Catalyst Coating and Process for Production Thereof

Abstract

A process for wet-chemical production of a catalyst coating on an electrically conductive support for electrodes for chloralkali or hydrochloric acid electrolysis with electrocatalytically active components based on noble metal oxides, in which the catalyst coating is produced by; producing a coating solution or dispersion comprising a precursor compound of a noble metal and/or a metal oxide of a noble metal, and a solvent or dispersant, with addition of one or more acids to the coating solution or dispersion, where the molar ratio of the total of the amounts of acid (in mol) present in the coating solution or dispersion to the sum of the amounts of the metals from the metal-containing components present in the coating solution or dispersion is at least 2:1; applying the coating solution or dispersion to the support; substantially freeing the layer applied of solvent or dispersant by drying; and subjecting the dried layer obtained to a thermal treatment to form the catalyst coating.

1 . A process for wet-chemical production of a catalyst coating on an electrically conductive support for electrodes for chloralkali or hydrochloric acid electrolysis with electrocatalytically active components based on at least one noble metal oxide and/or noble metal of the noble metals of transition group VIIIa of the Periodic Table of the Elements, and optionally additionally at least one valve metal oxide, especially titanium oxide, characterized in that the catalyst coating is produced by
 a) producing a coating solution or dispersion at least comprising a precursor compound of a noble metal and/or a metal oxide of a noble metal, especially of ruthenium and/or iridium, and optionally additionally a tin and/or valve metal compound, preferably a titanium compound, and a solvent or dispersant, with addition of one or more acids to the coating solution or dispersion, where the molar ratio of the total of the amounts of acid (in mol) present in the coating solution or dispersion to the sum of the amounts of the metals from the metal-containing components present in the coating solution or dispersion is at least 2:1,   b) applying the coating solution or dispersion to the conductive support,   c) substantially freeing the layer applied of solvent or dispersant by drying and   d) then subjecting the dried layer obtained to a thermal treatment at a temperature of at least 300° C., and optionally in the presence of oxygen-containing gases, to form the catalyst coating.   
     
     
         2 . The process according to  claim 1  wherein the coating solution or dispersion according to step a) comprises one or more acids having a pK a  in aqueous solution of not more than 12. 
     
     
         3 . The process according to  claim 1  wherein the coating solution or dispersion according to step a) additionally comprises one or more precursor compounds and/or a metal and/or a metal oxide of one or more doping elements selected from the group consisting of: aluminium, antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin and zirconium. 
     
     
         4 . The process according to  claim 1  wherein the thermal treatment d) is performed in the presence of air or in the presence of a mixture of oxygen and a protective gas selected from the group consisting of nitrogen, helium, neon, argon or krypton. 
     
     
         5 . The process according to  claim 1  wherein the drying c) is performed in the presence of air or in the presence of a mixture of oxygen and a protective gas, especially at least one protective gas selected from the group consisting of nitrogen, helium, neon, argon and krypton. 
     
     
         6 . The process according to  claim 1  wherein the precursor compound used in step a) is a chloride of ruthenium and/or iridium. 
     
     
         7 . The process according to  claim 1  wherein the acid used in the coating solution in a) is an organic or inorganic acid or a combination of organic and inorganic acids. 
     
     
         8 . The process according to  claim 7 , wherein the acid used in the coating solution is a combination of organic and inorganic acid, where the molar ratio of the amount of organic acid (in mol) present in the coating solution or dispersion to the amount of mineral acid is 20:80 to 100:0. 
     
     
         9 . The process according to  claim 7  wherein the acid used in the coating solution in a) is hydrochloric acid and/or a C 1  to C 4 -carboxylic acid selected from the group consisting of: hydrochloric acid, formic acid, acetic acid and propionic acid. 
     
     
         10 . The process according to  claim 1  wherein the solvent or dispersant used is one or more from the group of: water and C 1 -C 6 -alcohol, preferably butanol or isopropanol. 
     
     
         11 . The process according to  claim 1  wherein the support is based on a valve metal selected from the group consisting of titanium, zirconium, tungsten, tantalum and niobium. 
     
     
         12 . The process according to  claim 1  wherein the surface of the support before the application b) of the coating solution or dispersion to the support is mechanically pre-cleaned and optionally subsequently etched with an acid such as hydrochloric acid or oxalic acid for further removal of oxides on the surface. 
     
     
         13 . The process according to  claim 1  wherein the coating solution or dispersion according to step a) additionally comprises at least one precursor compound selected from the group of the compounds of: aluminium, antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin, and zirconium, tin, indium, manganese and antimony, optionally also as a precursor compound for a doping, in which case the proportion of doping elements is ultimately preferably up to 20 mol % based on the total content of metals in the coating solution or dispersion. 
     
     
         14 . The process according to  claim 13 , wherein the coating solution or dispersion according to step a) comprises one or more precursor compounds selected from the group of the compounds of: aluminium, antimony, tantalum, niobium, tin, indium, more preferably from the group of tin(IV) chloride (SnCl 4 ), indium(III) chloride (InCl 3 ), antimony(III) chloride (SbCl 3 ) and manganese(II) chloride (MnCl 2 ). 
     
     
         15 . The process according to  claim 1  wherein the additional valve metal precursor compounds used are solvent-soluble fluorides, chlorides, iodides, bromides, nitrates, phosphates, sulphates, acetates, acetylacetonates or alkoxides of the elements titanium, zirconium, tungsten, tantalum and niobium. 
     
     
         16 . The process according to  claim 1  wherein a catalyst coating is produced with binary mixed oxides formed from titanium oxide and ruthenium oxide by using a coating solution in a) comprising titanium(IV) butoxide and ruthenium(III) chloride (RuCl 3 ) and a mixture of acetic acid and water. 
     
     
         17 . An electrode having a catalyst coating obtained by a process according to  claim 1 . 
     
     
         18 . A method for using the electrode according to  claim 17  for electrochemical production of chlorine from hydrogen chloride or alkali metal chloride solutions, especially from sodium chloride solutions, or for generation of electrical power, preferably in fuel cells and batteries, in redox capacitors, for electrolysis of water, for regeneration of chromium baths, or in the case of use of fluoride-containing electrolytes in hydrogen peroxide, ozone of the peroxodisulphate production.