Identification of Intermediates in the Reaction Pathway of SO2 on the CaO surface: From Physisorption to Sulfite to Sulfate.

The interaction of CaO and Ca(OH)2 with solvated or gaseous SO2 plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO2 from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO2 with a CaO(001) single crystal. First, the surface chemistry of SO2 was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO2 solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.