Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof

Definitions

• the invention relates to a catalytic coating of valve metal articles suitable for use in highly aggressive electrolytic environments, for example in hydrochloric acid electrolysis cells.
• Hydrochloric acid electrolysis is an electrochemical process gaining increasing interest at present, being hydrochloric acid the typical by-product of all major industrial processes making use of chlorine: the increase in the production capacity of plants of new conception entails the formation of significant amounts of acid, whose placement on the market presents significant difficulties.
• the electrolysis of the acid typically carried out in two-compartment electrolytic cells separated by an ion-exchange membrane, leads to the formation of chlorine at the anode compartment, which can be recycled upstream resulting in a substantially closed cycle of negligible environmental impact.
• valve metals such as titanium, niobium and zirconium are preferably employed, optionally alloyed titanium being the most common example for reasons of cost and ease of machining.
• Titanium alloys containing nickel, chromium and small amounts of noble metals such as ruthenium and palladium, like the AKOT ® alloy commercialised by Kobe Steel, are for instance of widespread use.
• the anodes whereon the anodic evolution of chlorine is carried out consist for example of a valve metal article such as a titanium alloy substrate coated with a suitable catalyst, typically consisting of a mixture of oxides of titanium and ruthenium, capable of lowering the overvoltage of the anodic discharge of chlorine.
• a suitable catalyst typically consisting of a mixture of oxides of titanium and ruthenium, capable of lowering the overvoltage of the anodic discharge of chlorine.
• the same type of coating is also used to protect from corrosion some components of the anodic compartment not directly involved in the evolution of chlorine, with particular reference to interstitial areas subject to electrolyte stagnation.
• the lack of a sufficient electrolyte renewal may in fact lead to a local discontinuity of the passivation layer directed at protecting the valve metal, triggering corrosion phenomena, which are the more dangerous the more they are localised in small areas.
• the invention relates to a coating of valve metal surfaces including a titanium-free catalytic layer and consisting of the mixture of two phases, namely an amorphous phase of Ta2O 5 in admixture with a tetragonal ditetragonal dipyramidal crystalline phase containing RuO2 , optionally in solid solution with SnO2 .
• the inventors have in fact observed that titanium -free coatings are more resistant to chloride attack in acidic solution, presumably because titanium oxides - whose function in a combination with ruthenium dioxide is to act as film-forming component - are present as a mixture of crystalline phases including an anatase T1O2 phase, substantially weaker than the others.
• the inventors have also observed that mixtures of oxides of tantalum and ruthenium in an amorphous phase do not contribute to solving the problem in a decisive manner, even if completely free from titanium.
• the coating is formed from a mixture of RUO2 in the typical crystalline form similar to rutile (i.e. tetragonal ditetragonal dipyramidal) and Ta2O 5 in a basically amorphous phase, the stability of the coating to acid attack is greatly increased.
• the overvoltage of the coating towards anodic chlorine evolution is surprisingly reduced.
• the weight ratio between the amorphous phase of Ta2O 5 and the crystalline phase is between 0.25 and 4, which defines the best range of functioning of the invention.
• the R11O2 component in the tetragonal ditetragonal dipyramidal crystalline phase is partially replaced by SnO2 (cassiterite).
• the two dioxides of tin and of ruthenium whose tetragonal ditetragonal dipyramidal crystalline form turns out to be the most stable, are capable of forming solid solutions in any weight ratio; in one embodiment, the Ru to Sn weight ratio in the tetragonal ditetragonal dipyramidal crystalline phase of the coating ranges between 0.5 and 2, which gives the best results in terms of protection of the substrate as well as of catalytic activity of the coating.
• the coating comprises two distinct catalytic layers, one as hereinbefore described in direct contact with the valve metal substrate coupled to an outermost one overlaid thereto with a higher content of ruthenium oxide.
• the inner catalytic layer has a weight ratio of amorphous Ta2O 5 phase to RuO2-containing crystalline phase (optionally including SnO2) ranging between 0.25 and 2.5 and the outer catalytic layer consists of an amorphous phase of
• Ta2O 5 mixed with a tetragonal ditetragonal dipyramidal crystalline phase of RUO2 with a Ru to Ta weight ratio between 3 and 5.
