EP2447395A2 - Electrode for producing chlorine through electrolysis - Google Patents

Abstract

Electrode comprises at least an electrically conductive substrate and a catalytically active coating. The catalytically active layer is based on two catalytically active components comprising at least iridium, ruthenium or titanium as a metal oxide and/or mixed oxide. The total amount of ruthenium and/or iridium based on the sum of iridium, ruthenium and titanium is at least 10 mole-%, preferably 10-20 mol% and up to half of the ruthenium and/or iridium is replaced by vanadium or zirconium. At least one oxide base layer is applied on the electrically conducting substrate. Electrode comprises at least an electrically conductive substrate and a catalytically active coating. The catalytically active layer is based on two catalytically active components comprising at least iridium, ruthenium or titanium as a metal oxide and/or mixed oxide. The total amount of ruthenium and/or iridium based on the sum of iridium, ruthenium and titanium is at least 10 mole-%, preferably 10-20 mol% and up to half of the ruthenium and/or iridium is replced by vanadium or zirconium, preferably vanadium. At least one oxide base layer is applied on the electrically conducting substrate and the oxide base layer is impermeable for aqueous electrolytes comprising sodium chloride and/or sodium hydroxide or hydrogen chloride. Independent claims are also included for: (1) producing the electrode, comprising applying sol-gel coating solution containing a solution or dispersion of metal compounds of at least one of ruthenium, iridium or titanium on the electrically conductive substrate, preferably by immersion, removing the solvent, and calcining the dried metal compound layer at elevated temperature, preferably at least 350[deg] C, preferably at least 400[deg] C in the presence of oxygen-containing gases and optionally the steps of applying the solution or dispersion, drying and calcinating is carried out once or repeated many times; and (2) an electrolyzer for electrolyzing the electrolysis of sodium chloride or hydrogen chloride-containing solutions, comprising the electrode as an anode.

Description

The invention is based on electrodes containing known noble metal catalysts for the electrolytic production of chlorine. The invention relates to a process for the preparation of a catalyst coating for electrodes for improved chlorine production, which is applied to an electrically conductive substrate and novel electrodes obtainable from the process. The coating consists in particular of a dense and crack-free base layer and a highly porous covering layer. The base and cover layers consist, for example, of mixed oxides which are based on Ti, Ru or Ir and one or more doping elements of the transition metal series. Both layers are applied to an electrically conductive support by means of a modified sol-gel process followed by thermal treatment.

Industrial chlorine production takes place via the electrolytic decomposition of sodium chloride or hydrochloric acid solutions. As an alternative to the previously used amalgam process, today only the diaphragm and ion exchange membrane processes are used. The biggest cost factor in chlorine production is electrical energy. The energy consumption for producing one ton of chlorine in chloralkali electrolysis (membrane process) is typically about 2500 kWh. The other two mentioned methods are far more energy-intensive. In order to reduce the costs of chlorine production, the design of the cells greatly reduces ohmic resistances which occur at membranes, in the electrolyte and at the electrodes. In order to further reduce energy consumption, catalysts are applied to the electrodes which reduce the decomposition voltage in the electrolysis. The overvoltage for the charge transfer at the anode depends strongly on the material, the morphology of the surface and the method of preparation of the catalyst from. Modern anodes are usually based on expanded metal sheets as supports made of titanium, nickel, tantalum or zirconium. On the surface of these supports are usually applied oxides or oxide mixtures of elements of the platinum group. It has been shown that ruthenium dioxide (RuO ₂ ) is a stable catalyst for chlorine evolution. A decisive disadvantage of RuO ₂ as an electrocatalytically active material is the poor long-term stability of known electrode coatings produced therefrom by dissolution of the catalyst coating during the electrolysis. As a result, the catalytic activity of the anode in use decreases steadily. To stabilize the active ruthenium component, for example, a mixture of RuO ₂ and titanium dioxide is used in the choral alkali electrolysis. Due to the same crystal structure, RuO ₂ and TiO _{2 form} a solid solution, which prevents the chemical attack and the discharge of RuO ₂ . This effect is described in the patent US Pat. No. 3,632,498 described. The authors of US 3,562,008 find a reduced cell voltage for chlorine deposition and improved durability compared to an uncoated Pt anode for an RuO ₂ -TiO ₂ -coated anode containing 50 mol% ruthenium.

The influence of the use of different materials and coating morphologies on the electrocatalytic effectiveness of the chlorine deposition is described in various documents, wherein the effects can not always be completely separated from each other.

The DE 40 32 417 A1 describes a RuO ₂ -TiO ₂ coating with a gradient structure. In this case, the ruthenium content decreases towards the anode surface in the layer from 40 to 20 mol%. The anodic oxidation should thus take place almost exclusively at the interface with the electrolyte and thus avoid a life-reducing volume erosion.

EP 0 867 527 A1 describes the preparation of a ternary oxide mixture of TiO ₂ , RuO ₂ and IrO ₂ for different electrode applications with a gradient from the first deposited layer to the surface increasing gradient of the ratio of metal oxides of the noble metals to the oxide of the valve metal from 13 to 100 mol% to the outermost applied Layer. Since the uppermost layer consists only of noble metal oxides, at least in the case of chlorine production, the abovementioned disadvantages of pure noble metal oxides, in particular the insufficient long-term stability, are to be expected.

An improved performance of the chlorine separation is in US Pat. No. 4,517,068 achieved by a combination of TiO ₂ RuO ₂ with palladium oxide.

TAF Lassali et al. (Electrochimica Acta 39, 1545 (1994 )) find a drastic increase of the electrochemically active surface for a replacement of TiO ₂ by PtO _x and thereby an increase of the electrocatalytic activity. The authors describe a layer composition of Ru _0.3 Pt _x Ti _0.7-x O ₂ . The catalyst layer is prepared by thermal decomposition of metal salts (eg metal halides). A disadvantage is the high cost of recovering the precious metal platinum. Furthermore, chemical instability of Pt-containing coatings (triggering of platinum in the form of hexachloroplatinate solutions) can reduce the lifetime of the coating and thus make a technical application more difficult.

Also by thermal decomposition of various metal salts is in WO 2006/028443 A1 describes the preparation of palladium-containing catalyst mixed oxide layers which, in addition to Ru and iridium oxides, may also contain antimony, tin and tantalum as doping elements. Here a lowering of the overpotential for chlorine evolution by 100 mV was obtained compared to undoped layers.

Furthermore, thermal decomposition methods for producing doped and undoped catalyst layers which are applied as a single layer to a metallic substrate are known from the following publications: J. Electrochem. Soc. 129 1689 (1982 ) Electrochimica Acta 42, 3525 (1997 ) J. Alloys and Compounds 261, 176 (1997 ).

S. Trasatti, Electrochim. Acta, 36, 225 (1991 ) and GRP Malpass et al, Electrochim. Acta, 52, 936 (2006 ) describe the typical morphologies of the electrocatalytically active oxide coatings prepared by the conventional thermal decomposition method. Defects in the oxide coating, such as cracks, pores, holes, and grain boundaries, provide the electrolyte with highly accessible internal catalytic sites, and therefore increase the apparent electrocatalytic activity for the chlorine-forming reaction.

