However, as reaction ( ) is much slower than reaction (7), one can assume that -without irradiation -the predominant oxidation state of Cu is +2, and most of the Cu(I) species are formed via reduction by the photogenerated electrons. In other words, the enhanced activity of a-TiO2-Cu SA can be attributed to a distinct type of a photo-Fenton reaction, which proceeds at the anatase TiO2 surface modified with Cu(II) SACs as a combination of effective trapping of photogenerated electrons at Cu(II), followed by the Fenton step (Eq. 7), i.e., the reaction of photoreduced Cu(I) with H2O2 back to Cu(II).