Competing reactions of selected atmospheric gases on Fe3O4 nanoparticles surfaces

Abstract

Heterogeneous reactions on atmospheric aerosol surfaces are increasingly considered important in understanding aerosol–cloud nucleation and climate change. To understand potential reactions in polluted atmospheres, the co-adsorption of NO₂ and toluene to magnetite (Fe₃O₄i.e. FeO·Fe₂O₃) nanoparticles at ambient conditions was investigated for the first time. The surface area, size distribution, and morphology of Fe₃O₄ nanoparticles were characterized by BET method and high-resolution transmission electron microscopy. Adsorption isotherms, collected by gas chromatography with flame ionization detection, showed that the presence of NO₂ decreased the adsorption of toluene. The analyses of the surface chemical composition of Fe₃O₄ by X-ray photoelectron spectroscopy (XPS) reveal that, upon the addition of NO₂, the surface is oxidized and a contribution at 532.5 ± 0.4 eV in the O1s spectrum appears, showing that NO₂ likely competes with toluene by dissociating on Fe²⁺ sites and forming NO₃⁻. Different competing effects were observed for oxidized Fe₃O₄; oxidation occurred when exposed solely to NO₂, whereas, the mixture of toluene and NO₂ resulted in a reduction of the surface i.e. increased Fe²⁺/Fe³⁺. Analyses by time of flight secondary ion mass spectrometry further suggest toluene reacts with Fe³⁺ sites forming oxygenated organics. Our results indicate that on reduced magnetite, NO₂ is more reactive and competes with toluene; in contrast, on oxidized Fe₃O₄, toluene is more reactive. Because magnetite can assume a range of oxidation ratios in the environment, different competing interactions between pollutants like NO₂ and toluene could influence atmospheric processes, namely, the formation of Fe²⁺ and the formation of atmospheric oxidants.