Water oxidation by nano-layered manganese oxides in the presence of cerium(IV) ammonium nitrate: important factors and a proposed self-repair mechanism

Nano-sized layered cadmium, magnesium and potassium-Mn oxides were synthesized and characterized by scanning electron microscopy, energy-dispersive X-ray mapping, transmission electron microscopy, X-ray diffraction spectroscopy, and atomic absorption spectroscopy. These oxides showed water oxidizing activity in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant. The formulae of the best catalysts are Cd0.16MnO2, Mg0.16MnO2 and K0.25MnO2 with an average oxidation state of similar to 3.7-3.8 for the Mn ions in the birnessite structure. We also prepared a mixture of redox-active metal ions containing Fe(II), Cu(II), Co(II), and Ni(II) instead of inert redox ions. Important factors and a proposed self-repair mechanism for the layered Mn oxides in the water oxidation reaction are reported.