EP2397579A1 - Electrode for obtaining chlorine through electrolysis - Google Patents

Abstract

Electrode comprises at least an electrically conductive substrate based on a valve metal, preferably titanium-, tantalum or niobium or its alloys. The main component of the valve metal and an electrocatalytically active coating comprises: a noble metal oxide and/or a noble metal oxide mixture; and titanium oxide. The coating exhibits a minimum proportion of oxides with an anatase structure, which is determined by a ratio of a signal height of intensive anatase reflexes (101) in a X-ray diffraction pattern after allowing a linear substrate to a signal height of intensive rutile reflexes (110). Electrode comprises at least one electrically conductive substrate based on a valve metal, preferably titanium-, tantalum or niobium metal or its alloys, where the main component of titanium, tantalum or niobium and an electrocatalytically active coating comprises (in mole-%): a noble metal oxide and/or a noble metal oxide mixture (50); and titanium oxide (at least 50). The coating exhibits a minimum proportion of oxides with an anatase structure, which is determined by a ratio of at least 0.6, preferably at least 1 of a signal height of intensive anatase reflexes (101) in a X-ray diffraction pattern (copper (K-alpha ) radiation) after allowing a linear substrate to a signal height of intensive rutile reflexes (110) in the same diffraction pattern. An independent claim is also included for producing the electrode with the electrocatalytically active coating on the electrically conductive substrate, comprising: dissolving noble metal salts in an organic solvent; adding a soluble titanium compound, preferably titanium isopropoxide in an organic- and/or an aqueous solution; mixing the solution; hydrolyzing the salts in water and/or an aqueous acid, preferably acetic acid, propionic acid, hydrochloric acid or nitric acid; applying the solution on the electrically conductive substrate in at least one stage; removing the solvent; thermally post-treating the formed layer at a temperature of not > 250[deg] C, preferably 100-250[deg] C and a pressure of 10-100 bar in the presence of water vapor and optionally lower alcohols; and subsequently calcining the formed layer in the presence of oxygen-containing gases at a temperature of more than 300[deg] C, preferably 450-550[deg] C.

Description

The invention is based on known electrodes, at least consisting of an electrically conductive substrate based on a valve metal and an electrocatalytically active coating of a noble metal oxide or Edelmetalloxidgemischs and titanium oxide.

1. 1. Die Umsetzung bei niedrigen Temperaturen erlaubt die Herstellung von sehr kleinen Nanostrukturen.
2. 2. Durch die Hydrolyse der Edukte entstehen homogen und auf molekularer Ebene verteilte Produkte (RuO₂-TiO₂ die durch chemische Wechselwirkungen (z.B. Bindungen) gebildet werden Durch die homogene Verteilung der resultierenden Oxide in der Elektrodenbeschichtung entstehen elektronische Leitungspfade, die für eine optimale Stromführung sorgen.

To obtain chlorine, electrode coatings are used according to the prior art, which consist of ruthenium-titanium oxide mixtures (eg, dimensionally stable anodes, DSA ^TM ). The composition of the coating, ie the ratio of ruthenium to titanium oxide, is the decisive factor deciding on the electrocatalytic activity. Commercial DSA ^™ consist of 30 mol% RuO ₂ and 70 mol% TiO _2. As described in J. Electrochem, Soc. 124, 500 (1977), the coating is composed of a main phase consisting of a TiO ₂ ruthenium oxide mixed crystal with rutile structure and secondary phases with pure ruthenium oxide and a pure anatase phase. US 3,562,008 describes a coating of predominantly amorphous titanium oxide with crystalline noble metal oxide or noble metal. Furthermore, as in Soot. J. Electrochem, 38, 657 (2002 ) and Mat. Chem. And Phys. 22, 203 (1989 hydrated ruthenium oxide besides amorphous, hydrated oxide phases. The publications Electrochimica Acta 40, 817 (1995 ) and Electrochimica Acta 48, 1885 (2003 ) show that RuO ₂ -TiO ₂ coatings prepared by a thermal decomposition process give a product having a short-range structural order. These heterogeneous layers contain microclusters of RuO ₂ and TiO ₂ domains, which are randomly distributed in the layer. The electronic conductivity of these layers can be determined by the percolation theory ( Journal of Solid State Chemistry 52, 22 (1984 ) ) to be discribed. The theory explains the conductivity of very finely divided and conductive particles (RuO ₂ domains) in an insulating matrix of TiO ₂ domains. According to this, the electronic properties are determined by the homogeneity of the mixed oxide. An increase in activity and an improvement in the lifetime of the coating can only be achieved by the homogeneous distribution of the active component RuO ₂ on a molecular scale. Such a distribution of RuO ₂ in a TiO ₂ matrix can, as in Journal of Sol-Gel Science and Technology 29, 81 (2004 ) and Colloids and Surface A 157, 269 (1999 ) can be achieved by the use of a sol-gel method. Here, the distribution of the components takes place at the molecular level by the hydrolysis of suitable precursor substances. The advantages of the sol-gel process are:

