Computer modelling of a molybdenum disulphide catalyst

Abstract

Small MoS₂ slabs, which are the active component of hydrotreating catalysts, have been modelled by molecular graphics and molecular mechanics. Slabs of various sizes with various degrees of edge unsaturation, have been investigated.

As the slab size was increased the minimised energy decreased with no minima in the range investigated. Therefore, a free slab does not adopt any particular optimum size. The finite and apparently preferred slab sizes observed experimentally in supported catalysts must therefore be determined by constraints on slab growth imposed, for example, by the support.

The structures, including distortions, and energies of slabs with different degrees of edge saturation, and of active sites with different degrees of coordinative unsaturation of edge molybdenum atoms, have been investigated. The saturated slab has a high strain energy and would be expected to lose its terminal HS groups easily (as found experimentally).

Active sites at molybdenum are created by removing edge sulphur atoms. Structures where two molybdenum atoms are not linked by at least one sulphur atom are unstable, owing to the repulsion between the molybdenum atoms. They distort to increase the distance between the molybdenum atoms.