Room Temperature and Shock Tube Study of the Reaction HCO + O₂ Using the Photolysis of Glyoxal as an Efficient HCO Source

The rate of the reaction 1, HCO + O₂ → HO₂ + CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbar < p < 500 mbar) and (ii) in the temperature range 739 K < T < 1108 K behind reflected shock waves (0.82 bar < p < 1.84 bar) employing a perturbation approach. Following the 193 nm excimer laser photolysis of mixtures of glyoxal in Ar, concentration−time profiles were measured using frequency modulation (FM) detection of HCO at a wavelength of λ = 614.752 nm. Observed differences between HCO concentration−time profiles measured with and without O₂ added to the reaction mixtures could be almost exclusively attributed to reaction 1. The determined rate constants, k₁(295 K) = (3.55 ± 0.05) × 10¹² cm³ mol^-1 s^-1, k₁(739−1108 K) = 3.7 × 10¹³ exp(−13 kJ mol^{- 1}/RT) cm³ mol^-1 s^-1 (Δ log k₁ = ± 0.16), reveal a slightly positive temperature dependence of reaction 1 at high temperatures. Furthermore, the 193 nm photolysis of glyoxal, (CHO)₂, has been proven to be an efficient HCO source. Besides HCO, photolysis of the precursor also produces H atoms. The ratio of initially generated H atoms and HCO radicals, f = [H]₀/[HCO]₀^total, was found to depend on the total density ρ. At room temperature, it varies from f = 1.6 at ρ = 8 × 10^-7 mol cm^-3 to f = 3.0 at ρ = 2 × 10^-5 mol cm^-3. H atoms are transformed via reaction 4, H + (CHO)₂ → H₂ + HCO + CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k₄(295 K) = (3.6 ± 0.3) × 10¹⁰ cm³ mol^-1 s^-1 and k₄(769−1107 K) = 5.4 × 10¹³exp(−18 kJ mol^{- 1}/RT) cm³ mol^-1 s^-1(Δ log k₄ = ±0.12).