CN120077166A

CN120077166A - Life-prolonging anode coating

Info

Publication number: CN120077166A
Application number: CN202380061714.0A
Authority: CN
Inventors: D·迪弗兰科; D·考尔菲尔德; T·博迪; G·怀特菲尔德
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 2022-08-24
Filing date: 2023-08-24
Publication date: 2025-05-30
Also published as: WO2024044701A3; KR20250056936A; JP2025528889A; EP4577684A2; WO2024044701A2

Abstract

本发明提供了一种阳极，所述阳极包含芯基板，所述芯基板包括多层涂层，所述多层涂层具有包括钯的基底层，所述基底层直接涂覆所述基板。The present invention provides an anode comprising a core substrate, wherein the core substrate comprises a multi-layer coating having a base layer comprising palladium, wherein the base layer directly coats the substrate.

Description

Life-prolonging anode coating

Cross Reference to Related Applications

The present application claims the benefit of priority from U.S. provisional application No. 63/400,668 filed on month 8 of 2022, 24, which is incorporated herein by reference in its entirety.

Technical Field

The present disclosure relates generally to electrode coatings on substrates intended to operate as anodes in electrochemical processes, referred to herein as anode coatings.

Background

Many commercial manufacturing processes employ electrochemical techniques. For example, chloralkali processes electrolyze aqueous solutions of sodium chloride or potassium chloride to form valuable commodity materials such as chlorine, sodium hydroxide (caustic soda) or potassium hydroxide and hydrogen. The water generates hydrogen and oxygen by electrolysis. Other electrochemical processes are used to prepare various commodity chemicals and intermediates for the chemical and pharmaceutical industries. Currently, commercial electrochemical processes strive to reduce energy consumption, reduce manufacturing costs, and improve electrode efficiency and durability.

A particular electrochemical process in the field of the invention described herein is a process in which chloride salts are present in solution and in which chlorine or hypochlorite is the predominant product. Such processes include chlor-alkali membrane cell and diaphragm cell processes, chlorate production, and hypochlorite production in dilute to concentrated brine for disinfection purposes. The use and composition of the gas evolving electrode can create different problems and results than other electrode uses such as batteries.

Most conductive materials can be used as electrodes. Preferably, the materials used to make the electrodes are resistant to corrosion by the electrolyte and/or the resulting products. Many other suitable electrode materials lack the ability to effectively catalyze the transfer of electrons to the electrolyte, which requires the use of additional power. And the greater the additional power used, the greater the cost of performing the electrochemical process. A coating may be applied to the electrode to promote electron transfer and reduce the overpotential required in the electrolytic process. Thus, the coating helps reduce the overall operating voltage and power consumption of the electrolytic process. Additional details regarding electrode coatings are described in International application No. PCT/US2020/037426, filed on 6/12/2020, which is incorporated herein by reference in its entirety.

Anode coatings known in the art have a limited lifetime and contain noble metals. The function of the anode coating is to reduce the voltage required for oxidation by acting as an electrocatalyst and protecting the substrate, thus imparting geometric stability to the anode. The anodic coating for oxidation of chloride ions provides a lower overpotential for oxidation of chloride to chlorine gas.

Anode coatings fail when the coating itself wears away over time, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes under the coating to form a passivation layer. Once the coating fails, the voltage rises rapidly and continued operation may result in heating or damage to the process equipment. In general, the lifetime of the coating is proportional to the precious metal loading of the coating and inversely proportional to the square of the current density in the cell. Iridium and ruthenium are the primary noble metals used in the prior art invention. Iridium is commonly used for anodes that operate for more than 4 years at current densities greater than about 3kA/m ², but iridium is much more expensive than ruthenium.

It is desirable to develop anodes with improved durability, reduced overpotential and/or extended operating life.

Disclosure of Invention

In an embodiment of the invention, the preferred anode substrate is a valve metal, in particular titanium or an alloy thereof. One advantage of embodiments of the present invention is that it provides a coating on a substrate that exhibits a lower overpotential than prior art coatings, thereby reducing the power requirements of the process.

Embodiments of the present invention achieve a longer lifetime at a given precious metal loading of the coating than would be expected. An embodiment of the invention is characterized in that long-life anode coatings with a current density of more than 3kA/m ² can be produced without the use of iridium. When such an exemplary anode coating is applied to a chlorate cell, chlorine generated at the anode will immediately form hypochlorite ions in the solution, while hydrogen generated at the cathode will leave the cell. A particular problem with chlorate cells is that oxygen is generated at the anode by the decomposition of hypochlorite in the catalytic solution or oxygen on the electrocatalytic coating. The coatings of the embodiments of the invention described herein achieve exceptionally low oxygen levels in hydrogen from chlorate cells. This is achieved by reducing the evolution of electrochemical oxygen, in particular on the newly activated electrode, and by avoiding contamination of the solution containing hypochlorite in the electrolyzer with impurities which catalyse the decomposition of hypochlorite to oxygen.

In the anode coating of the embodiments of the present invention, a coating formulation containing titanium chloride or titanium oxychloride is produced in an aqueous/alcoholic solution containing hydrochloric acid. In certain embodiments, titanium alkoxides may be used in combination with pre-bake, secondary/tertiary alcohols, and/or oxidizing agents to achieve the same effect as titanium oxychloride or titanium chloride. Salts of ruthenium, palladium and optionally platinum and iridium are also dissolved in the solution, preferably in the form of chloride salts. Optionally, a chloride salt of a transition metal element may be added.

In a preferred embodiment of the present invention, in preparing the inventive coating comprising palladium, the various factors, alone and in combination with two or more factors, surprisingly contribute to extending the lifetime of an anode having the inventive anode coating:

1. primary alcohols are avoided in the preparation of the coating solutions.

