ADSORPTION OF METHANE ON MAGNESIUM-OXIDE SURFACES UNDER ULTRAVIOLET-IRRADIATION

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 (10(15) site m-2). Many similarities were found between M1 and IM adsorptions and both types of active site are believed to involve Mg(LC)2+ and O(LC)2- 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 CH3- and H+. However, the locations of both the LC ions are different for the two types of site; Mg(LC)2+ and O(LC)2- 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 conditions; UV-irradiation causes activation of the site to produce O(LC)- from O(LC)2- prior to the adsorption process.