ANODE FOR ELECTROLYTIC EVOLUTION OF CHLORINE

Abstract

The invention relates to a process for obtaining a electrode usable as a anode in electrolytic cells for the production of chlorine. The electrode thus obtained comprises a catalytic layer containing oxides of tin, ruthenium, iridium and titanium applied to a substrate of a valve metal.

Claims

1. An electrode for gas evolution in electrolytic processes comprising a valve metal substrate and a catalytic coating containing 5-40% of tin, 3.6-15% of iridium, 18-40% of ruthenium and 30-70% of titanium, in the form of metals or their oxides in molar percentage referred to the elements, said catalytic coating obtained by thermal decomposition of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium.

2. The electrode according to claim 1, wherein said catalytic coating contains 6-30% of tin, 3.7-12% of iridium, 20-30% of ruthenium and 50-70% of titanium, in the form of metals or their oxides in molar percentage referred to the elements.

3. The electrode according to claim 1, wherein said catalytic coating contains 8-18% of tin, 4-10% of iridium, 18-36% of ruthenium and 45-65% of titanium, in the form of metals or their oxides in molar percentage referred to the elements.

4. The electrode according to claim 1, wherein said catalytic coating has a specific load of noble metal expressed as the sum of iridium and ruthenium comprised between 6 and 12 g/m.sup.2.

5. The electrode according to claim 1, wherein said catalytic coating is obtained by thermal decomposition an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium, said solution containing 5-40% of tin, 3.6-15% of iridium, 18-40% of ruthenium and 30-70% of titanium, in molar percentage referred to the elements.

6. A method for the production of an electrode as defined in claim 1, comprising the following steps: a) applying to a valve metal substrate of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium, subsequent drying at 50-60° C. and thermal decomposition at 450-600° C. for a time of 5 to 30 minutes until reaching a specific noble metal loading expressed as the sum of iridium and ruthenium between 0.4 and 1 g/m.sup.2; b) repeating step a) until obtaining a catalytic coating with a specific noble metal loading of 6 to 12 g/m.sup.2; c) heat treating at 450-600° C. for a time of 50 to 200 minutes.

7. The method according to claim 6, wherein said acidic solution contains 5-40% of tin, 3.6-15% of iridium, 18-40% of ruthenium and 30-70% of titanium, preferably 6-30% of tin, 3.7-12% of iridium, 20-30% of ruthenium and 50-70% of titanium and more preferably 8-18% of tin, 4-10% of iridium, 18-36% of ruthenium and 45-65% of titanium, in molar percentage referred to the elements.

8. A method for the production of an electrode for gas evolution in electrolytic processes comprising the following steps: a) applying to a valve metal substrate of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium complexes containing 5-40% of tin, 3.6-15% of iridium, 18-40% of ruthenium and 30-70% of titanium, in molar percentage referred to the elements; subsequent drying at 50-60° C. and thermal decomposition at 450-600° C. for a time of 5 to 30 minutes until reaching a specific noble metal loading expressed as the sum of iridium and ruthenium of 0.4 to 1 g/m.sup.2; b) repeating step a) until obtaining a catalytic coating with a specific noble metal loading of 6 to 12 g/m.sup.2; c) heat treating at 450-600° C. for a time of 50 to 200 minutes.

9. The method according to claim 8, wherein said acidic solution contains 6-30% of tin, 3.7-12% of iridium, 20-30% of ruthenium and 50-70% of titanium and preferably 8-18% of tin, 4-10% of iridium, 18-36% of ruthenium and 45-65% of titanium, in molar percentage referred to the elements.

10. The method according to claim 6 where the temperature of said thermal decomposition in steps a) and c) is between 480 and 550° C.

11. A cell for the electrolysis of solutions of alkaline chlorides comprising an anodic compartment and a cathodic compartment where the anodic compartment is equipped with the electrode according to claim 1.

12. The cell for electrolysis according to claim 11 wherein said anodic compartment and said cathodic compartment are separated by a diaphragm or an ion-exchange membrane.

13. An electroyzer for the production of chlorine and alkali from alkali chloride solutions comprising a modular arrangement of cells according to claim 12.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0032] FIG. 1 is the Scanning Electron Microscope image of a cross-section of the electrode described in Example #1;

[0033] FIG. 2 is the Scanning Electron Microscope image of a cross-section of the electrode described in Counter-example #2C.

EXAMPLES

[0034] The following examples are included to demonstrate particular embodiments of the invention, whose feasibility has been amply verified in the range of values claimed. It will be obvious to a person skilled in the art that the compositions and the techniques described in the examples given hereunder represent compositions and techniques for which the inventors have found good operation of the invention in practice; however, a person skilled in the art will also appreciate that in the light of the present description, various changes may be made to the various embodiments described, still giving rise to identical or similar results while remaining within the scope of the invention.

Example 1

[0035] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0036] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 20% Ru, 7% Ir, 17% Sn and 56% Ti. The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0037] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 8 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0038] The electrode thus obtained was identified as sample #1.

Example 2

[0039] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0040] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 30% Ru, 4% Ir, 15% Sn and 51% Ti.

[0041] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0042] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 8 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0043] The electrode thus obtained was identified as sample #2.

Example 3

[0044] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0045] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 23% Ru, 12% Ir, 19% Sn and 46% Ti. The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0046] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 8 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0047] The electrode thus obtained was identified as sample #3.

Example 4

[0048] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0049] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 20% Ru, 10% Ir, 16% Sn and 54% Ti. The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0050] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 12 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0051] The electrode thus obtained was identified as sample #4.

