Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S-O bonds

Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are use d to examine the interaction of SO2 with pure and modified surfaces of magn esium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to for m SO3 and SO4 species. The bonding interactions with the Mg cations are wea k and do not lead to cleavage of S-O bonds. An identical result is found af ter adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (T iO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the o ccupied cations bands are too stable for effective bonding interactions wit h the LUMO of SO2. To activate an oxide for S-O bond cleavage, one has to c reate occupied metal states above the valence band of the oxide. DF calcula tions predict that in the presence of these "extra" electronic states the a dsorption energy of SO2 should increase, and there should be a significant oxide --> SO2(LUMO) charge transfer that facilitates the cleavage of the S- O bonds. In this article, we explore three different approaches (formation of O vacancies, promotion with alkali metals, and doping with transition me tals) that lead to the activation of SO2 and S-O bond breaking on MgO and o xides in general. Basic principles for a rational design of catalysts with a high efficiency for the destruction of SO2 are presented. (C) 2001 Americ an Institute of Physics.