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

Abstract

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.