Effect of Fe₂O₃ nanoparticles on combustion of coal surrogate (Anisole): Enhanced ignition and formation of persistent free radicals

This contribution explores the effect of nanoparticles of iron (III) oxide (Fe₂O₃) on the combustion of coal surrogate, i.e., anisole, identifying the changes in ignition features as well as the occurrence of persistent organic pollutants in the initiation channels. The method applies packed-bed reactor coupled with Fourier transform infrared (FTIR) spectroscopy to quantitate the ignition temperature under typical fuel-rich conditions, in-situ electron paramagnetic resonance (EPR) to elucidate the formation of environmentally-persistent free radicals (EPFR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to monitor the chemisorption of organic substrates on the nanoparticles, as well as X-ray diffraction for particles characterisation (PXRD). We employ cluster-based quantum mechanical calculation to map the reaction pathway within the scope of the density functional theory. The results of Fe₂O₃-mediated combustion of anisole depict an excessive reduction in ignition temperature from 500 °C around 220 °C at λ = 0.8. As confirmed both from EPR and DRIFTS measurements, the chemisorption of anisole on α-Fe₂O₃ surfaces follows the direct dissociation of the O-CH₃ (and OCH₂-H), leading to the formation of surface-bound phenoxy radicals at temperatures as low as 25 °C and incurring an estimated energy barrier of E_a = 18 kJ mol^-1 and a preexponential factor of A = 2.7 × 10¹² M^-1 s^-1. This insight applies to free-radical chain reactions that induce spontaneous fires of coal, as coal comprises ferric oxide nanoparticles, and equally to coexistence of aromatic fuels with thermodynamically reactive Fe₂O₃ surface, e.g., in fly ash, at the cooled-down tail of combustion stacks.