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–O2–LC. CH–3 thus formed reacts with O2 and then an oxide ion O2– to give OCH2–3. Upon subsequent heating in vacuo, part of the OCH–3 decomposes into CO and H2, and the rest is further oxidized into CO2–3. 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–O2–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 O2– 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 CO2–3.