• a further protective pre-layer consisting of a mixture of oxides of titanium and
• tantalum This can have the advantage of improving the anchoring of the catalytic layer to the substrate, at the expense of a resistive penalty deriving from the modest electrical conductivity of mixtures of titanium and tantalum oxides.
• the magnitude of such resistive penalty can be however very limited, provided the pre-layer has a suitably limited thickness.
• a total loading of titanium and tantalum oxides of 0.6 to 4 g/m 2 is a suitable value for a pre-layer to be combined with a catalytic layer containing 20 g/m 2 of total oxides.
• the invention relates to a method for the manufacturing of a coating as hereinbefore described comprising the optional application of a solution of titanium and tantalum compounds, for example T1OCI2 , T1CI3 and TaCI 5 , to a valve metal substrate in one or more coats, with subsequent thermal decomposition after each coat; the application of a solution of compounds of tantalum, ruthenium and optionally tin in one or more coats, with subsequent thermal decomposition after each coat, until obtaining a first catalytic layer; the optional application of a solution of compounds of tantalum and ruthenium upon the first catalytic layer with subsequent thermal decomposition after each coat, until obtaining a second catalytic layer.
• a solution of titanium and tantalum compounds for example T1OCI2 , T1CI3 and TaCI 5
• the compounds of ruthenium and tin applied in view of the subsequent thermal decomposition are hydroxyacetochloride complexes; this can have the advantage of obtaining more regular and compact layers, having a more homogeneous composition, compared to hydrochloric or other precursors.
• the thermal decomposition step after each coat can be effected between 350 and 600 °C, depending on the selected precursor compounds.
• thermal decomposition may for example be carried out between 450 and 550 °C.
• a 1 mm thick AKOT ® titanium alloy mesh was degreased with acetone in a ultrasonic bath and etched in 20% HCI at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of 10 cm x 10 cm size for the subsequent preparation of electrode samples.
• a solution of precursors for the preparation of the protective pre-layer was obtained by mixing 150 g/l of T1OCI2 and 50 g/l of TaCI 5 in 10% wt. hydrochloric acid.
• a first series of catalytic solutions was obtained by mixing 20% by weight RuCb and 50 g/l TaCI 5 in 10% wt. hydrochloric acid according to various proportions.
• Solutions of hydroxyacetochloride complexes of Ru (0.9 M) and Sn (1 .65 M) were obtained by dissolving the corresponding chlorides in 10% vol. aqueous acetic acid, evaporating the solvent, taking up with 10% aqueous acetic acid with subsequent evaporation of the solvent for two more times, finally dissolving the product again in 10% aqueous acetic acid to obtain the specified concentration.
• a second series of catalytic solutions was obtained by mixing the hydroxyacetochloride complexes of Ru and Sn according to various proportions.
• Electrode samples were obtained at different formulations with the following procedure:
• a protective pre-layer was applied to the samples cut out of the titanium mesh by brushing the solution containing T1OCI2 and TaCI 5 precursors in two coats, with subsequent drying at 50 °C for 5 minutes and thermal decomposition treatment at 515 °C for 5 minutes after each coat, until obtaining a deposit of oxides of tantalum and titanium with a loading of about 1 g/m 2 ;
• catalytic solutions of the first series were applied by brushing in 8-10 coats and subjected to subsequent drying at 50 °C for 10 minutes and thermal decomposition treatment at 500 °C for 5 minutes after each coat, until obtaining a deposit of oxides of tantalum and ruthenium with a total ruthenium loading of about 20 g/m 2 .
• the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500° C, until obtaining a crystalline tetragonal ditetragonal dipyramidal ruthenium dioxide phase mixed with the amorphous tantalum oxide phase, as verified by means of a subsequent XRD investigation.
• Some samples of electrodes thus obtained are indicated in Table 1 as RuTa type.
• the catalytic solutions of the second series have been applied by brushing in 8-10 coats and subjected to subsequent drying at 60 °C for 10 minutes and thermal
• the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 °C, until obtaining a solid solution of ruthenium dioxide and tin dioxide in a crystalline tetragonal ditetragonal dipyramidal phase mixed with the amorphous phase of tantalum oxide, as verified by a subsequent XRD investigation.
