KINETIC-STUDIES OF THE CATALYTIC-OXIDATION OF METHANE .1. METHYL RADICAL PRODUCTION ON 1-PERCENT SR/LA2O3

The kinetics of methyl radical production on 1% Sr/La2O3 has been studied by using a heatable tubular flow reactor, containing the catalyst, coupled to a photoionization mass spectrometer. The experimental technique, which permits direct detection of stable species and free radicals and measurements of their absolute concentrations along the reactor, is described. Experiments were conducted using conditions which isolated the initial CH3(g) production step. Absolute rates of CH3(g) production were determined from measurements of CH3(g) concentration profiles along the reactor. Rates were measured as a function of T (982-1138 K), [CH4(g)], and [O2(g)]. The information obtained was used to deduce the rate law for methyl radical production and to identify the most likely kinetic mechanism of this process. The body of data obtained and the rate law indicate the establishment and maintenance of an equilibrium between O2(g) and surface-bound oxygen atoms (endothermic dissociative adsorption). CH3(g) is formed in a rate-determining step between CH4(g) and a surface-bound oxygen atom. The temperature dependence of the constants in the rate law indicates that DELTA-H-degrees = 44 +/- 10 kcal mol-1 for the endothermic adsorption (O2(g) --> 2(-O(surface)), and the activation energy for the gas-surface reaction responsible for the production of CH3(g) is 21 +/- 5 kcal mol-1 (CH4(g) + -O(surface) --> CH3(g) + HO(surface)). Heterogeneous loss of CH3(g) did not occur to an observable degree under the conditions of these experiments.