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