Photocatalytic reduction of Cr(VI) on the new hetero-system CuCr2O4/ZnO

Self photodriven chromate reduction and oxalic acid oxidation are concomitantly achieved over the new hetero-system CuCr 2 O 4 /ZnO. The absorption of light promotes electrons in the sensitizer conduction band (CuCr 2 O 4 -CB) with a potential (−0.95 V SCE ), too negative to be involved in an electron exchange with HCrO 4 − species. The improved activity is due to the electron injection from activated CuCr 2 O 4 into ZnO-CB (−0.58 V SCE ). The nitrate route is suitable to prepare active ZnO on which the chromate reduction occurs. A reduction of more than 60% of HCrO 4 − is achieved in air equilibrated solution under optimal conditions (pH∼3.5, 1 mg catalyst mL −1 , 25 °C). The experimental data are fitted adequately with the modified Langmuir–Hinshelwood model. The reduction follows a pseudo first order kinetic with a half life of 53 min for a concentration of 10 −4 M. The oxidation of Cr 3+ by photo holes and the competitive water reduction are though to be the main reasons of the regression in the photoactivity. The hetero-system favours hydrogen formation, with a rate evolution of 15 μmol h −1 mg −1 .