1. 1. The reaction at low temperatures allows the production of very small nanostructures.
2. 2. Hydrolysis of the starting materials gives rise to homogeneous and molecularly distributed products (RuO ₂ -TiO ₂ which are formed by chemical interactions (eg bonds)). The homogeneous distribution of the resulting oxides in the electrode coating results in electronic conduction paths which ensure optimum current conduction to care.

In contrast to coatings produced by thermal decomposition of labile starting materials, layers produced by the sol-gel process show better electronic and mechanical properties based on the homogeneity of the mixing process. As a result, a higher stability of the layers is achieved in addition. As in Journal of Electroanalytical Chemistry 579, 67 (2005 ), samples prepared by sol-gel techniques show that the impedance of the samples increases less with chlorine evolution than with samples prepared by thermal decomposition. This observation suggests a higher sol-gel activity close the prepared samples. A disadvantage of the sol-gel route is the limited possibility of varying the phase composition in the binary RuO ₂ TiO ₂ layer. By varying the pH, the educt composition and the sintering temperature, it is possible to control the phase composition to a small extent. These possibilities are in Materials Chemistry and Physics 110, 256 (2008 ) Journal of the European Ceramic Society 27, 2369 (2007 ) Journal of Thermal Analysis and Calorimetry 60, 699 (2000 ) Chem. Mater. 12, 923 (2000 ) and J. sol-gel. Sci, Techn. 39, 211 (2006 ). The phase-forming behavior between RuO ₂ -TiO ₂ is in Journal of the Electrochemical Society 124, 500 (1977 ). TiO ₂ occurs in two polymorphic phases, rutile and anatase. While anatase is stable at low temperatures, rutile occurs only at high temperatures. The phases can be converted into each other by thermal treatment. Another possibility of the conversion is the addition of a second component in the form of a doping. This doping builds up on the TiO ₂ structure and thereby influences the coordination, which leads to the formation of a homogeneous rutile or anatase phase. Due to the very good lattice matching of tetragonal RuO ₂ to tetragonal TiO ₂ (rutile), its formation is preferred. For this reason, conventional coatings show a major constituent consisting of a solid mixture of RuO ₂ / TiO ₂ with corresponding tetragonal structure. Depending on the manufacturing process, small amounts of anatase phase may be present in layers with a RuO ₂ content of 20-40 mol%. The thermodynamic stability of the structure, ie, the binding behavior of the MO ₆ octahedra of Ru and Ti, depends on the surface free energy of the nanoparticles which is influenced by the surface chemistry (oxide and hydroxide formation, water adsorption) ( Nano Letter 5, 1261 (2005 )). In general, the thermally induced crystallization of amorphous phases under oxidizing conditions leads to a coating structure with a predominant rutile phase.

This process is caused by oxygen surface adsorption. So far, no electrocatalytically active coating systems are known with a major proportion of anatase.

Surprisingly, it has been found that coatings with an increased anatase content show increased electrocatalytic activity for chlorine evolution compared to rutile structure-based coatings. For this reason, it was the object of this invention to produce electrocatalytically active coatings with a major amount of anatase phase.

The invention relates to the production of an electrodeposition coating for electrolytic chlorine, which consists of a noble metal oxide and an anatase-rutile titanium oxide with a certain Mindestanatasanteil.

