Water splitting is any reaction converting water into hydrogen and oxygen for example by electrolysis. More specifically in photocatalytic water splitting the energy source is light and in artificial photosynthesis carbon dioxide is added to water forming oxygen and a carbohydrate. Given the need for clean and cheap energy (hydrogen) the hunt is on for a suitable photocatalyst to go with it.
In a recent Science report Yin et al.(DOI) describe the cobalt cluster compound Na10(Co4(H2O)2(PW9O34)2)·27H2O. Cobalt containing catalysts have already been investigated by others but are heterogeneous and the first homogeneous catalyst ever found is based on expensive ruthenium. The homogeneous cobalt compound was rediscovered in an old 1973 article (Weakley et al. DOI) and can be made by mixing and refluxing in water sodium tungstate Na2WO4, sodium biphosphate Na2HPO4 and cobalt(II) nitrate Co(NO3)2· The resulting cluster compound can be envisioned as having a Co4O16 core lined with PO4 tetrahedra and WO6 octahedra.
In order to save lab time the catalyst was only tested with respect to water oxidation in water against the oxidized form of tris(bipyridine)ruthenium(II) chloride in this reaction:
4 (Ru(bpy)3)3+ + H2O -> 4 (Ru(bpy)3)2+ + 4H+ + O2
By measuring the amount of oxygen generated a turnover number for the catalyst was measured of 80 per second with ultimately 70% oxygen yield. A total of 7 very related catalysts did not work at all and this catalyst only worked in a narrow pH region demonstrating that finding a catalyst at all is not that easy. Contrary to many organic-based catalysts no catalyst degradation was observed. For comparison regular cobalt nitrate scores a TON of only 10 and 33% oxygen yield (converted to gram conversion it would still win out).