Predicting the Formation Pathways and Morphologies of Oxygenated Carbonaceous Nanoparticle Precursors in Premixed Flames

Abstract

Organic nanoparticles are an inevitable by-product of combustion phenomena that have deleterious health and environmental effects. They are carcinogenic because they damage biological cells due to their small size and their presence in the atmosphere contributes to global warming. We would be better able to effectively manage the harmful effects of these nanoparticles if we better understood their formation mechanisms and chemical compositions at an atomic level. The complexities of the reaction chemistry involved along with the difficulties of experimental techniques to capture the atomic level details of nanoparticles and their chemical precursor molecules during flame synthesis, has led to a gap in the understanding of their formation pathways and molecular structures. This work presents a novel chemical kinetic reaction scheme and utilizes a computational approach to model laboratory-scale flames in order to elucidate the compositions and morphologies of organic nanoparticle precursors. Organic nanoparticles formed during combustion have long been assumed to comprise only hydrogen and carbon atoms, however, recent work has noted the presence of oxygen atoms. Using the first model to account for oxygenation of aromatic precursors, this work demonstrates that oxygen chemistry is key to understanding the formation pathways and morphologies of nanoparticles and their chemical precursors. Kinetic oxygenation pathways capture the influence of alcohol-doped-fuel on particle formation in premixed flames by identifying the fuel’s effect on precursor growth.

Stochastic simulations reveal an abundance of previously unconsidered oxygenated aromatic species to be present in premixed aromatic- and aliphatic-fuel flames. Key morphologies of oxygenated precursor species predicted by the model were confirmed in experiments, including a significant presence of furanic compounds. Similarly, simulations led to experiments that confirmed model predictions that large oxygenated aromatic molecules are important participants in particle formation. The model developed in this work demonstrates for the first time that inclusion of oxygenation pathways is necessary and vital in order to represent the chemical kinetic growth of nanoparticle precursors in premixed flames. The recognition of the previously unexpected importance of oxygenated aromatic precursors and their influence on nanoparticle formation in flames constitutes a notable advancement in the field of combustion-generated nanoparticle chemistry. The impact of the present findings are considerable to the efforts to investigate combustion generated particle formation with the aim to reduce their deleterious health and environmental effects.