Oxidative Coupling of Methane over Na 2WO 4/CeO 2 and Related Catalysts

Abstract

Na 2WO 4/CeO 2 is an active and selective catalyst for the oxidative coupling of methane (OCM). At 780°C and using a reactant feed of CH 4:O 2:He = 4.8:1.0:5.6, a C 2 selectivity in excess of 70% can be achieved over a 9.4 mol% Na 2WO 4/CeO 2 catalyst at a CH 4 conversion of 22%. By contrast, the C 2 selectivity exhibited by pure CeO 2 under the same reaction conditions, in the absence of Na 2WO 4 promoter, is <10%. The promoted catalyst is relatively insensitive to deactivation by formation of surface carbonate species. A comparable effect occurs for Na 2WO 4 on Pr 6O 11 which is also a nonselective catalyst in the absence of promoter. Characterization of Na 2WO 4/CeO 2 by X-ray powder diffraction, both after calcination and after use for the OCM reaction at 780°C, confirms that both the Na 2WO 4 and CeO 2 remain as discrete phases, and that no new bulk compounds or solid solutions are formed. X-ray photoelectron spectra demonstrate that all surface oxygen exists as lattice O 2− on the calcined catalysts and reveal no evidence for additional surface oxygen species, such as O 2− 2 or O −, that might serve as sites for CH 4 activation. Pulse reaction experiments show that bulk lattice oxygen species do not participate directly in the OCM reaction, and that the active oxygen species involved in the activation of methane exist only in the presence of gas phase oxygen. Ion scattering spectroscopy and in situ Raman spectroscopy indicate that the initial CeO 2 surface of the calcined catalyst is completely covered by one or more layers of Na 2WO 4, which exists in the molten state under reaction conditions.