Effects of different Cu loadings on photocatalytic activity of $TiO_{2}-SiO_{2}$ prepared at low temperature oxidation of organic pollutants in water

The objective of this research is to examine how copper modification can improve the photocatalytic activity of TiO2-SiO2, and to explain the correlation between Cu concentration and chemical state of Cu cations in the TiO2-SiO2 matrix, and the photocatalytic activity under the UV/solar irradiation. The Cu-modified TiO2-SiO2 photocatalysts were prepared by a low temperature sol-gel method based on organic copper, silicon and titanium precursors with varied Cu concentrations (from 0.05 to 3 mol%). The sol-gels were dried at 150 °C to obtain the photocatalysts in the powder form. The photocatalytic activity was determined by a fluorescence-based method of terephthalic acid decomposition. Up to three times increase in photocatalytic activity is obtained when TiO2-SiO2 matrix is modified with Cu in a narrow concentration range from 0.05 to 0.1 mol%. At higher Cu loadings the photocatalytic activity of Cu modified photocatalyst is smaller than in the unmodified reference TiO2-SiO2 photocatalyst. XRD analysis shows that all Cu modified TiO2-SiO2 composites with different Cu concentrations have the same crystalline structure as unmodified TiO2-SiO2 composites. The addition of Cu does not change the relative ratio between anatase and brookite phase or unit cell parameters of the two TiO2 crystalline structures. The Cu K-edge XANES and EXAFS analysis is used to determine valence state and local structure of Cu cations in Cu-modified TiO2-SiO2 photocatalyst. The results elucidate the mechanism responsible for the improved photocatalytic activity. In the samples with low Cu content, which exhibit largest activity, Cu-O-Ti connections are formed, suggesting that the activity enhancement is due to Cu(II) cations attachment on the surface of the photocatalytically active TiO2 nanoparticles, so Cu(II) cations may act as free electron traps, reducing the intensity of recombination between electrons and holes at the TiO2 photocatalyst’s surface. At higher Cu loadings no additional Cu-O-Ti connections are formed, instead only Cu-O-Cu connections are established, indicating the formation of amorphous or nanocrystalline Cu oxide, which hinders the photocatalytic activity of TiO2