On the other hand, the electrolytes can penetrate through the defects of the electrocatalytic active oxide layer to the titanium substrate and attack this, which causes a detachment of the electrocatalytically active oxide layer and further promotes the growth of an electrically insulating titanium oxide intermediate layer between the metallic support material and the active oxide layer, which subsequently results in increased ohmic losses and deactivation of the anode, such as LM Da Silva, et al., J. Electroanal. Chem. 532, 141 (2002 ) is described.

Various strategies have been developed to avoid the passivation of the support material caused by contact with electrolytes used in the electrolysis, for example aqueous solutions of sodium chloride or hydrogen chloride in chlorine production.

As in EP 0046449 Al Usually, many coating application / sintering cycles are used to increase the layer thickness and increase the life of the coating. The cracks and pores of the coating layer applied last are at least partially filled with the coating solution during the next coating application. The number of internal defects is reduced with each additional order cycle.

A drawback of this method is the large number of job cycles required with high material consumption. Another disadvantage is that because of the lower defect number, the electrochemically active area and thus the electrocatalytic activity decreases, combined with an increased consumption of electrical energy in the electrolysis. Furthermore, the lower electrical conductivity compared to the carrier material can lead to increased ohmic losses and to increased cell voltages in the electrolysis when layer thicknesses are too high.

To avoid the formation of electrically or poorly conducting titanium oxide interlayers, the concept of an interlayer located on the valve metal support and below the electrocatalytically active outer layer has been developed. Such an intermediate layer may also be described by the terms undercoat layer, barrier layer, protective layer or base layer, and the outer layer may also be referred to as cover layer.

One possibility of a base layer is the application of metallic platinum, as used, for example, for the cathodic protection of Titan in Hayfield, Precious Metal Review 27 (1) 1983_2-8 is described. Disadvantages of an application in chlorine production are, in particular, the chemical instability (formation of hexachloroplatinate solutions), which requires a great deal of effort to recover the noble metal platinum.

Other publications describe the production of electrochemically more stable noble metal-free intermediate layers via thermal decomposition, plasma jet or plasma spraying. In US 3,882,002 For example, antimony-doped tin oxide is applied as a protective layer to the valve metal substrate with an outer noble metal layer. US Pat. No. 3,950,240 describes an anode with niobium-doped tin oxide as an intermediate layer and a cover layer of ruthenium oxide. US Pat. No. 7 211 177 B2 describes an intermediate layer of titanium carbide or titanium boride for the electrolysis of hydrochloric acid.

Disadvantage of all mentioned intermediate layers are a non-optimal adhesion between the protective layer with the carrier material and the intermediate layer with the electrocatalytically active cover layer.

The increase in the surface area of ruthenium oxide-based electrode layers was reported by Y. Takasu (Electrochemica Acta 45, 4135 (2000 )) using five different methods, (1) the addition of sodium carbonate to an alcoholic dipping solution for dip-coated RuO ₂ -Ti electrodes, (2) the production of RuO ₂ -MO _x (M: doping metal) by dip coating, (3 ) the production of RuO ₂ -RO _x (R: rare earth element) layers on titanium electrodes by dip coating followed by chemical dissolution of the rare earth oxide by acid, (4) the addition of ammonium bicarbonate salt as a catalyst in the sol-gel process Production of ultrafine RuO ₂ particles and (5) the conversion of RuO ₂ into an H _x RuO _y layer structure were used.

The characterization of the surface structure was carried out electrochemically by determining the cyclovoltammetric charge on the electrodes produced. For mixed oxide electrodes of the type RuO ₂ -TiO ₂ , the cyclovoltammetric charge is 12 mC / cm ² and for RuO ₂ -SnO ₂ is 13 mC / cm ² . The highest cyclovoltammetric charges were obtained for the systems RuO ₂ -VO _x (162 mC / cm ² ), RuO ₂ -MoO ₃ (120 mC / cm ² ), RuO ₂ -CaO (130 mC / cm ² ) usually no more homogeneous mixed oxides obtained but a heterogeneous mixture of different oxides, with the above-mentioned disadvantages for pure RuO ₂ as a catalyst for chlorine production. These disadvantages also include coating compositions obtained by the above-mentioned methods (1), (3), (4) and (5). Application examples for the application of the coatings for the chlorine production are not described.

The processes presented so far enable the preparation of catalyst layers having a typical noble metal content between 20 and 50 mol%. Since noble metal salts and the precious metal itself have to be obtained in a very complex manner in many process steps, the technical outlay for use in large-scale industrial processes is very high and limits its use.

It could be shown that in the simple RuO ₂ -TiO ₂ system with the partial replacement of RuO ₂ by transition metal oxides (Ce, Nb, Sn, V, Cr, Mn, Co, Sb, Zr, Mo, W, Ta) Reduction of the ruthenium content is possible. In addition, by combining various transition metal oxides with a synergistic effect, an improvement in activity, selectivity and stability can be achieved. Experimental data is from SV Evdokimov et al. in Russian Journal of Electrochemistry 38, 657 (2002 ). The addition of 10 mol% of CrNbO ₄ reduced the RuO ₂ content in the standard system RuO ₂ -TiO ₂ from 70 to 30 mol%. The electrocatalytic activity is comparable to the standard system.

US 3,776,834 describes that the overpotential for the chlorine deposition with respect to the system 33.3 mol% RuO ₂ -66.7 mol% TiO _{2 is achieved} by 40 mV by using the ternary system 19 mol% RuO ₂ , 13 mol% SnO ₂ , 68 mol% TiO ₂ , by partially replacing the RuO ₂ with SnO ₂ . The use of doping elements can also increase the chemical corrosion resistance. US 4,039,409 and US 3,948,751 describe the preparation of doped catalyst layers by mixing the salt of a doping element with thermally unstable Ru salts and applied directly to titanium in an application process. The final thermal treatment results in a doped catalyst layer. The composition in the layer can be controlled by the amount of salt of the doping element. This technique is detailed in YE Roginskaya et al., Electrochimica Acta 40, 817 (1995 ). A disadvantage is the very inhomogeneous microstructures thus obtained with a coating composed of several components. These observations are also in US 4,668,531 described. At temperatures above 500 ° C., a dense and electrically insulating TiO ₂ layer is formed between the catalyst layer and the support. Such Intermediate layers cause a deterioration of the electrode performance. This connection is in WO 2008/046784 A1 described.

It has been shown that the sol-gel technique is a very good alternative to the thermal decomposition of noble metal salts. In this case, mixed oxides can be prepared by the controlled hydrolysis and condensation of precursor compounds.

According to the CN 1900368 A1 are introduced into an anode coating consisting of RuO ₂ and a high content of CeO ₂ dopants of Sn, Ir, Mn, and cobalt oxide by means of a sol-gel process. The authors observed in the doped ternary or quaternary coatings (with 45 mol% RuO ₂ ) increased activity for the chlorine evolution with respect to the binary RuO ₂ / CeO ₂ system. As in VV Panic et al., Colloids and Surfaces A 157, 269 (1999 ), catalyst coatings prepared by the sol-gel technique exhibit increased stability and durability compared to conventionally prepared coatings (thermal salt decomposition). Also report Y. Zeng et al., Ceramics International 33, 1087 (2007 ) that RuO ₂ -IrO ₂ -TiO ₂ catalyst layers, which were prepared by sol-gel technique, an increased activity due to the reduced particle size is observed.

