Anode for electrolytic evolution of chlorine

Abstract

An electrode suitable as chlorine-evolving anode in electrolytic cells and a method for obtaining thereof is provided. The electrode has a metal substrate coated with a catalytic composition made of thin layers based on oxides of tin, iridium and ruthenium and combines excellent characteristics of anodic potential and selectivity with respect to the reaction of chlorine evolution without resorting to the use of dopants such as platinum and palladium.

Claims

1. An electrode for evolution of gaseous products in electrolytic cells consisting of a metal substrate provided with a catalytic coating made up of layers, each layer of said catalytic coating consisting of a mixture of oxides consisting of 55-70% by weight of tin, 5-10% by weight of iridium and 20-40% by weight of ruthenium referred to the metals, each layer of the catalytic coating having an average thickness of 0.1 to 0.4 m, said catalytic coating having a total noble metal loading expressed as the sum of iridium and ruthenium referred to the metals of the coating of 2 to 8 g/m.sup.2, wherein the ratio Ir/Ru is 0.3 to 0.4 by weight referred to the metals of the catalytic coating, wherein each layer of said catalytic coating is obtained by applying a solution containing precursors of tin, iridium and ruthenium in the form of acetic solution containing SnHAC, IrHAC and RuHAC to the metal substrate with subsequent drying at 50-60 C. and thermal decomposition at 450-600 C. until reaching a noble metal loading, and upon reaching the total noble metal loading applying a final heat treatment at 500-550 C. for 20-200 minutes.

2. The electrode according to claim 1, wherein the noble metal loading of each layer of the catalytic coating expressed as the sum of iridium and ruthenium referred to the metals of the catalytic coating is 0.2 to 1.4 g/m.sup.2.

3. The electrode according to claim 1, wherein the ratio Ir/Ru is 0.3 to 0.4 by weight referred to the metals of the catalytic coating.

4. The electrode according to claim 1, wherein said catalytic coating consists of a mixture consisting of 55-65% by weight of tin oxide, 15-20% by weight of iridium oxide and 20-25% by weight of ruthenium oxide referred to the metals of the catalytic coating.

5. A method for manufacturing an electrode according to claim 1 comprising the execution of the following sequential steps on a metal substrate: a) applying the acetic solution containing precursors of the components of said catalytic coating in the form of SnHAC, IrHAC and RuHAC to the metal substrate with subsequent drying at 50-60 C. and thermal decomposition at 450-600 C. until reaching a specific noble metal loading of 0.2 to 1.4 g/m.sup.2; b) repeating step a) until obtaining a catalytic coating with a specific noble metal loading of 2 to 25 g/m.sup.2; and c) finally heat treating at 500-550 C. for a time of 50 to 200 minutes.

6. The method according to claim 5 comprising an intermediate heat treatment at 500-550 C. for a time of 50 to 200 minutes carried out upon reaching the application of half the total loading of noble metal.

7. An electrolysis cell of alkali chloride solutions comprising an electrode according to claim 1 as chlorine-evolving anode.

Description

EXAMPLE 1

(1) A sample of titanium mesh of 10 cm10 cm area was rinsed three times in deionised water at 60 C., changing the liquid every time. The rinsing was followed by a 2 hour heat treatment at 350 C. The mesh was then subjected to a treatment in a boiling 20% HCl solution for 30 minutes.

(2) 100 ml of an acetic solution containing Ru hydroxyacetochloride complex (hereinafter: RuHAC), Ir hydroxyacetochloride complex (in the following: IrHAC) and Sn hydroxyacetochloride complex (in the following: SnHAC) were then prepared according to the procedure disclosed in WO 2005/014885, with a molar composition of 32% Ru, 8% Ir and 60% Sn.

(3) The solution was applied to the titanium mesh sample by brushing in 14 coats. After each coat, a drying step was carried out at 50-60 C. for about 10 minutes, followed by a 10 minute heat treatment at 500 C. The sample was cooled in air every time before applying the next coat.

(4) The procedure was repeated until reaching a total noble metal loading of 8 g/m.sup.2 expressed as the sum of Ir and Ru referred to the metals. A final heat treatment at 500 C. was then carried out for 100 minutes.

(5) The electrode thus obtained was identified as sample #1.

EXAMPLE 2

(6) A sample of titanium mesh of 10 cm10 cm area was rinsed three times in deionised water at 60 C., changing the liquid every time. The rinsing was followed by a 2 hour heat treatment at 350 C. The mesh was then subjected to a treatment in a boiling 20% HCl solution for 30 minutes.

