Transition metals, aqueous heterogeneous reduction

Scherson, Palenscar, Tolmachev and Stefan provide a critical review of transition metal macrocycles, in both intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. An introduction is provided to fundamental aspects of electrocatalysis, oxygen reduction, and transition metal macrocydes. Since the theoretical and experimental tools used for investigation of homogeneous and heterogeneous electrocatalysis are considerably different, these topics are given separate discussion. The influence of the electrode surface on adsorbed macrocydes, and their influence on mechanism and rates of 02 reduction is treated in detail. Issues related to pyrolyzed macrocydes are also described. 

The reduction on aqueous transition metal species at the surfaces of Fe(II)-containing oxides is defined in terms of heterogeneous redox reactions that occur concurrently with oxidation and weathering of the mineral surfaces. Electrochemical measurements made on mineral electrodes document that such reactions are coupled half cells in which reductive dissolution can be decoupled and substituted for reduction of aqueous species. This is confirmed experimentally in aqueous/mineral suspensions in which Cr(VI), V(V) Fe(III) and Cu(II) are reduced to lower valance states. 

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