Interaction of SO2 with MgO(100) and Cu/MgO(100): Decomposition Reactions and the Formation of SO3 and SO4

Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFT-GGA) were used to study the interaction of SO 2 with clean and modified (OH, O δ-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of MgO was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO 2,gas + O lattice → SO 3,ads ). No evidence is found for bonding of SO 2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a η 3 -S,O,O adsorption configuration leads to a SO 3 -like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO 4 . On a flat MgO(100) substrate, the formation of SO 4 is not energetically viable. A SO 3 → SO 4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. From 450 to 650 K, the adsorbed SO 3 /SO 4 species decompose and SO 2 desorbs back into gas phase. The presence of OH groups and O δ- (δ < 2) species on MgO favors the formation of SO 4 at the expense of SO 3 . On the other hand, the creation of O vacancies in MgO(100) by ion sputtering leads to decomposition of SO 2 . The chemistry of SO 2 on Cu/MgO(100) surfaces is rich. At 150 K, the SO 2 molecule chemisorbs intact on the supported Cu particles and forms SO 3 on the oxide substrate. Heating to room temperature induces full decomposition of SO 2 and the formation of SO 4 . The Cu adatoms facilitate the decomposition of SO 2 by providing electronic states that are very efficient for interactions with the lowest unoccupied molecular orbital (S-O antibonding) of the molecule.