HK1158274B - Electrode for electrolysis cell - Google Patents

Description

Electrode for electrolytic cell

Technical Field

The present invention relates to an electrode suitable for use as an anode in an electrolysis cell, for example as an anode for chlorine evolution in a chlor-alkali electrolysis cell.

Background

The electrolysis of chlor-alkali brines, such as sodium chloride brines for chlorine and caustic soda production, often uses a solution made of ruthenium dioxide (RuO)₂) Surface-activated titanium or other valve metal (valvemetal) -based anode, said ruthenium dioxide (RuO)₂) Has the performance of reducing overvoltage of anodic chlorine evolution reaction. Typical formulations of catalysts for chlorine evolution consist, for example, of RuO₂And TiO₂With a sufficiently reduced anodic chlorine evolution overvoltage. In addition to the need to adopt very high ruthenium loadings to obtain satisfactory lifetimes under normal process conditions, these formulations also have similarly reduced overpotentiation of anodic oxygen evolution reactionsThe disadvantage of pressing; this results in an inability to effectively suppress the concurrent anodic oxygen evolution reaction, such that the product chlorine exhibits an oxygen content that is too high for some uses.

The same considerations apply to the use of a catalyst based on mixed SnO₂RuO of (2)₂The formulations of (a) or ternary mixtures of oxides suitable for ruthenium, titanium and tin; in general, catalysts capable of sufficiently reducing the overvoltage of the chlorine evolution reaction to ensure acceptable energy efficiency tend to have the same effect on the concurrent oxygen evolution reaction, resulting in products of unsuitable purity. In this connection, known examples are given by palladium-containing catalyst formulations which, in addition to their limited lifetime, are also capable of chlorine evolution at significantly reduced potentials, but with a significantly higher oxygen content in the chlorine.

For example by adding SnO mixed with a certain amount of a second noble metal selected from iridium and platinum as described in EP 0153586₂RuO of (2)₂Formulations which are capable of achieving partial improvements in terms of persistence and inhibition of oxygen evolution. The activity of this electrode is still not ideal for the economic significance of large-scale industrial production in terms of cell voltage and thus energy consumption.

It has therefore become necessary to identify catalyst formulations for electrodes suitable for use as chlorine evolving anodes in industrial electrolysis cells, which exhibit the feature of improved anodic chlorine evolution potential combined with a product chlorine of sufficient purity.

Disclosure of Invention

Aspects of the invention are set out in the appended claims.

In one embodiment, the invention relates to an electrode comprising a substrate of titanium, titanium alloy or other valve metal having applied over the surface an outer catalyst coating comprising a mixture of oxides of tin, ruthenium, iridium, palladium and niobium, in terms of molar ratios, involving elements of Sn50-70%, Ru5-20%, Ir5-20%, Pd1-10%, Nb 0.5-5%. The simultaneous addition of palladium and niobium in the concentrations described above to the catalytic layer of the oxide-based formulation based on tin, ruthenium and iridium, shows the performance of significantly reducing the potential of the anodic chlorine evolution reaction, while keeping the potential of the anodic oxygen evolution reaction high, resulting in the double advantage of permitting a reduction in the energy consumption per unit of product and at the same time increasing the purity of the chlorine obtained. As previously mentioned, no practical application of the catalytic action of palladium towards anodic chlorine evolution reactions has been found in the industry in electrolysers due to the weak chemical resistance and in particular the high quality of oxygen produced by the relative co-existing anodic reactions; the inventors have surprisingly found that the addition of a small amount of niobium oxide in the catalyst layer, even in the presence of palladium, has an effective effect in the reaction preventing the oxygen evolution, allowing to operate at cell voltages lower by a few tens mV than the processes of the prior art, without losing any purity of the product chlorine. 0.5 mol% Nb addition is sufficient to obtain a significant effect of preventing the anodic oxygen evolution reaction; in one embodiment, the molar content of the element concerned Nb is between 1 and 2%.

