Partial oxidation of methane with oxygen over magnesium oxide at low temperatures

Abstract

Partial oxidation of CH₄ by O₂ on MgO pretreated at 673 and 1123 K has been investigated near room temperature by temperature-programmed desorption (TPD), infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy. The UV-irradiation effect on this reaction was also studied. In the dark, this oxidation reaction proceeded only on MgO pretreated at 1123 K and is initiated by the heterolytic dissociation of CH₄ into CH^–₃ and H⁺ on a low-coordinated surface ion pair, Mg²⁺_LC–O^2–_LC. CH^–₃ thus formed reacts with O₂ and then an oxide ion O^2– to give OCH^2–₃. Upon subsequent heating in vacuo, part of the OCH^–₃ decomposes into CO and H₂, and the rest is further oxidized into CO^2–₃. Under UV-irradiation, the oxidation reaction proceeded on MgO irrespective of its pretreatment temperature. This reaction seems to be initiated by an active oxygen species, O^–, which can be formed on MgO in the presence of O₂ under UV-irradiation. The initial event in O^– formation is photon absorption at OH^– and Mg²⁺_LC–O^2–_LC sites on MgO pretreated at 673 and 1123 K, respectively. Such a site reacts with O₂ to produce O^–. The O^– so formed can abstract an H atom from CH₄ to give a CH₃ radical. This CH₃ radical reacts with a surface oxide ion O^2– to produce OCH^–₃, which is mainly further oxidized into HCO^–₂ by adsorbed oxygen species during UV-irradiation. A part of the HCO^–₂ decomposes to give CO during subsequent heating in vacuo, and the remainder is further oxidized into CO^2–₃.