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 O2–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 O2–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 O2–LC prior to the adsorption process.