Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts

Recent development of new methods of preparing cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidation catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorganic materials for catalytic water oxidation reported recently from other laboratories, in particular electrodeposits generated from Co phosphate solutions, a molecular water oxidation catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.