Layered manganese oxides for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion

Abstract

In reaction sequences for light driven water-splitting into H₂ and O₂, water-oxidation is a crucial reaction step. In vivo, the process is catalysed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn₄ cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca²⁺ is removed from its structure. Activity can be restored not only by the addition of Ca²⁺ but also Sr²⁺ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compounds for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, preparing a series of K-, Ca-, Sr- and Mg-containing birnessites. Structural motifs within these materials were determined using X-ray absorption spectroscopy (XAS) showing that all materials have similar atomic structures despite their different elemental compositions. Water-oxidation experiments were carried out to elucidate structure-reactivity relations. These experiments demonstrated that the oxides—like the OEC—require the presence of calcium in their structures to reach maximum catalytic activity. As another similarity to the OEC, Sr²⁺ is the “second best choice” for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca²⁺ in biological water-oxidation. Additionally, they offer valuable hints for the development of synthetic, manganese-based water-oxidation catalysts for artificial photosynthesis.