The anodic potential has a tendency to decrease when the content of palladium oxide in the catalytic coating is increased; the amount of 1% is sufficient to give a significant catalytic effect, while the upper limit of 10% is set primarily for stability in a chlorine-rich environment and not for reasons of increasing oxygen production. The addition of not more than 10% by moles of Pd, combined with the presence of a defined level of niobium oxide, allows in any case to obtain electrodes with a durability perfectly matched to the requirements of industrial applications, which seems to be due to the formation of mixed crystalline phases with stabilizing effect.

The inventors have also noted that the deposition of the catalytic layer can be carried out at a lower temperature than in the case of the known formulations based on tin, ruthenium and iridium, for example at 440-. Without wishing to limit the invention to any particular theory, the inventors believe that, due to the lower temperatures required for the heat treatment after application of the coating, partial beneficial effects on the electrode potential and thus on the cell voltage can be obtained with the indicated compositions: it is known in practice that in the case of general formulations, generally lower decomposition temperatures are associated with lower anode potentials.

In one embodiment, the electrode has a TiO-containing layer interposed between the substrate and the external catalytic layer described above₂The intermediate layer of (1). This may have the advantage of giving some protection against attack by the chemical environment to which the electrode is exposed during operation, for example by reducing passivation of the substrate valve metal or by preventing corrosion thereof. In one embodiment, the TiO is₂Mixed with small amounts (e.g. 0.5-3%) of other oxides, such as tantalum, niobium or bismuth oxides. Addition of such oxides to TiO₂In addition to increasing its conductivity by doping effects, it also has the advantage of giving a better bonding of the external catalytic layer to the protective interlayer, which leads to a further increase in the lifetime of the electrode under normal operating conditions.

In one embodiment, the electrode according to the above description is produced by oxidative pyrolysis of a precursor solution in the form of a complex of hydroxyacetyl chloride, such as sn (oh)₂Ac_(2-x)Cl_x、Ir(OH)₂Ac_(2-x)Cl_x、Ru(OH)₂Ac_(2-x)Cl_xContains tin, iridium and ruthenium. With more commonly used precursors such as SnCl₄This may have the advantage of stabilizing the composition of the various elements and in particular tin throughout the thickness of the coating, compared to what occurs (its volatility leads to almost uncontrolled variations in concentration). Precise control of the composition of the various components helps their inclusion into a single phase crystal that can play a positive role in the stabilization of palladium.

In one embodiment, an optionally hydroalcoholic solution comprising Sn, Ru, and Ir hydroxyacetyl chloride complexes of a soluble Pd species and a soluble Nb species is applied to a valve metal substrate in multiple coatings, followed by a heat treatment at a maximum temperature of 400 to 480 ℃ for a period of 15 to 30 minutes after each coating. The maximum temperatures indicated above generally correspond to the temperature at which the thermal decomposition of the precursor is completed with the formation of the relevant oxide; this step may be performed prior to the drying step at lower temperatures, e.g., 100-. The use of hydroalcoholic solutions may show advantages in terms of ease of application and effectiveness of solvent removal in the drying step.

In one embodiment, the soluble Pd species in the precursor solution consists of Pd (NO) in aqueous nitric acid₃)₂And (4) forming.

In one embodiment, the soluble Pd species in the precursor solution is formed from PdCl in ethanol₂And (4) forming.

In one embodiment, the soluble Nb species in the precursor solution is formed from NbCl in butanol₅And (4) forming.

In one embodiment, an electrode comprising a protective interposer and an external catalytic layer is fabricated according to the above-described procedure by: oxidative thermal decomposition of a first hydroalcoholic solution comprising titanium, for example in the form of a hydroxyacetyl chloride complex, and at least one of tantalum, niobium and bismuth, for example in the form of a soluble salt, until a protective interlayer is obtained; subsequently, the catalytic layer is obtained by oxidative pyrolysis of a precursor solution applied to the protective intermediate layer.