2. The titanium surface of the titanium or titanium alloy substrate is pre-oxidized or pre-baked.

3. In preparing the coating solution, the presence of the peroxide brings both titanium and ruthenium in the +4 oxidation state in other embodiments, and in preparing the coating solution, other oxidants may be used in place of or in addition to the peroxide to produce the +4 oxidation state of titanium and ruthenium, including nitric acid, chromates, halogens, chlorine dioxide, chloric acid, and/or ozone, among others. Thus, where reference is made to the use of peroxides as disclosed herein, it is to be understood that other oxidizing agents that may be used are also contemplated.

4. Titanium is preferably used as the titanium oxychloride solution. However, in embodiments, titanium alkoxides may be used in combination with pre-bake, secondary/tertiary alcohols, and/or oxidizing agents to achieve the same effect as titanium oxychloride or titanium chloride.

5. Tin is avoided from being used with palladium to avoid the formation of PdSn2 which behaves like a metal. In other embodiments, it is desirable to avoid forming PdSn4 compounds that behave like metal alloys and form phases that are separate from the rutile phase of the coating.

In an embodiment considered unique, hydrogen peroxide is optionally added to the mixed salt anodic coating solution in an amount that increases the oxidation potential of the coating and does not reduce the palladium to a metallic state during the coating procedure. Another optional ingredient in the coating solution is a secondary and/or tertiary alcohol, preferably isopropanol (2-propanol). In other embodiments, 2-butanol and/or t-butanol (t-butanol) is another optional component of the coating solution. A significant feature of the coatings of embodiments of the invention is that the molar ratio of titanium to noble metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of the other noble metals is between about 0.04 and 0.3.

Importantly, in embodiments of the present invention, the anode coating comprises titanium, ruthenium, and palladium, wherein the palladium is distributed in a fine scale, wherein all three metals are of the same crystal structure, and palladium is avoided from being single phase and separated as in prior art coatings comprising palladium.

In preparing anode coatings for high current density applications, if the amount of iridium is reduced, the thickness of the coating must be increased and the wear rate must be reduced. The coating is applied in multiple layers with drying and baking steps between each layer. An advantage of embodiments of the present invention is that a coating with a low wear rate, which contains mainly ruthenium as noble metal, can be achieved using a fewer layer of coating.

Coating adhesion is typically measured by a tape test, in which a piece of clear tape is applied to the coated anode surface and then peeled off quickly and the removed coating is observed. Adhesive tape testing is typically assessed by appearance, but may also be assessed quantitatively using X-ray fluorescence measurements of the adhesive tape. Quantitative tape test results the most useful is to quantify based on the percentage of total coating removed. It is an object of embodiments of the present invention to achieve a coating with quantitative tape test results, wherein the tape removes less than 5%, preferably less than 2%, and more preferably less than 1% of the coating.

In other anode coating inventions of the prior art, it was found that palladium or platinum containing coatings reduce the voltage required for oxidation of chloride ions to chlorine or hypochlorite, thereby reducing the power required and also reducing the undesirable generation of oxygen. However, in this prior art, the ratio of palladium or platinum to the low cost ruthenium component in the coating is greater than 3:10, and the palladium and platinum are lost from the coating faster than the ruthenium. When Pd or Pt in the coating is lost, the voltage will rise. The invention has the advantage that smaller amounts of Pd or Pt can be used as effective additives for the coating and maintain lower voltages for longer periods of time.

The anode substrate in the embodiments of the present invention is prepared by methods known in the art to roughen and etch the surface to remove oxide and embedded grit. Preferably, the light oxide film is then restored by baking at a temperature of 400 to 550C for a time sufficient to form orange to dark blue or light gray on the titanium surface, thereby rendering the substrate surface more hydrophilic.

The coating is then formed on the substrate by dip coating, roll coating, brush coating, spray coating, or electrostatic spray coating the substrate with the coating solution. The coating is completely dried, preferably at a temperature of less than about 110 ℃, and preferably at about 50 ℃, and then baked at a temperature of 400 ℃ to 550 ℃ for 10 to 20 minutes. Then, additional layers are applied by repeating the application, drying and baking processes.

Optionally, after final coating, a longer baking step may be used for an extended time, known as post baking.

The noble metals used in the coatings of the embodiments of the present invention are ruthenium, palladium, and optionally platinum and iridium. In the anode of the embodiment of the present invention, ruthenium is the main noble metal used in the coating, and the molar ratio of Pd to the total noble metal is preferably about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range has been shown to reduce the voltage for chlorine evolution while reducing oxygen evolution in the coating and while extending anode life. At higher levels of palladium, especially those proposed in the palladium coating examples of the prior art, rutile formation is reduced and the anatase phase is advantageous, thereby shortening the coating lifetime. Thus, in prior art coatings, the extended lifetime is not due to the presence of palladium in the coating, as higher levels of the coating promote anatase formation.

Regarding palladium salts and platinum salts, the problem not addressed in the prior art is that the palladium salts and platinum salts are more easily reduced to metallic form than ruthenium salts or iridium salts. Palladium and platinum can be reduced to metallic form when in intimate contact with a titanium metal substrate. Another aspect of embodiments of the present invention is to apply the coating after preparing the titanium surface by oxidizing (pre-baking) in air at elevated temperature and avoiding the use of primary alcohols. When the titanium oxide film is exposed to an acidic coating solution, it may be dissolved, and if dried at an elevated temperature, bare titanium may be exposed to the coating solution. By first pre-baking (pre-oxidizing) the substrate surface, the coating can effectively wet the surface and create an advantageous adhesion of the coating to the substrate surface.

