Investigation of photocatalytic activity of Fe, Co, Zn substituted polyoxometalates/magnetite MCM-41 for degradation of organic dyes in aqueous solutions

In this study, three polyoxometalates of substituted Keggin-type [(n-C₄H₉)₄ N)]₅PMo₂W₉O₃₉ (X²⁺.OH₂). nH₂O (X; Fe, Co, Zn), FePOM, CoPOM, and ZnPOM were synthesized and individually immobilized on a magnetite MCM-41 support (FeMCM-41). The resulting catalysts were characterized using various methods, including FTIR, DR UV–Vis, XRD, FESEM, EDX, texture analysis (BET), and ICP. TEM images showed spherical particles, mesoporous FeMCM-41, and FeMCM-41@FePOM agglomeration. BET showed a type IV isotherm (mesoporous), 373.5 m²/g surface area, and 10.1 nm pore diameter. The photocatalytic performance of the heterogeneous catalysts (FeMCM-41@XPOM) was evaluated by the degradation of the cationic dyes methylene blue (MB) and rhodamine B (RhB) under visible light irradiation. The kinetics model for dye degradation was controlled by first-order reactions according to the Langmuir-Hinshelwood equation. The photocatalytic degradation activity of the heterogeneous species was superior to that of the homogeneous species. The dye degradation rate of the obtained materials followed the order of FeMCM-41@FePOM > FeMCM-41@CoPOM > FeMCM-41@ZnPOM. This work achieved 98 % MB degradation in 50 min and 97 % RhB degradation in 40 min using 5 mg of the FeMCM-41@FePOM catalyst under 100 W white LED light. This performance is comparable to other catalysts (support@POM, where the support is magnetite, TiO₂, or ZnO), some of which require longer times or higher catalyst loadings. In addition, these systems can be easily separated with an external magnet and reused in four catalytic cycles with a slight reduction in performance.