High efficiency hypochlorite anode coating

Abstract

The invention relates to an electrocatalytic coating and an electrode having the coating thereon, wherein the coating is a mixed metal oxide coating, preferably platinum group metal oxides, with or without a valve metal oxide. The electrocatalytic coating can be used especially as an anode component of an electrolysis cell and in particular a cell for the electrolysis of aqueous hypochlorite solutions.

1 . An electrode for use in the electrolysis of an aqueous solution for the production of hypochlorite, said electrode having an electrocatalytic coating thereon, with said electrode comprising: 
 a valve metal electrode base;    a coating layer of an electrochemically active coating on said valve metal electrode base, said coating comprising a mixed metal oxide coating of platinum group metal oxides and a valve metal oxide, said mixed metal oxide coating consisting essentially of platinum group metal oxides of ruthenium, palladium, and iridium, and a valve metal oxide of titanium; wherein    (a) the molar ratio of said valve metal oxide to said platinum group metal oxides is from about 90:10 to about 40:60;    (b) the molar ratio of said ruthenium to said iridium is from about 90:10 to about 50:50 and    (c) the molar ratio of said palladium oxide to ruthenium plus iridium oxides is from about 5:95 to about 40:60, basis 100 mole percent of the metals present in the coating;    whereby said electrode operates at a high current efficiency to produce hypochlorite concentrations of at least 8 grams per liter.    
   
   
       2 . An electrode according to  claim 1 , wherein said valve metal electrode base comprises a valve metal mesh, sheet, blade, tube, punched plate or wire member.  
   
   
       3 . An electrode according to  claim 2 , wherein said valve metal electrode base comprises one or more of titanium, tantalum, aluminum, hafnium, niobium, zirconium, molybdenum or tungsten, their alloys and intermetallic mixtures thereof.  
   
   
       4 . An electrode according to  claim 3 , wherein a surface of said valve metal electrode base comprises a roughened surface.  
   
   
       5 . An electrode according to  claim 4 , wherein said surface is prepared as by one or more of intergranlular etching, grit blasting, or thermal spraying.  
   
   
       6 . An electrode according to  claim 4 , wherein there is established a ceramic oxide barrier layer as a pretreatment layer on said roughened surface.  
   
   
       7 . An electrode according to  claim 4 , wherein the molar ratio of ruthenium oxide to iridium oxide is about 1:1.  
   
   
       8 . An electrode according to  claim 7 , wherein the molar ratio of said platinum group metal oxides and said valve metal oxide is within the range of from about 4:1 to about 1:4.  
   
   
       9 . An electrode according to  claim 1 , wherein said electrode comprises an anode utilized in the electrolysis of seawater.  
   
   
       10 . An electrode according to  claim 1 , wherein said electrode operates at a current efficiency within the range of from about from about 90% to about 100% over a hypochlorite concentration of from 16 to 0 grams per liter.  
   
   
       11 . A process for the electrolysis of an aqueous solution in an electrolytic cell having at least one anode therein, said anode having an electrocatalytic coating thereon, said process comprising the steps of: 
 providing an unseparated electrolytic cell;    establishing in said cell an electrolyte containing chloride;    providing said anode in said cell in contact with said electrolyte, said anode having said electrocatalytic coating comprising a mixed metal oxide coating of platinum group metal oxides and a valve metal oxide, said mixed metal oxide coating consisting essentially platinum group metal oxides of ruthenium, palladium, and iridium, and a valve metal oxide of titanium; wherein    a) the molar ratio of said valve metal oxides to said platinum group metal oxides is from about 90:10 to about 40:60;    (b) the molar ratio of said ruthenium to said iridium is from about 90:10 to about 50:50 and    (c) the molar ratio of said palladium oxide to ruthenium plus iridium oxides is from about 5:95 to about 40:60, basis 100 mole percent of the metals present in the coating;    impressing an electric current on said anode; and    oxidizing chloride at said anode to produce hypochlorite at concentrations of at least 8 grams per liter.    
   
   
       12 . A process according to  claim 11 , wherein said chloride electrolyte in said cell comprises one or more of sodium chloride or potassium chloride.  
   
   
       13 . A process according to  claim 11 , wherein a surface of said anode comprises a roughened surface prepared by one or more steps of intergranular etching, grit blasting, or thermal spraying.  
   
   
       14 . The process of  claim 13  wherein said anode surface comprises titanium and said electrocatalytic coating is provided on said titanium member by a procedure including electrostatic spray application, brush application, roller coating, dip application and combinations thereof.  
   
   
       15 . A process according to  claim 12 , wherein said ruthenium oxide and iridium oxide are present in a molar proportion of from about 1:3 to about 4:1.