UC Berkeley

This dissertation presents a newly established, detailed model of elementary reactions of soot oxidation. Surface oxidation is the primary mechanism by which carbon is removed from soot. Previously, soot oxidation has been modeled using a one-step global mechanism based on chemical analogy to phenyl oxidation. The one-step model, however, is limited to simple descriptions of surface geometry. Studies of soot growth revealed that chemical analogy alone is insufficient to describe the reactions taking place at the particle surface. Steric effects, neighboring sites, and substrate size must also be considered. For soot growth, studies performed by the Frenklach group found a much richer chemistry than previously thought to exist, most notably the incorporation of five-member rings into the substrate leading to curvature of the initially planar aromatics. Recent experimental studies have found that the soot oxidation rate depends on soot particle curvature, so a one-step global mechanism is insufficient to explain the experimental observations. The purpose of developing a detailed kinetic model is to gain a qualitative and quantitative understanding of oxidation processes of soot particles, large polycyclic aromatics, and graphene edges.

To build the oxidation model, the energetics and kinetics of key elementary oxidation reactions were investigated. Reaction rate coefficients and product branching ratios were calculated for the oxidation of six- and five-member ring graphene-edges by molecular oxygen at temperatures and pressures relevant to combustion. The reactions and their corresponding rates were added to an existing model of soot surface growth along with reactions for oxidation of six-member rings by OH, thermal decomposition of oxyradicals, and O, H, and OH abstraction and addition.

Detailed kinetic Monte Carlo (KMC) simulations of high-temperature oxidation by molecular oxygen of a graphene sheet were performed using the newly established model of graphene-edge reactions. The KMC results revealed two principal pathways for an oxyradical site: oxidation and regeneration of an aromatic radical site. The overall oxidation rate is computed to be time-dependent, with reactivity decreasing over time as the ratio of reactive edge sites decreases relative to the number of basal-plane carbon atoms. At the same time, the oxidation rate was found to be higher for graphene with a higher initial curvature. Both results are in accord with experimental observations. Analysis showed that distinct aspects of graphene-edge morphology are responsible for curvature either raising or reducing the oxidative reactivity of the graphene edge.

Oxidation pathways for graphene-edge five-member rings reacting with atomic oxygen were investigated. The rate coefficient for oxidation by atomic oxygen exceeded that of for oxidation by molecular oxygen by several orders of magnitude. The detailed surface oxidation model was augmented to include this reaction along with several more abstraction and oxidation reactions.

KMC simulations were then performed in an evolving gas phase environment that was coupled to the surface chemistry for conditions analogous to those of shock tube experiments of soot oxidation at high temperatures. The KMC results showed that the oxidation rate was dependent on several factors. CO concentration profiles calculated from the KMC model were found to be in agreement with experimental measurements.