

Very thin layers of the metal oxide are effective. Satisfactory results have been achieved with the application of only three coats of palladium oxide prior to applying the spine. X-ray data indicate that palladium oxide and ruthenium oxide coatings with thicknesses from about 2 to 10 micro-inches are sufficient to give the desired result. By way of comparison, a fairly thick, uniform palladium oxide layer of greater than about 20 micro-inches is required when the palladium oxide is the outer surface. After the electrode has received a metal oxide coating, it is ready to receive a spinel coating, applied as described hereinabove.

While the invention has been particularly described with reference to a titanium substrate which serves as the electroconductive base, it is to be understood that other materials may be used for the substrate. For example, the spinel coating may be applied to a graphite base having the contour of the desired anode. The graphite, if desired, may first be coated with the platinum group metal oxide interlayer as described above before applying the spinel coating.

Moreover, the electrode base may be steel or other electroconductive base clad with a thin sheet or coating of titanium and in electrical contact therewith. The titanium coating or sheet may thus be coated with the platinum group metal oxide type interlayer and then with the spinel.

While an interlayer consisting essentially of an oxide of a second transition series platinum group metal is especially valuable because such materials are themselves resistant to anodic corrosion and have good electroconductivity and low overvoltage, other materials may be used in combination therewith. As an example, the following electroconductive materials may be deposited upon the titanium or like chemically-resistant base in combination with the oxide of palladium, ruthenium or rhodium, and the spinel deposited above the coating of these materials in combination with the oxide of palladium, rhodium or ruthenium; sulfides of platinum group metals, as platinum sulfide, osmium sulfide, irridium sulfide, ruthenium sulfide, rhodium sulfide, or palladium sulfide; metal hydrides as titanium hydride, tantalum hydride, zirconium hydride, hafnium hydride, and the like; carbides, borides, nitrides, oxides, and sulfides of the metals used in forming the binding agent, as carbides, borides, nitrides, oxides, and sulfides of titanium, tantalum, tungsten, zirconium hafnium, aluminum, vanadium, columbium, and the like. Additionally, oxides of chromium, manganese, iron, cobalt, nickel, molybdenum, and the like may be present with the oxide of palladium, ruthenium, or rhodium.

When such other materials are present in the intermediate coating with the oxide of palladium, rhodium, or ruthenium, such other material should constitute less than 70 percent of the intermediate coating on a molar basis, and preferably less than 50 percent on a molar basis.

The palladium, ruthenium, or rhodium oxide intermediate layer of this invention may also be used with electrodes having an electroconductive titanium hydride substrate. Such titanium hydride substrates may be prepared by powder metallurgy techniques, or by chemical reaction of the titanium. When a titanium hydride member is used as the electroconductive substrate, the spinel outer surface may be applied by any of the methods described above, as decomposition of an organic liquid containing the ground-up spinel, or electroless deposition, or cathodic electrodeposition. Alternatively, the titanium hydride substrate may be titanium hydride surface upon a titanium metal substrate. Such a surface may be provided by etching the titanium metal substrate in a strong inorganic acid, as concentrated hydrochloric acid.

The following examples are illustrative:

EXAMPLE I

An electrode was prepared having a cobalt aluminate spinel surface on a titanium metal substrate with a palladium oxide intermediate layer therebetween.

A 2-inch by 2-inch by 1/16-inch titanium coupon was scrubbed with household cleanser and rinsed with distilled water and with acetone. It was then dipped in 1 percent hydrofluoric acid solution at room temperature for 1 minute. The coupon was then etched in 37 percent hydrochloric acid solution for 16 hours. The etching temperature varied from 45.degree.C. to 55.degree.C. After etching in the hydrochloric acid solution, the titanium coupon was immersed in running distilled water for 2 minutes and air dried.

The coupon so treated was then immersed in a solution of 8.84 grams of palladium chloride (PdCl.sub.2), 2.25 grams of ammonium chloride (NH.sub.4 Cl), 30 cubic centimeters of concentrated hydrochloric acid (HCl), and 200 cubic centimeters of distilled water. The coupon was subjected to a current of 7 amperes per square foot for 2 minutes. It was then heated in a furnace open to the atmosphere for 1 hour at 550.degree.C., thereby providing a titanium coupon having a palladium oxide surface thereon.

