Hence, the Cu(II) ions a-TiO2-Cu SA act effectively as SACs for a secondary reaction in the photodegradation mechanism, namely a distinct type of photo-Fenton process, converting H2O2 to hydroxyl radicals and hydroxyl ions. In this context, we recall again the superior performance of a-TiO2(400)-V SA with respect to the overall photocatalytic degradation yield and the operational stability during several photodegradation cycles (Figure ). Our results suggest that this superior performance is directly related to the ability of vanadate SACs to catalyze the primary reaction with dioxygen. In contrast, the a-TiO2-Cu SA material exhibits both lower activity and stability. This can be ascribed to the poor reactivity of reduced Cu(I) SAC sites with dioxygen, which results in the significant portion of SAC being in the reduced Cu(I) state during the photocatalytic cycle. This, in turn, not only limits the enhancement of charge separation as the trapping of photogenerated electrons is less effective, but also increases the tendency of copper ions to leach into the solution since the propensity of first-row transition metal ions towards dissolution in aqueous solutions is known to increase significantly upon reduction. We, therefore, conclude that the ability of the SAC to provide fast catalytic turnover in reaction with dioxygen is highly beneficial with respect to the overall performance of SACmodified TiO2 photocatalysts for oxidative photocatalytic degradation of organic compounds.