An electronic structure study of H2and CH4interactions with MgO and Lihyphen;doped MgO clusters

摘要

Anabinitiostudy is presented concerning the chemisorption of hydrogen on a model of the (100) surface of MgO and Lihyphen;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 Hartreendash;Fock (UHF) level of theory. Correlation energy was treated at the secondhyphen;order unrestricted Moslash;llerndash;Plesset (UMP2) level at the UHF optimized geometry. It was found that H2undergoes heterolytic dissociation at neighboring threehyphen;coordinated Mg and O sites (denoted Mg3cand O3c) in MgO with activation energies of 4.2 and 2.4 kcal/mol at the UHF and UMP2 levels, respectively. Lihyphen;doped MgO did not support heterolytic dissociation at neighboring Mg and O sites. Instead H2was found to dissociate homolytically without barrier at two O3csites and to undergo hydrogen atom abstraction at O3cand O4csites. At the UHF/UMP2 level, it was found that at O3csites, abstraction occurs with a 17.9/3.0 kcal/mol barrier, and at O4csites, abstraction occurs with a 15.7/0.6 kcal/mol activation energy.