Cobalt aluminate spinel (CoAl.sub.2 O.sub.4) was then prepared. 42.4 grams of Co0 were ground to minus 200 mesh and 57.6 grams of Al.sub.2 O.sub.3 were ground to minus 200 mesh. The two ground oxides were mixed together and placed in an alundum crucible. The alundum crucible containing the two ground oxides was placed in a furnace open to the atmosphere. The ground oxides were then heated to 1,200.degree.C. for 24 hours. The resulting product was deep blue in color and had the X-ray diffraction pattern reported in the literature to be characteristic of cobalt aluminate spinel.

The coupon having a palladium oxide surface was then coated with a solution prepared from 0.5 gram of the cobalt aluminate spinel (CoAl.sub.2 O.sub.4) so prepared, 0.5 gram DuPont "Ludox" (Trademark), (a 42.5 weight percent solution of SiO.sub.2, particle size 100 to 150 microns; in water, adjusted to a pH of 9), and 2.0 grams of distilled water. Ten coats were brushed on. Coats 1 to 9 were heated at a rate of 50.degree.C. per 5 minutes to a temperature of 400.degree.C. and held at 400.degree.C. for 5 minutes. The tenth coat was heated at a rate of 50.degree.C. per 5 minutes to 600.degree.C. and held at 600.degree.C. for 30 minutes, thereby yielding an electrode having a cobalt aluminate spinel (CoAl.sub.2 O.sub.4) surface on a titanium metal substrate with an intermediate palladium oxide layer therebetween.

The resulting electrode was used as the anode of a beaker chlorate cell. The chlorate cell was a 1,500 milliliter beaker. The beaker contained a 300 grams/liter solution of sodium chloride at a temperature of 45.degree.C. to 55.degree.C. The cathode of the cell was platinized titanium having the same surface area as the anode. Electrolysis was usually conducted at a current density of 500 Amperes per square foot based on the surface of the anode under test. It gave an initial cell voltage of 4.55 volts at a current density of 500 Amperes per square foot.

The X-ray diffraction pattern of the anode produced according to Example I is shown in the FIGURE. Particularly to be noted is the peak at 33.92.degree. two theta corresponding to PdO.

EXAMPLE II

An electrode was prepared having a cobalt aluminate surface on a titanium metal substrate with a palladium oxide layer therebetween.

A 2 inch by 2 inch by 1/16 inch titanium coupon was scrubbed with household cleanser and rinsed with distilled water and with acetone. It was then dipped in 1 percent hydrofluoric acid solution at room temperature for 1 minute. The coupon was then etched in 37 percent hydrochloric acid solution for 19 hours. The etching temperature varied from 45.degree.C. to 55.degree.C. After etching in the hydrochloric acid solution, the titanium coupon was immersed in running distilled water for 2 minutes and air dried. The coupon so treated was then immersed in a plating solution prepared from 33 milliliters of a 7.4 grams per 100 cubic centimeters solution of palladium chloride (PdCl.sub.2), and 120 grams of potassium hydroxide (KOH) diluted to 500 milliliters by distilled water. The coupon was subjected to a current of 4.8 amperes per square foot for 5 minutes. It was rinsed in water, then acetone, and dried at room temperature.

Cobalt aluminate spinel (CoAl.sub.2 O.sub.4) was then prepared according to the procedure described in Example I.

A solution of 5 grams of cobalt aluminate spinel (CoAl.sub.2 O.sub.4), 10 grams of titanium resinate (containing 4.2 weight per cent titanium calculated as the metal), and 30 grams of toluene was applied to the coupon. The individual coats were brushed on. After the application of coats 1 through 5 and 7 through 9, the coupon was heated at a rate of 50.degree.C. per 5 minutes to a temperature of 400.degree.C. for 10 minutes. Coats 6 and 10 were heated to a temperature of 500.degree.C. for a period of 10 minutes, also at a rate of 50.degree.C. for 5 minutes. By this procedure an electrode was provided having a cobalt aluminate spinel surface on a titanium metal substrate, with an intervening palladium oxide layer therebetween.

