Manganese oxide nanorods as a robust Fenton-like catalyst at neutral pH: Crystal phase-dependent behavior

Manganese oxide nanorods with different crystal phases (α-, β-, γ-, and δ-MnO₂) were prepared via a mild hydrothermal method, and examined as heterogeneous catalysts for the decomposition of H₂O₂ into active radicals and concomitant degradation of methylene blue (MB) dye at neutral pH. The catalytic activity of MnO₂ nanorods was highly dependent upon their crystal structures, following the order of γ-MnO₂ > β-MnO₂ > α-MnO₂ > δ-MnO₂. Complete degradation of MB was achieved with γ-MnO₂ in a short duration of 20 min. Furthermore, γ-MnO₂ exhibited much higher activity compared to iron-containing nanoparticles (Fe° and Fe₃O₄) and several reported catalysts in the literature. The observed trend in catalyst performance was discussed in terms of surface area, degree of crystallinity, and exposed facets. It was found that γ-MnO₂ showed considerable activity over a broad range of pH and temperature as well as desired reusability for four consecutive cycles. Quenching studies indicated that hydroxyl radicals ([rad]OH) played a major role in the degradation of MB. These results provide important insight into the design of more efficient manganese oxide catalysts for wastewater treatment.