High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2 → Products

The two-channel thermal decomposition of formaldehyde [CH2O], (1a) CH2O + Ar → HCO + H + Ar, and (1b) CH2O + Ar → H2 + 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–O2–Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C3H6O3] was used as the CH2O precursor in the current experiments. H-atoms formed upon CH2O and HCO decomposition rapidly react with O2 to produce OH via H + O2 → 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 k1a and k1b, applicable in this temperature range, are:k1a=5.85×1014exp(-32100/T[K])[cm3mol-1s-1]k1b=4.64×1014exp(-28700/T[K])[cm3mol-1s-1] Uncertainty limits for k1a and k1b were estimated to be ∼±25%. The reaction between CH2O and O2, (2) CH2O + O2 → HO2 + HCO, was also investigated in shock tube experiments. The rapid thermal decomposition of HCO and HO2 generate H-atoms that react with O2 to produce OH. Rate coefficients were, as in the CH2O 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:k2=5.08×1014exp(-23300/T[K])[cm3mol-1s-1] The uncertainty in k2 was estimated to be ∼±35%.