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

The alpha-Mn2O3 nanocrystals with uniform morphology prepared by calcining a self-assembly Mn3O4 precursor show higher activity toward CO oxidation. Operand Raman spectroscopy is used to probe the near-surface structure of alpha-Mn2O3 nanocrystals during the adsorption and oxidation of CO for the first time. A surface phase-transformation from alpha-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 degrees 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 degrees 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 O-2 (TPD-O-2) and temperature-programmed surface reaction (TRSR) indicate that the oxidizing of CO may proceed through the Langmuir-Hinshelwood mechanism (<220 degrees C) to Mars-van-Krevelen mechanism (>350 degrees C) with the increasing of reaction temperature. In particular, the weakly adsorbed oxygen is deduced to be responsible for CO oxidation at low temperatures.