Dynamic kinetics of CO oxidation over magnesium oxide

Abstract

The dynamic kinetics of CO oxidation on magnesium oxide have been studied using the transient response method due to the concentration jump and temperature jump techniques at temperatures in the range 150–171 °C. Two different types of absorbed CO₂, strongly and weakly adsorbed, were recognized on the surface during the reaction. A small fraction of active sites, θ= 0.019, are available for the weakly adsorbed CO₂ and CO oxidation, and its adsorption obeys a Langmuir isotherm with a heat of adsorption of Q= 8.7 kcal mol^–1. This partly inhibits the rate of CO oxidation and raises the activation energy for the reaction. The strongly adsorbed CO₂, on the other hand, gives θ= 0.087, Q= 14 kcal mol^–1 and one fourth the adsorption rate constant of CO₂ compared to the weakly adsorbed one at 171 °C.

An Eley–Rideal mechanism was proposed as the best reaction mechanism and the kinetic parameters of all elementary steps have been calculated as a function of temperature by the computer simulation technique. The parameters obtained consistently explained all the isothermal transient response curves and the steady-state rate data. From the monotonic mode of the response curves and a comparison of k_j values, it is concluded that the reaction is controlled by the surface reaction and the desorption of CO₂. The non-isothermal transient response curves due to the temperature jump, however, could not be explained by the proposed mechanism. The reason for this non-fitting has been interpreted in terms of desorption of the strongly adsorbed CO₂.