Manganese Oxides in Heterogeneous (Photo)Catalysis: Possibilities and Challenges

The aim to develop active photocatalysts based on abundant elements for solar energy conversion reactions has sparked wide interest in manganese oxides as visible light-absorbing alternative to TiO₂. Today, a multitude of different routes are available for the synthesis of MnO_x species with specific stoichiometry, crystal structure, morphology, size or surface properties. Still, even for the bulk manganese oxides, some controversy remains, for example, with respect to the band gap, which hinders the targeted development of specific manganese oxide catalysts for photocatalysis. In classical heterogeneous catalysis and electrocatalysis, manganese oxides have been successfully used for a wide range of reactions, in particular in the field of (selective) oxidations. Photocatalytic applications have also been reported, but a true photocatalyst for the famous water-splitting reaction, deep insight into the prevailing mechanisms and an understanding of the involved processes has yet to be found. With this review, we aim to give a comprehensive overview over the structural, physical and catalytic properties of manganese oxides, together with an overview over suitable synthesis procedures. This will then serve as a basis for the discussion of the state of the art in the application of manganese oxides in catalysis and photocatalysis.