Isotopic oxygen exchange and EPR studies of superoxide species on the SrF2/La2O3 catalyst

Abstract

By using the in situ IR spectroscopy, the superoxide species (O (2) over bar), characterized by the O-O stretching peak at 1130 cm(-1), was detected on the SrF2/La2O3 catalyst at temperatures up to 973 K. The introduction of O-18(2) isotope caused the 1130 cm(-1) peak to shift to lower wavenumber (1095 and 1064 cm(-1)), consistent with the assignment of the spectra to the superoxide species. A good correlation between the rate of the disappearance of the O (2) over bar species and that of the formation of C2H4 was observed, suggesting that O (2) over bar was the active oxygen species responsible for the oxidative coupling of methane (OCM) on the SrF2/La2O3 catalyst. This conclusion was reinforced by the EPR experiments (g(xx) = 2.0001, g(yy) = 2.0045, g(zz) = 2.0685), showing that O (2) over bar was the only paramagnetic oxygen species detectable on the O-2-preadsorbed SrF2/La2O3 catalyst. These results suggest that superoxide O (2) over bar can be a stable active oxygen species. whose role in the OCM reaction cannot be overlooked.