A THEORETICAL-STUDY OF THE ADSORPTION AND REACTION OF SO2 AT SURFACE AND STEP SITES OF THE MGO(100) SURFACE

The interaction Of SO2 With acid and basic sites of a clean, fully dehydroxilated, MgO(100) surface has been investigated by means of ab initio cluster model calculations. Full geometry optimization of the surface complexes has been carried out at the SCF level. The nature of the bonding has been analyzed in detail by performing a decomposition of the interaction energy as the sum of electrostatics, polarization and charge transfer contributions. SO2 adsorbs molecularly at five-coordinated Mg-5c(2+) surface cations forming an electrostatically bound complex where SO2 is bridge-bonded to two adjacent Mg2+ centres. The formation of a strongly chemisorbed sulphite, SO32-, species occurs when SO2 interacts with the basic 02- ions. However, a very different reactivity is observed for five-coordinated, surface, and four-coordinated, step, 02- sites. In fact, the O-5c(2-) ions of the surface are rather unreactive while the formation of the very stable SO32- species at O-4c(2-) step sites is a non-activated process. The different reactivity of the regular and defect basic sites of the MgO surface is explained in terms of the different Madelung potential at these sites hence of the different energetic cost for the MgO-SO2 charge transfer. No evidence is found for the formation of sulphate SO42-, species. The computed vibrational and photoemission features of the two forms of adsorbed SO2, physisorbed and chemisorbed, are consistent with the corresponding experimental spectra.