However, the known sol-gel technique shows a particular disadvantage with respect to the solubility of inorganic salts and alkoxides in organic solvents. In order to obtain sufficient solubility, modification with strong acids and sonication is necessary. These procedures dramatically extend the manufacturing process. Some of these coating solutions prepared in this way have a low resistance, since the less soluble components precipitate prematurely (especially at high concentrations). These solutions can not be stored and lead in extreme cases to inhomogeneous coatings of the electrodes.

It was searched for new coatings and associated preparation methods, which do not show the disadvantages of the coatings described above - a high noble metal content, insufficient selectivity and stability and lack of electrical conductivity - and the manufacturing process.

For the reasons described above, it was an object of this invention, while avoiding the abovementioned disadvantages based on the sol-gel process, to develop a simple and versatile method for producing a catalyst layer for electrodes, in particular for anodes for chlorine production. This layer consists of an electrocatalytically active component and a stabilization, which is intended to ensure long-term stability. Farther should the anodes in comparison to the prior art have a reduction of the chlorine overvoltage and the reduction of the noble metal content.

The invention relates to a new electrode at least consisting of an electrically conductive substrate and a catalytically active coating, characterized in that the catalytic active layer is based on two catalytically active components containing at least iridium, ruthenium or titanium as the metal oxide or mixed oxide or mixtures of said Contains oxides, wherein the total content of ruthenium and / or iridium based on the sum of the elements iridium, ruthenium and titanium, at least 10 mol%, preferably 10 to 28 mol%, preferably 10 to 20 mol%, and that at least an oxidic base layer is provided, which is applied to the electrically conductive support and which is impermeable to aqueous electrolytes comprising NaCl and / or NaOH or HCl.

The electrically conductive substrate is preferably based on a valve metal, particularly preferably on a metal of the series titanium, tantalum, niobium and nickel or an alloy of these metals with a main constituent of titanium, tantalum or niobium.

The new catalyst coating for chlorine evolution consists for example of an active noble metal component, preferably RuO ₂ or RuO ₂ / IrO ₂ with a content of 10 - 20 mol% calculated on the Gesamtmetalloxidmenge, a component for stabilization (preferably TiO ₂ ) and a doping in the form a transition metal oxide (preferably tin, lanthanum, vanadium, zirconium, chromium, molybdenum). The concentration of the doping is in particular 5 to 15 mol%.

A new electrode is preferred, which is characterized in that the base layer is impermeable to aqueous hydrochloric acid solution, sodium chloride solution or sodium hydroxide solution. This is the case, for example, in a cyclovoltammetric experiment (in 3.5 molar aqueous sodium chloride solution at pH = 3 or in 0.5 molar hydrochloric acid, room temperature, Ag / AgCl reference electrode, scanning range 0.2 to 1.0 V (vs. Ag / AgCl ). The cyclovoltammetric capacitive charge (q _a ) determined by integration of the anodic branch of the cyclic voltammogram in a potential scan speed range of 5 mV / s to 200 mV / s is always less than 10 mC / cm ² , preferably less than 5 mC / cm ² and more preferably less than 2 mC / cm ² .

Particularly preferred is an embodiment of the electrode, in which an additional cover layer is provided, which has a cyclovoltammetrische capacitive charge ( q _a ) which is greater than that of the base layer. The cyclovoltammetric capacitive charge of the cover layer ( q _a ) is preferably at least 10 mC / cm ² , more preferably at least 20 mC / cm ² .

The base layer has in particular a surface charge (as oxide) of 0.1 to 20 g / m ² , preferably 0.5 to 10 g / m ² , the outer layer in particular a surface charge (as oxide) of at least 2 g / m ² , preferably at least 5 g / m ² .

A new electrode is preferred in which the loading (basis weight) of the cover layer is at least 2 g / m ² , preferably at least 5 g / m ² . A particular embodiment of the new electrode is characterized in that the cover layer, viewed in a cross section through the layer thickness, has a varying amount ratio of iridium to titanium and / or ruthenium to the titanium component.

In a preferred variant of the novel electrode, the ratio of iridium to titanium and / or ruthenium to titanium in the cover layer decreases in a cross section through the layer thickness as viewed from the outside in the direction of the electrically conductive support.

As starting compounds for the preparation of Edelmetalloxid- and stabilizing component (eg TiO ₂ ) of the anode coating chlorides, nitrates, alkoxides, acetylacetonates are preferably used. As precursor salts for the production of the noble metal component, preference is given to using ruthenium acetylacetonates, iridium acetylacetonates or iridium acetates. TiO ₂ is available for example from titanium isopropoxide or titanium butoxide. The doping agents are more preferably introduced via the precursor salts vanadium acetylacetonate, vanadium tetrabutoxide, zirconium n-propoxide, zirconium nitrate, molybdenum acetate, tin acetate, zinc isopropoxide, lanthanum acetylacetonate, lanthanum nitrate.

A further preferred embodiment of the new electrode is characterized in that the covering layer comprises the components of the catalytically active layer and additionally contains pore-forming compounds, in particular compounds of lanthanum, in particular lanthanum oxide or polymers, in particular polyvinylpyrrolidone.

In another preferred embodiment of the novel electrode, the base layer is electrically conductive and has a conductivity of at least 10 S / m, preferably at least 1000 S / m, more preferably at least 10000 S / m.

In addition to the electrode structure described above, a further novel electrode was found, which is the subject of the invention, and at least consists of an electrically conductive substrate and a catalytically active coating, characterized in that the catalytic active layer is based on two catalytically active components, containing at least iridium, ruthenium or titanium as metal oxide or mixed oxide or mixtures of said oxides, wherein the total content of ruthenium and / or iridium based on the sum of Elements iridium, ruthenium and titanium, at least 10 mol%, preferably 10 to 28 mol%, preferably 10 to 20 mol%, and that up to half of the ruthenium and / or iridium by vanadium, zirconium or molybdenum, is preferred replaced by vanadium.

This selected structure may preferably be combined with the object of the electrode described first, so that in this case also an oxide base layer is provided, which is applied to the electrically conductive support and impermeable to aqueous electrolytes comprising NaCl and / or NaOH or HCl is.

Another object of the invention is a method for producing an electrode, in particular a new electrode described above, characterized in that in a first step to an electrically conductive carrier one or more times sol-gel coating solution containing a solution or dispersion of metal compounds of one or more the metals: Ruthenium, iridium and titanium are applied to the support in particular by immersion and then freed of solvent, and then the dried metal compound layer at elevated temperature, in particular at least 350 ° C, preferably at least 400 ° C, calcined in the presence of oxygen-containing gases and, if appropriate, the steps: application of the solution or dispersion, drying and calcination are repeated one or more times.

Preference is given to a process in which one or more times a solution or dispersion of metal salts of the metals ruthenium, iridium and titanium is applied to the base layer to remove a solvent, and at elevated temperature, in particular at least 350 ° C, preferably at least 450 ° C, is calcined in the presence of oxygen-containing gases.

