High temperature thermal decomposition of acetylene and diacetylene at low relative concentrations

Abstract

The initial steps involved in the thermal decomposition of acetylene and diacetylene in the temperature range 1850 to 3000 K have been investigated by monitoring the hydrogen-atom production behind reflected shock waves in highly diluted mixtures of argon (5–400 ppm, total densities 6 × 10 −6 to 2.7 × 10 −5 mol/cm 3). The technique of atomic resonance absorption spectrophotometry (ARAS) was used to measure the H-atom concentration. In the low pressure region for unimolecular decomposition the initial step of the reaction can be characterized as: C 2 H 2+M → k 1 C 2 H+ H+M for which the rate coefficient k 1=(3.6±0.6)10 16 exp ( −E 1 RT ) cm 3 mol −1 s −1 E 1 R = 53600±400 K T:2100−3000 K M:8×10 −6−2.6×10 −5 mol cm −3 was obtained. The measurements also favor the recent value of 127 kcal/mol for the formation enthalpy of C 2H published by Okabe and Dibeler. In addition, it was possible to derive for the reaction C 4 H 2 → k 3 C 4 H+ H a rate coefficient k 3=(2.2±0.6)10 14 exp ( −E 3 RT ) s −1 E 3 R = 58700±700 K T:1850−2300 K M:1×10 −5−2.4×10 −5 mol cm −3 which is not far from the high pressure limit. It was also possible to derive rate coefficients in certain regimes of the investigated temperature interval at total densities around 10 −5 mol cm −3 for the following reactions: C 2 H 2+ C 2 H → k 2 C 4 H 2+ H k 2=3−4·10 13 cm 3 mol −1 s −1 T:2300−2700 K C 4 H → k 4 C 4+ H k 4=0.5−2.2·10 14 exp ( E 3 RT ) s −1 T:2100−2300 K C 2 H 2+ C 2 H 2 → k 6 C 4 H 2+2 H k 6=(3±1)·10 8 cm 3 mol −1 s −1 T:1850−2000 K C 4 H 2+ C 4 H → k 7 C 8 H 2+ H k 7=1.5−5.5·10 13 cm 3 mol −1 s −1 T:1850−2300 K C 4 H 2+ C 2 H → k 9 C 6 H 2+ H k 9=1−4·10 13 cm 3 mol −1 s −1 T:2400−2700 K