Characterization of Li-Doped MgO Catalysts for Oxidative Coupling of Methane by Means of Mg K-Edge XANES

To characterize active defect sites in the near-surface and bulk phase of MgO in Li-doped MgO, structural analyses were carried out by means of XRD, XPS, Mg K-edge XANES, and SEM techniques. For Li−MgO calcined at 873 K, Li doping at a low content (2.5 wt % as Li) brings about the formation of defect species only in the near-surface, which is due to the localization of doped 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 than 7.5 wt %, Li ions penetrate the MgO bulk, and its crystallinity decreases. The defect species possibly exist in the bulk phase. After oxidative coupling of methane reaction, the defect species are formed in the near-surface over MgO and Li−MgO. However, the C2 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 C2 compounds with high selectivity.