A particular electrode is characterized in that the coating has a proportion of anatase structure, characterized in that in each case after deduction of a linear background, the peak height of the most intense anatase reflex (Reflex (101)) in the X-ray diffractogram (radiation Cu _κα ) at least 60% of Height of the most intense rutile reflex (reflex (110)) in the X-ray diffractogram. The targeted adjustment of the composition and the influence of the microstructure on the electrode coating is achieved, for example, by a two-stage process. In this case, a thermally stabilized and amorphous educt phase, which is produced in a sol-gel process, is first crystallized in a solvothermal treatment and then with a thermal after-treatment.

A material with an anatase structure is understood here simply as a material having a structure of the anatase structure type.

A solvothermal treatment in the sense of the invention is understood to mean a treatment at elevated pressure relative to the ambient pressure and elevated temperature relative to room temperature.

In contrast to the prior art, the described process control achieves a coating with a higher anatase content, which leads to a direct increase in efficiency in the extraction of chlorine.

For this purpose, a solvothermal process with a process temperature of at most 250 ° C., preferably 100 to 250 ° C. and a process pressure of 1 to 10 MPa, has proven suitable for the crystallization of an amorphous educt mixture.

The invention relates to an electrode, at least consisting of an electrically conductive substrate based on a valve metal, in particular a metal of titanium, tantalum, niobium or an alloy of these metals, with a major constituent of titanium, tantalum or niobium and an electrocatalytically active coating with up to 40 mol% of a noble metal oxide or Edelmetalloxidgemischs and at least 6 mol% of titanium oxide, characterized in that the coating has a minimum proportion of oxides with anatase structure, which is determined by the ratio of the signal height of the most intense anatase reflex (101) in the X-ray diffractogram ( Radiation CuKα) to the signal level of the most intense rutile reflection (110) in each case after subtraction of a linear background in the same diffractogram, wherein the ratio has a value of at least 0.6, preferably at least 1.

Preference is given to an electrode which is characterized in that the noble metal oxide selected is an oxide of one or more metals from the group of ruthenium, iridium, platinum, gold, rhodium, palladium, silver, rhenium. Oxides of ruthenium or iridium are particularly preferred as the noble metal oxide.

The electrocatalytically active layer preferably has 10 to 50 mol% of the noble metal oxide or noble metal oxide mixture, more preferably 15 to 50 mol%.

The proportion of the titanium oxide component in a preferred embodiment of the electrode is 50 to 90 mol%, particularly preferably 50 to 85 mol%.

The invention further provides a process for producing an electrode having an electrocatalytically active coating on an electrically conductive substrate, in particular a new electrode described above, comprising the steps:

Dissolving precious metal salts, in particular noble metal chlorides, in an aqueous solvent, adding a soluble titanium compound, in particular Ti (iOPr) ₄ in organic and / or aqueous solution, mixing the solution, hydrolyzing the salts by means of water and / or aqueous acids, in particular acetic acid, Propionic acid, HCl or HNO ₃ , applying the solution to an electrically conductive substrate in one or more stages, removal of the solvent, thermal aftertreatment of the layer formed at the temperature of at most 250 ° C, preferably 100 to 250 ° C and at a pressure of 10 to 100 bar (1 to 10 MPa) in the presence of water vapor and optionally lower alcohols and then calcining the layer formed in the presence of oxygen-containing gases at a temperature of more than 300 ° C, preferably 400 to 600 ° C, more preferably 450 ° C to 550 ° C.

By way of example, electrocatalytically active layers consisting of a 15-40 mol% noble metal component (eg RuO ₂ or RuO ₂ / IrO ₂ mixtures) and a TiO ₂ phase having a predominantly anatase structure can be produced by the process according to the invention.

A major proportion of anatase structure is present when, after subtracting a linear background, the height of the most intense reflex of the anatase structure (Reflex (101)) in the X-ray diffraction divided by the height of the most intense reflection of the rutile structure (Reflex (110)) is a value of greater than or equal to 0.6.