For the production of the base layer, in a preferred embodiment of the method, after the application of the metal salt solution, drying is carried out at elevated temperature, in particular at least 200 ° C., preferably at least 240 ° C.

In a preferred variant of the method, a lower carboxylic acid, in particular propionic acid, C _{1 to} C ₅ alcohols or ketones or mixtures thereof is added to the metal compound solutions for the production of the base layer and / or the cover layer.

To produce a homogeneous and stable coating solution, the so-called propionic acid-sol-gel method is particularly preferably used. Here, the solvent used for the abovementioned compounds is a mixture of propionic acid with alcohols (methanol, ethanol, n-propanol, isopropanol, butanol) in various concentrations. The resolution of Precursor salts occur separately for each precursor salt at a temperature above 130 ° C with stirring. The duration of the dissolution process is about 1 hour. This results in a transparent and stable sol solution. The use of propionic acid leads to the complexation of the metal cations used and thus allows a controlled hydrolysis.

After cooling, the solutions of the individual precursor salts are mixed and then stirred. From this mixture, the coating solution for the production of electrocatalytically active chlorine evolution anodes is formed. This coating solution may be applied to substrates such as e.g. Titanium sheets or titanium mesh (eg titanium expanded metal) can be applied. Before application, the substrates must be mechanically, chemically or electrochemically cleaned, polished and roughened. This results in improved adhesion of the coating.

• ● niedrige Prozesstemperatur
• ● Mischoxide mit kontrollierter Stöchiometrie können durch das Zusammenmischen von Solen unterschiedlicher Verbindungen leicht hergestellt werden
• ● große Produkthomogenität, weil die Durchmischung der Edukte auf Molekularer Ebene erfolgt
• ● Eine große Produktreinheit resultiert aus der vollständigen Entfernung einfacher organischer Reste durch eine finale Temperaturbehandlung
• ● Durch die abschließende thermische Behandlung kann Einfluss auf die Mikrostruktur (Porosität) der hergestellten Schicht genommen werden.
• ● Beschichtung komplexer Geometrien durch Tauch-, Spray- oder Spinncoatingverfahren Der Sol-Gel Prozess ermöglicht die Herstellung von Beschichtungslösungen bestehend aus einem oder mehreren leicht hydrolysierbaren Alkoxiden, Acetylacetonaten oder anorganischen Salzen, welche in einem Lösungsmittel (Methanol, Ethanol, Isopropanol, Butanol) gelöst sind. Um stabile Kolloide (Sol) herzustellen, wird die Lösung säure- (Salz-, Salpeter- oder Essigsäure) oder basenkatalysiert (Ammoniak, Natronlauge) hydrolysiert. Durch die Hydrolyse entsteht ein Netzwerk von hydroxy- und oxoverbrückten Metallatomen.

Compared with conventional thermal salt decomposition, the sol-gel method has the following advantages:

• ● low process temperature
• ● Mixed oxides with controlled stoichiometry can be easily prepared by mixing together brines of different compounds
• ● great product homogeneity, because the mixing of the educts takes place at the molecular level
• ● High product purity results from the complete removal of simple organic residues by a final heat treatment
• ● The final thermal treatment can influence the microstructure (porosity) of the produced layer.
• ● Coating of complex geometries by immersion, spray or spin coating The sol-gel process enables the production of coating solutions consisting of one or more readily hydrolyzable alkoxides, acetylacetonates or inorganic salts dissolved in a solvent (methanol, ethanol, isopropanol, butanol) are. In order to produce stable colloids (sol), the solution is hydrolyzed acid (hydrochloric, nitric or acetic acid) or base catalysis (ammonia, caustic soda). Hydrolysis creates a network of hydroxy- and oxo-bridged metal atoms.

Substrates can now be coated with this coating solution. In the subsequent drying process, the sol is fixed on the substrate. In a final sintering step, the organic components are removed and the mixed oxide crystallizes in nanostructured particles. The introduction of a doping can be done by means of the sol-gel technique. These dopings can increase the activity of the catalyst and thus allow a reduction of the noble metal content.

The coating solution can be applied by dipping, brushing, dropping, spraying or spinning. The layer is then dried, in particular at room temperature, and then sintered, for example, at 250 ° C. for at least 10 minutes and then at 450 ° C. for at least 5 minutes. The final sintering step for stabilizing the layer takes place, for example, at 450 ° C. for 12 to 30 minutes. For improved oxidation of the components, pure oxygen or an oxygen-enriched atmosphere may be used. By a different sequential repetition of the process described, the layer thickness can be varied. By this procedure, multilayers are accessible.

The invention further provides an electrolyzer for the electrolysis of solutions containing sodium chloride or hydrogen chloride, characterized in that a new electrode described above is provided as the anode of the electrolyzer.

The invention also relates to the use of the new electrode described above as an anode in electrolysers for the sodium chloride or hydrogen chloride electrolysis for the electrochemical production of chlorine.

The invention will be illustrated by the following embodiments and figures, which by no means limit the invention.

Figur 1.

Rasterelektronenmikroskopische Aufnahmen der Oberfläche der Beschichtung aus Beispiel 6. Maßstab: a) 10 µm,

Figur 2.

Rasterelektronenmikroskopische Aufnahme der Oberfläche der Beschichtung aus Beispiel 7. Maßstab: 10 µm,.

Figur 3.

Rasterelektronenmikroskopische Aufnahmen der Oberfläche der Beschichtung aus Beispiel 8. Maßstab: 10 µm

Figur 4.

Rasterelektronenmikroskopische Aufnahme der Oberfläche der Beschichtung aus Beispiel 9b. Maßstab: 10 µm

Figur 5.

Die voltammetrische Ladung (q _a) aufgetragen als Funktion der Potential-Scangeschwindigkeit (v) für die dichte Basisschicht (durchgezogene Linie) and Riss-Struktur (gepunktete Linie) aus Beispiel 9.

Figur 6.

Rasterelektronenmikroskopische Aufnahme der Oberfläche der Beschichtung aus Beispiel 10. Maßstab: 10 µm.

Figur 7.

Die voltammetrische Ladung (q _a) aufgetragen als Funktion der Zahl der Cyclovoltammogramme für die Ru_0.4Ti_0.6O₂ Beschichtung (durchgezogene Linie) und die Ru_0.4T1₀.₄₅La₀.₁₅O₂ Beschichtung (gepunktete Linie) aus Beispiel 11. Die Cyclovoltammetrie wurde in 0.5 molarer Salzsäure bei Raumtemperatur mit einer Ag/AgCl Bezugselektrode durchgeführt. Das Potential wurde im Bereich von 0,2 bis 1,0 V (vs. Ag/AgCl) mit einer Potential-Scangeschwindigkeit von v = 50 mV/s variiert.

Figur 8.

Die voltammetrische Ladung (q _a) aufgetragen als Funktion der Potential-Scangeschwindigkeit (v) für die Beschichtungen aus Beispiel 12. Die Cyclovoltammetrie wurde in 3,5 molarer Kochsalzlösung bei Raumtemperatur mit einer Ag/AgCl Bezugselektrode durchgeführt. Das Potential wurde im Bereich von 0,2 bis 1,0 V (vs. Ag/AgCl) variiert.

