Role of reduction in the photocatalytic degradation of TNT

The reduction of TNT by electrochemical and photoelectrochemical techniques has been investigated to obtain insight into the role of reductive transformations in a photocatalytic degradation process. TNT was observed to be labile to reductive transformation by a platinum electrode at electrochemical potentials commensurate with the flat band potential of TiO₂, and aminodinitrotoluene species were detected as early reduction products. Oxygen did not influence the rate of reductive TNT transformation, but byproducts were more stable in aerated than deaerated conditions. Photocatalytic reduction of TNT was analyzed using CdS as a chromophore coupled to TiO₂ and visible light excitation so that direct photolysis of TNT was precluded. Under deaerated conditions, reduction of TNT occurred through both direct and sensitized mechanisms, predominantly forming aminodinitrotoluene compounds as relatively stable byproducts. Photocatalytic transformation and mineralization of TNT was also examined with particulate films of TiO₂ immobilized on optically transparent electrodes. Varying levels of positive bias were applied to the photocatalyst in order to decrease the availability of photoexcited electrons. The rate of TNT degradation was found to decrease with increasing positive bias, indicating that conduction band electrons facilitate overall compound degradation. Results from studies with TiO₂ thin films support the proposal that photocatalytic TNT destruction proceeds through oxidative pathways, where molecular oxygen accelerates byproduct degradation, and reductive pathways in which byproduct degradation is retarded by oxygen.