Direct observation of the rate of H-atom formation in the thermal decomposition of Ortho-, Meta-, and Para-xylene behind shock waves between 1300 and 1800 K

Abstract

The rate of H-atom formation in the thermal decomposition of o-, m- , and p -xylene was studied behind reflected shock waves between 1300 and 1800 K and at pressures of 1.6 to 4 bar by using atomic resonance absorption spectroscopy at 121.6 nm. For the thermal decomposition of p -xylene, m -xylene, and o -xylene p -CH 3 C 6 H 4 CH 3 → p -CH 3 C 6 H 4 CH 2 +H (R1) m -CH 3 C 6 H 4 CH 3 → m -CH 3 C 6 H 4 CH 2 +H (R2) o -CH 3 C 6 H 4 CH 3 → o -CH 3 C 6 H 4 CH 2 +H (R3) the following rate constants were determined: k 1 =1×10 16 exp(−380 kJ mol −1 /RT) s −1 , k 2 =1×10 16 exp(−382 kJ mol −1 /RT) s −1 , and k 3 =1×10 16 exp(−378 kJ mol −1 /RT) s −1 . The rate constants are all close to the high-pressure limit, and the accuracy was estimated to be 30%. The H-atom-concentration time profiles were modeled using a simple mechanism in which the parallel C-C bond split, and secondary reactions have been taken into account. We found that within our experimental conditions, the C-C bond scission contributes about 15% to the overall decomposition rate constant of the xylenes.