Brookite vs Anatase TiO2 in the Photocatalytic Activity for Organic Degradation in Water

Brookite is the least studied TiO2, and its photocatalytic activity higher or lower than that of anatase still remains unclear. In this work, three different model reactions have been used for the activity assessment. Phase-pure brookite and anatase were homemade at different temperatures (Ts = 200-500 degrees C), as confirmed by X-ray diffraction and Raman spectroscopy. For phenol oxidation in aerated aqueous solution, brookite showed an apparent activity higher and lower than that of anatase at low and high T-s, respectively. For chromate reduction in aerated aqueous suspension, the apparent activity of brookite was always lower than that of anatase. However, with the same amount of Cr(VI) or Ag(I) adsorbed on the oxide in water for Cr(VI) reduction or for phenol degradation under ND the intrinsic activities of brookite and anatase not only became similar at given Ts but also increased with the increase of Ts. Moreover, for O-2 reduction to H2O2 in the presence of excess phenol, the BET surface area normalized activity of brookite was always higher than that of anatase, the trend of which was similar to that observed from phenol degradation in aerated aqueous solution. It is proposed that brookite has a stronger affinity to O-2 in water than anatase. Then, the observed difference between brookite and anatase in the apparent photocatalytic activity for phenol degradation is ascribed to the combined effect of T-s, surface area, and sorption capacity toward the dissolved O-2 in water.