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

The kinetics of methane combustion was investigated on bulk and alpha-Mn2O3 nanocrystal catalysts using a fixed bed microreactor. The kinetic behavior is profoundly affected by changes in the particle size of alpha-Mn2O3 from 0.5 mu m 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 alpha-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 alpha-Mn2O3 nanocrystals only at or above 450 degrees 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 alpha-Mn2O3 nanocrystal catalyst might be responsible for the decrease of Ea and the near zero-order dependence on oxygen partial pressure, and might be associated with the activation of the C-H bonds in methane. (c) 2007 Elsevier Inc. All rights reserved.