Theoretical aspects of methane chemisorption on MgO surfaces. Modelling of impurity-induced trapping of a hole, surface defects and site dependence of methane chemisorption on (MgO)9,12 clusters

Abstract

The geometry and energetics of CH₄ chemisorption on various ion pair sites of the bare, Schottky defected and Li-doped (MgO)₉ and (MgO)₁₂ cluster models have been investigated at the MNDO-PM3 and modified ASED-MO levels of theory. The local environment (coordination geometry) of Mg_nc and O_nc ions at the chemisorption site (n is the coordination number of the respective ion site) was shown to be of crucial importance in modulating the mechanism and the energetic profile of the methane activation process. The computed chemisorption energy follows the trend: Mg_3c²⁺(corner) > Mg_4c²⁺(edge) > Mg_5c²⁺(face). Depending on the morphology of the catalyst and the reaction conditions the mechanism of methane activation by MgO catalysts can be described either by a homolytic or heterolytic chemisorption process. The homolytic chemisorption of methane is promoted by a hole or open-shell catalysis onto electrophilic oxide ion reactive centres of the catalyst exhibiting a high radical character. In order to effect the open-shell catalytic process, electronic holes in the topmost filled O 2p band of the catalyst may be generated by doping the catalyst with Li or introducing cationic vacancies in the crystal lattice. On the other hand, methane can also dissociate heterolytically at low-coordinated neighbouring Mg_nc and O_nc ion pair sites of the MgO catalyst in a more selective and stereospecific process. Furthermore, depending on the reaction conditions, methane heterolytic chemisorption can afford C₁ oxygenates, such as formaldehyde and methanol, rather than C₂ hydrocarbon derivatives. In both chemisorption processes, the catalytic activity increases with increasing concentration of the electrophilic oxide ion sites acquiring low coordination numbers. Catalysts exhibiting a high number of defect corner sites and high porosity are predicted to be the most reactive towards methane activation.