High-temperature measurements of the rates of the reactions CH 2O + Ar → Products and CH 2O + O 2 → Products

Abstract

The two-channel thermal decomposition of formaldehyde [CH 2O], (1a) CH 2O + Ar → HCO + H + Ar, and (1b) CH 2O + Ar → H 2 + CO + Ar, was studied in shock tube experiments in the 2258–2687 K temperature range, at an average total pressure of 1.6 atm. OH radicals, generated on shock heating trioxane–O 2–Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C 3H 6O 3] was used as the CH 2O precursor in the current experiments. H-atoms formed upon CH 2O and HCO decomposition rapidly react with O 2 to produce OH via H + O 2 → O + OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k 1a and k 1b, applicable in this temperature range, are: k 1 a = 5.85 × 10 14 exp ( - 32100 / T [ K ] ) [ cm 3 mol - 1 s - 1 ] k 1 b = 4.64 × 10 14 exp ( - 28700 / T [ K ] ) [ cm 3 mol - 1 s - 1 ] Uncertainty limits for k 1a and k 1b were estimated to be ∼±25%. The reaction between CH 2O and O 2, (2) CH 2O + O 2 → HO 2 + HCO, was also investigated in shock tube experiments. The rapid thermal decomposition of HCO and HO 2 generate H-atoms that react with O 2 to produce OH. Rate coefficients were, as in the CH 2O decomposition experiments, inferred by matching measured and modeled OH time-histories behind the reflected shock, under conditions where interference from secondary chemistry is minimal. A two-parameter, least-squares fit of the current data, valid over the 1480–2367 K temperature range, yields the following rate expression: k 2 = 5.08 × 10 14 exp ( - 23300 / T [ K ] ) [ cm 3 mol - 1 s - 1 ] The uncertainty in k 2 was estimated to be ∼±35%.