METHANE DISSOCIATION ON A NONPLANAR MGO(001) SURFACE - THEORETICAL MODELING OF SURFACE-DEFECTS

Accurate ab initio quantum mechanical calculations are presented for the abstraction of hydrogen from methane over three- and four-coordinated oxygen sites at an imperfect MgO surface. A local portion of the lattice close to the reaction site is described using a cluster model, whereas the surrounding crystal is represented by its Madelung potential. The four-coordinated oxygen has a closed-shell electronic structure and shows no activity towards hydrogen abstraction. The three-coordinated oxygen is a singly charged radical with high hydrogen affinity and the abstraction takes place here with a barrier of 15 kcal/mol. Subsequent adsorption of the methyl radical is found to be feasible and the adsorption geometry is partly optimized. An extension of the point-charge model in terms of projection operators is found useful when describing Pauli repulsion between the adsorbed methyl entity and neighboring Mg2+ cores.