Hydrated layered manganese dioxide: III. Role of bismuth oxide on the redox behaviour of hydrated layered manganese dioxides

The previous studies to the hydrated layered manganese dioxides were extend ed to show the role of bismuth oxide in the mechanism of redox reactions. T he steps identified for the mechanism of the redox reactions were suggested to be: (1) a short reversible one-phase reaction of charge transfer n cong ruent to 0.23 electron per formula unit; (2) a reversible two-phase reactio n from the birnessite structure to a hydrotalcite-carbonate phase of an ave rage Mn oxidation state of + 2.6; and, finally, (3) another two-phase react ion from the hydrotalcite-like intermediate to manganese hydroxide. In rech arging of the later phase, a two-phase reaction took place yielding beta -M nOOH (Feitknechtite) which may be further oxidised to birnessite. The bismu th oxide was found to stabilise the intermediate phases (hydrotalcite durin g reduction or beta -MnOOH during oxidation) through the adsorption of the dissolved Bi(III) species on the active sites found mainly on the sides of the layered oxide. This adsorption formed a protective layer against the ag gressive species which cause the transformation to the inactive 3D structur e (e.g. Mn3O4). The protection is still effective as long as the adsorption equilibrium of bismuth species predominates. (C) 2001 Published by Elsevie r Science B.V.