The preferred reduced metal oxides for chlorine production are reduced oxide of ruthenium or iridium. The electrocatalyst may be a single, reduced platinum group metal oxide such as ruthenium oxide, iridium oxide, platinum oxide, etc. It has been found, however, that mixtures or alloys of reduced platinum group metal oxides are more stable. Thus, one electrode of reduced ruthenium oxides containing up to 25% of reduced oxides of iridium, and preferably 5 to 25% of iridium oxide by weight, has been found very stable. In a preferred composition, graphite may be added in an amount up to 50% by weight, preferably 10-30%. Graphite has excellent conductivity with a low halogen overvoltage and is substantially less expensive than plantinum group metals so that a substantially less expensive, yet highly effective electrode is possible.
ELECTRODES
As pointed out in the aforesaid LaConti application, the anode electrode for hydrogen halide electrolysis is preferably a particulate mass of Teflon bonded, graphite activated with oxides of the platinum metal group, and preferably temperature stabilized, reduced oxides of those metals to minimize chlorine overvoltage. As one example, ruthenium oxides, preferably reduced oxides of ruthenium, are stabilized against chlorine to produce an effective, long-lived anode which is stable in acids and has low chlorine overvoltage. Stabilization is effected by temperature stabilization and by alloying or mixing with oxides of iridium or with oxides of titanium or oxides of tantalum. Ternary alloys of the oxides of titanium, ruthenium and iridium are also very effective as a catalytic anode. Other valve metals such as niobium, zirconium or hafnium can readily be substituted for titanium or tantalum.