Ce-Driven Ce-MnO_x/Na₂WO₄/SiO₂ Composite Catalysts for Low-Temperature Oxidative Coupling of Methane

Abstract

Mn/Na 2 WO 4 /SiO 2 catalyst is widely used in the oxidative coupling of methane (OCM) reactions because of its high catalytic performance and stability. Nevertheless, the complex elemental composition makes the role of each active component in these catalysts still controversial. Herein, we conducted separate studies on Mn/Na 2 WO 4 / SiO 2 catalyst by separating it into MnO x and Na 2 WO 4 /SiO 2 (NaWSi). This provided a persuasive strategy to turn manganese oxide from a combustion catalyst into a selective catalyst for the OCM at low temperatures through a synergistic effect between the Ce-MnO x catalyst and Na 2 WO 4 /SiO 2 catalyst. The introduction of Ce is a key factor in improving the low-temperature OCM activity of the catalyst. The temperature-programmed desorption of oxygen (O 2 -TPD) and 18 O isotope labeling experiments confirm that surface lattice oxygen is the main active oxygen species in the OCM reaction for the Ce-driven catalyst. The presence of Ce strengthens the surface lattice oxygen cycle process on the catalyst, resulting in higher oxygen exchange ability and improved migration of active lattice oxygen. Furthermore, in situ Raman spectroscopy shows that both 3Ce-MnO x and NaWSi catalysts can enhance resistance to carbon deposition in the catalysts. Hence, the 3Ce-MnO x /NaWSi composite catalyst achieved 38.4% conversion of CH 4 and a 15.6% yield of C 2 at 700 °C and the catalytic activity remained stable for at least 25 h.