Modelling the Adsorption of Thiophene and Benzene on MoS₂: on the Importance of a Good Surface Description

S. Cristol^a, J.-F. Paul^a, E. Payen^a, E. J. M. Hensen^b, R. Koide^c, R. A. van Santen^b

^aUCCS UMR 8181, USTL, Villeneuve d'Ascq, France

^bSK, Eindhoven University of Technology, Eindhoven, The Netherlands

^cJapan Energy Corporation, Saitama, Japan.

E-mail sylvain.cristol@univ-lille1.fr

Hydrodesulfurization (HDS) is a key step of the petroleum industry that consists in the removal the heteroatoms through a high temperature treatment of the feedstock under a high hydrogen pressure. The mechanism of this reaction is still under debate, even if it is widely accepted that the reaction takes place on the edges of the molybdenum sulfide nanoparticles. First theoretical studies have focused on the adsorption of the reactive molecules on the perfect surfaces, some of which presenting highly unsaturated metallic atoms. On the chemical point of view though, such surfaces are very unlikely to be present on the edges of the active phase as H₂S present in the gas phase will react with the unsaturated metal atoms. We have therefore undertaken a systematic study of thiophene and benzene adsorption both on the perfect surfaces and on more realistic surfaces that take into account the surrounding gas phase (1). As far as thiophene is concerned, on the perfect surface the adsorption takes place in a flat eta5 mode in which all the atoms of the cycle are interacting with surface metal atoms.

On the other hand, on more realistic surfaces, the adsorption mainly takes place in a eta1 mode, only the sulphur atom interacting with the surface. Comparisons between computed vibrational frequencies and the FTIR spectroscopy shows that this adsorption mode is indeed the most favoured. Such an adsorption mode can only be explained by a complete saturation of the surface of the active phase with sulphur atoms and confirms the importance of a good surface description in the catalytic conditions. Such adsorption is impossible for benzene and we tested its adsorption of different sites. Some of them allow the adsorption in eta2 or eta5 mode but require the departure of one sulphur atom from the stable surface that is highly unfavourable. The question of the hydrogenation site in HDT catalysts is then still open although it has been proposed recently (2) that this reaction could take place on the basal plane of the MoS₂ crystallites.

(1) S. Cristol et al., J. Catal., 239 (2006) 145.

(2) P. G. Moses et al., J. Catal., 248 (2007) 188.