Adsorption of NO and NO2 on terrace and step sites and on oxygen vacancies of the CaO(1 0 0) surface

Adsorption of NO and NO2 molecules on the surface of CaO has been investigated by means of density functional theory (DFT) cluster model calculations. Different embedding schemes (shell models and point charges) have been compared. Beside adsorption sites at flat terraces, defect sites like steps and oxygen vacancies (F and F+ centers) have been studied. The spin properties of the adsorbed molecules have been monitored by computing hyperfine coupling constants. At variance with MgO, NO is strongly bound to the oxide anions at terraces (≈0.8 eV) and steps (≈1.4 eV). The spin is localized on the NO molecule and the bonding has covalent character. NO2 adsorption is more complex. On the terrace sites, for the N-down adsorption we found two minima with similar energies but different surface-NO2 distances. In one case (short distance) the spin is entirely on NO2; in the second one (long distance), the bonding has partial charge-transfer character, with localization of 50% of the spin density on the surface oxide anions. The relative stability of the two structures depends on the level of treatment. In another isomer the molecule is oriented O-down and the bonding has a net charge transfer character (formation of NO2−). On step sites the N-down mode is definitely preferred, the O–N distance is short and the spin is on NO2. Finally, F and F+ centers on CaO(1 0 0) have similar characteristics as on MgO(1 0 0) and exhibit a strong reactivity towards NO and NO2.