The resulting electrode was utilized as the anode in a beaker chlorate cell as hereinbefore described. At a temperature of 45.degree.C. and a current density of 500 amperes per square foot, the initial voltage was 3.27 volts, and the voltage after 450 hours was 3.63 volts.

EXAMPLE III

An electrode was prepared having a cobalt aluminate spinel surface on a titanium metal substrate with a palladium oxide layer therebetween.

A 2 inch by 2 inch by 1/16 inch titanium coupon was scrubbed with household cleanser and rinsed with distilled water and with acetone. It was then dipped in 1 percent hydrofluoric acid solution at room temperature for 1 minute. The coupon was then etched in 37 percent hydrochloric acid solution for 19 hours. The etching temperature varied from 45.degree.C. to 55.degree.C. After etching in the hydrochloric acid solution, the titanium coupon was immersed in running distilled water for 2 minutes and air dried.

The coupon so treated was then immersed in a plating solution prepared from 33 milliliters of a 7.4 grams per 100 cubic centimeters solution of palladium chloride (PdCl.sub.2), and 120 grams of potassium hydroxide (KOH) diluted to 500 milliliters by distilled water. The coupon was subjected to a current of 4.8 amperes per square foot for 5 minutes. It was rinsed in water, then acetone, and dried at room temperature.

Cobalt aluminate spinel was then prepared for use as the outer surface thereof. 42.4 grams of CoO were ground to minus 200 mesh and 57.6 grams of Al.sub.2 O.sub.3 were ground to minus 200 mesh. The two ground oxides were mixed together and placed in an alundum crucible. The alundum crucible containing the two ground oxides was placed in a furnace open to the atmosphere. The ground oxides were then heated to 1,350.degree.C. for 16 hours. The resulting product was deep blue in color and had the X-ray diffraction pattern reported in the literature to be characteristic of cobalt aluminate spinel.

A solution of 5 grams of cobalt aluminate spinel (CoAl.sub.2 O.sub.4), 10 grams of titanium resinate (containing 4.2 weight percent titanium calculated as the metal), and 30 grams of toluene was applied to the coupon. The individual coats were brushed on. Coats 1 through 4 and 6 through 9 were heated at a rate of 50.degree.C. per 5 minutes to a temperature of 400.degree.C. and held at the temperature for 10 minutes. Coats 5 and 10 were heated at a rate of 50.degree.C. per 5 minutes to a temperature of 500.degree.C. and held at the temperature for 10 minutes.

The electrode was then utilized as the anode in a beaker chlorate cell as hereinbefore described. At a current density 500 amperes per square feet and a temperature of 43.5.degree.C., the initial voltage was 3.15 volts. After 413 hours the voltage was 3.50 volts.

EXAMPLE IV

An electrode was prepared having a cobalt aluminate spinel surface on a titanium metal substrate with an intervening ruthenium oxide layer therebetween.

A 2 inch by 2 inch by 1/16 inch titanium coupon was scrubbed with household cleanser and rinsed with distilled water and with acetone. It was then dipped in 1 percent hydrofluoric acid solution at room temperature for 1 minute. The coupon was then etched in 37 percent hydrochloric acid solution. The etching temperature varied from 45.degree.C. to 55.degree.C. After etching in the hydrochloric acid solution, the titanium coupon was immersed in running distilled water for 2 minutes and air dried.

The resulting etched titanium metal coupon was then immersed in a solution comprising 10 grams of ruthenium nitroso chloride, 21.5 grams of sulfuric acid, and sufficient distilled water to make one liter of solution. The coupon was subjected to a current density of 18 amperes per square foot for 8 minutes.

The coupon was then coated with a solution comprising 1 gram of cobalt aluminate spinel (CoAl.sub.2 O.sub.4), prepared according to the procedure described in Example III, 2 grams of titanium resinate, containing 4.2 percent of titanium calculated as the metal, and 6 grams of toluene. Five coats were brushed on. Coats 1 to 4 were heated at a rate of 50.degree.C. per 5 minutes to a temperature of 400.degree.C. and maintained at 400.degree.C. for 10 minutes. The fifth coat was heated at a rate of 50.degree.C. per 5 minutes to a temperature of 550.degree.C. and held at 550.degree.C. for 15 minutes.

