Mn2O3 structures dependent on the synthesis route: a comparative study of dark oxidative dye degradation.

Abstract

Manganese oxide structures attract considerable attention due to the diversity of their redox properties, morphology, crystal structure, size, and surface properties compared to those of other transition-metal oxide materials. The controllability of these properties, depending on the synthesis method, enables their use in many different areas, such as catalytic applications, Li-ion batteries, supercapacitors, and the removal of various types of pollutants. Within the scope of the study, Mn2O3 structures were prepared using three different methods. In the first method, manganese-based metal-organic frameworks (MnMOFs) were prepared as precursors, and then Mn2O3 structures were synthesized via thermal decomposition. All three routes yielded phase-pure α-Mn2O3 with no secondary phases, so that the crystal structure remained constant while texture and surface state varied. Sea-urchin-like MnMOF rods (5-10 µm, versus 17.9 and 6.2 m2 g-1) together with the highest surface Mn3+/Mn4+ (1.10) and Oads/Olatt (0.86) ratios. Methylene blue was oxidatively removed in the dark, without any light source or external oxidant; removal was strongly pH-dependent (91% at pH 3 versus 12% at pH 5) and followed the order Mn2O3(t) (91%) > Mn2O3(p) (80%) > Mn2O3(h) (51%), matching the trend in surface area and surface Mn3+ content. Mn2O3(t) retained 85% relative removal over three cycles with its structure preserved and, additionally, removed 94% of rhodamine B and 82% of crystal violet. These findings show that even for the same crystal phase, the synthesis route governs the oxidative reactivity of Mn2O3 through its surface area and surface Mn(iii) content, and that MOF-derived Mn2O3 can act as an effective direct oxidant for the removal of various dyes without any external oxidant or light.

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