The promoting effect of ceria on Li/MgO catalysts for the oxidative coupling of methane

The effects of ceria loading (0.1–6.0 wt%) on the properties of Li/MgO catalysts and on their activities for oxidative coupling of methane (OCM) were studied. The catalysts were characterised by surface area measurement, XRD, XPS, SEM and CO2 temperature programmed desorption. The catalyst activity study suggested that the addition of 0.5–1.0 wt% ceria improved the methane conversion of Li/MgO catalyst significantly at reaction temperatures below 800 °C with little change in C2 selectivity compared to un-promoted Li/MgO. Further increases in ceria loading resulted in lower methane conversion and C2 selectivity. The CH4 conversion was related to the surface distribution of O− and O2− on the catalysts. An increase in reaction temperature beyond 800 °C or a decrease in CH4:O2 ratio from 7:1 to 3:1 imparted negative effects on the methane conversion and C2 selectivity of the ceria (0.5 wt%) doped Li/MgO catalyst. All ceria doped catalysts did not show any methane coupling activity in the absence of gaseous oxygen, suggesting that they were ineffective to work with alternate feeds of CH4 and O2 (redox) mode. Based on the OCM activity and characterisation results, a new pathway that describes the formation of active sites through the electron transfer between Ce4+/Ce3+and Li/MgO was proposed to help explain the improved OCM activity by using ceria doped Li/MgO catalysts.