Based on these differences we can rationalize the mechanistic differences in photocatalytic enhancement at a-TiO2(400)-V SA and a-TiO2-Cu SA (Figure ). At a-TiO2(400)-V SA , the photogenerated electrons trapped at V(IV) sites just below the CBE of anatase TiO2 at approximately -0.5 V vs. NHE (at pH 7) have a reduction potential negative enough to react very efficiently with O2 to form the superoxide anion radical (standard reduction potential 33 V vs. NHE). This explains the very fast recovery of V(V) upon exposure of reduced V(IV) sites to O2 (Figure ). In other words, the vanadate species act as an effective SAC for the reduction of O2 to -