Elementary Processes in the Catalytic Combustion of Methane (Mechanism, Intermediates, and Controlling Reactions). Annual Report October 1983 - September 1984

摘要

The surface reactions involved in the catalytic oxidation of methane have been investigated using single crystal metal surfaces, supported metal catalysts, and binary metal-oxide catalysts. For Ni(111) the rate of reaction of surface-adsorbed oxygen (at fractional monolayer coverage) with gaseous methane is greatly reduced in the presence of coadsorbed sulfur. The oxidative removal of sulfur adatoms requires elevated temperatures (> 600 K). Also, the formation of an oxide layer overgrowth can occur on top of the sulfur adlayer. During methane oxidation (O2/CH4 > 2) catalyzed at elevated temperatures (> 600 K) by dispersed metals (Ir, Pt) on an alumina support, the introduction of H2S (at < 1 ppm) into the feed stream has been found to have a negligible effect. Presumably the sulfur species is converted to SO2, thereby preventing a sulfur buildup on the metal surface. Experimental studies of surface intermediates by electron energy loss spectroscopy have revealed the formation of a hydroformyl radical (OCH2) formed by reaction of gaseous methane with preadsorbed oxygen (initial surface coverage less than 0.5 monolayer). Also, its presence has been deduced from temperature programmed desorption studies leading to the simultaneous formation of CO and H2 in equimolar amounts.