(273c) Hydrogen Binding Versus Dissociation during Soot Formation: Insights from Reactive Molecular Dynamics

It has been proposed that two reactions are prominent in the earliest stages of soot nanoparticle formation; hydrogen abstraction via collision between a growing PAH (polyaromatic hydrocarbon) cluster and a hydrogen atom (yielding H₂ and a dehydrogenated cluster) and carbon addition via the binding of an alkane/alkene/alkyne chain to the cluster. We examine the first part of this process via reactive molecular dynamics using REAXFF C-H potentials. In simulations, cluster structures determined using the previously developed Atomistic Model for Particle Inception (AMPI) code of Violi and coworkers are equilibrated (at a predefined temperature) and a hydrogen atom with a specific velocity and impact parameter is driven towards the cluster. Depending upon the velocity, impact parameter, and collision orientation, we find that pure hydrogen abstraction is rare and instead at low velocities hydrogen binds to clusters. At increasing velocities, hydrogens rebound from the cluster surface and can drive hydrogen facilitated collision induced dissociation (CID) of the cluster. CID events yield smaller alkane/aklene/alkyne chains, which may serve to bind other clusters. Integration over all impact parameters and velocities for collisions (in accordance with the Maxwell Boltzmann distribution) yields the hydrogen binding rate coefficient and hydrogen induced dissociation rate coefficient for a given cluster structure. We discuss how these rates vary with temperature and cluster size.