Adsorption of CO on MgO supported alkali monolayers: a periodic density functional local density approximation and generalized gradient approximation study

The adsorption of CO on the MgO supported Na and K monolayers was investigated using a periodic density functional method. We found that the cooperative (non-pairwise) interaction present among the three constituents of the complex system, the MgO slab, the alkali metal monolayer, and the CO monolayer, is very small and that the only role of MgO is to support the alkali metal monolayer. Our calculations predict weakening of the CO bond due to the presence of the alkali metal monolayer, reflected by an elongation of the CO bond by 0.005 Å and a redshift of the CO stretching frequency by ca. 30 cm−1 in comparison with the corresponding gas phase values. The calculated density of states for the Mg⋯M, CO, and Mg⋯M⋯CO systems (M=Na, K) indicates that the electron transfer from the valence band of the alkali metal to the antibonding π∗ orbital of CO is the source of the CO bond weakening. Our estimate of the binding energy for CO on the MgO⋯M substrate is 5.1 and 5.0 kcal/mol for M=Na and K, respectively.