Redox-controlled Ni-doped ε-MnO₂ nanosheet catalysts for low-temperature oxidation of toluene and ethyl acetate.

Manganese dioxide (MnO₂)-based catalysts have emerged as promising candidates for the complete oxidation of volatile organic compounds (VOCs). However, insufficient oxygen mobility and the limited availability of reactive oxygen species restrict their low-temperature catalytic performance. Herein, Ni-doped ε-MnO₂ nanosheet catalysts were synthesized via a redox-precipitation route under two distinct synthesis redox environments by tuning the KMnO₄/Mn2+ ratio. The redox-controlled synthesis generates distinct surface chemistries, leading to different catalytic behaviors toward aromatic and oxygenated VOCs. Comprehensive structural, spectroscopic, and redox characterizations reveal that the catalyst synthesized under Mn2+-rich conditions possesses oxygen-vacancy-rich surfaces together with enhanced oxygen mobility and MnONi redox interactions, which correlate with superior low-temperature oxidation of toluene. In contrast, the catalyst prepared under KMnO₄-rich conditions exhibits more pronounced carbonate-modified surface chemistry, which is associated with enhanced ethyl acetate oxidation. The optimized catalysts achieve T₉₀ values of 238 °C for toluene and 198 °C for ethyl acetate oxidation. Catalytic tests under an inert atmosphere further support the direct participation of catalyst oxygen species, consistent with a Mars-van Krevelen-type oxidation mechanism. Moreover, the catalyst synthesized under Mn2+-rich conditions maintains nearly complete ethyl acetate conversion for 48 h at 220 °C. These results demonstrate that controlling the synthesis redox environment provides an effective strategy to tailor surface chemistry and oxygen activation in Mn-based catalysts, offering new insights into the rational design of efficient catalysts for VOC oxidation.