Preparation and characterization of mixed caesium-tin mixed salt of Keggin-type phosphovanadomolybdate. Application to photocatalytic chromate reduction

In this study, we are interested by the caesium-tin mixed salt of 1-vanado-11-molybdophosphoric acid (CsSnPMo11VO40) with Cs/Sn ~ 1, micro and mesoporous material with a large specific surface area and a good thermal stability. Caesium atoms in Cs3HPMo11VO40 are partially substituted by tin in order to facilitate the electron transfer in the redox processes. The synthesis and characterization by BET measurement, inductively coupled plasma (ICP), X-ray diffraction (XRD), 31P NMR, thermal analysis (TGA and DTA), UV-Vis and Raman spectroscopies, infrared technique of the Keggin-type compound CsSnPMo11VO40 are reported. The forbidden band (1.72 eV) is due to the crystal field splitting of Mo6+: 4d orbital in octahedral site and the compound exhibits a chemical stability at neutral pH and semiconducting properties due to molybdenum mixed valences. The intensity-potential (J-E) characteristic shows a small hysteresis while the capacitance measurement (C−2 - E) of the junction CsSnPMo11VO40/electrolyte in Na2SO4 electrolyte exhibits a rectifying behavior with a Schottky type barrier; the positive slope is characteristic of n type conduction with a flat band potential of −0.30 VSCE. The electrochemical impedance spectroscopy (EIS) shows two overlapped semicircles attributed to the bulk contribution (Rb=168 Ω cm2) and grains boundaries (Rgb=806 Ω cm2). The depletion angle (−16°) is ascribed to a constant phase element (CPE, 67 µF cm−2). The photosensitivity in the visible light involves d-d transition and the energy diagram shows the feasibility of the material for the HCrO4- reduction upon visible light (109 mW cm−2). The disappearance rate is found to be 4 mmol mn−1 (mg catalyst)−1 and the chromate reduction obeys to a first order kinetic with a half-life of 83 mn.