SULFUR BONDING IN MOS2 AND CO-MO-S STRUCTURES

The structure and bonding in small MoS2 structures with and without Co is studied theoretically using self-consistent density functional the ory with a non-local exchange-correlation energy. The structures model the catalysts used extensively in hydrotreating. We study in detail t he structure and binding energies as a function of the amount of sulfu r. The calculations show that extensive reconstructions occur at the t wo types of MoS2 edges where the sulfur dimerizes and occupies non-lat tice positions. These structures are shown to be in good agreement wit h available experimental data. We also study the energy required to fo rm sulfur vacancies, which are believed to be the active sites for man y hydrotreating reactions. The presence of Co atoms at the edges is sh own to lead to a significant lowering of the metal-sulfur binding ener gy. This imposes an increase in the concentration of active sites for the reactions and may thus explain the promoting effect of Co.