This leads to very low reactivity of electrons trapped at surface Cu(I) sites towards dioxygen, unless the temperature is increased (see Figure ). As already discussed above, the enhancement of photocatalysis at a-TiO2-Cu SA can be therefore ascribed to a combined effect of fast trapping of photogenerated electrons at Cu(II), followed by the Fenton reaction, i.e., the reaction of reduced Cu(I) with H2O2 back to Cu(II) (Eq. 7). is the standard reduction potential for one-electron reduction of O2 to superoxide anion radical (assuming the standard state with unit fugacity, i.e. the partial pressure of O2 ~ 1 atm). EC, EV, *EFn and *EFp stand for the conduction and valence band edge and the quasi-Fermi of electrons and holes, respectively. For details see the text.