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

Abstract

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 lambda=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)x10(12) cm3 mol-1 s-1, k1(739-1108 K)=3.7x10(13) exp(-13 kJ mol-1/RT) cm3 mol-1 s-1 (Delta 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, was found to depend on the total density rho. At room temperature, it varies from f=1.6 at rho=8x10(-7) mol cm-3 to f=3.0 at rho=2x10(-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)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).

Glyoxal Oxidation Mechanism: Implications for the Reactions HCO + O2 and OCHCHO + HO2

“…The theoretically predicted, 2 orders of magnitude lower rate constant value for OH formation reported by da Silva 34 is also supported by a reassessment of the previous glyoxal/O 2 As discussed above, the reaction of OCHCHO with HO 2 is believed to be of importance for the generation of chain carriers in oxidation of glyoxal. Figure 5 compares the available rate constant data for the rate of OCHCHO + HO 2 .…”

“…The results are in striking agreement with the previous shock tube/photolysis experiments of Colberg and Friedrichs. 2 The absolute values as well as the overall temperature dependence, which is stronger than predicted theoretically, could be confirmed.…”

“…3.0 51 (line marked with black filled circles) is in agreement with the new high-temperature results, but the temperature dependence is quite underestimated. The RRKM/VTST calculations of Hsu et al 36 (curve marked with down triangle) underpredict the onset of the high-temperature direct abstraction channel, resulting in about 2 times lower absolute k 1 values at temperatures around 1500 K. Other experimental data for intermediate temperatures and at room temperature 2,50,52,53 reveal a more or less temperature-independent rate constant, which is consistent with the expected capture controlled process of the indirect abstraction channel with an initiating recombination step and a low-lying exit barrier to the products CO + HO 2 . As a reasonable fit of the overall temperature dependence of the available data, an extended Arrhenius expression is recommended over the temperature range 295 K < T < 1705 K (red curve) The overall rate constant is independent of pressure.…”

“…The included red error bar corresponds to the ±75% uncertainty of a single data point; the 2σ standard deviation of the data with respect to the final Arrhenius fit (red curve) is about ±40%. For a more complete comparison of available literature data and a critical assessment of available room temperature data we refer to our previous publication 2 and the paper of DeSain et al 50 Within the scatter of the data, the experiments behind the incident (open circles) and reflected shock waves (star symbols) are consistent; hence, no dependence of the rate constant on the total density could be identified. In contrast, a weak positive temperature The original data and error bars of the latter study are included as blue plus symbols.…”

“…1,2 Moreover, glyoxal is discussed as an important component in tropospheric chemistry. 3,4 Glyoxal can be formed from oxidation of C 2 H 2 at low to medium temperatures, 5−9 as well as in the atmosphere, 10 Previous studies of OCHCHO chemistry include thermal decomposition in static reactors 17 and shock tubes 18,19 as well as low-temperature oxidation 20−22 and determination of explosion limits in static reactors.…”