Anode for electrochlorination and method for making the same

Abstract

The present invention related to an anode comprising a substrate and a catalytic coating, said catalytic coating consisting essentially of ruthenium oxide and tin oxide, wherein said ruthenium oxide is present in an amount comprised between 20% and 60% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide, wherein said tin oxide is present in an amount comprised between 40% and 80% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide. The present invention also related to the use of an electrolyser comprising the anode of the present invention in seawater electrochlorination.

Description

[0001] INDUSTRIE DE NORA S . p . A .

[0002] ANODE FOR ELECTROCHLORINATION AND METHOD FOR MAKING THE

[0003] SAME

[0004] TECHNICAL FIELD

[0005] The present invention relates to an anode suitable for electrochlorination or chloralkali , a method of making the same and its use in an electrolyser . Preferably, an electrolyser for seawater electrochlorination .

[0006] BACKGROUND OF THE INVENTION

[0007] Electrochlorination is the process of producing hypochlorite by passing electric current through brine . The resulting sodium hypochlorite can be exploited in numerous applications relating to water disinfection and oxidation, such as water treatment for drinking water, or microbiological control in cooling towers .

[0008] Contrary to chlorine gas or tablets , which may be added to water to achieve similar results , sodium hypochlorite can be produced on site , thus avoiding transportation, environmental and / or storage issues .

[0009] The process is carried out by applying a suitable current to an electrolytic cell comprising at least two electrodes and an electrolyte containing brine , i . e . a mixture of salt and water at varying concentrations depending on the application .

[0010] 1

[0011] M / 66032 -PCT ( 410 PCT ) INDUSTRIE DE NORA S.p.A.

[0012] The use of mixed metal oxide coatings such as platinumbased formulations on the cathode or the anode for the generation of hypochlorite by electrolyzing brine solutions is widely known in the art. However, those compositions are not economically feasible due to the high price of platinum.

[0013] Currently, anodes comprising catalytic layers of ruthenium oxide and titanium oxide are commonly used. Those layers have an improved lifetime and operating efficiency for the generation of hypochlorite.

[0014] Nevertheless, ruthenium oxide and titanium oxide formulations have two main downsides. The first one is their high cost due to the presence of ruthenium oxide in high amount (price of ruthenium in February 2024 is 425.00 USD / ozt) . The second one is the fact that titanium does not tolerate manganese ions, which may be present in high quantity in some seawater. In this case, manganese ions can be deposited on the coating and leads to the reduction of the active surface of the electrode. Consequently, the current density and the electrical consumption for the hypochlorite production increases. Furthermore, manganese deposition on the anode results in drastic reduction of the lifetime of the anode.

[0015] Thus, it would be desirable to provide an anode for the production of hypochlorite that can solve the aforementioned drawbacks without affecting the performance and the lifetime of the anode.

[0016] 2

[0017] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0018] US 2017 / 306512 Al describes an electrode for chlorine evolution in alkaline chloride electrolysis, comprising a metal substrate, e.g. a titanium substrate, with a catalytic coating of mixed oxides containing tin, iridium and ruthenium.

[0019] CN 188141 A describes a seawater pollution resisting oxide anode comprising a titanium or titanium alloy substrate having a coating comprising ruthenium oxide, iridium oxide and tin oxide in combination with other optional metal oxides such as platinum oxide and tantalum oxide .

[0020] WO 95 / 05498 Al describes an electrode for chlorine evolution comprising a valve metal substrate and a coating with an outer layer of an electrocatalytically active material, including at least ruthenium oxide and a non-noble metal oxide, e.g. tin oxide or titanium oxide. In one example a coating comprising tin oxide and ruthenium oxide is described. The outer layer is prepared by thermal-spraying a mixture of powders of metal oxide mixtures onto the substrate.

[0021] US 2018 / 127887 Al describes an oxygen evolution anode having a tin-based protective layer and an iridium-based catalytic layer.

[0022] Thus, it would also be desirable to provide an cost- effective anode for the production of hypochlorite which

[0023] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A. exhibits a high catalytic efficiency and an extended lifetime, particularly in manganese-rich seawater electro chlorination.

[0024] SUMMARY OF THE INVENTION

[0025] From what is stated above, the present application aims at providing a cost-effective and active anode for chloralkali or the production of hypochlorite via electrochlorination analogous to those currently used in the industry such as anodes comprising ruthenium oxide and titanium oxide formulations.

[0026] This result is achieved using an anode comprising a substrate and a catalytic coating which consists essentially of ruthenium oxide and tin oxide at specific percentages .

[0027] The presence of tin instead of titanium permits a reduction of the ruthenium oxide load with no penalty on free available chlorine (FAC) efficiency and with a sensible improvement of the lifetime compared to traditional coating.

