Observation of the reversible phase-transformation of α-Mn2O3 nanocrystals during the catalytic combustion of methane by in situ Raman spectroscopy

Real-time in situ Raman spectroscopy is used to probe the near-surface structure of alpha-Mn2O3 nanocrystals during methane combustion for the first time. A surface phase-transformation from alpha-Mn2O3 to Mn3O4-like species, as evidenced by the formation of a single band at 648 cm(-1), was observed only at or above 400 degrees C in the presence of pure He or in the mixture Of O-2 + CH4. 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 phase-change at the near-surface is suggested to be associated with the activating C-H bond of methane.