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

The temperature dependent kinetics of Ni⁺ + O₃ and of NiO⁺ + CH₄/CD₄ are measured from 300 to 600 K using a selected-ion flow tube apparatus. Together, these reactions comprise a catalytic cycle converting CH₄ to CH₃OH. The reaction of Ni⁺ + O₃ proceeds at the collisional limit, faster than previously reported at 300 K. The NiO⁺ product reacts further with O₃, also at the collisional limit, yielding both higher oxides (up to NiO₅⁺ is observed) as well as undergoing an apparent reduction back to Ni⁺. This apparent reduction channel is due to the oxidation channel yielding NiO₂⁺* with sufficient energy to dissociate. ⁴NiO⁺ + CH₄ (CD₄) (whereas ⁴NiO⁺ refers to the quartet state of NiO⁺) proceeds with a rate constant of (2.6 ± 0.4) × 10^-10 cm³ s^-1 [(1.8 ± 0.5) × 10^-10 cm³ s^-1] at 300 K and a temperature dependence of ∼ T^-0.7±0.3 (∼ T^-1.1±0.4), producing only the ²Ni⁺ + ¹CH₃OH 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 ³NiOH⁺ + ²CH₃ channel to become increasingly important at higher temperatures.