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.