Photocatalytic reduction of Cr(VI) on the new hetero-system CuAl2O4/TiO2

Visible light driven HCrO4− reduction was successfully achieved over the new hetero-system CuAl2O4/TiO2. The spinel, elaborated by nitrate route, was characterized photo electrochemically. The optical gap was found to be 1.70 eV and the transition is directly allowed. The conduction band (−1.05 VSCE) is located below that of TiO2, more negative than the HCrO4−/Cr3+ level (+0.58 VSCE) yielding a thermodynamically feasible chromate reduction upon visible illumination. CuAl2O4 is stable against photo corrosion by holes consumption reaction involving salicylic acid which favors the charges separation. There is a direct correlation between the dark adsorption and the photo activity. A reduction of more than 95% of chromate was achieved after 3 h irradiation at pH 2 with an optimal mass ratio (CuAl2O4/TiO2) equal to 1/3. The reduction follows a first order kinetic with a half life of ∼1 h and a quantum yield of 0.11% under polychromatic light. Prolonged illumination was accompanied by a deceleration of the Cr(VI) reduction thanks to the competitive water discharge. The hydrogen evolution, an issue of energetic concern, took place with a rate of 3.75 cm3 (g catalyst)−1 h−1.