In the figures describe:

FIG. 1

Scanning electron micrographs of the surface of the coating from Example 6. Scale: a) 10 μm,

FIG. 2.

Scanning electron micrograph of the surface of the coating of Example 7. Scale: 10 μm.

FIG. 3.

Scanning electron micrographs of the surface of the coating of Example 8. Scale: 10 μm

FIG. 4.

Scanning electron micrograph of the surface of the coating of Example 9b. Scale: 10 μm

FIG. 5.

The voltammetric charge ( q _a ) plotted as a function of the potential scan rate ( v ) for the dense base layer (solid line) and crack structure (dotted line) of Example 9.

FIG. 6.

Scanning electron micrograph of the surface of the coating of Example 10. Scale: 10 μm.

FIG. 7.

The voltammetric charge ( q _a ) plotted as a function of the number of cyclic voltammograms for the Ru _0.4 Ti _0.6 O ₂ coating (solid line) and the Ru _0.4 T1 ₀ . ₄₅ La ₀ . ₁₅ O ₂ coating (dotted line) from Example 11. The cyclic voltammetry was carried out in 0.5 molar hydrochloric acid at room temperature with an Ag / AgCl reference electrode. The potential was varied in the range of 0.2 to 1.0 V (vs. Ag / AgCl) with a potential scan rate of v = 50 mV / s.

FIG. 8.

The voltammetric charge ( q _a ) plotted as a function of the potential scan rate ( v ) for the coatings of Example 12. Cyclic voltammetry was performed in 3.5 molar saline at room temperature with an Ag / AgCl reference electrode. The potential was varied in the range of 0.2 to 1.0 V (vs. Ag / AgCl).

Examples example 1

Titanium plates with a diameter of 15 mm (thickness 2 mm) were sandblasted for cleaning and roughening the surface and then etched in 10% oxalic acid at 80 ° C (2 hours), then cleaned with isopropanol and dried in a stream of nitrogen.

To prepare the coating solutions, 99.6 mg ruthenium acetylacetonate (Ru (acac) ₃ ), 207.2 μL titanium isopropoxide (Ti (i-OPr) ₄ ) and 13.3 mg vanadyl acetylacetonate (VO (acac) ₂ ) were dissolved in 1.45 mL isopropanol and 1.45 mL propionic acid, and Heated under reflux for 30 minutes. After cooling to room temperature, the three solutions were mixed to give a homogeneous and transparent solution of burgundy color. Using a micropipette, 50 μL of this coating solution was applied to the titanium substrate and then dried in air. The layer was sintered for 10 minutes at 250 ° C and then for 10 minutes at 450 ° C in air. These procedures (coating solution coating, drying, sintering) were repeated 8 times. After the 9th coating step, the coated titanium substrate was sintered at 450 ° C for one hour. This resulted in a sample of the composition 25 mol% Ru / 70 mol% Ti / 5 mol% V based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 6.4 g / m ² . This corresponds to a total coating loading of 23.7 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , V ₂ O ₅ ).

The electrocatalytic activity for chlorine evolution was determined by chronoamperometry (reference electrode: Ag / AgCl, electrolyte: 3.5 M NaCl, pH: 3, room temperature). In the experiment, a steady state current density of 1 kA / m ^{2 was} set. The resulting voltage was 1.17 volts.

Example 1b (comparative example)

The titanium substrates are pretreated analogously to Example 1.

To prepare a thermal decomposition coating, a coating solution containing 2.00 g of ruthenium (III) chloride hydrate (Ru content 40.5% by weight), 21.56 g of n-butanol, 0.94 g of concentrated hydrochloric acid and 5.93 g Titansäuretetrabutylester Ti- (O-Bu) ₄ ). A part of the coating solution was applied to a titanium plate by means of a brush. This was air-dried at 80 ° C for 10 minutes and then at 470 ° C for 10 minutes in air treated. This process (solution coating drying, heat treatment) was carried out a total of eight times. Subsequently, the plate was treated for one hour at 520 ° C in air. The ruthenium surface charge was determined from the consumption of the coating solution at 16 g / m ² , which corresponds to a total coating loading of 49.2 g / m ² (as oxides) at a composition of 31 mol% RuO ₂ and 69 mol%. TiO ₂ .

The potential for the evolution of chlorine (measured as in Example 1) was 1.25 volts in this sample.

Example 2

The titanium substrates were pretreated analogously to Example 1.

In each case, 60 mg of ruthenium acetylacetonate (Ru (acac) ₃ ), 236.8 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) and 13.3 mg of vanadyl acetylacetonate (VO (acac) ₂ ) were dissolved in 1.45 ml of isopropanol and 1.45 ml of propionic acid and then for 30 minutes heated at 150 ° C under reflux (with stirring). After cooling to room temperature, the three solutions were mixed to give a homogeneous and transparent solution of burgundy color. Coating and sintering processes were carried out as described in Example 1.

This resulted in a sample of the composition 15 mol% Ru / 80 mol% Ti / 5 mol% V based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 3.9 g / m ² . This corresponds to a total coating loading of 22.7 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , V ₂ O ₅ ).

The potential for the evolution of chlorine (measured as in Example 1) was 1.17 volts for this sample.

Example 3

The titanium substrates were pretreated analogously to Example 1.

In each case 99.6 mg ruthenium acetylacetonate (Ru (acac) ₃ ), 207.2 μL titanium isopropoxide (Ti (i-OPr) ₄ ) and 22.4 μL zirconium n-propoxide (70 wt .-% in n-propanol) in 1.44 mL Dissolved isopropanol and 1.44 mL of propionic acid and then heated at 150 ° C for 30 minutes under reflux (with stirring). After cooling to room temperature, the three solutions were mixed to give a homogeneous and transparent solution of burgundy color. Coating and sintering processes were carried out as described in Example 1.

This resulted in a sample of the composition 25 mol% Ru / 70 mol% Ti / 5 mol% Zr based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 6.4 g / m ² . This corresponds to a total coating loading of 24.2 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , ZrO ₂ ).

The potential for the evolution of chlorine (measured as in Example 1) was 1.25 volts in this sample.

Example 4

The titanium substrates were pretreated analogously to Example 1.

In each case 99.6 mg of ruthenium acetylacetonate (Ru (acac) ₃ ), 192.4 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) and 42.8 mg of molybdenum acetate (Mo ₂ (OCOCH ₃ ) ₄ ) in 1.45 mL of isopropanol and 1.45 mL Propionic acid and then heated at 150 ° C for 30 minutes under reflux (with stirring). After cooling to room temperature, the three solutions were mixed to give a homogeneous and transparent solution of burgundy color. Coating and sintering processes were carried out as described in Example 1.

This resulted in a sample of the composition 25 mol% Ru / 65 mol% Ti / 10 mol% Mo based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 6.4 g / m ² . This corresponds to a total coating loading of 25.2 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , MoO ₃ ).

The potential for the evolution of chlorine (measured in accordance with Example 1) was 1.18 volts in this sample.

Example 5

The titanium substrates were pretreated analogously to Example 1.

