Adsorption of methane on magnesium oxide surfaces under ultraviolet irradiation

Abstract

CH₄ adsorption on MgO under the influence of UV-irradiation near room temperature was studied, mainly using temperature-programmed desorption. In addition to an adsorbed species (type M₁) observed in the dark adsorption, an adsorbed species (type IM), which was desorbed at a higher temperature range of 350–800 K than M₁ in a TPD run, was found. The active site IM was formed by pretreatment of MgO above 973 K in vacuo, and the number of sites increased remarkably with increase in pretreatment temperature, although the maximum number of sites observed was still very small (10¹⁵ site m^–2). Many similarities were found between M₁ and IM adsorptions and both types of active site are believed to involve Mg²⁺_LC and O^2–_LC ions, where LC (low coordination) refers mainly to 3C and partly 4C (three-coordinated and four-coordinated sites), on which CH₄ is heterolytically dissociated into CH^–₃ and H⁺. However, the locations of both the LC ions are different for the two types of site; Mg²⁺_LC and O^2–_LC are separately located in IM while they constitute the nearest lattice pair in M₁. Therefore, the IM site can adsorb CH₄ only under more severe adsorption condition; UV-irradiation causes activation of the site to produce O^–_LC from O^2–_LC prior to the adsorption process.