The adsorption and oxidation of SO2 on MgO surface: Experimental and DFT calculation studies
The heterogeneous oxidation of sulfur dioxide (SO2) to sulfate on the surface of MgO particles was investigated by in situ diﬀuse reﬂectance infrared Fourier transform spectroscopy (DRIFTS) and Density Functional Theory (DFT). The in situ DRIFTS spectra show that the major products of SO2 adsorption on MgO particle are sulfite, bisulfite and sulfate, while the coexisting NO2 and O2 promote the conversion of sulfite/bisulfite into sulfate. DFT calculations show that the main adsorption products of SO2 on perfect, hydroxylated and step sites of MgO (001) surface are sulfite (SO2, gas+Olattice→SO3, ads), bisulfite and sulfate, respectively. The oxidation of sulfite to sulfate was hardly promoted by presence of O2 in the room temperature, because the process needs to overcome an energy barrier of 0.67 eV. The coexistence of NO2 couldn’t promote the formation of sulfate, because the direct oxidation of sulfite into sulfate by NO2 is difficult (ΔEa=1.08 eV). However, coexisting NO2 can facilitate the oxidation of SO2 by O2, which illustrates the micro-mechanism of synergistic effect between SO2 and NO2 on MgO. In contrast to sulfite, surface bisulfite formed through the adsorption of SO2 on surface OH could be oxidized by O2 or NO2 via barrierless processes. This mechanism highlights the synergistic effects of O2/NO2 as well as surface hydroxyl on the heterogeneous oxidation of SO2 on the MgO surface. Atmospheric implications of heterogeneous oxidation of SO2 on MgO under ambient atmosphere are discussed.