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 VSCE), 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 VSCE). 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∼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 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 μmol h−1 mg−1.

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► The visible light activity is mediated by ZnO for the chromate reduction where 60% are reduced.
► The pseudo first order kinetic of the chromium removal obeys to the Langmuir Hinshelwood model.
► The chromate reduction is successfully achieved under direct solar radiation.
► The hetero-system CuCr2O4/ZnO favours the hydrogen formation.