Preparation, characterization and application of CuCrO2/ZnO photocatalysts for the reduction of Cr(VI)
Résumé
The delafossite CuCrO2 elaborated by sol-gel from 40 nm diameter colloid is optically active in the visible region. It is characterized physically and photoelectrochemically. The microstructure is fairly homogenous with a mean crystallite size of ca. 2 μm. The optical gap (1.30 eV), determined from the diffuse reflectance, is well suited to the sunlight spectrum. The Mott Schottky plot is characteristic of P-type conductivity with a flat band potential of -0.26 VSCE. As application, the photoreduction of chromate is successfully achieved in air-equilibrated suspension CuCrO2/ZnO (1/1). CuCrO2 is photoactivated by visible light and the electrons in the conduction band (-1.34 VSCE) are injected to ZnO. In the presence of salicylic acid, a conversion of Cr(VI) to Cr(III) of 57% is obtained under optimal conditions (pH 3 at 25°C, 5×10−4 mol/L) because of the HCrO4 − dark adsorption onto ZnO (4HCrO4 − + 3C7H6O3 + 18O2 + 16H+ −→ 4Cr3+ + 21CO2 + 19H2O, ΔG0 = -557 kcal/mol). Prolonged illumination is accompanied by a deceleration in the photoactivity owing to the competitive water reduction, an issue of energetic concern. The hetero-system exhibits self sensitization for hydrogen production with an evolution rate of 149 μmol/(hr*g).