[0028] These and other objects and advantages of the invention will become obvious from the following detailed description .

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] 4

[0031] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0032] A first object of the invention therefore relates to an anode, which comprises a substrate and a catalytic coating, wherein the catalytic coating consists essentially of ruthenium oxide and tin oxide, wherein said ruthenium oxide is present in an amount comprised between 20% and 60% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide, wherein said tin oxide is present in an amount comprised between 40% and 80% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide.

[0033] As used herein, the expression "consists essentially of" means that the catalytic coating contains combined more than 95% ruthenium oxide and tin oxide by weight referred to metals, i.e. less than 5% by weight of other metals. Preferably the catalytic coating contains more than 98% ruthenium oxide and tin oxide by weight referred to metals, i.e. less than 2% by weight of other metals.

[0034] In one embodiment, the catalytic coating consists of ruthenium oxide and tin oxide. In the present context, the expression "consists" means that the catalytic coating contains combined more than 99% ruthenium oxide and tin oxide by weight referred to metals, i.e. less than 1% by weight of other ingredients. Preferably the catalytic coating contains more than 99.5% ruthenium oxide and tin oxide by weight referred to metals, i.e.

[0035] 5

[0036] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S . p . A . less than 0 . 5% by weight of other metals .

[0037] Preferably, said ruthenium oxide is present in an amount comprised between 40% and 55% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide , and said tin oxide i s present in an amount comprised between 45% and 60% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide .

[0038] As mentioned, the use of tin instead of titanium in combination with ruthenium in the catalytic coating permits a reduction of ruthenium oxide load with no penalty on free available chlorine ( FAC ) ef ficiency and with a sensible improvement of the li fetime compared to the traditional coating .

[0039] Additionally, the greater thickness of tin permits a better dispersion of the Ruthenium in the matrix guaranteeing a less ruthenium oxide consumption during the operation with consequent improvement of li fetime . Furthermore , tin is a semiconductor, which allows tin to maintain stable ef ficiency .

[0040] As will be explained in more detail below, the anode is typically obtained by repeating multiple cycles of application and thermal treatment of a precursor solution, resulting in a catalytic coating having multiple layers . In the present description, the coating layer adj acent to the substrate is also denoted as the

[0041] M / 66032 -PCT ( 410 PCT ) INDUSTRIE DE NORA S.p.A.

[0042] "inner layer" and the outermost layer of the catalytic coating facing away from the substrate towards the electrolyte is denoted as the "outer layer".

[0043] Without wishing to limit the invention to a particular theory, the reduction of the content of ruthenium oxide in Ru-Sn based anode with no effect on efficiency with respect to Ru-Ti based anode may be related to the distribution of the elements in the coating. Indeed, an SEM and EDX elemental analyses of both Ru-Ti and Ru-Sn coatings show significant differences. In Ru-Ti coating, the elemental analysis depicts that ruthenium and titanium are distributed homogeneously through the thickness of the coating, i.e. from the layer adjacent to the substrate to the outer layer. In Ru-Sn coating, on the other hand, the elemental analysis shows concentration gradient of both ruthenium and tin atoms, wherein tin content is the lowest in the layer adjacent to the substrate and tends to increase going to outer layer, whereas ruthenium content is the highest in the layer adjacent to the substrate and tends to decrease going to outer layer. In particular, in certain embodiments the new coating is applied in such a way that, starting from the inner layer, for 1 / 3 of its thickness, the composition maintains a constant ruthenium to tin weight ratio in the range of 50:50 to 70:30. At the outer layer, i.e. the interface to the electrolyte, the composition has a ruthenium to tin weight ratio in the range of 30:70 to 40:60, with a regular gradient in composition across the remaining 2 / 3

[0044] 7

[0045] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A. of its thickness. Preferably, the average composition of the catalytic coating of such embodiments exhibits a ruthenium to tin weight ratio of approximately 50:50.

[0046] The catalytic coating of the present invention has the advantage of operating for longer periods in chloride diffusion conditions and under very high current density compared to traditional coatings such as coatings comprising titanium and ruthenium.

[0047] Furthermore, tin can tolerate the presence of organic and inorganic pollutants, thus the catalytic composition of the present invention is particularly suitable for the chlorination of seawater. Specifically, seawater having high content of manganese ions.

[0048] The presence of manganese can affect electrode performance by depositing on the electrode itself. This deposition may reduce the active surface area of the electrode, leading to an increase in current density and higher electrical consumption for the hypochlorite production required for the process. As a result, the electrode operates unevenly, with locally higher currents producing more oxygen and consequently more chlorate, drastically reducing the electrode's life.

[0049] According to a preferred aspect, the catalytic coating has a total noble metal load comprised between 0.5 and 20 g / m2.

