Kinetic and spectroscopic study of methane combustion over α-Mn2O3 nanocrystal catalysts

The kinetics of methane combustion was investigated on bulk and α-Mn2O3 nanocrystal catalysts using a fixed bed microreactor. The kinetic behavior is profoundly affected by changes in the particle size of α-Mn2O3 from 0.5 μm to ca. 50 nm. Real-time in situ Raman spectroscopy was used to probe the structure of both catalysts during the reaction. A reversible phase-transformation from α-Mn2O3 to a Mn3O4-like species, as evidenced by the presence of a band at 648 cm−1 in He flow and at 660 cm−1 in methane combustion, was observed for α-Mn2O3 nanocrystals only at or above 450 °C. This modification is probably due to the loss of lattice oxygen at high temperatures, leading to structural reconstruction. We infer from the study that the phase change of the α-Mn2O3 nanocrystal catalyst might be responsible for the decrease of EaEa and the near zero-order dependence on oxygen partial pressure, and might be associated with the activation of the CH bonds in methane.