Role of Cl− ions in photooxidation of propylene on TiO2 surface

The effect of Cl− ions on photooxidation of propylene on TiO2 semiconductor was investigated. Cl−/TiO2 catalysts were prepared by annealing Degussa P25 TiO2 in the gas flow of N2 and Cl2 under 100-400 °C. The photocatalytic oxidation of propylene was carried out in a continuous flow system, with the chromatograph to analyze the products on line. The experimental results showed that the activity of Cl−/TiO2 catalysts increased as heat-treated temperature decreased. The activity of the sample heat-treated at 100 °C was about two times higher than that of pure TiO2. Moreover, as to TiO2, the main product of the propylene photocatalytic oxidation was CO2, but with Cl−/TiO2 catalysts, not only CO2 but also trace CO was determined. The adsorbed species on TiO2 surface before and after reaction were analyzed by X-ray photoelectron spectroscopy (XPS) and thermogravimetric/differential thermal analyses (TG-DTA) coupled to a mass spectrometer (MS). XPS analysis showed that there was Cl− absorbed on the Cl−/TiO2 surface, and the absorption amount of Cl− decreased after the photooxidation reaction of propylene. TG-DTA-MS analysis confirmed chlorine absorbed on the surface of TiO2 in the form of Cl− ion. These results illuminated that absorbed Cl− on the surface of TiO2 formed a weak physical absorption on TiO2 at low temperature, and subsequently participated in the photooxidation of propylene, finally removed from TiO2 surface.