Molecular Insights into NO-Promoted Sulfate Formation on Model TiO2 Nanoparticles with Different Exposed Facets.

In this study, molecular insights into NO-promoted SO2 oxidation on model TiO2 with well-defined (001), (010), and (101) facets are investigated in the laboratory. The adsorbed SO2 is significantly promoted to transform into sulfate by the coexisting NO on the three facets. The appearance of active oxygen species indicates an active oxygen species-initiated NO oxidation. The resulting NO2 acts as an oxidant to convert adsorbed sulfite on hydroxyls to sulfate species. The (101) facet presents the best performance in the NO-promoted sulfate formation possibly owing to its desirable structure to accommodate SO32-, NO2, and molecular water. The uptake coefficient (γt) of SO2 increases by the coexistence of NO on the (001) facet at 0% RH and is relative humidity (RH) dependent, which first decreases from 0% RH to 32% RH but then increases in the range of 32%-80% RH. The probable explanation is the combined contribution of the blocking effect of water and the hydration of SO2. The finding in this study provides insight into the possibility of its occurrence on common mineral dusts and requires further investigation.