Temperature and Isotope Dependent Kinetics of Nickel-Catalyzed Oxidation of Methane by Ozone.

The temperature dependent kinetics of Ni + O and of NiO + CH/CD are measured from 300 to 600 K using a selected-ion flow tube apparatus. Together, these reactions comprise a catalytic cycle converting CH to CHOH. The reaction of Ni + O proceeds at the collisional limit, faster than previously reported at 300 K. The NiO product reacts further with O, also at the collisional limit, yielding both higher oxides (up to NiO is observed) as well as undergoing an apparent reduction back to Ni. This apparent reduction channel is due to the oxidation channel yielding NiO* with sufficient energy to dissociate. NiO + CH (CD) (whereas NiO refers to the quartet state of NiO) proceeds with a rate constant of (2.6 ± 0.4) × 10 cm s [(1.8 ± 0.5) × 10 cm s] at 300 K and a temperature dependence of ∼ T (∼ T), producing only the Ni + CHOH channel up to 600 K. Statistical modeling of the reaction based on calculated stationary points along the reaction coordinate reproduces the experimental rate constant as a function of temperature but underpredicts the kinetic isotope shift. The modeling was found to better represent the data when the crossing from quartet to doublet surface was incomplete, suggesting a possible kinetic effect in crossing from quartet to doublet surfaces. Additionally, the modeling predicts a competing NiOH + CH channel to become increasingly important at higher temperatures.