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.
Journal of Physical Chemistry A
Digital Object Identifier (DOI)
Sweeny, Ard, S. G., Melko, J. J., Ruliffson, J. E., White, M. C., Viggiano, A. A., & Shuman, N. S. (2018). Temperature and Isotope Dependent Kinetics of Nickel-Catalyzed Oxidation of Methane by Ozone. The Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, & General Theory, 122(33), 6655–6662. https://doi.org/10.1021/acs.jpca.8b02513