The Rate-Determining Step in the Selective Reduction of Nitric-Oxide by Methane over a Co-ZSM5 Catalyst in the Presence of Oxygen

The relative reaction rates of CH₄ and CD₄ with NO in the presence of excess oxygen over a Co-ZSM5 catalyst have been determined. The rate ratio, CH₄ consumption versus CD₄ consumption, is nearly 2.4 when using a dry feed at 375°C. Experiments at 431°C using a feed containing 1.6% H₂O show a rate ratio of 2.05. These values are close to those expected for a primary kinetic isotope effect at the respective temperatures, and it is therefore concluded that the rate-determining step is the breaking of a carbon hydrogen bond. The kinetic isotope effect is slightly less for the selective reaction with NO than for the concurrent direct oxidation of methane by oxygen. The two reactions have very similar kinetic orders in methane, suggesting that a common intermediate is involved in the two pathways. Hydrogen-deuterium exchange between CD₄ and H₂O to make CH₃D is negligible under reaction conditions. Similarly, mixed feeds of CH₄ and CD₄ show very little exchange. One interpretation consistent with the results is that the rate-determining step involves detachment of hydrogen from methane by adsorbed nitrogen dioxide to form a methyl species which reacts further with a nitrogen oxide in one pathway but with oxygen in the other.