JP2017522457A - Catalyst coating and method for producing the same - Google Patents

Abstract

The present invention relates to a catalytic coating on the surface of a valve metal, such as titanium, which is suitable for operation in a highly erosive electrolysis atmosphere such as a hydrochloric acid electrolytic cell. The coating can be used as a catalyst activation of the electrode, for example for the generation of chlorine anodes or to protect against crevice corrosion of electrolytic cell flanges and other components that are prone to liquid stagnation. [Selection figure] None

Description

  The present invention relates to a catalyst coating of a valve metal article suitable for use in a highly erodible electrolysis atmosphere, for example, in a hydrochloric acid electrolytic cell.

  Hydrochloric acid electrolysis is an electrochemical process of current interest for hydrochloric acid, which is a common by-product of all major industrial processes using chlorine, and a significant increase in production capacity for the new concept plant With the formation of acid, it is very difficult to put it on the market. Acid electrolysis, usually performed in a two-compartment electrolyzer separated by an ion exchange membrane, results in the formation of chlorine in the anode compartment, which is essentially a recycle of upstream and negligible environmental impact. A closed cycle can be given. The material of the anode compartment must be able to withstand an erodible atmosphere combined with acidic, wet chlorine, and anode polarization while maintaining suitable electrical conductivity. For such purposes, valve metals such as titanium, niobium and zirconium are preferably used, and in some cases alloyed titanium is the most common example for reasons of cost and ease of machining. For example, titanium alloys containing nickel, chromium, and small amounts of noble metals such as ruthenium and palladium, such as AKOT® alloys sold by Kobe Steel, are widely used. The anode on which the anodic release of chlorine takes place is, for example, a titanium alloy substrate coated with a suitable catalyst, usually composed of a mixture of titanium and ruthenium oxides, which can reduce the anodic release of chlorine. It is comprised from the valve metal articles like. The same type of coating has also been used to protect some components of the anode compartment that are not directly involved in chlorine release from corrosion, particularly with respect to gap regions that are prone to electrolyte stagnation. Insufficient electrolyte regeneration can actually cause local discontinuities in the passivation layer responsible for protecting the valve metal, which can lead to corrosion phenomena, It is more dangerous to be localized more in a small area. An example of a region where the gap is likely to be defined is given by the peripheral flanges of both the anode and cathode compartments, on which a seal gasket is usually mounted. In the most convenient case encountered in industrial practice, a titanium alloy coated with a catalyst formulation based on ruthenium and titanium oxides deactivates the anode structure and / or cell member in the flange region. Continuous operation in a hydrochloric acid electrolysis plant in the range of 24 to 48 months can be ensured before the corrosion problem that causes leakage of water occurs. In order to improve the competitiveness of industrial hydrochloric acid electrolysis processes, it is necessary to further increase the useful life of these components.

  Various aspects of the invention are set out in the accompanying claims.

In one form, the present invention includes a titanium-free catalyst layer and includes two phases, a tetragonal ditetragonal dipyramidal crystal phase, optionally containing RuO ₂ in solid solution with SnO ₂ , and It relates to a coating on the valve metal surface, which is composed of a mixture of mixed Ta ₂ O ₅ amorphous phases. We have actually observed that titanium-free coatings are more resistant to chloride attack in acidic solutions. This is probably a mixture of crystalline phases containing titanium oxide (its function is to function as a film-forming component in combination with ruthenium dioxide) but includes anatase TiO ₂ phase that is substantially more fragile than the others. Because it exists as. The inventors have also observed that a mixture of tantalum and ruthenium oxides in the amorphous phase does not contribute decisively to solve this problem, even when completely free of titanium. . However, if the coating is formed from a typical crystal form similar to rutile (ie, tetragonal ditetragonal dipyramidal RuO ₂ ), and a mixture of essentially amorphous phase Ta ₂ O ₅ , The stability of the coating against acid attack is greatly increased. As a further advantage, the overvoltage of the coating contributing to anodic chlorine release is surprisingly reduced. In one embodiment, the weight ratio between the amorphous phase and the crystalline phase of Ta ₂ O ₅ is between 0.25 to 4, which defines the best range of features of the present invention. In one aspect, the RuO ₂ component in the tetragonal ditetragonal dipyramidal crystal phase is partially replaced by SnO ₂ (stannite). Two dioxides, tin and ruthenium, whose tetragonal ditetragonal dipyramidal crystal form has been found to be most stable, can form solid solutions in any weight ratio. In one embodiment, the Ru / Sn weight ratio in the tetragonal ditetragonal dipyramidal crystalline phase of the coating is in the range of 0.5-2, which is best in terms of substrate protection and coating catalytic activity. Give a good result. In one aspect, the coating includes two separate catalyst layers, one described above is in direct contact with a valve metal substrate that binds to the outermost layer coated thereon having a higher content of ruthenium oxide. This is necessary, for example, when the coating is used for catalytic activation of the anode structure whose outer surface is in direct contact with the electrolyte, on the one hand the protective function on the substrate surface, on the other hand the catalyst in the outermost layer And the advantage of increasing the conduction properties. In one aspect, the inner catalyst layer has a weight ratio of amorphous Ta ₂ O ₅ phase / RuO ₂ containing crystalline phase (optionally including SnO ₂ ) in the range between 0.25 and 2.5; The outer catalyst layer is composed of an amorphous phase of Ta ₂ O ₅ mixed with a tetragonal ditetragonal dipyramidal crystal phase of RuO ₂ with a Ru / Ta weight ratio between 3 and 5. In one embodiment, a further protective preliminary layer composed of a mixture of titanium and tantalum oxides is interposed between the coating described above (one or two coatings) and the substrate. This can provide the advantage of improving the anchoring of the catalyst layer to the substrate at the expense of resistance penalty due to the moderate electrical conductivity of the titanium and tantalum oxide mixture. However, the magnitude of such resistance penalty can be greatly limited if the pre-layer has a suitably limited thickness. A total loading of 0.6-4 g / m ² of titanium and tantalum oxide is a suitable value for a pre-layer combined with a catalyst layer containing 20 g / m ² of total oxide.