[0050] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0051] In one embodiment, the catalytic coating has a total noble metal load comprised between 0.5 and 15 g / m2, preferably comprised between 1 and 9 g / m2. As described above, the presence of tin permits a reduction of the ruthenium oxide load with no penalty on free available chlorine (FAC) efficiency and with a sensible improvement of the lifetime compared to traditional coating .

[0052] In another embodiment, the catalytic coating has a total noble metal load comprised between 16 and 20 g / m2. This higher noble metal loading range is particularly advantageous for applications requiring enhanced durability and performance under demanding conditions, such as high current density operations or environments with elevated levels of contaminants like manganese ions. This loading range maintains the balance between cost-effectiveness and performance, ensuring that the anode remains economically viable while providing superior resistance to wear and corrosion, as well as sustained efficiency in hypochlorite production.

[0053] Preferably, the substrate comprises titanium. Titanium substrates ensure low and stable cell voltage of the electrolysis process.

[0054] A second object of the invention relates to a method for making the anode of the present invention, the method comprising the steps of: providing a substrate

[0055] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A. applying a coating solution comprising precursors of ruthenium and tin to said substrate thus obtaining a coated substrate, baking said coated substrate at a temperature comprised between 300 and 700°C, preferably 300 and 550 °C, optionally repeating former steps.

[0056] Preferably, the precursors of ruthenium and tin are organoacetic complexes. More specifically, these organoacetic complexes can be selected from a group that includes hydroxyacetic chlorate, acetate, chloroacetate, and nitric acetate. The preference for organoacetic complexes arises from the high volatility of tin, which can lead to uncontrolled losses during heat treatments. By using organoacetic complexes, it is possible to mitigate the problem of tin sublimation during the baking phase, allowing for a catalytic coating with a well- controlled chemical composition.

[0057] A third object of the invention relates to an electrolyzer, preferably an electrochlorination system, comprising :

[0058] - a housing provided with an inlet and an outlet suitable for circulating brine,

[0059] - a cathode and the anode of the present invention.

[0060] A fourth object of the invention relates to a process for hypochlorite production in the electrolyzer of the present invention, wherein the process comprises:

[0061] 10

[0062] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0063] - circulating the brine between a cathode and the anode of the present invention,

[0064] - applying an external voltage between the cathode and the anode thereby producing hypochlorite.

[0065] A fifth object of the invention relates to the use of the electrolyser of the present invention in seawater electro chlorination.

[0066] The use in seawater electrochlorination is particularly preferred, especially those having high content of manganese .

[0067] In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared.

[0068] For the purposes of the present discussion the term "to comprise" or "to include" also comprises the term "to consist in" or "essentially consisting of".

[0069] For the purposes of the present invention the definitions of the ranges always comprise the extreme values unless otherwise specified.

[0070] The following examples are provided for illustrative purposes only of the present invention and must not be understood as limiting the scope of protection defined by the appended claims.

[0071] 11

[0072] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0073] EXAMPLES.

[0074] Example 1 - preparation of an anode according to the invention

[0075] A solution was prepared starting from ruthenium organoacetic complex and tin organoacetic complex, and using acetic acid as solvent, in which the ratio of Sn:Ru is 50:50 by weight.

[0076] A substrate of titanium was annealed, washed and dried to be ready for the deposition.

[0077] The coating solution is applied to the substrate until achieving a final load of 8 g / m2of ruthenium as metal. For any cycle, the coated substrate is then dried and backed at 600°C for 6 hours.

[0078] Example 2 - preparation of an anode according to the invention

[0079] The anode of example 2 was prepared in the same way as of example 1.

[0080] A solution was prepared starting from ruthenium organoacetic complex and tin organoacetic complex, and using acetic acid as solvent, in which the ratio of Sn:Ru is 75:25 by weight.

[0081] Same surface and thermal treatments of example 1 were applied .

[0082] Comparative example 1 - preparation of an anode coated with a catalytic coating comprising ruthenium and

[0083] 12

[0084] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A. titanium oxide

[0085] The anode was prepared in the same way as of example 1, replacing tin with titanium.

[0086] A solution was prepared starting from ruthenium hexachloride acid and titanium trichloride, and using acid chloride as solvent, in which the ratio of Ti:Ru is 50:50 by weight.

[0087] The coating solution is applied to the substrate until achieved a final load of 16 g / m2of ruthenium.

[0088] Same surface and thermal treatments of example 1 were applied .

[0089] Characterization studies

[0090] The following characterization studies were conducted on the samples of the examples and comparative example.

[0091] Cell voltage

[0092] The cell voltage of the electrodes was measured after 2000 hours of operation in an aqueous solution of 30 g / 1 of NaCl . The solution was at a temperature of 30°C ± 2°C and the electrodes operated at a current density of 2,2 kA / m2. The results are indicated in Table 1.