The coating solutions are obtained, for example, via a sol-gel process, preference being given to using as precursor salts chlorides, nitrates, alkoxides or acetylacetonates of the abovementioned noble metals which are dissolved in a solvent from the series of C ₁ to C ₈ alcohols, in particular methanol, n -Propanol, i-propanol, n-butanol or t-butanol are dissolved under stirring and sonication. To avoid spontaneous hydrolysis and condensation between the educts, complexing agents such as acetylacetone or 4-hydroxy-4-methyl-2-pentanone are added. For the hydrolysis and condensation of the precursors, water and / or acids such as acetic acid, propionic acid, HCl or HNO _{3 are} added. The coating solution prepared in this way is used for the coating of electronically conductive materials such as titanium, tantalum and niobium or their alloys. These materials can be in different geometries z. B: sheets; Wires or nets are present A mechanical, chemical or electrochemical treatment of the substrates may be necessary to remove any oxide layers present and to achieve a mechanical adhesion of the coating by increasing the substrate surface. To apply the coating solution, methods such as dripping, spin-coating, spraying, dipping or brushing can be used. The resulting layer is air dried and then thermally stabilized at 100-250 ° C. Thicker layers can be obtained by repeating the steps previously described several times. After thermal stabilization, the coatings exhibit an amorphous structure which is crystallized by the process of the present invention.

The solvothermal process is carried out, for example, in a steel cylinder which can be sealed and heated. The necessary process pressure is achieved by an evaporable liquid, which is located in a Tefloneinsatz inside the steel cylinder. The sample itself hangs or lies in a glass vessel, which is in Tefloneinsatz. The process pressure can be adjusted by the amount of liquid and by the applied temperature. As liquids, water, solvents or dilute sol solutions may be used For example, the closed steel cylinder is heated to 150 - 200 ° C at a rate of 10 ° / min for a period of 3-24 hours. This creates a pressure of 1-10 MPa inside the steel cylinder. After cooling the coated sample to room temperature, a thermal after-treatment for 1-2 hours at more than 300 ° C, preferably 400 to 600 ° C, preferably at 450 ° C to 550 ° C instead.

Subsequently, electrochemical tests (eg cyclic voltammetry) for the characterization of the chlorine evolution can be carried out with the aid of the resulting electrode.

It has been found in such tests that the thermal aftertreatment significantly improves the performance of such electrodes over known electrodes. As the embodiments show, the samples with solvothermal pretreatment have a significantly higher electrocatalytic activity compared to purely thermally treated samples.

Another object of the invention is the use of the electrode according to the invention as an anode in electrolyzers for the electrolysis of (aqueous) sodium chloride or hydrogen chloride solutions for the electrochemical production of chlorine.

The invention furthermore relates to an electrolyzer for the electrolysis of solutions containing sodium chloride or hydrogen chloride, characterized in that an electrode according to the invention is provided as the anode.

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

FIG. 1 shows an X-ray diffractogram of the solvothermal pretreated sample of Example 1

• A: Reflex (101) der Phase vom Anatas-Strukturtyp
• R: Reflexe (110) der Phase vom Rutil-Strukturtyp

Herein mean:

• A: Reflex (101) of the anatase structural type phase
• R: reflections (110) of the rutile structural type phase