In another form, the present invention optionally applies a solution of titanium and tantalum compounds such as TiOCl ₂ , TiCl ₃ , and TaCl _{5 to} the valve metal substrate in one or more coatings, and then each After the coating, thermal decomposition is performed; a solution of the compound of tantalum, ruthenium, and possibly tin is applied in one or more coatings until the first catalyst layer is obtained, and then the heat is applied after each coating. Performing decomposition; in some cases, a solution of the tantalum and ruthenium compound is applied over the first catalyst layer until a second catalyst layer is obtained, followed by thermal decomposition after each coating; And a method for producing the coating as described above. In one aspect, the ruthenium and tin compounds applied in view of subsequent pyrolysis are hydroxyacetochloride complexes, which are more regular with a more uniform composition compared to hydrochloric acid or other precursors. The advantage of obtaining a dense layer can be given. The pyrolysis step after each coating can be performed between 350-600 ° C. depending on the precursor compound selected. In the case of decomposition of a mixture of precursors composed of tantalum chloride and a hydroxyacetochloride complex of ruthenium and possibly tin, the thermal decomposition can be carried out, for example, between 450 and 550 ° C.

  The following examples are included to demonstrate particular embodiments of the invention, and their feasibility has been generally demonstrated within the scope of the claimed values. It will be appreciated by those skilled in the art that the compositions and techniques disclosed in the following examples represent compositions and techniques that have been discovered by the inventors to function well in the practice of the present invention. In view of the present disclosure, it will be appreciated that many changes may be made in the particular embodiments disclosed and still obtain similar or similar results without departing from the scope of the invention.

Example 1:
A 1 mm thick AKOT® titanium alloy mesh was degreased with acetone in an ultrasonic bath and etched in 20% HCl at boiling temperature for 15 minutes. Subsequently, in order to produce an electrode sample, the mesh was cut into a plurality of pieces having dimensions of 10 cm × 10 cm.

By mixing 150 g / L TiOCl ₂ and 50 g / L TaCl ₅ in 10% by weight hydrochloric acid, a solution of the precursor for producing the protective pre-layer was obtained.
A first set of catalyst solutions was obtained by mixing 20 wt% RuCl ₃ and 50 g / L TaCl ₅ in various proportions in 10 wt% hydrochloric acid.

  The corresponding chloride is dissolved in 10% by volume aqueous acetic acid, the solvent is evaporated, the solvent is evaporated after two more additions of 10% aqueous acetic acid and finally the product is again dissolved in 10% aqueous acetic acid. By obtaining the prescribed concentration, a solution of Ru (0.9 M) and Sn (1.65 M) hydroxyacetochloride complex was obtained.

A second set of catalyst solutions was obtained by mixing the hydroxyacetochloride complexes of Ru and Sn according to various proportions.
The following procedure yielded electrode samples with different formulations.

Brush the solution containing the TiOCl ₂ and TaCl ₅ precursors in _two coats until a tantalum and titanium oxide coating is obtained at a loading of about 1 g / m ² , and then after each coating The protective preliminary layer was applied to the sample cut from the titanium mesh by drying at 50 ° C. for 5 minutes and pyrolyzing at 515 ° C. for 5 minutes.

