Kinetics and mechanism of oxidative dehydrogenation of ethane and small alkanes with nitrous oxide over cobalt-doped magnesium oxide

Abstract

The kinetics of the decomposition of N₂O and the partial oxidation of C₂H₆ and other alkanes with N₂O have been studied over Co²⁺-doped MgO catalysts mainly at 473 K and below atmospheric pressure. The initial rate of N₂O decomposition is proportional to the estimated amount of surface Co²⁺ ion, which suggests the Co²⁺ ion to be the active centre. Since the amounts of adsorbed oxygen are much greater than the estimated amounts of surface Co²⁺ ion, the adsorbed oxygen is inferred to migrate from the Co²⁺ ion centre to the MgO surface. No O₂ was produced during the oxidation of C₂H₆ and the initial rate of the oxidation reaction was almost identical to those of N₂O decomposition. Thus, it was concluded that the oxidation of C₂H₆ was controlled by the decomposition of N₂O. Ethoxide is inferred to be a common intermediate, giving C₂H₄ and surface oxidized species. Pure alkanes from C₁ to C₄ react at much the same rate with N₂O, because the decomposition of N₂O is the rate-determining step. If, however, mixed alkanes are used, differences in inherent reactivities appear. The reactivity sequence (n-C₄H₁₀ > C₃H₈≈ C₂H₄ > C₂H₆ > CH₄) obtained is almost identical to that with oxygen atoms in the gas phase.