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 CuCr2O4/ZnO. The absorption of light promotes electrons in the sensitizer conduction band (CuCr2O4-CB) with a potential (-0.95 V-SCE), too negative to be involved in an electron exchange with HCrO4- species. The improved activity is due to the electron injection from activated CuCr2O4 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 HCrO4- is achieved in air equilibrated solution under optimal conditions (pH similar to 3.5, 1 mg catalyst mL(-1), 25 degrees 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 Cr3+ 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 mu mol h(-1) mg(-1). (C) 2011 Elsevier B.V. All rights reserved.