Theoretical Calculation of Reaction Rates and Combustion Kinetic Modeling Study of Triethyl Phosphate (TEP)

Triethyl
phosphate (TEP) is an organophosphorus compound used as
a simulant for highly toxic nerve agents such as sarin GB. A high
temperature decomposition pathway during TEP pyrolysis has been proposed
previously and takes place via seven concerted elimination reactions.
A computational study to investigate the kinetics of these seven reactions
was carried out at the CBS-QB3 level of theory. The transition state
optimization was done at the B3LYP/6-311G­(2d,d,p) theory level, and
CanTherm was used to derive the Arrhenius coefficients. The pre-exponential
factors of the rate constant of these reactions were found to be up
to 50 times lower than the estimated values from the literature. In
addition, kinetics of reaction of the trioxidophosphorus radical (PO3) with H2 (H2 + PO3 →
HOPO2 + H), which is one of the important reactions in
predicting CO formation during TEP decomposition, was also investigated
computationally at the same theory level. The new kinetic parameters
derived from the computational study were used with the TEP kinetic
model proposed recently by our group. In addition, an alternative
decomposition pathway for TEP decomposition via H-abstraction, radical
decomposition, and recombination reactions was added. The proposed
mechanism was validated with the literature’s experimental
data, that is, intermediate CO time-history data from pyrolysis and
oxidation experiments and ignition delay times. Fairly good agreement
with experiments was obtained for pyrolysis and oxidation CO yield
within 1200–1700 K. The model was able to predict the ignition
times of the rich TEP mixture (φ = 2) within 25% of the experimental
results, while the discrepancies for stoichiometric and rich mixtures
were larger. Discussions on results of sensitivity and reaction pathway
analysis are presented to identify the important phosphorus reactions
and to understand the effect of addition of the alternative TEP decomposition
pathway.