Water Photo-Oxidation over TiO2—History and Reaction Mechanism

Photocatalytic water oxidation over titanium dioxide (TiO2) was overviewed by surveying briefly the history of water photo-oxidation, followed by profiling the research for the molecular mechanism of oxygen evolution reaction (OER) at the TiO2 surface. As the experimental approach to investigate the reaction mechanism, ESR, NMR, and STM were described as well as FTIR spectroscopy. Detection of reactive oxygen species, which are the intermediate species in the OER, was also involved in discussing the mechanism. As the theoretical approach to the reaction mechanism, some research with density functional theory (DFT) for anatase (101) surface was illustrated. Since the OER activity of rutile TiO2 is higher than that of anatase, and the rutile (011) surface has been assigned to the oxidation facet, we performed a DFT calculation for a (011) surface model molecule. The results were successfully discussed with the reported mechanism. The first oxidation step occurs at the bridging OH site, which faces a Ti5C site. The water molecule which coordinates both sites is oxidized, and the resultant radical coordinates the Ti5C site to form a trapped hole Ti-O•. In the second step, a coordinated water molecule is oxidized at the Ti-O• site to form a Ti-OOH structure.