Selecting between two transition states by which water oxidation intermediates decay on an oxide surface

Abstract

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) andTST pre-factors (10 11 -10 12 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 nfor

Reaction kinetics and interplay of two different surface states on hematite photoanodes for water oxidation

“…The kinetics and interplay of both surface states on hematite correspond well with many other oxide-based water oxidation catalysts monitored under operando conditions 52,[58][59][60] . This points to high-energetic oxo species (after one-hole injection) as a common feature to initiate OER directly, with no need for extensive accumulation once they reach a sufficient lifetime.…”

Measuring Theoretical Descriptors of Water Oxidation: Free Energy Differences Between Intermediates Using Time-Resolved Optical Spectroscopy

“…An increasingly more exoergic ΔGOH*(UVB) on the RHE scale would be needed to obtain a half-rise closer to 8. The weak-binding and the downhill pathway towards O-O bond formation observed previously 21 supports the full multi-site reaction mechanism of Fig. 4a, where the first electron-transfer creates four Ti-OH * intermediates from four H2O and then subsequent thermo-chemical reaction steps form O2 and release two H2O.…”

“…While vibrational spectroscopies determine these detailed bonding geometries [18][19] , optical spectroscopy 12,20 revealed easily-trackable electronic levels created in the middle of the semiconducting band gap that target the total population of M-OH * . Ultrafast studies of SrTiO3 (STO) followed the picosecond formation of the Ti-OH * population from delocalized semiconductor holes 12 and its subsequent, microsecond decay towards the next reaction step 21 .…”

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

“…It should be emphasized that the combination of many different techniques for multi-dimensional surface and structural information is required for the establishment of reliable and complete catalytic cycles. 23 For example, time-resolved FTIR can provide molecular information of the active sites in action, 208,209 while operando optical and XAS spectroscopy can uncover the electronic structure of the involved metal centres. 210…”