Paramagnetic centers in particulate formed from the oxidative pyrolysis of 1-methylnaphthalene in the presence of Fe(III)2O3 nanoparticles

Abstract

The identity of radical species associated with particulate formed from the oxidative pyrolysis of 1-methylnaphthalene (1-MN) was investigated using low temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), a specialized technique that provided a method of sampling and analysis of the gas-phase paramagnetic components. A superimposed EPR signal was identified to be a mixture of organic radicals (carbon and oxygen-centered) and soot. The carbon-centered radicals were identified as a mixture of the resonance-stabilized indenyl, cyclopentadienyl, and naphthalene 1-methylene radicals through the theoretical simulation of the radical's hyperfine structure. Formation of these radical species was promoted by the addition of Fe(III)₂O₃ nanoparticles. Enhanced formation of resonance stabilized radicals from the addition of Fe(III)₂O₃ nanoparticles can account for the observed increased sooting tendency associated with Fe(III)₂O₃ nanoparticle addition.

Keywords: Annealing studies; Cryogenic trapping; EPR; Environmentally Persistent Free Radicals (EPFRs); Nanoparticles; Particulate.

Fig. 1

Dual-zone reactor capable of generating metal oxide nanoparticles (Zone 1) in the presence of a high sooting 1-MN fuel (Zone 2) coupled with a cold finger for condensation of paramagnetic species. Soot was collected through a moveable probe stationed at sampling ports denoted I–IV in the isothermal region as well as sampling points A (700 °C) and B (475 °C) in the quenching zone of reactor, and sampling point C (80 °C) in the exhaust stream.

Fig. 2

(A) EPR spectra of particulate accumulated at sampling port C using the cryogenic method. (B) Spectrum A on expanded scale to show detail. (C) EPR spectra of particulate collected on a cellulose-ester filter at the same sampling position.

Fig. 3

(A) EPR spectra of particulate collected on Dewar cold finger. (B) EPR spectra after gradual annealing of the matrix. (C) EPR spectra after further annealing. (D) The subtraction spectrum, A and B, provides a residue spectrum of the mixture of annihilated organic radicals.

Fig. 4

Power dependence of spectra of particulate collected on Dewar cold finger. At low microwave power (0.1 mW), the organic radical is resolved.

Fig. 5

Normalized EPR spectra of soot particles detected in Zone 2, in the quenching zone of the reactor (ports A and B) and in the exhaust stream, port C.

Fig. 6

Optimized geometries of radicals mentioned in the text; C atoms are gray, H atoms are red. On the right side; simulated spectra of radicals formed from the oxidative pyrolysis of 1-MN. The Win-EPR Simfonia simulation software was used with parameters: ΔH_p–p for each individual line − 2.5 G, Lorenzian lineshape, g = 2.0030. The hyperfine splitting constants for cyclopentadienyl radical (with five equivalent protons) was 6.2 G [36]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Comparison of experimental EPR spectra with simulated spectrum composed of indeyl, cyclopentadienyl, and naphthalene 1-methylene radical. The Win-EPR Simfonia simulation software was used with parameters: ΔH_p–p for each individual line − 2.5 G, Lorenzian lineshape, g = 2.0030.

Fig. 8

Observed enhancement in radical formation as a result of Fe(III)₂O₃ nanoparticle introduction prior to soot inception.

Fig. 9

Accumulated Fe(II)₂O₃ nanoparticles on the Dewar cold-finger exhibited the same spectral features as a ng quantity of an Fe(II)₂O₃ standard.