Contrary Role of H₂O and O₂ in the Kinetics of Heterogeneous Photochemical Reactions of SO₂ on TiO₂

Although the heterogeneous conversion of SO2 on TiO2 has been considered a potential source of sulfate in the troposphere, little is known about the kinetics of SO2 uptake on TiO2 under ambient conditions. In this study, heterogeneous reactions of SO2 on a TiO2 surface at 298 K were investigated by a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The effect of UV irradiation, relative humidity (RH), and O2 on the uptake coefficients and adsorption amount of SO2 have been analyzed comprehensively. UV irradiation exhibits a slight effect on the initial uptake coefficient (γBET), which increases from 1.23 × 10–6 to 1.48 × 10–6 in air conditions and 5.89 × 10–7 to 6.42 × 10–7 in a N2 stream under dry conditions. Surface water exhibits a competition effect on the adsorption of SO2 on TiO2. An increase in RH (in the range of 0–75%) led to a decrease in the γBET and adsorption amount of SO2 both in the dark and the light reaction. In contrast, oxygen plays an important role in the uptake of SO2. Both the uptake coefficients and adsorption amount of SO2 on TiO2 decreased in the N2 stream compared to those in air conditions. Only sulfite was formed during the adsorption of SO2 in dark and dry conditions. The presence of UV irradiation was found to promote the conversion of SO2 to sulfate and the adsorption amount of sulfur species on the TiO2 surface. These results suggest that heterogeneous reaction on TiO2 as a sink for SO2 is not important in the atmosphere