-By selectively applying the first or second set of catalyst solutions, variously formulated catalyst layers were applied over the protective pre-layer of the sample. The first set of catalyst solutions was brushed with 8-10 coatings until a tantalum and ruthenium oxide coating was obtained with a total ruthenium loading of about 20 g / m ² , and then each coating Later, it was applied by drying at 50 ° C. for 10 minutes and thermal decomposition at 500 ° C. for 5 minutes. At the end of the pyrolysis process, the electrodes are subsequently obtained until a crystalline tetragonal ditetragonal dipyramidal ruthenium dioxide phase is obtained that is mixed with an amorphous tantalum oxide phase, as demonstrated by subsequent XRD analysis. Was subjected to a 2 hour thermal cycle at 500 ° C. Several samples of the electrodes thus obtained are shown in Table 1 as RuTa type. The second set of catalyst solutions is brushed with 8-10 coatings until a tantalum, tin, and ruthenium oxide coating is obtained with a total ruthenium loading of about 20 g / m ² , and then each The coating was performed by drying at 60 ° C. for 10 minutes and thermal decomposition at 500 ° C. for 5 minutes. Again, at the end of the pyrolysis process, the crystalline tetragonal ditetragonal dipyramidal phase dioxide subsequently mixed with the amorphous phase of tantalum oxide as demonstrated by subsequent XRD analysis. The electrode was subjected to a 2 hour thermal cycle at 500 ° C. until a solid solution of ruthenium and tin dioxide was obtained. Several samples of the electrodes thus obtained are shown in Table 1 as RuTaSn type.
Another electrode sample with a catalyst coating composed of two layers was obtained by selectively applying a first or second set of catalyst solutions. The first set of catalyst solutions is brushed with 6-7 coatings until a first coating of ruthenium and tantalum oxide is obtained, and then 5 minutes to dry at 50 ° C. after each coating. Applying a pyrolysis treatment at 500 ° C. for 5 minutes; then a first type solution having a Ru / Ta weight ratio equal to 4 is obtained with a total ruthenium loading of about 20 g / m ² Up to 2 coats were applied by brushing and subjected to the same drying and pyrolysis cycle after each coating. At the end of the pyrolysis process, until a crystalline tetragonal ditetragonal dipyramidal phase of ruthenium dioxide mixed with an amorphous phase of tantalum oxide is subsequently obtained, as demonstrated by subsequent XRD analysis. The electrode was subjected to a 2 hour thermal cycle at 500 ° C. Several samples of the electrodes thus obtained are shown in Table 1 as RuTa-TOP types. The second set of catalyst solutions was brushed with 6-7 coatings until a tantalum, tin, and ruthenium oxide coating was obtained, and then each coating was dried at 60 ° C. for 5 minutes. A first having a weight ratio of Ru / Ta equal to 4 until a two layer catalyst coating is obtained with a total ruthenium loading of about 20 g / m ² . Ruthenium and tantalum oxide coatings obtained by brushing a solution of the type in two coatings and subjecting each coating to drying at 50 ° C. for 5 minutes and thermal decomposition at 500 ° C. for 10 minutes. It was coated on it. At the end of the pyrolysis process, the tetragonal ditetragonal dipyramidal crystalline phase ruthenium and tin dioxide mixed with the amorphous phase of tantalum oxide in the inner layer, as evidenced by subsequent XRD, and the outer layer The electrode was subjected to a 2 hour thermal cycle at 500 ° C. until a solid solution of tetragonal ditetragonal dipyramidal ruthenium dioxide crystal phase mixed with an amorphous phase of tantalum oxide was obtained. Several samples of the electrodes thus obtained are shown in Table 1 as RuTaSn-TOP types.

Comparative Example 1:
A 1 mm thick AKOT® titanium alloy mesh was degreased with acetone in an ultrasonic bath and etched in 20% HCl at boiling temperature for 15 minutes. Subsequently, in order to produce an electrode sample, the mesh was cut into a plurality of pieces having dimensions of 10 cm × 10 cm.

By mixing 150 g / L TiOCl ₂ and 50 g / L TaCl ₅ in 10% hydrochloric acid, a solution of the precursor for producing the protective pre-layer was obtained.
A set of catalyst solutions was obtained by mixing 20% by weight RuCl ₃ and 150 g / L TiOCl ₂ in various proportions in 10% hydrochloric acid.

