Photochemistry

Previously reported abiotic processes have much slower oxidation kinetics than biotic processes, and in environmental systems, abiotic inorganic processes can oxidize Mn²⁺ only to Mn(III). Also, although photo-catalysis using disordered δ–MnO₂ nanosheets has been attracting interest for use in water splitting, most studies have focused on photoreduction only. Here, using the photolysis of nitrate, we show the fast oxidation of Mn²⁺ (aq) to Mn(IV) (s), and the formation of δ–MnO₂ nanosheets. Although they have previously been thought unlikely, even comparable to those of biotic processes. The findings suggest a new and previously unconsidered abiotic explanation for the occurrence of Mn(IV) oxide in environmental systems. I also studied the enhanced formation rate of MnO₂ and the structural change of MnO₂ under varied concentrations of an inorganic ligand, pyrophosphate. I found that pyrophosphate plays a catalytic role by making Mn(III)-PP, as an intermediate form, between Mn²⁺ and Mn(IV). The different concentrations of soluble Mn(III) under varied concentrations of pyrophosphate also controlled the amount of Mn(III) in layered birnessite, and consequently resulted in structural changes. These findings open the possibility of using photochemically-assisted green chemistry to control the properties of disordered δ–MnO₂ nanosheets for use in more effective catalytic reactions, such as water oxidation, and suggest the importance of Mn(III)-PP complexes for the formation of δ–MnO₂ nanosheets in nature.