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

The temperature dependent kinetics of Ni+ + O-3 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+ + O-3 proceeds at the collisional limit, faster than previously reported at 300 K. The NiO+ product reacts further with O-3, also at the collisional limit, yielding both higher oxides (up to NiO5+ is observed) as well as undergoing an apparent reduction back to Nit. This apparent reduction channel is due to the oxidation channel yielding NiO2+* with sufficient energy to dissociate. (NiO+)-Ni-4 + CH4 (CD4) (whereas (NiO+)-Ni-4 refers to the quartet state of NiO+) proceeds with a rate constant of (2.6 +/- 0.4) X 10(-10) cm(3) s(-1) [(1.8 +/- 0.5) x 10(-10) cm(3) s(-1)] at 300 K and a temperature dependence of similar to T-0.7 +/- 0.3 (similar to T-1.1 +/- 0.4) producing only the Ni-2(+) + (CH3OH)-C-1 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+)-Ni-3 + (CH3)-C-2 channel to become increasingly important at higher temperatures.