[0093] At the end of the experiment the hypochlorite concentration generated in the aqueous solution was determined by iodometric titration.

[0094] The efficiency of the electrode was determined as the

[0095] 13

[0096] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A. ratio between the actual concentration of NaOCl versus the theoretical concentration, calculated according to Faraday law of electrolysis, expressed in percentage. The results are indicated in Table 2.

[0097] Table 1 :

[0098] Hypo efficiencies

[0099] Table 2 :

[0100] Accelerated test

[0101] The electrodes were characterized in an accelerated life-test under hypochlorite production.

[0102] The electrodes of the examples were operated at a current density of 6000 A / m2 and were kept in testing conditions until cell voltage exceeded 8,5 volts (the "Accelerated test", measured in hours on line) . The results are indicated in Table 3.

[0103] 14

[0104] M / 66032-PCT (410PCT) INDUSTRIE DE NORA S.p.A.

[0105] Table 3:

[0106] Manganese pickup

[0107] The manganese pickup of the electrodes was measured after 1000 hours of operation in an electrolyte consisting of an aqueous solution containing 30 g / 1 of NaCl and 1000 ppb of Mn. The solution was at a temperature of 30±2°C and the electrodes operated at a current density of 1,5 kA / m2.

[0108] At the end of the experiment the manganese pickup was determined by SEM and EDX elemental analyses, measured in kcps . The results are indicated in Table 4.

[0109] Table 4 :

[0110] M / 66032-PCT (410PCT)

Claims

INDUSTRIE DE NORA S . p . A .CLAIMS1 . Anode comprising a substrate and a catalytic coating, said catalytic coating consisting essentially o f ruthenium oxide and tin oxide , wherein said ruthenium oxide is present in an amount comprised between 20% and 60% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide , wherein said tin oxide is present in an amount comprised between 40% and 80% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide .2 . Anode according to claim 1 , wherein said catalytic coating consisting essentially of ruthenium oxide and tin oxide3 . Anode according to any one of claims 1 or 2 , wherein said ruthenium oxide is present in an amount comprised between 40% and 55% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide , wherein said tin oxide is present in an amount comprised between 45% and 60% by weight referred to metals with respect to the total weight of said ruthenium oxide and tin oxide .4 . Anode according to any one of the preceding16M / 66032 -PCT ( 410 PCT )INDUSTRIE DE NORA S.p.A. claims, wherein said catalytic coating exhibits a concentration gradient of ruthenium and tin, such that the tin content increases from the inner layer of the coating toward the outer layer of the coating, while the ruthenium content decreases from the inner layer of the coating toward the outer layer of the coating.

5. Anode according to claim 4, wherein said catalytic coating has a composition such that, starting from the inner layer, for one-third of its thickness, the composition maintains a constant ruthenium to tin weight ratio in the range of 50:50 to 70:30, and at the outer layer, the composition has a ruthenium to tin weight ratio in the range of 30:70 to 40:60, with a regular gradient in composition across the remaining two-thirds of its thickness.

6. Anode according to claim 5, wherein the average composition of said catalytic coating exhibits a ruthenium to tin weight ratio of approximately 50:50.

7. Anode according to any one of the preceding claims, wherein said catalytic coating has a total noble metal load comprised between 0.5 and 20 g / m2.

8. Anode according to claim 7, wherein said catalytic coating has a total noble metal load comprised between 0.5 and 15 g / m2, preferably comprised between 1 and 9 g / m2.17M / 66032-PCT (410PCT)INDUSTRIE DE NORA S.p.A.

9. Anode according to claim 7, wherein said catalytic coating has a total noble metal load comprised between 16 and 20 g / m2.

10. Anode according to any one of the preceding claims, wherein said substrate comprises titanium.

11. Method for making an anode according to anyone of the preceding claims, said method comprising the steps of : providing a substrate applying a catalytic coating solution comprising precursors of ruthenium and tin on said substrate thus obtaining a coated substrate, baking said coated substrate at a temperature comprised between 300 and 700°C, optionally repeating former steps12. Method according to claim 11, wherein said precursors of ruthenium and tin are organoacetic complexes, preferably selected from a group consisting of hydroxyacetic chlorate, acetate, chloro acetate and nitric acetate complex.

13. Electrolyzer comprising:- a housing provided with an inlet and an outlet suitable for circulating brine,- a cathode and an anode according any one of claims1 to 10.18M / 66032-PCT (410PCT)INDUSTRIE DE NORA S.p.A.

14. Process for hypochlorite production in the electrolyzer according to claim 13, wherein said process comprises :- circulating said brine between said cathode and said anode,- applying an external voltage between said cathode and said anode thereby producing hypochlorite.

15. Use of the electrolyser according to claim 13 in seawater electrochlorination.19M / 66032-PCT (410PCT)

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