Selecting Between Two Transition States by Which Water Oxidation Intermediates on an Oxide Surface Decay

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 population of charge-trapping intermediates 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 with band-gap, pulsed excitation, the intermediates’ microsecond decay reflects transition state theory (TST) through: (1) two distinct and reaction dependent (pH, T, Ionic Strength, and H/D exchange) time constants, (2) a primary kinetic salt effect on each activation barrier and an H/D kinetic isotope effect on one, and (3) realistic activation barrier heights (~0.4-0.5 eV) and TST pre-factors (~10¹¹ -10¹² Hz). A photoluminescence from mid-gap states in n-SrTiO₃ reveals the reaction dependent decay; the same spectrum was previously assigned by us to hole-trapping at parallel Ti-O^·-Ti (bridge) and perpendicular Ti-O^· (oxyl) O-sites using in-situ ultrafast vibrational and optical spectroscopy. Therefore, the two transition states are naturally associated with the decay of these respective intermediates. 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 necessary for water oxidation, which is necessary for designing water oxidation catalysts for high O₂ turn-over. Moreover, in isolating it, we identify competing mechanisms for O₂ evolution at surfaces and show how to use reaction conditions to select between them.