Researchers at Carnegie Mellon University have developed a new catalyst—iron-centered tetraamido macrocyclic ligand (Fe-TAML)—that efficiently catalyzes water oxidation. Water oxidation is the second of two requisite half-reactions in the photolysis of water, the other being the reduction of protons to dihydrogen.
Proton reduction is well-understood, with catalysts achieving quantum yields of 34% when driven by visible light, the researchers note in a new paper published online 22 July in the Journal of the American Chemical Society. Water oxidation, on the other hand, is much less advanced, typically involving expensive metal centers and rarely working in conjunction with a photochemically powered system.
When the new catalyst is combined in unbuffered solution with ceric ammonium nitrate, its turnover frequency exceeds 1.3 s-1.
At pH 0.7, one of five tested Fe-TAMLs, 5, activates ceric ammonium nitrate to liberate oxygen from water at a rate that is among the highest in the literature; 2-4 are also active. Notably, Fe-TAMLs have low molecular weights (~500 g/mol) and are based on iron, the safest and most abundant transition metal, which helps to set this research on an affordable trajectory for large-scale conversion of solar to chemical energy.
Equally important, Fe-TAMLs are very soluble in water and have low molar extinction coefficients in the visible spectrum, qualities that could be very helpful in attaining unimpeded photon harvesting in a future photosynthetic process. It is well-established that synthetic modification of TAMLs allows the electronic properties of derivative complexes to be widely tuned. Studies are underway to improve the usefulness of Fe-TAMLs for water splitting.
—Ellis et al.
W. Chadwick Ellis, Neal D. McDaniel, Stefan Bernhard and Terrence J. Collins (2010) Fast Water Oxidation Using Iron. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja104766