The effect of lanthanum(III) and cerium(III) ions between layers of manganese oxide on water oxidation.

Manganese oxide structure with lanthanum(III) or cerium(III) ions between the layers was synthesized by a simple method. The ratio of Mn to Ce or La in samples was 0.00, 0.04, 0.08, 0.16, 0.32, 0.5, 0.82, or 1.62. The compounds were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction studies, and atomic absorption spectroscopy. The compounds show efficient catalytic activity of water oxidation in the presence of cerium(IV) ammonium nitrate with a turnover frequency of 1.6 mmol O2/mol Mn.s. In contrast to the water-oxidizing complex in Photosystem II, calcium(II) has no specific role to enhance the water-oxidizing activity of the layered manganese oxides and other cations can be replaced without any significant decrease in water-oxidizing activities of these layered Mn oxides. Based on this and previously reported results from oxygen evolution in the presence of H 2 (18) O, we discuss the mechanism and the important factors influencing the water-oxidizing activities of the manganese oxides.

The effect of different metal ions between nanolayers of manganese oxide on water oxidation.

Here, we used a strategy to answer to the question that whether Ca(II) ion is specific for water oxidation or not? In the procedure, first we synthesized layered Mn oxides with K(I) between layers and then replaced K(I) by Ca(II), K(I), Mg(II), La(III) or Ni(II). We proposed that Ca(II), K(I), Mg(II), La(III) and Ni(II), between layers are important to form efficient water-oxidizing catalyst, but not specific in water oxidation. However, Cu(II) ions decrease water-oxidizing activity of layered Mn oxides. The result is important to find critical factors in water oxidation by low-cost and environmentally friendly nanolayered Mn oxides.

Water exchange in manganese-based water-oxidizing catalysts in photosynthetic systems: from the water-oxidizing complex in photosystem II to nano-sized manganese oxides.

The water-oxidizing complex (WOC), also known as the oxygen-evolving complex (OEC), of photosystem II in oxygenic photosynthetic organisms efficiently catalyzes water oxidation. It is, therefore, responsible for the presence of oxygen in the Earth's atmosphere. The WOC is a manganese-calcium (Mn4CaO5(H2O)4) cluster housed in a protein complex. In this review, we focus on water exchange chemistry of metal hydrates and discuss the mechanisms and factors affecting this chemical process. Further, water exchange rates for both the biological cofactor and synthetic manganese water splitting are discussed. The importance of fully unveiling the water exchange mechanism to understand the chemistry of water oxidation is also emphasized here. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.