Kinetics and mechanism of CCl4 photoreductive degradation on TiO2: The role of trichloromethyl radical and dichlorocarbene

The mechanism of photoreduction of CCl4 on illuminated TiO2 surfaces was investigated by selectively trapping transient free radical intermediates. Dichlorocarbene and trichloromethyl radical were trapped with 2,3-dimethyl-2-butene during the photocatalytic degradation of CCl4. The rate of formation of trapped :CCl2 and CCl3 was found to be a function of [H2O], pH, [CCl4], the nature of the dissolved gas, and light intensity. Dissolved oxygen was not essential for the degradation of CCl4. The production rate of trapped dichlorocarbene showed light intensity dependencies of second, first, and half order with progressively increasing light intensity. A two-electron photoreductive pathway (via dichlorocarbene formation) was found to be the dominant mechanism leading to the full degradation of CCl4. Since dichlorocarbene is hydrolyzed under basic conditions, the pH and water concentration were found to be integral parameters controlling the complete degradation of CCl4 to CO, CO2, and HCl. Kinetic equations describing the formation of trapped dichlorocarbene were derived from a proposed mechanism. The comparison of the predicted rate expression to the observed data suggested that the observed two-electron transfer occurred consecutively.