In embodiments of the present invention, when the pre-bake conditions cause the titanium substrate to appear blue and the coating dries at a temperature of less than 110 ℃, and preferably at about 50 ℃, a palladium-containing coating may be formed without forming a metallic palladium phase. Alternatively, the preferred coating solutions of embodiments of the present invention contain some peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to a +4 state in solution. The reduced palladium or platinum metals in the coating are readily oxidized to soluble chloride salts in the process to which the embodiments of the invention are applied, and thus these metal phases in the coating shorten their lifetime. Thus, prior art palladium or platinum containing coatings do not achieve an extended lifetime when producing the metallic phase.

In further embodiments of the invention, the anodic coating may include iridium to promote rutile formation and extend the life of the coating. In coatings having iridium to ruthenium molar ratios greater than about 0.1, the prior art has determined that the coating lifetime is primarily a function of the iridium loading of the coating. Unexpectedly, the presence of palladium at a molar ratio of 0.04 to 0.3 relative to the total of ruthenium and iridium significantly increases the anode lifetime even in coatings having iridium to ruthenium molar ratios greater than 0.1, and the coatings of the embodiments of the invention described herein can achieve more than twice the lifetime of prior art coatings having a total iridium-like loading.

In embodiments of the present invention, additional dopants selected from transition metals such as Fe, ni, or Co may be added to the coating because these metals are known to promote rutile formation and are known in the art to increase the conductivity of the coating. However, the anode coating of embodiments of the present invention may be produced in the absence of dopants. While increasing conductivity, the addition of dopants has not been found to increase the life of the coating. Furthermore, in chlorate-generating applications, nickel and cobalt are known to catalyze the decomposition of hypochlorite to oxygen, and therefore these dopants are not present in the coating for chlorate generation.

In embodiments of the disclosed invention, it has been found that the use of titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, particularly when a portion of the solvent of the coating is an alcohol and the coating is completely dry at an air temperature of less than 110 ℃. It has been found that the coating solution has an optimal titanium oxychloride content of 0.25 to 5 mass% titanium, and an optimal peroxide content of 0.1 to 2.0 molar ratio of peroxide to titanium, and an optimal alcohol content of 5 to 75 mass% of the coating solution. The alcohol is preferably a secondary and/or tertiary alcohol, preferably isopropanol (2-propanol), to avoid reaction with the peroxide before the solvent evaporates. In other embodiments, 2-butanol and/or t-butanol (t-butanol) is another optional component of the coating solution. In an embodiment, the alcohol may be replaced with another water-soluble and volatile organic solvent if compatible with the oxidizing properties of the salt.

The invention makes it possible to realize a breakthrough in performance and to reduce the manufacturing costs of all the plating materials used in chlor-alkali processes and chlorate production. Another broad field of application is hypochlorite generators for swimming pool disinfection, municipal water treatment, wastewater treatment or bilge water disinfection. The potential use is the global market.

Other features and iterations of the present invention are described in more detail below.

Detailed Description

When introducing elements of the embodiments(s) described herein, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

One aspect of the present disclosure encompasses an anode comprising (a) a core substrate, preferably a valve metal, such as titanium and alloys thereof, and (b) a coating prepared, applied to and adhered to the core substrate, the core substrate comprising titanium (Ti), ruthenium (Ru) and palladium (Pd), and optionally in embodiments platinum and/or iridium. The coating is prepared to avoid palladium segregation and in a single phase as in prior art coatings, but the palladium is well distributed on a fine scale with Ti-Ru-Pd in the same crystal structure, i.e. the palladium is well dispersed throughout the coating.

Embodiments of the present invention relate to electrode coatings on substrates intended to operate as anodes in electrochemical processes, referred to herein as anode coatings. A particular electrochemical process in the field of embodiments of the invention is a process in which chloride salts are present in solution and in which chlorine or hypochlorite is the primary product (however, other fields, and indeed, the global market may benefit from the invention). Such processes include chlor-alkali membrane cell and diaphragm cell processes, chlorate production, and hypochlorite production in dilute to concentrated brine for disinfection purposes. Preferred substrates are valve metals, in particular titanium or alloys thereof. Anode coatings known in the art have a limited lifetime and contain noble metals. The function of the anode coating is to reduce the voltage required for oxidation by acting as an electrocatalyst and protecting the substrate, thus imparting geometric stability to the anode. The anodic coating for oxidation of chloride ions provides a lower overpotential for oxidation of chloride to chlorine gas. One advantage of embodiments of the present invention is that it provides a lower overpotential than prior art coatings, thereby reducing the power requirements of the process. Anode coatings fail when the coating itself wears away over time, loses conductivity, loses adhesion to the substrate, or the substrate oxidizes under the coating to form a passivation layer. Once the coating fails, the voltage rises rapidly and continued operation may result in heating or damage to the process equipment. In general, the lifetime of the coating is proportional to the precious metal loading of the coating and inversely proportional to the square of the current density in the cell. Embodiments of the present invention achieve a longer lifetime at a given precious metal loading of the coating than would be expected. Iridium and ruthenium are the primary noble metals used in the prior art invention. Iridium is commonly used for anodes that operate for more than 4 years at current densities greater than about 3kA/m2, but iridium is much more expensive than ruthenium because iridium is rare and less abundant in the crust. An embodiment of the invention is characterized in that long-life anode coatings with a current density of more than 3kA/m ² can be produced without the use of iridium. When such an anode coating is applied to a chlorate cell, chlorine generated at the anode will immediately form hypochlorite ions in the solution, while hydrogen generated at the cathode will leave the cell. A particular problem with chlorate cells is that oxygen is generated at the anode by the decomposition of hypochlorite in the catalytic solution or oxygen on the electrocatalytic coating. The coatings of the embodiments of the present invention achieve exceptionally low oxygen levels in hydrogen from chlorate cells. This is achieved by reducing the evolution of electrochemical oxygen, in particular on the newly activated electrode, and by avoiding contamination of the solution containing hypochlorite in the electrolyzer with impurities which catalyse the decomposition of hypochlorite to oxygen.

