Study On The Mechanism Of Acidity In The Process Of Heterogeneous Photocatalysis

TiO2, TiO2-ZrO2 solid acid and SO42-/TiO2 solid superacid were prepared using modified sol-gel method. Structure, surface acidity, optical properties and photocatalytic performance of the above three series of catalysts were investigated by decomposition of ethylene and methyl bromide and by X-ray diffraction, surface area measurement, X-ray photoelectron spectroscopy, FTIR spectroscopy, surface photo-voltage spectroscopy, and Hammett indicator method. In addition, the nature of surface acidity and the relationship between surface acidity and photocatalytic performance were studied by in-situ FTIR technology using pyridine as bi-functional probe of reactant and basic probe for acidic site. The result revealed that increasing surface acidity of TiO2, especially, when it reached superacidity, could dramatically enhance photocatalytic performance including activity, selectivity and water-resistant ability, which may be resulted from the following three factors: 1. Bettering physical and chemical properties of catalyst, such as increasing surface area, decreasing crystal size, and inhibiting phase transformation from anatase to rutile which are favorable for increasing photoactivity; 2. Accelerating the separation rate of photogenerated electron-hole pairs. Sulfation greatly increased catalyst's Bronsted acidic sites that may serve as trapping sites for photogenerated holes, while the bridged bitentate sulfur complex bounded on catalyst could serve as capturing sites for light-induced electrons; 3. Activating H2O molecules produced from photocatalytic reaction by adsorbing them on Lewis acidic site of catalyst then transfer to Bronsted acidic sites to generate highly active OH radicals. In the end, a model of the mechanism of superacidity in the process of heterogeneous photocatalysis was put forward. The model was confirmed as reasonable by a designed in- situ experiment.