Role of the crystallite phase of TiO2 in heterogeneous photocatalysis for phenol oxidation in water

A series of TiO2 samples with different anatase-to-rutile ratios was prepar ed by calcination, and the roles of the two crystallite phases of titanium( IV) oxide (TiO2) on the photocatalytic activity in oxidation of phenol in a queous solution were studied. High dispersion of nanometer-sized anatase in the silica matrix and the possible bonding of Si-O-Ti in SiO2/TiO2 interfa ce were found to stabilize the crystallite transformation from anatase to r utile. The temperature for this transformation was 1200 degrees C for the s ilica-titania (ST) sample, much higher than 700 degrees C for Degussa P25, a benchmarking photocatalyst. It is shown that samples with higher anatase- to-rutile ratios have higher activities for phenol degradation. However, th e activity did not totally disappear after a complete crystallite transform ation for P25 samples, indicating some activity of the rutile phase. Furthe rmore, the activity for the ST samples after calcination decreased signific antly, even though the amount of anatase did not change much. The activity of the same samples with different anatase-to-rutile ratios is more related to the amount of the surface-adsorbed water and hydroxyl groups and surfac e area. The formation of rutile by calcination would reduce the surface-ads orbed water and hydroxyl groups and surface area, leading to the decrease i n activity.