CoMn₂O₄: A new spinel photocatalyst for hydrogen photo-generation under visible light irradiation

CoMn2O4 synthetized by co-precipitation, was successfully tested as photo-catalyst in the H2 photo-evolution under soft experimental conditions. The single phase CoMn2O4 was confirmed by X-ray diffraction (XRD), FTIR spectroscopy and SEM analysis. The black oxide exhibits a good activity in the visible range with a forbidden band (Eg) of 1.61 eV. The latter is obtained from the diffuse reflectance spectrum and assigned to d-d transition of the normal spinel. The BET analysis of CoMn2O4 gave a specific surface area of 24 m2/g. The electrical and photo-electrochemical properties have also been studied; the flat band potential is determined from the capacitance measurement (Efb = 0.17 Vo). According to the chrono-amperometry curve, CoMn2O4 exhibits p-type conduction with an activation energy of 0.18 eV relative to the Arrhenius-type law. The product turns out to be photo-electrochemically active under visible light, CoMn2O4-CB (-0.88 Vo) offers the possibility of spontaneous H2-generation under soft conditions {pH -13, 50 degrees C, [S2O3 -2] = 0.025 mol/L, dose CoMn2O4 = 1.25 mg/mL}. An evolution rate of 0.175 mu mol mg-1 min-1 of H2 was obtained with a quantum efficiency of 1.47% under a radiation flux of 19 mW cm-2 (1.38 x 1019 photons s-1).