An Investigation on the Role of NO₂ in the Oxidation of Methane to Formaldehyde

Experimental results and a kinetic modeling were previously reported for the gas phase oxidation of methane in the presence of NO₂ at short residence time (20–80 ms) in a microreactor. A detailed kinetic analysis is carried out in the present study in means of flow rate and sensitivity analysis, in order to investigate the role of NO₂. A comparison is made of the reaction systems with and without NO₂. The concentration profiles of several key species are presented and analyzed in the present study. The CH₃O· radicals are present in far larger quantities in the CH₄/O₂/NO₂ system than in the CH₄/O₂ system. With NO₂, the dominant route to HCHO is through CH₃O·, primarily involving the reaction: CH₃· + NO₂ ⇔ CH₃O· + NO, followed by: CH₃O· (+M) ⇔ CH₂O + H· (+M) and CH₃O· + O₂ ⇔ HO₂· + CH₂O. The promoting effects of NO₂ can be explained by the interconversion NO₂/NO, which is mainly related to: HO₂· + NO ⇔ NO₂ + OH·, NO₂ + H· ⇔ NO + OH·, NO + CH₃O₂· ⇔ CH₃O· + NO₂ and NO₂ + CH₃· ⇔ CH₃O· + NO.