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

Abstract

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-tBu-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.