Characterization of Li-doped MgO catalysts for oxidative coupling of methane by means of MgK-edge XANES

To characterize active defect sites in the near-surface and bulk phase of M gO in Li-doped MgO, structural analyses were carried out by means of XRD, X PS, Mg K-edge XANES, and SEM techniques. For Li-MgO calcined at 873 K, Li d oping at a low content (2.5 wt % as Li) brings about the formation pf defec t species only in the near-surface, which is due to the localization of dop ed Li ions in the surface. In this case, the catalytic species containing a Li+-O- center exists in the surface region. At higher contents of more tha n 7.5 wt %, Li ions penetrate the MgO bulk, and its crystallinity decreases . The defect species possibly exist in the bulk phase. After oxidative coup ling of methane reaction, the defect species are formed in the near-surface over MgO and Li-MgO. However, the C-2 selectivity is much higher on Li-MgO than on MgO. It is concluded that the defect species containing Li+-O- in both surface and bulk can act as the active species for producing Ct compou nds with high selectivity.