Influence of in-situ and ex-situ Cu-Fe doping in K-OMS-2 catalysts on dye degradation via Fenton-like reaction with focus on catalytic properties and performances

Abstract The aims of this work were to synthesize Cu and Fe doping on manganese-based octahedral molecular sieves (K-OMS-2) via the in-situ hydrothermal method (Cu-Fe-OMS-2) and impregnation on K-OMS-2 for ex-situ techniques (Cu-Fe/OMS-2). The catalytic activities of prepared samples were evaluated through the degradation of reactive red 120 (RR-120) applying a Fenton-like reaction with peroxymonosulfate (PMS). The physicochemical properties of the catalysts were investigated using X-ray diffraction (XRD), N2 adsorption-desorption, Field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) techniques. The effects of catalyst loading, initial pH of the solution and PMS concentration were examined by applying the design of the experiment (DOE) based on the Box-Behnken design (BBD) on RR-120 degradation. Results demonstrated that cryptomelane crystalline phases of the Cu-Fe-OMS-2 and Cu-Fe/OMS-2 catalysts were successfully synthesized, confirmed by XRD measurement. The morphology of the in-situ catalyst represented in rod-like shapes and their size apparently decreased when doped with Cu and Fe metals. On the other hand, the surface morphology of the ex-situ catalyst displayed the aggregation of copper and iron oxides particles on the rod-like of K-OMS-2. The specific surface area of in-situ catalyst showed higher than ex-situ catalyst about 6 times. The oxidation states of Mn, Cu and Fe atoms were analyzed using XPS and XANES techniques. The valence states of the Mn atom in all prepared samples presented Mn3+ and Mn4+, while the Fe atom demonstrated Fe3+ in both modified samples. Simultaneously, the Cu atom in both modified samples illustrated different valence states, ex-situ catalyst (Cu+ and Cu2+) and in-situ catalyst (Cu2+ and Cu3+). The RR-120 removal by Cu-Fe-OMS-2 was more effective than Cu-Fe/OMS-2 in every run order. The significant effects on dye degradation for Cu-Fe-OMS-2 were the initial pH of the solution and catalyst loading. For Cu-Fe/OMS-2, only the catalyst loading effect was significant. The significant interaction term was found only Cu-Fe-OMS-2 catalyst between the initial pH of the solution and PMS concentration. The degradation percentage of the Cu-Fe-OMS-2 catalyst demonstrated higher than the Cu-Fe/OMS-2 catalyst about 5% at pH 4, catalyst loading of 0.85 g/L and PMS of 4.70 mM conditions.