Selecting between two transition states by which water oxidation intermediates on an oxide surface decay
X. Chen, D. Aschaffenburg, and T. Cuk*
Nature Catalysis 2019 2, 820-827. DOI:
See Blog at Nature Catalysis: .
While catalytic mechanisms on electrode surfaces have been proposed for decades, the pathways by which the product’s chemical bonds evolve from the initial charge-trapping intermediates have not been resolved in time. Here, we discover a reactive intermediate population with states in the middle of a semiconductor’s band-gap to reveal the dynamics of two parallel transition state pathways for their decay. Upon photo-triggering the water oxidation reaction from the n-SrTiO3 surface, the intermediates’ microsecond decay reflects transition state theory (TST) through: two distinct time constants, primary kinetic salt and H/D kinetic isotope effects, and realistic activation barrier heights and TST pre-factors. Furthermore, we show that reaction conditions select between the two pathways, one of which reflects a labile intermediate facing the electrolyte (the oxyl) and the other a lattice oxygen (the bridge). Altogether, we experimentally isolate an important activation barrier within multi-electron transfer water oxidation and in so doing, identify competing mechanisms for O2 evolution at surfaces.