Mössbauer study of ferric hydroxides and hydroxo complexes and their bahaviour in alkaline electrolytes

• Published in: Electrochimica acta 1991, Vol.36 (8), p.1253-1257
• Main Authors: Kamnev, Alexander A., Ezhov, Boris B., Kopelev, Nikolai S., Kiselev, Yurii M., Perfilyev, Yurii D.
• Format: Article
• Language: English
• Subjects: alkaline electrolytes; binary NiFe hydroxides; ferric hydroxides; ferric hydroxo complexes; transmission Mössbauer spectroscopy
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/0013-4686(91)80003-Q

Some ferric hydroxides including synthetic goethite, amorphous ferric hydroxide, ferric hydroxo complexes, as well as binary NiFe hydroxides and their anodic oxidation products in alkaline electrolytes have been studied by Mössbauer spectroscopy combined with other methods. The results obtained reveal the specificity of lithium cation in strongly alkaline solutions, both as pure LiOH and as an additive to KOH solutions, consisting in inhibition of the dissolution of poorly crystallized fraction contained in the synthetic goethite sample during its long term contact with alkaline electrolytes. The study of soluble ferric hydroxo forms in strong alkali demonstrated that transformation of polynuclear forms, which are characterized by a symmetric Mössbauer doublet, to mononuclear hydroxo complexes under certain conditions results in the change of the initial doublet to a symmetric singlet with the same parameters, which has not been observed for ferric oxide/hydroxide systems before. It obviously corresponds to the formation of a symmetric coordination sphere of mononuclear ferric hydroxo complexes statistically distributed in the NaOH·H 2O matrix and is consistent with our earlier electron spectroscopic observations. The study of electrooxidation of the NiFe(4:1 molar ratio) hydroxide system in 8 M KOH solution showed that the position of its resonant absorption remained constant indicating that the ferric component is electrochemically inactive remaining in the Fe(III) form, despite the nickel component oxidation. Nevertheless, the soluble products can be anodically oxidized in the oxygen evolution potential region, as well as reduced on the cathode.