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 SO₃^2-, NO₂, and molecular water. The uptake coefficient (γ_t) 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.