Under pathway II, #-Obr -dominates at all light intensities for both models. This is because it can only react if an OH radical is supplied to it. We note that the reaction between an OH radical and #-Obr -is also dominant on the anatase surface. In Model 1, which is well-mixed, this is a second-order reaction that depends nonlinearly on reactant concentrations. In Model 2, OH transfer is a unimolecular rearrangement, which has more favorable kinetics. Indeed, pathway II is dominant for Model 2 because of relatively facile OH transfer to a bridging O -. The buildup of #-Obr -indicates that formation of that species is faster than the OH transfer step at any hole mobility condition, however, Roberts et al. have noted that loss of a proton from surface Ti-OOH intermediates, which is a thermal process, is a slow step that controls the overall rate of reaction. The present work provides additional evidence that thermal processes are very important, and provide additional insights to how they are involved in this complex mechanism. It is interesting to compare the kinetic predictions with the thermodynamics study by Bajdich et al. of the influence of photogenerated polarons on OER on TiO2 44 using a mechanism significantly simplified compared to that of Wang et al. In the Bajdich study it was found that polarons stabilized surface O, OH and OOH intermediates, with the OOH stabilization proposed to significantly reduce the overpotential required to drive the reaction on Ti active sites. As discussed above, a buildup of OOH-type intermediates is consistent with infrared measurements. Model 2 most closely mimics the influence of polarons by forcing holes to be stationary once they are trapped by surface species, and the influence of the polarons can be assessed by comparison to Model 1. The kinetics simulations predict a buildup of hydroxo species via the Ti pathway (pathway I) at low light intensities, whose concentrations in both models are similar (Figure ). The concentration of Ti-peroxo species from pathway I declines rapidly at higher light intensity. Because the hydroperoxo species only builds up in the bridge oxygen pathway (pathway II), its concentration is much higher in Model 2. This buildup reflects an imbalance between hole trapping rates across both catalytic pathways. Increasing light intensity (hole formation rate) in Model 2 shifts the overall OER reaction to bridge sites (Figure ) because hole trapping by Ti-OH -to form Ti-OH (which loses OH to a bridge O site) becomes highly favored (Figure ). This process is not favored in Model 1, in which formation of TiO - and continued reaction along pathway I dominates. As the light intensity increases, the ratio of the photocurrent generated by pathway II of Model 2 to the photocurrent generated by pathway I of Model 1 increases from 1.8 to 30, as shown in Figure . These kinetics reveal that a polaronic-type environment, where holes are not very mobile, significantly accelerates the overall photo-OER reaction rate relative to highly mobile holes. Examination of the Model 2 mechanism indicates that this acceleration is attributable not to the presence of immobilized holes per se, but to the fact that reactants in thermal processes are adjacent to one another. The increased rates of thermal processes such as TiO -coupling and OH transfer involve entirely local rearrangements rather than stabilization of intermediates. It would be of great interest to extend the thermodynamics analysis of the role of polarons to a kinetic one applicable to photocatalysis where there is no formal overpotential in order to understand in greater detail how polarons influence surface reactivity beyond whether or not they can diffuse.