Thermal Decomposition of Phosgene and Diphosgene

Phosgene (COCl2) is a toxic compound used or formed
in a wide range of applications. The understanding of its thermal
decomposition for destruction processes or in the event of accidental
fire of stored reserves is a major safety issue. In this study, a
detailed chemical kinetic model for the thermal decomposition and
combustion of phosgene and diphosgene is proposed for the first time.
A large number of thermo-kinetic parameters were calculated using
quantum chemistry and reaction rate theory. The model was validated
against experimental pyrolysis data from the literature. It is predicted
that the degradation of diphosgene is mainly ruled by a pericyclic
reaction producing two molecules of phosgene and, to a lesser extent,
by a roaming radical reaction yielding CO2 and CCl4. Phosgene is much more stable than diphosgene under high-temperature
conditions, and its decomposition starts at higher temperatures. Decomposition
products are CO and Cl2. An equimolar mixture of the latter
molecules can be considered as a surrogate of phosgene from the kinetic
point of view, but the important endothermic effect of the decomposition
reaction can lead to different behaviors, for instance, in the case
of autoignition under high pressure and high temperature.