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EPR Spectroscopy of Metal-Oxos: Probing Reactive Sites for Water Oxidation

A large number of heterogeneous metal (hydr)oxides are known to catalyze water oxidation in aqueous base. Several of these catalysts have been developed and studied by CCI Solar groups, including systems based on iron, cobalt, and nickel. In many systems, the site of water (or hydroxide) coordination and the mechanisms of O-O bond formation are unknown. We have endeavored to explore these materials using electron paramagnetic resonance (EPR) in order to gain an atomistic understanding of how these materials function.

In nickel- and iron-rich (hydr)oxides, the paramagnetism of the bulk metal ions complicates magnetic resonance characterizations. To solve this problem, we have looked at VT CW EPR of these materials and developed an understanding of their superparamagnetism and blocking temperature. We have also prepared materials with small concentrations of paramagnetic ions doped into diamagnetic lattices (e.g. ZnAlOx layered double hydroxide). In the case of iron doping, we find that there are multiple types of redox-active Fe(III) sites, of which only a subset appear to have labile water or hydroxide.

We have recently expanded our toolkit to detect minute quantities of oxygen generated during sequential turnover events. This EPR-microwave hybrid experiment is extremely sensitive and provides detailed information about the isotopic distribution of dioxygen as a function of turnover number. We will present our progress on this new front.