The effect of water on the heterogeneous reactions of SO2 and NH3 on the surfaces of α-Fe2O3 and γ-Al2O3

Abstract

In this study, a wall-coated flow tube reactor, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC) were used to investigate the effect of water on the heterogeneous reactions of SO₂ and NH₃ on the surfaces of α-Fe₂O₃ and γ-Al₂O₃. Flow tube results show that water exhibited little effect on the initial uptake coefficients (γ_BET) of SO₂ but the addition of NH₃ increased the γ_BET at various relative humidity (RH) values on both α-Fe₂O₃ and γ-Al₂O₃. The γ_BET of SO₂ in the presence of NH₃ was negatively RH dependent on α-Fe₂O₃, which decreased from (8.80 ± 2.30) × 10⁻⁶ at 9% RH to (2.24 ± 0.71) × 10⁻⁶ at 85% RH. In contrast, the γ_BET of SO₂ in the presence of NH₃ on γ-Al₂O₃ increased from (1.32 ± 0.45) × 10⁻⁶ at 10% RH to (2.56 ± 0.39) × 10⁻⁶ at 74% RH. Surface product analysis by DRIFTS and IC shows that ammonium sulfate-like species formed, whereas the variation of SO₄²⁻ and NH₄⁺ with RH differed on Fe₂O₃ and Al₂O₃. With the increase of RH, sulfate tended to transform into bisulfate on Fe₂O₃ with copresence of NH₃, whereas surface-coordinated sulfite transformed to water-solvated sulfite and water-solvated sulfate species on Al₂O₃ regardless of the presence of NH₃. During this process, the amount of sulfate decreased with RH on Fe₂O₃, as opposed to that on Al₂O₃; yet the NH₄⁺ was inhibited to form by adsorbed water on both surfaces. The different interaction mechanisms among water, NH₃, and SO₂ on Fe₂O₃ and Al₂O₃ as well as the atmospheric implications have been proposed.