Studies On Electrochemistry Characterization With Transitional-metal-substituted POMs

The polyoxometalates constitute an extremely large and diverse class of compounds. Their remarkable chemical and physical properties have led to a variety of potential applications in medicine, catalysis, solid-state technology, and chemical analysis. The interest in the class of transition-metal-substituted POMs (TMSPs), has been growing because of the unique reactivity depending on the composition and structure of the molecules.Transition metal substituted polyoxometalates (TMSPs) can be considered as inorganic analogs of the well known metalloporphyrin complexes, and they have distinct advantages over organometallic species, e.g. they are rigid, hydrolytically stable and thermally robust, resulting in their potential applications as catalysts. So far, a large number of TMSPs have been discovered and their catalytic properties have been studied. In most cases the accommodated transition metal ions are the active sites for the electrocatalysis process. Therefore, the complete electrochemical investigation into the particular TMSP is very significant in order to design an electrocatalyst for an expected reaction.In the present thesis, redox electrochemistry of [(β₂-SiW₁₁MnO₃₈OH)₃]^15- and [Cd₄(H₂O)₂(GeW₉O₃₄₂]^12- has been undertaken in solution. The two compounds belong to the Keggin-typetrimer and dimer derivatives. The oxidized forms of these heteropolymetallates were found to be stable from pH 1 to 7 for [((32-SiW11MnO38OH)3]15" and pH 1 to pH 5 for [Cd^hfeOMGeWgOMb]12', with the redox processes all exhibiting pH dependence. [(p2-SiWnMnO38OH)3]15' exhibits not only successive reduction processes of the addenda atoms (W) in a negative potential range, but some of them also involve redox reactions originating from the substituted transition metals (Mn) such as the oxidation of Mn" at a more positive potential. However, for [Cd^hkO^GeWgO^]12", in a negative potential range there is well-defined redox wave originating from the tungsten-oxo framework. In the positive potential range no redox waves of Cd" are observed, suggesting that the Cd" within the sandwich-type heteropolyanion have no electroactivity under our experimental conditions. In addition, the two complexes are shown to have electrocatalytic properties towards the reduction of nitrite, bromate and hydrogen peroxide.