Elementary Kinetics of Soot Oxidation by OH
The goal of this dissertation is to elucidate detailed pathways, products, and rates leading to soot oxidation by OH. Such information would supplement current soot modeling which has generally focused more on growth reactions. Indeed, OH oxidation of soot has received no detailed theoretical analysis. The results presented here are a first step to fill this gap in soot modeling.
Due to the presumed importance of oxyradical decomposition to soot oxidation, the thermal decomposition of armchair oxyradicals was pursued. A number of different armchair prototype reactions were investigated to determine how rates vary based on oxyradical location and prototype size. Potential energy surfaces were explored and thermal decomposition rates were calculated. The results indicated that armchair edge oxyradicals decompose at rates similar to zigzag edge oxyradicals. These results were used in subsequent OH oxidation study.
Next, OH oxidation was studied on a series of prototype soot surfaces to identify which edge sites would lead to rapid oxidation by OH. The potential energy surfaces of three different reactions were explored. The carbon radical site was identified as the most likely candidate for a single OH radical attack leading to CO expulsion. A phenanthrene radical was selected as the prototype structure for this site, and the kinetics of this most promising reaction, OH + phenanthrene radical, were explored. This system, including a number of barrierless reactions, was explored using both chemical-activation and thermal decomposition master-equation simulations. The results confirmed the assumption that oxyradicals are key intermediates; multiple oxyradicals were found, all leading to CO expulsion. The overall rate coefficient of phenanthrene radical oxidation by OH forming CO was found to be insensitive to pressure and temperature and was approximately 1E14 cm3 mol-1 s-1. The OH + phenanthrene radical was also studied on the triplet surface. The rate in this case was found to be about a factor of 6 smaller than the singlet surface results.
The two principal processes involved in phenanthrene radical oxidation by OH were H atom migration/elimination followed by oxyradical decomposition. H atom migration/elimination made possible the relatively rapid rearrangement of the PAH edge, forming kinetically favorable oxyradicals which then decomposed. These same two processes are expected to be present in soot surface oxidation by OH. As a preliminary comparison of these theoretical results to soot oxidation experimental results, the collisional efficiency of OH with a phenanthrene radical was calculated and found to be a close match to the experimentally calculated collision efficiency of OH with a soot particle.