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

M. Colberg, and G. Friedrichs
J. Phys. Chem. A 110 (2006) 160-170.

 

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 cm3mol-1s-1
k1(739 - 1108 K) = 3.7 · 1013 exp(-13 kJ mol-1 / RT) cm3mol-1s-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]0total, which was found to depend on the total density ρ, 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 cm3mol-1s-1
k4(769 - 1107 K) = 5.4 · 1013 exp(-18 kJ mol-1 / RT) cm3mol-1s-1 (Δ log k4 = ± 0.12)

 

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