Unimportance of the Surrounding Lattice in the Adsorption of CO on Low-Coordinated Mg Sites of MgO

Abstract

The frequency shifts and binding energies of CO adsorbed on three-coordinated (3c) and four-coordinated (4c) Mg sites of MgO via the C-end are computed using bare, stoichiometric cluster models of progressively increasing size: CO·MgnOn, n = 4, 10, 13, 19 for the Mg3c site and n = 6, 10, 12, 20 for the Mg4c site. Calculations are performed by the SCF and DFT methods with a combined basis set which effectively provides the 6-311+G(2df) level of treatment. No marked dependence of the calculated properties on the model size is found. All the bare cluster models reproduce the related experimental frequency shifts and binding energies of CO on MgO to about the same accuracy as the embedded cluster models based on the total ion ab initio model potential (AIMP) formalism [Nygren, M. A.; Pettersson, L. G. M. J. Chem. Phys. 1996, 105, 9339]. Unlike the common assumption that the surrounding lattice makes a significant contribution to the adsorption characteristics of CO on Mg3c and Mg4c sites of MgO, these results suggest its role to be unimportant. The analysis of the electric field (EF) in the adsorption region of these sites of the bare MgO clusters allows us to conclude that, due to the very steep decrease of the EF on moving away from the surface, both the frequency shift and binding energy of CO are mainly defined by the EF in the CO lone pair subregion nearby the adsorbing Mg. This finding justifies the adequacy of the bare cluster models: as the EF in the vicinity of the adsorbing Mg should be dominated by the smallest stoichiometric cluster including the Mg and its nearest surroundings, the effect of the residual lattice is insignificant.