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.
• EP 2 757179 A1 describes chlorine evolution anodes which, in addition to amorphous ruthenium oxide, may comprise crystalline ruthenium oxide in an intermediate layer or in a catalytic layer.
• US 3 875 043 A describes a catalytic coating comprising tantalum oxide and ruthenium oxide.
• US 3,853,739 A describes a coating made from a solid solution of platinum group metal oxides in an amorphous tantalum binder. Carl-Erik Boman et al. describe in Acta Chemica Scandinavica, vol. 24, 1 January 1970, pp. 116-122 the preparation and characterization of crysatls of ruthenium dioxide having a rutile structure.
• US 2011/0209992 A1 describes an electrode for an electrolysis cell comprising a catalytic layer containing tin, ruthenium, iridium, palladium and niobium oxides using precursor solutions of hydroxyacetochloride complexes of tin, iridium or ruthenium.
• the invention relates to a coated valve metal substrate, having a coating as defined in claim 1.
• the coating includes a titanium-free catalytic layer and consisting of the mixture of two phases, namely an amorphous phase of Ta 2 O 5 in admixture with a tetragonal ditetragonal dipyramidal crystalline phase containing RuO 2 , optionally in solid solution with SnO 2 .
• 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 TiO 2 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 RuO 2 in the typical crystalline form similar to rutile (i.e. tetragonal ditetragonal dipyramidal) and Ta 2 O 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 Ta 2 O 5 and the crystalline phase is between 0.25 and 2.5, which defines the best range of functioning of the invention.
• the RuO 2 component in the tetragonal ditetragonal dipyramidal crystalline phase is partially replaced by SnO 2 . (cassiterite).
• SnO 2 . 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.
• This can have the advantage of enhancing on one hand the protective function at the substrate surface and on the other hand the catalytic and conductive properties of the outermost layer, as required for example in the case wherein the coating is used for the catalytic activation of an anodic structure whose outer surface is in direct contact with the electrolyte.
• the inner catalytic layer has a weight ratio of amorphous Ta 2 O 5 phase to RuO 2 -containing crystalline phase (optionally including SnO 2 ) ranging between 0.25 and 2.5 and the outer catalytic layer consists of an amorphous phase of Ta 2 O 5 mixed with a tetragonal ditetragonal dipyramidal crystalline phase of RuO 2 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.
• 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 in another aspect, relates to a method for the manufacturing of a coated valve metal substrate as hereinbefore described comprising the optional application of a solution of titanium and tantalum compounds, for example TiOCl 2 , TiCl 3 and TaCl 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 TiOCl 2 , TiCl 3 and TaCl 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 TiOCl 2 and 50 g/l of TaCl 5 in 10% wt. hydrochloric acid.
• a first series of catalytic solutions was obtained by mixing 20% by weight RuCl 3 and 50 g/l TaCl 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 1 mm thick AKOT® titanium alloy mesh was degreased with acetone in a ultrasonic bath and etched in 20% HCl 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 TiOCl 2 and 50 g/l of TaCl 5 in 10% hydrochloric acid.
• a series of catalytic solutions was obtained by mixing 20% by weight RuCl 3 and 150 g/l TiOCl 2 in 10% hydrochloric acid according to various proportions.
• 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. HCl 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. HCl 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. HCl at 45 °C under nitrogen stream, to simulate electrolyte stagnation conditions; a second (control) series was immersed in the same volume of 20% wt. HCl at 40 °C under a stream of oxygen, in order to maintain passivation.
• the test was repeated with another set of samples, confirming a substantial increase in the corrosion resistance for the formulations of the invention.

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• Chemical & Material Sciences (AREA)
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• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Chemically Coating (AREA)
• Catalysts (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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• 2015
  • 2015-06-18 TW TW104119668A patent/TWI679256B/zh not_active IP Right Cessation
  • 2015-07-21 AR ARP150102307A patent/AR101828A1/es active IP Right Grant
  • 2015-07-28 WO PCT/EP2015/067273 patent/WO2016016243A1/en not_active Ceased
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  • 2015-07-28 CN CN201580034498.6A patent/CN106471159B/zh active Active
  • 2015-07-28 JP JP2017505073A patent/JP6714576B2/ja not_active Expired - Fee Related
  • 2015-07-28 EP EP15742289.0A patent/EP3175019B1/en active Active
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  • 2015-07-28 HU HUE15742289A patent/HUE041583T2/hu unknown
  • 2015-07-28 RU RU2017106084A patent/RU2689985C2/ru active
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