MOLECULAR MECHANICS STUDY OF THE INTERACTION OF MOLYBDENUM-DISULFIDE LAYERS WITH A GAMMA-ALUMINA SUPPORT IN HYDROTREATING CATALYSTS

The non-bonded interactions of MoS2 slabs supported on gamma-Al2O3, wh ich are the active phase of a hydrotreatment catalyst, have been model led by molecular mechanics. Free MoS2 sheets, containing 39 molybdenum atoms, have been used to test the reliability of the force field. The edge MoS2 planes and the support surfaces have been ideally modelled in order to investigate the influence on energy of geometrical paramet ers such as the orientation of the MoS2 layered cluster on gamma-Al2O3 or the distance between the support and the MoS2 slabs. Whatever the support surface, a single MoS2 slab lying flat on the support is more stable than a slab perpendicular to the support, however, the energy d ifference between these two configurations decreases as the number of stacked MoS2 sheets increases. The energy of the interactions between two contiguous layers or between a sheet and the exposed surface of th e support shows that single sheets lying flat are favoured on the [111 ] plane whereas the [110] plane induces a stacking of several slabs. T he intercalation of one layer between a slab and the support or betwee n two contiguous MoS2 slabs leads to a screen effect, characterized by a decrease of about two orders of magnitude in the interaction energy of the separated surfaces. This effect increases with the number of i nserted sheets.