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

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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 CO2– 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 CO2– 3. The CO2– 3 species thus formed ultimately are desorbed as CO2 above 600–700 K.

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