MECHANISTIC STUDY OF OXIDATIVE COUPLING OF METHANE OVER MN2O3-NA2WO4/SIO2 CATALYST

EPR and Raman spectroscopies have been used to characterize the active oxygen species on Mn2O3Na2WO4/SiO2 catalyst. The results show that the lattice oxygen O2- is responsible for the activation of methane. F-centers with an EPR parameter g = 2.0046 form when gas-phase oxygen is admitted at room temperature onto the catalyst reduced with methane at 800 degrees C. Molecular oxygen plays an inducing role in the two-electrons transfer from W4+ to the oxygen ion vacancy which is produced by methane reduction. The EPR peak at g = 2.0046 disappears with increasing temperature. Raman spectra give further information about the production of O2- from molecular oxygen. For the catalyst reduced with methane, no Raman lines are observed, because the top W-O bonds are broken by the reduction, W-O-Si species are slightly ionic, and they are Raman inactive. When the temperature is increased to 80 degrees C in the presence of gas-phase oxygen, Raman lines are obtained which are the same as those of the catalyst before reduction, No O-O stretching modes are observed. We suggest that molecular oxygen is activated by an F-center to produce lattice oxygen O2-. A possible redox model for the Mn2O3-Na2WO4/SiO2 catalyst has been proposed.