Detailed Kinetic Model for the Thermal Decomposition of Hydrazine Nitrate in Nitric Acid Solution Based on Quantum Chemistry Calculations Combined with the Polarizable Continuum Model

The decomposition mechanism of hydrazine
nitrate in nitric acid
solutions was investigated using quantum chemistry calculations combined
with the polarizable continuum model at the CBS-QB3//ωB97X-D/SMD
level of theory. These calculations provided a detailed kinetic model
incorporating rate coefficients and thermodynamic data. Rate coefficients
were determined using traditional transition state theory, while diffusion-limited
reactions were modeled based on the Einstein–Stokes equations.
The resulting model comprised the kinetics for 108 reactions and thermodynamic
data for 58 species. This model was validated by comparing simulations
of the variations in chemical species during the decomposition process
to experimental data acquired under isothermal conditions at 100 °C.
The model was found to accurately reproduce the concentration changes
of N2H4, HN3, and NH3 and
also explained the reaction mechanism. The thermal decomposition was
found to proceed via two parallel paths: N2H4 + HNO3 → H2O + HONO + N2H2 and N2H4 + HONO → HN3 + 2H2O. Following these reactions, a portion of
the HN3 decomposes to produce NH3 through a
multistep process. A sensitivity analysis showed that the rate of
decomposition is greatly affected by the pH of the solution.