A quantum chemical study of carbon monoxide adsorption on models of the Mg0 (001) surface.

Abstract

The adsorption of CO molecules on models of the MgO (001) surface was investigated using GAUSSIAN 92. Different sized cluster models and basis sets were used to determine the optimized geometries of the CO molecule, the dipole moment and the adsorption energy, and for some clusters, the surface geometry. Clusters of MgO one layer thick with nine atoms in the layer (3 x 3 x 1), one layer thick with twenty-five atoms in the layer (5 x 5 x 1), and two layers thick with nine atoms in each layer (3 x 3 x 2) were used as cluster models with the STO-3G, LANL1DZ and 3-21G* basis sets to determine the optimized CO position above the surface. These values were compared to values determined experimentally and by other computational models. The 3 x 3 x 1 cluster model with Mg or O as the central atom, the presence of an atom (possibly a defect) under the central atom, the charge on the cluster, and the orientation of the CO molecule were investigated for each basis set, and the results on the optimized geometry were compared.

The 3-21G* basis set and the 3 x 3 x 1 cluster model were used to determine dipole moments and binding energies for each of the above mentioned conditions. To model catalyst defects, a sulfur atom was placed below the central Mg and the geometry optimizations, dipole moment, and adsorption energy were determined using the 3-21G* basis set.

Finally, the Laplacian of the electron distribution was used to investigate the chemical properties of the interaction of the CO molecule with the MgO (001) surface. The effect of the different cluster conditions and CO orientations are demonstrated using contour plots of the Laplacian of the electron density -12r .