PHYSISORBED AND CHEMISORBED CO2 AT SURFACE AND STEP SITES OF THE MGO(100) SURFACE

The interaction of CO2 with regular and defect sites of the MgO(100) surface has been investigated by means of ab initio cluster model SCF and correlated calculations. Full geometry optimization of the surface complexes has been carried out at the SCF level. CO2 adsorbs molecularly at the Mg2+ surface and step sites, forming an end-on linear complex with the molecular axis normal to the surface; a ''horizontal'' orientation where the molecule lies flat on the surface and interacts with two Mg2+ cations, is only slightly less favorable. The interaction of CO2 With surface oxide anions, O2-, and the consequent formation of surface carbonates has also been considered. While five-coordinated surface O2- ions are rather unreactive, carbonates form at low-coordinated defect sites. The carbonate formation at defect sites is a non-activated process. The local electrostatic interactions between the surface ions and the chemisorbed CO2 play an important role in determining the orientation of the surface carbonate. The analysis of the vibrational frequencies and of the core level binding energies of physisorbed and chemisorbed CO2 is fully consistent with the experimental observation that two distinct species coexist at the surface of MgO at low temperatures.