Oxidative Coupling of Methane over Na2WO4/CeO2 and Related Catalysts

Article abstract of DOI:10.1006/jcat.1995.1157

Na2WO4/CeO2 is an active and selective catalyst for the oxidative coupling of methane (OCM).At 780 deg C and using a reactant feed of CH4:O2:He=4.8:1.0:5.6, a C₂ selectivity in excess of 70percent can be achieved over a 9.4 molpercent Na2WO4/CeO2 catalyst at a CH4 conversion of 22percent.By contrast, the C₂ selectivity exhibited by pure CeO2 under the same reaction conditions, in the absence of Na2WO4 promoter, is <10percent.The promoted catalyst is relatively insensitive to deactivation by formation of surface carbonate species.A comparable effect occurs for Na2WO4 on Pr6O11, which is also a nonselective catalyst in the absence of promoter.Characterization of Na2WO4/CeO2 by X-ray powder diffraction, both after calcination and after use for the OCM reaction at 780 deg C, confirms that both the Na2WO4 and CeO2 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 O2^2- or O^-, that might serve as sites for CH4 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 CeO2 surface of the calcined catalyst is completely covered by one or more layers of Na2WO4, which exists in the molten state under reaction conditions.