Roles of oxygen and carbon dioxide on methane oxidative coupling over CaO and Sm2O3 catalysts

Abstract

The effect of partial pressures of oxygen and carbon dioxide on the rate of methane conversion over CaO and Sm2O3 catalysts has been investigated by measuring the concentration gradients of products and reactants along the catalyst bed. The results show that the oxidative reaction of methane mainly occurred in the reaction zone of the bed where the methane, oxygen and catalyst co-exist. In this zone, although the oxygen concentration decreases significantly, the rate of methane conversion remains constant over sm2O3 catalyst at 873 K. No inhibition of the catalyst activity by carbon dioxide was observed over Sm2O3 catalyst. By contrast, at the same reaction temperature of 873 K, CaO catalyst can be completely deactivated behind the reaction zone where the partial pressure of carbon dioxide produced was only 0.8 KPa. When the reaction temperature was elevated to 973 K and 1073 K, the rate of conversion of methane increased sharply to a maximum at the entrance of the bed and then slowly dropped with increasing bed height due to carbon dioxide formation. The curve of rate distribution displays a sharp drop at the point where the concentration of oxygen reaches zero. Based on the above results, a kinetic equation for the methane conversion rate has been suggested which is zero order in oxygen and first order in methane, and the isotherm for carbon dioxide adsorption can be described by a Langmuir equation.