In the anode coating of the present invention, the coating formulation contains titanium chloride or titanium oxychloride in a water/alcohol solution containing hydrochloric acid. Salts of ruthenium, palladium and optionally platinum and iridium are also dissolved in the solution, preferably in the form of chloride salts. Optionally, a chloride salt of a transition metal element may be added. An embodiment of the invention is unique in that hydrogen peroxide is optionally added to the solution of the mixed salt in an amount that increases the oxidation potential of the coating and does not reduce the palladium to a metallic state during the coating procedure. Another optional ingredient in the coating solution is a secondary and/or tertiary alcohol, preferably isopropanol (2-propanol). In other embodiments, 2-butanol and/or t-butanol (t-butanol) is another optional component of the coating solution. A significant feature of the coatings of embodiments of the invention is that the molar ratio of titanium to noble metals (including ruthenium, palladium, platinum, and iridium) is between 3 and 5, and the molar ratio of palladium to the sum of the other noble metals is between about 0.04 and 0.3. The substrate is prepared by methods known in the art to roughen and etch the surface to remove oxide and embedded grit. Preferably, the light oxide film is then restored by baking at a temperature of 400 to 550 ℃ for a time sufficient to form an orange to dark blue or light grey film on the titanium surface, thereby rendering the surface more hydrophilic. The coating is then formed by dipping, rolling, brushing, spraying or electrostatically spraying the substrate with the coating solution. The coating is completely dried, preferably at a temperature of less than about 110 ℃, and more preferably at about 50 ℃, and then baked at a temperature of 400 to 550 ℃ for 10 to 20 minutes. Then, additional layers are applied by repeating the application, drying and baking processes. Optionally, after final coating, a longer baking step may be used for an extended time, known as post baking.

The noble metals used in the coating are ruthenium, palladium, and optionally platinum and iridium. In the anode of the embodiment of the present invention, ruthenium is the main noble metal used in the coating, and the molar ratio of Pd to the total noble metal is preferably about 0.04 to 0.3, preferably about 0.12. Palladium in the optimal range has been shown to reduce the voltage for chlorine evolution while reducing oxygen evolution in the coating and while extending anode life. At higher levels of palladium, especially those proposed in the palladium coating examples of the prior art, rutile formation is reduced and the anatase phase is advantageous, thereby shortening the coating lifetime. Thus, in prior art coatings, the extended lifetime is not due to the presence of palladium in the coating, as higher levels of the coating promote anatase formation.

Regarding palladium salts and platinum salts, the problem not addressed in the prior art is that the palladium salts and platinum salts are more easily reduced to metallic form than ruthenium salts or iridium salts. Palladium and platinum can be reduced to metallic form when in intimate contact with a titanium metal substrate. Another aspect of embodiments of the present invention is to apply the coating after preparing the titanium surface by oxidation (pre-bake) in air at elevated temperature. When the titanium oxide film is exposed to an acidic coating solution, it may be dissolved, and if dried at an elevated temperature, bare titanium may be exposed to the coating solution. In embodiments of the present invention, when the pre-bake conditions cause the titanium substrate to appear blue and the coating dries at a temperature below 110 ℃, a palladium-containing coating may be formed without forming a metallic palladium phase. Alternatively, the preferred coating solution contains some peroxide. The peroxide forms a stable complex with titanium and oxidizes ruthenium to a +4 state in solution. The reduced palladium or platinum metals in the coating are readily oxidized to soluble chloride salts in the process to which the embodiments of the invention are applied, and thus these metal phases in the coating shorten their lifetime. Thus, prior art palladium or platinum containing coatings do not achieve an extended lifetime when producing the metallic phase.

In coatings of embodiments of the present invention, iridium may be used to promote rutile formation and extend the life of the coating. In coatings having iridium to ruthenium molar ratios greater than about 0.1, the prior art has determined that the coating lifetime is primarily a function of the iridium loading of the coating. Unexpectedly, the presence of palladium at a molar ratio of 0.04 to 0.3 relative to the total of ruthenium and iridium significantly increases the anode lifetime even in coatings having iridium to ruthenium molar ratios greater than 0.1, and the coatings of embodiments of the invention can achieve more than twice the coating lifetime of prior art coatings having a total iridium-like loading. Additional dopants selected from transition metals such as Fe, ni, or Co may be added to the coating because these are known to promote rutile formation and are known in the art to increase the conductivity of the coating. However, the anode coating of embodiments of the present invention may be produced in the absence of dopants. These dopants have been found not to increase the coating lifetime. Furthermore, in chlorate-generating applications, nickel and cobalt are known to catalyze the decomposition of hypochlorite to oxygen, and therefore these dopants are not present in the coating for chlorate generation. In embodiments of the present invention, it has been found that the use of titanium in the form of titanium oxychloride in combination with hydrogen peroxide increases rutile formation, particularly when a portion of the solvent of the coating is an alcohol and the coating is completely dry at an air temperature of less than 110 ℃. It has been found that the coating solution has an optimal titanium oxychloride content of 0.25 to 5% titanium, and an optimal peroxide content of 0.1 to 2.0 peroxide to titanium molar ratio, and an optimal alcohol content of 5 to 75% of the coating solution. The alcohol is preferably a secondary and/or tertiary alcohol, such as 2-butanol and/or tertiary butanol, to avoid reaction with the peroxide prior to evaporation of the solvent. It is obvious that these alcohols can be replaced by another water-soluble and volatile organic solvent if compatible with the oxidizing properties of the salt.

