A novel MgCr2O4/WO3 hetero-junction photocatalyst for solar photo reduction of hexavalent chromium Cr(VI)

WO3/MIL-125 (Ti) composite material for enhancing the reduction of Cr(vi) under visible light

In wastewater containing heavy metals, Cr(vi) is a potentially toxic metal, mainly derived from production and processing processes such as textile printing, dyeing, ore mining, battery applications, metal cleaning and electroplating. WO3 is widely used in photocatalytic degradation and reduction, and its utilization rate of visible light is high. However, the rapid recombination of photogenerated electron-hole pairs of WO3 limits its use. In this work, the composite material (WxMy) of WO3 and MIL-125 (Ti) was prepared by the ball milling method, and the catalyst was used to photocatalytically reduce Cr(vi). After using W90M10 as a photocatalyst for 50 min, the reduction rate of Cr(vi) can reach 99.2%, and the reduction rate is 2.3 times that of WO3. After 5 cycles of use, the reduction rate can still reach 91.3%. It is mainly due to the formation of a II-type heterojunction between WO3 and MIL-125 (Ti), which promotes the separation of photogenerated electron-hole pairs, thus improving the efficiency of photocatalytic reduction of Cr(vi).

Solar photodegradation of Rhodamine B dye by Cu2O/TiO2 heterostructure: experimental and computational studies of degradation and toxicity

CONTEXT AND RESULTS

In this study, the heterojunction Cu₂O/TiO₂ is used for the degradation of a cationic dye, Rhodamine B, under solar light irradiation over a wide pH range. The physical and optical properties of both semiconductors Cu₂O and TiO₂ are correlated with the photo-electrochemical characterization to establish the energy diagram of the heterojunction Cu₂O/TiO₂. X-ray diffraction, UV-visible, SEM, EDX, and BET analyses are conducted for both photocatalysts. The band gap (E_g) of 3.26 eV is obtained for TiO₂ with an indirect optical transition. In the case of Cu₂O, the transition is directly allowed at 2.05 eV. According to the BET analysis, the specific surface area of TiO₂ particles is higher (82.65 m² g^-1) than that of Cu₂O (29.81 m² g^-1). The flat band potentials, determined from the Mott-Schottky plots, are 0.3 and - 0.32 V_SCE for TiO₂ and Cu₂O, respectively. The photocatalytic activity is directly proportional to the mass ratio, and the best result is obtained for the mass ratio 1:1 of Cu₂O/TiO₂.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

Furthermore, a theoretical study is conducted by using density functional theory to optimize the structure, reactivity sites of the RhB molecule, and physical parameters like the energy of the frontier molecular orbitals and electronegativity and to predict the proposed mechanism of RhB degradation as well as its intermediates. Also, molecular dynamics simulation is used to determine the adsorption behavior of RhB on TiO₂ (101) and Cu₂O (111) surfaces. The ecotoxicity evaluation showed that degradation products have significantly lower acute toxicity than RhB.