In one embodiment, a hydroalcoholic solution of T i hydroxyacetyl chloride complex containing a soluble substance, such as a soluble salt, of at least one element selected from among Ta, Nb and Bi is applied to a valve metal substrate in multiple coatings, after each coating, a heat treatment at a maximum temperature of 400 to 480 ℃ for a period of 15 to 30 minutes; an optionally hydroalcoholic solution of Sn, Ru and Ir hydroxyacetyl chloride complexes comprising a Pd-soluble species and a Nb-soluble species is then applied to the valve metal substrate in multiple coatings, after each coating, a heat treatment at a maximum temperature of 400 to 480 ℃ for a period of 15 to 30 minutes. Also in this case, the above-mentioned maximum temperature generally coincides with the temperature at which the thermal decomposition of the precursor is completed with the formation of the relevant oxide; this step may be performed prior to the drying step at lower temperatures, e.g., 100-.

In one embodiment, BiCl is added₃The substance is dissolved in an acetic acid solution of a hydroxyacetyl chloride complex of Ti, to which NbCl dissolved in butanol is subsequently added₅。

In one embodiment, TaCl dissolved in butanol is added to an acetic acid solution of a hydroxyacetyl chloride complex of Ti₅。

Example 1

Titanium mesh bodies of dimensions 10cm x 10cm were grit blasted with diamond grit and the treated residue was cleaned by a jet of compressed air. The flakes were then degreased in an ultrasonic bath using acetone for about 10 minutes. After the drying step, the tablets were placed in a bath containing 250g/l NaOH and 50g/l KNO at about 100 ℃₃Is soaked in the aqueous solution of (1) for 1 hour. After the alkaline treatment, the tablets were rinsed three times with deionized water at 60 ℃ with each liquid change. A final rinse step was performed, adding a small amount of HCl (about 1ml/l solution). Air drying is carried out due to TiO_xThe film grows and the formation of a brown color is observed.

A 100ml hydroalcoholic solution of a 1.3M Ti-based precursor suitable for the deposition of a protective layer with a molar composition of 98% Ti, 1% Bi, 1% Nb was then prepared using the following composition:

65ml of a 2M solution of Ti hydroxyacetyl chloride complex;

32.5ml of ethanol and pure reagent;

0.41g of BiCl₃；

1.3ml of 1M NbCl₅And (4) butanol solution.

A2M solution of a hydroxyacetyl chloride complex of Ti was obtained by: 220ml of TiCl are added₄Dissolved in 600ml of 10% by volume aqueous acetic acid, the temperature was controlled by ice bath below 60 ℃ and the solution obtained was placed in a volume (volume) of the same 10% acetic acid until the above concentration was reached. Under stirring, adding BiCl₃Hydroxy acetyl chloride dissolved in TiTo the complex solution, NbCl was added₅Solution and ethanol. The resulting solution was then placed in a volume of 10% aqueous acetic acid. A volumetric dilution of about 1: 1 resulted in a Ti final concentration of 62 g/l.

The solution obtained was applied to the titanium flakes previously prepared by multi-coat brushing until about 3g/m²Of TiO 2₂And (4) loading. After each coating, a drying step was performed at 100-110 ℃ for about 10 minutes, followed by a heat treatment at 420 ℃ for 15-20 minutes. Each time the sheet was cooled in air before applying a subsequent coating. The desired loading was achieved by applying two coats of the above hydroalcoholic solution. After the application was completed, a matte gray electrode was obtained.

100ml of precursor solution suitable for the deposition of a catalytic layer with a molar composition of 20% Ru, 10% Ir, 10% Pd, 59% Sn, 1% Nb was prepared using the following components:

42.15ml of a 1.65M Sn hydroxyacetyl chloride complex solution;

12.85ml of a 0.9M Ir hydroxyacetyl chloride complex solution;

25.7ml of a 0.9M solution of Ru hydroxyacetyl chloride complex;

12.85ml of 0.9M Pd (NO) acidified with nitric acid₃)₂A solution;

1.3ml of 1M NbCl₅A butanol solution;

5ml of ethanol, reagent pure.

Preparing a Sn hydroxyacetyl chloride complex solution according to the procedure disclosed in WO 2005/014885; a solution of a hydroxyacetylchloride complex of Ir and Ru is obtained by: the relevant chloride was dissolved in 10% by volume aqueous acetic acid, the solvent was evaporated, washed with 10% by volume aqueous acetic acid, and then the solvent was evaporated twice more, and finally the product was dissolved again in 10% aqueous acetic acid to obtain the specified concentration.

