Reactive Vs. Adsorbed Oxygen in the Heterogeneous Oxidation of Methane Over Li/MgO

Abstract

The kinetics of O_2(g) adsorption, desorption, and CH_4(g) or H_2(g) oxidation on 7% Li-doped MgO were investigated by on-line, modulated beam mass spectrometry in a low-pressure, continuous flow reactor between 930–1130 K. Under these conditions, which suppress gas-phase reactions, only purely heterogenous processes take place in this system. Dosage of the amounts of bound oxygen removed at several stages of evacuation, or consumed by reaction with methane or hydrogen, indicates that oxygen adsorption is a reversible process leading to at least two major types of chemisorbed species. The kinetics of O_2(g) exchange is a slow process in the time scale of the experiments, that can not be described by Langmuir-type equilibrium isotherms involving either dissociative or nondissociative adsorption on one or two types of active sites. Fast adsorption on a precursor state, followed by slow conversion into a more strongly bound species seem essential to model oxygen take up by the solid. On an oxygen saturated oxide, but in the absence of O_2(g), methane reacts rapidly with about 14% of the pool of exchangeable oxygen, after which a slower oxidation reaction sets in. Hydrogen behaves similarly. During the slow oxidation stage methane is oxidized into carbon oxides and ethane, but not water, and partially decomposes into unvolatile carbon-containing species without releasing H_2(g).