In each case, 49.8 mg of ruthenium acetylacetonate (Ru (acac) ₃ ), 62.3 mg of iridium acetylacetonate, 16.6 mg of vanadyl acetylacetonate (VO (acac) ₂ ), 207.6 mg of zinc isopropanol (Sn (i-OPr) ₄ · C ₃ H ₇ OH) and 129.5 μL titanium isopropoxide (Ti (i-OPr) ₄ ) dissolved in 1.11 mL isopropanol and 1.11 mL propionic acid and then heated for 30 minutes at 150 ° C under reflux (with stirring). After cooling to room temperature, the five solutions were mixed to obtain a homogeneous and transparent solution with a wine red color.

Coating and sintering processes are carried out as described in Example 1.

This resulted in a sample of the composition 10 mol% Ru, 10 mol% Ir, 5 mol% V, 40 mol% Sn, 35 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 3.2 g / m ² . This corresponds to a total coating loading of 33.6 g / m ² (as the sum of the oxides RuO ₂ , IrO ₂ , V ₂ O ₅ , SnO ₂ , TiO ₂ ).

The potential for the evolution of chlorine (measured in accordance with Example 1) was 1.22 volts in this sample.

Example 6

The titanium substrates were pretreated analogously to Example 1.

In each case 149.4 mg ruthenium acetylacetonate (Ru (acac) ₃ ) and 333.1 μL titanium isopropoxide (Ti (i-OPr) ₄ ) were dissolved in 3.25 mL isopropanol and 3.25 mL propionic acid and then at reflux for 30 minutes at 150 ° C. (with stirring) heated. After cooling to room temperature, the two solutions were mixed to give a homogeneous and transparent solution with a wine-red color

The crack-free and dense coatings were prepared by dip coating. For this purpose, the titanium substrates were immersed in the coating solution for a period of 20 seconds and then pulled out vertically from the coating solution at a rate of 167 mm per minute and then dried in air. The layer was first sintered at 250 ° C for 10 minutes and then at 450 ° C for 5 minutes in air. The process of dip coating-drying-sintering was repeated 5 times.

This resulted in a sample of the composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 0.16 g / m ² . This corresponds to a total coating loading of 0.59 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

The resulting surface morphology of the coating produced was characterized by scanning electron microscopy, s. FIG. 1 ,

Example 7

The titanium substrates were pretreated analogously to Example 1.

The coating solutions are identical to Example 6

The crack-free and dense coatings were obtained analogously to Example 6. The process of dip coating-drying-sintering was repeated 15 times. This resulted in a sample of the composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 0.50 g / m ² . This corresponds to a total coating loading of 1.84 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

The resulting surface morphology of the coating produced was characterized by scanning electron microscopy, s. FIG. 2 ,

Example 8

The titanium substrates were pretreated analogously to Example 1.

The coating solutions were obtained by dissolving 421.1 mg of ruthenium (III) chloride hydrate (RuCl ₃ .xH 2 O, 36% Ru) in 4.62 mL of isopropanol and stirring overnight (solution A). 1,332 ml of titanium isopropoxide (Ti (i-OPr) ₄ ) were added to a premixed solution of 2.246 ml of 4-hydroxy-4-methyl-2-pentanone and 5 ml of isopropanol and stirred for 30 minutes (solution B). Solution A and Solution B were mixed by sonication and a clear solution was obtained. Subsequently, 25.8 μL of acetic acid and 108 μL of deionized water were added to the solution. The solution thus prepared was covered and stirred overnight at room temperature.

The crack-free and dense coatings were prepared by dip coating. For this purpose, the titanium substrates were immersed in the coating solution for 20 seconds, and then pulled out vertically from the coating solution at a rate of 193 mm per minute, followed by drying in air. The layer was sintered for 30 minutes at 90 ° C and then for 10 minutes at 450 ° C in air. The process of dip coating-drying-sintering was repeated 6 times.

This resulted in a sample of the composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 1.0 g / m ² . This corresponds to a total coating loading of 3.69 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

The resulting surface morphology of the coating produced was characterized by scanning electron microscopy, s. FIG. 3 ,

Example 9

The titanium substrates were pretreated analogously to Example 1.

The coating solutions were obtained by dissolving 210.6 mg of ruthenium (III) chloride hydrate (RuCl ₃ .xH 2 O, 36% Ru) in 4.81 mL of isopropanol and stirring overnight (solution A). 666.1 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) were added to a premixed solution of 1.123 ml of 4-hydroxy-4-methyl-2-pentanone and 20 ml of isopropanol and stirred for 30 minutes (solution B). Solution A and Solution B were mixed by sonication and a clear solution was obtained. Subsequently, 12.9 μL of acetic acid and 54 μL of deionized water were added to the solution. The solution thus prepared was covered and stirred overnight at room temperature.

The crack-free and dense coatings were prepared by dip coating. To do so, the titanium substrates were immersed in the coating solution for 20 seconds and then withdrawn vertically from the coating solution at a rate of 193 mm per minute. The wet coating was dried in air and sintered for 30 minutes at 90 ° C and then for 10 minutes at 450 ° C in air. The process of dip coating-drying-sintering was repeated 50 times, with a final sintering for 1 hour at 450 ° C.

This resulted in a sample of the composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 2.0 g / m ² . This corresponds to a total coating loading of 7.38 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

Example 9b (comparative example)

For comparison, a coating having a mud-crack structure was prepared, with the same coating solutions as in Example 9.

The titanium substrates were pretreated analogously to Example 1, and 50 .mu.l of the coating solution were applied by means of a dropping method by means of a micropipettor.

The wet coating was dried in air and sintered for 30 minutes at 90 ° C and then for 10 minutes at 450 ° C in air. The process of drop coating-drying-sintering was repeated 4 times, with a final sintering for 1 hour at 450 ° C.

This resulted in a sample of the composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 3.4 g / m ² . This corresponds to a total coating loading of 12.54 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

The resulting surface morphology of the coating produced was characterized by scanning electron microscopy, s. FIG. 4

The electrodes with the crack-free and the dry-crack structure were investigated by cyclic voltammetry. The measurements were carried out in 3.5 molar saline solution at pH = 3 and room temperature with a silver / silver chloride reference electrode. The potential was traversed in the range of 0.2 to 1.0 V with different potential scan speeds (v) in the range of 5 to 200 mV / s. The voltammetric capacitive charge (qa) was obtained by EC-Lab software by integration of the anodic branches of the cyclic voltammograms. The voltammetric charge (q _a ) as a function of the potential scan rate (v) for the crack-free coating and the coating with the dry cracking structure is in FIG. 5 applied. The voltammetric charge drops very much as the potential scan rate (v) is increased from 5 to 50 mV / s and then assumes an approximately constant value for the coating with the dry crack structure (dotted line in FIG FIG. 5 ). This is consistent with the fact that at low potential scan speeds (v), the electrolyte penetrates through the cracks into the inner cracks and voids, thus obtaining a high capacitive charge value, while at the higher potentials. Scan speeds only the outermost surface is accessible. In contrast, the capacitive loading of the crack-free coating is virtually independent of the potential scan rate (solid line in FIG FIG. 5 ), which proves the compact and dense character of the coating.

