Mechanism for the promotional formation of NH4+ by SO2 on different mineral dust surfaces

The promotion of the heterogeneous conversion of NH3 to NH4+ by SO2 on different mineral dust surfaces displays remarkable discrepancies, especially on MgO and α-Fe2O3 surfaces, however, the underlying mechanisms are not well known. Here, using periodic density functional theory (DFT) calculation and Born-Oppenheimer molecular dynamics (BOMD) simulation, we explored the heterogeneous adsorption of NH3 on MgO (110) and α-Fe2O3 (001) surfaces in the presence and absence of SO2.

The results show that on MgO (110) surface, hydrogen-bonding interactions of NH3 on both adsorbed hydroxyl or bisulfite/bisulfate sites are observed no matter whether SO2 is present or not. While, on the α-Fe2O3 (001) surface, significant conversion of NH3 to NH4+ occurs with the coexistence of SO2, which is due to the hydrogen transfer reaction from surface HSO4 to N in NH3. The fundamental reason may be that the stronger electron affinity of Fe3+ than Mg2+ results in adsorbed bisulfate and/or bisulfite with greater acidity on α-Fe2O3 surface than MgO surface.