The anodic coating of the present invention achieves longer life in accelerated wear testing by a previously unexpected mechanism. One explanation for this behavior is that the peroxide-titanium complex has a surprising stability, survives the drying process, and affects the phase behavior of the oxide coating formed during baking. These oxides are converted to the rutile structure at lower bake temperatures than do coatings without peroxide. It has also been demonstrated that the coating lifetime is significantly prolonged in proportion to the palladium content of the coating.

The lifetime of an oxide coating formed with a solution containing hydrogen peroxide appears to be 2 to 8 times the lifetime of a similar coating without the addition of hydrogen peroxide or palladium. One theory that may explain this longer lifetime is that the oxygen to metal ratio of the prior art coating is close to 2:1, rather than at least a 2.5:1 ratio as in the present invention. During electrolysis, particularly under conditions where oxygen may volatilize, some oxygen may enter the surface of the coating, forming a crystalline structure, with excess oxygen being greater than the volume of the underlying coating. Thus, in prior art anode coatings, severe mechanical stresses build up on the surface of the coating, which can be relieved only with oxygen evolution catalysts such as iridium, which would otherwise gradually lose the coating over time due to damage. In the present invention, an excess of oxygen is already present in the coating and thus no stress is generated by oxygen evolution, since the surface of the coating cannot absorb more oxygen.

The coating of the embodiments of the present invention is characterized in that it comprises a mixed solid solution of titanium oxide, ruthenium oxide and palladium oxide, predominantly in the form of rutile crystals. The prior art coatings contain anatase crystals and some also contain significant amounts of precious metals in the oxide or metal phase alone, not in solid solution with rutile. It has been determined in research that these alternating phases, if present, wear faster and disappear before the rutile phase of the coating.

In embodiments of the present invention, it has been found that when the crystalline form of the coating contains more than about 80% rutile, the coating wear rate is unexpectedly greatly reduced, and even lower when the coating contains more than about 85% rutile. This is achieved in a coating having a specific ratio of titanium to ruthenium and palladium to ruthenium, and rutile formation is enhanced when peroxide (and/or potentially other oxidants) is present in the coating. In prior art coatings, it has been widely accepted that the optimal molar ratio of titanium to noble metal is between 1.5 and 2.5, but experiments have shown that at these molar ratios titanium is insufficient to form a solid solution of noble metal in the rutile phase and separate phases in the form of RuO2 or RuO2+ IrO2—single electrode potential (SEP) tests, which have shown that such separate phases would reduce the single electrode potential of oxygen evolution, thereby increasing the faraday inefficiency of oxygen evolution of the coating, which is undesirable in applications where products of chlorine, hypochlorite and chlorate are desired. In addition, in coating formulations containing palladium, when the molar ratio of titanium to noble metal exceeds about 5, a separate titanium anatase phase is formed. Noble metal oxides with less titanium have a faster loss than rutile, resulting in faster wear of the coating. The anatase phase of titanium is also more rapidly lost than rutile, resulting in faster coating wear.

Example Palladium coating formulation

Example 1

Multivariate tests (MVTs) were designed to evaluate and optimize some of the uncertain aspects of chlorate coating formulation. Variables considered for the MVT include the ratio of titanium to noble metal (ruthenium, palladium) (Ti ratio), the ratio of palladium to ruthenium (Pd ratio), the pre-bake temperature and the post-bake temperature. A customized experimental design was created using software that accounts for all expected non-linear and bi-directional interactions of the variables described above. The operation is performed by depositing an aqueous solution of noble metal chloride and titanium oxychloride on a titanium metal plate. As shown in table 1, the following 12 rounds of experimental design were performed, in which most potential interactions and first order effects were orthogonal, but not fully balanced:

TABLE 1 MVT test run

In the experimental design, there were seven unique coating formulations in which the molar ratio of palladium to ruthenium was varied in three options 0.02, 0.06 and 0.1, the molar ratio of titanium to noble metal (combination of palladium and ruthenium) was varied in three options 2.6, 3.8 and 5, the pre-bake temperature was varied between 420 and 490 ℃, and the post-bake temperature was varied between 490 and 525 ℃.

As a result, formulations B, C, F and G are examples of embodiments of the invention, exhibit favorable wear properties, and the remaining formulations A, D and E are counter examples.

All coating formulations were made with the same concentration of ruthenium metal in solution, 34 g/l, and the dopant concentrations (Ni, fe and Co) were all set at 1 g/l. The palladium and titanium contents vary according to the formulation, and thus the molar ratios of the coating components in the formulation are as follows. Subsequent experiments will reveal that the molar ratio of hydrogen peroxide is important for the success of the coating, and thus these ratios are included in table 2 below.

Table 2. Coating metal composition (in mole%) for each chlorate MVT formulation.

Formulation of	Ru	Ti	Pd	Fe	Ni	Co	H₂O₂:Ru	H₂O₂:Ti
									A	26.15%	69.36%	0.52%	1.32%	1.32%	1.32%	1.01	0.38
B	19.81%	76.79%	0.40%	1.00%	1.00%	1.00%	1.01	0.26
									C	15.95%	81.32%	0.32%	0.80%	0.81%	0.80%	1.01	0.20
D	25.20%	69.46%	1.51%	1.27%	1.28%	1.27%	1.01	0.36
									E	24.32%	69.56%	2.43%	1.23%	1.23%	1.23%	1.01	0.35
F	18.41%	76.96%	1.84%	0.93%	0.93%	0.93%	1.01	0.24
									G	14.81%	81.46%	1.48%	0.75%	0.75%	0.75%	1.01	0.18

Each coating formulation had equal weight concentrations of ruthenium, hydrogen peroxide, and dopant metal salts of Fe, ni, and Co. Finally, hydrochloric acid acts as a stabilizer in the titanium solution, so the HCl content in each coating solution will vary with the titanium ratio, although each formulation contains a separate addition of 4.32 wt% aqueous HCl.

