Solvothermal synthesis of α-MnO2 and Mn2O3 for efficient catalytic dye degradation

Abstract

The increasing industrial demand for synthetic dyes, particularly in textiles as well as pulp and paper sectors, is a significant factor that intensified environmental pollution due to dye-contaminated wastewater. Therefore, this study aimed to develop efficient manganese oxide-based catalysts for methylene blue (MB) degradation through a one-step solvothermal synthesis route. Phase evolution of the synthesized MnO₂ materials was primarily governed by heating temperature and duration, rather than pH adjustment. Samples A-8 and A-9 of Mn₂O₃ were synthesized at 140 °C for 24 h, while A-10 and A-11 of α-MnO₂ were synthesized at 105 °C for 6 h. The results of morphological analysis showed that A-8 had rod-like structures, while A-9 contained a hierarchical architecture comprising rods and cubes. Acid-treated samples (A-8 and A-11) had higher crystallinity than A-9 and A-10 synthesized under alkaline conditions. The catalytic performance in MB degradation correlated with the presence of Mn³⁺/Mn²⁺ redox pairs and oxygen vacancies, where A-9 presented superior stability across a broad pH range of 1–11. Furthermore, acidic conditions significantly enhanced degradation efficiency compared to systems applying H₂O₂. Fragmentation of MB into simpler molecules confirmed the proposed degradation mechanism. The best performance was achieved using A-9 under 200 mg L^-1 MB, pH 1, 0.007 g L^-1 catalyst, and 30 min reaction time, signifying the potential for practical dye wastewater treatment applications.