Fe(III)2O3 Nanoparticle Mediated Molecular Growth and Soot Particle Inception from the Oxidative Pyrolysis of 1-Methylnaphthalene

Abstract

The potential for Fe(III)2O3 nanoparticles to participate in the molecular growth and particle inception of soot from 1-methylnapthalene (1-MN) was studied in a dual-zone, high-temperature flow reactor. An iron substituted, dendrimer template was oxidized in the Zone 1 reactor to generate ~5nm Fe(III)2O3 nanoparticles, that were continually seeded to a high sooting 1-MN fuel under oxidative pyrolysis conditions. The Fe(III)2O3 nanoparticles could then influence the otherwise gas-phase reactions of 1-MN. Increased PAH formation, soot number density, soot mass yield, and soot radical intensity were observed as a result of Fe(III)2O3 nanoparticle introduction prior to soot inception. Low Temperature Matrix Isolation-Electron Paramagnetic Resonance (LTMI-EPR) spectroscopy was used to determine the superimposed signal of particulate from the oxidative pyrolysis of 1-MN. The EPR spectrum of particulate was found to be a mixture of organic carbon-centered radicals, oxygen-centered radicals, and soot. The carbon-centered radicals were identified through simulation of the hyperfine structure to be the resonance-stabilized indenyl, cyclopentadienyl, and naphthalene 1-methylene radicals. The species of iron oxide nanoparticles generated from the iron substituted dendrimer was also determined with the LTMI-EPR technique to be Fe(III)2O3 nanoparticles clusters and superoxide anion-radicals, O2•- adsorbed on nanoparticle surfaces in form of Fe (IV)-O2•-. Superoxide anion-radicals initiate radical chain reactions through abstraction of hydrogen whereas Fe(III)2O3 nanoparticles are capable of the stabilization of the initially formed radicals species. Subsequent formation of soot also resulted in a partial reduction of the Fe(III)2O3 nanoparticles. These studies contained herein suggest Fe(III)2O3 nanoparticles can mediate the formation and stabilization of resonance stabilized radicals and subsequent reactions can result in the molecular growth of PAHs into soot. At the high concentrations in the flow reactor, these resonance–stabilized free radicals can undergo surface-mediated, radical-radical, molecular growth reactions and contribute to molecular growth and soot particle inception.