Molecular adsorption of methane and methyl onto MgO(100) - An embedded-cluster study

Embedded cluster models have been used to model the molecular adsorption of methane and methyl at the perfect (100) surface of MgO. Energies are compu ted using the modified coupled pair functional method, with corrections for basis set superposition errors applied both during geometry optimization a nd to adsorption energies. In the case of methane, interadsorbate interacti ons are taken into account through ab initio pair-interaction energies, fac ilitating adsorption energies at monolayer coverage. Methane is found to ad sorb preferentially at magnesium sites, in a dipod configuration, i.e. with two hydrogen atoms pointed down and towards oxide anions. At monolayer cov erage, neighboring methane molecules are rotated 90 degrees relative to eac h other, keeping the dipod orientation. An adsorption energy of 8.5 kJ mol( -1) is obtained for methane, which is somewhat low compared to experimental estimates. Methyl adsorbs preferably over Mg2+ sites, with a binding energ y only slightly higher than for methane. No elements of covalency was detec ted in the bond between the radical and the MgO(100) surface. (C) 1999 Publ ished by Elsevier Science B.V. All rights reserved.