High-Temperature Gas-Kinetic Study of Carbonyl Sulfide Pyrolysis Performed with a Shock Tube and Quadrupole Mass Filter

A quadrupole mass filter coupled to a shock tube, described in Part I, was used to study the chemical kinetics of the thermal decomposition of carbonyl sulfide in the temperature range 2000° to 3200°K at total densities of 2 to 3×10−6 mole/cc. The equipment previously described was unable to detect concentrations of carbonyl sulfide low enough to prevent clogging of the sampling leak by solid sulfur. This difficulty was eliminated by the design of a new rapid-pumping ion source, described here, which can detect 5×10−10 mole/cc OCS in the shock tube. Mixtures of 0.5%, 2%, and 4% OCS in argon were heated by reflected shocks. In the 0.5% mixtures the ion currents of all the principal species, OCS, CO, S, S2, CS, and SO, were monitored. For the 2% and 4% mixtures only the ion current of OCS was monitored. In each dynamic experiment, the mass filter was tuned to follow the concentration of a single species. A large number of experiments was performed at different temperatures for each species with the following results: (a) The mechanism between 2000° and 3200°K at low pressures involves a unimolecular, second-order rupture followed by two bimolecular atom abstractions: OCS+Ar→ lim k1CO+S+Ar, ΔH00=71.5 kcal/mole,S+OCS→ lim k2CO+S2, ΔH00=−29.5 kcal/mole,S+OCS→ lim k3CS+SO, ΔH00=22.4 kcal/mole.(b) In the 0.5% mixtures, S atom did not approach a steady-state value in the available testing time. (c) For 0.5% mixtures of OCS, k1 is approximately half the apparent k1 for the 2% and 4% mixtures. This implies that for the high concentrations, k2 (S) rapidly approaches k1 (Ar). (d) The critical activation energy for the rate-determining step is not known. However, for 0.5% mixtures and an assumed critical activation energy of Ea=71.5 kcal/mole, k1 fit a quantum RRKM calculation with 1% collisional deactivation efficiency. This result is well represented by the following Kassel expression: k1=1.11×1011(T/deg)1/271.5(kcal/mole)/RT1.87×exp−71.5(kcal/mole)/RTcc/mole·sec.Further arguments suggest Ea to lie between 71.5 and 82 kcal/mole. (e) An estimate of k2=6.0×1011 cc/mole·sec at 2570°K is consistent with all the behavior observed. From k2 an activation energy of 27±6 kcal/mole is estimated. (f) The difference in activation energy between Reactions (3) and (2) is ∼19 kcal/mole.