OXIDATIVE COUPLING OF METHANE OVER NA2WO4 CEO2 AND RELATED CATALYSTS/

Na2WO4/CeO2 is an active and selective catalyst for the oxidative coup ling of methane (OCM). At 780 degrees C and using a reactant feed of C H4:O-2:He = 4.8:1.0:5.6, a C-2 selectivity in excess of 70% can be ach ieved over a 9.4 mol% Na2WO4/CeO2 catalyst at a CH4 conversion of 22%. By contrast, the C-2 selectivity exhibited by pure CeO2 under the sam e reaction conditions, in the absence of Na2WO4 promoter, is <10%. The promoted catalyst is relatively insensitive to deactivation by format ion of surface carbonate species. A comparable effect occurs for Na2WO 4 on Pr6O11 which is also a nonselective catalyst in the absence of pr omoter. Characterization of Na2WO4/CeO2 by X-ray powder diffraction, b oth after calcination and after use for the OCM reaction at 780 degree s 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 p hotoelectron spectra demonstrate that all surface oxygen exists as lat tice O2- On the calcined catalysts and reveal no evidence for addition al surface oxygen species, such as O-2(2-) or O-, that might serve as sites for CH4 activation. Pulse reaction experiments show that bulk la ttice oxygen species do not participate directly in the OCM reaction, and that the active oxygen species involved in the activation of metha ne exist only in the presence of gas phase oxygen. Ion scattering spec troscopy 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 con ditions. (C) 1995 Academic Press, Inc.