Photo-Induced Electron Transfers to CCl_4 and its Degradation Mechanism on TiO_2 Particles

Abstract

The photoreductive degradation of CCl_4 in TiO_2 particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. CHCl_3, C_2Cl_4, and C_2Cl_6 are detected as intermediates during photolysis. The rate of CCl_4 dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. Kinetic isotope effects and structure-reactivity relationships show that hydrogen-abstraction by hydroxyl radicals plays an important role in the hole-scavenging mechanism. The pH of the TiO_2 suspension influences the rate of CCl_4 reduction either by altering the electrostatic interactions of electron donors on the TiO_2 surface or by changing the reduction potential of the conduction band electron in a Nernstian fashion. Since dissolved oxygen is shown to be nonessential for the complete mineralization of CCl_4, a mechanism, which involves dichlorocarbene formation through a two-electron transfer, is proposed.