Theoretical studies of photo-OER on TiO2 have examined the mechanism and identified some of the key steps facilitated by holes using their free energies. There are significant differences in the conclusions reached in those works. Imanishi et al. proposed a reaction mechanism for rutile TiO2 where O-O coupling occurs via surface O and hydroxyl radicals on neighboring Ti atoms via a bridging oxygen site; while Liu et al. proposed that O-O coupling occurs via an O -and surface hydroxyl on the same Ti atom at a bridging oxygen site for rutile and anatase mixed phase. Wang et al. proposed that the water oxidation mechanism on rutile TiO2 proceeds via holes oxidizing negatively charged intermediates and that the mechanism involves 2 parallel pathways beginning at a Ti row or bridging O on the TiO2 surface. Wang's group then studied the water oxidation kinetics on anatase TiO2, and demonstrated that the chemistry is very similar to that on rutile TiO2, except that under low light intensity, O-O coupling occurs on a single Ti atom in the anatase phase instead of on adjacent Ti atoms. At high light intensity the chemistry of the two phases is more similar. Of these investigations, only the study for rutile phase by Wang et al. reports a set of energies that is sufficiently complete to calculate rate coefficients for each step in the mechanism and perform kinetic modeling. Their theoretical and microkinetic calculations indicated that the low concentration of photo-holes on TiO2 limits the efficiency of the OER on TiO2(110), rather than the intrinsic catalytic activity captured in the reaction barriers. They concluded that enhancing the concentration of surfacereaching holes is the most effective approach to accelerating the rate of photo-OER.