Isotopic Oxygen Exchange and EPR Studies of Superoxide Species on the SrF 2 /La 2 O 3 Catalyst
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Isotopic Oxygen Exchange and EPR Studies of Superoxide Species on the SrF2/La2O3 Catalyst Lihua Wang Æ Xiaodong Yi Æ Weizheng Weng Æ Chunxi Zhang Æ Xin Xu Æ Huilin Wan
Received: 30 January 2007 / Accepted: 22 June 2007 / Published online: 17 July 2007 Springer Science+Business Media, LLC 2007
Abstract 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. Keywords Isotopic oxygen exchange In situ IR EPR Superoxide species Methane oxidative coupling
L. Wang X. Yi W. Weng X. Xu (&) H. Wan State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China e-mail: [email protected] H. Wan e-mail: [email protected] C. Zhang Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China e-mail: [email protected]
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1 Introduction As one of the important potential routes to convert an abundant hydrocarbon resource to more useful chemicals, the oxidative coupling of methane (OCM) to C2 hydrocarbons has been intensively studied since the pioneer work of Keller and Bhasin [1] (For some reviews we refer to references [2–6]). The reaction mechanism has now been well established. The reaction is initiated by the hydrogen abstraction from CH4 to liberate CH3, using the active oxygen species on the surfaces of the metal oxide catalysts; followed by the coupling of the methyl radicals in the gas phase to give ethane as the primary product, which is then dehydrogenated to form ethylene. However, consensus has not yet reached on the nature of the active oxygen species. Lunsford and co-workers established many of the generally accepted principles concerning the nature of the active sites [7, 8]. In their work on the OCM reaction over Li/MgO, surface O– species have been concluded as the active species based on the EPR results [7, 8]. In the case of pure alkaline earth or rare earth oxides or their composite compounds, there are evidences pointing to the surface peroxide O2– 2 ions as the active species. Otsuka et al. found that Na2O2, SrO2 and BaO2 were capable of convert
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