Examples example 1

Titanium discs 15 mm in diameter (thickness: 2 mm) are sandblasted and then etched in 10% oxalic acid at 80 ° C for 2 hours. Thereafter, the platelets are removed from the acid and washed with 2-propanol. The drying process takes place in a stream of nitrogen. To prepare the first component (solution A) of the sol solution, 168.5 mg RuCl ₃ .xH ₂ O (36% Ru) are dissolved in 6 ml 2-propanol and stirred for 12 hours. Solution B is prepared from 333.1 μL of Ti (i-OPr) ₄ and 561.5 μ of 4-hydroxy-4-methyl-2-pentanone previously dissolved in 7.52 mL of 2-propanol. For homogenization, it is stirred for 30 minutes. Solutions A and B are combined under ultrasound. This creates a transparent solution. For hydrolysis, 12.9 μl acetic acid and 27 μl deionized water are then added. The resulting mixture is stirred for 12 hours at room temperature. Before this mixture can be used as a coating solution, it is diluted with 26.67 ml of 2-propanol. 50 μl of this solution are dropped onto the titanium platelets described above and then dried in air. This process is repeated 24 times, with thermal stabilization at 200 ° C for 10 minutes after every fourth application. This results in an amorphous coating having a chemical composition of 40 mol% RuO ₂ and 60 mol% TiO ₂ . This corresponds to a ruthenium loading of 10.3 g / m ² . The solvothermal treatment is carried out in the steel autoclave described above with a 250 ml Teflon insert filled with 30 ml of coating solution (37.5 mmol). The coated sample is placed in a glass jar, which is placed in the Teflon insert. The sealed autoclave is heated at a heating rate of 10 ° C / min to 150 ° C and left at this temperature for 24 hours. After cooling to room temperature, the coated substrate is thermally post-treated at 450 ° C. in air for 1 hour. The control sample without solvothermal pretreatment is thermally treated at 450 ° C in air for only one hour. The phase analysis is carried out by means of X-ray diffractometry. Fig. 1 shows the X-ray diffractogram of a sample with solvothermal pretreatment. It can be seen that predominantly anatase structure is present in the coating. After subtracting a linear background, the ratio of the height of the most intense reflex of the anatase structure (Reflex (101)) in the X-ray diffractogram to the height of the most intense reflex of the rutile structure (Reflex (110)) is 3.96. Without solvothermal pretreatment, only the rutile phase occurs. The electrocatalytic activity for chlorine evolution was investigated by chronoamperometry (reference electrode: Ag / AgCl, 3.5 mo1 / 1 NaCl, pH: 3, T: 25 ° C). A current density of 1 kA / m ^{2 was} applied and the potential was determined. For the solvothermal pretreated sample, a potential of 1.18 V results and for the purely thermally treated sample has a potential of 1.32 V.

Example 2

The titanium substrates are treated as described in Example 1. To prepare the first component (solution A) of the sol solution, 105.3 mg of RuCl ₃ H ₂ O (36% Ru) are dissolved in 488 ml of 2-propanol and stirred for 12 hours. Solution B is prepared from 333.1 Ti (i-OPr) ₄ and 561.5 _μ l of 4-hydroxy-4-methyl-2-pentanone previously dissolved in 7.52 ml of 2-propanol. For homogenization, it is stirred for 30 minutes. Solutions A and B are combined under ultrasound. This creates a transparent solution. For hydrolysis, 12.9 μl acetic acid and 27 μl deionized water are then added. The resulting mixture is stirred for 12 hours at room temperature. Before this mixture can be used as a coating solution, it is diluted with 26.67 ml of 2-propanol. 50 μl of this solution are dropped onto the titanium platelets described above and then dried in air. This process is repeated 24 times, with thermal stabilization being carried out at 100 ° C. for 10 minutes after every fourth application. This results in an amorphous coating having a chemical composition of 25 mol% RuO ₂ and 75 mol% TiO ₂ . This corresponds to a ruthenium loading of 6.4 g / m ² . The solvothermal pretreatment and the thermal aftertreatment are carried out as described in Example 1. The control sample without solvothermal pretreatment is thermally treated at 450 ° C in air for only one hour. The phase analysis is carried out by means of X-ray diffractometry.

From the X-ray diffractogram of a sample without solvothermal pretreatment, it can be seen that a rutile anatase structure mixture with a predominant rutile content is present. After subtracting a linear background, the ratio of the height of the most intense reflex of the anatase structure (Reflex (101)) in the X-ray diffractogram to the height of the most intense reflex of the rutile structure (Reflex (110)) is 0.18. It can be seen from the X-ray diffractogram of a sample with solvothermal pretreatment that predominantly an anatase structure fraction occurs in the coating. After subtracting a linear background, the ratio of the height of the most intense reflex of the anatase structure (Reflex (101)) in the X-ray diffractogram to the height of the most intense reflex of the rutile structure (Reflex (110)) is 1.81. The electrocatalytic activity for chlorine evolution was examined by chronoamperometry (reference electrode: Ag / AgCl, 3.5 mol / 1 NaCl, pH: 3, T: 25 ° C). A current density of 1 kA / m ^{2 was} applied and the potential determined. For the solvothermal pretreated sample a potential of 1.23 V results and for the purely thermally treated sample a potential of 1.42 V.

