Preparation of composite catalyst Fe₃O₄/ZIF-9 and its rapid degradation rates toward methylene blue

To enhance the degradation rate of methylene blue (MB) in water, ultrasonication and solvothermal methods were used to prepare Fe₃O₄/ZIF-9 composite catalysts. The morphology of the catalyst revealed the attachment of cubic ZIF-9 particles to spherical-like Fe₃O₄ particles. The particle sizes of these ZIF-9s slightly varied, which might be due to the influence of the increased number of nucleation sites originating from the addition of Fe₃O₄ on the nucleation and growth of ZIF-9s as well as the effects of the variation in Fe₃O₄/Co salt ratio on the nucleation quantity and growth rate of ZIF-9s. Using this catalyst at a Fe₃O₄/Co salt molar ratio of 1∶1, 95.1% of the MB was degraded within 30 min. Its pseudo-first-order kinetic constant for a 10-min catalytic reaction reached 0.101 min⁻¹, and it remained stable and had high catalytic performance within the pH range from 5 to 9. X-ray photoelectron spectroscopy results showed that the valence state changes of Fe and Co in Fe₃O₄/ZIF-9 after MB degradation, verifying the electron transfer between the Fe and Co sites. Meanwhile, the divalent unsaturated Fe and Co sites in the Fe₃O₄/ZIF-9 structure were likely to synergistically transfer electrons based on the fast degradation kinetics mentioned above. Consequently, compared with the pure ZIF-9, the addition of Fe ions can effectively reduce the reduction potential of Co ions and accelerate the valence state changes of Co ions and the corresponding redox cycle. Furthermore, electron paramagnetic resonance results demonstrated that Fe₃O₄/ZIF-9 can activate peroxymonosulfate to generate singlet oxygen (¹O₂), sulfate radicals(SO_4^- ·), and hydroxyl radicals (·OH), with ¹O₂ being the primary active species, as further confirmed by the free radical quenching test. Hence, such a redox cycle of Fe and Co sites continuously generated ¹O₂, SO_4^- ·, and ·OH, which degraded the MB molecules into carbon dioxide and water. Moreover, a saturation magnetization value of 7.6 A·m²·kg⁻¹ was measured using the vibrating sample magnetometer method, indicating that the Fe₃O₄ imparts good ferromagnetic properties to the composite catalyst, facilitating its collection and recovery after the catalytic reaction. In addition, after four cycles of use, the catalyst maintained its structure and high degradation rate without significant changes, thereby demonstrating excellent reusability. In conclusion, this study systematically explored the influence of the Fe₃O₄/Co salt molar ratio on the structure and catalytic performance of the catalyst, qualitatively analyzed the active species during catalysis, and further elucidated the mechanism by which Fe₃O₄/ZIF-9 improves the catalytic efficiency. This study offers insight into techniques for the organic dye remediation.