Cyanamide route to calcium–manganese oxide foams for water oxidation

Abstract

In nature, photosynthetic water oxidation is efficiently catalysed at a protein-bound μ-oxido Mn₄Ca cluster. This cluster consists of earth abundant, non-toxic elements and serves as a paragon for development of synthetic catalysts. In this study we developed porous calcium–manganese oxides with a unique foam-like nanostructure prepared via a facile and robust synthetic route using cyanamide as a porogen. A series of such oxide foams annealed at different temperatures was characterized by TEM, SEM, XRD, N₂ physisorption, and X-ray absorption spectroscopy (XAS) in order to correlate crystallinity, atomic structure, surface area and oxidation state of the materials with catalytic activity. Some of the resulting Ca–Mn oxides show high activity as catalysts for water oxidation in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant. An amorphous calcium–manganese-oxide foam with 130 m² g⁻¹ surface area and Mn oxidation state of +3.6 was identified to be most active; its activity is superior to previously reported Ca–Mn oxides. At the atomic level, this material shares structural motifs with the biological paragon as revealed by dual-edge XAS at the Mn and Ca K-edge. Rather than nanostructure and surface area, the atomic structure of the Ca–Mn oxide and the extent of structural order appear to be crucial determinants of catalytic activity. Fully disordered low-valent Mn materials as well as high-valent but crystalline Mn–Ca oxides are unreactive. Highly disordered variants of layered manganese oxide with Ca and water molecules interfacing layer fragments are most reactive.