Using molecular dynamics simulations with a ReaxFF reactive force field to develop a kinetic mechanism for ammonia borane oxidation

Ammonia borane is a hydrogen rich compound recently studied as a means for high density hydrogen storage. Ammonia borane also has the potential for increasing performance in a propulsion system when introduced as a fuel. The chemical kinetics of ammonia borane oxidation have been studied using molecular dynamics simulations performed with a ReaxFF reactive force field, which in turn, is based on ab initio data. This approach allows for the development of a continuum model of ammonia borane oxidation, which after refinement can be used to model fundamental experiments, without any prior experimental data. The results of molecular dynamics simulations elucidate the pertinent chemical pathways and intermediate species needed to define the elementary reactions of a simple chemical kinetic mechanism. These simulations show that the gas phase ammonia borane molecule first undergoes two hydrogen elimination steps. Subsequently, the H₂ reacts with the O₂ in the system, while the boron side of the remaining HNBH molecule is attacked by oxygen, eventually leading to the cleavage of the B-N bond and the formation of the equilibrium products H₂O, HOBO, and N₂. Density functional theory calculations were used to calculate unknown thermochemical properties, and simple collision theory was used to estimate reaction rate constants. The resulting continuum kinetic model was used to perform simple closed reactor, constant pressure and energy calculations in CHEMKIN, the results of which are consistent with the observations made at the atomistic level in the molecular dynamics simulations. This work demonstrates how a combination between ab initio calculations, molecular dynamics and thermodynamics equilibrium calculations can be employed to elucidate complex combustion reaction kinetics.