High Oxidation State of Iron in Molten Hydroxides

The increased interest in iron in the high oxidation state 6+ arises from its potential as an environment friendly cleaner for remediation processes, as an perspective alternative for “green” battery cathodes, and as a suitable oxidant for organic synthesis. The effective synthesis of ferrates(VI) became the challenging task for several research groups all over the world. Three main methods of ferrates(VI) synthesis have been proposed: electrochemical, wet chemical and thermal method. Electrochemical one can be considered as a “green” method because no harmful and expensive chemicals to oxidize Fe to the 6+ oxidation state are used. Two main environments are suitable for ferrates(VI) electrochemical synthesis: strong alkaline aqueous solutions and molten hydroxides. The electrochemical way of ferrates(VI) production provides product of high purity. The synthesis of ferrates(VI) by an anodic dissolution of metallic iron proceeds typically in the transpassive potential region. At these conditions, the surface of the iron anode is covered by a partly disintegrated (e.g. containing cracks and/or pores) oxo-hydroxide layer. Efficiency of the synthesis is strongly influenced by the protective properties of this layer. These can be affected into a significant degree by the anode material used and/or by the reaction conditions, i.e. by the electrolyte concentration, composition, temperature and the cell arrangement.