Heterogeneous photocatalytic oxidation of nitrotoluenes

Abstract

The photocatalytic oxidation rates of nitrotoluene (2‐NT and 3‐NT) and dinitrotoluene (2,4‐DNT and 2,6‐DNT) in aqueous suspensions of Degussa P‐25 were found to be pH independent because these compounds do not exhibit any pH‐dependent speciation. The reaction rates of mononitrotoluenes were found to be 60 to 80% greater than those of dinitrotoluenes. A Langmuir–Hinshelwood (L–H) rate form is used to describe the kinetics of the photocatalytic oxidation of these compounds. The L–H constant k 1 has been found to be essentially independent of the compound being degraded. At low concentrations, the reaction rates of nitrotoluenes decrease with increasing nitration. This has been explained through the electron‐withdrawing characteristics of the nitro group, employing the Hammett relationship, with the reaction rates following the sequence Toluene > 3‐NT > 2‐NT > 2,4‐DNT > 2,6‐DNT > TNT (trinitrotoluene, predicted) The photocatalytic oxidation of 2,6‐DNT produces ammonium and nitrate ions. Attempts at augmenting the reaction rate by addition of hydrogen peroxide or Cu 2+ ions did not significantly increase the reaction rate.