Kinetics and Mechanism of CCl_4 Photoreductive Degradation on TiO_2: The Role of Trichloromethyl Radical and Dichlorocarbene
The mechanism of photoreduction of CCl_4 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 CCl_4. The rate of formation of trapped :CCl_2 and •CCl_3 was found to be a function of [H_2O], 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.
© 1996 American Chemical Society. Received: May 23, 1995; In Final Form: October 30, 1995. Financial support from the Advanced Research Projects Agency (ARPA) and the Office of Naval Research (ONR) (Grant N0014-92-J-1901) under the auspices of the Department of Defense-University Research Initiative Program (DOD-URI) is gratefully acknowledged.