Isotopic Oxygen Exchange and EPR Studies of Superoxide Species on the SrF2/La2O3 Catalyst
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By using the in situ IR spectroscopy, the superoxide species (O 2 − ), 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 18O2 isotope caused the 1130 cm−1 peak to shift to lower wavenumbers (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 − species and that of the formation of C2H4 was observed, suggesting that O 2 − 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 (gxx = 2.0001, gyy = 2.0045, gzz = 2.0685), showing that O 2 − was the only paramagnetic oxygen species detectable on the O2-preadsorbed SrF2/La2O3 catalyst. These results suggest that superoxide O 2 − can be a stable active oxygen species, whose role in the OCM reaction cannot be overlooked.
KeywordsIsotopic oxygen exchange In situ IR EPR Superoxide species Methane oxidative coupling
This work was supported by the Ministry of Science and Technology of China (Grant No. 2005CB221408, 2004CB719902) and NSFC (20433030, 20525311, 20533030, 20423002).
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