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

Abstract

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 CO 2 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 C 2 selectivity compared to un-promoted Li/MgO. Further increases in ceria loading resulted in lower methane conversion and C 2 selectivity. The CH 4 conversion was related to the surface distribution of O − and O 2− on the catalysts. An increase in reaction temperature beyond 800 °C or a decrease in CH 4:O 2 ratio from 7:1 to 3:1 imparted negative effects on the methane conversion and C 2 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 CH 4 and O 2 (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 Ce 4+/Ce 3+and Li/MgO was proposed to help explain the improved OCM activity by using ceria doped Li/MgO catalysts.