(476aa) Understanding the Aqueous Phase Oxidation of Hydroxylamine by Nitric and Nitrous Acids Using Computational Chemistry

The oxidation of hydroxylamine to nitrous oxide and nitrous acid in
aqueous solutions of nitric acid is important in the Plutonium-Uranium
Reduction Extraction (PUREX) process.  A combination of computational chemistry
calculations and limited experimental data was used to construct a detailed
chemical kinetic model of the system.  Previously proposed models fit
experimental data over the narrow range where it was collected, but were much
simplified.  A method for estimating the thermochemistry of aqueous species
based on a combination of ab initio calculations and empirical parameters is
outlined.  A high-level CBS-QB3 gas-phase calculation was corrected for
solvation energy at the B3LYP/CBSB7 level.  Empirical bond-additivity
corrections were included to reduce the error between predicted and
experimental values.  The solvation entropy, which is very important to
non-equal molar reactions, was estimated using Pieriotti's equation for the
free energy of solvation and an empirical part to account for solute-solvent
interactions.  Traditional transition state theory was used to estimate rate
constants in solution, along with many diffusion-limited reaction rates.  A
kinetic model of the system was constructed using these estimates, with several
layers of modifications.  The first layer was altering the thermochemistry
slightly to match experimentally-determine pK_A values.  The second
layer was including experimental activity coefficients for the major species,
which modified the equilibrium constants of a number of reactions, especially
at high acid concentrations.  The final modification involved modest corrections
to several rate parameters, guided by sensitivity analyses, to achieve good
agreement with much of the available experimental yield data.  The results show
that with a small amount of experimental thermochemical data, the methodologies
outlined here may yield qualitatively reasonable predictions.  The resulting
models can be used to understand complicated systems, and may help guide
experiments or more precise solution phase rate estimation techniques.  The
model does not predict the stability of the system accurately, which is the
main deficiency in the model.  It is hoped that the present study will
encourage the development of more accurate, practical methods for estimating
energies and entropies of solvation needed for accurate modeling in condensed
phases.