Surface WO4 tetrahedron: the essence of the oxidative coupling of methane over MWMn/SiO2 catalysts

A series of MWMn/SiO2 catalysts (M = Li, Na, K, Ba, Ca, Fe, Co, Ni, and Al) have been prepared and their catalytic performance for the oxidative coupling of methane (OCM) was evaluated in a continuous-flow microreactor. The structural properties of the catalysts have been studied using X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). In the trimetallic catalysts studied, there was evidence for WO4 tetrahedron on the surface in the Li, Na, and KWMn/SiO2 catalysts, which is mainly present in the subsurface of the BaWMn/SiO2 catalyst. It appears that the WO4 has a strong interaction with the α-cristobalite support and is stabilized in the Na and KWMn/SiO2 catalysts. However, the WO4 species appear to be less stable in Li or BaWMn/SiO2 catalysts, in which the support turns into quartz SiO2 or amorphous SiO2. The WO4 tetrahedron on the catalyst surface appears to play an essential role in achieving high CH4 conversion and high C2 hydrocarbon selectivity in the OCM reaction. Calculations suggest that the WO4 tetrahedron interacts with the CH4, giving suitable geometry and energy matching with CH4, and this may account for the high OCM activities.