According to XRF, different coating formulations are applied to give approximately the same total loading of metallic ruthenium, requiring 8-13 impregnations. Coating solutions with higher titanium concentrations have higher formulation viscosities and thus require fewer layers to achieve the desired minimum loading of 500ug/cm 2. The average weight gain per impregnation was found to vary with the coating formulation, and mainly with the titanium concentration.

It will be appreciated that the coating composition is determined by methods known in the art, including non-destructive determination by XRF (X-ray fluorescence spectroscopy) or using electron microscopy with EDS (energy dispersive X-ray spectroscopy).

Representative preparation and application procedures for plating formulation a are summarized below.

Surface preparation

Preliminary surface treatment experiments performed on titanium plates established optimal preparation, including a combination of light sand blasting with fine sand and oxalic acid etching. For chlorate MVT, duplicate samples were applied for each of the 12 design runs, yielding a total of 24 flat chlorate anode samples. A representative description of the preparation of one of them is described in detail. The 4"x 0.025" titanium plate (grade 2) was blasted with 220 alumina blasting media. The nozzle size was 6mm and the grit blast pressure was set at 25psi. To ensure a uniform surface, the spray gun sprays six times on each side at an angle of 60 to 90 degrees. The sandblasted plate was then rinsed with DI water and dried, after which it was etched with 20gpl oxalic acid dihydrate at 80 ℃ for 1-1.5 hours. After etching, the plate was rinsed again with DI water, where grey oxide was observed to have been removed, and then pre-baked for 20 minutes at 420 ℃ or 490 ℃ to produce a yellow or dark blue surface, respectively. The sample was then dip coated on one side. After pre-baking, a thin protective layer of TiO2 is formed, and under ideal storage conditions, the shelf stability of the titanium substrate can be as long as at least one month, even indefinitely.

Preparation of coating solution

Coating solution a specifies a molar ratio of titanium metal to palladium and ruthenium metal of 2.6 and a molar ratio of palladium metal to ruthenium metal of 0.02. Formulation a was used to coat 4 anodes, thus targeting 300g of coating solution. Distilled water (133.61 g) was mixed with aqueous HCl (36% assay, 36.01 g), followed by addition of cobalt (II) chloride hexahydrate (24.6% Co assay, 1.22 g), iron (III) chloride hexahydrate (20.29% Fe assay, 1.48 g) and then nickel (II) chloride hexahydrate (37.18% Ni assay, 0.814 g). Ruthenium (III) chloride hydrate (40.88% Ru assay, zhuang Xinmo Feng (Johnson Matthey), 24.91 g) was then added followed by a solution of titanium oxychloride (14.08% Ti assay, 34.5% HCl, kang Nuosi company (Kronos), 91.01 g). Note that the titanium oxychloride solution contains HCl for stabilization, so for formulation a the total HCl content is about 14.8 wt% (4.32% from aqueous HCl and 10.47% from TiOCl 2). The resulting solution was stirred until all solids were significantly dissolved, about 1 hour, but in some cases the solution was stirred overnight. Hydrogen peroxide (32% assay, 10.73 g) was then added. After the peroxide addition, care was taken to periodically vent the solution bottles. The solution was stirred for at least one hour before palladium (II) chloride (59.7% Pd assay, 1.81 g) was added to ensure peroxide reaction. As a good practice, after all ingredients are added, the coating solution should be stirred for at least one hour before impregnation, and the coating solution should be stirred between impregnations.

Dip coating procedure

For dip coating the anode substrate, a Pyrex glass container or similar container of sufficient size is selected to pave the sample. Desirably, the container is airtight with a rubber gasket to prevent evaporation. A sufficient amount of coating solution is added so that the substrate can be completely immersed in the solution upon immersion, which typically requires a solution depth of at least 0.25-0.5 ".

Two holes were drilled along the top and bottom edges within the boundary of 0.5 "prior to coating the flat substrate. Ideally, the holes are centered so as to suspend the sample vertically in a balanced manner. Note that titanium wires are used to suspend the anode sample during impregnation to prevent corrosion and contamination. The first layer was applied by hanging the sample above the dipping vessel and then gently immersing the bottom edge of the plate in the solution, after which the plate was laid flat and completely immersed. It is important to avoid touching the sample, and it is preferable to manipulate the sample through titanium wire. Furthermore, it is critical to move slowly and smoothly to avoid the generation of bubbles. After the sample is immersed, the above action is reversed and the sample is slowly lifted to a vertical position, after which the bottom edge of the sample is lifted off the solution. The sample was placed over the dipping vessel while draining excess solution and then hung on a titanium rack to air dry for 20 minutes in the well ventilated designated area, although subsequent experiments showed that 40-60 minutes was ideal. The samples were then dried in an oven at 110 ℃ for 20 minutes, after which the temperature was raised to 490 ℃ and baked for an additional 20 minutes. The sample is carefully removed, preferably with pliers and heat resistant gloves, and cooled to room temperature. It was found that the samples coated in the formulations with higher titanium ratios had a strip of loose oxide powder along the bottom edge of the titanium plate, which was removed by brushing. The sample is then rotated 180 degrees and hung with opposite edges. The above impregnation, drying and baking steps were repeated until the minimum loading of ruthenium was 500 μg/cm2 as measured by XRF. After achieving the target load, the samples were dried and then baked at the specified post bake temperature of 490 ℃ or 525 ℃ for 2 hours.