Example 3

The titanium substrates are treated as described in Example 1. To prepare the first component (solution A) of the sol solution, 105.3 mg of RuCl ₃ -xH ₂ O (36% Ru) are dissolved in 4.88 ml of 2-propanol and, for 12 hours, solution B is prepared from 333.1 Ti (i-OPr ) ₄ and 561.5 μ1 of 4-hydroxy-4-methyl-2-pentanone previously dissolved in 7.52 ml of 2-propanol. For homogenization, the mixture is stirred for 30 minutes. Solutions A and B are combined under ultrasound. This creates a transparent solution. For the hydrolysis, 12.9 μl acetic acid and 27 μl deionized water are added. The resulting mixture is stirred for 12 hours at rum temperature. Before this mixture can be used as a coating solution, it is diluted with 26.67 ml of 2-propanol. 50 μl of this solution are dropped onto the titanium platelets described above and then dried in air. This process is repeated 24 times with thermal stabilization at 250 ° C for 10 minutes after every fourth application. This results in an amorphous coating having a chemical composition of 25 mol% RuO ₂ and 75 mol% TiO ₂ . This corresponds to a ruthenium loading of 6.4 g / m ² . The solvothermal treatment is carried out as described in Example 1, in a steel autoclave with a 250 ml Teflon insert, which is filled with 30 ml of coating solution (37.5 mmol). The coated sample is placed in a glass jar, which is placed in the Teflon insert. The sealed autoclave with a heating rate of 10 ° C / min. heated to 150 ° C and left at this temperature for 24 hours. After cooling to room temperature, the coated substrate is thermally post-treated for one hour at 450 ° C. in air. The control sample without solvothermal pretreatment is thermally treated at 450 ° C in air for only one hour. The phase analysis is carried out by means of X-ray diffractometry. The X-ray diffractogram of a sample without solvothermal pretreatment shows that only one rutile phase is present. From the X-ray diffractogram of the sample with solvothermal pretreatment, it can be seen that an anatase structure fraction occurs next to the rutile component in the coating. After subtracting a linear background, the ratio of the height of the most intense reflex of the anatase structure (reflex (101)) in the X-ray diffractogram to the height of the most intense reflex of the rutile structure (reflex (110)) is 0.21.

The electrocatalytic activity for chlorine evolution was examined by chronoamperometry (reference electrode: Ag / AgCl, 3.5 mol / l NaCl, pH: 3, T: 25 ° C). A current density of 1 kA / m ^{2 was} applied and the potential was determined. For the solvothermal pretreated Sample results in a potential of 1.32 V and for the purely thermally treated sample has a potential of 1.41 V.

Example 4

The titanium substrates are treated as described in Example 1. To prepare the first component (solution A) of the sol solution, 63.2 mg RuCl ₃ -xH ₂ O (36% Ru) are dissolved in 1.26 ml 2-propanol and stirred for 12 hours. Solution B is prepared from 377.5 ml of Ti (i-OPr) ₄ and 561.5 μl of 4-hydroxy-4-methyl-2-pentanone previously dissolved in 11.1 ml of 2-propanol. For homogenization, it is stirred for 30 minutes. Solutions A and B are combined under ultrasound. This creates a transparent solution. For hydrolysis, 12.9 μl acetic acid and 27 μl deionized water are then added. The resulting mixture is stirred for 12 hours at room temperature. Before this mixture is used as a coating solution, it is diluted with 26.67 ml of 2-propanol. 50 μl of this solution are dropped onto the titanium platelets described above and then dried in air. This process is repeated 8 times, with thermal stabilization at 200 ° C for 10 minutes after each application. This results in an amorphous coating having a chemical composition of 15 mol% RuO ₂ and 85 mol% TiO ₂ . This corresponds to a ruthenium loading of 3.86 g / m ² . The solvothermal treatment is carried out as described in Example 1 in a steel autoclave with a 250 ml Teflon insert, which is filled with 30 ml of coating solution (37.5 mmol). The coated sample is placed in a glass jar, which is placed in the Teflon insert. The sealed autoclave is heated at a heating rate of 10 ° C / min to 150 ° C and left at this temperature for 3 hours. After cooling to room temperature, the coated substrate is thermally treated for 10 minutes at 250, 300, 350, 400 and 450 ° C in air. From the X-ray diffractogram of the sample it can be seen that a rutile / anatase mixture with a high content of rutile phase is present. After subtracting a linear background, the ratio of the height of the most intense reflex of the anatase structure (reflex (101)) in the X-ray diffractogram to the height of the most intense reflex of the rutile structure (reflex (110)) is 0.10. The electrocatalytic activity for chlorine evolution was investigated by chronoamperometry (reference electrode: Ag / AgCl, 3.5 mol / 1 NaCl, pH: 3, T: 25 ° C.). A current density of 1 kA / m ^{2 was} applied and the potential was determined. A potential of 1.27 V was determined.

