Validation of a thermal decomposition mechanism of formaldehyde by detection of CH 2 O and HCO behind shock waves

The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH 2 O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH 2 O + M → HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k 1a = 5.0 × 10 15 exp(‐308 kJ mol −1 / RT ) cm 3 mol −1 s −1 . The pressure dependence, the rate of the competing molecular channel (1b), CH 2 O + M → H 2 + CO + M, and the branching fraction β = k 1a /( k A 1a + k 1b ) was characterized by a two‐channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1–50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH 2 O, CO, and H atom measurements at temperatures of 1200–3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self‐reaction of two CH 2 O molecules. At high initial CH 2 O mole fractions the reverse of reaction (6), CH 2 OH + HCO ⇌ CH 2 O + CH 2 O becomes noticeable. The rate of the forward reaction was roughly measured to be k 6 = 1.5 × 10 13 cm 3 mol −1 s −1 . © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 157–169 2004