The presence of palladium in the base layer increases anode life

In addition to the anode physical properties that were found to be improved by the inclusion of palladium in the improved anode coating described above, the examples of the invention described in the examples that follow surprisingly show that the inclusion of palladium in the coating, particularly in the base layer adjacent the anode substrate, provides increased anode lifetime (see accelerated lifetime test results in table 3). It is conventionally thought that it is most beneficial to include life-extending materials such as palladium in the outermost layer of the coating. However, the coatings and tests of the present invention determined that the inclusion of palladium in the innermost layer or layers adjacent to the anode substrate was more beneficial than the inclusion of palladium in the outermost layer, including even where no palladium was present in the outermost layer. Thus, a key finding of the present invention is that palladium included in the base layer of the multilayer coating provides improved anode life even without palladium in the outermost layer. The palladium in the innermost layer of the anode coating may interact with the anode substrate with a synergistic effect, resulting in a longer duration (i.e., longer lifetime) of the substrate. For example, when a titanium anode substrate is present with palladium in the innermost (base) coating of the multilayer coating, a titanium/palladium alloy may be gradually formed that alters the characteristics of the individual titanium substrate to increase the anode service life, including anodes preferably used to generate gases.

Example 2 (counter example)

Titanium mesh samples were prepared by sand blasting, etching and pre-baking. Coating solution "Z" was prepared using isopropanol containing 0.24% titanium, 0.22% ruthenium and 0.24% iridium with 5.1% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (trade name Tyzor TPT), an alcoholic solution containing 16.8% titanium, ruthenium (III) chloride hydrate crystals containing 40.9% ruthenium, iridium (IV) chloride dihydrate, an alcoholic solution containing 5.1% iridium, anhydrous HCl in isopropanol containing 22.6% HCl, and dry isopropanol for diluting the solution to the desired final concentration. The coating is applied by the following steps:

The web is immersed in the coating solution and then hung vertically, allowing excess coating to flow over the surface.

The coating was allowed to dry completely at 50 ℃ for typically 20 minutes and then baked at 490 ℃ for 20 minutes.

Steps 1-3 were repeated 9 times and the web was periodically flipped vertically. Final baking was performed for 40 minutes after the 9 th impregnation. The sample is labeled ID 13.

Example 3 (counter example)

The procedure of counter example 2 was followed except that the titanium mesh was prepared by sand blasting and washing. A total of 6 cycles were applied by the same procedure using a coating solution "Z x" of Iridium (IV) chloride dihydrate crystals containing 52.0% iridium. The sample is labeled ID 6.

Example 4 (counter example)

Titanium mesh samples with low palladium content in the substrate layer were prepared by sand blasting, etching and pre-baking.

Base layer coating solution "X" was prepared using isopropyl alcohol containing 0.40% titanium, 0.14% ruthenium, 0.16% iridium, and 0.027% palladium with 5.6% hydrochloric acid. The materials used to prepare this solution were tetrapropyl titanate (trade name Tyzor TPT), an alcoholic solution containing 16.8% titanium, ruthenium (III) chloride hydrate crystals containing 39.9% ruthenium, hydrogen (IV) hexachloroiridium (hci) hydride containing 39.2% iridium, anhydrous HCl in isopropanol containing 22.6% HCl, and dry isopropanol for diluting the solution to the desired final concentration. The base layer coating was applied following the procedure outlined in counter example 1 for a total of 4 cycles (ID 1) and 6 cycles (ID 22).

Top coat solution "Y" was prepared using the same materials, using isopropyl alcohol containing 0.40% titanium, 0.13% ruthenium, 0.15% iridium and 0.048% palladium with 5.6% hydrochloric acid. The top coat was applied for a total of 3 cycles (ID 1) and 5 cycles (ID 22), followed by a final bake of 2 hours.

Example 5

The procedure of counter example 4 was followed, except that only Tu Dingceng coating solutions "Y" were applied for a total of 7 cycles (ID 5) and 11 cycles (ID 14).

Example 6

The procedure of counter example 2 was followed, except that the Pd-containing coating solution "Y" was applied as the base layer for 5 cycles and the Pd-free coating solution "Z" was applied as the top layer for 4 cycles, followed by final baking at 490 ℃ for 40 minutes. The sample is labeled ID 34.

The coated anodes of examples 1-5 were subjected to accelerated life testing and XRF measurements were also performed to determine the average ruthenium loading for each sample. A comparison of the results from this test is summarized in table 3 below. In general, in the sample group with the same coating formulation, the number of AC lifetime hours increased with increasing total precious metal loading (ID 6 and ID 13, ID 1 and ID 22, ID 5 and ID 14). Notably, even if the top layer does not contain palladium, any amount of palladium in the base layer will extend the accelerated corrosion life, as is the case with ID 34. The coating lifetime is substantially prolonged in proportion to the palladium content, in particular in the coating close to the anode substrate.

Table 3. Comparative test of the effect of palladium on AC lifetime in anode coating.

Anode life effect by palladium together with peroxide for coating preparation

In further embodiments, it has been found that in industries where oxygen generation is detrimental, formulations with palladium benefit from the presence of both peroxide and prebaking, which can prevent unwanted oxygen generation while extending the lifetime of the anode. The results of examples 7-11 subsequently demonstrate the advantage of adding palladium to the coating of the gas generating anode.

Example 7 (counter example)

Titanium plate samples were prepared by sand blasting, etching and pre-baking. With 4.6 wt% titanium in the solution, the molar ratio of titanium metal to noble metal was 3.6, a coating solution was developed. Distilled water (71 g) was combined with aqueous HCl (23 g,36% assay) followed by the addition of a solution of titanium oxychloride (76 g,12.06% Ti assay, 16% HCl). Ruthenium (III) chloride hydrate (13 g,40.78% Ru assay) was then added. The resulting solution was stirred until all solids were significantly dissolved, for about 1 hour. Hydrogen peroxide (6 g,30% assay) was then added followed by isopropanol (10 g,99.5% assay).