(7) 100 ml of an acetic solution containing RuHAC, IrHAC and SnHAC were then prepared according to the procedure disclosed in WO 2005/014885, with a molar composition of 27% Ru, 10% Ir, 63% Sn.

(8) The solution was applied to the titanium mesh sample by brushing in 12 coats. After each coat, a drying step was carried out at 50-60 C. for about 10 minutes, followed by a 10 minute heat treatment at 500 C. The sample was cooled in air every time before applying the next coat.

(9) The procedure was repeated until reaching a total noble metal loading of 8 g/m.sup.2 expressed as the sum of Ir and Ru referred to the metals, carrying out an intermediate heat treatment for 1 hour at 500 C. after applying half the total loading and a final heat treatment at 500 C. for 100 minutes upon reaching the total loading.

(10) The electrode thus obtained was identified as sample #2.

COUNTEREXAMPLE 1

(11) A sample of titanium mesh of 10 cm10 cm area was rinsed three times in deionised water at 60 C., changing the liquid every time. The rinsing was followed by a 2 hour heat treatment at 350 C. The mesh was then subjected to a treatment in a boiling 20% HCl solution for 30 minutes.

(12) 100 ml of a hydroalcoholic solution containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiCl.sub.3 in a solution of isopropanol with a molar composition of 30% Ru, 19% Ir, 51% Ti referred to the metals were then prepared.

(13) The solution was applied to the titanium mesh sample by brushing in 10 coats. After each coat, a drying step was carried out at 35-50 C. for about 5 minutes, followed by a 10 minute heat treatment at 460-470 C. for the first coat and at 480-500 C. for the subsequent coats. The sample was cooled in air every time before applying the next coat.

(14) At the end of the whole process, a total noble metal loading of 8 g/m.sup.2 was achieved, expressed as the sum of Ru and Ir referred to the metals.

(15) The electrode thus obtained was identified as sample #C1.

COUNTEREXAMPLE 2

(16) A sample of titanium mesh of 10 cm10 cm area was rinsed three times in deionised water at 60 C., changing the liquid every time. The rinsing was followed by a 2 hour heat treatment at 350 C. The mesh was then subjected to a treatment in a boiling 20% HCl solution for 30 minutes.

(17) 100 ml of a hydroalcoholic solution containing RuCl.sub.3.3H.sub.2O, H.sub.2IrCl.sub.6.6H.sub.2O and C.sub.16H.sub.30O.sub.4Sn (stannous 2-ethylhexanoate) having a molar composition of 20% Ru, 10% Ir, 70% Sn were then prepared.

(18) The solution was applied by brushing followed by drying and heat treatment at 500 for 10 minutes. The brushing, drying and heat treatment cycle was repeated four times until obtaining the electrode referred to as sample #C2.

EXAMPLE 3

(19) The samples of the preceding Examples were characterised as anodes for chlorine evolution in a 1 dm.sup.2 active area zero-gap laboratory cell fed with a sodium chloride brine at a concentration of 200 g/l and at a temperature of 89 C. with a 32% by weight NaOH catholyte. The following Table shows the cell voltage of the samples measured at a current density of 4 kA/m.sup.2 as an indication of their catalytic activity for chlorine evolution and the volume percent of oxygen in product chlorine as an indication of their selectivity. The noble metal wear rate was measured using a laboratory membrane cell with 0.2 dm.sup.2 anodic active area at 8 kA/m.sup.2 (accelerated test) after elapsing 4000 and 8000 hours in operation (HOL). The test was carried out with a 210 g/l NaCl anolyte and a 32% by weight NaOH catholyte at a temperature of 89 C. The average layer thickness of the catalytic coating of each sample was calculated according to the designated procedure hereinbefore described. The average thickness of the catalytic coating was measured on the unused samples 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.

(20) Each sample for the cross section measurement was prepared according to the following steps: 6 portions of the sample electrode having a 20 mm width were cut using a precision cutting machine, each portion was embedded in a phenolic hot mounting resin with carbon filler using a press and subsequently polished.

(21) TABLE-US-00001 TABLE Time to reach Cell mg/kAh steady-state Average layer voltage O.sub.2/Cl.sub.2 after 7000 conditions thickness Samples (V) (Vol %) HOL (days) (m) 1 2.77 1.5% 0.021 <5 0.4 2 2.78 1.0% 0.006 <5 0.3 C1 2.90 1.4% 0.025 7 0.6 C2 2.85 3% 0.026 7 1.3

(22) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(23) Throughout the description and claims of the present application, the term comprise and variations thereof such as comprising and comprises are not intended to exclude the presence of other elements, components or additional process steps.

(24) The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.