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 extended to show the role of bismuth oxide in the mechanism of redox reactions. The steps identified for the mechanism of the redox reactions were suggested to be: (1) a short reversible one-phase reaction of charge transfer n congruent to 0.23 electron per formula unit; (2) a reversible two-phase reaction from the birnessite structure to a hydrotalcite-carbonate phase of an average Mn oxidation state of + 2.6; and, finally, (3) another two-phase reaction from the hydrotalcite-like intermediate to manganese hydroxide. In recharging of the later phase, a two-phase reaction took place yielding beta -MnOOH (Feitknechtite) which may be further oxidised to birnessite. The bismuth oxide was found to stabilise the intermediate phases (hydrotalcite during 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 aggressive species which cause the transformation to the inactive 3D structure (e.g. Mn3O4). The protection is still effective as long as the adsorption equilibrium of bismuth species predominates. (C) 2001 Published by Elsevier Science B.V.