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Partial oxidation of CH, by 0, 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 CH, into CH, and Hf on a low-coordinated surface ion pair, Mg:;-Of,.CH; thus formed reacts with 0, and then an oxide ion 0'to give OCH,. Upon subsequent heating in vacuo, part of the OCH; decomposes into CO and H, , and the rest is further oxidized into COZ-. 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, 0 -, which can be formed on MgO in the presence of 0, under UV-irradiation. The initial event in 0-formation is photon absorption at OHand MgfS-0;; sites on MgO pretreated at 673 and 1123 K, respectively. Such a site reacts with 0, to produce 0 -. The 0 -so formed can abstract an H atom from CH, to give a CH, radical. This CH, radical reacts with a surface oxide ion 0'-to produce OCH,, which is mainly further oxidized into HCO; by adsorbed oxygen species during UV-irradiation. A part of the HCO; decomposes to give CO during subsequent heating in vacuo, and the remainder is further oxidized into COS-.Partial oxidation of CH, by 0, on metal oxides has been investigated by many researchers.' Ito has shown that Lidoped MgO has high activity for this reaction., However, all the catalysts that have high activity for this reaction require high reaction temperatures, usually above 900 K. High temperatures are considered necessary to produce active sites, such as 0 -, which can abstract H from CH,3 or to activate CH, molecules in the vibrational mode.4 If this reaction can proceed at lower temperatures, especially near room temperature, we can save energy and suppress complete oxidation, i.e., the production of CO, and H,O.Garrone et aL5 reported that MgO could not adsorb CH, in dissociated form. However, we have already shown that MgO which had previously been outgassed at high temperatures (above 973 K) can dissociate CH, into CH; and Hf at room temperature.6 This fact indicates that MgO outgassed above 973 K can activate CH, at room temperature without excess energy. In fact, we have already reported that previously adsorbed CH;, which had been formed by heterolytically dissociative adsorption of CH, on low-coordinated surface ions, was easily oxidized by subsequent oxygen admission even at 223 K.7 Therefore, we may expect to obtain valuable partially oxidized products in greater amounts if the reaction is carried out using a mixture of CH, and 0, over MgO.Recent reports from our group have indicated that UVirradiation of the MgO surface in the presence of a gas gives pronounced effects on the adsorptive nature : oxygen species such as 0 -, 0, and O,, which can react with hydrocarbons...

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