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

Abstract

Partial oxidation by 0, of methane, which had previously been adsorbed at room temperature on MgO pretreated at 1123 K in uucuo, 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;+H+), and then oxidized to a methoxide species, OCH;(Z), on the admission of 0, at room temperature. On heating under evacuation, OCHi(2) is either decomposed into H, and CO in the temperature range 495-535 K or further oxidized by coexisting 0; into HCO; below 473 K. The HCO; formed gives either CO or bidentate COiat cu. 600 K. Another more stable methoxide species, OCH;(l), is also sometimes formed either directly on room-temperature oxidation or through a transformation of the less stable species, OCH;(2), on heating at ca. 500 K. Part of the OCHi(1) present then decomposes into H, and CO at 760 K and the rest changes into bidentate CO:-. The COi-species thus formed ultimately are desorbed as CO, above 600-700 K.

Continuous wave electron paramagnetic resonance investigation of the hyperfine structure of O2−17 adsorbed on the MgO surface

“…The largest splitting corresponds to A xx ϭ76 G, which is in agreement with previous reports. 5,16,26 However, in the case of the sample with 63% 17 O enrichment, the ( 17 O- 16 O͒ Ϫ and ( 17 O-17 O͒ Ϫ hyperfine patterns appear simultaneously ͓see stick diagram in Fig. 2͑c͔͒.…”

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

“…[8] However, Ito et al reported that low coordination ions on the surface of MgO play an important role in the dissociation of adsorbed methane. [9] Defects will definitely fulfill a key function with respect to the activation of either methane or oxygen by changing the electronic structure of the wide band gap magnesium oxide particularly with regard to facilitate the transfer of electrons between the solid surface and the adsorbed molecules, which undergo a redox reaction. [10][11][12][13][14][15] The present work addresses relations between the nature and abundance of morphological surface defects such as steps and corners on pure magnesium oxide and its reactivity in the oxidative coupling of methane.…”

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

“…cannot adsorb anymore, unless high reaction temperatures are applied to assure a partially dehydroxylated surface under reaction conditions. In addition, Ito et al reported that methyl radicals from the gas phase could react with surface oxygen under the formation of strongly bonded methoxide species [40] according to Eq. (2).…”

“…(3) [41]. Methoxide species can be transformed into carbonates at room temperature and decompose only at high temperature (T > 700 K) to produce either syngas or carbon dioxide and water [40]. Eq.…”