Photoreduction of Chromate Cr(VI) under Visible Light Using CuCr₂O₄/WO₃ Hetero-system Photocatalyst

This work aims to develop a CuCr₂O₄ nanoparticle at 850°C in air, and WO₃ nanoparticles by one-step hydrothermal method. The novel CuCr₂O₄/WO₃ hetero-system is successfully used for the photoreduction of Cr(VI) (30 mg/l) under sunlight. Both materials are characterized by the structural, physical and electrochemical techniques. The energy of the spinel gap (CuCr₂O₄)(E_g = 1.78 eV) is determined by diffuse reflectance and corresponds to the solar spectrum. Capacitance measurements indicate p-type conduction with a flat band potential of 0.62V_SCE. For WO₃: a direct gap (E_g = 2.90 eV) with n-type behavior with an electron density (N_D = 5.4 × 10¹⁶ cm^–3) and a flat band potential of 0.25V_SCE. Optimization of the experimental conditions of chromate photoreduction at pH ~4 (adipic acid = 5 × 10^–1 mol /L, CuCr₂O₄/WO₃ (1/1) and a dose of 1mg/L) led to a conversion of 92% of 30 mg/L of Cr (IV) in Cr(III) within 180 min under sunlight. The photocatalytic performance is due to the synergy of the p–n hetero-system whose electrons generated in the conduction band of CuCr₂O₄ excited by solar light are injected into CB-WO₃). The role of adipic acid is essential; it improves the reduction of Cr(VI) under solar irradiation. The photoreduction kinetics follows a pseudo-first order model with a constant rate of 5.51 × 10^–3 min^–1.