Coadsorption of sodium and SO2 on MgO(100): alkali promoted S-O bond cleavage

The coadsorption of Na and SO2 on MgO(100) was studied using synchrotron ba sed high-resolution photoemission, X-ray absorption near-edge spectroscopy and first-principles density functional calculations. The adsorption of SO2 on a MgO(100) single crystal and MgO(100) films, grown epitaxially on a Mo (100) substrate, produces SO3 and SO4 species at 300 It. In general, the ep itaxial films are a good model for studying the chemistry of SO2 on MgO(100 ). Defects and imperfections in the MgO(100) films facilitate SO4 formation , but no dissociation of SO2 is observed on the films ol MgO(100) single cr ystal, Dissociation of S-O bonds is seen after coadsorbing SO2 with Na on t he MgO(100) surfaces. The Na adatoms provide occupied electronic states abo ve the MgO valence band which are very efficient for bonding interactions w ith SO2. In the {Na + SO2)/MgO(100) system, Na transfers electrons into the lowest-unoccupied molecular orbital of SO2 inducing a weakening of the S-O bonds that facilitates dissociation of the molecule, (C) 2000 Published by Elsevier Science B.V.