Wide temperature range (T = 295 K and 770–1305 K) study of the kinetics of the reactions HCO + NO and HCO + NO2 using frequency modulation spectroscopy

Abstract

The rate constants for reaction (1), HCO + NO → HNO + CO, and reaction (2), HCO + NO₂ → products, have been measured at temperatures between 770 K < T < 1305 K behind reflected shock waves and, for the purpose of a consistency check, in a slow flow reactor at room temperature. HCO radicals were generated by 193 nm excimer laser photolysis of diluted gas mixtures containing glyoxal, (CHO)₂, and NO or NO₂ in argon and were monitored using frequency modulation (FM) absorption spectroscopy. Kinetic simulations based on a comprehensive reaction mechanism showed that the rate constants for the title reactions could be sensitively extracted from the measured HCO profiles. The determined high temperature rate constants are k₁(769–1307 K) = (7.1 ± 2.7) × 10¹² cm³ mol⁻¹ s⁻¹ and k₂(804–1186 K) = (3.3 ± 1.8) × 10¹³ cm³ mol⁻¹ s⁻¹. The room temperature values were found to be in very good agreement with existing literature data and show that both reactions are essentially temperature independent. The weak temperature dependence of reaction (1) can be explained by the interplay of a dominating direct abstraction pathway and a complex-forming mechanism. Both pathways yield the products HNO + CO. In contrast to reaction (1), no evidence for a significant contribution of a direct high temperature abstraction channel was found for reaction (2). Here, the observed temperature independent overall rate constant can be described by a complex-forming mechanism with several product channels. Detailed information on the strongly temperature dependent channel branching ratios is provided. Moreover, the high temperature rate constant of reaction (7), OH + (CHO)₂, has been determined to be k₇ ≈1.1 × 10¹³ cm³ mol⁻¹ s⁻¹.