Photoelectrochemical H 2-generation over Spinel FeCr 2O 4 in X 2− solutions (X 2− = S 2− and [formula omitted

Abstract

Photocatalytic H 2-production has been observed over FeCr 2O 4, synthesized from Cr(NO 3) 3 and Fe(NO 3) 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 V SCE is found. The valence band, made up of cationic 3d orbital, is located at 5.2 eV (0.45 V SCE) below vacuum. Hence, the conduction band (−0.98 V SCE) allows a thermodynamically feasible H 2 liberation under visible light. In aqueous electrolytes, the oxide is stabilized by hole consumption involving X 2− species and the best photoactivity for H 2-production is obtained in S 2− solution with an evolution rate of 8.26 cm 3 g −1 h −1. The tendency towards saturation is due to the competitive reduction of the end products namely S n 2 - and S 2 O 6 2 - with water and to the yellow color of S n 2 - . On the other hand and for a comparative purpose, the catalytic activity of unreduced FeCr 2O 4 for the water–gas shift reaction (CO + H 2O → CO 2 + H 2) is studied in the temperature range 523–723 K. At 723 K, the rate of H 2-formation and the degree of CO conversion are respectively 280 μmol g −1 s −1 and 44%.