Experimental determination of the temperature dependence of the absolute rate coefficients of the HCCO + NO2 and HCCO + H2 reactions

Abstract

The absolute rate coefficients of the gas-phase reactions HCCO + NO₂ and HCCO + H₂ were experimentally determined for the first time over extended temperature ranges: 293 K to 769 K and 438 K to 761 K, respectively. HCCO radicals were generated by pulsed-laser photolysis of CH₂CO at 193 nm. Their subsequent decay, under pseudo-first-order conditions, was monitored in real-time using a laser-photofragment/laser-induced fluorescence technique. The rate coefficient of HCCO + NO₂ exhibits a negative temperature dependence similar to that of the HCCO + NO reaction, but the Arrhenius A-factor is 1.4 times larger; k(T)_(HCCO+NO2) = (2.3 ± 0.4) × 10⁻¹¹ exp (340 ± 40) K/T) cm³ s⁻¹. It is argued that, if the major product channels yield N, NH or NCO, the HCCO + NO₂ reaction should be a significant removal route of NO_x in stationary combustion systems under fuel-rich conditions at temperatures below ca. 1300 K. The rate coefficient for the HCCO + H₂ reaction was determined as k(T)_(HCCO+H2) = (2.2 ± 1.4) × 10⁻¹¹ exp(−2000 ± 400)K/T). In fuel-rich combustion environments, given the high concentrations of H₂, this reaction is likely to be a significant loss process for HCCO radicals: k(1500 K)_HCCO+H2 = (6^+0.4_−0.2) × 10⁻¹² cm⁻³ s⁻¹, a factor of three greater than k(1500 K)_HCCO+O2.