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 th e lattice oxygen O2- is responsible for the activation of methane. F-c enters 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-ele ctrons transfer from W4+ to the oxygen ion vacancy which is produced b y methane reduction. The EPR peak at g = 2.0046 disappears with increa sing temperature. Raman spectra give further information about the pro duction of O2- from molecular oxygen. For the catalyst reduced with me thane, no Raman lines are observed, because the top W-O bonds are brok en by the reduction, W-O-Si species are slightly ionic, and they are R aman inactive. When the temperature is increased to 80 degrees C in th e presence of gas-phase oxygen, Raman lines are obtained which are the same as those of the catalyst before reduction, No O-O stretching mod es 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.