Electronic properties of the CaO/MgO mixed oxide surfaces. NO and SO2 adsorption

M. M. BRANDA, H. A. RODRIGUEZ, N. J. CASTELLANI

Conferencia; 13th International Conference on Solid Films and Surfaces; 2006

         The relationship between surface electronic and chemical properties of mixed oxides composed by Ca and Mg is studied in this work. For that purpose, the adsorption and reactivity of small molecules on both basic and acid surface sites were considered. From a technological point of view, it is important to have a deeper knowledge of these properties and to achieve the best Ca/Mg mixing to eliminate pollutant molecules. In particular, oxide surfaces with different Ca/Mg ratios were studied. The corresponding crystalline structures were those of pure oxides or those where the second metal ions behave as substitutional defects.          The NO and SO2 adsorption processes on these oxides were studied in the framework of DFT (Density Functional Theory) and using the cluster approach. The effect of solid matrix was taken into account by embedding the clusters with ECPs (effective pseudo-potentials core) and point charges. The clusters plus ECPs were:  Ca30-nMgnO30CA25 and Mg30-nCanO30MG25, where Ca and Mg are calcium and magnesium atoms and CA and MG are calcium and magnesium pseudopotentials respectively. The calculations were performed employing the Gaussian03 code.                 NO does only physisorb on MgO(100). This weak interaction is mainly due to electrostatic forces. When the Ca/Mg surface ratio increases, the NO interaction with the oxide surface is greater. This molecule binds quite strongly with the O5c anions, forming a relatively short bond with an O5c-NO distance of ~1.5 Å, and it is tilted from the surface normal giving an O-N-O5c angle of ~108°. The O atom of NO acquires negative charge from the surface and it is equidistant from the two neighboring Ca cations on the surface (Me-ON) ≈ 2.2 Å. The spin is completely localized on the NO molecule. Despite that the bonding between NO and oxide surface has a covalent character, a significant charge delocalization over NO is present, as it is evidence by the net atomic charges obtain from the Mülliken analysis.                 Planar SO3 species are found on the oxide surfaces. A link is formed between a basic surface oxygen ion and the S atom of the SO2 molecule. Our results indicate that in order to attain an efficient removal of SO2 the oxide surfaces must have oxidant sites to convert the SO2 to SO3, which then can be removed as a sulfate.