A detailed reaction mechanism (reaction steps and information for all rate coefficients) for the rutile phase has been reported by Wang et al. The anatase phase is used while measuring photo-OER kinetics, however a similarly detailed mechanism with corresponding rate coefficients has not been reported for that surface. As described in detail in the Supporting Information (SI), the photophysics and chemistry for rutile and anatase phases are very similar (SI Section 1) except that at low light intensities, O-O coupling across adjacent Ti atoms on the surface of the anatase phase is inactive (SI Section 2). The specific surface structure of these two phases differs, however both contain the same types of atomic arrangements that are important for this reaction, namely TiOx centers and O atoms that bridge two Ti atoms. When the two mechanisms are compared, Wang et al. concluded that from a surface chemistry perspective, the rutile and anatase phases are very similar. Because the rutile mechanism is more detailed and enables quantitative kinetics to be simulated, we chose it for the purpose of studying photo-driven OER. mobility in determining the photo-OER rate at a given light intensity. In order to assess the influence of mobility on the chemistry, we have examined two models for photo-OER that both use the full Wang mechanism, shown in