Reaction of preadsorbed methane with oxygen over magnesium oxide at low temperatures

Abstract

Partial oxidation by O₂ of methane, which had previously been adsorbed at room temperature on MgO pretreated at 1123 K in vacuo, has been examined near room temperature by t.p.d., i.r. and e.s.r. spectroscopies and compared with a methanol adsorption system. The oxidation reaction of methane preadsorbed either in the dark or under u.v.-irradiation proceeds according to the same reaction mechanism, where low-coordination surface ions of MgO play an important role. Methane is adsorbed in a heterolytically dissociated form (CH^–₃+ H⁺), and then oxidized to a methoxide species, OCH^–₃(2), on the admission of O₂ at room temperature. On heating under evacuation, OCH^–₃(2) is either decomposed into H₂ and CO in the temperature range 495–535 K or further oxidized by coexisting O^–₂ into HCO^–₂ below 473 K. The HCO^–₂ formed gives either CO or bidentate CO^2–₃ at ca. 600 K. Another more stable methoxide species, OCH^–₃(1), is also sometimes formed eithere directly on room-temperature oxidation or through a transformation of the less stable species, OCH^–₃(2), on heating at ca. 500 K. Part of the OCH^–₃(1) present then decomposes into H₂ and CO at 760 K and the rest changes into bidentate CO^2–₃. The CO^2–₃ species thus formed ultimately are desorbed as CO₂ above 600–700 K.