AN ELECTRONIC-STRUCTURE STUDY OF H-2 AND CH4 INTERACTIONS WITH MGO AND LI-DOPED MGO CLUSTERS

An ab initio study is presented concerning the chemisorption of hydrogen on a model of the (100) surface of MgO and Li-doped MgO. The local surface environment was modeled employing cubic and tetragonal clusters composed of 8 and 12 atoms, respectively. The lattice constant for the clusters was fixed at the experimentally determined value for bulk MgO and the geometry of the adsorbate was optimized at the unrestricted Hartree-Fock (UHF) level of theory. Correlation energy was treated at the second-order unrestricted Moller-Plesset (UMP2) level at the UHF optimized geometry. It was found that H-2 undergoes heterolytic dissociation at neighboring three-coordinated Mg and 0 sites (denoted Mg3c and O3c) in MgO with activation energies of 4.2 and 2.4 kcal/mol at the UHF and UMP2 levels, respectively. Li-doped MgO did not support heterolytic dissociation at neighboring Mg and O sites. Instead H-2 was found to dissociate homolytically without barrier at two O3c sites and to undergo hydrogen atom abstraction at O3c and O4c sites. At the UHF/UMP2 level, it was found that at O3c sites, abstraction occurs with a 17.9/3.0 kcal/mol barrier, and at O4c sites, abstraction occurs with a 15-7/0.6 kcal/mol activation energy.