Example 5

[0052] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0053] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 21% Ru, 7% Ir, 32% Sn and 40% Ti.

[0054] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0055] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 9 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0056] The electrode thus obtained was identified as sample #5.

Example 6

[0057] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0058] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium, hydroxyacetochloride complex of iridium, and hydroxyacetochloride complex of titanium and having a molar composition equal to 21% Ru, 9% Ir, 29% Sn and 41% Ti.

[0059] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0060] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 9 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0061] The electrode thus obtained was identified as sample #6.

Counter-Example 1

[0062] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0063] Then 100 ml of an aqueous-alcoholic solution was prepared containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiCl.sub.3 in an isopropanol solution, having a molar composition equal to 27% Ru, 12% Ir, 61% Ti.

[0064] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0065] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 13 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0066] The electrode thus obtained was identified as sample #1C.

Counter-Example 2

[0067] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0068] Then 100 ml of an aqueous-alcoholic solution was prepared containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiCl.sub.3, SnCl.sub.4 in an isopropanol solution, having a molar composition equal to 20% Ru, 7% Ir, 17% Sn and 56% Ti.

[0069] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0070] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 8 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0071] The electrode thus obtained was identified as sample #2C.

Counter-Example 3

[0072] A piece of titanium mesh with dimensions of 10 cm×10 cm was washed three times in deionized water at 60° C., changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350° C. The mesh was then treated in a 20% solution of HCl, with boiling for 30 minutes.

[0073] 100 ml of an acetic solution was then prepared containing hydroxyacetochloride complex of tin, together with hydroxyacetochloride complex of ruthenium and hydroxyacetochloride complex of iridium and having a molar composition equal to 35% Ru, 6% Ir, 59% Sn.

[0074] The solution was applied on the piece of titanium mesh by brushing, in 14 coats. After each coat, drying was carried out at 50-60° C. for about 10 minutes, followed by heat treatment for 10 minutes at 500° C. The piece was air-cooled each time before applying the next coat.

[0075] The procedure is repeated until we reach a total loading of noble metal, expressed as the sum of Ir and Ru referred to the metals, equal to 8 g/m.sup.2. A final heat treatment is then carried out at 500° C. for 100 minutes.

[0076] The electrode thus obtained was identified as sample #3C.

Sample Characterization

[0077] Evidence of the high volatility of tin tetrachloride and the related uncontrolled loss of the latter during the heat treatment has been confirmed by SEM EDX analysis of two samples of the electrode of Example 1 (sample 1) and the electrode of Counter-Example 2 (sample 2C), respectively.

[0078] The analyses were performed with a scanning electrode microscope (the commercial SEM/FEG Inspect F 50 by FEI with EDAX microanalysis system), equipped with a Everhart-Thornley detection system used in backscattering mode; the working distance was set at 10 mm, the acceleration voltage at 20 kV, and the magnification ranged between 10000× and 100000×.

[0079] FIGS. 1 and 2 show images of cross sections of the catalytic coatings of the prepared electrodes of Example 1 and Counter-Example 2, respectively, and corresponding composition profiles determined using the ZAF correction method (“ZAF” refers to the correction of matrix effects of the specimen; specifically: Z refers to the atomic number effect, A refers to X-ray absorption effect and F refers to X-ray fluorescence effect).

[0080] As can be taken from FIG. 1 relating to the electrode of Example 1 (sample 1), tin is present throughout the cross-section of the catalytic coating. In addition, the metal composition detected by EDX Analysis is comparable to the molar composition of the initial acetic solution containing hydroxyacetochloride complexes of tin, ruthenium, iridium, and titanium equal to 20% Ru, 7% Ir, 17% Sn and 56% Ti of sample 1.

[0081] As can be taken from FIG. 2 relating to the electrode of Counter-Example 2 (sample 2C), there is only exiguous presence of tin in the entire cross-section of the catalytic coating. The metal composition detected by EDX Analysis proves the loss of most of tin as compared to the molar composition of the initial aqueous-alcoholic solution containing tin, ruthenium, iridium, and titanium equal to 20% Ru, 7% Ir, 17% Sn and 56% Ti of sample 2C.

Chlorine Evolution Test

[0082] The samples from the examples were characterized as anodes for evolution of chlorine in a laboratory cell supplied with sodium chloride brine at a concentration of 200 g/l, closely controlling the pH to a value of 3.

[0083] Table 1 shows the chlorine overvoltage measured at a current density of 3 kA/m.sup.2, the percentage of oxygen by volume in the chlorine produced and the resistance to reversals expressed as percentage of noble metal lost.

TABLE-US-00001 TABLE 1 Resistance to Samples ηCl.sub.2 (mV) O.sub.2/Cl.sub.2 (vol %) reversals (%) 1 30 0.2 3 2 40 0.2 2 3 30 0.3 3 4 40 0.2 3 5 30 0.3 3 6 30 0.3 3 1C 50 0.4 4 2C 50 0.4 8 3C 40 0.3 7

[0084] The foregoing description is not intended to limit the invention, which may be used according to various embodiments but without deviating from the aims, its scope being defined unambiguously by the appended claims.

[0085] In the description and in the claims of the present application, the term “comprises” and “contains” and their variants such as “comprising” and “containing” are not intended to exclude the presence of other additional elements, components or process steps.

[0086] The discussion of documents, actions, materials, devices, articles and the like is included in this description solely for the purpose of supplying a context for the present invention. It is not suggested or represented that some or all of these arguments would form part of the prior art or would be general common knowledge in the field relevant to the present invention before the priority date of each claim of this application.