Amorphous Cobalt Oxide Nanoparticles as Active Water-Oxidation Catalysts

Artificial photosynthesis requires the practical use of efficient, robust, and economical water-oxidation catalysts (WOCs) for chemical-fuel production. The synthesis of amorphous cobalt oxide nanoparticles (ca. 2 nm) is reported as a WOC with a turnover frequency up to 8.6 s⁻¹ in the photocatalytic Ru(bpy)₃ ²⁺–Na₂S₂O₈ system (bpy=2,2′-bipyridyl). This activity is unprecedented in heterogeneous cobalt-based WOCs and is even comparable to that of a state-of-the-art homogeneous cobalt-based polyoxometalate catalyst. With the help of experimental and computational X-ray absorption spectroscopy, the atomic structure of the synthesized amorphous cobalt oxide nanoparticles was characterized, and it consists of a one-dimensional chain of dimeric edge-sharing CoO₆ octahedra. Theoretical calculations suggest that this structure was able to promote O−O bond coupling, unlike crystalline cobalt oxide WOCs, which led to the enhanced water-oxidation activity.