The resulting electrode having a cobalt aluminate spinel surface and titanium metal substrate with an intermediate ruthenium oxide layer therebetween was utilized as the anode in the beaker chlorate cell as hereinbefore described. It gave a cell voltage of 3.52 volts at a current density of 500 amperes per square foot and an electrolyte temperature of 45.degree.C.

Although this invention has been described with particular reference to anodes for electrolysis of aqueous alkali metal chloride solutions, it is not limited to such use. The anodes herein contemplated may be used in electrochemical reactions wherever a corrosion-resistant anode or at least one having long life is desired. Thus, the electrolyte in the cell may be a salt of a metal which may be electrodeposited and this electrolyte electrolyzed between the spinel surface anode and a cathode to electrodeposit the metal on the cathode. Copper, nickel, iron, manganese, and the like may be so deposited in these salts. The electrolytic oxidation of organic compounds, e.g., propylene to propylene oxide or propylene glycol, may be performed using such anodes. In each case, the cell comprises the spinel surface anode having an intermediate palladium, ruthenium, or rhodium oxide layer herein contemplate, a cathode, and means to establish an external voltage or electromotive force between the anode and cathode whereby the anode is positively charged with reference to the cathode. Moreover, metal structures such as ships' hulls may be cathodically protected using these anodes.

It is to be understood that although the invention has been described with specific reference to specific details of particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

Claims

I claim:

1. An electrode comprising:

an electroconductive substrate;

an electroconductive surface comprising a spinel; and

a layer between and in contact with said substrate and said surface, said layer comprising an oxygen-containing compound of a second transition series platinum group metal.

2. The electrode of claim 1 wherein the substrate is a valve metal.

3. The electrode of claim 1 wherein the spinel is selected from the group consisting of CoAl.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, CuAl.sub.2 O.sub.4, CuCo.sub.2 O.sub.4, CuCr.sub.2 O.sub.4, CuFe.sub.2 O.sub.4, Fe.sup.II Fe.sup.III.sub. 2 O.sub. 4, Cr.sup.II Cr.sup.III.sub.2 O.sub.4, Mn.sup.II Mn.sup.III.sub. 2 O.sub.4, Co.sup. II Co.sup.III.sub. 2 O.sub.4, Fe.sup.II AlFe.sup.III O.sub.4, MgFe.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, NiCr.sub.2 O.sub.4, and mixtures thereof.

4. The electrode of claim 1 wherein the oxygen-containing compound of a second transition series platinum group metal is selected from the group consisting of oxides of the second transition series platinum group metals, alkali earth oxides of the second transition series platinum group metals, rare earth oxides of the second transition series platinum group metals, and spinels of the second transition series platinum group metals.

5. The electrode of claim 1 wherein the layer comprising an oxygen-containing compound of a second transition series platinum group metal is from about 2 to about 10 micro-inches thick.

6. In an electrolytic cell for the electrolysis of aqueous alkali metal chloride solutions and having a cathode and an anode and means for imposing an electromotive force therebetween, the improvement which comprises:

the said anode comprising an electroconductive substrate;

an electroconductive surface comprising a spinel; and

an intermediate layer comprising an oxygen-containing compound of a second transition series platinum group metal between and in contact with the substrate and the electroconductive surface.

7. The electrolytic cell of claim 6 wherein the oxygen-containing compound of a second transition series platinum group metal is selected from the group consisting of oxides of the second transition series platinum group metals, alkali earth oxides of the second transition series platinum group metals, rare earth oxides of the second transition series platinum group metals, and spinels of the second transition series platinum group metals.

8. A process for the electrolysis of aqueous alkali metal chlorides comprising:

causing an electric current to pass to a cathode from an anode having an electroconductive substrate, an outer surface comprising a spinel, and a layer comprising an oxygen-containing compound of a second transition series platinum group metal between and in contact with the substrate and the outer surface.

9. The method of claim 8 wherein the oxy-compound of a second transition series platinum group metal is selected from the group consisting of oxides of the second transition series platinum group metals, alkali earth oxides of the second transition series platinum group metals, rare earth oxides of the second transition series platinum group metals, and spinels of the second transition series platinum group metals.