Kinetics and Mechanism of CCl₄ Photoreductive Degradation on TiO₂:  The Role of Trichloromethyl Radical and Dichlorocarbene

The mechanism of photoreduction of CCl 4 on illuminated TiO 2 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 CCl 4 . The rate of formation of trapped:CCl 2 and • CCl 3 was found to be a function of [H 2 O], pH, [CCl 4 ], the nature of the dissolved gas, and light intensity. Dissolved oxygen was not essential for the degradation of CCl 4 . 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 CCl 4 . Since dichlorocarbene is hydrolyzed under basic conditions, the pH and water concentration were found to be integral parameters controlling the complete degradation of CCl 4 to CO, CO 2, 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.