Mechanistic study on the photocatalytic degradation of rhodamine B <i>via</i> Mn–Schiff-base-modified Keggin-type polyoxometalate composite materials

Organic-inorganic hybrid materials have significant potential in the photocatalytic degradation of Rhodamine B (RhB). In this study, five hybrid materials were successfully synthesized by modifying silicotungstic acid (H₄SiW₁₂O₄₀, abbreviated SiW₁₂O₄₀) with diverse MnL [L = Salen(L¹), 5-Br-Salen(L²), 5-Cl-Salen(L³), 3-Me-Salen(L⁴), di-<i>t</i>Bu-Salen(L⁵)] complexes, specifically derivatives featuring various substituents. All the compounds are characterized by IR spectra, elemental analyses and thermogravimetric analyses (TGA). The band gaps of 1.33-1.52 eV and energy bands are obtained through the measurement of UV-Vis DRS and Mott-Schottky. Photocatalytic experiments of (MnL¹)₄SiW₁₂O₄₀ (compound 1), (MnL²)₄SiW₁₂O₄₀ (compound 2), (MnL³)₄SiW₁₂O₄₀ (compound 3), (MnL⁴)₄SiW₁₂O₄₀ (compound 4) and (MnL⁵)₄SiW₁₂O₄₀ (compound 5) indicate that compound 2 catalyst exhibits the best photocatalytic properties (RhB degrades to 6% during 70 min) while all of them possess catalytic activity for photodegradation of RhB under UV irradiation. Free radical trapping experiments show that the addition of PBQ (·O₂ ^- trapping agents) makes the RhB residual ratio increase from 6% to 60% and indicates that ·O₂ ^- is playing a pivotal role. A possible mechanism of RhB photodegradation in the presence of compound 2 is proposed based on free radical trapping experiments and the energy bands. Future work could focus on fine-tuning the molecular architecture through strategic modification of the organic ligands and precise control of the metal-to-POM ratio, potentially leading to optimized electronic structures and enhanced charge transfer kinetics for superior photocatalytic performance in environmental applications.