Partial Oxidation of Methane, Methanol, Formaldehyde, and Carbon Monoxide Over Silica: Global Reaction Kinetics

Oxidation of methane (848-898K), methanol (648-748K), formaldehyde (623-673K), and carbon monoxide (673-833K) over a precipitated silica catalyst has been examined over a range of reactant and oxygen partial pressures. Conversion-selectivity relationships are used to assess the reaction network and differential reactor experiments are employed to determine the global reaction kinetics. All reactions exhibited a positive-order dependence on oxygen, partial pressure consistent with reaction of chemisorbed oxygen. This implies that the chemisorption of oxygen on the reduced sites occurs at a rate comparable to that of substrate oxidation, and may therefore limit the oxidation rate. Self-inhibition was observed for oxidation of carbon monoxide, as PCO was increased, the rate of carbon monoxide oxidation decreased. A conceptual model where CO and O2 compete for surface oxygen vacancies is proposed. Additionally, it is shown that in methane partial oxidation, reactions of methanol, formaldehyde, and CO can consume oxidized surface sites, thought to be the active sites for methane partial oxidation. A simple model of the degree of surface reduction is presented.