Earth-abundant metal complexes as catalysts for water oxidation; is it homogeneous or heterogeneous?

Recent developments in the oxidation of water to dioxygen using metal complexes as catalysts or precatalysts are presented, as well as the conversion of homogeneous catalysts into nanoparticles or heterogeneous catalysts during the oxidation of water. Homogeneous catalysts are advantageous in the careful design and elucidation of the mechanisms for catalytic water oxidation. In distinction, the use of heterogeneous catalysts in water oxidation is advantageous for applications as a result of their robust catalytic activity. Although detailed studies on homogeneous and heterogeneous water oxidation reactions are performed rather often, understanding the link between homogeneous catalysts and heterogeneous ones is becoming an additional requirement in order to produce economically attractive water oxidation catalysts (WOCs). This minireview focuses on the aspects that determine whether particular catalysts for the oxidation of water are homogeneous or heterogeneous.