OXIDATIVE COUPLING OF METHANE OVER PRASEODYMIUM OXIDE CATALYSTS

The catalytic properties of PrO(x) and Li/PrO(x) 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(x) 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 including 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/PrO(x) surface. These carbonates are associated with lithium since none were formed on PrO(x), but the TGA experiments indicate that gaseous oxygen is necessary for their formation. XPS measurements of PrO(x) and Li/PrO(x) samples variously oxidized have confirmed the evolution in the oxidation state of praseodymium oxide and the carbonate formation on Li/PrO(x). The XPS results suggest that the addition of lithium on praseodymium oxide leads to the formation of a mixed carbonate layer and that these carbonates stabilize the Pr3+ oxidation state and suppress the Pr4+ ions which are associated with the labile oxygens responsible for surface total oxidation of methane and C2.