The water oxidation mechanism, catalyzed by mononuclear ruthenium complexes, produces protons that can be used as a source of clean and renewable carbon-free fuel. This mechanism includes proton-coupled electron transfer (PCET) steps and an O-O bond forming step, similar to the oxygen evolving complex in plants. Ruthenium is too rare and costly of a metal for wide application. Iron is similar to ruthenium and much more abundant and inexpensive. Accordingly, we employ density functional theory (DFT) calculations of eleven mononuclear ruthenium catalysts and compare these to analogous mononuclear iron catalysts to investigate energetics for the steps along the water oxidation mechanism. Individual bond lengths and atomic charges were also analyzed to locate significant differences among the sets of metal catalysts that differ in their organic ligands as well as between the two metal centers. The relevant thermodynamic and kinetic values for the steps along the mechanism vary from one catalyst to the next with different steps serving as the most significant hurdle depending on the ligands’ donating or withdrawing character. Typically, less than 10 kcal/mol is required to release O2 in the final step to complete the cycle from the ruthenium and iron catalysts we investigated. Our results suggest that different ligands subtly influence the intermolecular features of the catalyst but can lead to dramatic impact on the energetics. The insights gained from the analysis of our calculations help inform design of functioning iron catalysts and target ligand modification to address the most challenging mechanistic steps.

Predictive design principles for earth abundant mononuclear water oxidation catalysts via <i>ab initio </i>calculations

251st National Meeting (2016)

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Predictive design principles for earth abundant mononuclear water oxidation catalysts via ab initio calculations