Heterogeneous conversion of SO2 on nano α-Fe2O3: the effects of morphology, light illumination and relative humidity

Abstract

Hematite is one of the most important components in atmospheric mineral aerosols, which always presents different morphologies due to its different formation processes and various sources. However, less attention has been paid to the influences of the particle morphologies on atmospheric heterogeneous reactions as well as the contributions from illumination and moisture. In the present study, the impacts of these factors on the heterogeneous conversion of SO₂ on nano α-Fe₂O₃ particles with different morphologies were investigated in detail using in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The results indicated that the heterogeneous reactivities were significantly different among four α-Fe₂O₃ samples with various shapes. Compared to the nanocapsule-like and hollow nanoring-like α-Fe₂O₃, hexagonal nanoplate-like and agglomerated nanoparticle-like α-Fe₂O₃ exhibited significantly greater uptake capacity of SO₂ probably due to the abundant reactive sites and larger BET surface area and pore size. In addition, light illumination enhanced the heterogeneous conversion of SO₂ on the four nano α-Fe₂O₃ samples; this might be due to the semiconducting property of hematite and comparatively low light intensity. Furthermore, moisture promoted the heterogeneous uptake of SO₂ on hexagonal nanoplate-like α-Fe₂O₃ in appropriate range; however, excess moisture was unfavorable for sulfate formation. Possible mechanisms were proposed in the present study. This study would be helpful in understanding the atmospheric heterogeneous processes and provides useful parameters for atmospheric modelling studies.