Water Oxidation by λ-MnO2: Catalysis by the Cubical Mn4O4 Subcluster Obtained by Delithiation of Spinel LiMn2O4

Nanocrystalline spinel LiMn2O4 has been prepared and treatment of LiMn2O4 with dilute nitric acid solution resulted in the delithiation of the framework, while maintaining the spinel structure, λ-MnO2. LiMn2O4 is not a catalyst for water oxidation. Upon removal of the lithium, the cubical Mn4O4 cores become active sites for oxidizing water to molecular oxygen, which was investigated with the photochemical [Ru2+(2,2′-bpy)3]/persulfate system at pH 5.8. The nanosize λ-MnO2 obtained from the nanocrystalline LiMn2O4, which was synthesized by the citrate route, shows a significantly higher water oxidation catalytic activity (Turnover Frequency: 3 × 10−5 mol O2/s/mol Mn) than that obtained via solid state reaction with micrometer and irregular particle sizes (Turnover Frequency: 5 × 10−6 mol O2/s/mol Mn).