Probing the Surface Structure of α‑Mn₂O₃ Nanocrystals during CO Oxidation by Operando Raman Spectroscopy

The α-Mn₂O₃ nanocrystals with uniform morphology prepared by calcining a self-assembly Mn₃O₄ precursor show higher activity toward CO oxidation. Operando Raman spectroscopy is used to probe the near-surface structure of α-Mn₂O₃ nanocrystals during the adsorption and oxidation of CO for the first time. A surface phase-transformation from α-Mn₂O₃ to MnO-like species, as evidenced by the formation of a single band at 498 cm^–1, was observed only at or above 300 °C in the presence of CO. This modification is probably due to the loss of lattice oxygen at high temperatures that leads to the surface reconstruction. Very interestingly, a reversible phase-transformation was observed with decreasing the temperature to 25 °C. The shift of the symmetric stretching of Mn₂O₃ groups (632 → 649 cm^–1) due to the adsorption of CO was observed even at room temperature. In addition, the results of the temperature-programmed desorption of O₂ (TPD-O₂) and temperature-programmed surface reaction (TRSR) indicate that the oxidizing of CO may proceed through the Langmuir–Hinshelwood mechanism (<220 °C) to Mars-van-Krevelen mechanism (>350 °C) with the increasing of reaction temperature. In particular, the weakly adsorbed oxygen is deduced to be responsible for CO oxidation at low temperatures