The pressure and temperature dependence of the OH+C2H2 reaction above 800 K

Abstract

The rate constant for the reaction OH+C2H2 has been measured at 900, 1100, and 1300 K. The experimental method was that of laser pyrolysis/laser fluorescence, in which a pulsed CO2 laser heats a mixture of SF6, N2, H2O2, and C2H2. The rate constant is determined from the rate of decay of laser-induced fluorescence signals in OH, formed by pyrolysis of the peroxide and consumed by reaction with the acetylene. At the two higher temperatures the rate constant is independent of pressure between 10 and 120 Torr but at 900 K it was observed to be pressure dependent over a similar range. The rate constant at 1100 K is 2.7±0.6×10−3 cm3 s−1, and rises to 5.8±0.8×10−13 cm3 s−1 at 1300 K. Calculations of the temperature and pressure dependence of the addition channel OH+C2H2+M→C2H2OH+M were made using Troe’s approach, based on flow tube data at a lower temperature. These theoretical calculations are consistent with the present results as well as previous experimental measurements, showing a decrease in the importance of the addition channel with increasing temperature and the onset of a direct route in the region of 1000 K. These considerations of combined pressure and temperature dependence of reaction rates, and changes in mechanism, must be properly taken into account in detailed combustion chemistry models which cover a large range in temperature and pressure.