Ag. Pelmenschikov et al., UNIMPORTANCE OF THE SURROUNDING LATTICE IN THE ADSORPTION OF CO ON LOW-COORDINATED MG SITES OF MGO, JOURNAL OF PHYSICAL CHEMISTRY B, 102(12), 1998, pp. 2226-2231
The frequency shifts and binding energies of CO adsorbed on three-coor
dinated (3c) and four-coordinated (4c) ME sites of MgO via the C-end a
re computed using bare, stoichiometric cluster models of progressively
increasing size: CO . MgnOn, n = 4, 10, 13, 19 for the Mg-3c site and
n = 6, 10, 12, 20 for the Mg-4c site, Calculations are performed by t
he SCF and DFT methods with a combined basis set which effectively pro
vides the 6-311+G(2df) level of treatment. No marked dependence of the
calculated properties on the model size is found. All the bare cluste
r models reproduce the related experimental frequency shifts and bindi
ng energies of CO on MgO to about the same accuracy as the embedded cl
uster models based on the total ion ab initio model potential (AIMP) f
ormalism [Nygren, M, A.; Pettersson, L. G. M. J. Chem. Phys. 1996, 105
, 9339], Unlike the common assumption that the surrounding lattice mak
es a significant contribution to the adsorption characteristics of CO
on Mg-3c and Mg-4c 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 Me and its nearest surr
oundings, the effect of the residual lattice is insignificant.