• Electrodes thus obtained are indicated in Table 1 as RuTaSn type; other electrode samples provided with a catalytic coating consisting of two layers were obtained by alternatively applying catalytic solutions of the first or of the second series.
• the catalytic solutions of the first series were applied by brushing in 6-7 coats and subjected to subsequent drying at 50 °C for 5 minutes and thermal decomposition treatment at 500 °C for 5 minutes after each coat, until obtaining a first deposit of oxides of ruthenium and tantalum; a subsequent solution of the first type with a Ru to Ta weight ratio equal to 4 was subsequently applied by brushing in 2 coats and subjected to the same drying and thermal decomposition cycle after each coat, until obtaining a total ruthenium loading of approximately 20 g/m 2 .
• the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500 °C, until obtaining a crystalline tetragonal ditetragonal dipyramidal phase of ruthenium dioxide mixed with the amorphous phase of tantalum oxide, as verified by a subsequent XRD investigation.
• Some samples of electrodes thus obtained are indicated in Table 1 as RuTa_TOP type.
• the catalytic solutions of the second series were applied by brushing in 6-7 coats and subjected to subsequent drying at 60 °C for 5 minutes and thermal decomposition treatment at 500 °C for 10 minutes after each coat, until obtaining a deposit of oxides of tantalum, tin and ruthenium; a deposit of oxides of ruthenium and tantalum, obtained upon brushing in 2 coats of a solution of the first type with a Ru to Ta weight ratio equal to 4, subjected to drying at 50 °C for 5 minutes and thermal decomposition at 500 °C for 10 minutes after each coat, was overlaid thereto, until obtaining a catalytic coating in two layers with a total ruthenium loading of about 20 g/m 2 .
• the electrodes were subjected to a subsequent thermal cycle of 2 hours at 500° C, until obtaining a solid solution of ruthenium dioxide and tin dioxide in a tetragonal ditetragonal dipyramidal crystalline phase mixed with the amorphous phase of tantalum oxide in the inner layer and of a tetragonal ditetragonal dipyramidal ruthenium dioxide crystal phase mixed with the amorphous phase of tantalum oxide in the outer layer, as verified by a subsequent investigation by XRD.
• Some samples of electrodes thus obtained are indicated in Table 1 as RuTaSn_TOP type.
• a 1 mm thick AKOT ® titanium alloy mesh was degreased with acetone in a ultrasonic bath and etched in 20% HCI at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of 10 cm x 10 cm size for the subsequent preparation of electrode samples.
• a solution of precursors for the preparation of the protective pre-layer was obtained by mixing 150 g/l of T1OCI2 and 50 g/l of TaCI 5 in 10% hydrochloric acid.
• catalytic layers of various formulations were applied on the protective pre-layer of the above samples by brushing the above catalytic solutions in 8-10 coats and subjected to subsequent drying at 50 °C for 5 minutes and thermal
• the electrode samples shown in the table were subjected to a test of standard potential under anodic evolution of chlorine at the current density of 3 kA/m 2 , in 15% wt. HCI at a temperature of 60 °C.
• the potential data obtained are reported in Table 3 (SEP).
• the table shows also the related data of an accelerated lifetime test, expressed in terms of hours of operation before deactivation under anodic evolution of chlorine at the current density of 6 kA/m 2 , in 20% wt. HCI at a temperature of 60 °C, using a zirconium cathode as counterelectrode.
• the deactivation of the electrode is defined by a 1 V increase in the cell with respect to the initial value.
• Duplicates of electrode samples 2, 6 and C2 were subjected to a corrosion test which simulates the crevice corrosion conditions that can occur on the flanges of electrolysers for the production of chlorine or other occluded zones.
• a first series of samples was immersed in a known volume of 20% wt. HCI at 45 °C under nitrogen stream, to simulate electrolyte stagnation conditions; a second (control) series was immersed in the same volume of 20% wt. HCI at 40 °C under a stream of oxygen, in order to maintain passivation.

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• Inorganic Chemistry (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
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• 2015
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