Claims (10)

Electrode, at least consisting of an electrically conductive substrate based on a valve metal, in particular a metal of the series titanium, tantalum, niobium or an alloy of these metals, with a main constituent of titanium, tantalum or niobium and an electrocatalytically active coating with up to 50 molar % of a noble metal oxide or Edelmetalloxidgemischs and at least 50 mol% of titanium oxide, characterized in that the coating has a minimum proportion of oxides with anatase structure, which is determined by the ratio of the signal height of the most intense anatase reflex (101) in the X-ray diffractogram (radiation Cu _Kα ) to the signal level of the most intense rutile reflection (110) in each case after subtraction of a linear background in the same diffractogram, wherein the ratio has a value of at least 0.6, preferably at least 1. An electrode according to claim 1, characterized in that as the noble metal oxide, an oxide of one or more metals from the group ruthenium, iridium, platinum, gold, rhodium, palladium, silver, rhenium, preferably an oxide of ruthenium or iridium is selected. An electrode according to claim 1 or 2, characterized in that the electrocatalytically active layer 10 to 50 mol% of the noble metal oxide or Edelmetalloxidgemischs, preferably 15 to 50 mol%, having. Electrode according to one of claims 1 to 3, characterized in that the proportion of the titanium oxide component is 50 to 90 mol%, preferably 50 to 85 mol%. Process for producing an electrode having an electrocatalytically active coating on an electrically conductive substrate, in particular an electrode according to one of Claims 1 to 4, by dissolving precious metal salts, in particular an organic solvent, adding a soluble titanium compound, in particular Ti (iOPr) ₄ in organic and / or aqueous solution, mixing the solution, hydrolyzing the salts by means of water and / or aqueous acids, in particular acetic acid, propionic acid, HCl or HNO ₃ , applying the solution to an electrically conductive substrate in one or more stages, removal of the solvent , thermal aftertreatment of the layer formed at the temperature of at most 250 ° C, preferably 100 to 250 ° C and at a pressure of 10 to 100 bar (1 to 10 MPa) in the presence of water vapor and optionally lower alcohols and then calcining the formed Layer in the presence of Oxygen-containing gases at a temperature of more than 300 ° C, preferably 400 to 600 ° C, more preferably 450 ° C to 550 ° C. A method according to claim 5, characterized in that are used as noble metal salts chlorides, nitrates, alkoxides or acetylacetonates of precious metals, especially noble metal chlorides. A method according to claim 5 or 5, characterized in that as organic solvent for the noble metal salt and / or the soluble titanium compound, a solvent from the series of C ₁ - to C ₈ alcohols, in particular methanol, n-propanol, i-propanol, n Butanol or t-butanol is used. An electrode obtained from a method according to any one of claims 5 to 7. Electrolyzer for the electrolysis of solutions containing sodium chloride or hydrogen chloride, characterized in that an electrode according to one of claims 1 to 4 or 8 is provided as the anode. Use of the electrode according to one of claims 1 to 4 or 8 as an anode in electrolyzers for the sodium chloride or hydrogen chloride electrolysis for the electrochemical production of chlorine.

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