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• Validation of a Thermal Decomposition Mechanism of Formaldehyde by Detection of CH2O and HCO behind Shock Waves

G. Friedrichs, D.F. Davidson, and R.K. Hanson
Int. J. Chem. Kinet. 36 (2004) 157-169

 

he 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 (CH2O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH2O + 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.

k1a = 5.0· 1015· exp(-308 (kJ/mol) /RT) cm3mol-1s-1 The pressure dependence, the rate of the competing molecular channel (1b), CH2O + M → H2 + CO + M, and the branching fraction beta = k1a/(k1a+k1b) 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 CH2O, CO and H atom measurements at temperatures from 1200 to 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 CH2O molecules. At high initial CH2O mole fractions the reverse of reaction (6), CH2OH + HCO ⇔ CH2O + CH2O, becomes noticeable. The rate of the forward reaction was roughly measured to be:
k6 = 1.5· 1013 cm3mol-1s-1

 

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