Preparation, characterization and application of CuCrO2/ZnO photocatalysts for the reduction of Cr(VI)

The delafossite CuCrO2 elaborated by sol-gel from 40 nut 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 mu m. 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 V-SCE. 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 V-SCE) 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 degrees C, 5x10(-4) mol/L) because of the HCrO4- dark adsorption onto ZnO (4HCrO(4)(-) + 3C(7)H(6)O(3) + 18O(2) + 16H(+) -> 4Cr(3+) + 21CO(2) + 19H(2)O, Delta G(0) = -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 mu mol/(hr.g).