Example 10

In order to produce a coating with a crack-free and dense base layer and a cracked and electrocatalytically active cover layer, the crack-free base layer was prepared analogously to Example 7. This resulted in a sample of the base layer composition 25 mol% Ru, 75 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 0.50 g / m ² . This corresponds to a total base layer loading of 1.84 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ )

The cracked overcoat applied to the crack-free basecoat was carried out with the same coating solutions as in Example 1 and the same coating and sintering procedure as in Example 1 except that a total of 4 coating-sintering cycles were performed.

This resulted in a sample of the topcoat composition 25 mol% Ru / 70 mol% Ti / 5 mol% V based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 2.6 g / m ² . This corresponds to a total covering layer loading of 9.64 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , V ₂ O ₅ ).

The resulting surface morphology of the coating produced was characterized by scanning electron microscopy, s. FIG. 6 ,

The electrode potential for chlorine evolution was determined under analogous conditions as in Example 1. The measured electrode potential was 1.21 V.

Example 11

In order to produce a porous and cracked electrocatalytically active coating, the titanium substrates were pretreated analogously to Example 1.

The coating solutions were obtained by dissolving 67.4 mg of ruthenium (III) chloride hydrate (RuCl ₃ .xH ₂ O, 36% Ru) in 2 mL of isopropanol and stirring overnight (solution A). 39 mg lanthanum (III) nitrate hexahydrate (La (NO ₃ ) ₃ 6H ₂ O) were added to 1 mL isopropanol and stirred for 30 minutes (solution B). 80 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) were added to a premixed solution of 224.6 μl of 4-hydroxy-4-methyl-2-pentanone and 0.66 ml of isopropanol and stirred for 10 minutes (solution C).

Solutions A, B and C were mixed by sonication and a clear solution was obtained. Subsequently, 5.15 μL of acetic acid and 10.8 μL of deionized water were added to the solution. The solution thus prepared was covered and stirred overnight at room temperature.

By means of dropping, 50 μl of the coating solution was applied to the titanium substrate by means of a micropipettor.

The wet coating was dried in air and sintered for 10 minutes at 250 ° C and then for 10 minutes at 450 ° C in air. The procedure drop-on-dry drying-sintering was repeated 5 times, with a final sintering for 1 hour at 450 ° C.

This resulted in a sample of the composition 40 mol% Ru / 45 mol% Ti / 15 mol% La based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 8.5 g / m ² . This corresponds to a total coating loading of 29.0 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ , La ₂ O ₃ ).

For comparison, a coating of composition Ru was ₀ . ₄ Ti ₀ . ₆ O ₂ produced. To this was added 67.4 mg of ruthenium (III) chloride hydrate (RuCl ₃ -xH ₂ O, 36% Ru) in 2 mL of isopropanol and stirred overnight (solution A). 106.6 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) were added to a premixed solution of 224.6 μl of 4-hydroxy-4-methyl-2-pentanone and 1.66 ml of isopropanol and stirred for 10 minutes (solution B). , Solution A and Solution B were mixed by sonication and a clear solution was obtained. Subsequently, 5.15 μL of acetic acid and 10.8 μL of deionized water were added to the solution. The solution thus prepared was covered and stirred overnight at room temperature.

By means of dropping, 50 μl of the coating solution was applied to the titanium substrate by means of a micropipettor.

The wet coating was dried in air and sintered for 10 minutes at 250 ° C and then for 10 minutes at 450 ° C in air. The procedure drop-on-dry drying-sintering was repeated 5 times, with a final sintering for 1 hour at 450 ° C.

This resulted in a sample of the composition 40 mol% Ru / 60 mol% Ti based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 8.5 g / m ² . This corresponds to a total coating loading of 21.3 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ ).

Cyclic voltammetry was carried out in 0.5 molar hydrochloric acid at room temperature using silver / silver chloride as the reference electrode. The potential was traversed in the range from 0.2 to 1.0 volt with a potential scanning speed of υ = 50 mV / s, the area exposed to the electrolyte being 1 cm ² . The voltammetric capacitive charge (q _a ) was obtained by EC-Lab software by integration of the anodic branches of the cyclic voltammograms. The voltammetric charge ( q _a ) as a function of the number of cyclic voltammetry cycles for the Ru _0.4 Ti _0.45 La _0.15 O ₂ coating (dotted line) and Ru _0.4 Ti _0.6 O ₂ (solid line) in FIG. 7 applied.

The cyclovoltammetric charge q _{a of} the Ru _0.4 Ti _0.6 O ₂ coating is independent of the number of cyclic voltammetry cycles, which is an indication that the character of the coating does not change. For the Ru _0.4 Ti _0.45 La _0.15 O ₂ coating, however, a continuous increase of the voltammetric charge from the second to the 79th potential cycle was observed. This is caused by a continuous dissolution of the lanthanum oxide from the oxide matrix during the cyclic voltammetry cycles with a simultaneous increase in the coating porosity.

Example 12

In order to produce a porous and cracked electrocatalytically active coating, the titanium substrates were pretreated analogously to Example 1.

The four coating solutions were respectively obtained by dissolving 37.9 mg of ruthenium (III) chloride hydrate (RuCl ₃ · x H ₂ O, 36% Ru) in 1.33 mL of isopropanol and stirred overnight (Solution A). The four solutions B were each obtained by adding 130.8 mg of zinc isopropanolate (Sn (i-OPr) ₄ .C ₃ H ₇ OH) to a mixture of 1.34 ml of isopropanol and 1.33 ml of propionic acid and subsequent boiling under reflux at 150 ° C for 30 minutes and strong stirring. Subsequently, a different amount of lanthanum (III) nitrate hexahydrate (La (NO ₃ ) ₃ 6H ₂ O), namely 39 mg, 29.2 mg, 9.7 mg and 0 mg was added to each of the four hot solutions B and stirred for an additional 20 minutes before the solutions were cooled to room temperature. Subsequently, these solutions were each added to the solutions A dropwise with stirring.

By means of the dropwise method, in each case 50 μl of the coating solution was applied to the titanium substrate by means of a micropipettor.

The wet coatings were air dried and sintered for 10 minutes at 250 ° C and then for 10 minutes at 450 ° C in air. The coated titanium plates were then immersed in 5% hydrochloric acid at a temperature of 60 ° C for 15 minutes with gentle stirring to dissolve the lanthanum oxide components of the coatings.

The process of drop coating-drying-sintering-dissolution was repeated 8 times each, with a final sintering for 1 hour at 450 ° C.

This resulted in 4 samples of the composition 30 mol% Ru / 70 mol% Sn based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 7.7 g / m ² each. This corresponds to a total coating loading of 36.9 g / m ² (as the sum of the oxides RuO ₂ , SnO ₂ ).

The resulting coatings were labeled La39, La29, La9, La0, corresponding to the different amounts of lanthanum (III) nitrate hexahydrate (39 mg, 29.2 mg, 9.7 mg or 0 mg) contained in the coating solution ..

The electrodes were examined by cyclic voltammetry as described in Example 9 and the voltammetric capacitive charge ( q _a ) was determined as in Example 9. The voltammetric charge ( q _a ) as a function of the potential scan velocity (υ) of the electrodes is in FIG. 8 played. The voltammetric charge ( q _a ) decreases more and more as the potential scan rate increases from 5 to 200 mV / s. This is an indication of the cracked and porous structure of the coatings. An increase in capacitive charge with increasing lanthanum (III) nitrate hexahydrate was observed. The dissolution of lanthanum oxide gives coatings of high porosity.