By acetylating hydroxyThe chloro complex solution is premixed and then NbCl is added with stirring₅Solution and ethanol.

The solution obtained was applied to the titanium flakes previously prepared by multi-coat brushing until about 9g/m²Is expressed as the sum of the elements Ir, Ru and Pd involved. After each coating, a drying step was carried out at 100-110 ℃ for about 10 minutes, followed by heat treatment of the first two coatings at 420 ℃ for 15 minutes, at 440 ℃ for the third and fourth coatings and at 460-470 ℃ for the subsequent coatings. Each time the sheet was cooled in air before applying a subsequent coating. The desired loading was achieved by applying six coats of precursor solution.

This electrode is labeled as sample a 01.

Example 2

Titanium mesh bodies of dimensions 10cm x 10cm were grit blasted with diamond grit and the treated residue was cleaned by a jet of compressed air. The tablets were then degreased in an ultrasonic bath using acetone for about 10 minutes. After the drying step, the tablets were placed in a bath containing 250g/l NaOH and 50g/l KNO at about 100 ℃₃Is soaked in the aqueous solution of (1) for 1 hour. After the alkaline treatment, the tablets were rinsed three times with deionized water at 60 ℃ with each liquid change. A final rinse step was performed, adding a small amount of HCl (about 1ml/l solution). Air drying is carried out due to TiO_xThe film grows and the formation of a brown color is observed.

A100 ml 1.3M hydroalcoholic solution of Ti-based precursors suitable for the deposition of a protective layer with a molar composition of 98% Ti, 2% Ta was then prepared using the following composition:

65ml of a 2M solution of Ti hydroxyacetyl chloride complex;

32.5ml of ethanol and pure reagent;

2.6ml of 1M TaCl₅And (4) butanol solution.

The hydroalcoholic solution of Ti hydroxyacetyl chloride complex was the same as in the previous examples.

Under stirring, adding TaCl₅The solution was added to a solution of Ti hydroxyacetyl chloride complex, followed by addition of ethanol. Next, the obtained solution was put into a volume with 10 vol% of aqueous acetic acid. A volumetric dilution of about 1: 1 resulted in a Ti final concentration of 62 g/l.

The solution obtained was applied to the previously prepared titanium sheet by multi-coat brushing until about 3g/m was reached²Of TiO 2₂The amount of the supported. After each coating, a drying step was performed at 100-110 ℃ for about 10 minutes, followed by a heat treatment at 420 ℃ for 15-20 minutes. Each time the sheet was cooled in air before applying a subsequent coating. The desired loading was achieved by applying two coats of the above hydroalcoholic solution. After the application was completed, a matte gray electrode was obtained.

The electrode was activated by a catalytic layer with a molar composition of 20% Ru, 10% Ir, 10% Pd, 59% Sn, 1% Nb as in example 1, with the only difference that PdCl was previously dissolved in ethanol₂Instead of adding Pd as nitrate in acetic acid solution.

This electrode is labeled as sample B01.

Comparative example

Titanium mesh bodies of dimensions 10cm x 10cm were grit blasted with diamond grit and the treated residue was cleaned by a jet of compressed air. The tablets were then degreased in an ultrasonic bath using acetone for about 10 minutes. After the drying step, the tablets were placed in a bath containing 250g/l NaOH and 50g/l KNO at about 100 ℃₃Is soaked in the aqueous solution of (1) for 1 hour. After the alkaline treatment, the tablets were rinsed three times with deionized water at 60 ℃ with each liquid change. A final rinse step was performed, adding a small amount of HCl (about 1ml/l solution). Drying in air due to TiO_xThe film grows and the formation of a brown color is observed.

A protective layer with a molar composition of 98% Ti, 2% Ta was then deposited on the electrode as in example 2.

Similar to the previous examples, starting from the relevant hydroxyacetyl chloride complex solution, the electrode was activated with a catalytic layer with a molar composition of 25% Ru, 15% Ir, 60% Sn. Also in this case, about 9g/m was applied using the same technique²Of the noble metal.

This electrode is labeled as sample B00.

