Since the discovery of photoinduced water splitting on TiO₂ electrodes in 1972,¹  several studies on photocatalysis by TiO₂ have focused on the solar energy conversion for hydrogen production. Recently, the TiO₂ photocatalysts have been practically devoted to environmental clean-up by decomposing organic pollutants. The difference in these two kinetic processes could be schematically illustrated in Figure 7.1.

Though the electrons photogenerated in the conduction band (CB) are consumed to reduce protons for water splitting, they are actually used to reduce oxygen in air for environmental clean-up. Instead, for water splitting, holes generated in valence band (VB) by four photons are used to oxidize water, while for environmental clean-up only one hole is required to oxidize organic pollutants. Thus, there are varieties of kinetic processes in photocatalysis and the initially employed reactants are also considerably diverse for individual applications. Therefore, in photocatalytic reactions, the primary processes of the photogenerated charge carriers are important. Though there are many review reports concerning the reactions of TiO₂ photocatalysts,^2–7  in the present chapter the primary reaction processes studied mainly in our research group will be reviewed in particular by focusing on the application to environmental clean-up.