Example 13

In order to produce a porous and cracked electrocatalytically active coating, the titanium substrates were pretreated analogously to Example 1.

In each case 99.6 mg of ruthenium acetylacetonate (Ru (acac) ₃ ), 192.4 μL of titanium isopropoxide (Ti (i-OPr) ₄ ) and 26.6 mg of vanadyl acetylacetonate (VO (acac) ₂ ) in 1.45 ml of isopropanol were used to prepare the coating solutions and 1.45 mL of propionic acid are dissolved and heated under reflux at 150 ° C for 30 minutes with vigorous stirring. After cooling to room temperature, the three solutions were mixed to give a homogeneous and transparent solution of burgundy color. 72.2 mg of polyvinylpyrrolidone K30 (PVP) (average molecular weight Mw = 40,000) was added to the solution and treated by sonication for 30 minutes. Using a micropipette, 50 μL of this coating solution was dropped on the titanium substrate and then dried in air. The layer was first sintered at 250 ° C for 15 minutes and then at 450 ° C for 20 minutes in air. These procedures (coating solution coating, drying, sintering) were repeated 8 times. Subsequently, the coated titanium substrate was sintered at 450 ° C for one hour.

This resulted in a sample of the composition 25 mol% Ru, 65 mol% Ti and 10 mol% V based on the metallic constituents. Calculated on the metal content this corresponds to a ruthenium loading of 6.4 g / m ² . This corresponds to a total coating loading of 23.9 g / m ² (as the sum of the oxides RuO ₂ , TiO ₂ and V ₂ O ₅ ).

Cyclic voltammetry was performed in 3.5 molar saline at room temperature with an Ag / AgCl reference electrode. The potential was varied in the range of 0.2 to 1.0 V (vs. Ag / AgCl) with a potential scanning speed of υ = 50 mV / s. The voltammetric capacitive charge ( q _a ) was determined as in Example 9. A value of 39.2 mC / cm ^{2 was obtained} for the porous and cracked layer.

For comparison, an electrode was manufactured with the same coating solution but without the addition of polyvinylpyrrolidone (PVP), otherwise the same manufacturing procedure and the voltammetric capacitive charge ( q _a ) determined in the same way. A value of 20.8 mC / cm ² for the voltammetric capacitive charge ( q _a ) of the coating without added PVP was obtained.

Claims (18)

Electrode at least consisting of an electrically conductive substrate and a catalytically active coating, characterized in that the catalytic active layer is based on two catalytically active components containing at least iridium, ruthenium or titanium as a metal oxide or mixed oxide or mixtures of said oxides, wherein the total content to ruthenium and / or iridium based on the sum of the elements iridium, ruthenium and titanium, at least 10 mol%, preferably 10 to 28 mol%, preferably 10 to 20 mol%, and that at least one oxidic base layer is provided, which is applied to the electrically conductive support and which is impermeable to aqueous electrolytes comprising NaCl and / or NaOH or HCl. An electrode according to claim 1, characterized in that the electrically conductive substrate is based on a valve metal, in particular a metal of the series titanium, tantalum, niobium and nickel or an alloy of these metals with a main constituent of titanium, tantalum or niobium. An electrode according to any one of claims 1 or 2, characterized in that the base layer is impermeable to aqueous hydrochloric acid solution, sodium chloride solution and sodium hydroxide solution. Electrode according to one of claims 1 to 3, characterized in that in addition a cover layer is provided, the cyclovoltammetric charge is greater than that of the base layer. Electrode according to one of claims 1 to 4, characterized in that the cover layer comprises the components of the catalytically active layer and additionally contains pore-forming compounds, in particular compounds of lanthanum, in particular lanthanum oxide or polymers, in particular polyvinylpyrrolidone. Electrode according to one of claims 1 to 5, characterized in that the layer thickness (surface charge as oxide) of the base layer of 0.1 to 20 g / m ^{2 is} preferably 0.5 to 10 g / m ² . Electrode according to one of claims 1 to 6, characterized in that the layer thickness (surface charge as oxide) of the cover layer, at least 2 g / m ² , preferably at least 5 g / m ² . Electrode according to one of claims 1 to 7, characterized in that the cover layer has a varying proportion of iridium to titanium and / or ruthenium to titanium component viewed in a cross section through the layer thickness. An electrode according to claim 8, characterized in that the ratio of iridium to titanium and / or ruthenium to titanium in the cover layer decreases in a cross section through the layer thickness viewed from the outside in the direction of the electrically conductive support. Electrode according to one of claims 1 to 9, characterized in that the base layer is electrically conductive and has a conductivity of at least 10 S / m, preferably at least 1000 S / m, more preferably at least 10000 S / m. Electrode at least consisting of an electrically conductive substrate and a catalytically active coating, characterized in that the catalytic active layer is based on two catalytically active components containing at least iridium, ruthenium or titanium as a metal oxide or mixed oxide or mixtures of said oxides, wherein the total content to ruthenium and / or iridium based on the sum of the elements iridium, ruthenium and titanium, at least 10 mol%, preferably 10 to 28 mol%, preferably 10 to 20 mol%, and that up to half of the ruthenium and / or iridium is replaced by vanadium, zirconium or molybdenum, preferably by vanadium. An electrode according to claim 11, characterized in that the electrode is constructed according to an electrode according to one of claims 1 to 10. Process for producing an electrode, in particular an electrode according to one of the preceding claims, characterized in that in a first step to an electrically conductive carrier one or more times sol-gel coating solution containing a solution or dispersion of metal compounds of one or more of the metals ruthenium, Iridium and titanium are applied to the support in particular by immersion and then freed of solvent, and then the dried metal compound layer at elevated temperature, in particular at least 350 ° C, preferably at least 400 ° C, calcined in the presence of oxygen-containing gases and optionally the steps : Application of the solution or dispersion, drying and calcination are repeated one or more times. A method according to claim 13, characterized in that for the preparation of a cover layer one or more times a solution or dispersion of metal salts of the metals from the series ruthenium, iridium and titanium is applied to the base layer, is freed from the solvent and calcined at elevated temperature, in particular at least 350 ° C, preferably at least 400 ° C, in the presence of oxygen-containing gases. Method according to one of claims 13 or 14, characterized in that for the production of the base layer after the application of the metal salt solution, a drying at elevated temperature, in particular at least 200 ° C, preferably at least 240 ° C, is performed. A method according to any one of claims 13 to 15, characterized in that the metal compound solutions for producing the base layer and / or the cover layer, a lower carboxylic acid, in particular propionic acid, C ₁ - to C ₅ alcohols or ketones or mixtures thereof is added. Electrolyzer for the electrolysis of sodium chloride or hydrogen chloride-containing solutions, characterized in that an electrode according to one of claims 1 to 12 is provided as the anode. Use of the electrode according to one of claims 1 to 12 as an anode in electrolyzers for the sodium chloride or hydrogen chloride electrolysis for the electrochemical production of chlorine.

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