Example 3

Starting from the pretreated titanium mesh sheet described above, having dimensions of 10cm x 10cm, a series of coupons, labeled a02-a11, were prepared using the reagents and methods as in example 1, with a protective layer having a molar composition of 98% Ti, 1% Bi, 1% Nb, and a catalytic layer having the composition and specific noble metal loading reported in table 1.

Example 4

Starting from the pretreated titanium mesh pieces of dimensions 10cm x 10cm described above, a series of test pieces labeled B02-B11 were prepared using the reagents and methods as in example 2, with protective layers of molar composition 98% Ti, 2% Ta, and catalytic layers of composition and specific noble metal loadings reported in table 1.

Example 5

The samples of the preceding examples were characterized by chlorine-evolving anodes in laboratory cells filled with sodium chloride brine at a concentration of 220g/l, strictly controlled to pH 2. Table 1 reports the values at 2kA/m²The current density of (a) and the volume percentage of oxygen in the product chlorine.

TABLE 1

The foregoing description should not be considered as limiting the invention, which may be used according to different embodiments without departing from its scope, which is defined solely by the appended claims.

In the description and claims of this application, the terms "comprise" and variations thereof such as "comprises" and "comprising" are not intended to exclude the presence of other elements or additives.

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.

Claims (10)

1. An electrode suitable for use as an anode in an electrolytic cell comprising a valve metal substrate and an external catalytic layer comprising oxides of tin, ruthenium, iridium, palladium and niobium in elemental molar ratios of Sn50-70%, Ru5-20%, Ir5-20%, Pd1-10%, nb0.5-5%.

2. The electrode of claim 1, comprising a TiO-containing layer interposed between the valve metal substrate and the outer catalytic layer₂The protective layer of (1).

3. The electrode of claim 2, wherein the TiO-containing is present in a total elemental molar ratio of 0.5-3%₂The protective layer of (2) is added with an oxide of tantalum, niobium or bismuth.

4. A method for producing an electrode according to claim 1, comprising: a precursor solution comprising hydroxyacetyl chloride complexes of Sn, Ir and Ru, at least one Pd-soluble species and at least one Nb-soluble species is applied to a valve metal substrate in multiple coatings, after each coating, a heat treatment at a temperature of 400 to 480 ℃ for a period of 15 to 30 minutes.

5. The process according to claim 4, wherein said at least one Pd soluble substance is chosen from Pd (NO) pre-dissolved in aqueous nitric acid₃)₂And PdCl pre-dissolved in ethanol₂And the at least one Nb-soluble substance is NbCl pre-dissolved in butanol₅。

6. A method for producing an electrode according to claim 2 or 3, comprising: a first hydroalcoholic solution comprising a titanium hydroxyacetyl chloride complex and at least one salt of tantalum, niobium or bismuth is applied to a valve metal substrate in multiple coatings, after each coating a heat treatment is carried out for 15-30 minutes at a temperature of 400 to 480 ℃, after which a second hydroalcoholic solution comprising a hydroxyacetyl chloride complex of Sn, Ir and Ru, at least one Pd-soluble substance and at least one Nb-soluble substance is applied in multiple coatings, after each coating a heat treatment is carried out for 15-30 minutes at a temperature of 400 to 480 ℃.

7. The method of claim 6, wherein the first hydroalcoholic solution is prepared by passing BiCl₃Dissolving in a solution of titanium hydroxyacetyl chloride complex in acetic acid and subsequent addition of NbCl in butanol₅And then the preparation.

8. The method of claim 6, wherein the first hydroalcoholic solution is prepared by dissolving TaCl in butanol₅Added to a solution of titanium hydroxyacetyl chloride complex in acetic acid.

9. An electrolysis cell comprising a cathode chamber housing a cathode and an anode chamber housing an anode separated by a membrane or membrane layer, the anode chamber being filled with an alkali chloride brine, wherein the anode of the anode chamber is an electrode according to any one of claims 1 to 3.

10. A process for producing chlorine and alkali comprising applying a potential difference between an anode and a cathode in an electrolytic cell according to claim 9 and evolving chlorine on the surface of said anode chamber.