Enhanced Reactivity toward Oxidation by Water Vapor: Interactions of Toluene and NO2 on Hydrated Magnetite Nanoparticles

Abstract

In the atmosphere, water vapor affects the interaction of trace gases and particles, influencing key processes including cloud nucleation, radiation, and heterogeneous chemistry. In this study, the effect of water vapor on the reactions of toluene and NO2 on magnetite, a component of atmospheric dust particles, is investigated using a suite of analytical techniques, namely, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS). Adsorption isotherms show that water vapor reduces the adsorption of toluene on magnetite. XPS spectra reveal that exposure to water vapor results in limited dissociation and molecular adsorption of water, and partial oxidation of magnetite. When toluene is added, enhanced dissociation of water and oxidation of the magnetite surface are observed, strongly suggesting the importance of intermolecular interactions between water molecules and the interaction of toluene with the H-bonded network of adsorbed water. Upon addition of NO2, enhanced oxidation and NO3 are observed in XPS and TOF SIMS spectra, respectively. In contrast, on oxidized magnetite, less dissociation and sorption of water is observed, and no enhanced oxidation is observed. Our results show that hydrated magnetite surfaces inactive toward further water dissociation can be reactivated depending on the surface chemistry, due to Fe2+. We show that the effect of water vapor on the interaction of toluene and NO2 on magnetite depends on the Fe2+/Fe3+ ratio, which can vary under environmental conditions. Different reactivity of the Fe3O4 in dust can thus be expected, with implications on the fate of pollutants in the atmosphere.