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

Visible light driven HCrO(4)(-) reduction was successfully achieved over the new hetero-system CuAl(2)O(4)/TiO(2). 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 V(SCE)) is located below that of TiO(2), more negative than the HCrO(4)(-)/Cr(3+) level (+0.58 V(SCE)) yielding a thermodynamically feasible chromate reduction upon visible illumination. CuAl(2)O(4) 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 (CuAl(2)O(4)/TiO(2)) 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 cm(3) (g catalyst)(-1) h(-1).