Pyrolysis of acetylene behind reflected shock waves

The thermal decomposition of acetylene has been studied in the temperature and pressure regimes of 1900–2500 K and 0.3–0.55 atm using a shock tube coupled to a time‐of‐flight mass spectrometer. A series of mixtures varying from 1.0–6.2% C 2 H 2 diluted in a Ne‐Ar mixture yielded a carbon atom density range of 0.24–2.0 × 10 17 atoms cm −3 in the reflected shock zone. Concentration profiles for C 2 H 2 , C 4 H 2 , and C 6 H 2 were constructed during typical observation times of 750 μs. C 8 H 2 and trace amounts of C 4 H 3 were found in relatively low concentrations at the high‐temperature end of this study. A mechanism for acetylene pyrolysis is proposed, which successfully models this work and the results obtained by several other groups employing a variety of analytical techniques. Two values of the heat of formation for C 2 H(134 ± 2 and 127 ± 1 kcal/mol) were employed in the modeling process; superior fits to the data were attained using the latter value. The initial step of acetylene decomposition involves competition between two channels. In mixtures (<200 ppm) where the acetylene concentrations are less than 2.18 × 10 −9 mol cm −3 , the decay is predominantly first order with respect to C 2 H 2 ; in mixtures >200 ppm, the dominant initial step is second order. The rate constant for the second‐order reaction is described by the equation Benzene concentrations predicted by the model are below the TOF detectability limit. C 4 H 3 was observed in the 6.2% C 2 H 2 mixture in accordance with the proposed mechanism.