Highly active CuO-modified TiO2 (Degussa P25) photocatalysts were prepared by an impregnation method. Efficient and rapid degradation of polybrominated diphenyl ethers were achieved on CuO/TiO2 photocatalysts. Both TiO2 and CuO/TiO2 yielded a rapid photocatalytic reduction of decabromodiphenyl (BDE209). However, the photo-reductive degradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE47) was impossible on TiO2, but became fast on CuO/TiO2. Moreover, the photo-reductive degradation of BDE47 on CuO/TiO2 needed a short induction time period (20 s) to initiate the reduction. A mechanism was proposed for the enhanced photo-reductive degradation of BDE47 on CuO/TiO2. During the induction time period, the CuO clusters trapped electrons from TiO2 to form Cu2O as confirmed by XPS analysis. Further surface photovoltage and phase spectroscopic analysis and flatband potential measurements were conducted for TiO2, CuO/TiO2 and Cu2O/TiO2. In the new mechanism, a "switching reduction potential by the valence state of copper" concept was proposed. Because of the higher reduction potential of Cu2O than CuO and TiO2, the in-situ composition change led to a Fermi level shift to a more reductive energy for injecting electrons to BDE47, resulting in the initiation of the photocatalytic reduction of BDE47 on CuO/TiO2. The proposed concept was useful to design and regulate photocatalysts for reductively degrading persistent pollutants. (C) 2015 Elsevier B.V. All rights reserved.