Identification of Intermediates in the Reaction Pathway of SO 2 on the CaO Surface: From Physisorption to Sulfite to Sulfate.

The interaction of CaO and Ca(OH) ₂ with solvated or gaseous SO ₂ plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO ₂ from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO ₂ with a CaO(001) single crystal. First, the surface chemistry of SO ₂ 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 SO ₂ 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 passivatating 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.
(© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)