Applied Catalysis, 71 (1991) 103-122 103 Elsevier Science Publishers B.V., Amsterdam Oxidative coupling of methane over praseodymium oxide catalysts Michel G. Poirier 1 and Raymond Breault 2 Institut de Recherche d'Hydro-Qudbec, P.O. Box 1000, Varennes (Quebec), J3X 1S1 (Canada) and Serge Kaliaguine 1. and Alain Adnot Chemical Engineering Department and GRAPS, Universit~ Laval, Qudbec (Qudbec), GIK 7P4 (Canada) (Received 28 August 1990, revised manuscript received 21 November 1990) Abstract The catalytic properties of PrOx and Li/PrO~ for the oxidative coupling of methane have been studied in fixed bed reactors. The results obtained, combined with those of the reaction of oxygen with methane, ethane, ethene and carbon monoxide in an empty reactor, allow us to suggest a reaction scheme for the catalytic process. The experimental results have shown that a fraction of the carbon oxides formed comes from the secondary reactions of C2, but with PrO~ the major part of the carbon oxides results from a reaction parallel to C2 formation. The two catalysts were studied by thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). TGA experiments in various atmospheres in- cluding methane-oxygen mixtures have shown that the oxidation state of praseodymium oxide increases with oxygen partial pressure in methane as it does in an inert gas. The TGA studies indicate carbonate formation on the Li/PrOx surface. These carbonates are associated with lithium since none were formed on Pr0~, but the TGA experiments indicate that gaseous oxygen is necessary for their formation. XPS measurements of PrOx and Li/PrOx samples variously oxidized have confirmed the evolution in the oxidation state of praseodymium oxide and the carbonate formation on Li/PrOx. The XPS results sug- gest that the addition of lithium on praseodymium oxide leads to the formation of a mixed carbonate layer and that these carbonates stabilize the Pr a+ oxidation state and suppress the Pr 4+ ions which are associated with the labile oxygens responsible for surface total oxidation of methane and C2. Keywords: methane conversion, praseodymium oxide, carbonate formation, lithium, catalyst character- ization (TGA, XPS) INTRODUCTION There are large reserves of natural gas throughout the world [ 1 ], which makes this gas abundant at a low price. This is the incentive for looking for a way to 1present address: CANMET/LRDE, 2082, Boul. Marie-Victorin, Suite 210, Varennes (Qudbec), J3X 1R3, Canada, tel. ( + 1-418) 6562708, fax. ( + 1-418) 6565993. 2Present address: Alcan International Limitde, Centre de Recherche et de DSveloppement Arvida, P.O. Box 1250, Jonqui~re (Qudbec), GTS 4K8, Canada. 0166-9834/91/$03.50 © 1991 Elsevier Science Publishers B.V.