The coating was applied to the prepared plate by the techniques previously described herein until a target ruthenium thickness of 500ug/cm2 was measured. Final baking was performed for 2 hours. The sample is labeled ID 2-1.

Example 8 (counter example)

The procedure of counter example 7 was followed except that peroxide was not added. The samples were labeled 2-3.

Example 9 (counter example)

The procedure of example 7 was reversed, with the following exceptions. The samples were not pre-baked. After ruthenium addition, the solution was stirred for at least one hour before palladium (II) chloride (0.7 g,59.71% Pd assay) and isopropanol (10 g,99.5% assay) were added. The sample was labeled 2-2-2.

Example 10

Titanium plate samples were prepared by sand blasting, etching and pre-baking. With 4.6 wt% titanium in the solution, the molar ratio of titanium metal to noble metal (Pd, ru) was 3.6 and the molar ratio of palladium metal to ruthenium metal was 0.08, a coating solution was developed. Distilled water (71 g) was combined with aqueous HCl (23 g,36% assay) followed by the addition of a solution of titanium oxychloride (76 g,12.06% Ti assay, 16% HCl). Ruthenium (III) chloride hydrate (13 g,40.78% Ru assay) was then added. The resulting solution was stirred until all solids were significantly dissolved, for about 1 hour. Hydrogen peroxide (6 g,30% assay) was then added. The solution was stirred for at least one hour before adding palladium (II) chloride (0.7 g,59.71% Pd assay) and isopropyl alcohol (10 g,99.5% assay) to ensure that the peroxide reacted.

The coating was applied to the prepared plate by the techniques previously described herein until a target ruthenium thickness of 500ug/cm2 was measured. Final baking was performed for 2 hours. Samples were labeled 2-0.

Example 11

The procedure of example 10 was followed except that peroxide was not added. The sample is labeled 2-2.

Examples 7-11 were evaluated for single electrode potential for chlorine and oxygen overvoltage and accelerated corrosion tests. A comparison of the results from these evaluations is summarized in table 4 below. Notably, the addition of palladium reduces oxygen production (by increasing oxygen overvoltage), extends lifetime, and reduces chlorine overvoltage (2-0 and 2-3). The use of peroxides in the absence of palladium is detrimental to the lifetime of the anode, in fact, in the absence of palladium the peroxide increases the probability of oxygen production (2-1). The lack of both peroxide and pre-bake in the presence of palladium is extremely detrimental to lifetime (2-2-2), in fact, in the absence of peroxide, pre-bake prevents reduction of palladium, thus a significant lifetime advantage is still observed (2-2).

Table 4.

Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention described herein.

Claims

1. An anode, comprising:

a core substrate comprising titanium or a titanium alloy, and

A coating having a molar ratio of titanium to noble metal of 3 to 5, wherein the noble metal comprises at least ruthenium and palladium, and a molar ratio of palladium to the sum of the other noble metals of 0.02 to 0.3.

2. The anode of claim 1, wherein the noble metal of the coating comprises iridium.

3. The anode of claim 2, wherein the titanium and the noble metal in the coating are in a crystalline structure, and wherein palladium is not in a single phase and is substantially dispersed throughout the coating.

4. The anode of claim 1, wherein the titanium and the noble metal in the coating are in a crystalline structure, and wherein palladium is not in a single phase and is substantially dispersed throughout the coating.

5. The anode of claim 1, wherein the coating is an innermost layer of a multilayer coating that directly contacts a surface of the core substrate.

6. The anode of claim 5, wherein the multilayer coating comprises an outermost layer that is free of palladium that does not directly contact the surface of the core substrate.

7. A method for preparing an anode includes mixing titanium, ruthenium, and palladium in a coating solution without using a primary alcohol in preparing the coating solution, and applying the coating solution to a surface of an anode substrate including titanium or a titanium alloy.

8. The method of claim 7, further comprising pre-baking the anode substrate surface of the titanium or titanium alloy prior to applying the coating solution.

9. The method of claim 7, further comprising mixing an oxidizing agent in the coating solution to place both titanium and ruthenium in a +4 oxidation state.

10. The method of claim 8, further comprising mixing an oxidizing agent in the coating solution to place both titanium and ruthenium in a +4 oxidation state.

11. The method of claim 10, wherein no tin is present in the coating solution.

12. The method of claim 9, wherein tin is absent from the coating solution.

13. The method of claim 8, wherein no tin is present in the coating solution.

14. The method of claim 7, wherein no tin is present in the coating solution.

15. A method for preparing an anode includes mixing titanium, ruthenium, and palladium in a coating solution, pre-baking an anode substrate surface of titanium or titanium alloy prior to applying the coating solution, and applying the coating solution to the anode substrate surface including titanium or titanium alloy.

16. A method for preparing an anode includes mixing titanium, ruthenium, palladium, and a peroxide sufficient to bring both titanium and ruthenium to a +4 oxidation state in a coating solution, and applying the coating solution to a surface of an anode substrate comprising titanium or a titanium alloy.

17. A method for preparing an anode includes mixing titanium, ruthenium, and palladium in a coating solution, wherein titanium is provided in the coating solution in the form of an oxide, and applying the coating solution to a surface of an anode substrate including titanium or a titanium alloy.

18. An anode, comprising:

a core substrate comprising valve metal, and

A multilayer coating comprising a base layer comprising palladium directly coating a surface of the core substrate.

19. The anode of claim 18, wherein the valve metal is titanium or a titanium alloy.

20. The anode of claim 18, wherein the base layer comprises ruthenium and titanium.

21. The anode of claim 20, wherein the base layer comprises iridium.