Pyrolysis and Oxidation of Formaldehyde in Shock Waves

Abstract

The pyrolysis and oxidation of formaldehyde have been studied in shock waves by mass spectrometry and by ionization measurements over the temperature range 1400° to 2100°K. The pyrolysis shows first‐order rate dependence with respect to both formaldehyde and inert gas after a temperature‐dependent induction period. Experiments with formaldehyde‐d₂ show that the pyrolysis proceeds by a radical chain mechanism. The formation of the radical H₃CO has been detected. The observed activation energy is 28±2.5 kcal/mole, which is surprisingly low; various mechanisms are suggested to explain this result. The initial stage of the oxidation consists mainly of an oxygen‐induced decomposition of formaldehyde into CO and H₂, followed by branching chain oxidation of the H₂ thus produced, as well as of remaining formaldehyde, and partial oxidation of the CO. Chemi‐ionization is observed in the oxidation reaction, the species H₃O⁺ and H₃CO⁺ being the only ions detected. The total yield of ions is virtually independent of temperature. Very good agreement is obtained between measurements in reflected shock waves with the TOF mass spectrometer and in incident waves with a Langmuir probe. The mechanism of the oxidation reaction and of ion formation is discussed.