A protective preliminary layer was applied to the sample cut out from the titanium mesh in the same manner as in Example 1.
• Brush the catalyst solution with 8-10 coatings until a ruthenium and titanium oxide coating is obtained with a total ruthenium loading of about 20 g / m ² , then 50 ° C. after each coating Various formulations of the catalyst solution were applied over the protective pre-layer of the above sample by subjecting to 5 minutes of drying at 500 ° C. and 5 minutes of thermal decomposition at 500 ° C. At the end of the pyrolysis process, the electrode was subsequently subjected to a 2 hour thermal cycle at 500 ° C. Some samples thus obtained are shown in Table 2 as RuTi type.

Example 2:
The electrode samples shown in the table were subjected to a standard potential test under anodic generation of chlorine at a current density of ³ kA / m ² in 15 wt% HCl at a temperature of 60 ° C. The resulting potential data is reported in Table 3 (SEP). The table also represents the operating time to deactivation under anodic generation of chlorine at a current density of 6 kA / m ² in 20 wt% HCl at a temperature of 60 ° C. with a zirconium cathode as counter electrode. Also shows relevant data for accelerated life testing. Electrode inactivation is defined by an increase of 1V relative to the initial value in the cell.

Example 3:
The same samples of electrode samples 2, 6, and C2 were subjected to a corrosion test that simulates crevice corrosion conditions that may occur on the flange or other closed area of the cell to produce chlorine. The first set of samples is immersed in a known volume of 20 wt% HCl at 45 ° C. under nitrogen flow to simulate electrolyte stagnation conditions; the second (control) set is passivated. Was maintained in an equal volume of 20 wt% HCl at 40 ° C. under a stream of oxygen. In both cases, the concentrations of chromium and nickel released from the substrate over the course of 24 hours were detected, and for samples 2 and 6, the concentration of both metals in that volume of HCl was less than 2 mg / L. On the other hand, sample C2 showed slightly higher concentrations than 2 mg / L Cr and 4 mg / L Ni under oxygen flow, which was significantly increased under nitrogen flow (up to 6.5 mg / L for nickel). .

The test was repeated with other sets of samples to confirm that the corrosion resistance was significantly increased for the formulations of the present invention.
The above description is not intended to limit the invention, which can be used in accordance with different embodiments without departing from the scope thereof, the scope of which is defined solely by the appended claims.

  Throughout the specification and claims of this application, the term “comprising” and variations thereof such as “comprising” and “including” shall include the presence of other members, components, or additional process steps. It is not intended to be excluded. Discussion of documents, acts, materials, devices, articles, etc. is included herein solely for the purpose of providing a context for the present invention. Any or all of these matters form part of the basis of the prior art or are common general knowledge in the field relevant to the present invention prior to the priority date of each claim of this application. It is neither suggested nor stated.

Claims (10)

Titanium comprising an amorphous phase of Ta ₂ O ₅ coated with a tetragonal ditetragonal dipyramidal crystal phase consisting of either RuO ₂ or a solid solution of RuO ₂ and SnO ₂ for a coating on a valve metal surface Said coating comprising a free first catalyst layer.   The coating of claim 1 wherein the weight ratio of the amorphous phase to the crystalline phase is in the range of 0.25-4.   The coating according to claim 1 or 2, wherein the weight ratio of Ru to Sn in the crystalline phase is in the range of 0.5-2. The first catalyst layer includes a second catalyst layer applied from the outside, and the first catalyst layer has a weight ratio of the amorphous phase to the crystal phase in the range of 0.25 to 2.5. The second catalyst layer comprises an amorphous phase of Ta ₂ O ₅ mixed with a tetragonal ditetragonal dipyramidal crystal phase of RuO ₂ at a weight ratio of Ru to Ta in the range of 3-5. The coating according to any one of 1 to 3.   The coating according to any one of claims 1 to 4, further comprising a protective pre-layer comprising a mixture of titanium and tantalum oxide interposed between the valve metal surface and the first catalyst layer.   The coating according to any one of claims 1 to 5, which is applied to a titanium or titanium alloy substrate.   The coating of claim 6, wherein the substrate is an anode substrate or flange of a chlorine producing electrolyzer.   The coating according to claim 7, wherein the electrolytic cell is a hydrochloric acid electrolytic cell. A method for producing the coating according to any one of claims 1 to 8,
Next simultaneous or sequential process:
-Optionally applying a solution of a compound of titanium and tantalum to the valve metal substrate in one or more coatings, followed by pyrolysis after each coating;
Applying a solution of a compound of tantalum, ruthenium, and possibly tin, in one or more coatings until a first catalyst layer is obtained, followed by pyrolysis after each coating;
In some cases, a solution of the tantalum and ruthenium compound is applied to the first catalyst layer in one or more coatings, and then pyrolyzed after each coating, until a second catalyst layer is obtained. Process;
A method for producing the coating, comprising:   The method of claim 9, wherein the ruthenium and tin compound is a hydroxyacetochloride complex.