Photoelectrochemical H2-generation over Spinel FeCr2O4 in X2- solutions (X2- = S2- and SO32-)

Photocatalytic H2-production has been observed over FeCr2O4, synthesized from Cr(NO3)3 and Fe(NO3)3. The oxide crystallizes in the normal spinel structure and the transport properties exhibit p-type conductivity ascribed to metal deficiency. An optical gap of 1.33 eV is determined from the reflectance diffuse spectrum. From photoelectrochemical measurements, a flat band potential of +0.23 VSCE is found. The valence band, made up of cationic 3d orbital, is located at 5.2 eV (0.45 VSCE) below vacuum. Hence, the conduction band (-0.98 VSCE) allows a thermodynamically feasible H2 liberation under visible light. In aqueous electrolytes, the oxide is stabilized by hole consumption involving X2- species and the best photoactivity for H2-production is obtained in S2- solution with an evolution rate of 8.26 cm3 g-1 h-1. The tendency towards saturation is due to the competitive reduction of the end products namely Sn2- and S2O62- with water and to the yellow color of Sn2-. On the other hand and for a comparative purpose, the catalytic activity of unreduced FeCr2O4 for the water-gas shift reaction (CO + H2O → CO2 + H2) is studied in the temperature range 523-723 K. At 723 K, the rate of H2-formation and the degree of CO conversion are respectively 280 μmol g-1 s-1 and 44%